Photonic band gap of 2D complex lattice photonic crystal
Institute of Scientific and Technical Information of China (English)
GUAN Chun-ying; YUAN Li-bo
2009-01-01
It is of great significance to present a photonic crystal lattice structure with a wide photonic bandgap. A two-dimension complex lattice photonic crystal is proposed. The photonic crystal is composed of complex lattices with triangular structure, and each single cell is surrounded by six scatterers in an hexagon. The photonic band gaps are calculated based on the plane wave expansion (PWE) method. The results indicate that the photonic crystal has tunable large TM polarization band gap, and a gap-midgap ratio of up to 45.6%.
Photonic band gap engineering in 2D photonic crystals
Indian Academy of Sciences (India)
Yogita Kalra; R K Sinha
2006-12-01
The polarization-dependent photonic band gaps (TM and TE polarizations) in two-dimensional photonic crystals with square lattices composed of air holes in dielectric and vice versa i.e., dielectric rods in air, using the plane-wave expansion method are investigated. We then study, how the photonic band gap size is affected by the changing ellipticity of the constituent air holes/dielectric rods. It is observed that the size of the photonic band gap changes with changing ellipticity of the constituent air holes/dielectric rods. Further, it is reported, how the photonic band gap size is affected by the change in the orientation of the constituent elliptical air holes/dielectric rods in 2D photonic crystals.
Tunable Photonic Band Gaps In Photonic Crystal Fibers Filled With a Cholesteric Liquid Crystal
Institute of Scientific and Technical Information of China (English)
Thomas; Tanggaard; Larsen; David; Sparre; Hermann; Anders; Bjarklev
2003-01-01
A photonic crystal fiber has been filled with a cholesteric liquid crystal. A temperature sensitive photonic band gap effect was observed, which was especially pronounced around the liquid crystal phase transition temperature.
Topological Design of Cellular Phononic Band Gap Crystals
Directory of Open Access Journals (Sweden)
Yang Fan Li
2016-03-01
Full Text Available This paper systematically investigated the topological design of cellular phononic crystals with a maximized gap size between two adjacent bands. Considering that the obtained structures may sustain a certain amount of static loadings, it is desirable to ensure the optimized designs to have a relatively high stiffness. To tackle this issue, we conducted a multiple objective optimization to maximize band gap size and bulk or shear modulus simultaneously with a prescribed volume fraction of solid material so that the resulting structures can be lightweight, as well. In particular, we first conducted the finite element analysis of the phononic band gap crystals and then adapted a very efficient optimization procedure to resolve this problem based on bi-directional evolutionary structure optimization (BESO algorithm in conjunction with the homogenization method. A number of optimization results for maximizing band gaps with bulk and shear modulus constraints are presented for out-of-plane and in-plane modes. Numerical results showed that the optimized structures are similar to those obtained for composite case, except that additional slim connections are added in the cellular case to support the propagation of shear wave modes and meanwhile to satisfy the prescribed bulk or shear modulus constraints.
Band gap engineering in simultaneous phononic and photonic crystal slabs
Energy Technology Data Exchange (ETDEWEB)
Djafari Rouhani, B.; Pennec, Y.; Vasseur, J.O.; Hassouani, Y.El; Li, C.; Akjouj, A. [Universite de Lille1 Sciences et Technologies, Cite Scientifique, Institut d' Electronique, de Microelectronique et de Nanotechnologie, UMR CNRS 8520, Villeneuve d' Ascq (France); Boudouti, E.H.El; Bria, D. [Universite de Lille1 Sciences et Technologies, Cite Scientifique, Institut d' Electronique, de Microelectronique et de Nanotechnologie, UMR CNRS 8520, Villeneuve d' Ascq (France); Universite d' Oujda, Laboratoire de Dynamique et d' Optique des Materiaux, Faculte des Sciences, Oujda (Morocco)
2011-06-15
We discuss the simultaneous existence of phononic and photonic band gaps in two types of phononic crystals slabs, namely periodic arrays of nanoholes in a Si membrane and of Si nanodots on a SiO{sub 2} membrane. In the former geometry, we investigate in detail both the boron nitride lattice and the square lattice with two atoms per unit cell (these include the square, triangular and honeycomb lattices as particular cases). In the latter geometry, some preliminary results are reported for a square lattice. (orig.)
Modeling of Photonic Band Gap Crystals and Applications
Energy Technology Data Exchange (ETDEWEB)
Ihab Fathy El-Kady
2002-08-27
In this work, the authors have undertaken a theoretical approach to the complex problem of modeling the flow of electromagnetic waves in photonic crystals. The focus is to address the feasibility of using the exciting phenomena of photonic gaps (PBG) in actual applications. The authors start by providing analytical derivations of the computational electromagnetic methods used in their work. They also present a detailed explanation of the physics underlying each approach, as well as a comparative study of the strengths and weaknesses of each method. The Plane Wave expansion, Transfer Matrix, and Finite Difference time Domain Methods are addressed. They also introduce a new theoretical approach, the Modal Expansion Method. They then shift the attention to actual applications. They begin with a discussion of 2D photonic crystal wave guides. The structure addressed consists of a 2D hexagonal structure of air cylinders in a layered dielectric background. Comparison with the performance of a conventional guide is made, as well as suggestions for enhancing it. The studies provide an upper theoretical limit on the performance of such guides, as they assumed no crystal imperfections and non-absorbing media. Next, they study 3D metallic PBG materials at near infrared and optical wavelengths. The main objective is to study the importance of absorption in the metal and the suitability of observing photonic band gaps in such structures. They study simple cubic structures where the metallic scatters are either cubes or interconnected metallic rods. Several metals are studied (aluminum, gold, copper, and silver). The effect of topology is addressed and isolated metallic cubes are found to be less lossy than the connected rod structures. The results reveal that the best performance is obtained by choosing metals with a large negative real part of the dielectric function, together with a relatively small imaginary part. Finally, they point out a new direction in photonic crystal
Institute of Scientific and Technical Information of China (English)
Yongjun He(何拥军); Zhongchao Wei(韦中超); Yongchun Zhong(钟永春); Jianwei Diao(刁建伟); Hezhou Wang(汪河洲)
2004-01-01
Experimental results demonstrate that the band gap of colloidal suspension crystal changes with both the thickness of crystal and the dispersity of micro-spheres.As the thickness decreases,a red shift of band gap is observed,and there is a maximum of red shift.The values of the maximum red shifts are dependent on the standard deviations of micro-spheres.The experimental results are consistent with theoretical calculation.As the colloidal suspension crystal is assembled from micro-spheres with a standard deviation of 8.4% in a thick cell,an incident angles independent broadband is observed,which is explained as an amorphous structure.Two amorphous models are discussed.
Chalmers, L.; Elford, D. P.; Kusmartsev, F. V.; Swallowe, G. M.
2010-12-01
We present a new type of sonic crystal technology offering a novel method of achieving broad acoustic band gaps. The proposed design of a locally resonating sonic crystal (LRSC) is constructed from "C"-shaped Helmholtz resonators as opposed to traditional solid scattering units. This unique construction enables a two band gap system to be generated in which the first -- a Bragg type band gap, arises due to the periodic nature of the crystal, whilst the second gap results from resonance of the air column within the resonators. The position of this secondary band gap is found to be dependent upon the dimensions of the resonating cavity. The band gap formation is investigated theoretically using finite element methods, and confirmed through experimental testing. It is noted that the resonance band gaps detected cover a much broader frequency range (in the order of kHz) than has been achieved to date. In addition the possibility of overlapping such a wide band gap with the characteristic Bragg gap generated by the structure itself could yield gaps of even greater range. A design of sonic crystal is proposed, that comprises of several resonators with differing cavity sizes. Such a structure generates multiple resonance gaps corresponding to the various resonator sizes, which may be overlapped to form yet larger band gaps. This multiple resonance gap system can occur in two configurations. Firstly a simple mixed array can be created by alternating resonator sizes in the array and secondly using a System coined the Matryoshka (Russian doll) array in which the resonators are distributed inside one another. The proposed designs of LRSC's offer a real potential for acoustic shielding using sonic crystals, as both the size and position of the band gaps generated can be controlled. This is an application which has been suggested and investigated for several years with little progress. Furthermore the frequency region attenuated by resonance is unrelated to the crystals lattice
Ultra-wide acoustic band gaps in pillar-based phononic crystal strips
Energy Technology Data Exchange (ETDEWEB)
Coffy, Etienne, E-mail: etienne.coffy@femto-st.fr; Lavergne, Thomas; Addouche, Mahmoud; Euphrasie, Sébastien; Vairac, Pascal; Khelif, Abdelkrim [FEMTO-ST Institute, Université de Franche-Comté, UBFC, CNRS, ENSMM, UTBM, 15B Av. des Montboucons, F-25030 Besançon (France)
2015-12-07
An original approach for designing a one dimensional phononic crystal strip with an ultra-wide band gap is presented. The strip consists of periodic pillars erected on a tailored beam, enabling the generation of a band gap that is due to both Bragg scattering and local resonances. The optimized combination of both effects results in the lowering and the widening of the main band gap, ultimately leading to a gap-to-midgap ratio of 138%. The design method used to improve the band gap width is based on the flattening of phononic bands and relies on the study of the modal energy distribution within the unit cell. The computed transmission through a finite number of periods corroborates the dispersion diagram. The strong attenuation, in excess of 150 dB for only five periods, highlights the interest of such ultra-wide band gap phononic crystal strips.
Thermal emissivity for finite three-dimensional photonic band gap crystals
Stimpson, Andrew J.; Dowling, Jonathan P.
2002-01-01
We discuss the results of computer model for the thermal emissivity of a three-dimensional photonic band gap (PBG) crystal, specifically an inverted opal structure. The thermal emittance for a range of frequencies and angles is calculated.
Yudistira, D; Boes, A; Djafari-Rouhani, B; Pennec, Y; Yeo, L Y; Mitchell, A; Friend, J R
2014-11-21
We theoretically and experimentally demonstrate the existence of complete surface acoustic wave band gaps in surface phonon-polariton phononic crystals, in a completely monolithic structure formed from a two-dimensional honeycomb array of hexagonal shape domain-inverted inclusions in single crystal piezoelectric Z-cut lithium niobate. The band gaps appear at a frequency of about twice the Bragg band gap at the center of the Brillouin zone, formed through phonon-polariton coupling. The structure is mechanically, electromagnetically, and topographically homogeneous, without any physical alteration of the surface, offering an ideal platform for many acoustic wave applications for photonics, phononics, and microfluidics. PMID:25479504
Study of periodic band gap structure of the magnetized plasma photonic crystals
Institute of Scientific and Technical Information of China (English)
ZHANG Hai-feng; MA Li; LIU Shao-bin
2009-01-01
The characteristics of the periodic band gaps of the one dimension magnetized plasma photonic crystals are studied with the piecewise linear current density recursive convolution (PLCDRC) finite-differential time-domain (FDTD) method. In fre-quency-domain, the transmission coefficients of electromagnetic Gaussian pulses are computed, and the effects of the periodic structure constant, plasma layer thickness and parameters of plasma on the properties of periodic band gaps of magnetized photonic crystals are analyzed. The results show that the periodic band gaps depend strongly on the plasma parameters.
Fathollahi Khalkhali, T.; Bananej, A.
2016-06-01
In this study, we analyze the tunability of complete photonic band gap of square and triangular photonic crystal slabs composed of square and hexagonal air holes in anisotropic tellurium background with SiO2 as cladding material. The non-circular holes are infiltrated with liquid crystal. Using the supercell method based on plane wave expansion, we study the variation of complete band gap by changing the optical axis orientation of liquid crystal. Our numerical results show that noticeable tunability of complete photonic band gap can be obtained in both square and triangular structures with non-circular holes.
Wave propagation in single column woodpile phononic crystals: Formation of tunable band gaps
Kim, Eunho; Yang, Jinkyu
2014-11-01
We study the formation of frequency band gaps in single column woodpile phononic crystals composed of orthogonally stacked slender cylinders. We focus on investigating the effect of the cylinders' local vibrations on the dispersion of elastic waves along the stacking direction of the woodpile phononic crystals. We experimentally verify that their frequency band structures depend significantly on the bending resonant behavior of unit cells. We propose a simple theoretical model based on a discrete element method to associate the behavior of locally resonant cylindrical rods with the band gap formation mechanism in woodpile phononic crystals. The findings in this work imply that we can achieve versatile control of frequency band structures in phononic crystals by using woodpile architectures. The woodpile phononic crystals can form a new type of vibration filtering devices that offer an enhanced degree of freedom in manipulating stress wave propagation.
Band Gap and Waveguide States in Two-Dimensional Disorder Phononic Crystals
Institute of Scientific and Technical Information of China (English)
LI Xiao-Chun; LIU Zheng-You; LIANG Hong-Yu; XIAO Qing-Wu
2006-01-01
@@ The influences of the configurational disorders on phononic band gaps and on waveguide modes are investigated for the two-dimensional phononic crystals consisting of water cylinders periodically arrayed in mercury. Two types of conflgurational disorders, relevant to the cylinder position and cylinder size respectively, are taken into account. It is found that the phononic band gap and the guide band are sensitive to the disorders, and generally become narrower with the increasing disorders. It is also found that the waveguide side walls without disorder can significantly prevent the guide modes in the waveguide from influence by the disorders in the crystals to a large amount.
Accurate evaluation of lowest band gaps in ternary locally resonant phononic crystals
Institute of Scientific and Technical Information of China (English)
Wang Gang; Shao Li-Hui; Liu Yao-Zong; Wen Ji-Hong
2006-01-01
Based on a better understanding of the lattice vibration modes, two simple spring-mass models are constructed in order to evaluate the frequencies on both the lower and upper edges of the lowest locally resonant band gaps of the ternary locally resonant phononic crystals. The parameters of the models are given in a reasonable way based on the physical insight into the band gap mechanism. Both the lumped-mass methods and our models are used in the study of the influences of structural and the material parameters on frequencies on both edges of the lowest gaps in the ternary locally resonant phononic crystals. The analytical evaluations with our models and the theoretical predictions with the lumped-mass method are in good agreement with each other. The newly proposed heuristic models are helpful for a better understanding of the locally resonant band gap mechanism, as well as more accurate evaluation of the band edge frequencies.
Modelling and design of complete photonic band gaps in two-dimensional photonic crystals
Indian Academy of Sciences (India)
Yogita Kalra; R K Sinha
2008-01-01
In this paper, we investigate the existence and variation of complete photonic band gap size with the introduction of asymmetry in the constituent dielectric rods with honeycomb lattices in two-dimensional photonic crystals (PhC) using the plane-wave expansion (PWE) method. Two examples, one consisting of elliptical rods and the other comprising of rectangular rods in honeycomb lattices are considered with a view to estimate the design parameters for maximizing the complete photonic band gap. Further, it has been shown that complete photonic band gap size changes with the variation in the orientation angle of the constituent dielectric rods.
Study on the vibration band gap and vibration attenuation property of phononic crystals
Institute of Scientific and Technical Information of China (English)
无
2008-01-01
Phononic crystals (PCs) are functional materials with periodic structures and elastic wave (vibration) band gaps, where propagation of vibrations with frequencies within band gaps is forbidden. PCs with finite periods can restrain the propagation of vibrations with frequencies in band gaps and thus has vibration attenuation property. Worldwide, many institutions and researchers are engaged in the research of PCs, however, studies on the vibration attenuation property of PCs are still limited. In this paper, we report our study of band gaps and vibration attenuation properties of 1) a simplified PC-periodic mass-spring structures, 2) longitudinal vibration of one-dimensional (1D-), 2D-, 3D-PCs, and 3) the flexural vibration of 1D- and 2D-PCs. These studies provide a foundation for the applications of PCs in vibration attenuation.
Study on the vibration band gap and vibration attenuation property of phononic crystals
Institute of Scientific and Technical Information of China (English)
2008-01-01
Phononic crystals (PCs) are functional materials with periodic structures and elas- tic wave (vibration) band gaps, where propagation of vibrations with frequencies within band gaps is forbidden. PCs with finite periods can restrain the propagation of vibrations with frequencies in band gaps and thus has vibration attenuation property. Worldwide, many institutions and researchers are engaged in the re- search of PCs, however, studies on the vibration attenuation property of PCs are still limited. In this paper, we report our study of band gaps and vibration attenua- tion properties of 1) a simplified PC—periodic mass-spring structures, 2) longitu- dinal vibration of one-dimensional (1D-), 2D-, 3D-PCs, and 3) the flexural vibration of 1D- and 2D-PCs. These studies provide a foundation for the applications of PCs in vibration attenuation.
Acoustic band gaps of the woodpile sonic crystal with the simple cubic lattice
Energy Technology Data Exchange (ETDEWEB)
Wu, Liang-Yu; Chen, Lien-Wen, E-mail: chenlw@mail.ncku.edu.t [Department of Mechanical Engineering, National Cheng Kung University, Tainan 70101, Taiwan (China)
2011-02-02
This study theoretically and experimentally investigates the acoustic band gap of a three-dimensional woodpile sonic crystal. Such crystals are built by blocks or rods that are orthogonally stacked together. The adjacent layers are perpendicular to each other. The woodpile structure is embedded in air background. Their band structures and transmission spectra are calculated using the finite element method with a periodic boundary condition. The dependence of the band gap on the width of the stacked rods is discussed. The deaf bands in the band structure are observed by comparing with the calculated transmission spectra. The experimental transmission spectra for the {Gamma}-X and {Gamma}-X' directions are also presented. The calculated results are compared with the experimental results.
Wavelet-based method for computing elastic band gaps of one-dimensional phononic crystals
Institute of Scientific and Technical Information of China (English)
YAN; ZhiZhong; WANG; YueSheng
2007-01-01
A wavelet-based method was developed to compute elastic band gaps of one-dimensional phononic crystals. The wave field was expanded in the wavelet basis and an equivalent eigenvalue problem was derived in a matrix form involving the adaptive computation of integrals of the wavelets. The method was then applied to a binary system. For comparison, the elastic band gaps of the same one-di- mensional phononic crystals computed with the wavelet method and the well- known plane wave expansion (PWE) method are both presented in this paper. The numerical results of the two methods are in good agreement while the computation costs of the wavelet method are much lower than that of PWE method. In addition, the adaptability of wavelets makes the method possible for efficient band gap computation of more complex phononic structures.
Robust topology optimization of three-dimensional photonic-crystal band-gap structures
Men, Han; Freund, Robert M; Peraire, Jaime; Johnson, Steven G
2014-01-01
We perform full 3D topology optimization (in which "every voxel" of the unit cell is a degree of freedom) of photonic-crystal structures in order to find optimal omnidirectional band gaps for various symmetry groups, including fcc (including diamond), bcc, and simple-cubic lattices. Even without imposing the constraints of any fabrication process, the resulting optimal gaps are only slightly larger than previous hand designs, suggesting that current photonic crystals are nearly optimal in this respect. However, optimization can discover new structures, e.g. a new fcc structure with the same symmetry but slightly larger gap than the well known inverse opal, which may offer new degrees of freedom to future fabrication technologies. Furthermore, our band-gap optimization is an illustration of a computational approach to 3D dispersion engineering which is applicable to many other problems in optics, based on a novel semidefinite-program formulation for nonconvex eigenvalue optimization combined with other techniq...
Gorisse, M.; Benchabane, S.; Teissier, G.; Billard, C.; Reinhardt, A.; Laude, V.; Defaÿ, E.; Aïd, M.
2011-06-01
We report on the observation of elastic waves propagating in a two-dimensional phononic crystal composed of air holes drilled in an aluminum nitride membrane. The theoretical band structure indicates the existence of an acoustic band gap centered around 800 MHz with a relative bandwidth of 6.5% that is confirmed by gigahertz optical images of the surface displacement. Further electrical measurements and computation of the transmission reveal a much wider attenuation band that is explained by the deaf character of certain bands resulting from the orthogonality of their polarization with that of the source.
Fabrication of Ceramic Layer-by-Layer Infrared Wavelength Photonic Band Gap Crystals
Energy Technology Data Exchange (ETDEWEB)
Henry Hao-Chuan Kang
2004-12-19
Photonic band gap (PBG) crystals, also known as photonic crystals, are periodic dielectric structures which form a photonic band gap that prohibit the propagation of electromagnetic (EM) waves of certain frequencies at any incident angles. Photonic crystals have several potential applications including zero-threshold semiconductor lasers, the inhibition of spontaneous emission, dielectric mirrors, and wavelength filters. If defect states are introduced in the crystals, light can be guided from one location to another or even a sharp bending of light in micron scale can be achieved. This generates the potential for optical waveguide and optical circuits, which will contribute to the improvement in the fiber-optic communications and the development of high-speed computers.
Ultrafast optical switching of three-dimensional Si inverse opal photonic band gap crystals
Euser, Tijmen G.; Wei, Hong; Kalkman, Jeroen; Jun, Yoonho; Polman, Albert; Norris, David J.; Vos, Willem L.
2007-01-01
We present ultrafast optical switching experiments on three-dimensional photonic band gap crystals. Switching the Si inverse opal is achieved by optically exciting free carriers by a two-photon process. We probe reflectivity in the frequency range of second order Bragg diffraction where the photonic
Subpicosecond shifting of the photonic band gap in a three-dimensional photonic crystal
Mazurenko, DA; Kerst, R; Dijkhuis, JI; Akimov, AV; Golubev, VG; Kaplyanskii, AA; Kurdyukov, DA; Pevtsov, AB
2005-01-01
We demonstrate spectral shifting of the photonic band gap in a three-dimensional photonic crystal within a time of less than 350 fs. Single 120 fs high-power optical pulses are capable to induce the transition from the semiconductor to the metallic phase of VO2 in the pores of our artificial silica
Experimental investigation of hollow-core photonic crystal fibers with five photonic band-gaps
Institute of Scientific and Technical Information of China (English)
YUAN Jin-hui; HOU Lan-tian; WEI Dong-bin; WANG Hai-yun; ZHOU Gui-yao
2008-01-01
The hollow-core photonic crystal fibers (HC-PCFs) with integrity structure have been fabricated with an improved twice stack-and-draw technique. The transmission spectrum shows that five photonic band-gaps within 450-1100 nm have been obtained.And the green light transmission in the HC-PCFs'has been observed remarkably.
Optimisation study of micro cantilevers for switching of photonic band gap crystals
Chakkalakkal Abdulla, S.; Berenschot, E.; Boer, de M.J.; Kauppinen, L.J.; Ridder, de R.M.; Krijnen, G.J.M.
2009-01-01
We propose to use electrostatically actuated micro bimorph cantilevers with tips for nanometric perturbations in the evanescent field of various resonators and photonic band gap crystals (PBG) using a self aligning technology. Since in PBG and in other high optical index contrast structures the inte
Reflectivity calculated for a 3D silicon photonic band gap crystal with finite support
Devashish, D; van der Vegt, J J W; Vos, Willem L
2016-01-01
We study numerically the reflectivity of three-dimensional (3D) photonic crystals with a complete 3D photonic band gap, with the aim to interpret recent experiments. We employ the finite element method to study crystals with the cubic diamond-like inverse woodpile structure. The high-index backbone has a dielectric function similar to silicon. We study crystals with a range of thicknesses up to ten unit cells ($L \\leq 10 c$). The crystals are surrounded by vacuum, and have a finite support as in experiments. The polarization-resolved reflectivity spectra reveal Fabry-P{\\'e}rot fringes related to standing waves in the finite crystal, as well as broad stop bands with nearly $100~\\%$ reflectivity, even for thin crystals. From the strong reflectivity peaks, it is inferred that the maximum reflectivity observed in experiments is not limited by finite size. The frequency ranges of the stop bands are in excellent agreement with stop gaps in the photonic band structure, that pertain to infinite and perfect crystals. ...
Crystal structure and band gap of AlGaAsN
Munich, D. P.; Pierret, R. F.
1987-09-01
Quantum dielectric theory is applied to the quaternary alloy Al xGa 1- xAs 1- yN y to predict its electronic properties as a function of Al and N mole fractions. Results are presented for the expected crystal structure, minimum electron energy band gap, and direction in k-space of the band gap minimum for all x and y values. The results suggest that, for a proper choice of x and y, Al xGa 1- xAs 1- yN y could exhibit certain advantages over Al xGa 1- xAs when utilized in field-effect transistor structures.
Research on the large band gaps in multilayer radial phononic crystal structure
Gao, Nansha; Wu, Jiu Hui; Guan, Dong
2016-04-01
In this paper, we study the band gaps (BGs) of new proposed radial phononic crystal (RPC) structure composed of multilayer sections. The band structure, transmission spectra and eigenmode displacement fields of the multilayer RPC are calculated by using finite element method (FEM). Due to the vibration coupling effects between thin circular plate and intermediate mass, the RPC structure can exhibit large BGs, which can be effectively shifted by changing the different geometry values. This study shows that multilayer RPC can unfold larger and lower BGs than traditional phononic crystals (PCs) and RPC can be composed of single material.
Synthesis and Band Gap Control in Three-Dimensional Polystyrene Opal Photonic Crystals
Institute of Scientific and Technical Information of China (English)
LIU Ye; ZHENG Zhong-Yu; QIN Fei; ZHOU Fei; ZHOU Chang-Zhu; ZHANG Dao-Zhong; MENG Qing-Bo; LI Zhi-Yuan
2008-01-01
High-quality three-dimensional polystyrene opal photonic crystals are fabricated by vertical deposition method.The transmission properties with different incident angles and different composite refractive index contrasts are experimentally and theoretically studied. Good agreement between the experiment and theory is achieved. We find that with the increasing incident angle, the gap position shifts to the short wavelength (blue shift) and the gap becomes shallower; and with the increase of refractive index of the opal void materials and decrease the contrast of refractive index, the gap position shifts to the long wavelength (red shift). At the same time, we observe the swelling effects when the sample is immerged in the solutions with different refractive indices, which make the microsphere diameter in solution become larger than that in air. The understanding of band gap shift behaviour may be helpful in designing optical sensors and tunable photonic crystal ultrafast optical switches.
International Nuclear Information System (INIS)
Composite right/left-handed transmission lines with lumped element series capacitors and shunt inductors are used to experimentally realize the one-dimensional photonic crystals composed of single-negative metamaterials. The simulated and experimental results show that a special photonic band gap corresponding to zero-effective-phase (zero-φeff) may appear in the microwave regime. In contrast to the Bragg gap, by changing the length ratio of the two component materials, the width and depth of the zero-φeff gap can be conveniently adjusted while keeping the center frequency constant. Furthermore, the zero-φeff gap vanishes when both the phase-matching and impedance-matching conditions are satisfied simultaneously. These transmission line structures provide a good way for realizing microwave devices based on the zero-φeff gap. -- Highlights: ► 1D photonic crystals with metamaterials were investigated experimentally. ► Both Bragg gap and zero-φeff gap were observed in the microwave regime. ► The width and depth of the zero-φeff gap were experimentally adjusted. ► Zero-φeff gap was observed to be close when two match conditions were satisfied.
Energy Technology Data Exchange (ETDEWEB)
Chen, Yihang, E-mail: eon.chen@yahoo.com.cn [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China); Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou (China); Wang, Xinggang [Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou (China); Yong, Zehui; Zhang, Yunjuan [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China); Chen, Zefeng [Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou (China); He, Lianxing; Lee, P.F.; Chan, Helen L.W.; Leung, Chi Wah [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China); Wang, Yu, E-mail: apywang@inet.polyu.edu.hk [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China)
2012-03-19
Composite right/left-handed transmission lines with lumped element series capacitors and shunt inductors are used to experimentally realize the one-dimensional photonic crystals composed of single-negative metamaterials. The simulated and experimental results show that a special photonic band gap corresponding to zero-effective-phase (zero-φ{sub eff}) may appear in the microwave regime. In contrast to the Bragg gap, by changing the length ratio of the two component materials, the width and depth of the zero-φ{sub eff} gap can be conveniently adjusted while keeping the center frequency constant. Furthermore, the zero-φ{sub eff} gap vanishes when both the phase-matching and impedance-matching conditions are satisfied simultaneously. These transmission line structures provide a good way for realizing microwave devices based on the zero-φ{sub eff} gap. -- Highlights: ► 1D photonic crystals with metamaterials were investigated experimentally. ► Both Bragg gap and zero-φ{sub eff} gap were observed in the microwave regime. ► The width and depth of the zero-φ{sub eff} gap were experimentally adjusted. ► Zero-φ{sub eff} gap was observed to be close when two match conditions were satisfied.
Direct band gap silicon crystals predicted by an inverse design method
Oh, Young Jun; Lee, In-Ho; Lee, Jooyoung; Kim, Sunghyun; Chang, Kee Joo
2015-03-01
Cubic diamond silicon has an indirect band gap and does not absorb or emit light as efficiently as other semiconductors with direct band gaps. Thus, searching for Si crystals with direct band gaps around 1.3 eV is important to realize efficient thin-film solar cells. In this work, we report various crystalline silicon allotropes with direct and quasi-direct band gaps, which are predicted by the inverse design method which combines a conformation space annealing algorithm for global optimization and first-principles density functional calculations. The predicted allotropes exhibit energies less than 0.3 eV per atom and good lattice matches, compared with the diamond structure. The structural stability is examined by performing finite-temperature ab initio molecular dynamics simulations and calculating the phonon spectra. The absorption spectra are obtained by solving the Bethe-Salpeter equation together with the quasiparticle G0W0 approximation. For several allotropes with the band gaps around 1 eV, photovoltaic efficiencies are comparable to those of best-known photovoltaic absorbers such as CuInSe2. This work is supported by the National Research Foundation of Korea (2005-0093845 and 2008-0061987), Samsung Science and Technology Foundation (SSTF-BA1401-08), KIAS Center for Advanced Computation, and KISTI (KSC-2013-C2-040).
Bagheri Nouri, Mohammad; Moradi, Mehran
2016-05-01
In this paper, a new heterostructure phononic crystal is introduced. The new heterostructure is composed of square and rhombus phononic crystals. Using finite difference method, a displacement-based algorithm is presented to study elastic wave propagation in the phononic crystal. In contrast with conventional finite difference time domain method, at first by using constitutive equations and strain-displacement relations, elastic wave equations are derived based on displacement. Then, these forms are discretized using finite difference method. By this technique, components of stress tensor can be removed from the updating equations. Since the proposed method needs less elementary arithmetical operations, its computational cost is less than that of the conventional FDTD method. Using the presented displacement-based finite difference time domain algorithm, square phononic crystal, rhombus phononic crystal and the new heterostructure phononic crystal were analyzed. Comparison of transmission spectra of the new heterostructure phononic crystal with those creating lattices, showed that band gap can be extended by using the new structure. Also it was observed that by changing the angular constant of rhombus lattice, a new extended band gap can be achieved.
Stable Band-Gaps in Phononic Crystals by Harnessing Hyperelastic Transformation Media
Liu, Yan; Feng, Xi-Qiao
2016-01-01
The band structure in phononic crystals (PCs) is usually affected by the deformations of their soft components. In this work, hyperelastic transformation media is proposed to be integrated in the PCs'design, to achieve stable elastic band-gaps which is independent with finite mechanical deformations. For a one-dimensional (1D) PC, we demonstrate the semi-linear soft component can keep all elastic wave bands unchanged with the external deformation field. While for neo-Hookean soft component, only S-wave bands can be precisely retained. The change of the P-wave bands can be predicted by using a lumped mass method. Numerical simulations are performed to validate our theory predictions and the robustness of the proposed PCs.
Analysis of band gap of non-bravais lattice photonic crystal fiber
Institute of Scientific and Technical Information of China (English)
Yichao MA; Heming CHEN
2009-01-01
This article designs a novel type ofnon-bravais lattice photonic crystal fiber. To form the nesting complexperiod with positive and negative refractive index materials respectively, a cylinder with the same radius and negative refractive index is introduced into the center of each lattice unit cell in the traditional square lattice air-holes photonic crystal fiber. The photonic band-gap of the photonic crystal fiber is calculated numerically by the plane wave expansion method. The result shows that compared with the traditional square photonic band-gap fiber (PBGF),when R/A is 0.35, the refractive index of the substrate, airhole, and medium-column are 1.30, 1.0, and -1.0,respectively. This new PBGF can transmit signal by the photonic band-gap effect. When the lattice constant Λvaries from 1.5 μm to 3.0 μm, the range of the wavelength ranges from 880 nm to 2300 nm.
Institute of Scientific and Technical Information of China (English)
无
2010-01-01
Absolute band gaps of a two-dimensional triangular-lattice photonic crystal are calculated with the finite-difference time-domain method in this paper.Through calculating the photonic band structures of the triangular-lattice photonic crystal consisting of Ge rods immersed in air with different shapes,it is found that a large absolute band gap of 0.098 (2c/a) can be obtained for the structures with hollow triangular Ge rods immersed in air,corresponding to 19.8% of the middle frequency.The influence of the different factors on the width of the absolute band gaps is also discussed.
Large band gaps in two-dimensional phononic crystal composed of periodic cross-boards in air
International Nuclear Information System (INIS)
Using the finite-element method, the band structure and the transmission coefficient spectrum of a new two-dimensional phononic crystal (PC) composed of periodic cross-boards in air were investigated. The effects of the geometry parameters on band gaps are discussed. Numerical results show that this PC can yield large band gaps in the low-frequency range. The location and width of the band gaps can be modulated by geometry parameters such as the margin width and cross-angle. Furthermore, the acoustic modes on the edge of the band gaps are calculated to find the role of the cross-board in the formation of band gaps. Results show that the edges of the band gaps are determined, respectively, by the acute or obtuse air regions divided by the cross-board.
Institute of Scientific and Technical Information of China (English)
Yan Hai-Tao; Wang Ming; Ge Yi-Xian; Yu Ping
2009-01-01
The macropore silica colloidal crystal templates were assembled orderly in a capillary glass tube by an applied electric field method to control silica deposition. In order to achieve the photonic band gap (PBG) of colloidal crystal in optical communication waveband, the diameter of silica microspheres is selected by Bragg diffraction formula. An experiment was designed to test the bandgap of the silica crystal templates. This paper discusses the formation process and the close-packed fashion of the silica colloidal crystal templates was discussed. The surface morphology of the templates was also analyzed. The results showed that the close-packed fashion of silica array templates was face-centered cubic (FCC) structure. The agreement is very good between the experimental data and the theoretical calculation.
Institute of Scientific and Technical Information of China (English)
Kong Xiao-Yan; Yue Lei-Lei; Chen Yu; Liu Ying-Kai
2012-01-01
The band structures of a new two-dimensional triangle-shaped array geometry of 4340 steel cylinders of square cross section in an epoxy resin were studied by the plane-wave expansion and supercell calculation method.The band gaps of this type of phononic crystals with different defects were calculated such as defect-free,60° crystal linear defect states,120° crystal linear defect states,and 180° crystal linear defect states.It was found that the band gap will emerge in different linear defects of the phononic crystals and the bandwidth of linear defect states is larger than that of the free-defect crystal by about 2.14 times within the filling fraction F ＝ 0.1-0.85.In addition,the influence of the filling fraction on the relative width of the minimum band gap is discussed.
Absolute photonic band gap in 2D honeycomb annular photonic crystals
International Nuclear Information System (INIS)
Highlights: • A two-dimensional honeycomb annular photonic crystal (PC) is proposed. • The absolute photonic band gap (PBG) is studied. • Annular PCs show larger PBGs than usual air-hole PCs for high refractive index. • Annular PCs with anisotropic rods show large PBGs for low refractive index. • There exist optimal parameters to open largest band gaps. - Abstract: Using the plane wave expansion method, we investigate the effects of structural parameters on absolute photonic band gap (PBG) in two-dimensional honeycomb annular photonic crystals (PCs). The results reveal that the annular PCs possess absolute PBGs that are larger than those of the conventional air-hole PCs only when the refractive index of the material from which the PC is made is equal to 4.5 or larger. If the refractive index is smaller than 4.5, utilization of anisotropic inner rods in honeycomb annular PCs can lead to the formation of larger PBGs. The optimal structural parameters that yield the largest absolute PBGs are obtained
Band gap and refractive index tunability in thallium based layered mixed crystals
Energy Technology Data Exchange (ETDEWEB)
Gasanly, N. M., E-mail: nizami@metu.edu.tr [Department of Physics, Middle East Technical University, Ankara 06800 (Turkey); Virtual International Scientific Research Centre, Baku State University, Baku 1148 (Azerbaijan)
2015-07-21
Compositional variation of the band gap energy and refractive index of TlMeX{sub 2}-type (Me = Ga or In and X = S or Se) layered mixed crystals have been studied by the transmission and reflection measurements in the wavelength range of 400–1100 nm. The analysis of absorption data of TlGa{sub 1-x}In{sub x}Se{sub 2}, TlGa(S{sub 1−x}Se{sub x}){sub 2}, TlGa{sub 1−x}In{sub x}S{sub 2}, and TlIn(Se{sub 1−x}S{sub x}){sub 2} mixed crystals revealed the presence of both optical indirect and direct transitions. It was found that the energy band gaps of mixed crystals decrease at the replacing of gallium atoms by indium and of sulfur atoms by selenium ones. Through the similar replacing of atoms (smaller atoms by larger ones) in the studied mixed crystals, the refractive index shows the quite opposite behavior.
Band gap control in a line-defect magnonic crystal waveguide
Energy Technology Data Exchange (ETDEWEB)
Morozova, M. A., E-mail: mamorozovama@yandex.ru; Grishin, S. V.; Sadovnikov, A. V.; Romanenko, D. V.; Sharaevskii, Yu. P.; Nikitov, S. A. [Laboratory ' Metamaterials,' Saratov State University, Astrakhanskaya 83, Saratov 410012 (Russian Federation)
2015-12-14
We report on the experimental observation of the spin wave spectrum control in a line-defect magnonic crystal (MC) waveguide. We demonstrate the possibility to control the forbidden frequency band (band gap) for spin waves tuning the line-defect width. In particular, this frequency may be greater or lower than the one of 1D MC waveguide without line-defect. By means of space-resolved Brillouin light scattering technique, we study the localization of magnetization amplitude in the line-defect area. We show that the length of this localization region depends on the line-defect width. These results agree well with theoretical calculations of spin wave spectrum using the proposed model of two coupled magnonic crystal waveguides. The proposed simple geometry of MC with line-defect can be used as a logic and multiplexing block for application in the novel field of magnonic devices.
Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals
Aslan, Ozgur Burak; Ruppert, Claudia; Heinz, Tony
2015-03-01
Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-infrared. Supported by the NSF through Grant DMR-1124894 for sample preparation and characterization by the O?ce of Naval Research for analysis. C.R. acknowledges support from the Alexander von Humboldt Foundation.
Directory of Open Access Journals (Sweden)
Xiao-Peng Wang
2015-10-01
Full Text Available In this paper, the tuning characteristics of band gaps and waveguides in a locally resonant phononic crystal structure, consisting of multiple square stubs deposited on a thin homogeneous plate, are investigated. Using the finite element method and supercell technique, the dispersion relationships and power transmission spectra of those structures are calculated. In contrast to a system of one square stub, systems of multiple square stubs show wide band gaps at lower frequencies and an increased quantity of band gaps at higher frequencies. The vibration modes of the band gap edges are analyzed to clarify the mechanism of the generation of the lowest band gap. Additionally, the influence of the stubs arrangement on the band gaps in multi-stub systems is investigated. The arrangements of the stubs were found to influence the band gaps; this is critical to understand for practical applications. Based on this finding, a novel method to form defect scatterers by changing the arrangement of square stubs in a multi-stub perfect phononic crystal plate was developed. Defect bands can be induced by creating defects inside the original complete band gaps. The frequency can then be tuned by changing the defect scatterers’ stub arrangement. These results will help in fabricating devices such as acoustic filters and waveguides whose band frequency can be modulated.
Self-imaging effect in photonic crystal multimode waveguides exhibiting no band gaps
Institute of Scientific and Technical Information of China (English)
Tianbao Yu; Xiaoqing Jiang; Qinghua Liao; Wei Qi; Jianyi Yang; Minghua Wang
2007-01-01
The properties of the propagating field in multimode photonic crystal waveguides (PCWs) exhibiting no photonic band gaps (PBGs) are investigated. The transmission spectrum shows that the input field can be guided with high efficiency, and resemble index-guided modes owing to the combination of total internal reflection (TIR) and distributed Bragg reflection (DBR). Self-imaging effect happens and the filling fraction determines the beating lengths. The rows of air holes decide DBR coming from the mirrors on both sides of the guiding region, which governs the transmission spectrum. It provides a new way to realize the components for both polarizations by combining PBG and TIR effects in PCWs.
Institute of Scientific and Technical Information of China (English)
Wang Gang; Liu Yao-Zong; Wen Ji-Hong; Yu Dian-Long
2006-01-01
The low-frequency band gap and the corresponding vibration modes in two-dimensional ternary locally resonant phononic crystals are restudied successfully with the lumped-mass method. Compared with the work of C. Goffaux and J. Sanchez-Dehesa (Phys. Rev. B 67 14 4301(2003)), it is shown that there exists an error of about 50% in their calculated results of the band structure, and one band is missing in their results. Moreover, the in-plane modes shown in their paper are improper, which results in the wrong conclusion on the mechanism of the ternary locally resonant phononic crystals. Based on the lumped-mass method and better description of the vibration modes according to the band gaps, the locally resonant mechanism in forming the subfrequency gaps is thoroughly analysed. The rule used to judge whether a resonant mode in the phononic crystals can result in a corresponding subfrequency gap is also verified in this ternary case.
Hypersonic crystal band gaps in Ni/Cu superlattice nanowire arrays
Hu, Jia-Guang; Shen, Tie
2016-03-01
The hexagonal and tetragonal ordered arrays were prepared by Ni/Cu superlattice nanowires on the porous anodic alumina membrane template, and their phonon band structures were calculated by using the plane wave expansion method. Numerical results show that the hypersonic band gaps can be acquired by adjusting the structural parameters. Along the different wave-vector directions, the width and position of band gap would vary. If the nanowires'filling fraction is increased continuously, the width of the first band gap firstly increases and then decreases within a certain range. The height of superlattice nanowire elementary unit can only affect the width of band gap within a quite narrow range. When the height of elementary unit remains unchanged, the decrease of the Cu-component ratio can contribute to the formation of a wider band gap. Additionally, the wide band gap is more easily formed in tetragonal structure than in hexagonal structure.
Directory of Open Access Journals (Sweden)
Feroz A. Mir
2014-01-01
Full Text Available Recently, we carried out structural, optical and dielectric studies on micro-crystals of Oxypeucedanin (C16H14O5, isolated from the roots of plant Prangos pabularia (Mir et al. (2014 [3,4]. The obtained trend in frequency exponent (s with frequency (ω indicates that the universal dynamic response is followed by this compound. From optical absorption spectroscopy, the optical band gap (Eg was estimated around 3.76 eV and system is showing indirect allowed transition. Using Eg in certain relation of s, a close value of s (as much close obtained by fitting ac conductivity was obtained. This method was further used for other similar systems and again same trend was obtained. So a general conclusion was made that the high transmitting wide band insulators or semiconductors may follow bipolaron hopping transport mechanism.
Men, Han; Freund, Robert M; Parrilo, Pablo A; Peraire, Jaume
2009-01-01
In this paper, we consider the optimal design of photonic crystal band structures for two-dimensional square lattices. The mathematical formulation of the band gap optimization problem leads to an infinite-dimensional Hermitian eigenvalue optimization problem parametrized by the dielectric material and the wave vector. To make the problem tractable, the original eigenvalue problem is discretized using the finite element method into a series of finite-dimensional eigenvalue problems for multiple values of the wave vector parameter. The resulting optimization problem is large-scale and non-convex, with low regularity and non-differentiable objective. By restricting to appropriate eigenspaces, we reduce the large-scale non-convex optimization problem via reparametrization to a sequence of small-scale convex semidefinite programs (SDPs) for which modern SDP solvers can be efficiently applied. Numerical results are presented for both transverse magnetic (TM) and transverse electric (TE) polarizations at several fr...
A Study of Properties of the Photonic Band Gap of Unmagnetized Plasma Photonic Crystal
Institute of Scientific and Technical Information of China (English)
LIU Song; ZHONG Shuangying; LIU Sanqiu
2009-01-01
In this study,the propagation of electromagnetic waves in one-dimensional plasma photonic crystals(PPCs),namely,superlattice structures consisting alternately of a homogeneous unmagnetized plasma and dielectric material,is simulated numerically using the finite-difference time-domain(FDTD) algorithm.A perfectly matched layer (PML) absorbing technique is used in this simulation.The reflection and transmission coefficients of electromagnetic(EM)waves through PPCs are calculated.The characteristics of the photonic band gap(PBG)are discussed in terms of plasma density,dielectric constant ratios,number of periods,and introduced layer defect.These may provide some useful information for designing plasma photonic crystal devices.
International Nuclear Information System (INIS)
The multilayer structure of TiO2/SiO2 (11 layers) as one dimensional photonic crystal (1D PC) has been designed and then fabricated by using asymmetric bipolar pulse DC magnetron sputtering technique for omnidirectional photonic band gap. The experimentally measured photonic band gap (PBG) in the visible region is well matched with the theoretically calculated band structure (ω vs. k) diagram. The experimentally measured omnidirectional reflection band of 44 nm over the incident angle range of 0°-70° is found almost matching within the theoretically calculated band
Cassagne, D.
Photonic band gap materials Photonic band gap materials are periodic dielectric structures that control the propagation of electromagnetic waves. We describe the plane wave method, which allows to calculate the band structures of photonic crystals. By symmetry analysis and a perturbative approach, we predict the appearance of the low energy photonic band gaps of hexagonal structures. We propose new two-dimensional structures called graphite and boron nitride. Using a transfer matrix method, we calculate the transmission of the graphite structure and we show the crucial role of the coupling with external modes. We study the appearance of allowed modes in the photonic band gap by the introduction of localized defects in the periodicity. Finally, we discuss the properties of opals formed by self-organized silica microspheres, which are very promising for the fabrication of three-dimensional photonic crystals. Les matériaux à bandes interdites photoniques sont des structures diélectriques périodiques qui contrôlent la propagation des ondes électromagnétiques. Nous décrivons la méthode des ondes planes qui permet de calculer les structures de bandes des cristaux photoniques. Par une analyse de la symétrie et une approche perturbative, nous précisons les conditions d'existence des bandes interdites de basse énergie. Nous proposons de nouvelles structures bidimensionnelles appelées graphite et nitrure de bore. Grâce à une méthode de matrices de transfert, nous calculons la transmission de la structure graphite et nous mettons en évidence le rôle fondamental du couplage avec les modes extérieurs. Nous étudions l'apparition de modes permis dans la bande interdite grâce à l'introduction de défauts dans la périodicité. Enfin, nous discutons les propriétés des opales constituées de micro-billes de silice auto-organisées, qui sont très prometteuses pour la fabrication de cristaux photoniques tridimensionnels.
DEFF Research Database (Denmark)
Borel, Peter Ingo; Frandsen, Lars Hagedorn; Thorhauge, Morten;
2003-01-01
We have investigated the properties of TM polarized light in planar photonic crystal waveguide structures, which exhibit photonic band gaps for TE polarized light. Straight and bent photonic crystal waveguides and couplers have been fabricated in silicon-on-insulator material and modelled using a...... in a simple photonic crystal coupler with a size of ~ 20 m × 20 m. These promising features may open for the realization of ultra-compact photonic crystal components, which are easily integrated in optical communication networks....
Chern, R. L.; Chang, C. Chung; Chang, Chien C.; Hwang, R. R.
2003-08-01
In this study, two fast and accurate methods of inverse iteration with multigrid acceleration are developed to compute band structures of photonic crystals of general shape. In particular, we report two-dimensional photonic crystals of silicon air with an optimal full band gap of gap-midgap ratio Δω/ωmid=0.2421, which is 30% larger than ever reported in the literature. The crystals consist of a hexagonal array of circular columns, each connected to its nearest neighbors by slender rectangular rods. A systematic study with respect to the geometric parameters of the photonic crystals was made possible with the present method in drawing a three-dimensional band-gap diagram with reasonable computing time.
Silica photonic crystals with quasi-full band gap in the visible region prepared in ethanol
Institute of Scientific and Technical Information of China (English)
ZHANG Hui; WANG Xidong; ZHAO Xiaofeng; LI Wenchao; TANG Qing
2003-01-01
Monodisperse silica spheres of 252 nm with a standard deviation of 5.7% are prepared by Stber method. By comparison of both of media, ethanol instead of water is used to assemble opal, and the artificial opal has been prepared by the sedimentation in ethanol of silica spheres. The structure of the opal prepared has been examined and discussed. The results show that the artificial opal has a structure similar to the face-centered cubic (fcc) type packed system with silica spheres. Transmission measurements of the artificial opal have been conducted, which shows that the artificial opal is quasi-full band gap silica photonic crystals in the visible region.
Institute of Scientific and Technical Information of China (English)
YUAN Jin-Hui; HOU Lan-Tian; ZHOU Gui-Yao; WEI Dong-Bin; CHEN Chao; WANG Qing-Yue; HU Ming-Lie; LIU Bo-Wen
2008-01-01
Based on the full-vector plane-wave method (FVPWM),a hollow-core photonic crystal fibre (HC-PCF) fabricated by using the improved stack-and-draw technique is simulated. Under given propagation constants β,several effective photonic band gaps with different sizes emerge within the visible wavelength range from 575 to 720hm.The fundamental mode and second-order mode lying in a part of PBGs are investigated.In the transmission spectrum tested,the positions of PBGs are discovered to be shifting to shorter wavelengths.The main reason is the existence of interstitial holes at nodes in the cladding region.In the later experiment,green light is observed propagating in the air-core region,and the result is more consistent with our theoretical simulation.
Band Gap Computation of Two Dimensional Photonic Crystal for High Index Contrast Grating Application
Directory of Open Access Journals (Sweden)
Gagandeep Kaur
2014-05-01
Full Text Available Two Dimensional Photonic Crystal (PHc is convenient type of PHc, It refers to the fact that the dielectric is periodic in Two directions. The study of photonic structure by a simulation method is extremely momentous. At optical frequencies the optical density contained by two dimensional PHc changes periodically. They have the property to strong effect the propagation of light waves at these optical frequencies. A typical linearization method which solves the common nonlinear Eigen values difficulties has been used to achieve structures of the photonic band. There are two method plane wave expansion method (PWE and Finite Difference Time Domain method (FDTD. These Methods are most widely used for band gap calculation of PHc’s. FDTD Method has more smoothness and directness and can be explored effortlessly for simulation of the field circulation inside the photonic structure than PWE method so we have used FDTD Method for Two dimensional PHc’s calculation. In simulation of Two Dimensional band structures, silicon material has 0.543nm lattice constant and 1.46refractive index.
Optimization of band gaps of 2D photonic crystals by the rapid generic algorithm
Institute of Scientific and Technical Information of China (English)
SUN Yun-tao
2011-01-01
@@ Based on the rapid genetic algorithm (RGA), the band gap structures of square lattices with square scatters are optimized.In the optimizing process, gene codes are used to express square scatters and the fitting function adopts the relative values of the largest absolute photonic band gaps (PBGs).By changing the value of filling factor, three cell forms with large photonic band gaps are obtained.In addition, the comparison between the rapid genetic algorithm and the general genetic algorithm (GGA) is analyzed.
2D photonic crystal complete band gap search using a cyclic cellular automaton refination
González-García, R.; Castañón, G.; Hernández-Figueroa, H. E.
2014-11-01
We present a refination method based on a cyclic cellular automaton (CCA) that simulates a crystallization-like process, aided with a heuristic evolutionary method called differential evolution (DE) used to perform an ordered search of full photonic band gaps (FPBGs) in a 2D photonic crystal (PC). The solution is proposed as a combinatorial optimization of the elements in a binary array. These elements represent the existence or absence of a dielectric material surrounded by air, thus representing a general geometry whose search space is defined by the number of elements in such array. A block-iterative frequency-domain method was used to compute the FPBGs on a PC, when present. DE has proved to be useful in combinatorial problems and we also present an implementation feature that takes advantage of the periodic nature of PCs to enhance the convergence of this algorithm. Finally, we used this methodology to find a PC structure with a 19% bandgap-to-midgap ratio without requiring previous information of suboptimal configurations and we made a statistical study of how it is affected by disorder in the borders of the structure compared with a previous work that uses a genetic algorithm.
Acoustic Band Gaps in Three-Dimensional NaCl-Type Acoustic Crystals
Institute of Scientific and Technical Information of China (English)
FANG Nong-Yu; WU Pu-Gen; ZHANG Xin
2008-01-01
@@ We present the acoustic band gaps (ABGs) for a geometry of three-dimensional complex acoustic crystals: the NaCl-type structure. By using the super cell method based on the plane-wave expansion method (PWE), we study the three configurations formed by water objects (either a sphere of different sizes or a cube) located at the vertices of simple cubic (SC) lattice and surrounded by mercury background. The numerical results show that ABGs larger than the original SC structure for all the three configurations can be obtained by adjusting the length-diameter ratio of adjacent objects but keeping the filling fraction (f=0.25) of the unit cell unchanged. We also compare our results with that of 3D solid composites and find that the ABGs in liquid composites are insensitive to the shapes as that in the solid composites. We further prove that the decrease of the translation group symmetry is more efficient in creating the ABGs in 3D water-mercury systems.
Photonic band gap of three dimensional magnetized photonic crystal with Voigt configuration
Zhang, Hai-Feng; Liu, Shao-Bin; Kong, Xiang-Kun; Li, Bing-Xiang
2013-08-01
In this paper, the properties of two types of three-dimensional magnetized plasma photonic crystals (MPPCs) composed of homogeneous magnetized plasma and dielectric with simple-cubic lattices are theoretically studied by a modified plane wave expansion (PWE) method, as the magneto-optical Voigt effects of magnetized plasma are considered. The equations for type-1 structures with simple-cubic lattices (dielectric spheres immersed in magnetized plasma background), are theoretically deduced. The influences of dielectric constant of dielectric, plasma collision frequency, filling factor, the external magnetic field and plasma frequency on the properties of photonic band gaps (PBGs) for both types of MPPCs are investigated in detail, respectively, and some corresponding physical explanations are also given. The characteristics of flatbands regions are also discussed. From the numerical results, it has been shown that the PBGs of both types of three-dimensional MPPCs can be manipulated by plasma frequency, filling factor, the external magnetic field and the relative dielectric constant of dielectric, respectively. However, the plasma collision frequency has no effects on the PBGs for two types of three-dimensional MPPCs. The locations of flatbands regions can not be tuned by any parameters except for plasma frequency and the external magnetic field.
Extremely low-frequency Lamb wave band gaps in a sandwich phononic crystal thin plate
Shen, Li; Wu, Jiu Hui; Liu, Zhangyi; Fu, Gang
2015-11-01
In this paper, a kind of sandwich phononic crystal (PC) plate with silicon rubber scatterers embedded in polymethyl methacrylate (PMMA) matrix is proposed to demonstrate its low-frequency Lamb wave band gap (BG) characteristics. The dispersion relationship and the displacement vector fields of the basic slab modes and the locally resonant modes are investigated to show the BG formation mechanism. The anti-symmetric Lamb wave BG is further studied due to its important function in reducing vibration. The analysis on the BG characteristics of the PC through changing their geometrical parameters is performed. By optimizing the structure, a sandwich PC plate with a thickness of only 3 mm and a lower boundary (as low as 23.9 Hz) of the first anti-symmetric BG is designed. Finally, sound insulation experiment on a sandwich PC plate with the thickness of only 2.5 mm is conducted, showing satisfactory noise reduction effect in the frequency range of the anti-symmetric Lamb BG. Therefore, this kind of sandwich PC plate has potential applications in controlling vibration and noise in low-frequency ranges.
Design of a 3D photonic band gap cavity in a diamond-like inverse woodpile photonic crystal
Woldering, Léon A; Vos, Willem L
2014-01-01
We theoretically investigate the design of cavities in a three-dimensional (3D) inverse woodpile photonic crystal. This class of cubic diamond-like crystals has a very broad photonic band gap and consists of two perpendicular arrays of pores with a rectangular structure. The point defect that acts as a cavity is centred on the intersection of two intersecting perpendicular pores with a radius that differs from the ones in the bulk of the crystal. We have performed supercell bandstructure calculations with up to $5 \\times 5 \\times 5$ unit cells. We find that up to five isolated and dispersionless bands appear within the 3D photonic band gap. For each isolated band, the electric-field energy is localized in a volume centred on the point defect, hence the point defect acts as a 3D photonic band gap cavity. The mode volume of the cavities resonances is as small as 0.8 $\\lambda^{3}$ (resonance wavelength cubed), indicating a strong confinement of the light. By varying the radius of the defect pores we found that o...
Surface transport and band gap structure of exfoliated 2H-MoTe2 crystals
Gutiérrez Lezama, Ignacio; Ubaldini, Alberto; Longobardi, Maria; Giannini, Enrico; Renner, Christoph; Kuzmenko, Alexey B.; Morpurgo, Alberto F.
2014-09-01
Semiconducting transition metal dichalcogenides (TMDs) have emerged as materials that can be used to realize two-dimensional (2D) crystals possessing rather unique transport and optical properties. Most research has so far focused on sulfur and selenium compounds, while tellurium-based materials have attracted little attention so far. As a first step in the investigation of Te-based semiconducting TMDs in this context, we have studied MoTe2 crystals with thicknesses above 4 nm, focusing on surface transport and a quantitative determination of the gap structure. Using ionic-liquid gated transistors, we show that ambipolar transport at the surface of the material is reproducibly achieved, with hole and electron mobility values between 10 and 30 cm2 V-1s-1 at room temperature. The gap structure is determined through three different techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy, which allow the measurement of the indirect gap (Eind), and optical transmission spectroscopy on crystals of different thickness, which enables the determination of both the direct (Edir) and the indirect gap. We find that at room temperature Eind = 0.88 eV and Edir = 1.02 eV. Our results suggest that thin MoTe2 layers may exhibit a transition to a direct gap before mono-layer thickness. They should also drastically extend the range of direct gaps accessible in 2D semiconducting TMDs.
Ultrawide low frequency band gap of phononic crystal in nacreous composite material
Energy Technology Data Exchange (ETDEWEB)
Yin, J.; Huang, J.; Zhang, S., E-mail: zhangs@dlut.edu.cn; Zhang, H.W.; Chen, B.S.
2014-06-27
The band structure of a nacreous composite material is studied by two proposed models, where an ultrawide low frequency band gap is observed. The first model (tension-shear chain model) with two phases including brick and mortar is investigated to describe the wave propagation in the nacreous composite material, and the dispersion relation is calculated by transfer matrix method and Bloch theorem. The results show that the frequency ranges of the pass bands are quite narrow, because a special tension-shear chain motion in the nacreous composite material is formed by some very slow modes. Furthermore, the second model (two-dimensional finite element model) is presented to investigate its band gap by a multi-level substructure scheme. Our findings will be of great value to the design and synthesis of vibration isolation materials in a wide and low frequency range. Finally, the transmission characteristics are calculated to verify the results. - Highlights: • A Brick-and-Mortar structure is used to discuss wave propagation through nacreous materials. • A 1D Bloch wave solution of nacreous materials with a tension-shear chain model is obtained. • The band structure and transmission characteristics of nacreous materials with the FE model are examined. • An ultrawide low frequency band gap is found in nacreous materials with both theory and FE model.
Crystal structure and band gap determination of HfO2 thin films
Cheynet, M.C.; Pokrant, S.; Tichelaar, F.D.; Rouvière, J.L.
2007-01-01
Valence electron energy loss spectroscopy (VEELS) and high resolution transmission electron microscopy (HRTEM) are performed on three different HfO2 thin films grown on Si (001) by chemical vapor deposition (CVD) or atomic layer deposition (ALD). For each sample the band gap (Eg) is determined by lo
Crystal growth and characterization of the narrow-band-gap semiconductors OsPn₂ (Pn = P, As, Sb).
Bugaris, Daniel E; Malliakas, Christos D; Shoemaker, Daniel P; Do, Dat T; Chung, Duck Young; Mahanti, Subhendra D; Kanatzidis, Mercouri G
2014-09-15
Using metal fluxes, crystals of the binary osmium dipnictides OsPn2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three-dimensional framework of corner- and edge-sharing OsPn6 octahedra, as well as [Pn2(4-)] anions. Raman spectroscopy shows the presence of P-P single bonds, consistent with the presence of [Pn2(-4)] anions and formally Os(4+) cations. Optical-band-gap and high-temperature electrical resistivity measurements indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP2 and OsSb2 are n-type semiconductors, whereas OsAs2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn-Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment.
International Nuclear Information System (INIS)
The properties of omnidirectional photonic band gaps (OBGs) in two-dimensional plasma photonic crystals (2D PPCs) are theoretically investigated by the modified plane wave expansion method. In the simulation, we consider the off-plane incident wave vector. The configuration of 2D PPCs is the triangular lattices filled with the nonmagnetized plasma cylinders in the homogeneous and isotropic dielectric background. The calculated results show that the proposed 2D PPCs possess a flatbands region and the OBGs. Compared with the OBGs in the conventional 2D dielectric-air PCs, it can be obtained more easily and enlarged in the 2D PPCs with a similar structure. The effects of configurational parameters of the PPCs on the OBGs also are studied. The simulated results demonstrate that the locations of OBGs can be tuned easily by manipulating those parameters except for changing plasma collision frequency. The achieved OBGs can be enlarged by optimizations. The OBGs of two novel configurations of PPCs with different cross sections are computed for a comparison. Both configurations have the advantages of obtaining the larger OBGs compared with the conventional configuration, since the symmetry of 2D PPCs is broken by different sizes of periodically inserted plasma cylinders or connected by the embedded plasma cylinders with thin veins. The analysis of the results shows that the bandwidths of OBGs can be tuned by changing geometric and physical parameters of such two PPCs structures. The theoretical results may open a new scope for designing the omnidirectional reflectors or mirrors based on the 2D PPCs
Energy Technology Data Exchange (ETDEWEB)
Zhang, Hai-Feng, E-mail: hanlor@163.com [Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Nanjing Artillery Academy, Nanjing 211132 (China); Liu, Shao-Bin; Li, Bing-Xiang [Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China)
2016-01-15
The properties of omnidirectional photonic band gaps (OBGs) in two-dimensional plasma photonic crystals (2D PPCs) are theoretically investigated by the modified plane wave expansion method. In the simulation, we consider the off-plane incident wave vector. The configuration of 2D PPCs is the triangular lattices filled with the nonmagnetized plasma cylinders in the homogeneous and isotropic dielectric background. The calculated results show that the proposed 2D PPCs possess a flatbands region and the OBGs. Compared with the OBGs in the conventional 2D dielectric-air PCs, it can be obtained more easily and enlarged in the 2D PPCs with a similar structure. The effects of configurational parameters of the PPCs on the OBGs also are studied. The simulated results demonstrate that the locations of OBGs can be tuned easily by manipulating those parameters except for changing plasma collision frequency. The achieved OBGs can be enlarged by optimizations. The OBGs of two novel configurations of PPCs with different cross sections are computed for a comparison. Both configurations have the advantages of obtaining the larger OBGs compared with the conventional configuration, since the symmetry of 2D PPCs is broken by different sizes of periodically inserted plasma cylinders or connected by the embedded plasma cylinders with thin veins. The analysis of the results shows that the bandwidths of OBGs can be tuned by changing geometric and physical parameters of such two PPCs structures. The theoretical results may open a new scope for designing the omnidirectional reflectors or mirrors based on the 2D PPCs.
Zero permeability and zero permittivity band gaps in 1D metamaterial photonic crystals
Energy Technology Data Exchange (ETDEWEB)
Depine, Ricardo A. [Grupo de Electromagnetismo Aplicado, Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon I, C1428EHA Buenos Aires (Argentina); Martinez-Ricci, Maria L. [Grupo de Electromagnetismo Aplicado, Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon I, C1428EHA Buenos Aires (Argentina); Monsoriu, Juan A. [Departamento de Fisica Aplicada, Universidad Politecnica de Valencia, 46022 Valencia (Spain)]. E-mail: jmonsori@fis.upv.es; Silvestre, Enrique [Departamento de Optica, Universidad de Valencia, 46100 Burjassot (Spain); Andres, Pedro [Departamento de Optica, Universidad de Valencia, 46100 Burjassot (Spain)
2007-04-30
We consider layered heterostructures combining ordinary positive index materials and dispersive metamaterials. We show that these structures can exhibit a new type of photonic gap around frequencies where either the magnetic permeability {mu} or the electric permittivity {epsilon} of the metamaterial is zero. Although the interface of a semi-infinite medium with zero refractive index (a condition attained either when {mu}=0 or when {epsilon}=0) is known to give full reflectivity for all incident polarizations, here we show that a gap corresponding to {mu}=0 occurs only for TE polarized waves, whereas a gap corresponding to {epsilon}=0 occurs only for TM polarized waves. These band gaps are scale-length invariant and very robust against disorder, although they may disappear for the particular case of propagation along the stratification direction.
Kim, Seong-Han; Kim, Soeun; Kee, Chul-Sik
2016-08-01
Photonic crystals composed of virtual pillars with magnetic walls are proposed. A virtual pillar with a magnetic wall can be created inside a parallel perfect electric conductor plate waveguide by introducing a circular perfect magnetic conductor patch in the upper perfect electric conductor plate of the waveguide. The virtual pillar mimics a perfect magnetic conductor pillar with a radius less than that of the circular patch because electromagnetic waves can slightly penetrate the wall. Furthermore, the photonic band structures of a triangular photonic crystal composed of virtual pillars for the transverse electromagnetic modes of the waveguide are investigated. They are very similar to those of a triangular photonic crystal composed of infinitely long perfect electric conductor cylinders for transverse magnetic modes. The similarity between the two different photonic crystals is well understood by the boundary conditions of perfect electric and magnetic conductor surfaces. A double Dirac cone at the center of the Brillouin zone is observed and thus the virtual pillar triangular photonic crystal can act a zero-refractive-index material at the Dirac point frequency.
Institute of Scientific and Technical Information of China (English)
Fan Chun-Zhen; Wang Jun-Qiao; He Jin-Na; Ding Pei; Liang Er-Jun
2013-01-01
We theoretically investigate the photonic band gap in one-dimensional photonic crystals with a graded multilayer structure.The proposed structure constitutes an alternating composite layer (metallic nanoparticles embedded in TiO2 film) and an air layer.Regarding the multilayer as a series of capacitance,effective optical properties are derived.The dispersion relation is obtained with the solution of the transfer matrix equation.With a graded structure in the composite layer,numerical results show that the position and width of the photonic band gap can be effectively modulated by varying the number of the graded composite layers,the volume fraction of nanoparticles and the external stimuli.
Band gap structures in two-dimensional super porous phononic crystals.
Liu, Ying; Sun, Xiu-zhan; Chen, Shao-ting
2013-02-01
As one kind of new linear cellular alloys (LCAs), Kagome honeycombs, which are constituted by triangular and hexagonal cells, attract great attention due to the excellent performance compared to the ordinary ones. Instead of mechanical investigation, the in-plane elastic wave dispersion in Kagome structures are analyzed in this paper aiming to the multi-functional application of the materials. Firstly, the band structures in the common two-dimensional (2D) porous phononic structures (triangular or hexagonal honeycombs) are discussed. Then, based on these results, the wave dispersion in Kagome honeycombs is given. Through the component cell porosity controlling, the effects of component cells on the whole responses of the structures are investigated. The intrinsic relation between the component cell porosity and the critical porosity of Kagome honeycombs is established. These results will provide an important guidance in the band structure design of super porous phononic crystals.
Broadening of Omnidirectional Photonic Band Gap in Graphene Based one Dimensional Photonic Crystals
Directory of Open Access Journals (Sweden)
Neetika Arora
2015-09-01
Full Text Available A simple design of one dimensional gradual stacked photonic crystal has been proposed. This structure exhibits a periodic array of alternate layers of Graphene and Silica. These are the materials of low and high refractive indices respectively. Here the structure considered has three stacks .Each stack has five alternate layers of Graphene and silica. The transfer matrix method has been used for numerical computation. In this paper, such a structure has wider reflection bands in comparison to a conventional dielectric PC structure and structure with Sio2 and Si layers for a constant gradual constant ϒ at different incident angle.
Thermal expansion of CuIn5S8 single crystals and the temperature dependence of their band gap
International Nuclear Information System (INIS)
Single crystals of the CuIn5S8 ternary compound are grown by planar crystallization of the melt (the vertical Bridgman method). The composition and structure of the crystals are established. The specific expansion is measured by the dilatometric technique, and the coefficients of thermal expansion are calculated. From the data, the Debye temperatures (ΘD) and the root-mean-square dynamic displacements of atoms (√(u-bar2)) in the CuIn5S8 compound are calculated. From the transmittance spectra recorded in the region of the fundamental absorption edge in the temperature range 20 to 300 K, the band gap is determined and its temperature dependence is constructed.
Zhu, Dapeng; Liu, Guolei; Xiao, Shuqin; Yan, Shishen; He, Shumin; Cai, Li; Li, Qinghao; Cao, Qiang; Hu, Shujun; Chen, Yanxue; Kang, Shishou; Mei, Liangmo
2013-05-01
Epitaxial (Zn1-xMgx)1-yMnyO thin films were grown on c-Al2O3 substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn1-xMgx)1-yMnyO films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn1-xMgx)1-yMnyO films can create free carriers and contribute to charge transfer transitions. The conduction band offset ΔEC = 0.13 eV and valence band offset ΔEV = 0.1 eV were obtained for ZnO/Zn0.8Mg0.2O heterostructures, which increase to ΔEC = 0.21 eV and ΔEV = 0.14 eV for ZnO/Zn0.7Mg0.3O heterostructures.
Energy Technology Data Exchange (ETDEWEB)
Zhang Haifeng [College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Nanjing Artillery Academy, Nanjing 211132 (China); Liu Shaobin [College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); State Key Laboratory of Millimeter Waves of Southeast University, Nanjing Jiangsu 210096 (China); Kong Xiangkun; Bian Borui; Dai Yi [College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China)
2012-11-15
In this paper, an omnidirectional photonic band gap realized by one-dimensional ternary unmagnetized plasma photonic crystals based on a new Fibonacci quasiperiodic structure, which is composed of homogeneous unmagnetized plasma and two kinds of isotropic dielectric, is theoretically studied by the transfer matrix method. It has been shown that such an omnidirectional photonic band gap originates from Bragg gap in contrast to zero-n gap or single negative (negative permittivity or negative permeability) gap, and it is insensitive to the incidence angle and the polarization of electromagnetic wave. From the numerical results, the frequency range and central frequency of omnidirectional photonic band gap can be tuned by the thickness and density of the plasma but cease to change with increasing Fibonacci order. The bandwidth of omnidirectional photonic band gap can be notably enlarged. Moreover, the plasma collision frequency has no effect on the bandwidth of omnidirectional photonic band gap. It is shown that such new structure Fibonacci quasiperiodic one-dimensional ternary plasma photonic crystals have a superior feature in the enhancement of frequency range of omnidirectional photonic band gap compared with the conventional ternary and conventional Fibonacci quasiperiodic ternary plasma photonic crystals.
Direct Band Gap Wurtzite Gallium Phosphide Nanowires
Assali, S.; Zardo, I.; Plissard, S.; Kriegner, D.; Verheijen, M.A.; Bauer, G.; Meijerink, A.; Belabbes, A.; Bechstedt, F.; Haverkort, J.E.M.; Bakkers, E.P.A.M.
2013-01-01
The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a di
Liu, Rongqiang; Zhao, Haojiang; Zhang, Yingying; Guo, Honghwei; Deng, Zongquan
2015-12-01
The plane wave expansion (PWE) method is used to calculate the band gaps of two-dimensional (2D) phononic crystals (PCs) with a hybrid square-like (HSL) lattice. Band structures of both XY-mode and Z-mode are calculated. Numerical results show that the band gaps between any two bands could be maximized by altering the radius ratio of the inclusions at different positions. By comparing with square lattice and bathroom lattice, the HSL lattice is more efficient in creating larger gaps.
One-Dimensional Anisotropic Band Gap Structure
Institute of Scientific and Technical Information of China (English)
无
2000-01-01
The band gap structure of one-dimensional anisotropic photonic crystal has been studied by means of the transfer matrix formalism. From the analytic expressions and numeric calculations we see some general characteristics of the band gap structure of anisotropic photonic crystals, each band separates into two branches and the two branches react to polarization sensitively. In the practical case of oblique incidence, gaps move towards high frequency when the angle of incidence increases. Under some special conditions, the two branches become degenerate again.
Sarantopoulou, E.; Kollia, Z.; Cefalas, A. C.; Semashko, V. V.; Yu. Abdulsabirov, R.; Naumov, A. K.; Korableva, S. L.; Szczurek, T.; Kobe, S.; McGuiness, P. J.
2002-07-01
The absorption and the laser-induced fluorescence spectra of Pr 3+ ion in YF 3, LaF 3, KY 3 F 10 and LiLuF 4, single crystal hosts were obtained in the vacuum ultraviolet region of the spectrum. The energy position and the spacing of the levels of the 4f 5d electronic configuration depend on the host matrix. In addition, strong vacuum ultraviolet emission bands were observed, following crystal excitation at 157 nm with the molecular fluorine laser. The emission bands were due to the interconfigurational 4 f 5 d→4 f2 dipole-allowed transitions in Pr 3+ ions, and they were assigned to the transitions between the edge of the lowest Stark component of the 4f 5d electronic configuration and the levels of the 4f 2 electronic configuration. The VUV spectra can be interpreted by applying the crystal field model, and taking into consideration that lanthanide contraction of the 4f n-1 5d electronic configurations of the rare earth ions, and shielding of the positive ion charge from the electrons in the 4f n electronic configuration is taking place. Finally, a new method for monitoring the concentration of the rare earth ions in wide band gap fluoride dielectric crystals in a non-destructive way, by measuring magnetic dipole moments with the vibrating sample magnetometer (VSM) method, is presented for the first time to our knowledge for this type of crystals.
Chang, Chien C.; Chi, J. Y.; Chern, R. L.; Chang, C. Chung; Lin, C. H.; Chang, C. O.
2004-08-01
In this study, we investigate the effect of metallic inclusion modeled as perfect conductor on a dielectric photonic crystal (silicon/air) with large full band gap. The dielectric crystal consists of a hexagonal array of circular dielectric columns, each connected to its nearest neighbors by slender rectangular rods. It is found that inclusion of small metallic components inside the circular dielectrics sharply “turns off“ the full band gap of the dielectric photonic crystal. By increasing the radius of metallic inclusion above a threshold value, the full band gap (of the metallodielectric photonic crystal) makes its appearance again and continues to grow in size. On the other hand, metallic inclusion in the air region shows an opposite trend that the full band gap is not turned off, and its size diminishes gradually to zero with increasing the radius of inclusion. These peculiar behaviors can be explained on a unified basis by examining different types of boundary conditions for TM and TE modes, and employing variational arguments based on Rayleigh’s quotients. Moreover, the free-electron model for metallic components is also considered for TM modes. At large plasma frequencies, these modes show very close band structures to those described above for the case of perfect conductors.
Crystal-field investigations of rare-earth-doped wide band gap semiconductors
Muller, S; Wahl, U
Crystal field investigations play a central role in the studies of rare earth doped semiconductors. Optical stark level spectroscopy and lattice location studies of radioactive rare earth isotopes implanted at ISOLDE have provided important insight into these systems during the last years. It has been shown that despite a major site preference of the probe atoms in the lattice, several defect configurations do exist. These sites are visible in the optical spectra but their origin and nature aren't deducible from these spectra alone. Hyperfine measurements on the other hand should reveal these defect configurations and yield the parameters necessary for a description of the optical properties at the atomic scale. In order to study the crystal field with this alternative approach, we propose a new concept for perturbed $\\gamma\\gamma$-angular correlation (PAC) experiments at ISOLDE based on digital signal processing in contrast to earlier analog setups. The general functionality of the spectrometer is explained ...
Marichy, Catherine; Froufe-Pérez, Luis S; Scheffold, Frank
2015-01-01
Photonic crystal materials are based on a periodic modulation of the dielectric constant on length scales comparable to the wavelength of light. These materials can exhibit photonic band gaps; frequency regions for which the propagation of electromagnetic radiation is forbidden due to the depletion of the density of states. In order to exhibit a full band gap, 3D PCs must present a threshold refractive index contrast that depends on the crystal structure. In the case of the so-called woodpile photonic crystals this threshold is comparably low, approximately 1.9 for the direct structure. Therefore direct or inverted woodpiles made of high refractive index materials like silicon, germanium or titanium dioxide are sought after. Here we show that, by combining multiphoton lithography and atomic layer deposition, we can achieve a direct inversion of polymer templates into TiO$_{2}$ based photonic crystals. The obtained structures show remarkable optical properties in the near-infrared region with almost perfect sp...
Conoscopic patterns in photonic band gap of cholesteric liquid crystal cells with twist defects
Egorov, Roman I
2009-01-01
We theoretically investigate into the effects of the incidence angles in light transmission of cholesteric liquid crystal two-layer sandwich structures with twist defects created by rotation of the one layer about the helical axis.The conoscopic images and polarization resolved patterns are obtained for thick layers by computing the intensity and the polarization parameters as a function of the incidence angles.In addition to the defect angle induced rotation of the pictures as a whole, the rings of defect mode resonances are found to shrink to the origin and disappear as the defect twist angle varies from zero to its limiting value and beyond.
Microstrip microwave band gap structures
Indian Academy of Sciences (India)
V Subramanian
2008-04-01
Microwave band gap structures exhibit certain stop band characteristics based on the periodicity, impedance contrast and effective refractive index contrast. These structures though formed in one-, two- and three-dimensional periodicity, are huge in size. In this paper, microstrip-based microwave band gap structures are formed by removing the substrate material in a periodic manner. This paper also demonstrates that these structures can serve as a non-destructive characterization tool for materials, a duplexor and frequency selective coupler. The paper presents both experimental results and theoretical simulation based on a commercially available finite element methodology for comparison.
Su, Ching-Hua
2007-01-01
Optical transmission measurements were performed on CdTe bulk single crystal. It was found that when a sliced and polished CdTe wafer was used, a white film started to develop when the sample was heated above 530 K and the sample became opaque. Therefore, a bulk crystal of CdTe was first grown in the window area by physical vapor transport; the optical transmission was then measured and from which the energy band gap was derived between 304 and 1067 K. The band gaps of CdTe can be fit well as a function of temperature using the Varshini expression: Eg (e V) = 1.5860 - 5.9117xl0(exp -4) T(sup 2)/(T + 160). Using the band gap data, the high temperature electron-hole equilibrium was calculated numerically by assuming the Kane's conduction band structure and a heavy-hole parabolic valance band. The calculated intrinsic carrier concentrations agree well with the experimental data reported previously. The calculated intrinsic Fermi levels between 270 and 1200 K were also presented.
Formation of Degenerate Band Gaps in Layered Systems
Directory of Open Access Journals (Sweden)
Alexey P. Vinogradov
2012-06-01
Full Text Available In the review, peculiarities of spectra of one-dimensional photonic crystals made of anisotropic and/or magnetooptic materials are considered. The attention is focused on band gaps of a special type—the so called degenerate band gaps which are degenerate with respect to polarization. Mechanisms of formation and properties of these band gaps are analyzed. Peculiarities of spectra of photonic crystals that arise due to the linkage between band gaps are discussed. Particularly, it is shown that formation of a frozen mode is caused by linkage between Brillouin and degenerate band gaps. Also, existence of the optical Borrmann effect at the boundaries of degenerate band gaps and optical Tamm states at the frequencies of degenerate band gaps are analyzed.
Direct band gap silicon allotropes.
Wang, Qianqian; Xu, Bo; Sun, Jian; Liu, Hanyu; Zhao, Zhisheng; Yu, Dongli; Fan, Changzeng; He, Julong
2014-07-16
Elemental silicon has a large impact on the economy of the modern world and is of fundamental importance in the technological field, particularly in solar cell industry. The great demand of society for new clean energy and the shortcomings of the current silicon solar cells are calling for new materials that can make full use of the solar power. In this paper, six metastable allotropes of silicon with direct or quasidirect band gaps of 0.39-1.25 eV are predicted by ab initio calculations at ambient pressure. Five of them possess band gaps within the optimal range for high converting efficiency from solar energy to electric power and also have better optical properties than the Si-I phase. These Si structures with different band gaps could be applied to multiple p-n junction photovoltaic modules. PMID:24971657
International Nuclear Information System (INIS)
Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ∼0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron
Yater, J. E.; Shaw, J. L.; Pate, B. B.; Feygelson, T. I.
2016-02-01
Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ˜0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron
Ha, Thi Dep; Bao, JingFu
2016-04-01
Phononic crystals (PnCs) and n-type doped silicon technique have been widely employed in silicon-based MEMS resonators to obtain high quality factor (Q) as well as temperature-induced frequency stability. For the PnCs, their band gaps play an important role in the acoustic wave propagation. Also, the temperature and dopant doped into silicon can cause the change in its material properties such as elastic constants, Young's modulus. Therefore, in order to design the simultaneous high Q and frequency stability silicon-based MEMS resonators by two these techniques, a careful design should study effects of temperature and dopant on the band gap characteristics to examine the acoustic wave propagation in the PnC. Based on these, this paper presents (1) a proposed silicon-based PnC strip structure for support tether applications in low frequency silicon-based MEMS resonators, (2) influences of temperature and dopant on band gap characteristics of the PnC strips. The simulation results show that the largest band gap can achieve up to 33.56 at 57.59 MHz and increase 1280.13 % (also increase 131.89 % for ratio of the widest gaps) compared with the counterpart without hole. The band gap properties of the PnC strips is insignificantly effected by temperature and electron doping concentration. Also, the quality factor of two designed length extensional mode MEMS resonators with proposed PnC strip based support tethers is up to 1084.59% and 43846.36% over the same resonators with PnC strip without hole and circled corners, respectively. This theoretical study uses the finite element analysis in COMSOL Multiphysics and MATLAB softwares as simulation tools. This findings provides a background in combination of PnC and dopant techniques for high performance silicon-based MEMS resonators as well as PnC-based MEMS devices.
Directory of Open Access Journals (Sweden)
Thi Dep Ha
2016-04-01
Full Text Available Phononic crystals (PnCs and n-type doped silicon technique have been widely employed in silicon-based MEMS resonators to obtain high quality factor (Q as well as temperature-induced frequency stability. For the PnCs, their band gaps play an important role in the acoustic wave propagation. Also, the temperature and dopant doped into silicon can cause the change in its material properties such as elastic constants, Young’s modulus. Therefore, in order to design the simultaneous high Q and frequency stability silicon-based MEMS resonators by two these techniques, a careful design should study effects of temperature and dopant on the band gap characteristics to examine the acoustic wave propagation in the PnC. Based on these, this paper presents (1 a proposed silicon-based PnC strip structure for support tether applications in low frequency silicon-based MEMS resonators, (2 influences of temperature and dopant on band gap characteristics of the PnC strips. The simulation results show that the largest band gap can achieve up to 33.56 at 57.59 MHz and increase 1280.13 % (also increase 131.89 % for ratio of the widest gaps compared with the counterpart without hole. The band gap properties of the PnC strips is insignificantly effected by temperature and electron doping concentration. Also, the quality factor of two designed length extensional mode MEMS resonators with proposed PnC strip based support tethers is up to 1084.59% and 43846.36% over the same resonators with PnC strip without hole and circled corners, respectively. This theoretical study uses the finite element analysis in COMSOL Multiphysics and MATLAB softwares as simulation tools. This findings provides a background in combination of PnC and dopant techniques for high performance silicon-based MEMS resonators as well as PnC-based MEMS devices.
DEFF Research Database (Denmark)
1999-01-01
An optical fibre having a periodicidal cladding structure provididing a photonic band gap structure with superior qualities. The periodical structure being one wherein high index areas are defined and wherein these are separated using a number of methods. One such method is the introduction...
Rellergert, Wade G; DeMille, D; Greco, R R; Hehlen, M P; Jackson, R A; Torgerson, J R; Hudson, Eric R
2010-01-01
We have recently described a novel method for the construction of a solid-state optical frequency reference based on doping $^{229}$Th into high energy band-gap crystals. Since nuclear transitions are far less sensitive to environmental conditions than atomic transitions, we have argued that the $^{229}$Th optical nuclear transition may be driven inside a host crystal resulting in an optical frequency reference with a short-term stability of $3\\times10^{-17}<\\Delta f/f <1\\times10^{-15}$ at 1 s and a systematic-limited repeatability of $\\Delta f/f \\sim 2 \\times 10^{-16}$. Improvement by $10^2-10^3$ of the constraints on the variability of several important fundamental constants also appears possible. Here we present the results of the first phase of these experiments. Specifically, we have evaluated several high energy band-gap crystals (Th:NaYF, Th:YLF, Th:LiCAF, Na$_2$ThF$_6$, LiSAF) for their suitability as a crystal host by a combination of electron beam microprobe measurements, Rutherford Backscatte...
Photonic band gap in thin wire metamaterials.
Hock, Kai Meng
2008-03-01
We investigate the band structure of a class of photonic crystals made from only thin wires. Using a different method, we demonstrate that a complete photonic band gap is possible for such materials. Band gap materials normally consist of space filling dielectric or metal, whereas thin wires occupy a very small fraction of the volume. We show that this is related to the large increase in scattering at the Brillouin zone boundary. The method we developed brings together the calculation techniques in three different fields. The first is the calculation of scattering from periodic, tilted antennas, which we improve upon. The second is the standard technique for frequency selective surface design. The third is obtained directly from low energy electron diffraction theory. Good agreements with experiments for left handed materials, negative materials, and frequency selective surfaces are demonstrated.
Energy Technology Data Exchange (ETDEWEB)
Bodnar, I. V., E-mail: chemzav@bsuir.by [Belarussian State University of Information and Radio Electronics (Belarus)
2012-05-15
Single crystals of the CuIn{sub 5}S{sub 8} ternary compound are grown by planar crystallization of the melt (the vertical Bridgman method). The composition and structure of the crystals are established. The specific expansion is measured by the dilatometric technique, and the coefficients of thermal expansion are calculated. From the data, the Debye temperatures ({Theta}{sub D}) and the root-mean-square dynamic displacements of atoms ({radical}(u-bar{sup 2})) in the CuIn{sub 5}S{sub 8} compound are calculated. From the transmittance spectra recorded in the region of the fundamental absorption edge in the temperature range 20 to 300 K, the band gap is determined and its temperature dependence is constructed.
Del Villar, I.; Matias, I. R.; Arregui, F. J.; Claus, Richard O.
2003-01-01
A theoretical analysis of a fiber optical photonic band gap based tunable wavelength filter is presented. The design presented here is based on the quarter wave reflector with a liquid crystal defect layer in the middle of the structure. The filter generated by the structure is shifted in wavelength as the voltage applied to the structure is modified. Some critical parameters are analyzed: the effect of the consideration of fiber as the first layer and not the input medium in the shape of the...
Energy Technology Data Exchange (ETDEWEB)
Zhu Dapeng; Liu Guolei; Xiao Shuqin; Yan Shishen; He Shumin; Cai Li; Li Qinghao; Hu Shujun; Chen Yanxue; Kang Shishou; Mei Liangmo [School of Physics, National Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100 (China); Cao Qiang [School of Physics, National Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100 (China); Department of Physics, Qufu Normal University, Qufu, Shandong 273165 (China)
2013-05-07
Epitaxial (Zn{sub 1-x}Mg{sub x}){sub 1-y}Mn{sub y}O thin films were grown on c-Al{sub 2}O{sub 3} substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn{sub 1-x}Mg{sub x}){sub 1-y}Mn{sub y}O films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn{sub 1-x}Mg{sub x}){sub 1-y}Mn{sub y}O films can create free carriers and contribute to charge transfer transitions. The conduction band offset {Delta}E{sub C} = 0.13 eV and valence band offset {Delta}E{sub V} = 0.1 eV were obtained for ZnO/Zn{sub 0.8}Mg{sub 0.2}O heterostructures, which increase to {Delta}E{sub C} = 0.21 eV and {Delta}E{sub V} = 0.14 eV for ZnO/Zn{sub 0.7}Mg{sub 0.3}O heterostructures.
Proceedings of wide band gap semiconductors
International Nuclear Information System (INIS)
This book contains the proceedings of wide band gap semiconductors. Wide band gap semiconductors are under intense study because of their potential applications in photonic devices in the visible and ultraviolet part of the electromagnetic spectrum, and devices for high temperature, high frequency and high power electronics. Additionally, due to their unique mechanical, thermal, optical, chemical, and electronic properties many wide band gap semiconductors are anticipated to find applications in thermoelectric, electrooptic, piezoelectric and acoustooptic devices as well as protective coatings, hard coatings and heat sinks. Material systems covered in this symposium include diamond, II-VI compounds, III-V nitrides, silicon carbide, boron compounds, amorphous and microcrystalline semiconductors, chalcopyrites, oxides and halides. The various papers addressed recent experimental and theoretical developments. They covered issues related to crystal growth (bulk and thin films), structure and microstructure, defects, doping, optoelectronic properties and device applications. A theoretical session was dedicated to identifying common themes in the heteroepitaxy and the role of defects in doping, compensation and phase stability of this unique class of materials. Important experimental milestones included the demonstrations of bright blue injection luminescence at room temperatures from junctions based on III-V nitrides and a similar result from multiple quantum wells in a ZnSe double heterojunction at liquid nitrogen temperatures
Photonic band gap structure simulator
Chen, Chiping; Shapiro, Michael A.; Smirnova, Evgenya I.; Temkin, Richard J.; Sirigiri, Jagadishwar R.
2006-10-03
A system and method for designing photonic band gap structures. The system and method provide a user with the capability to produce a model of a two-dimensional array of conductors corresponding to a unit cell. The model involves a linear equation. Boundary conditions representative of conditions at the boundary of the unit cell are applied to a solution of the Helmholtz equation defined for the unit cell. The linear equation can be approximated by a Hermitian matrix. An eigenvalue of the Helmholtz equation is calculated. One computation approach involves calculating finite differences. The model can include a symmetry element, such as a center of inversion, a rotation axis, and a mirror plane. A graphical user interface is provided for the user's convenience. A display is provided to display to a user the calculated eigenvalue, corresponding to a photonic energy level in the Brilloin zone of the unit cell.
Hopping magnetotransport of the band-gap tuning Cu2Zn(Sn x Ge1-x )Se4 crystals.
Lähderanta, E; Hajdeu-Chicarosh, E; Shakhov, M A; Guc, M; Bodnar, I V; Arushanov, E; Lisunov, K G
2016-11-16
Resistivity, ρ(T, x), of Cu2Zn(Sn x Ge1-x )Se4 (CZTGeSe) single crystals with x = 0-1, investigated at temperatures between T ~ 10-320 K, exhibits an activated character within the whole temperature range, attaining a minimum at x = 0.47. Magnetoresistance (MR) of CZTGeSe with x = 0.26, 0.47 and 0.64 is positive (pMR) in all measured fields of B up to 20 T at any T between ~40-320 K, whereas MR of samples with x = 0 and 1 contains a negative contribution (nMR). The dependence of ρ(T) at B = 0 gives evidence for a nearest-neighbor hopping (NNH) conductivity in high-temperature intervals within T ~ 200-320 K depending on x, followed by the Mott variable-range hopping (VRH) charge transfer with lowering temperature. The pMR law of lnρ(B) [Formula: see text] B (2) is observed in both hopping conductivity regimes above, provided that the nMR contribution is absent or saturated. Analysis of the ρ(T) and MR data has yielded the values of the NNH activation energy and the VRH characteristic temperature, as well as those of the acceptor band width, the acceptor concentration, the localization radii of holes and the density of the localized states (DOS) at the Fermi level. All the parameters above exhibit a systematic non-monotonous dependence on x. Their extremums, lying close to x = 0.64, correspond to the minimum of a lattice disorder along with the maximum of DOS and of the acceptor concentration, as well as a highest proximity to the metal-insulator transition. PMID:27619988
Hopping magnetotransport of the band-gap tuning Cu2Zn(Sn x Ge1-x )Se4 crystals.
Lähderanta, E; Hajdeu-Chicarosh, E; Shakhov, M A; Guc, M; Bodnar, I V; Arushanov, E; Lisunov, K G
2016-11-16
Resistivity, ρ(T, x), of Cu2Zn(Sn x Ge1-x )Se4 (CZTGeSe) single crystals with x = 0-1, investigated at temperatures between T ~ 10-320 K, exhibits an activated character within the whole temperature range, attaining a minimum at x = 0.47. Magnetoresistance (MR) of CZTGeSe with x = 0.26, 0.47 and 0.64 is positive (pMR) in all measured fields of B up to 20 T at any T between ~40-320 K, whereas MR of samples with x = 0 and 1 contains a negative contribution (nMR). The dependence of ρ(T) at B = 0 gives evidence for a nearest-neighbor hopping (NNH) conductivity in high-temperature intervals within T ~ 200-320 K depending on x, followed by the Mott variable-range hopping (VRH) charge transfer with lowering temperature. The pMR law of lnρ(B) [Formula: see text] B (2) is observed in both hopping conductivity regimes above, provided that the nMR contribution is absent or saturated. Analysis of the ρ(T) and MR data has yielded the values of the NNH activation energy and the VRH characteristic temperature, as well as those of the acceptor band width, the acceptor concentration, the localization radii of holes and the density of the localized states (DOS) at the Fermi level. All the parameters above exhibit a systematic non-monotonous dependence on x. Their extremums, lying close to x = 0.64, correspond to the minimum of a lattice disorder along with the maximum of DOS and of the acceptor concentration, as well as a highest proximity to the metal-insulator transition.
Semiconductor resonator solitons above band gap
Taranenko, V. B.; Weiss, C. O.; Stolz, W.
2001-01-01
We show experimentally the existence of bright and dark spatial solitons in semiconductor resonators for excitation above the band gap energy. These solitons can be switched on, both spontaneously and with address pulses, without the thermal delay found for solitons below the band gap which is unfavorable for applications. The differences between soliton properties above and below gap energy are discussed.
Del Villar, Ignacio; Matias, Ignacio; Arregui, Francisco; Claus, Richard
2003-03-10
A theoretical analysis of a fiber optical photonic band gap based tunable wavelength filter is presented. The design presented here is based on the quarter wave reflector with a liquid crystal defect layer in the middle of the structure. The filter generated by the structure is shifted in wavelength as the voltage applied to the structure is modified. Some critical parameters are analyzed: the effect of the consideration of fiber as the first layer and not the input medium in the shape of the filter, the number of layers of the structure, and the thickness of the defect layer. This last parameter determines the width of the wavelength sweep of the filter, but is limited by the creation of more defects. Some rules of practical implementation of this device are also given. PMID:19461749
Zhang, Hai-Feng; Liu, Shao-Bin; Li, Bing-Xiang
2013-10-01
In this paper, the properties of photonic band gaps (PBGs) for three-dimensional magnetized plasma photonic crystals (MPPCs) composed of anisotropic dielectric (the uniaxial material) spheres immersed in homogeneous magnetized plasma background with simple-cubic lattices are theoretically investigated by the plane wave expansion (PWE) method, as the Voigt effects of magnetized plasma are considered. The equations for calculating the anisotropic PBGs in the first irreducible Brillouin zone are theoretically deduced. The anisotropic PBGs and two flatband regions can be obtained. The effects of the ordinary-refractive index, extraordinary-refractive index, filling factor, plasma frequency and plasma cyclotron frequency on the characteristics of anisotropic PBGs for the three-dimensional MPPCs are studied in detail and some corresponding physical explanations are also given. The numerical results show that the anisotropy can open partial band gaps in simple-cubic lattices and the complete PBGs can be found compared to the conventional three-dimensional MPPCs doped by the isotropic material. The bandwidths of PBGs can be enlarged by introducing the magnetized plasma into three-dimensional PCs containing the uniaxial material. It is also shown that the anisotropic PBGs can be manipulated by the ordinary-refractive index, extraordinary-refractive index, filling factor, plasma frequency and plasma cyclotron frequency. The locations of flatband regions cannot be tuned by any parameters except for the plasma frequency and plasma cyclotron frequency. Introducing the uniaxial material in three-dimensional magnetized plasma-dielectric photonic crystals can enlarge the PBGs and also provide a way to obtain the complete PBGs as the three-dimensional MPPCs with high symmetry.
Band gaps of primary metallic carbon nanotubes
Kleiner, Alex; Eggert, Sebastian
2000-01-01
Primary metallic, or small gap semiconducting nanotubes, are tubes with band gaps that arise solely from breaking the bond symmetry due to the curvature. We derive an analytic expression for these gaps by considering how a general symmetry breaking opens a gap in nanotubes with a well defined chiral wrapping vector. This approach provides a straightforward way to include all types of symmetry breaking effects, resulting in a simple unified gap equation as a function of chirality and deformati...
Low band gap polymers for organic photovoltaics
DEFF Research Database (Denmark)
Bundgaard, Eva; Krebs, Frederik C
2007-01-01
Low band gap polymer materials and their application in organic photovoltaics (OPV) are reviewed. We detail the synthetic approaches to low band gap polymer materials starting from the early methodologies employing quinoid homopolymer structures to the current state of the art that relies...
Directory of Open Access Journals (Sweden)
Hai-Feng Zhang
2016-08-01
Full Text Available In this paper, the properties of photonic band gaps (PBGs in two types of two-dimensional plasma-dielectric photonic crystals (2D PPCs under a transverse-magnetic (TM wave are theoretically investigated by a modified plane wave expansion (PWE method where Monte Carlo method is introduced. The proposed PWE method can be used to calculate the band structures of 2D PPCs which possess arbitrary-shaped filler and any lattice. The efficiency and convergence of the present method are discussed by a numerical example. The configuration of 2D PPCs is the square lattices with fractal Sierpinski gasket structure whose constituents are homogeneous and isotropic. The type-1 PPCs is filled with the dielectric cylinders in the plasma background, while its complementary structure is called type-2 PPCs, in which plasma cylinders behave as the fillers in the dielectric background. The calculated results reveal that the enough accuracy and good convergence can be obtained, if the number of random sampling points of Monte Carlo method is large enough. The band structures of two types of PPCs with different fractal orders of Sierpinski gasket structure also are theoretically computed for a comparison. It is demonstrate that the PBGs in higher frequency region are more easily produced in the type-1 PPCs rather than in the type-2 PPCs. Sierpinski gasket structure introduced in the 2D PPCs leads to a larger cutoff frequency, enhances and induces more PBGs in high frequency region. The effects of configurational parameters of two types of PPCs on the PBGs are also investigated in detail. The results show that the PBGs of the PPCs can be easily manipulated by tuning those parameters. The present type-1 PPCs are more suitable to design the tunable compacted devices.
Amri, R.; Sahel, S.; Manaa, C.; Bouaziz, L.; Gamra, D.; Lejeune, M.; Clin, M.; Zellama, K.; Bouchriha, H.
2016-08-01
Hybrid One-dimensional photonic crystal coated from a mixture of an organic compound (HMDSO) and oxygen (O2) is elaborated by PECVD technique. The originality of the method consists in obtaining layers of different permittivity with the same gas mixture, but with different flow. The change in flow is optimized to obtain organic/inorganic layers of good quality with high and low refractive index of 2.1 and 1.4 corresponding respectively to HMDSO and SiO2 materials as assigned by IR measurement. Evidence of the photonic band gap is obtained by measuring the transmissions and reflections spectra which show that it appears only after 13 periods with a width of 325 nm corresponding to energy 3.8 eV. We have also introduced a defect in this photonic structure by changing the thickness of central layer, and observed the presence of a frequency mode corresponding to this defect. Our results are interpreted by using a theoretical model based on transfer matrix wich well reproduced the experimental data.
Energy Technology Data Exchange (ETDEWEB)
Zhang, Hai-Feng, E-mail: hanlor@163.com [Key Laboratory of Radar Imaging and Microwave Photonics (Nanjing University of Aeronautics and Astronautics), Ministry of Education, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China); Nanjing Artillery Academy, Nanjing 211132 (China); Ding, Guo-Wen; Li, Hai-Ming; Liu, Shao-Bin [Key Laboratory of Radar Imaging and Microwave Photonics (Nanjing University of Aeronautics and Astronautics), Ministry of Education, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China)
2015-02-15
In this paper, the properties of complete photonic band gaps (CPBGs) and tunable self-collimation in two-dimensional plasma photonic crystals (2D PPCs) with a new structure in square lattices, whose dielectric fillers (GaAs) are inserted into homogeneous and nomagnetized plasma background are theoretically investigated by a modified plane wave expansion (PWE) method with a novel technique. The novel PWE method can be utilized to compute the dispersion curves of 2D PPCs with arbitrary-shaped cross section in any lattices. As a comparison, CPBGs of PPCs for four different configurations are numerically calculated. The computed results show that the proposed design has the advantages of achieving the larger CPBGs compared to the other three configurations. The influences of geometric parameters of filled unit cell and plasma frequency on the properties of CPBGs are studied in detail. The calculated results demonstrate that CPBGs of the proposed 2D PPCs can be easily engineered by changing those parameters, and the larger CPBGs also can be obtained by optimization. The self-collimation in such 2D PPCs also is discussed in theory under TM wave. The theoretical simulations reveal that the self-collimation phenomena can be found in the TM bands, and both the frequency range of self-collimation and the equifrequency surface contours can be tuned by the parameters as mentioned above. It means that the frequency range and direction of electromagnetic wave can be manipulated by designing, as it propagates in the proposed PPCs without diffraction. Those results can hold promise for designing the tunable applications based on the proposed PPCs.
Bulk band gaps in divalent hexaborides
Energy Technology Data Exchange (ETDEWEB)
Denlinger, Jonathan; Clack, Jules A.; Allen, James W.; Gweon, Gey-Hong; Poirier, Derek M.; Olson, Cliff G.; Sarrao, John L.; Bianchi, Andrea D.; Fisk, Zachary
2002-08-01
Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB6 additionally provide a stringent test case for proper inclusion of many-body effects in quasi-particle band calculations.
Wide band gap semiconductor templates
Energy Technology Data Exchange (ETDEWEB)
Arendt, Paul N. (Los Alamos, NM); Stan, Liliana (Los Alamos, NM); Jia, Quanxi (Los Alamos, NM); DePaula, Raymond F. (Santa Fe, NM); Usov, Igor O. (Los Alamos, NM)
2010-12-14
The present invention relates to a thin film structure based on an epitaxial (111)-oriented rare earth-Group IVB oxide on the cubic (001) MgO terminated surface and the ion-beam-assisted deposition ("IBAD") techniques that are amendable to be over coated by semiconductors with hexagonal crystal structures. The IBAD magnesium oxide ("MgO") technology, in conjunction with certain template materials, is used to fabricate the desired thin film array. Similarly, IBAD MgO with appropriate template layers can be used for semiconductors with cubic type crystal structures.
Diluted II-VI oxide semiconductors with multiple band gaps.
Yu, K M; Walukiewicz, W; Wu, J; Shan, W; Beeman, J W; Scarpulla, M A; Dubon, O D; Becla, P
2003-12-12
We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.
Energy bands and gaps near an impurity
Mihóková, E.; Schulman, L. S.
2016-10-01
It has been suggested that in the neighborhood of a certain kind of defect in a crystal there is a bend in the electronic band. We confirm that this is indeed possible using the Kronig-Penney model. Our calculations also have implications for photonic crystals.
Maximizing band gaps in plate structures
DEFF Research Database (Denmark)
Halkjær, Søren; Sigmund, Ole; Jensen, Jakob Søndergaard
2006-01-01
Band gaps, i.e., frequency ranges in which waves cannot propagate, can be found in elastic structures for which there is a certain periodic modulation of the material properties or structure. In this paper, we maximize the band gap size for bending waves in a Mindlin plate. We analyze an infinite...... periodic plate using Bloch theory, which conveniently reduces the maximization problem to that of a single base cell. Secondly, we construct a finite periodic plate using a number of the optimized base cells in a postprocessed version. The dynamic properties of the finite plate are investigated...
Labbani, Amel; Benghalia, Abdelmadjid
2012-06-01
Using the Maxwell-Garnett theory, the evolution of the refractive index of titanium dioxide (TiO2) doped with zinc sulfide (ZnS) particles is presented. The presence of the nano-objects in the host matrix allows us to obtain a new composite material with tunable optical properties. We find that the filling factor of ZnS nanoparticles greatly alters photonic band gaps (PBGs). We have calculated also the photonic band structure for electromagnetic waves propagating in a structure consisting of ZnS rods covered with the air shell layer in 2D hexagonal and square lattices by the finite difference time domain (FDTD) method. The rods are embedded in the TiO2 background medium with a high dielectric constant. Such photonic lattices present complete photonic band gaps (CPBGs). Our results show that the existence of the air shell layer leads to larger complete photonic gaps. We believe that the present results are significant to increase the possibilities for experimentalists to realize a sizeable and larger CPBG.
Derzsi, Mariana; Stasiewicz, Juliusz; Grochala, Wojciech
2011-09-01
We demonstrate that DFT calculations performed with the local density approximation (LDA) allow for significantly better reproduction of lattice constants, the unit cell volume and the density of Ag(II)SO(4) than those done with generalized gradient approximation (GGA). The LDA+U scheme, which accounts for electronic correlation effects, enables the accurate prediction of the magnetic superexchange constant of this strongly correlated material and its band gap at the Fermi level. The character of the band gap places the compound on the borderline between a Mott insulator and a charge transfer insulator. The size of the band gap (0.82 eV) indicates that AgSO(4) is a ferrimagnetic semiconductor, and possibly an attractive material for spintronics. A bulk modulus of 27.0 GPa and a compressibility of 0.037 GPa(-1) were determined for AgSO(4) from the third-order Birch-Murnaghan isothermal equation of state up to 20 GPa. Several polymorphic types compete with the ambient pressure P-1 phase as the external pressure is increased. The P-1 phase is predicted to resist pressure-induced metallization up to at least 20 GPa.
Derzsi, Mariana; Stasiewicz, Juliusz; Grochala, Wojciech
2011-09-01
We demonstrate that DFT calculations performed with the local density approximation (LDA) allow for significantly better reproduction of lattice constants, the unit cell volume and the density of Ag(II)SO(4) than those done with generalized gradient approximation (GGA). The LDA+U scheme, which accounts for electronic correlation effects, enables the accurate prediction of the magnetic superexchange constant of this strongly correlated material and its band gap at the Fermi level. The character of the band gap places the compound on the borderline between a Mott insulator and a charge transfer insulator. The size of the band gap (0.82 eV) indicates that AgSO(4) is a ferrimagnetic semiconductor, and possibly an attractive material for spintronics. A bulk modulus of 27.0 GPa and a compressibility of 0.037 GPa(-1) were determined for AgSO(4) from the third-order Birch-Murnaghan isothermal equation of state up to 20 GPa. Several polymorphic types compete with the ambient pressure P-1 phase as the external pressure is increased. The P-1 phase is predicted to resist pressure-induced metallization up to at least 20 GPa. PMID:21267751
Institute of Scientific and Technical Information of China (English)
章海锋; 蓝鹏飞; 杨国华; 肖正泉
2011-01-01
In order to study property of prohibit band gaps in 1-D ternary unmagnetized plasma photonic crystals, the transfer matrix method was applied to simulate the electromagnetic wave propagation through 1-D ternary umnagnetizod plasma photonic crystals. Based on the calculated transmission coefficients, effect of periodic constant, dielectric constant, thickness of dielectric layer and parameter of plasma on the property of prohibited band gaps was analyzed. The results illustrate that the band gaps can be tuned by changing dielectric constant, thickness of dielectric layer and plasma frequency, the bandwidth of prohibited band gaps can not be broadened by increasing periodic and plasma collision frequency. The results are good theoretical references to design I-D ternary unmagnetizod plasma photonic crystal devices.%为了研究1维3元非磁化等离子体光子晶体的禁带特性,采用传输矩阵法仿真计算了电磁波在1维3元非磁化等离子体光子晶体的传播规律,用计算得到的电磁波透射系数讨论了周期常数、介电常数、介质层厚度和等离子体参量对其禁带特性的影响.结果表明,改变介电常数、介质层厚度和等离子体频率可以实现对带隙的调制,增加周期数和等离子体频率不能实现对禁带的拓展.这一结果为设计1维3元非磁化等离子体光子晶体器件提供了理论参考.
Phononic band gap structures as optimal designs
DEFF Research Database (Denmark)
Jensen, Jakob Søndergaard; Sigmund, Ole
2003-01-01
In this paper we use topology optimization to design phononic band gap structures. We consider 2D structures subjected to periodic loading and obtain the distribution of two materials with high contrast in material properties that gives the minimal vibrational response of the structure. Both in-plane...... and out-of-plane vibrations are considered....
Institute of Scientific and Technical Information of China (English)
李未; 陈小玲
2011-01-01
利用二维三角晶格介质柱光子晶体TE偏振的禁带与介质柱半径的变化关系,分析了二维光子晶体的带隙分布及斜边耦合特性.结果表明,光子禁带的大小受到构成光子晶体的介电材料的空间排列分布以及介质柱半径大小的影响;束缚在光子晶体中的光波可以在波导和谐振腔中进行传输,达到选择输出光波的目的.%The paper study the relation between two dimensional triangular lattice photonic crystal band gap for TE polarizationand dielectric cylinder radius, and study distribution of two dimensional photonic crystal defect state. Results show, the photonic crystal band gaps were distributed by dielectric material space distribution and medium size of the radius; Tied in the photon crystals of light waves can transmission in waveguides and resonator cavity to select the output of light waves.
Sculpting the band gap: a computational approach.
Prasai, Kiran; Biswas, Parthapratim; Drabold, D A
2015-01-01
Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated with the gap states can be used to find atomic coordinates that yield desired electronic density of states. Using tight-binding models, we show that this approach may be used to arrive at electronically designed models of amorphous silicon and carbon. We provide a simple recipe to include a priori electronic information in the formation of computer models of materials, and prove that this information may have profound structural consequences. The models are validated with plane-wave density functional calculations. PMID:26490203
Mechanism of photonic band gap, optical properties, tuning and applications
International Nuclear Information System (INIS)
Mechanism of occurrence of Photonic Band Gap (PBG) is presented for 3-D structure using close packed face centered cubic lattice. Concepts and our work, specifically optical properties of 3-D photonic crystal, relative width, filling fraction, effective refractive index, alternative mechanism of photonic band gap scattering strength and dielectric contrast, effect of fluctuations and minimum refractive index contrast, are reported. The temperature tuning and anisotropy of nematic and ferroelectric liquid crystal infiltrated opal for different phase transitions are given. Effective dielectric constant with filling fraction using Maxwell Garnet theory (MG), multiple modified Maxwell Garnet (MMMG) and Effective Medium theory (EM) and results are compared with experiment to understand the occurrence of PBG. Our calculations of Lamb shifts including fluctuations are given and compared with those of literature values. We have also done band structure calculations including anisotropy and compared isotropic characteristic of liquid crystal. A possibility of lowest refractive index contrast useful for the fabrication of PBG is given. Our calculations for relative width as a function of refractive index contrast are reported and comparisons with existing theoretical and experimental optimal values are briefed. Applications of photonic crystals are summarized. The investigations conducted on PBG materials and reported here may pave the way for understanding the challenges in the field of PBG. (author)
Photonic band gap materials: design, synthesis, and applications
International Nuclear Information System (INIS)
Full text: Unlike semiconductors which facilitate the coherent propagation of electrons, photonic band gap (PBG) materials execute their novel functions through the coherent localization of photons. I review and discuss our recent synthesis of a large scale three-dimensional silicon photonic crystal with a complete photonic band gap near 1.5 microns. When a PBG material is doped with impurity atoms which have an electronic transition that lies within the gap, spontaneous emission of light from the atom is inhibited. Inside the gap, the photon forms a bound state to the atom. Outside the gap, radiative dynamics in the colored vacuum is highly non Markovian. I discuss the influence of these memory effects on laser action. When spontaneous emission is absent, the next order radiative effect (resonance dipole dipole interaction between atoms) must be incorporated leading to anomalous nonlinear optical effects which occur at a much lower threshold than in ordinary vacuum. I describe the collective switching of two-level atoms near a photonic band edge, by external laser field, from a passive state to one exhibiting population inversion. This effect is forbidden in ordinary vacuum. However, in the context of a PBG material, this effect may be utilized for an all-optical transistor. Finally, I discuss the prospects for a phase sensitive, single atom quantum memory device, onto which information may be written by an external laser pulse
Theoretical study of relative width of photonic band gap for the 3-D dielectric structure
Indian Academy of Sciences (India)
G K Johri; Akhilesh Tiwari; Saumya Saxena; Rajesh Sharma; Kuldeep Srivastava; Manoj Johri
2002-03-01
Calculations for the relative width (/0) as a function of refractive index and relative radius of the photonic band gap for the fcc closed packed 3-D dielectric microstructure are reported and comparison of experimental observations and theoretical predictions are given. This work is useful for the understanding of photonic crystals and occurrence of the photonic band gap.
DEFF Research Database (Denmark)
Svane, Axel; Christensen, Niels Egede; Gorczyca, I.;
2010-01-01
The electronic band structures of InN, GaN, and a hypothetical ordered InGaN2 compound, all in the wurtzite crystal structure, are calculated using the quasiparticle self-consistent GW approximation. This approach leads to band gaps which are significantly improved compared to gaps calculated....... The band gap of InGaN2 is considerably smaller than what would be expected by linear interpolation implying a significant band gap bowing in InGaN alloys....
Optical band gaps of organic semiconductor materials
Costa, José C. S.; Taveira, Ricardo J. S.; Lima, Carlos F. R. A. C.; Mendes, Adélio; Santos, Luís M. N. B. F.
2016-08-01
UV-Vis can be used as an easy and forthright technique to accurately estimate the band gap energy of organic π-conjugated materials, widely used as thin films/composites in organic and hybrid electronic devices such as OLEDs, OPVs and OFETs. The electronic and optical properties, including HOMO-LUMO energy gaps of π-conjugated systems were evaluated by UV-Vis spectroscopy in CHCl3 solution for a large number of relevant π-conjugated systems: tris-8-hydroxyquinolinatos (Alq3, Gaq3, Inq3, Al(qNO2)3, Al(qCl)3, Al(qBr)3, In(qNO2)3, In(qCl)3 and In(qBr)3); triphenylamine derivatives (DDP, p-TTP, TPB, TPD, TDAB, m-MTDAB, NPB, α-NPD); oligoacenes (naphthalene, anthracene, tetracene and rubrene); oligothiophenes (α-2T, β-2T, α-3T, β-3T, α-4T and α-5T). Additionally, some electronic properties were also explored by quantum chemical calculations. The experimental UV-Vis data are in accordance with the DFT predictions and indicate that the band gap energies of the OSCs dissolved in CHCl3 solution are consistent with the values presented for thin films.
The band-gap enhanced photovoltaic structure
Tessler, Nir
2016-05-01
We critically examine the recently suggested structure that was postulated to potentially add 50% to the photo-conversion efficiency of organic solar cells. We find that the structure could be realized using stepwise increase in the gap as long as the steps are not above 0.1 eV. We also show that the charge extraction is not compromised due to an interplay between the contact's space charge and the energy level modification, which result in a flat energy band at the extracting contact.
Highly dispersive photonic band-gap prism
International Nuclear Information System (INIS)
We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime. We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action. Such a PBG prism is very compact if operated in the optical regime, ∼20μm in size for λ∼700nm, and can serve as a dispersive element for building ultracompact miniature spectrometers. copyright 1996 Optical Society of America
Highly dispersive photonic band-gap prism.
Lin, S Y; Hietala, V M; Wang, L; Jones, E D
1996-11-01
We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime. We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action. Such a PBG prism is very compact if operated in the optical regime, ~20 mm in size for lambda ~ 700 nm, and can serve as a dispersive element for building ultracompact miniature spectrometers. PMID:19881796
Size effects in band gap bowing in nitride semiconducting alloys
DEFF Research Database (Denmark)
Gorczyca, I.; Suski, T.; Christensen, Niels Egede;
2011-01-01
Chemical and size contributions to the band gap bowing of nitride semiconducting alloys (InxGa1-xN, InxAl1-xN, and AlxGa1-xN) are analyzed. It is shown that the band gap deformation potentials of the binary constituents determine the gap bowing in the ternary alloys. The particularly large gap...
Xu, Wen; Bai, Xue; Zhu, Yongsheng; Liu, Tong; Xu, Sai; Dong, Biao; Song, Hongwei
2013-05-14
Changes in the excitation spectra of luminescent species inserted in photorefractive crystals as a function of changes in the high-order photonic band gap (PBG) have not been previously observed. In this communication, we present our results monitoring the excitation band of Eu(TTA)3(TPPO)2 inserted in the PMMA opal photonic crystals as a function of the changes in the high-order PBG of the crystals. We find shifts in the complex excitation band and changes in the integrated emission intensity that correlates with shifts in the high-order PBG through coupling to the excitation transition.
Enlargement of Photonic Band Gaps and Physical Picture of Photonic Band Structures
Institute of Scientific and Technical Information of China (English)
ZHANG Yan; SHI Jun-Jie
2006-01-01
@@ Light propagation in a one-dimensional photonic crystal (PC), consisting of alternative slabs with refractive indices (layer thicknesses) n1 (a) and n2 (b), is investigated. An important optimal parameter matching condition,n1a ≈ n2b, is obtained for the largest photonic band gap (PBG). Moreover, we find that the exact analytical solutions for the electric/magnetic field eigenmodes at the band edges are standing waves with odd or even symmetry about the centre of each layer. The electric/magnetic field eigenfunctions at the top and bottom of the nth band have n and n - 1 nodes in one period of PC, respectively. The PBG arises from the symmetric differences of the field eigenfunctions at the band edges.
Peculiarities of Tamm states formed in degenerate photonic band gaps
Energy Technology Data Exchange (ETDEWEB)
Merzlikin, A.M., E-mail: merzlikin_a@mail.r [Institution for Theoretical and Applied Electromagnetics, Russian Academy of Science, 13/19 Izhorskaya ul., 125412 Moscow (Russian Federation); Vinogradov, A.P.; Lagarkov, A.N. [Institution for Theoretical and Applied Electromagnetics, Russian Academy of Science, 13/19 Izhorskaya ul., 125412 Moscow (Russian Federation); Levy, M. [Department of Physics, Michigan Technological University, 1400 Townsend Drive Houghton, MI 49931-1295 (United States); Bergman, D.J. [Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978 (Israel); Strelniker, Y.M. [Department of Physics, Bar-Ilan University, IL-52900 Ramat-Gan (Israel)
2010-07-15
The structure of the Tamm state localized at the interface between anisotropic magnetophotonic crystal (anisotropic MPC) and a photonic crystal (PC) made of isotropic dielectrics is studied. It is shown that if the frequency of this state appears within the degenerate band gap then its structure qualitatively differs from the structure of a well-known Tamm state localized at the interface between two one-dimensional PC made of isotropic materials. Since inside the degenerate BG the real part of the Bloch wavenumber differs from the Brillouin value, two Bloch waves with different signs in the real part of the wavenumber and the same sign in the imaginary part have different input impedance values. Moreover, contrary to the case of a PC made of isotropic materials the impedance of each Bloch wave is a tensor. As a consequence to construct a surface state at least three evanescent Bloch waves are required. The conditions that determine the Tamm state frequency also change.
Phononic First Band Gap of Quaternary Layered Periodic Structure with the Lumped-Mass Method
Directory of Open Access Journals (Sweden)
Chen Yuan
2014-01-01
Full Text Available Existing band gap analysis is mostly focused on the binary structure, while the researches on the quaternary layered periodic structure are still lacking. In this paper, the unidimensional lumped-mass method in the phonic crystal theory is firstly improved so that the material viscoelasticity can be taken into consideration. Then, the binary layered periodic structure is converted into a quaternary one and band gaps appear at low frequency range. Finally, the effects of density, elastic modulus, damping ratio, and the thickness of single material on the first band gap of the quaternary layered periodic structure are analyzed after the algorithm is promoted. The research findings show that effects of density, elastic modulus, and thickness of materials on the first band gap are considerable but those of damping ratio are not so distinct. This research provides theoretical bases for band gap design of the quaternary layered periodic structure.
Directory of Open Access Journals (Sweden)
A. Kumar
2015-09-01
Full Text Available The single crystal growth of tetrachloromercurate (II [HM-1] and tetrachlorozincate (II [HM-2] with 2-amino-5-chloropyridine has been performed by slow cooling (SC crystal growth technique of solution growth methodin which needle shaped transparent single crystals (0.5 x 0.2 x 0.2mm were obtained. The crystal structures of these hybrid materials have been studied by X-ray diffraction, experimental and computational methods. [HgCl4]2- anions have a distorted tetrahedral geometry and the tetrahedra hybrid structure exhibit interwoven inorganic-organic layers mingled through N-Hδ+...Clδ- hydrogen bonding interactions. The mercurophilic interactions [Hg...Hg = 3.984(5Å] and halogen interactions [Cl...Cl = 3.406(2Å] form 2D parallelogram pattern of secondary interactions in [HM-1] whereas for [HM-2] crystal structure is stabilized by Cl...Cl = 3.357(2Å interactions. UV-vis absorption spectra depict the change in optical band gap from 3.01 eV to 3.42 eV on replacing the metal halide group, could be due to increase in optical absorption as a function of wavelength. The Raman and Hyper-Raman tensors calculations were performed based on single crystal X-ray data and the Lagrangian strain tensor calculations show the degree of lattice distortion = 1.794 between [HM-1] and [HM-2] which are useful tools for the optical response properties of inorganic-organic hybrid derivatives. The photoluminescence emission spectra peaks were observed in the wavelength range of 371 to 598 nm for material [HM-1] and in the wavelength range of 384 to 600 nm for material [HM-2] and lie in the visible range for both materials.
Anhydrous crystals of DNA bases are wide gap semiconductors
Maia, F. F.; Freire, V. N.; Caetano, E. W. S.; Azevedo, D. L.; Sales, F. A. M.; Albuquerque, E. L.
2011-05-01
We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results.
Review of wide band-gap semiconductors technology
Directory of Open Access Journals (Sweden)
Jin Haiwei
2016-01-01
Full Text Available Silicon carbide (SiC and gallium nitride (GaN are typical representative of the wide band-gap semiconductor material, which is also known as third-generation semiconductor materials. Compared with the conventional semiconductor silicon (Si or gallium arsenide (GaAs, wide band-gap semiconductor has the wide band gap, high saturated drift velocity, high critical breakdown field and other advantages; it is a highly desirable semiconductor material applied under the case of high-power, high-temperature, high-frequency, anti-radiation environment. These advantages of wide band-gap devices make them a hot spot of semiconductor technology research in various countries. This article describes the research agenda of United States and European in this area, focusing on the recent developments of the wide band-gap technology in the US and Europe, summed up the facing challenge of the wide band-gap technology.
Incomplete photonic band gap as inferred from the speckle pattern of scattered light waves.
Apalkov, V M; Raikh, M E; Shapiro, B
2004-06-25
Motivated by recent experiments on intensity correlations of the waves transmitted through disordered media, we demonstrate that the speckle pattern from disordered photonic crystal with incomplete band gap represents a sensitive tool for determination of the stop-band width. We establish the quantitative relation between this width and the angular anisotropy of the intensity correlation function.
Isotropic properties of the photonic band gap in quasicrystals with low-index contrast
Rose, Priya; Abbate, G; Andreone, A
2011-01-01
We report on the formation and development of the photonic band gap in two-dimensional 8-, 10- and 12-fold symmetry quasicrystalline lattices of low index contrast. Finite size structures made of dielectric cylindrical rods were studied and measured in the microwave region, and their properties compared with a conventional hexagonal crystal. Band gap characteristics were investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence from 0 \\degree to 30\\degree were used in order to investigate the isotropic nature of the band gap. The arbitrarily high rotational symmetry of aperiodically ordered structures could be practically exploited to manufacture isotropic band gap materials, which are perfectly suitable for hosting waveguides or cavities.
Isotropic properties of the photonic band gap in quasicrystals with low-index contrast
Priya Rose, T.; di Gennaro, E.; Abbate, G.; Andreone, A.
2011-09-01
We report on the formation and development of the photonic band gap in two-dimensional 8-, 10-, and 12-fold symmetry quasicrystalline lattices of low-index contrast. Finite-size structures made of dielectric cylindrical rods were studied and measured in the microwave region, and their properties were compared with a conventional hexagonal crystal. Band-gap characteristics were investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence from 0∘ to 30∘ were used to investigate the isotropic nature of the band gap. The arbitrarily high rotational symmetry of aperiodically ordered structures could be practically exploited to manufacture isotropic band-gap materials, which are perfectly suitable for hosting waveguides or cavities.
Functionally Graded Thermoelectric Material though One Step Band Gap and Dopant Engineering
DEFF Research Database (Denmark)
Jensen, Ellen Marie; Borup, Kasper Andersen; Cederkrantz, Daniel;
gradients. It has previously been shown that a large functionally graded thermoelectric single crystal can be synthesized by the Czochralski method (1). Utilizing element gradients inherent to the Czochralski process we have synthesized a Ge1-xSix:B crystal with a continuously varying x, band gap, and...
Maximizing phononic band gaps in piezocomposite materials by means of topology optimization.
Vatanabe, Sandro L; Paulino, Glaucio H; Silva, Emílio C N
2014-08-01
Phononic crystals (PCs) can exhibit phononic band gaps within which sound and vibrations at certain frequencies do not propagate. In fact, PCs with large band gaps are of great interest for many applications, such as transducers, elastic/acoustic filters, noise control, and vibration shields. Previous work in the field concentrated on PCs made of elastic isotropic materials; however, band gaps can be enlarged by using non-isotropic materials, such as piezoelectric materials. Because the main property of PCs is the presence of band gaps, one possible way to design microstructures that have a desired band gap is through topology optimization. Thus in this work, the main objective is to maximize the width of absolute elastic wave band gaps in piezocomposite materials designed by means of topology optimization. For band gap calculation, the finite element analysis is implemented with Bloch-Floquet theory to solve the dynamic behavior of two-dimensional piezocomposite unit cells. Higher order frequency branches are investigated. The results demonstrate that tunable phononic band gaps in piezocomposite materials can be designed by means of the present methodology.
Flexural vibration band gaps in thin plates with two-dimensional binary locally resonant structures
Institute of Scientific and Technical Information of China (English)
Yu Dian-Long; Wang Gang; Liu Yao-Zong; Wen Ji-Hong; Qiu Jing
2006-01-01
The complete flexural vibration band gaps are studied in the thin plates with two-dimensional binary locally resonant structures, i.e. the composite plate consisting of soft rubber cylindrical inclusions periodically placed in a host material. Numerical simulations show that the low-frequency gaps of flexural wave exist in the thin plates. The width of the first gap decreases monotonically as the matrix density increases. The frequency response of the finite periodic thin plates is simulated by the finite element method, which provides attenuations of over 20dB in the frequency range of the band gaps. The findings will be significant in the application of phononic crystals.
Solid state dielectric screening versus band gap trends and implications
Ravichandran, Ram; Wang, Alan X.; Wager, John F.
2016-10-01
High-frequency (optical) and low-frequency (static) dielectric constant versus band gap trends, as well as index of refraction versus band gap trends are plotted for 107 inorganic semiconductors and insulators. These plots are describable via power-law fitting. Dielectric screening trends that emerge from this analysis have important optical and electronic implications. For example, barrier lowering during Schottky emission, phonon-assisted or Fowler-Nordheim tunneling, or Frenkel-Poole emission from a trap is found to be significantly more pronounced with increasing band gap due to a reduction in the optical dielectric constant with increasing band gap. The decrease in the interface state density with increasing band gap is another optical dielectric constant trend. The tendency for a material with a wider band gap to be more difficult to dope is attributed to an increase in the ionization energy of the donor or acceptor dopant, which in turn, depends on the optical dielectric constant and the effective mass. Since the effective mass for holes is almost always larger than that for electrons, p-type doping is more challenging than n-type doping in a wide band gap material. Finally, the polar optical phonon-limited mobility depends critically upon the reciprocal difference of the optical and the static dielectric constant. Consequently, electron and hole mobility tend to decrease with increasing band gap in a polar material.
Large area modules based on low band gap polymers
DEFF Research Database (Denmark)
Bundgaard, Eva; Krebs, Frederik C
2010-01-01
The use of three low band gap polymers in large area roll-to-roll coated modules is demonstrated. The polymers were prepared by a Stille cross coupling polymerization and all had a band gap around 1.6 eV. The polymers were first tested in small area organic photovoltaic devices which showed...
Band structures in Sierpinski triangle fractal porous phononic crystals
Wang, Kai; Liu, Ying; Liang, Tianshu
2016-10-01
In this paper, the band structures in Sierpinski triangle fractal porous phononic crystals (FPPCs) are studied with the aim to clarify the effect of fractal hierarchy on the band structures. Firstly, one kind of FPPCs based on Sierpinski triangle routine is proposed. Then the influence of the porosity on the elastic wave dispersion in Sierpinski triangle FPPCs is investigated. The sensitivity of the band structures to the fractal hierarchy is discussed in detail. The results show that the increase of the hierarchy increases the sensitivity of ABG (Absolute band gap) central frequency to the porosity. But further increase of the fractal hierarchy weakens this sensitivity. On the same hierarchy, wider ABGs could be opened in Sierpinski equilateral triangle FPPC; whilst, a lower ABG could be opened at lower porosity in Sierpinski right-angled isosceles FPPCs. These results will provide a meaningful guidance in tuning band structures in porous phononic crystals by fractal design.
Strain modulated band gap of edge passivated armchair graphene nanoribbons
Peng, Xihong
2011-01-01
First principles calculations were performed to study strain effects on band gap of armchair graphene nanoribbons (AGNRs)with different edge passivation, including H, O, and OH group. The band gap of the H-passivated AGNRs shows a nearly periodic zigzag variation under strain. For O and OH passivation, the zigzag patterns are significantly shifted by a modified quantum confinement due to the edges. In addition, the band gap of the O-passivated AGNRs experiences a direct-to-indirect transition with sufficient tensile strain (~5%). The indirect gap reduces to zero with further increased strain.
Institute of Scientific and Technical Information of China (English)
闫海涛; 王鸣; 葛益娴; 喻平; 刘青
2009-01-01
基于对光纤传输特性和胶体光子晶体制备方法的研究,提出了用外加电场控制的方法制备光子带隙位于通讯波段的FCC结构的胶体光子晶体,并用光纤系统测试胶体光子晶体的带隙特性.采用RSOFT模拟了胶体光子晶体的带隙,分析了带隙位于通讯波段时所需的胶体微球的基本参量(微球折射率和直径).采用自组装的方法,用步进电机控制玻璃基片向上的拉升速率.速率为5 μm/s,同时外加一电场.用扫描电镜观测胶体晶体的表面形貌,并设计了单模光纤系统测量胶体光子晶体的带隙特性.测试的透射谱线表明胶体光子晶体的带隙中心波长为1552 nm.测试结果和模拟结果具有很好的一致性,误差只有2 nm.%Based on the transmission characteration of optical fiber and the methods of fabrication colloidal photonic crystals, electric field-controlled method is used to fabricate colloidal photonic crystals with the photonic band-gap (PBG) in communications wavelength. RSOFT is used to simulate PBG characteration of colloidal photonic crystals and the parameters of colloidal microsphere are obtained. By the self-assembled method, the stepping motor is used to control the glass substrate, with the rate of 5 μm/s. At the same time an electric field is applied in the process of the colloidal crystals growth. The scanning electron microscopy is used to observe the surface of colloidal photonic crystals. An optical fiber system is designed to test PBG center wavelength of colloidal photonic crystals. Transmission spectrum shows the PBG of this colloidal crystals at 1552 nm. The agreement is very good between the experimental results and the simulation results, with the error of only 2 nm.
Computational investigation on tunable optical band gap in armchair polyacenes
International Nuclear Information System (INIS)
Polyacenes in their armchair geometry (phenacenes) have recently been found to possess appealing electronic and optical properties with higher chemical stability and comparatively larger band gap as compared to linear polyacenes. They also behave as high-temperature superconductors upon alkali metal doping. Moreover, the optical properties of crystalline picene can be finely tuned by applying external pressure. We investigated the variation of optical gap as a function of altering the interplanar distances between parallel cofacial phenacene dimers. We employed both time-dependent density functional theory and density matrix renormalization group (DMRG) technique to investigate the lowest singlet excitations in phenacene dimer. Our study showed that the lowest singlet excitation in these systems evolved as a function of interplanar separation. The optical excitation energy gap decreases as a function of inverse interplanar separation of the phenacene dimer. The distant dependent variation of optical absorption at the dimer level may be comparable with experimental observation in picene crystal under pressure. DMRG study also demonstrates that besides picene, electronic properties of higher phenacenes can also be tunable by altering interplanar separation
Band Gaps of an Amorphous Photonic Materials
Institute of Scientific and Technical Information of China (English)
WANG Yi-Quan; FENG Zhi-Fang; HU Xiao-Yong; CHENG Bing-Ying; ZHANG Dao-Zhong
2004-01-01
@@ A new kind of amorphous photonic materials is presented. Both the simulated and experimental results show that although the disorder of the whole dielectric structure is strong, the amorphous photonic materials have two photonic gaps. This confirms that the short-range order is an essential factor for the formation of the photonic gaps.
Reducing support loss in micromechanical ring resonators using phononic band-gap structures
Energy Technology Data Exchange (ETDEWEB)
Hsu, Feng-Chia; Huang, Tsun-Che; Wang, Chin-Hung; Chang, Pin [Industrial Technology Research Institute-South, Tainan 709, Taiwan (China); Hsu, Jin-Chen, E-mail: fengchiahsu@itri.org.t, E-mail: hsujc@yuntech.edu.t [Department of Mechanical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin 64002, Taiwan (China)
2011-09-21
In micromechanical resonators, energy loss via supports into the substrates may lead to a low quality factor. To eliminate the support loss, in this paper a phononic band-gap structure is employed. We demonstrate a design of phononic-crystal (PC) strips used to support extensional wine-glass mode ring resonators to increase the quality factor. The PC strips are introduced to stop elastic-wave propagation by the band-gap and deaf-band effects. Analyses of resonant characteristics of the ring resonators and the dispersion relations, eigenmodes, and transmission properties of the PC strips are presented. With the proposed resonator architecture, the finite-element simulations show that the leaky power is effectively reduced and the stored energy inside the resonators is enhanced simultaneously as the operating frequencies of the resonators are within the band gap or deaf bands. Realization of a high quality factor micromechanical ring resonator with minimized support loss is expected.
Sub-band-gap laser micromachining of lithium niobate
DEFF Research Database (Denmark)
Christensen, F. K.; Müllenborn, Matthias
1995-01-01
Laser processing of insulators and semiconductors is usually realized using photon energies exceeding the band-gap energy. This makes laser processing of insulators difficult since high photon energies typically require either a pulsed laser or a frequency-doubled continuous-wave laser. A new...... method is reported which enables us to do laser processing of lithium niobate using sub-band-gap photons. Using high scan speeds, moderate power densities, and sub-band-gap photon energies results in volume removal rates in excess of 106µm3/s. This enables fast micromachining of small piezoelectric...
Band gap effects of hexagonal boron nitride using oxygen plasma
Energy Technology Data Exchange (ETDEWEB)
Sevak Singh, Ram; Leong Chow, Wai [School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); Yingjie Tay, Roland [School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); Temasek Laboratories-NTU, 50 Nanyang Avenue, Singapore 639798 (Singapore); Hon Tsang, Siu [Temasek Laboratories-NTU, 50 Nanyang Avenue, Singapore 639798 (Singapore); Mallick, Govind [Temasek Laboratories-NTU, 50 Nanyang Avenue, Singapore 639798 (Singapore); Weapons and Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (United States); Tong Teo, Edwin Hang, E-mail: htteo@ntu.edu.sg [School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore)
2014-04-21
Tuning of band gap of hexagonal boron nitride (h-BN) has been a challenging problem due to its inherent chemical stability and inertness. In this work, we report the changes in band gaps in a few layers of chemical vapor deposition processed as-grown h-BN using a simple oxygen plasma treatment. Optical absorption spectra show a trend of band gap narrowing monotonically from 6 eV of pristine h-BN to 4.31 eV when exposed to oxygen plasma for 12 s. The narrowing of band gap causes the reduction in electrical resistance by ∼100 fold. The x-ray photoelectron spectroscopy results of plasma treated hexagonal boron nitride surface show the predominant doping of oxygen for the nitrogen vacancy. Energy sub-band formations inside the band gap of h-BN, due to the incorporation of oxygen dopants, cause a red shift in absorption edge corresponding to the band gap narrowing.
Singh, Bipin K; Pandey, Praveen C
2016-07-20
Engineering of thermally tunable terahertz photonic and omnidirectional bandgaps has been demonstrated theoretically in one-dimensional quasi-periodic photonic crystals (PCs) containing semiconductor and dielectric materials. The considered quasi-periodic structures are taken in the form of Fibonacci, Thue-Morse, and double periodic sequences. We have shown that the photonic and omnidirectional bandgaps in the quasi-periodic structures with semiconductor constituents are strongly depend on the temperature, thickness of the constituted semiconductor and dielectric material layers, and generations of the quasi-periodic sequences. It has been found that the number of photonic bandgaps increases with layer thickness and generation of the quasi-periodic sequences. Omnidirectional bandgaps in the structures have also been obtained. Results show that the bandwidths of photonic and omnidirectional bandgaps are tunable by changing the temperature and lattice parameters of the structures. The generation of quasi-periodic sequences can also change the properties of photonic and omnidirectional bandgaps remarkably. The frequency range of the photonic and omnidirectional bandgaps can be tuned by the change of temperature and layer thickness of the considered quasi-periodic structures. This work will be useful to design tunable terahertz PC devices. PMID:27463924
Degraeve, S.; Granger, C.; Dubus, B.; Vasseur, J. O.; Pham Thi, M.; Hladky-Hennion, A.-C.
2014-05-01
An homogeneous piezoelectric rod is shown to exhibit Bragg band gaps when an electrical boundary condition is applied periodically with the help of metallic electrodes. An analytical model is developed which formulation depends on the applied electric boundary condition and reveals that Bragg band gaps occurring in this very peculiar phononic crystal are related to the electric charge located on the electrodes. Moreover, via an accurate boundary condition (electrodes connected in short circuit, in open circuit, or through an external capacitance), full tunability of the Bragg band gaps can be achieved. Measurements of ultrasonic transmission present an overall excellent agreement with the theoretical results. This phononic crystal can be easily manufactured and presents many potential applications as frequency filters especially for radio frequency telecommunications.
II-VI wide band gap semiconductors under hydrostatic pressure
Baquero, R.; Decoss, R.; Olguin, D.
1993-08-01
We set an analytical expression for the gap as a function of hydrostatic deformation, E(sub g)(epsilon), by diagonalizing in Gamma the corresponding empirical tight-binding Hamiltonian (ETBH). In the ETBH we use the well known d(exp -2) Harrison scaling law (HSL) to adjust the TB parameter (TBP) to the changes in interatomic distances. We do not consider cation-anion charge transfer. We calculate E(sub g)(epsilon) for wide band gap II-VI semiconductors with zincblende crystal structure for deformations under pressure up to -5 percent. Results are in good agreement with experiment for the compounds of lower ionicity but deviate as the ionicity of the compound increases. This is due to the neglect of charge transfer which should be included self-consistently. Within the approximation we always find a positive second derivative of E(sub g)(epsilon) with respect to epsilon, independent of the material. Furthermore, the inclusion of deviations from HSL appear to be unimportant to this problem.
Tunable Band Gap of Boron Nitride Interfaces under Uniaxial Pressure
Moraes, Elizane; Manhabosco, Taise; de Oliveira, Alan; Batista, Ronaldo
2013-03-01
In this work we show, by means of a density functional theory formalism, that the interaction between hydrogen terminated boron nitride surfaces gives rise to a metallic interface with free carries of opposite sign at each surface. A band gap can be induced by decreasing the surface separation. The size of the band gap changes continuously from zero up to 4.4 eV with decreasing separation, which is understood in terms of the interaction between surface states.Due to the high thermal conductivity of cubic boron nitride and the coupling between band gap and applied pressure, such tunable band gap interfaces may be used in high stable electronic and electromechanical devices. In addition, the spacial separation of charge carries at the interface may lead to photovoltaic applications. The authors thank tha brazilian agencies Fapemig, CNPq and Capes
Acoustic band gaps in two-dimensional square arrays of semi-hollow circular cylinders
Institute of Scientific and Technical Information of China (English)
T.; Kim
2009-01-01
Concave surfaces focus sound while convex surfaces disperse sound. It is therefore interesting to know if it is possible to make use of these two opposite characteristics to enhance the band gap performance of periodic arrays of solid cylinders in air. In this paper, the band gap characteristics of a 2-D square array of semi-hollow circular cylinders embedded in air are investigated, both experimentally and theoretically. In comparison with the types of inclusion studied by previous researchers, a semi-hollow circular cylinder is unique in the sense that it has concave inner surfaces and convex outer surfaces. The finite difference time domain (FDTD) method is employed to study the propagation behavior of sound across the new phononic crystal of finite extent, and the influences of sample size and inclusion orientation on band gap characteristics are quantified in order to obtain the maximum band gap. For reference, the band gap behaviors of solid circular cylinder/air and hollow circular cylinder/air systems are considered and compared with those of semi-hollow circular cylinder/air systems. In addition to semi-hollow circular cylinders, other inclusion topologies such as semi-hollow triangular and square cylinders are also investigated. To validate the theoretical predictions, experimental measurements on square arrays of hollow Al cylinders in air and semi-hollow Al cylinders in air are carried out. The results demonstrate that the semi-hollow circular cylinder/air system has the best overall band gap performance.
Strongly nonparabolic variation of the band gap in In x Al1‑x N with low indium content
Zubialevich, Vitaly Z.; Dinh, Duc V.; Alam, Shahab N.; Schulz, Stefan; O’Reilly, Eoin P.; Parbrook, Peter J.
2016-02-01
80–120 nm thick In x Al1‑x N epitaxial layers with 0 growth temperature. The composition dependence of the band gap was estimated from the photoluminescence excitation absorption edge for 0 crystal-field splitting of the highest valence band states. Our results indicate also that the ordering of the valence bands is changed at much lower In contents than one would expect from linear interpolation of the valence band parameters. These findings on band gap bowing and valence band ordering are of direct relevance for the design of InAlN-containing optoelectronic devices.
Theoretical study on the two-band degenerate-gaps superconductors: Application to SrPt3P
Huang, Hai; Hou, Li-Chao; Zhao, Bin-Peng
2016-09-01
We study the magnetic properties of two-band degenerate-gaps superconductors with two-band isotropic Ginzburg-Landau theory. The exact solutions of upper critical field and London penetration depth are obtained, and the calculations reproduce the experimental data of the recently observed superconducting crystal SrPt3P in a broad temperature range. It directly underlies that SrPt3P is a multi-band superconductor with equal gaps in two Fermi surface sheets.
Limitations to band gap tuning in nitride semiconductor alloys
DEFF Research Database (Denmark)
Gorczyca, I.; Suski, T.; Christensen, Niels Egede;
2010-01-01
Relations between the band gaps of nitride alloys and their lattice parameters are presented and limits to tuning of the fundamental gap in nitride semiconductors are set by combining a large number of experimental data with ab initio theoretical calculations. Large band gap bowings obtained...... theoretically for GaxAl1-xN, InxGa1-xN, and InxAl1-xN for uniform as well as clustered arrangements of the cation atoms are considered in the theoretical analysis. It is shown that indium plays a particular role in nitride alloys being responsible for most of the observed effects....
Optimum design of band-gap beam structures
DEFF Research Database (Denmark)
Olhoff, Niels; Niu, Bin; Cheng, Gengdong
2012-01-01
of a single, linearly elastic material without damping. Numerical results are presented for different combinations of classical boundary conditions, prescribed orders of the upper and lower natural frequencies of maximized natural frequency gaps, and a given minimum constraint value for the beam cross......The design of band-gap structures receives increasing attention for many applications in mitigation of undesirable vibration and noise emission levels. A band-gap structure usually consists of a periodic distribution of elastic materials or segments, where the propagation of waves is impeded...
Band Gap Engineering of Two-Dimensional Nitrogene
Li, Jie-Sen; Wang, Wei-Liang; Yao, Dao-Xin
2016-01-01
In our previous study, we have predicted the novel two-dimensional honeycomb monolayers of pnictogen. In particular, the structure and properties of the honeycomb monolayer of nitrogen, which we call nitrogene, are very unusual. In this paper, we make an in-depth investigation of its electronic structure. We find that the band structure of nitrogene can be engineered in several ways: controlling the stacking of monolayers, application of biaxial tensile strain, and application of perpendicular electric field. The band gap of nitrogene is found to decrease with the increasing number of layers. The perpendicular electric field can also reduce the band gap when it is larger than 0.18 V/Å, and the gap closes at 0.35 V/Å. A nearly linear dependence of the gap on the electric field is found during the process. Application of biaxial strain can decrease the band gap as well, and eventually closes the gap. After the gap-closing, we find six inequivalent Dirac points in the Brillouin zone under the strain between 17% and 28%, and the nitrogene monolayer becomes a Dirac semimetal. These findings suggest that the electronic structure of nitrogene can be modified by several techniques, which makes it a promising candidate for electronic devices. PMID:27680297
Institute of Scientific and Technical Information of China (English)
ZHANG Yan; SHI Jun-Jie
2008-01-01
A two-dimensional photonic crystal model with a periodic square dielectric background is proposed.The photonic band modulation effects due to the two-dimensional periodic background are investigated jn detail.It is found that periodic modulation of the dielectric background greatly alters photonic band structures,especially for the Epolarization modes.The number,width and position of the photonic band gaps sensitively depend on the dielectric constants of the two-dimensional periodic background.Complete band gaps are found,and the dependence of the widths of these gaps on the structural and material parameters of the two alternating rods/holes is studied.
Optical band gap of Sn0.2Bi1.8Te3 thin films
Indian Academy of Sciences (India)
P H Soni; M V Hathi; C F Desai
2003-12-01
Sn0.2Bi1.8Te3 thin films were grown using the thermal evaporation technique on a (001) face of NaCl crystal as a substrate at room temperature. The optical absorption was measured in the wave number range 500–4000 cm-1. From the optical absorption data the band gap was evaluated and studied as a function of film thickness and deposition temperature. The data indicate absorption through direct interband transition with a band gap of around 0.216 eV. The detailed results are reported here.
Indirect-to-direct band gap crossover in few-layer MoTe₂.
Lezama, Ignacio Gutiérrez; Arora, Ashish; Ubaldini, Alberto; Barreteau, Céline; Giannini, Enrico; Potemski, Marek; Morpurgo, Alberto F
2015-04-01
We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications. PMID:25803208
Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2
Lezama, Ignacio Gutiérrez; Arora, Ashish; Ubaldini, Alberto; Barreteau, Céline; Giannini, Enrico; Potemski, Marek; Morpurgo, Alberto F.
2015-04-01
We study the evolution of the band-gap structure in few-layer MoTe$_2$ crystals, by means of low-temperature micro-reflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that, in complete analogy with other semiconducting transition metal dichalchogenides (TMDs), the dominant PL emission peaks originate from direct transitions associated to recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe$_2$ behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer and it starts decreasing for trilayers. A quantitative analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe$_2$ being direct band-gap semiconductors, with tetralayer MoTe$_2$ being an indirect gap semiconductor, and with trilayers having nearly identical direct and indirect gaps.This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides, for which only monolayers are found to be direct band-gap semiconductors, with thicker layers having indirect band gaps that are significantly smaller, by hundreds of meV, than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.
Zhang, Hai-Feng; Liu, Shao-Bin; Ding, Guo-Wen
2014-10-01
In this paper, the magneto-optical Voigt effects in surface plasmon modes and anisotropic photonic band gaps (PBGs) of the three-dimensional (3D) magnetized plasma photonic crystals (MPPCs) with face-centered-cubic lattices are theoretically investigated based on the modified plane wave expansion (PWE) method, which are the homogeneous Te (tellurium) spheres immersed in the homogeneous magnetized plasma background, as the mixed polarized modes are considered. The more general condition is considered, and the anisotropic PBGs are not only for the extraordinary and ordinary modes but also for mixed polarized modes. The equations for computing such anisotropic PBGs are theoretically deduced. Theoretical simulations show that the anisotropic PBGs and a flatbands region can be observed in the dispersive curve. Compared to the similar 3D MPPCs containing the isotropic dielectric or uniaxial material spheres, the larger PBGs can be obtained as the extraordinary axis of the inserted uniaxial material is along the Г-H symmetry line although the region of flatbands is also different. However, the relative bandwidths of PBGs for such two cases are almost the same. The interesting properties of surface plasmon modes can also be found, which are that the upper edge of flatbands region cannot be tuned by the filling factor but can almost linearly increase on increasing the plasma frequency and plasma cyclotron frequency (the external magnetic field), respectively. The effects of the filling factor, plasma frequency and plasma cyclotron frequency on the anisotropic PBGs are investigated in detail, respectively. Theoretical calculations also show that such PBGs can be manipulated by the parameters as mentioned above.
Acoustic band gaps of two-dimensional three-component composite
Institute of Scientific and Technical Information of China (English)
ZHANG Shu; CHENG Jianchun
2003-01-01
Numerical analyses of the band-gaps in the two-dimensional three-component periodic composite are presented in this paper. The three-component composite is composed of an array of coated cylinders embedded in a resin. The coated cylinder consists of a hard inner core and a soft coating, which has much smaller wave velocity and mass density than the matrix and the inner material. The calculated band gaps appear in a lower frequency domain in contrast to the two-component counterpart composite because of the localized resonance in the elastically soft coating ring. The influence of the properties of the different coating materials and the geometry of the crystal on the band gaps is also investigated.
Energy band gaps in graphene nanoribbons with corners
Szczȩśniak, Dominik; Durajski, Artur P.; Khater, Antoine; Ghader, Doried
2016-05-01
In the present paper, we study the relation between the band gap size and the corner-corner length in representative chevron-shaped graphene nanoribbons (CGNRs) with 120° and 150° corner edges. The direct physical insight into the electronic properties of CGNRs is provided within the tight-binding model with phenomenological edge parameters, developed against recent first-principle results. We show that the analyzed CGNRs exhibit inverse relation between their band gaps and corner-corner lengths, and that they do not present a metal-insulator transition when the chemical edge modifications are introduced. Our results also suggest that the band gap width for the CGNRs is predominantly governed by the armchair edge effects, and is tunable through edge modifications with foreign atoms dressing.
HAC: Band Gap, Photoluminescence, and Optical/Near-Infrared Absorption
Witt, Adolf N.; Ryutov, Dimitri; Furton, Douglas G.
1996-01-01
We report results of laboratory measurements which illustrate the wide range of physical properties found among hydrogenated amorphous carbon (HAC) solids. Within this range, HAC can match quantitatively the astronomical phenomena ascribed to carbonaceous coatings on interstellar grains. We find the optical band gap of HAC to be well correlated with other physical properties of HAC of astronomical interest, and conclude that interstellar HAC must be fairly hydrogen-rich with a band gap of E(sub g) is approx. greater than 2.0 eV.
Enhancing the Precision of Parameter Estimation in Band Gap
Huang, J.; Zhan, Q.; Liu, Z. K.
2016-09-01
Recently, the dynamics of quantum Fisher information(QFI) in various environment are investigated and many kinds of schemes to overcome the drawback of decoherence are designed. Here we propose the pseudomode method to enhance the phase parameter precision of optimal quantum estimation of a qubit coupled to a non-Markovian structured environment. We find that the QFI can be enhanced in the weak-coupling regime with non-perfect band gap and can be trapped permanently with a large value in the perfect band gap. The effects of qubit-pseudomode detuning and the spectrum of reservoir are discussed, a reasonable physical explanation is given, too.
Amorphous Photonic Lattices: Band Gaps, Effective Mass and Suppressed Transport
Rechtsman, Mikael; Dreisow, Felix; Heinrich, Matthias; Keil, Robert; Nolte, Stefan; Segev, Mordechai
2010-01-01
We present, theoretically and experimentally, amorphous photonic lattices exhibiting a band-gap yet completely lacking Bragg diffraction: 2D waveguides distributed randomly according to a liquid-like model responsible for the absence of Bragg peaks as opposed to ordered lattices containing disorder, which always exhibit Bragg peaks. In amorphous lattices the bands are comprised of localized states, but we find that defect states residing in the gap are more localized than the Anderson localization length. Finally, we show how the concept of effective mass carries over to amorphous lattices.
The band gap of II-Vi ternary alloys in a tight-binding description
Energy Technology Data Exchange (ETDEWEB)
Olguin, Daniel; Blanquero, Rafael [Instituto Politecnico Nacional, Mexico, D.F (Mexico); De Coss, Romeo [Instituto Politecnico Nacional, Yucatan (Mexico)
2001-02-01
We present tight-binding calculations for the band gap of II-Vi pseudobinary ternary alloys. We use an sp{sup 3} s* tight-binding Hamiltonian which include spin-orbit coupling. The band gap composition dependence is calculated using a extended version of the virtual crystal approximation, which introduce an empirical correction factor that takes into account the non-linear dependence of the band gap with the composition. The results compare quite well with the experimental data, both for the ternary alloys with wide band gap and for the narrow band gap ones. [Spanish] Presentamos el calculo de la banda de energia prohibida de aleaciones ternarias de compuestos II-VI. El calculo, que incluye interaccion espin-orbita, se hace con el metodo de enlace fuerte, utilizando una base ortogonal de cinco orbitales atomicos por atomo (sp{sup 3} s*), en conjunto con la aproximacion del cristal virtual. En la aproximacion del cristal virtual, incluimos un factor de correccion que toma en cuenta la no linealidad de la banda de energia prohibida como funcion de la concentracion. Con esta correccion nuestros resultados reproducen aceptablemente los datos experimentales hallados en la literatura.
Obtaining the band structure of a complicated photonic crystal by linear operations
Institute of Scientific and Technical Information of China (English)
吴良; 叶卓; 何赛灵
2003-01-01
Absolute band gaps can be created by lifting the degeneracy in the bands of a photonic crystal.To calculate the band structure of a complicated photonic crystal generated by e.g.symmetry breaking,general forms of all possible linear operations are presented in terms of matrices and a procedure to combine these operations is given.Other forms of linear operations(such as the addition,subtraction,and translation transforms) are also presented to obtain an explicit expression for the Fourier coefficient of the dielectric function in the plane-wave expansion method.With the present method,band structures for various complicated photonic crystals(related through these linear operations) can be obtained easily and quickly.As a numerical example,a large absolute band gap for a complicated photonic crystal structure of GaAs is found in the high region of normalized frequency.
The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique
Energy Technology Data Exchange (ETDEWEB)
Kevin Jerome Sutherland
2001-05-01
Photonic band gap (PBG) crystals are periodic dielectric structures that manipulate electromagnetic radiation in a manner similar to semiconductor devices manipulating electrons. Whereas a semiconductor material exhibits an electronic band gap in which electrons cannot exist, similarly, a photonic crystal containing a photonic band gap does not allow the propagation of specific frequencies of electromagnetic radiation. This phenomenon results from the destructive Bragg diffraction interference that a wave propagating at a specific frequency will experience because of the periodic change in dielectric permitivity. This gives rise to a variety of optical applications for improving the efficiency and effectiveness of opto-electronic devices. These applications are reviewed later. Several methods are currently used to fabricate photonic crystals, which are also discussed in detail. This research involves a layer-by-layer micro-transfer molding ({mu}TM) and stacking method to create three-dimensional FCC structures of epoxy or titania. The structures, once reduced significantly in size can be infiltrated with an organic gain media and stacked on a semiconductor to improve the efficiency of an electronically pumped light-emitting diode. Photonic band gap structures have been proven to effectively create a band gap for certain frequencies of electro-magnetic radiation in the microwave and near-infrared ranges. The objective of this research project was originally two-fold: to fabricate a three dimensional (3-D) structure of a size scaled to prohibit electromagnetic propagation within the visible wavelength range, and then to characterize that structure using laser dye emission spectra. As a master mold has not yet been developed for the micro transfer molding technique in the visible range, the research was limited to scaling down the length scale as much as possible with the current available technology and characterizing these structures with other methods.
Band gap bowing in quaternary nitride semiconducting alloys
DEFF Research Database (Denmark)
Gorczyka, Isabela; Suski, T.; Christensen, Niels Egede;
2011-01-01
Structural properties of InxGayAl1−x−yN alloys are derived from total-energy minimization within the local-density approximation (LDA). The electronic properties are studied by band structure calculations including a semiempirical correction for the “LDA gap error.” The effects of varying...
Design for maximum band-gaps in beam structures
DEFF Research Database (Denmark)
Olhoff, Niels; Niu, Bin; Cheng, Gengdong
2012-01-01
This paper aims to extend earlier optimum design results for transversely vibrating Bernoulli-Euler beams by determining new optimum band-gap beam structures for (i) different combinations of classical boundary conditions, (ii) much larger values of the orders n and n-1 of adjacent upper and lower...
Strain sensitivity of band gaps of Sn-containing semiconductors
DEFF Research Database (Denmark)
Li, Hong; Castelli, Ivano Eligio; Thygesen, Kristian Sommer;
2015-01-01
attribute the ultrahigh strain sensitivity to the pure Sn s-state character of the conduction-band edges. Other Sn-containing compounds may show both increasing and decreasing gaps under tensile strain and we show that the behavior can be understood by analyzing the role of the Sn s states in both...
Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films.
Herklotz, Andreas; Rus, Stefania Florina; Ward, Thomas Zac
2016-03-01
The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques. PMID:26836282
Robust band gap of TiS3 nanofilms.
Kang, Jun; Wang, Lin-Wang
2016-06-01
First-principles calculations have been performed on the band structure of mono- and few-layer TiS3 nanofilms. It is found that the band gap character of the TiS3 films is quite robust, almost independent of layer thickness, vertical strain and stacking order, which is in sharp contrast to most other two-dimensional materials, such as MoS2. The robustness of the band gap originates from the location of the CBM and VBM states, which are at the center atoms of TiS3, and are thus unaffected by the layer-layer coupling. Such a property of TiS3 nanofilms promises good application potential in nanoelectronics and optoelectronics, and also makes TiS3 a good platform to study the electronic properties of a material in the two-dimensional limit. PMID:27029227
Low Loss Plastic Terahertz Photonic Band-Gap Fibres
Institute of Scientific and Technical Information of China (English)
GENG You-Fu; TAN Xiao-Ling; ZHONG Kai; WANG Peng; YAO Jian-Quan
2008-01-01
We report a numerical investigation on terahertz wave propagation in plastic photonic band-gap fibres which are characterized by a 19-unit-cell air core and hexagonal air holes with rounded corners in cladding. Using the finite element method, the leakage loss and absorption loss are calculated and the transmission properties are analysed.The lowest loss of 0.268 dB/m is obtained. Numerical results show that the fibres could liberate the constraints of background materials beyond the transparency region in terahertz wave band, and efficiently minimize the effect of absorption by background materials, which present great advantage of plastic photonic band-gap fibres in long distance terahertz delivery.
Substrate-induced band gap opening in epitaxial graphene
Energy Technology Data Exchange (ETDEWEB)
Zhou, S.Y.; Gweon, G.-H.; Fedorov, A.V.; First, P.N.; de Heer,W.A.; Lee, D.-H.; Guinea, F.; Castro Neto, A.H.; Lanzara, A.
2007-09-08
Graphene has shown great application potential as the hostmaterial for next-generation electronic devices. However, despite itsintriguing properties, one of the biggest hurdles for graphene to beuseful as an electronic material is the lack of an energy gap in itselectronic spectra. This, for example, prevents the use of graphene inmaking transistors. Although several proposals have been made to open agap in graphene's electronic spectra, they all require complexengineering of the graphene layer. Here, we show that when graphene isepitaxially grown on SiC substrate, a gap of ~;0.26 eV is produced. Thisgap decreases as the sample thickness increases and eventually approacheszero when the number of layers exceeds four. We propose that the originof this gap is the breaking of sublattice symmetry owing to thegraphene-substrate interaction. We believe that our results highlight apromising direction for band gap engineering of graphene.
Acoustic band gaps in two-dimensional square arrays of semi-hollow circular cylinders
Institute of Scientific and Technical Information of China (English)
LU TianJian; GAO GuoQin; MA ShouLin; JIN Feng; T.Kim
2009-01-01
Concave surfaces focus sound while convex surfaces disperse sound. It is therefore interesting to know if it is possible to make use of these two opposite characteristics to enhance the band gap per-formance of periodic arrays of solid cylinders in air. In this paper, the band gap characteristics of a 2-D square array of semi-hollow circular cylinders embedded in air are investigated, both experimentally and theoretically. In comparison with the types of inclusion studied by previous researchers, a semi-hollow circular cylinder is unique in the sense that it has concave inner surfaces and convex outer surfaces. The finite difference time domain (FDTD) method is employed to study the propagation behavior of sound across the new phononic crystal of finite extent, and the influences of sample size and inclusion orientation on band gap characteristics are quantified in order to obtain the maximum band gap. For reference, the band gap behaviors of solid circular cylinder/air and hollow circular cyl-inder/air systems are considered and compared with those of semi-hollow circular cylinder/air systems. In addition to semi-hollow circular cylinders, other inclusion topologies such as semi-hollow triangular and square cylinders are also investigated. To validate the theoretical predictions, experimental meas-urements on square arrays of hollow AI cylinders in air and semi-hollow AI cylinders in air are carried out. The results demonstrate that the semi-hollow circular cylinder/air system has the best overall band gap performance.
Ultrafast band-gap oscillations in iron pyrite
Energy Technology Data Exchange (ETDEWEB)
Kolb, B; Kolpak, AM
2013-12-20
With its combination of favorable band gap, high absorption coefficient, material abundance, and low cost, iron pyrite, FeS2, has received a great deal of attention over the past decades as a promising material for photovoltaic applications such as solar cells and photoelectrochemical cells. Devices made from pyrite, however, exhibit open circuit voltages significantly lower than predicted, and despite a recent resurgence of interest in the material, there currently exists no widely accepted explanation for this disappointing behavior. In this paper, we show that phonons, which have been largely overlooked in previous efforts, may play a significant role. Using fully self-consistent GW calculations, we demonstrate that a phonon mode related to the oscillation of the sulfur-sulfur bond distance in the pyrite structure is strongly coupled to the energy of the conduction-band minimum, leading to an ultrafast (approximate to 100 fs) oscillation in the band gap. Depending on the coherency of the phonons, we predict that this effect can cause changes of up to +/- 0.3 eV relative to the accepted FeS2 band gap at room temperature. Harnessing this effect via temperature or irradiation with infrared light could open up numerous possibilities for novel devices such as ultrafast switches and adaptive solar absorbers.
Dual-band electromagnetic band gap structure for noise isolation in mixed signal SiP
Rotaru, M. D.; Sykulski, J. K.
2010-01-01
A compact dual-band electromagnetic band-gap (EBG) structure is proposed. It is shown through numerical simulation using 3D electromagnetic finite element modelling that by adding a slit to the classical mushroom shape an extra resonance is introduced and thus dual-band EBG structures can be built by cascading these new elements. It is also demonstrated that this novel approach can be used to isolate noise in a system such as a dual band transceiver integrated into a mixed signal system in a ...
Band gap determination of Ni–Zn ferrites
Indian Academy of Sciences (India)
G P Joshi; N S Saxena; R Mangal; A Mishra; T P Sharma
2003-06-01
Nanocomposites of Ni–Zn with copolymer matrix of aniline and formaldehyde in presence of varying concentrations of zinc ions have been studied at room temperature and normal pressure. The energy band gap of these materials are determined by reflection spectra in the wavelength range 400–850 nm by spectrophotometer at room temperature. From the analysis of reflection spectra, nanocomposites of copolymer of aniline and formaldehyde with Ni$_{1–x}$Zn$_x$Fe2O4 ( = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) have been found to have direct band gaps ranging from 1.50–1.66 eV.
Band gap engineering of MoS2 upon compression
López-Suárez, Miquel; Neri, Igor; Rurali, Riccardo
2016-04-01
Molybdenum disulfide (MoS2) is a promising candidate for 2D nanoelectronic devices, which shows a direct band-gap for monolayer structure. In this work we study the electronic structure of MoS2 upon both compressive and tensile strains with first-principles density-functional calculations for different number of layers. The results show that the band-gap can be engineered for experimentally attainable strains (i.e., ±0.15). However, compressive strain can result in bucking that can prevent the use of large compressive strain. We then studied the stability of the compression, calculating the critical strain that results in the on-set of buckling for free-standing nanoribbons of different lengths. The results demonstrate that short structures, or few-layer MoS2, show semi-conductor to metal transition upon compressive strain without bucking.
On acoustic band gaps in homogenized piezoelectric phononic materials
Directory of Open Access Journals (Sweden)
Rohan E.
2010-07-01
Full Text Available We consider a composite medium made of weakly piezoelectric inclusions periodically distributed in the matrix which ismade of a different piezoelectricmaterial. Themediumis subject to a periodic excitation with an incidence wave frequency independent of scale ε of the microscopic heterogeneities. Two-scale method of homogenization is applied to obtain the limit homogenized model which describes acoustic wave propagation in the piezoelectric medium when ε → 0. In analogy with the purely elastic composite, the resulting model allows existence of the acoustic band gaps. These are identified for certain frequency ranges whenever the so-called homogenized mass becomes negative. The homogenized model can be used for band gap prediction and for dispersion analysis for low wave numbers. Modeling such composite materials seems to be perspective in the context of Smart Materials design.
Twisted bilayer blue phosphorene: A direct band gap semiconductor
Ospina, D. A.; Duque, C. A.; Correa, J. D.; Suárez Morell, Eric
2016-09-01
We report that two rotated layers of blue phosphorene behave as a direct band gap semiconductor. The optical spectrum shows absorption peaks in the visible region of the spectrum and in addition the energy of these peaks can be tuned with the rotational angle. These findings makes twisted bilayer blue phosphorene a strong candidate as a solar cell or photodetection device. Our results are based on ab initio calculations of several rotated blue phosphorene layers.
Microscopic theory of photonic band gaps in optical lattices
Samoylova, M; Bachelard, R; Courteille, Ph W
2013-01-01
We propose a microscopic model to describe the scattering of light by atoms in optical lattices. The model is shown to efficiently capture Bragg scattering, spontaneous emission and photonic band gaps. A connection to the transfer matrix formalism is established in the limit of a one-dimensional optical lattice, and we find the two theories to yield results in good agreement. The advantage of the microscopic model is, however, that it suits better for studies of finite-size and disorder effects.
One-dimensional photonic band gaps in optical lattices
Samoylova, Marina; Holynski, Michael; Courteille, Philippe Wilhelm; Bachelard, Romain
2013-01-01
The phenomenon of photonic band gaps in one-dimensional optical lattices is reviewed using a microscopic approach. Formally equivalent to the transfer matrix approach in the thermodynamic limit, a microscopic model is required to study finite-size effects, such as deviations from the Bragg condition. Microscopic models describing both scalar and vectorial light are proposed, as well as for two- and three-level atoms. Several analytical results are compared to experimental data, showing a good agreement.
Zhang, Shengli; Yan, Zhong; Li, Yafei; Chen, Zhongfang; Zeng, Haibo
2015-03-01
The typical two-dimensional (2D) semiconductors MoS2, MoSe2, WS2, WSe2 and black phosphorus have garnered tremendous interest for their unique electronic, optical, and chemical properties. However, all 2D semiconductors reported thus far feature band gaps that are smaller than 2.0 eV, which has greatly restricted their applications, especially in optoelectronic devices with photoresponse in the blue and UV range. Novel 2D mono-elemental semiconductors, namely monolayered arsenene and antimonene, with wide band gaps and high stability were now developed based on first-principles calculations. Interestingly, although As and Sb are typically semimetals in the bulk, they are transformed into indirect semiconductors with band gaps of 2.49 and 2.28 eV when thinned to one atomic layer. Significantly, under small biaxial strain, these materials were transformed from indirect into direct band-gap semiconductors. Such dramatic changes in the electronic structure could pave the way for transistors with high on/off ratios, optoelectronic devices working under blue or UV light, and mechanical sensors based on new 2D crystals.
Two novel silicon phases with direct band gaps.
Fan, Qingyang; Chai, Changchun; Wei, Qun; Yang, Yintang
2016-05-14
Due to its abundance, silicon is the preferred solar-cell material despite the fact that many silicon allotropes have indirect band gaps. Elemental silicon has a large impact on the economy of the modern world and is of fundamental importance in the technological field, particularly in the solar cell industry. Looking for direct band gap silicon is still an important field in material science. Based on density function theory with the ultrasoft pseudopotential scheme in the frame of the local density approximation and the generalized gradient approximation, we have systematically studied the structural stability, absorption spectra, electronic, optical and mechanical properties and minimum thermal conductivity of two novel silicon phases, Cm-32 silicon and P21/m silicon. These are both thermally, dynamically and mechanically stable. The absorption spectra of Cm-32 silicon and P21/m silicon exhibit significant overlap with the solar spectrum and thus, excellent photovoltaic efficiency with great improvements over Fd3[combining macron]m Si. These two novel Si structures with direct band gaps could be applied in single p-n junction thin-film solar cells or tandem photovoltaic devices. PMID:27104737
Band gap engineering of indium zinc oxide by nitrogen incorporation
Energy Technology Data Exchange (ETDEWEB)
Ortega, J.J., E-mail: jjosila@hotmail.com [Unidad Académica de Física, Universidad Autónoma de Zacatecas, Calzada Solidaridad esq. Paseo la Bufa, Fracc. Progreso, C.P. 98060 Zacatecas (Mexico); Doctorado Institucional de Ingeniería y Ciencia de Materiales, Universidad Autónoma de San Luis Potosí, Av. Salvador Nava, Zona Universitaria, C.P. 78270 San Luis Potosí (Mexico); Aguilar-Frutis, M.A.; Alarcón, G. [Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada del Instituto Politécnico Nacional, Unidad Legaría, Calz. Legaría No. 694, Col. Irrigación, C.P. 11500 México D.F. (Mexico); Falcony, C. [Departamento de Física, Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional campus Zacatenco, Av. Instituto Politécnico Nacional 2508, Col. San Pedro Zacatenco, C.P. 07360 México D.F. (Mexico); and others
2014-09-15
Highlights: • IZON thin films were deposited by RF reactive sputtering at room temperature. • The effects of nitrogen on physical properties of IZO were analyzed. • Optical properties of IZON were studied by SE and UV–vis spectroscopy. • Adachi and classical parameters were quantitative and qualitatively congruent. • Nitrogen induces a gradual narrowing band gap from 3.5 to 2.5 eV on IZON films. - Abstract: The effects of nitrogen incorporation in indium zinc oxide films, as grown by RF reactive magnetron sputtering, on the structural, electrical and optical properties were studied. It was determined that the variation of the N{sub 2}/Ar ratio, in the reactive gas flux, was directly proportional to the nitrogen percentage measured in the sample, and the incorporated nitrogen, which substituted oxygen in the films induces changes in the band gap of the films. This phenomenon was observed by measurement of absorption and transmission spectroscopy in conjunction with spectral ellipsometry. To fit the ellipsometry spectra, the classical and Adachi dispersion models were used. The obtained optical parameters presented notable changes related to the increment of the nitrogen in the film. The band gap narrowed from 3.5 to 2.5 eV as the N{sub 2}/Ar ratio was increased. The lowest resistivity obtained for these films was 3.8 × 10{sup −4} Ω cm with a carrier concentration of 5.1 × 10{sup 20} cm{sup −3}.
Photonic band structure of two-dimensional metal/dielectric photonic crystals
International Nuclear Information System (INIS)
An improved plane wave expansion method for the numerical calculation of photonic bands of metal/dielectric photonic crystal (PC) are presented. This method is applied to two-dimensional PCs with frequency-dependent dielectric constants. We obtained the photonic band structure of three kinds of structures: sawtooth, cylinder and hole PCs. The results show that the lowest band-1 is relatively flat, and does not approach zero. Also, there is no complete band-gap that extends throughout the first Brillouin zone for these three structures. However, there are partial band-gaps in different directions in the first Brillouin zone. For the complementary cylinder and hole PCs, their photonic bands are similar except for the lowest three bands; the hole PC’s lowest frequency of band-1 is larger than that of cylinder PC for the configuration R/d = 0.2. (paper)
Electron Elevator: Excitations across the Band Gap via a Dynamical Gap State.
Lim, A; Foulkes, W M C; Horsfield, A P; Mason, D R; Schleife, A; Draeger, E W; Correa, A A
2016-01-29
We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap. PMID:26871327
Group IV direct band gap photonics: Methods, Challenges and Opportunities
Directory of Open Access Journals (Sweden)
Richard eGeiger
2015-07-01
Full Text Available The concept of direct band gap group IV materials offers a paradigm change for Si-photonics concerning the monolithic implementation of light emitters: The idea is to integrate fully compatible group IV materials with equally favorable optical properties as the chemically incompatible group III-V-based systems. The concept involves either mechanically applied strain on Ge or alloying of Ge with Sn and permits to drastically improve the insufficient radiative efficiency of Ge. The favorable optical properties result from a modified band structure transformed from an indirect to a direct one. The first demonstration of such a direct band gap laser, accomplished in GeSn, exemplifies the capability of this new concept. These systems may permit a qualitative as well as a quantitative expansion of Si-photonics into traditional but also new areas of applications, provided they can be operated energy efficiently, under ambient conditions and integrated with current Si technologies. This review aims to discuss the challenges along this path in terms of fabrication, characterization and fundamental understanding, and will elaborate on evoking opportunities of this new class of group IV-based laser materials.
Priya Rose, T.; Di Gennaro, E.; Andreone, A.; Abbate, G.
2010-05-01
Photonic quasicrystals (PQCs) have neither true periodicity nor translational symmetry, however they can exhibit symmetries that are not achievable by conventional periodic structures. The arbitrarily high rotational symmetry of these materials can be practically exploited to manufacture isotropic band gap materials, which are perfectly suitable for hosting waveguides or cavities. In this work, formation and development of the photonic bandgap (PBG) in twodimensional 8-, 10- and 12-fold symmetry quasicrystalline lattices of low dielectric contrast (0.4-0.6) were measured in the microwave region and compared with the PBG properties of a conventional hexagonal crystal. Band-gap properties were also investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence from 0° to 30° were used in order to investigate the isotropic nature of the band-gap.
Band gap and partial density of states for ZnO: Under high pressure
Energy Technology Data Exchange (ETDEWEB)
Saoud, Fatma Saad, E-mail: f_saoud@yahoo.fr [Department of Science and Technique, Faculty of Sciences and Technology, University of Bordj Bou-Arreridj, LMRN (Algeria); Plenet, Jean Claude [University of Lyon, University of Lyon I, LPMCN, CNRS UMR 5586, 69622 Villeurbanne (France); Henini, Mohamed [School of Physics and Astronomy, and Nottingham Nanotechnology and Nanoscience Centre (NNNC), University of Nottingham, Nottingham NG7 2RD (United Kingdom)
2015-01-15
Highlights: • B4 and B3 phases having a direct band gap but B1 and B2 phases having a indirect band gap under pressure. • Strong contribution of d orbitals of Zn atoms in the structure of the band. • Study of the mechanism of change of gap from direct to indirect in the system. • Effects of pressure and SO coupling on the bands structure of ZnO in the four phases. - Abstract: In this work, the effect of pressure on the electronic band structure, partial density of states (PDOS), and the band gap of the four phases of ZnO, namely B4 (wurtzite), B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type), has been investigated using the plane-wave pseudo-potential code CASTEP with three different schemes: the generalized gradient approximation (GGA) in the latter approach Perdew–Burk–Ernzerhof (PBE) with and without spin–orbit (SO) coupling, the new hybrid exchange–correlation functional named B3LYP functional and Hartree–Fock + Local Density Approximation (HF + LDA). These schemes are employed in order to treat the exchange–correlation effects. In addition, we will illustrate how the orbital motion of crystal electrons is affected by spin–orbit (SO) coupling. To our knowledge, this is the first theoretical study reported on ZnO using the B3LYP method. Our investigation shows that the increase of the pressure causes the nature of the band gap to change from direct to indirect. The mechanism responsible for this change of band structure is analyzed. The wide band gap of the B4 (wurtzite) phase at p = 0 as determined by the precedent methods is ∼3.221 and 3.222 eV (PBE) with and without spin–orbit (SO) coupling, respectively, 9.186 eV (HF + LDA) and 2.451 eV (B3LYP). The first two approaches provide the best agreement with the experiments. The band gap of B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type) and the strong contribution of d orbitals of Zn atoms on the structure of the bands will be discussed. The SO coupling effect on the band structure
Dolgonos, Alex; Mason, Thomas O.; Poeppelmeier, Kenneth R.
2016-08-01
The direct optical band gap of semiconductors is traditionally measured by extrapolating the linear region of the square of the absorption curve to the x-axis, and a variation of this method, developed by Tauc, has also been widely used. The application of the Tauc method to crystalline materials is rooted in misconception-and traditional linear extrapolation methods are inappropriate for use on degenerate semiconductors, where the occupation of conduction band energy states cannot be ignored. A new method is proposed for extracting a direct optical band gap from absorption spectra of degenerately-doped bulk semiconductors. This method was applied to pseudo-absorption spectra of Sn-doped In2O3 (ITO)-converted from diffuse-reflectance measurements on bulk specimens. The results of this analysis were corroborated by room-temperature photoluminescence excitation measurements, which yielded values of optical band gap and Burstein-Moss shift that are consistent with previous studies on In2O3 single crystals and thin films.
Institute of Scientific and Technical Information of China (English)
郝东山
2013-01-01
为了研究Compton散射对1维3元未磁化等离子体光子晶体中TE波禁带影响，采用Compton散射模型和传输矩阵法，进行了理论分析和实验验证，取得了一些重要数据。结果表明，随着等离子体频率增大，左旋和右旋极化波禁带展宽比散射前减小0.09GHz，禁带主频率向高频区域移动增大0.48GHz。随着等离子体碰撞频率增大，两种极化波禁带宽度发生一定变化。随着等离子体回旋频率、填充率、光入射角和介质相对介电常数增大，左旋和右旋极化波禁带明显调谐效应。这一结果对等离子体光子晶体应用是有帮助的。%In order to study the effect of Compton scattering on TE wave prohibited band gaps of 1-D ternary un-magnetized plasma photonic crystals ,based on the model of Compton scattering and transfer matrix method ,some important data was obtained after the theoretical analysis and experimental verification .The broadening width of prohibited band gap of the left circle polarization wave and the right circle polarization wave were decreased 0.09GHz along with the increasing of plasma frequency after Compton scattering .The movement from the central frequency area of prohibited band gap to the high frequency area was increased 0.48GHz.The change of prohibited band gaps widths of the left circle polarization wave and the right circle polarization wave happened along with the increasing of plasma collision frequency .The significant tuning effect of prohibited band gaps of the left circle polarization wave and the right circle polarization wave was induced by Compton scattering along with the increasing of plasma circle frequency ,filling index,light incident angle and relative dielectric constant.The result is helpful for the application of the plasma photonic crystals .
CZTS stoichiometry effects on the band gap energy
Energy Technology Data Exchange (ETDEWEB)
Malerba, Claudia, E-mail: claudia.malerba-1@ing.unitn.it [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Biccari, Francesco [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Azanza Ricardo, Cristy Leonor [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); Valentini, Matteo [Sapienza – University of Rome, Department of Physics, p.le A. Moro 5, 00185 Roma (Italy); ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Chierchia, Rosa [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Müller, Melanie [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); Max Planck Institute for Solid State Research, Heisenberg str. 1, 70569 Stuttgart (Germany); Santoni, Antonino [ENEA, Frascati Research Center, via E. Fermi 45, 00044 Frascati (Italy); Esposito, Emilia [ENEA, Portici Research Center, Piazzale E. Fermi, 80055 Portici (Napoli) (Italy); Mangiapane, Pietro [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Scardi, Paolo [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); Mittiga, Alberto [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy)
2014-01-05
Highlights: • CZTS films with different compositions were grown from stacked-layer precursors. • The band-gap energy varies from 1.48 to 1.63 eV as the [Sn]/[Cu] ratio increases. • The Zn content seems not to be a critical parameter for the optical properties. • PDS data show an increase of the sub-gap absorption as the Sn content is reduced. • Formation of defects at low Sn content was proposed to explain the Eg variation. -- Abstract: The considerable spread of Cu{sub 2}ZnSnS{sub 4} (CZTS) optical properties reported in the literature is discussed in terms of material stoichiometry. To this purpose, kesterite thin films were prepared by sulfurization of multilayered precursors of ZnS, Cu and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray (EDX) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy were used for structural and compositional analysis. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric and Photothermal Deflection Spectroscopy (PDS) measurements to assess the absorption coefficient of samples with different compositions. The PDS data show an increase of the sub-band absorption as the Sn content decreases. The results are interpreted assuming the formation of additional defects as the tin content is reduced. Those defects can also be responsible for the decrease of the band gap energy value as the Sn/Cu ratio is decreased.
Experimental studies of surface plasmon polariton band gap effect
DEFF Research Database (Denmark)
Volkov, V. S.; Bozhevolnyi, S. I.; Leosson, K.;
2003-01-01
Surface plasmon polaritons (SPPs) propagation at a gold film surface covered by periodic arrays of ~40-nm-high scatterers arranged in a triangular lattice of different periods containing straight line defects is studied using collection scanning near-field optical microscopy. The results reveal...... the dependence of the SPP band gap (SPPBG) effect manifested via the SPP reflection and guiding (along line defects) on the parameters of the surface structures (period, filling factor and lattice orientation). We find that the SPPBG effect is stronger along &ggr;K direction for all investigated periodic...
Graded band gap GaInNAs solar cells
Energy Technology Data Exchange (ETDEWEB)
Langer, F.; Perl, S.; Kamp, M. [Technische Physik, Physikalisches Institut and Wilhelm Conrad Röntgen Research Center for Complex, Material Systems, University of Würzburg, Am Hubland, Würzburg D97074 (Germany); Höfling, S. [Technische Physik, Physikalisches Institut and Wilhelm Conrad Röntgen Research Center for Complex, Material Systems, University of Würzburg, Am Hubland, Würzburg D97074 (Germany); SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS (United Kingdom)
2015-06-08
Dilute nitride GaInN(Sb)As with a band gap (E{sub g}) of 1.0 eV is a promising material for the integration in next generation multijunction solar cells. We have investigated the effect of a compositionally graded GaInNAs absorber layer on the spectral response of a GaInNAs sub cell. We produced band gap gradings (ΔE{sub g}) of up to 39 meV across a 1 μm thick GaInNAs layer. Thereby, the external quantum efficiency—compared to reference cells—was increased due to the improved extraction of photo-generated carriers from 34.0% to 36.7% for the wavelength range from 900 nm to 1150 nm. However, this device figure improvement is accompanied by a small decrease in the open circuit voltage of about 20 mV and the shift of the absorption edge to shorter wavelengths.
Transmission and photonic band gaps in Fibonacci superlattices
Directory of Open Access Journals (Sweden)
J. Garus
2013-12-01
Full Text Available Purpose: The purpose of the article was to broaden the knowledge about the behavior of Fibonacci superlattices as filters electromagnetic waves. Design/methodology/approach: Simulations of multi-layer systems is usually carried out by using two complementary methods. The first, matrix method which allows the study of the properties of structures using transmission maps and the second method used is the Finite-Difference Time Domain (FDTD algorithm allows on the study of electromagnetic wave propagation in the structure. Findings: It can be seen that the lighting of the filter with monochromatic light in the wavelength range of the band gap filter at the output causes propagation of low intensity in the range other than the wavelength of the incident beam. Research limitations/implications: The simulation was not considered the impact of losses in the material. Practical implications: Present clear differences depending on the polarization allow the use of superlattices as polarizers for specific ranges of wavelengths and angles of incidence. Originality/value: Fibonacci superlattices have been pre-tested in. The purpose of the article was to broaden the knowledge about the behavior of these structures as filters electromagnetic waves with a wavelength range from the near infrared, the effect of the material surrounding the transmission and increasing knowledge of the formation of band gaps.
Band structure characteristics of T-square fractal phononic crystals
Institute of Scientific and Technical Information of China (English)
Liu Xiao-Jian; Fan You-Hua
2013-01-01
The T-square fractal two-dimensional phononic crystal model is presented in this article.A comprehensive study is performed for the Bragg scattering and locally resonant fractal phononic crystal.We find that the band structures of the fractal and non-fractal phononic crystals at the same filling ratio are quite different through using the finite element method.The fractal design has an important impact on the band structures of the two-dimensional phononic crystals.
Photonic gap vanishing in one-dimensional photonic crystals with single-negative metamaterials
Energy Technology Data Exchange (ETDEWEB)
Chen, Yihang, E-mail: kallenmail@sina.com [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China); Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou (China); Wang, Yu; Leung, C.W.; Hu, Mingzhe; Chan, H.L.W. [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China)
2011-06-13
The properties of photonic band gap in one-dimensional photonic crystals composed of single-negative metamaterials are studied theoretically. Our study shows that the photonic gap will vanish at a certain incident angle when both the phase-match and impedance-match conditions are satisfied simultaneously, suggesting that the bandwidth and location of the photonic gap are strongly dependent on the incident angle and polarization. However, the photonic gap will not vanish and may become insensitive to the incident angle when the two match conditions cannot be met. Our study also shows that losses in metamaterials have little effect on the properties of the photonic gap. -- Highlights: → Photonic gap of 1D photonic crystal containing metamaterials was investigated. → The gap can be designed to be sensitive or insensitive to the incident angle. → The gap can be designed to be close at a specific incident angle. → Conditions for photonic gap vanishing were proposed. → Losses of metamaterials have little effect on the properties of the photonic gap.
Nonlocal gap soliton in liquid infiltrated photonic crystal fibres
DEFF Research Database (Denmark)
Bennet, F.H.; Rosberg, C.R.; Rasmussen, Per Dalgaard;
We report on the observation of nonlocal gap solitons in infiltrated photonic crystal fibres. We employ the thermal defocusing nonlinearity of the liquid to study soliton existence and effect of boundaries of the periodic structure.......We report on the observation of nonlocal gap solitons in infiltrated photonic crystal fibres. We employ the thermal defocusing nonlinearity of the liquid to study soliton existence and effect of boundaries of the periodic structure....
Band Gap Engineering and Layer-by-Layer Band Gap Mapping of Selenium-doped Molybdenum Disulfide
Energy Technology Data Exchange (ETDEWEB)
Gong, Yongji [Rice University; Liu, Zheng [Rice University; Lupini, Andrew R [ORNL; Lin, Junhao [ORNL; Pantelides, Sokrates T [ORNL; Pennycook, Stephen J [ORNL; Zhou, Wu [ORNL; Ajayan, Pullikel M [Rice University
2014-01-01
Ternary two-dimensional dichalcogenide alloys exhibit compositionally modulated electronic structure and hence, control of dopant concentration within each layer of these layered compounds provides a powerful way to modify their properties. The challenge then becomes quantifying and locating the dopant atoms within each layer in order to better understand and fine-tune the desired properties. Here we report the synthesis of selenium substitutionally doped molybdenum disulfide atomic layers, with a broad range of selenium concentrations, resulting in band gap modulations of over 0.2 eV. Atomic scale chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local band gaps. Furthermore, in a bilayer structure, the dopant distribution of each layer is imaged independently. We demonstrate that each layer in the bilayer contains similar doping levels, randomly distributed, providing new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers. The results show that growth of uniform, ternary, two-dimensional dichalcogenide alloy films with tunable electronic properties is feasible.
Hydrogen production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2
Waterhouse, G. I. N.
2013-10-10
Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO
Band gap tuning of epitaxial SrTiO{sub 3-δ}/Si(001) thin films through strain engineering
Energy Technology Data Exchange (ETDEWEB)
Cottier, Ryan J.; Steinle, Nathan A.; Currie, Daniel A.; Theodoropoulou, Nikoleta, E-mail: ntheo@txstate.edu [Physics Department, Texas State University, San Marcos, Texas 78666 (United States)
2015-11-30
We investigate the effect of strain and oxygen vacancies (V{sub O}) on the crystal and optical properties of oxygen deficient, ultra-thin (4–30 nm) films of SrTiO{sub 3-δ} (STO) grown heteroepitaxially on p-Si(001) substrates by molecular beam epitaxy. We demonstrate that STO band gap tuning can be achieved through strain engineering and show that the energy shift of the direct energy gap transition of SrTiO{sub 3-δ}/Si films has a quantifiable dimensional and doping dependence that correlates well with the changes in crystal structure.
Method for implantation of high dopant concentrations in wide band gap materials
Energy Technology Data Exchange (ETDEWEB)
Usov, Igor (Los Alamos, NM); Arendt, Paul N. (Los Alamos, NM)
2009-09-15
A method that combines alternate low/medium ion dose implantation with rapid thermal annealing at relatively low temperatures. At least one dopant is implanted in one of a single crystal and an epitaxial film of the wide band gap compound by a plurality of implantation cycles. The number of implantation cycles is sufficient to implant a predetermined concentration of the dopant in one of the single crystal and the epitaxial film. Each of the implantation cycles includes the steps of: implanting a portion of the predetermined concentration of the one dopant in one of the single crystal and the epitaxial film; annealing one of the single crystal and the epitaxial film and implanted portion at a predetermined temperature for a predetermined time to repair damage to one of the single crystal and the epitaxial film caused by implantation and activates the implanted dopant; and cooling the annealed single crystal and implanted portion to a temperature of less than about 100.degree. C. This combination produces high concentrations of dopants, while minimizing the defect concentration.
Half-oxidized phosphorene: band gap and elastic properties modulation
International Nuclear Information System (INIS)
Based on a first principles approach, we study structural, electronic and elastic properties, as well as stabilities of all possible half-oxidized phosphorene conformers. Stability analysis reveals that oxygen chemisorption is an exothermic process in the six configurations despite the formation of interstitial oxygen bridges in three of them. Electronic structure calculations show that oxidation induces a band gap modulation ranging between 0.54 and 1.57 eV in the generalized gradient approximation corrected to 1.19 and 2.88 eV using GW. The mechanical response of the conformers is sensitively dependent on direction and indicates that the new derivatives are incompressible materials and one configuration has an auxetic behavior. The present results provide a basis for tailoring the electronic and elastic properties of phosphorene via half oxidation. (paper)
Study of optical band gap of zinc-borate glasses
International Nuclear Information System (INIS)
The present study deals with binary xZnO-(100-x)B/sub 2/O/sub 3) (x=50-60%) and ternary yV/sub 2/O/sub 5/-50ZnO-(50-y)B/sub 2/O/sub 3/ (y=5-10%) glass systems. The mass density, oxygen packing density, molar volume and optical energy gap of these glasses were measured at room temperature. The density varies in the range of 2.92 - 3.57 g cm/sup 03/. The absorption spectra of these glasses were recorded in the UV-visible range. The optical band gap energies for these glasses were estimated from absorption data using the Mott and Davis relation and found to be in the range of 1.14 approx. equal to 2.92 eV. These results show that E/sub opt/ decreases with decreasing concentration of ZnO in binary glasses. In ternary glasses E/sub opt/ decreases with increasing concentration of V/sub 2/O/sub 5/ for a fixed amount of ZnO.(author)
Two-dimensional silica: Structural, mechanical properties, and strain-induced band gap tuning
International Nuclear Information System (INIS)
Two-dimensional silica is of rising interests not only for its practical applications as insulating layers in nanoelectronics, but also as a model material to understand crystals and glasses. In this study, we examine structural and electronic properties of hexagonal and haeckelite phases of silica bilayers by performing first-principles calculations. We find that the corner-sharing SiO4 tetrahedrons in these two phases are locally similar. The robustness and resilience of these tetrahedrons under mechanical perturbation allow effective strain engineering of the electronic structures with band gaps covering a very wide range, from of that for insulators, to wide-, and even narrow-gap semiconductors. These findings suggest that the flexible 2D silica holds great promises in developing nanoelectronic devices with strain-tunable performance, and lay the ground for the understanding of crystalline and vitreous phases in 2D, where bilayer silica provides an ideal test-bed
Two-dimensional silica: Structural, mechanical properties, and strain-induced band gap tuning
Gao, Enlai; Xie, Bo; Xu, Zhiping
2016-01-01
Two-dimensional silica is of rising interests not only for its practical applications as insulating layers in nanoelectronics, but also as a model material to understand crystals and glasses. In this study, we examine structural and electronic properties of hexagonal and haeckelite phases of silica bilayers by performing first-principles calculations. We find that the corner-sharing SiO4 tetrahedrons in these two phases are locally similar. The robustness and resilience of these tetrahedrons under mechanical perturbation allow effective strain engineering of the electronic structures with band gaps covering a very wide range, from of that for insulators, to wide-, and even narrow-gap semiconductors. These findings suggest that the flexible 2D silica holds great promises in developing nanoelectronic devices with strain-tunable performance, and lay the ground for the understanding of crystalline and vitreous phases in 2D, where bilayer silica provides an ideal test-bed.
Two-dimensional silica: Structural, mechanical properties, and strain-induced band gap tuning
Energy Technology Data Exchange (ETDEWEB)
Gao, Enlai; Xie, Bo [Applied Mechanics Laboratory, Department of Engineering Mechanics, and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084 (China); Xu, Zhiping, E-mail: xuzp@tsinghua.edu.cn [Applied Mechanics Laboratory, Department of Engineering Mechanics, and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084 (China); State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016 (China)
2016-01-07
Two-dimensional silica is of rising interests not only for its practical applications as insulating layers in nanoelectronics, but also as a model material to understand crystals and glasses. In this study, we examine structural and electronic properties of hexagonal and haeckelite phases of silica bilayers by performing first-principles calculations. We find that the corner-sharing SiO{sub 4} tetrahedrons in these two phases are locally similar. The robustness and resilience of these tetrahedrons under mechanical perturbation allow effective strain engineering of the electronic structures with band gaps covering a very wide range, from of that for insulators, to wide-, and even narrow-gap semiconductors. These findings suggest that the flexible 2D silica holds great promises in developing nanoelectronic devices with strain-tunable performance, and lay the ground for the understanding of crystalline and vitreous phases in 2D, where bilayer silica provides an ideal test-bed.
Band gap energy and bowing parameter of In-rich InAlN films grown by magnetron sputtering
International Nuclear Information System (INIS)
The crystal structure, band gap energy and bowing parameter of In-rich InxAl1-xN (0.7 xAl1-xN films were obtained from absorption spectra. Band gap tailing due to compositional fluctuation in the films was observed. The band gap of the as-grown InN measured by optical absorption method is 1.34 eV, which is larger than the reported 0.7 eV for pure InN prepared by molecular beam epitaxy (MBE) method. This could be explained by the Burstein-Moss effect under carrier concentration of 1020 cm-3 of our sputtered films. The bowing parameter of 3.68 eV is obtained for our InxAl1-xN film which is consistent with the previous experimental reports and theoretical calculations.
Parhoodeh, Saeed; Kowsari, Mohammad
2016-10-01
In this study, effects of vanadium doping in crystal lattice structure of indium oxide (In2O3) were investigated. Indium oxide nanoparticles with different amounts of dopant concentrations were fabricated by a facile and cost effective method. X-ray diffraction (XRD) analysis revealed the formation of cubic phase for doped and undoped samples. It was observed that the lattice parameters of doped samples were decreased respect to the pure indium oxide, but the crystallite sizes and the particles' sizes of doped samples were increased in result of substitution of vanadium in crystal lattice of In2O3. The scanning electron microscope (SEM) images of samples showed that all samples have spherical shapes, and their distribution sizes are between 10 and 70 nm. It was found that the average sizes of nanoparticles were increased linearly with the amounts of dopant concentration. A red shift was founded in the band gap of vanadium doped samples respect to pure In2O3. The maximum of the band gap shift was observed for samples with 0.025 M concentration of dopant. Based on impedance spectroscopy data, it was found that impedances of samples are increased by increasing of dopant concentration for all frequencies which were tested in this study.
Vidal Faez, Angelo
2012-07-01
Researchers have argued for many years that one of the conditions for omnidirectional reflection in a one-dimensional photonic crystal is a strong refractive index contrast between the two constituent dielectric materials. Using numerical simulations and the theory of Anderson localization of light, in this work we demonstrate that an omnidirectional band gap can indeed be created utilizing low refractive index contrast materials when they are arranged in a disordered manner. Moreover, the size of the omnidirectional band gap becomes a controllable parameter, which now depends on the number of layers and not only on the refractive index contrast of the system, as it is widely accepted. This achievement constitutes a major breakthrough in the field since it allows for the development of cheaper and more efficient technologies. Of particular interest is the case of high index contrast one-dimensional photonic crystal fibers, where the propagation losses are mainly due to increased optical scattering from sidewall roughness at the interfaces of high index contrast materials. By using low index contrast materials these losses can be reduced dramatically, while maintaining the confinement capability of the waveguide. This is just one of many applications that could be proven useful for this discovery.
Band Gap Engineering of PbI2 by Incommensurate Van der Waals Epitaxy
Wang, Yiping; Shi, Jian
Van der Waals epitaxial growth had been thought to have trivial contribution on inducing substantial epitaxial strain in thin films due to its weak nature of Van der Waals interfacial energy. Due to this, electrical and optical structure engineering via Van der Waals epitaxial strain has been rarely studied. However, by appropriate film-substrate selection, we show that significant band structure engineering could be achieved in a soft thin film material PbI2 via Van der Waals epitaxy. The thickness dependent photoluminescence of single crystal PbI2 flakes was studied and attributed to the substrate-film coupling effect via incommensurate Van der Waals epitaxy. It is proposed that the Van der Waals strain is resulted from the soft nature of PbI2 and large Van der Waals interaction due to the involvement of heavy elements. Such strain plays vital roles in modifying the band gap of PbI2. The deformation potential theory is used to quantitatively unveil the correlation between thickness, strain and band gap change. Our hypothesis is confirmed by the subsequent mechanical bending test and Raman characterization.
Band gap engineering in silicene: A theoretical study of density functional tight-binding theory
Zaminpayma, Esmaeil; Nayebi, Payman
2016-10-01
In this work, we performed first principles calculations based on self-consistent charge density functional tight-binding to investigate different mechanisms of band gap tuning of silicene. We optimized structures of silicene sheet, functionalized silicene with H, CH3 and F groups and nanoribbons with the edge of zigzag and armchair. Then we calculated electronic properties of silicene, functionalized silicene under uniaxial elastic strain, silicene nanoribbons and silicene under external electrical fields. It is found that the bond length and buckling value for relaxed silicene is agreeable with experimental and other theoretical values. Our results show that the band gap opens by functionalization of silicene. Also, we found that the direct band gap at K point for silicene changed to the direct band gap at the gamma point. Also, the functionalized silicene band gap decrease with increasing of the strain. For all sizes of the zigzag silicene nanoribbons, the band gap is near zero, while an oscillating decay occurs for the band gap of the armchair nanoribbons with increasing the nanoribbons width. At finally, it can be seen that the external electric field can open the band gap of silicene. We found that by increasing the electric field magnitude the band gap increases.
True photonic band-gap mode-control in VCSEL structures
DEFF Research Database (Denmark)
Romstad, F.; Madsen, M.; Birkedal, Dan;
2003-01-01
Photonic band-gap mode confinement in novel nano-structured large area VCSEL structures is confirmed by the amplified spontaneous emission spectrum. Both guide and anti-guide VCSEL structures are experimentally characterised to verify the photonic band-gap effect.......Photonic band-gap mode confinement in novel nano-structured large area VCSEL structures is confirmed by the amplified spontaneous emission spectrum. Both guide and anti-guide VCSEL structures are experimentally characterised to verify the photonic band-gap effect....
Photonic band structures of two-dimensional photonic crystals with deformed lattices
Institute of Scientific and Technical Information of China (English)
Cai Xiang-Hua; Zheng Wan-Hua; Ma Xiao-Tao; Ren Gang; Xia Jian-Bai
2005-01-01
Using the plane-wave expansion method, we have calculated and analysed the changes of photonic band structures arising from two kinds of deformed lattices, including the stretching and shrinking of lattices. The square lattice with square air holes and the triangular lattice with circular air holes are both studied. Calculated results show that the change of lattice size in some special ranges can enlarge the band gap, which depends strongly on the filling factor of air holes in photonic crystals; and besides, the asymmetric band edges will appear with the broken symmetry of lattices.
Miniaturization of electromagnetic band gap structures for mobile applications
Goussetis, G.; Feresidis, A. P.; Palikaras, G. K.; Kitra, M.; Vardaxoglou, J. C.
2005-12-01
It is well known that interference of the human body affects the performance of the antennas in mobile phone handsets. In this contribution, we investigate the use of miniaturized metallodielectric electromagnetic band gap (MEBG) structures embedded in the case of a mobile handset as a means of decoupling the antenna from the user's hand. The closely coupled MEBG concept is employed to achieve miniaturization of the order of 15:1. Full wave dispersion relations for planar closely coupled MEBG arrays are presented and are validated experimentally. The performance of a prototype handset with an embedded conformal MEBG is assessed experimentally and is compared to a similar prototype without the MEBG. Reduction in the detuning of the antenna because of the human hand by virtue of the MEBG is demonstrated. Moreover, the efficiency of the handset when loaded with a human hand model is shown to improve when the MEBG is in place. The improvements are attributed to the decoupling of the antenna from the user's hand, which is achieved by means of suppressing the fields in the locality of the hand.
Photonic band gap materials: Technology, applications and challenges
International Nuclear Information System (INIS)
Last century has been the age of Artificial Materials. One material that stands out in this regard is the semiconductor. The revolution in electronic industry in the 20th century was made possible by the ability of semiconductors to microscopically manipulate the flow of electrons. Further advancement in the field made scientists suggest that the new millennium will be the age of photonics in which artificial materials will be synthesized to microscopically manipulate the flow of light. One of these will be Photonic Band Gap material (PBG). PBG are periodic dielectric structures that forbid propagation of electromagnetic waves in a certain frequency range. They are able to engineer most fundamental properties of electromagnetic waves such as the laws of refraction, diffraction, and emission of light from atoms. Such PBG material not only opens up variety of possible applications (in lasers, antennas, millimeter wave devices, efficient solar cells photo-catalytic processes, integrated optical communication etc.) but also give rise to new physics (cavity electrodynamics, localization, disorder, photon-number-state squeezing). Unlike electronic micro-cavity, optical waveguides in a PBG microchip can simultaneously conduct hundreds of wavelength channels of information in a three dimensional circuit path. In this article we have discussed some aspects of PBG materials and their unusual properties, which provided a foundation for novel practical applications ranging from clinical medicine to information technology. (author)
Study on temperature property of band structures in onedimensional photonic crystals
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
Using transfer matrix method, the optical transmission properties in one-dimensional (1-D) photonic crystal is analyzed.When the temperature varies, not only the refractive index of the optical medium is changed because of the thermo-optical effect, but also the thickness of the optical medium is changed due to the thermal-expansion effect. Thus, the structure of 1/4 wave-plate stack in original photonic crystal is destroyed and the band structure varies. In this work, the effects of the temperature variation on the first and second band gap in a 1-D photonic crystal are analyzed in detail. It is found that the changes of the starting wavelength, the cut-off wavelength and the forbidden band width depend linearly on the temperature.
Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.
Zhang, Nanlin; Neo, Darren C J; Tazawa, Yujiro; Li, Xiuting; Assender, Hazel E; Compton, Richard G; Watt, Andrew A R
2016-08-24
The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.
Growth, Band Structure and Optical Properties of LiSrBO3 Crystal
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
The bulk crystal of LiSrBO3(8.39 g) with a size of 21mm × 20mm × 15mm was grown by high temperature solution growth method. The relationship between growth habit and crystal structure was discussed. The transmission spectrum shows an UV absorption edge at about 300 nm. The melting temperature of this crystal was determined to be 942 ℃ by DTA-TG measurement. The band structure of the LiSrBO3 crystal was studied by means of the first principle method. An indirect band gap was found to be about 4.0 eV, and a low dielectric constant was estimated to be about 1.9 in terms of theoretical results.
The optical band gap investigation of PVP-capped ZnO nanoparticles synthesized by sol-gel method
Yuliah, Yayah; Bahtiar, Ayi; Fitrilawati, Siregar, Rustam E.
2016-02-01
ZnO Nanoparticles (NPs) has unique natures on their crystal structure, direct band gap and high exciton binding energy, consequently applied in optoelectronic devices such as solar cells, optical wave guide and light emitting diodes (LED). However the drawback was ZnO NPs tend to agglomerate and turn to nano-structured materials with poor properties. Effort to avoid agglomerations generally resolved by surface modification of ZnO NPs to obtain well-dispersed suspension. However changes in the surface of ZnO NPs may change the electronic structure and density of states of ZnO NPs, in turn may change the optical band gap. Thus, the objective of current research is investigation of optical band gap of ZnO NPs due to surface modification by capping agent of poly-4-vinylpyrrolidone (PVP) molecules. Uncapped and PVP-capped ZnO nanoparticles were prepared by sol-gel method. The characteristics of surface modifications were investigated by UV-Vis and Photo Luminescence spectroscopy and Transmission Electron Microscope (TEM). The results shows the surface modification has change the band gap of ZnO NPs obtained at second precipitated stage. In contrast, the change of the optical band gap did not observe due to the surface modification of ZnO NPs obtained at the first stage. It was concluded that PVP capping on ZnO NPs did not affect on the band gap when the capping was performed on first stage. It is emphasized that this statement also supported by TEM images observations.
Localized crystallization in shear bands of a metallic glass
Yan, Zhijie; Song, Kaikai; Hu, Yong; Dai, Fuping; Chu, Zhibing; Eckert, Jürgen
2016-01-01
Stress-induced viscous flow is the characteristic of atomic movements during plastic deformation of metallic glasses in the absence of substantial temperature increase, which suggests that stress state plays an important role in mechanically induced crystallization in a metallic glass. However, it is poorly understood. Here, we report on the stress-induced localized crystallization in individual shear bands of Zr60Al15Ni25 metallic glass subjected to cold rolling. We find that crystallization in individual shear bands preferentially occurs in the regions neighboring the amorphous matrix, where the materials are subjected to compressive stresses demonstrated by our finite element simulations. Our results provide direct evidence that the mechanically induced crystallization kinetics is closely related with the stress state. The crystallization kinetics under compressive and tensile stresses are interpreted within the frameworks of potential energy landscape and classical nucleation theory, which reduces the role of stress state in mechanically induced crystallization in a metallic glass.
LC Filter Design for Wide Band Gap Device Based Adjustable Speed Drives
DEFF Research Database (Denmark)
Vadstrup, Casper; Wang, Xiongfei; Blaabjerg, Frede
2014-01-01
This paper presents a simple design procedure for LC filters used in wide band gap device based adjustable speed drives. Wide band gap devices offer fast turn-on and turn-off times, thus producing high dV/dt into the motor terminals. The high dV/dt can be harmful for the motor windings and bearings...
Band-gap engineering of functional perovskites through quantum confinement and tunneling
DEFF Research Database (Denmark)
Castelli, Ivano Eligio; Pandey, Mohnish; Thygesen, Kristian Sommer;
2015-01-01
An optimal band gap that allows for a high solar-to-fuel energy conversion efficiency is one of the key factors to achieve sustainability. We investigate computationally the band gaps and optical spectra of functional perovskites composed of layers of the two cubic perovskite semiconductors BaSnO3...
Low Band Gap Polymers for Roll-to-Roll Coated Polymer Solar Cells
DEFF Research Database (Denmark)
2010-01-01
We present the synthesis of a low band gap copolymer based on dithienothiophene and dialkoxybenzothiadiazole (poly(dithienothiophene-co-dialkoxybenzothiadiazole), PDTTDABT). The optical properties of the polymer showed a band gap of 1.6 eV and a sky-blue color in solid films. The polymer was...
High-power picosecond pulse delivery through hollow core photonic band gap fibers
DEFF Research Database (Denmark)
Michieletto, Mattia; Johansen, Mette Marie; Lyngsø, Jens Kristian;
2015-01-01
We demonstrated robust and bend insensitive fiber delivery of high power pulsed laser with diffraction limited beam quality for two different kind of hollow core photonic band gap fibers......We demonstrated robust and bend insensitive fiber delivery of high power pulsed laser with diffraction limited beam quality for two different kind of hollow core photonic band gap fibers...
Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures.
Hu, Xiaohui; Kou, Liangzhi; Sun, Litao
2016-01-01
The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A'B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA', AB and AB' stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs. PMID:27528196
Band-gap engineering by Bi intercalation of graphene on Ir(111)
Warmuth, Jonas; Bruix, Albert; Michiardi, Matteo; Hänke, Torben; Bianchi, Marco; Wiebe, Jens; Wiesendanger, Roland; Hammer, Bjørk; Hofmann, Philip; Khajetoorians, Alexander A.
2016-04-01
We report on the structural and electronic properties of a single bismuth layer intercalated underneath a graphene layer grown on an Ir(111) single crystal. Scanning tunneling microscopy (STM) reveals a hexagonal surface structure and a dislocation network upon Bi intercalation, which we attribute to a √{3 }×√{3 }R 30∘ Bi structure on the underlying Ir(111) surface. Ab initio calculations show that this Bi structure is the most energetically favorable and illustrate that STM measurements are most sensitive to C atoms in close proximity to intercalated Bi atoms. Additionally, Bi intercalation induces a band gap (Eg=0.42 eV) at the Dirac point of graphene and an overall n doping (˜0.39 eV ) as seen in angular-resolved photoemission spectroscopy. We attribute the emergence of the band gap to the dislocation network which forms favorably along certain parts of the moiré structure induced by the graphene/Ir(111) interface.
Band gap control of phononic beam with negative capacitance piezoelectric shunt
Institute of Scientific and Technical Information of China (English)
Chen Sheng-Bing; Wen Ji-Hong; Yu Dian-Long; Wang Gang; Wen Xi-Sen
2011-01-01
Periodic arrays of negative capacitance shunted piezoelectric patches are employed to control the band gaps of phononic beams. The location and the extent of induced band gap depend on the mismatch in impedance generated by each patch. The total impedance mismatch is determined by the added mass and stiffness of each patch as well as the shunting electrical impedance. Therefore, the band gap of the shunted phononic beam can be actively tuned by appropriately selecting the value of negative capacitance. The control of the band gap of phononic beam with negative capacitive shunt is demonstrated numerically by employing transfer matrix method. The result reveals that using negative capacitive shunt to tune the band gap is effective.
Understanding Band Gaps of Solids in Generalized Kohn-Sham Theory
Perdew, John P; Burke, Kieron; Yang, Zenghui; Gross, Eberhard K U; Scheffler, Matthias; Scuseria, Gustavo E; Henderson, Thomas M; Zhang, Igor Ying; Ruzsinszky, Adrienn; Peng, Haowei; Sun, Jianwei
2016-01-01
The fundamental energy gap of a periodic solid distinguishes insulators from metals and characterizes low-energy single-electron excitations. But the gap in the band-structure of the exact multiplicative Kohn-Sham (KS) potential substantially underestimates the fundamental gap, a major limitation of KS density functional theory. Here we give a simple proof of a new theorem: In generalized KS theory (GKS), the band gap equals the fundamental gap for the approximate functional if the GKS potential operator is continuous and the density change is delocalized when an electron or hole is added. Our theorem explains how GKS band gaps from meta-generalized gradient approximations (meta-GGAs) and hybrid functionals can be more realistic than those from GGAs or even from the exact KS potential, It also follows from earlier work. The band edges in the GKS one-electron spectrum are also related to measurable energies. A linear chain of hydrogen molecules provides a numerical illustration.
Lebens-Higgins, Z; Scanlon, D O; Paik, H; Sallis, S; Nie, Y; Uchida, M; Quackenbush, N F; Wahila, M J; Sterbinsky, G E; Arena, Dario A; Woicik, J C; Schlom, D G; Piper, L F J
2016-01-15
We have directly measured the band gap renormalization associated with the Moss-Burstein shift in the perovskite transparent conducting oxide (TCO), La-doped BaSnO_{3}, using hard x-ray photoelectron spectroscopy. We determine that the band gap renormalization is almost entirely associated with the evolution of the conduction band. Our experimental results are supported by hybrid density functional theory supercell calculations. We determine that unlike conventional TCOs where interactions with the dopant orbitals are important, the band gap renormalization in La-BaSnO_{3} is driven purely by electrostatic interactions. PMID:26824566
Grain size dependent optical band gap of CdI2 films
Indian Academy of Sciences (India)
Pankaj Tyagi; A G Vedeshwar
2001-06-01
The thermally evaporated stoichiometric CdI2 films show good -axis alignment normal to substrate plane for film thickness up to 200 nm. The optical absorption data indicate an allowed direct interband transition across a gap of 3.6 eV in confirmation with earlier band structure calculations. However, part of the absorption data near band edge can be fitted to an indirect band gap of 3 eV. The dependence of band gap on film thickness (> 200 nm) can be explained qualitatively in terms of decreasing grain boundary barrier height with grain size.
International Nuclear Information System (INIS)
Preliminary results from our charge self-consistent LCAO band structure (CSCBS) calculations with Bloch sums as the basis reveal that a noncorrosive reduced band gap electrode for photoelectrochemical solar cells may be produced from a (1:1) mixture of β-PbO2 and TiO2 (both rutile). The band gaps for the constituents (β-PbO2 and TiO2) and the 1:1 mixture are calculated and a detailed characterization of the valence and the conduction bands is undertaken to offer a possible mechanism for the reduction of the band gap of the mixture. The band gap for the perovskite PbTiO3 is also calculated to offer a guideline for selecting from the competing pathways to the fabrication of noncorrosive photoelectrochemical electrodes
Band-gap measurements of bulk and nanoscale hematite by soft x-ray spectroscopy
DEFF Research Database (Denmark)
Gilbert, B.; Frandsen, Cathrine; Maxey, E.R.;
2009-01-01
Chemical and photochemical processes at semiconductor surfaces are highly influenced by the size of the band gap, and ability to control the band gap by particle size in nanomaterials is part of their promise. The combination of soft x-ray absorption and emission spectroscopies provides band......-gap determination in bulk and nanoscale itinerant electron semiconductors such as CdS and ZnO, but this approach has not been established for materials such as iron oxides that possess band-edge electronic structure dominated by electron correlations. We performed soft x-ray spectroscopy at the oxygen K-edge to...... reveal band-edge electronic structure of bulk and nanoscale hematite. Good agreement is found between the hematite band gap derived from optical spectroscopy and the energy separation of the first inflection points in the x-ray absorption and emission onset regions. By applying this method to two sizes...
The band gap of Cu2ZnSnSe4: Effect of order-disorder
Rey, G.; Redinger, A.; Sendler, J.; Weiss, T. P.; Thevenin, M.; Guennou, M.; El Adib, B.; Siebentritt, S.
2014-09-01
The order-disorder transition in kesterite Cu2ZnSnSe4 (CZTSe), an interesting material for solar cell, has been investigated by spectrophotometry, photoluminescence (PL), and Raman spectroscopy. Like Cu2ZnSnS4, CZTSe is prone to disorder by Cu-Zn exchanges depending on temperature. Absorption measurements have been used to monitor the changes in band gap energy (Eg) of solar cell grade thin films as a function of the annealing temperature. We show that ordering can increase Eg by 110 meV as compared to fully disordered material. Kinetics simulations show that Eg can be used as an order parameter and the critical temperature for the CZTSe order-disorder transition is 200 ± 20 °C. On the one hand, ordering was found to increase the correlation length of the crystal. But on the other hand, except the change in Eg, ordering did not influence the PL signal of the CZTSe.
Schuster, M.; Klein, N.
2003-03-01
We report on the controlled excitation of line and point defect modes in a two-dimensional hexagonal electromagnetic band-gap structure made of rods of dielectric material (aluminium oxide). We compared simulation performed with a numerical field simulation software and experimental measurements at microwave frequencies with regard to coupling from external waveguides to line defects and subsequent coupling to resonant modes. We observed that for a line defect in the photonic crystal the impedance matching to a waveguide is strongly dependent on the defect width. We furthermore demonstrated that the coupling to a localized defect resonance can be strongly influenced by the variation of certain single lattice elements, affecting transmission behavior and quality factor of the resonant modes.
Characterization of self-phase modulation in liquid filled hollow core photonic band gap fibers
Huy, Minh Chau Phan; Lebrun, Sylvie; Frey, Robert; Delaye, Philippe
2010-01-01
We experimentally and theoretically study self-phase modulation by Kerr effect in a liquid filled hollow core photonic crystal fiber. We perform a complete characterization of the linear optical properties of the hollow core photonic band gap fiber filled with deuterated acetone to determine all the characteristics of the propagation mode. The nonlinear coefficient of the fiber is determined by fitting the output spectra broadened by self-phase modulation with a new analytical expression giving the spectra of a secant hyperbolic pulse transmitted through a Kerr media. The experiment allows a precise determination of the nonlinear index change of acetone-d6 equal to (1.15$\\pm$0.17)x10-19 m2W-1.
Energy Technology Data Exchange (ETDEWEB)
Sedghi, Aliasghar [Islamic Azad University, Shabestar (Iran, Islamic Republic of); Valiaghaie, Soma [Islamic Azad University, Sanandaj (Iran, Islamic Republic of); Soufiani, Ahad Rounaghi [Islamic Azad University, Sufian (Iran, Islamic Republic of)
2014-10-15
By virtue of the efficiency of the Dirichlet-to-Neumann map method, we have calculated, for H-polarization (TE mode), the band structure of 2D photonic crystals with a square lattice composed of metallic rods embedded in an air background. The rod in the unit cell is chosen to be circular in shape. Here, from a practical point of view, in order to obtain maximum band gaps, we have studied the band structure as a function of the size of the rods. We have also studied the flat bands appearing in the band structures and have shown that for frequencies around the surface plasmon frequency, the modes are highly localized at the interface between the metallic rods and the air background.
Energy Technology Data Exchange (ETDEWEB)
Malashchonak, M.V., E-mail: che.malasche@gmail.com [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Streltsov, E.A., E-mail: streltea@bsu.by [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Mazanik, A.V. [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Kulak, A.I., E-mail: kulak@igic.bas-net.by [Institute of General and Inorganic Chemistry, National Academy of Sciences of Belarus, Surganova str., 9/1, Minsk 220072 (Belarus); Poznyak, S.K. [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Stroyuk, O.L., E-mail: stroyuk@inphyschem-nas.kiev.ua [L.V. Pysarzhevsky Institute of Physical Chemistry of National Academy of Sciences of Ukraine, 31 prosp. Nauky, 03028 Kyiv (Ukraine); Kuchmiy, S.Ya. [L.V. Pysarzhevsky Institute of Physical Chemistry of National Academy of Sciences of Ukraine, 31 prosp. Nauky, 03028 Kyiv (Ukraine); Gaiduk, P.I. [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus)
2015-08-31
Cadmium sulfide nanoparticle (NP) deposition by the successive ionic layer adsorption and reaction (SILAR) method on the surface of mesoporous ZnO micro-platelets with a large specific surface area (110 ± 10 m{sup 2}g{sup −1}) results in the formation of ZnO/CdS heterostructures exhibiting a high incident photon-to-current conversion efficiency (Y) not only within the region of CdS fundamental absorption (Y{sub max} = 90%; 0.1 M Na{sub 2}S + 0.1 M Na{sub 2}SO{sub 3}), but also in the sub-band-gap (SBG) range (Y{sub max} = 25%). The onset potentials of SBG photoelectrochemical processes are more positive than the band-gap (BG) onset potential by up to 100 mV. A maximum incident photon-to-current conversion efficiency value for SBG processes is observed at larger amount of deposited CdS in comparison with the case of BG ones. The Urbach energy (E{sub U}) of CdS NPs determined from the photocurrent spectra reaches a maximal value on an early deposition stage (E{sub U} = 93 mV at SILAR cycle number N = 5), then lowers somewhat (E{sub U} = 73 mV at N = 10) and remains steady in the range of N from 20 to 300 (E{sub U} = 67 ± 1 mV). High efficiency of the photoelectrochemical SBG processes are interpreted in terms of light scattering in the ZnO/CdS heterostructures. - Highlights: • ZnO/CdS films demonstrate high quantum efficiency (25%) for sub-band-gap transitions. • Onset photocurrent potentials for sub-band-gap processes differ than those for band-gap ones. • Sub-band-gap transitions are caused by band-tail states in CdS nanoparticles.
Vibronic interaction in crystals with the Jahn-Teller centers in the elementary energy bands concept
Directory of Open Access Journals (Sweden)
D.M. Bercha
2015-09-01
Full Text Available The order-disorder type phase transition caused by the vibronic interaction (collective Jahn-Teller effect in a monoclinic CuInP2S6 crystal is analyzed. For this purpose, a trigonal protostructure model of CuInP2S6 is created, through a slight change in the crystal lattice parameters of the CuInP2S6 paraelectric phase. In parallel to the group-theoretical analysis, the DFT-based ab initio band structure calculations of the CuInP2S6 protostructure, para-, and ferriphases are performed. Using the elementary energy bands concept, a part of the band structure from the vicinity of the forbidden energy gap, which is created by the d-electron states of copper has been related with a certain Wyckoff position where the Jahn-Teller's centers are localized. A construction procedure of the vibronic potential energy matrix is generalized for the case of crystal using the elementary energy bands concept and the group theoretical method of invariants. The procedure is illustrated by the creation of the adiabatic potentials of the Γ5-Γ5 vibronic coupling for the protostructure and paraphase of the CuInP2S6 crystal. A structure of the obtained adiabatic potentials is analyzed, followed by conclusions on their transformation under a phase transition and the discussion on the possibility for the spontaneous polarization to arise in this crystal.
Band gap bowing and electron localization of (GaxIn1-x)N
Energy Technology Data Exchange (ETDEWEB)
Lee, Byounghak; Wang, Lin-Wang
2006-05-09
The band gap bowing and the electron localization ofGaxIn1-xN are calculated using both the local density approximation (LDA)and screened-exchange local density functional (sX-LDA) methods. Thecalculated sX-LDA band gaps are in good agreement with the experimentallyobserved values, with errors of -0.26 and 0.09 eV for bulk GaN and InN,respectively. The LDA band gap errors are 1.33 and 0.81 eV for GaN andInN, in order. In contrast to the gap itself, the band gap bowingparameter is found to be very similar in sX-LDA and LDA. We identify thelocalization of hole states in GaxIn1-xN alloys along In-N-In chains. Thepredicted localizationis stronger in sX-LDA.
Qi, Jingshan; Li, Xiao; Qian, Xiaofeng
2016-06-01
Electrically controlled band gap and topological electronic states are important for the next-generation topological quantum devices. In this letter, we study the electric field control of band gap and topological phase transitions in multilayer germanane. We find that although the monolayer and multilayer germananes are normal insulators, a vertical electric field can significantly reduce the band gap of multilayer germananes owing to the giant Stark effect. The decrease of band gap eventually leads to band inversion, transforming them into topological insulators with nontrivial Z2 invariant. The electrically controlled topological phase transition in multilayer germananes provides a potential route to manipulate topologically protected edge states and design topological quantum devices. This strategy should be generally applicable to a broad range of materials, including other two-dimensional materials and ultrathin films with controlled growth.
Low-frequency band gap mechanism of torsional vibration of lightweight elastic metamaterial shafts
Li, Lixia; Cai, Anjiang
2016-07-01
In this paper, the low-frequency band gap mechanism of torsional vibration is investigated for a kind of light elastic metamaterial (EM) shafts architecture comprised of a radial double-period element periodically as locally resonant oscillators with low frequency property. The dispersion relations are calculated by a method combining the transfer matrix and a lumped-mass method. The theoretical results agree well with finite method simulations, independent of the density of the hard material ring. The effects of the material parameters on the band gaps are further explored numerically. Our results show that in contrast to the traditional EM shaft, the weight of our proposed EM shaft can be reduced by 27% in the same band gap range while the vibration attenuation is kept unchanged, which is very convenient to instruct the potential engineering applications. Finally, the band edge frequencies of the lower band gaps for this light EM shaft are expressed analytically using physical heuristic models.
Photonic bands and defect modes in metallo-dielectric photonic crystal slabs
Zanotto, Simone; Sorba, Lucia; Tredicucci, Alessandro
2016-01-01
Photonic components based on structured metallic elements show great potential for device applications where field enhancement and confinement of the radiation on a subwavelength scale is required. In this paper we report a detailed study of a prototypical metallo-dielectric photonic structure, where features well known in the world of dielectric photonic crystals, like band gaps and defect modes, are exported to the metallic counterpart, with interesting applications to infrared science and technology, as for instance in quantum well infrared photodetectors, narrow-band spectral filters, and tailorable thermal emitters.
X-Band Photonic Band-Gap Accelerator Structure Breakdown Experiment
Energy Technology Data Exchange (ETDEWEB)
Marsh, Roark A.; /MIT /MIT /NIFS, Gifu /JAERI, Kyoto /LLNL, Livermore; Shapiro, Michael A.; Temkin, Richard J.; /MIT; Dolgashev, Valery A.; Laurent, Lisa L.; Lewandowski, James R.; Yeremian, A.Dian; Tantawi, Sami G.; /SLAC
2012-06-11
In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at X band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of 65 MV/m yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above 110 MV/m was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of 100 MV/m and a surface magnetic field of 890 kA/m. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electric field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about 14 MV/m, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.
Synthesis and Characterization of Small Band-gap Conjugated Polymers - Poly(pyrrolyl methines)
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
A kind of small band-gap conjugated polymers-poly (pyrrolyl methines) and their precursors-(poly pyrrolyl methanes) have been synthesized by a simple method and characterized by 1HNMR, FT-IR, TGA and UV-Vis. These polymers can be dissolved in high polar solvents such as DMSO, DMF or NMP. The results reveals that the band-gap of the synthesized conjugated polymers are in the range of 0.96～1.14 eV and they all belong to the small band-gap polymers. The conductivity of doped products with iodine is in the range of semiconductor.
Chegel, Raad
2016-06-01
By using the third nearest neighbor modified tight binding (3NN-TB) method, the electronic structure and band gap of BNNRs under transverse electric fields are explored. The band gap of the BNNRs has a decreasing with increasing the intensity of the applied electric field, independent on the ribbon edge types. Furthermore, an analytic model for the dependence of the band gap in armchair and zigzag BNNRs on the electric field is proposed. The reduction of E g is similar for some N a armchair and N z zigzag BNNRs independent of their edges.
Topological description for gaps of one-dimensional symmetric all-dielectric photonic crystals.
Shi, Xi; Xue, Chunhua; Jiang, Haitao; Chen, Hong
2016-08-01
We propose a topological description for gaps of one-dimensional symmetric all-dielectric photonic crystals (PCs). It is shown that, in the propagating direction, the effective electromagnetic parameters of PCs can be derived from one unit cell with mirror symmetry. Besides, at the frequencies of gaps, these symmetric PCs can be described as photonic insulators with effective negative permittivity or negative permeability. Moreover, based on the mapping of Maxwell's equations to the Dirac equation and the band inversion achieved by tuning the material and structural parameters, we demonstrate that the gaps of PCs with effective negative permittivity or negative permeability possess different topological orders. Lastly, we show that a bound state is robust against the disorder under a zero-average-effective-mass condition in a heterostructure made of two PCs with different topological orders. PMID:27505821
Band structures and localization properties of aperiodic layered phononic crystals
Energy Technology Data Exchange (ETDEWEB)
Yan Zhizhong, E-mail: zzyan@bit.edu.cn [Department of Applied Mathematics, Beijing Institute of Technology, Beijing 100081 (China); Zhang Chuanzeng [Department of Civil Engineering, University of Siegen, D-57078 Siegen (Germany)
2012-03-15
The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.
Vibrational renormalisation of the electronic band gap in hexagonal and cubic ice
Energy Technology Data Exchange (ETDEWEB)
Engel, Edgar A., E-mail: eae32@cam.ac.uk; Needs, Richard J. [TCM Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Monserrat, Bartomeu [TCM Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019 (United States)
2015-12-28
Electron-phonon coupling in hexagonal and cubic water ice is studied using first-principles quantum mechanical methods. We consider 29 distinct hexagonal and cubic ice proton-orderings with up to 192 molecules in the simulation cell to account for proton-disorder. We find quantum zero-point vibrational corrections to the minimum electronic band gaps ranging from −1.5 to −1.7 eV, which leads to improved agreement between calculated and experimental band gaps. Anharmonic nuclear vibrations play a negligible role in determining the gaps. Deuterated ice has a smaller band-gap correction at zero-temperature of −1.2 to −1.4 eV. Vibrations reduce the differences between the electronic band gaps of different proton-orderings from around 0.17 eV to less than 0.05 eV, so that the electronic band gaps of hexagonal and cubic ice are almost independent of the proton-ordering when quantum nuclear vibrations are taken into account. The comparatively small reduction in the band gap over the temperature range 0 − 240 K of around 0.1 eV does not depend on the proton ordering, or whether the ice is protiated or deuterated, or hexagonal, or cubic. We explain this in terms of the atomistic origin of the strong electron-phonon coupling in ice.
Effect of thermal annealing on structure and optical band gap of amorphous Se72Te25Sb3 thin films
Dwivedi, D. K.; Pathak, H. P.; Kumar, Vipin; Shukla, Nitesh
2014-04-01
Thin films of a-Se72Te25Sb3 were prepared by vacuum evaporation technique in a base pressure of 10-6 Torr on to well cleaned glass substrate. a-Se72Te25Sb3 thin films were annealed at different temperatures below their crystallization temperatures for 2h. The structural analysis of the films has been investigated using X-ray diffraction technique. The optical band gap of as prepared and annealed films as a function of photon energy in the wavelength range 400-1100 nm has been studied. It has been found that the optical band gap decreases with increasing annealing temperatures in the present system.
Costa, C H O; Vasconcelos, M S
2013-07-17
We employ a microscopic theory to investigate spin wave (magnon) propagation through their dispersion and transmission spectra in magnonic crystals arranged to display deterministic disorder. In this work the quasiperiodic arrangement investigated is the well-known generalized Fibonacci sequence, which is characterized by the σ(p,q) parameter, where p and q are non-zero integers. In order to determine the bulk modes and transmission spectra of the spin waves, the calculations are carried out for the exchange dominated regime within the framework of the Heisenberg model and taking into account the random phase approximation. We have considered magnetic materials that have a ferromagnetic order, and the transfer-matrix treatment is applied to simplify the algebra. The results reveal that spin wave spectra display a rich and interesting magnonic pass- and stop-bands structures, including an almost symmetric band gap distribution around of a mid-gap frequency, which depends on the Fibonacci sequence type. PMID:23779133
Damage and ablation of large band gap dielectrics induced by a 46.9 nm laser beam
Energy Technology Data Exchange (ETDEWEB)
Ritucci, A; Tomassetti, G; Reale, A; Arrizza, L; Zuppella, P; Reale, L; Palladino, L; Flora, F; Bonfigli, F; Faenov, A; Pikuz, T; Kaiser, J; Nilsen, J; Jankowski, A F
2006-03-08
We applied a 0.3 mJ, 1.7 ns, 46.9 nm soft X-ray Argon laser to ablate the surface of large band gap dielectrics: CaF{sub 2} and LiF crystals. The ablation versus the fluence of the soft X-ray beam has been studied varying the fluence in the range of 0.05-3 J/cm{sup 2}. An ablation threshold of 0.06 and 0.1 J/cm{sup 2} and an ablation depth of 14 and 20 nm have been found for CaF{sub 2} and LiF, respectively. These results define new ablation conditions for these large band gap dielectrics, which can be of interest for the fine processing of these materials.
Electronic Bands Behaviour at Sinusoidal Potential Presence of Incommensurate Crystals
Vlokh R.; Vlokh O.; Lukiyanets B.
2004-01-01
On the basis of solving the Schrodinger and Mathieu equations, for the case of crystal field perturbed by one-dimensional sinusoidal potential of the modulated phase in uniaxial ferroelectrics, it has been shown that the positions of electronic levels are sensitive to the ratio of periods of the crystal field and the perturbation potential. Considering the energy states as prototypes of bands normalized by perturbation, one can come to the conclusion that the level of the states is the same a...
Systematic analysis of the unique band gap modulation of mixed halide perovskites.
Kim, Jongseob; Lee, Sung-Hoon; Chung, Choong-Heui; Hong, Ki-Ha
2016-02-14
Solar cells based on organic-inorganic hybrid metal halide perovskites have been proven to be one of the most promising candidates for the next generation thin film photovoltaic cells. Mixing Br or Cl into I-based perovskites has been frequently tried to enhance the cell efficiency and stability. One of the advantages of mixed halides is the modulation of band gap by controlling the composition of the incorporated halides. However, the reported band gap transition behavior has not been resolved yet. Here a theoretical model is presented to understand the electronic structure variation of metal mixed-halide perovskites through hybrid density functional theory. Comparative calculations in this work suggest that the band gap correction including spin-orbit interaction is essential to describe the band gap changes of mixed halides. In our model, both the lattice variation and the orbital interactions between metal and halides play key roles to determine band gap changes and band alignments of mixed halides. It is also presented that the band gap of mixed halide thin films can be significantly affected by the distribution of halide composition. PMID:26791587
Modification of Band Gap of β-SiC by N-Doping
Institute of Scientific and Technical Information of China (English)
LIU Hong-Sheng; FANG Xiao-Yong; SONG Wei-Li; HOU Zhi-Ling; LU Ran; YUAN Jie; CAO Mao-Sheng
2009-01-01
The geometrical and electronic structures of nitrogen-doped β-SiC are investigated by employing the first principles of plane wave ultra-soft pseudo-potential technology based on density functional theory.The structures of SiC1-xNx (x=0,1/32,1/16,1/8,1/4) with different doping concentrations are optimized.The results reveal that the band gap of β-SiC transforms from an indirect band gap to a direct band gap with band gap shrinkage after carbon atoms are replaced by nitrogen atoms.The Fermi level shifts from valence band top to conduction band by doping nitrogen in pure β-SiC,and the doped β-SiC becomes metallic.The degree of Fermi levels entering into the conduction band increases with the increment of doping concentration;however,the band gap becomes narrower.This is attributed to defects with negative electricity occurring in surrounding silicon atoms.With the increase of doping concentration,more residual electrons,more easily captured by the 3p orbit in the silicon atom,will be provided by nitrogen atoms to form more defects with negative electricity.
Phononic band gaps and vibrations in one- and two-dimensional mass-spring structures
DEFF Research Database (Denmark)
Jensen, Jakob Søndergaard
2003-01-01
The vibrational response of finite periodic lattice structures subjected to periodic loading is investigated. Special attention is devoted to the response in frequency ranges with gaps in the band structure for the corresponding infinite periodic lattice. The effects of boundaries, viscous damping......, and imperfections are studied by analyzing two examples; a 1-D filter and a 2-D wave guide. In 1-D the structural response in the band gap is shown to be insensitive to damping and small imperfections. In 2-D the similar effect of damping is noted for one type of periodic structure, whereas for...... another type the band gap effect is nearly eliminated by damping. In both 1-D and 2-D it is demonstrated how the free structural boundaries affect the response in the band gap due to local resonances. Finally, 2-D wave guides are considered by replacing the periodic structure with a homogeneous structure...
Vibration band-gap properties of three-dimensional Kagome lattices using the spectral element method
Wu, Zhi-Jing; Li, Feng-Ming; Zhang, Chuanzeng
2015-04-01
The spectral element method (SEM) is extended to investigate the vibration band-gap properties of three-dimensional (3D) Kagome lattices. The dynamic stiffness matrix of the 3D element which contains bending, tensional and torsional components is derived. The spectral equations of motion of the whole 3D Kagome lattice are then established. Comparing with frequency-domain solutions calculated by the finite element method (FEM), the accuracy and the feasibility of the SEM solutions are verified. It can be shown that the SEM is suitable for analyzing the vibration band-gap properties. Due to the band-gap characteristics, the periodic 3D Kagome lattice has the performance of vibration isolation. The influences of the structural and material parameters on the vibration band-gaps are discussed and a new type of 3D Kagome lattice is designed to obtain the improved vibration isolation capability.
Mondal, Rajib
2009-01-01
Removing the adjacent thiophene groups around the acceptor core in low band gap polymers significantly enhances solar cell efficiency through increasing the optical absorption and raising the ionization potential of the polymer. © 2009 The Royal Society of Chemistry.
Robust band gap and half-metallicity in graphene with triangular perforations
DEFF Research Database (Denmark)
Gregersen, Søren Schou; Power, Stephen; Jauho, Antti-Pekka
2016-01-01
Ideal graphene antidot lattices are predicted to show promising band gap behavior (i.e., EG ≅ 500 meV) under carefully specified conditions. However, for the structures studied so far this behavior is critically dependent on superlattice geometry and is not robust against experimentally realistic...... disorders. Here we study a rectangular array of triangular antidots with zigzag edge geometries and show that their band gap behavior qualitatively differs from the standard behavior which is exhibited, e.g., by rectangular arrays of armchair-edged triangles. In the spin unpolarized case, zigzag......, reducing the band gaps compared to the unpolarized case. This behavior is also found to be robust in the presence of disorder. Our results highlight the possibilities of using triangular perforations in graphene to open electronic band gaps in systems with experimentally realistic levels of disorder...
Shape optimization of phononic band gap structures using the homogenization approach
Vondřejc, Jaroslav; Heczko, Jan
2016-01-01
The paper deals with optimization of the acoustic band gaps computed using the homogenized model of strongly heterogeneous elastic composite which is constituted by soft inclusions periodically distributed in stiff elastic matrix. We employ the homogenized model of such medium to compute intervals - band gaps - of the incident wave frequencies for which acoustic waves cannot propagate. It was demonstrated that the band gaps distribution can be influenced by changing the shape of inclusions. Therefore, we deal with the shape optimization problem to maximize low-frequency band gaps; their bounds are determined by analyzing the effective mass tensor of the homogenized medium. Analytic transformation formulas are derived which describe dispersion effects of resizing the inclusions. The core of the problem lies in sensitivity of the eigenvalue problem associated with the microstructure. Computational sensitivity analysis is developed, which allows for efficient using of the gradient based optimization methods. Num...
Crystal structure, energy band and optical properties of dysprosium monophosphate DyPO{sub 4}
Energy Technology Data Exchange (ETDEWEB)
Khadraoui, Z.; Bouzidi, C., E-mail: bouzidtc@yahoo.fr; Horchani-Naifer, K.; Ferid, M.
2014-12-25
Graphical abstract: The monophosphate DyPO{sub 4} has been synthesized by high temperature solid-state reaction method and was structurally characterized by single crystal X-ray diffraction. DyPO{sub 4} crystallizes in the tetragonal system (I4{sub 1}/Iamd). The energy-band structure, density of states and the chemical bonds have been investigated by density functional methods (DFT). - Highlights: • The DyPO{sub 4} has been synthesized by high temperature solid-state reaction method. • DFT was used to determine the electronic structure and optical properties of DyPO{sub 4}. • The monophosphate DyPO{sub 4} is an insulator with direct band gap (6.38 eV). - Abstract: A rare earth monophosphate crystal of DyPO{sub 4} has been synthesized by high temperature solid-state reaction method and was structurally characterized by single crystal X-ray diffraction. Atomic arrangement of DyPO{sub 4} structure is based on corner and edge sharing PO{sub 4} tetrahedra and DyO{sub 8} polyhedra. The FTIR, Raman, Scanning electron microscopy, diffuse reflectance and emission spectra of the compound have been investigated. Density functional calculation using a Generalized Gradient Approximation was used to determine the electronic structure and optical properties. The calculated total and partial densities of states indicate that the top of valance band is mainly built upon O-2p states with P-3p states via σ (P–O) interactions, and the low conduction bands mostly originates from Dy-5d. The results show that the monophosphate DyPO{sub 4} is an insulator with a calculated band gap (5.8 eV) closer to the experimental value (6.38 eV)
Variable band-gap semiconductors as the basis of new solar cells
Energy Technology Data Exchange (ETDEWEB)
Morales-Acevedo, Arturo [Centro de Investigacion y de Estudios Avanzados del IPN, Electrical Engineering Department, Avenida IPN No. 2508, 07360 Mexico, DF (Mexico)
2009-09-15
Some basic concepts related to variable band-gap absorbing semiconductors in solar cell structures, such as the associated quasi-electric field, will be discussed. The effects of this quasi-electric field upon the minority carrier drift-diffusion length and the back surface recombination velocity may induce a larger generated carrier collection at the junction with the corresponding increase of the illumination current density. It will also be shown that an additional improvement of the open-circuit voltage is possible when the band-gap is reduced within the space charge region so that the dark saturation current density is reduced there. Our estimation is that in the case of a solar cell where the band-gap is changed about 0.5 eV within the space charge region, an increase of the open-circuit voltage around 115 mV will be observed with respect to the single minimum band-gap absorbing material case. A similar band-gap variation in the bulk of the material will cause an increase of the minority carrier drift-diffusion length by a factor of 10 with respect to the single band-gap material. Therefore, based on these physical concepts, two possible structures with variable band-gap layers are proposed in order to have higher efficiencies than for cells without any band-gap grading. It will be shown that these concepts can be applied to II-VI, III-V chalcopyrite and even amorphous semiconductor solar cells. (author)
Amir Reza Sadrolhosseini; Suraya Abdul Rashid; Noor, A. S. M.; Alireza Kharazmi; Lim, H. N.; Mohd Adzir Mahdi
2016-01-01
A polypyrrole-nanoparticles reduced graphene oxide nanocomposite layer was prepared using electrochemical method. The prepared samples were characterized using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and UV-visible spectroscopy. The band gap of nanocomposite layers was calculated from UV-visible spectra and the thermal diffusivity of layers was measured using a photoacoustic technique. As experimental results, the optical band gap was in the range...
Quasiparticle Band Gaps of Graphene and Graphone on Hexagonal Boron Nitride Substrate
Kharche, Neerav; Nayak, Saroj
2012-02-01
Graphene holds great promise for post-silicon electronics; however, it faces two main challenges: opening up a band gap and finding a suitable substrate material. Graphene on hexagonal boron nitride (hBN) substrate provides a potential system to overcome these challenges. While theoretical studies suggested a possibility of a finite band gap of graphene on hBN, recent experimental studies find no band gap. We have studied graphene-hBN system using the first-principles density functional method and the many-body perturbation theory within GW approximation [1]. A Bernal stacked graphene on hBN has a band gap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits band gaps larger than 2.5 eV. The graphone band gap is due to chemical functionalization and is robust in the presence of misalignment, however, it reduces by about 1 eV due to the polarization effects at the graphone/hBN interface.[4pt] [1] N. Kharche and S. K. Nayak, Nano Lett., DOI: 10.1021/nl202725w, (2011).
Influence of process parameters on band gap of AI-doped ZnO film
Institute of Scientific and Technical Information of China (English)
Diqiu HUANG; Xiangbin ZENG; Yajuan ZHENG; Xiaojin WANG; Yanyan YANG
2013-01-01
This paper presents the influence of process parameters, such as argon （Ar） flow rate, sputtering power and substrate temperature on the band gap of Al-doped ZnO film, Al-doped ZnO thin films were fabricated by radio frequency （RF） magnetron sputtering technology and deposited on polyimide and glass substrates. Under different Ar flow rates varied from 30 to 70 sccm, the band gap of thin films were changed from 3.56 to 3.67 eV. As sputtering power ranged from 125 to 200 W, the band gap was varied from 3.28 to 3.82 eV; the band gap was between 3.41 and 3.88 eV as substrate temperature increases from 150℃ to 300℃. Furthermore, the correlation between carrier concentration and band gap was investigated by HALL. These results demonstrate that the band gap of the Al-doped ZnO thin film can be adjusted by changing the Ar flow rate, sputtering power and substrate temperature, which can improve the performance of semiconductor devices related to Al-doped ZnO thin film.
Optical band gap tuning of Sb-Se thin films for xerographic based applications
Kaur, Ramandeep; Singh, Palwinder; Singh, Kulwinder; Kumar, Akshay; Thakur, Anup
2016-10-01
In the present paper we have studied the effect of Sb addition on the optical band gap tuning of thermally evaporated SbxSe100-x (x = 0, 5, 20, 50 and 60) thin films. The structural investigations revealed that all thin films were amorphous in nature. Transmission spectrum was taken in the range 400-2500 nm shows that all films are highly transparent in the near infrared region. The fundamental absorption edge shifts towards longer wavelength with Sb incorporation. The optical band gap decreases with addition of antimony in a-Se thin films. A good correlation has been drawn between experimentally estimated and theoretically calculated optical band gap. The decrease in optical band gap of thin films has been explained using chemical bond approach and density of states model. Decrease in optical band gap with Sb addition increases the concentration of electron deep traps which increases the X-ray sensitivity of Sb-Se thin films. Thus by tuning the optical band gap of Sb-Se alloy, it could be utilized for xerographic based applications.
Optofluidic tuning of photonic crystal band edge lasers
DEFF Research Database (Denmark)
Bernal, Felipe; Christiansen, Mads Brøkner; Gersborg-Hansen, Morten;
2007-01-01
We demonstrate optofluidic tuning of polymer photonic crystal band edge lasers with an imposed rectangular symmetry. The emission wavelength depends on both lattice constant and cladding refractive index. The emission wavelength is shown to change 1 nm with a cladding refractive index change of 10...
Band gap engineering for single-layer graphene by using slow Li+ ions
Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook
2016-08-01
In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li+ ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li+ ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li+ ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li+ ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices.
Band gap engineering for single-layer graphene by using slow Li(+) ions.
Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook
2016-08-01
In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li(+) ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li(+) ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li(+) ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li(+) ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices. PMID:27345294
Towards a complete photonic band gap in the visible
Velikov, K.P.
2002-01-01
The first part of the thesis describes the fabrication and the characterization of face-centered-cubic (fcc) photonic crystals (PCs) of dielectric (core-shell) spheres in a low-dielectric host (air). We demonstrate the synthesis and optical characterization of the PC's building blocks: well-defined
THz Photonic Band-Gap Prisms Fabricated by Fiber Drawing
DEFF Research Database (Denmark)
Busch, Stefan F.; Xu, Lipeng; Stecher, Matthias;
2012-01-01
We suggest a novel form of polymeric based 3D photonic crystal prisms for THz frequencies which could be fabricated using a standard fiber drawing technique. The structures are modeled and designed using a finite element analyzing technique. Using this simulation software we theoretically study...
Analysis of photonic band-gap (PBG) structures using the FDTD method
DEFF Research Database (Denmark)
Tong, M.S.; Cheng, M.; Lu, Y.L.;
2004-01-01
In this paper, a number of photonic band-gap (PBG) structures, which are formed by periodic circuit elements printed oil transmission-line circuits, are studied by using a well-known numerical method, the finite-difference time-domain (FDTD) method. The results validate the band-stop filter...
DEFF Research Database (Denmark)
Gorczyca, I.; Kamińska, A.; Staszczak, G.;
2010-01-01
The pressure-induced changes in the electronic band structures of In-containing nitride alloys, InxGa1-xN and InxAl1-xN are examined experimentally as well as by ab initio calculations. It is found that the band gap pressure coefficients, dEg/dp, exhibit very large bowing with x, and calculations...
Design Analysis of An Electromagnetic Band Gap Microstrip Antenna
Directory of Open Access Journals (Sweden)
M. S. Alam
2011-01-01
Full Text Available Problem statement: Wideband compact antenna is highly demandable due to the dynamic development in the wireless technology. Approach: A simple, compact EBG microstrip antenna is proposed in this study that covers a wideband of 250 GHz and the design is conformal with the 2.45 GHz ISM band (WLAN, IEEE 802.11b and g/Bluetooth/RFID applications. Results: A 6×6 array of square unit cell formed the EBG structure which is incorporated with the radiating patch to enhance the antenna performances. This design achieved an impedance bandwidth of 10.14% (2.34-2.59 GHz at -10 dB return loss and VSWR ≤ 2. Simulated radiation pattern is almost omnideirectional. Conclusion/Recommendations: The simulated results prove the compatibility of the EBG antenna with the 2.45 GHz ISM band applications. Further enhancement of the antenna performance with improved design is under consideration.
Robust band gap and half-metallicity in graphene with triangular perforations
Gregersen, Søren Schou; Power, Stephen R.; Jauho, Antti-Pekka
2016-06-01
Ideal graphene antidot lattices are predicted to show promising band gap behavior (i.e., EG≃500 meV) under carefully specified conditions. However, for the structures studied so far this behavior is critically dependent on superlattice geometry and is not robust against experimentally realistic disorders. Here we study a rectangular array of triangular antidots with zigzag edge geometries and show that their band gap behavior qualitatively differs from the standard behavior which is exhibited, e.g., by rectangular arrays of armchair-edged triangles. In the spin unpolarized case, zigzag-edged antidots give rise to large band gaps compared to armchair-edged antidots, irrespective of the rules which govern the existence of gaps in armchair-edged antidot lattices. In addition the zigzag-edged antidots appear more robust than armchair-edged antidots in the presence of geometrical disorder. The inclusion of spin polarization within a mean-field Hubbard approach gives rise to a large overall magnetic moment at each antidot due to the sublattice imbalance imposed by the triangular geometry. Half-metallic behavior arises from the formation of spin-split dispersive states near the Fermi energy, reducing the band gaps compared to the unpolarized case. This behavior is also found to be robust in the presence of disorder. Our results highlight the possibilities of using triangular perforations in graphene to open electronic band gaps in systems with experimentally realistic levels of disorder, and furthermore, of exploiting the strong spin dependence of the system for spintronic applications.
Strain-induced band-gap engineering of graphene monoxide and its effect on graphene
Pu, H. H.; Rhim, S. H.; Hirschmugl, C. J.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J. H.
2013-02-01
Using first-principles calculations we demonstrate the feasibility of band-gap engineering in two-dimensional crystalline graphene monoxide (GMO), a recently reported graphene-based material with a 1:1 carbon/oxygen ratio. The band gap of GMO, which can be switched between direct and indirect, is tunable over a large range (0-1.35 eV) for accessible strains. Electron and hole transport occurs predominantly along the zigzag and armchair directions (armchair for both) when GMO is a direct- (indirect-) gap semiconductor. A band gap of ˜0.5 eV is also induced in graphene at the K' points for GMO/graphene hybrid systems.
Band-Gap Modulation of GeCH3 Nanoribbons Under Elastic Strain: A Density Functional Theory Study
Ma, ShengQian; Li, Feng; Jiang, ChunLing
2016-06-01
Using the density functional theory method, we researched the band-gap modulation of GeCH3 nanoribbons under uniaxial elastic strain. The results indicated that the band gap of GeCH3 nanoribbons could be tuned along two directions, namely, stretching or compressing ribbons when ɛ was changed from -10% to 10% in 6-zigzag, 10-zigzag, 13-armchair, and 17-armchair nanoribbons, respectively. The band gap greatly changed with strain. In the case of tension, the amount of change in the band gap was bigger. But in the case of compression, the gradient was steeper. The band gap had a nearly linear relationship when ɛ ranges from 0% to 10%. We also investigated if the band gap is changed with widths. The results showed variation of the band gap did not rely on widths. Therefore, the GeCH3 nanoribbons had the greatest potential application in strain sensors and optical electronics at the nanoscale.
Energy Technology Data Exchange (ETDEWEB)
Kong, Lingping; Liu, Gang; Gong, Jue; Hu, Qingyang; Schaller, Richard D.; Dera, Przemyslaw; Zhang, Dongzhou; Liu, Zhenxian; Yang, Wenge; Zhu, Kai; Tang, Yuzhao; Wang, Chuanyi; Wei, Su-Huai; Xu, Tao; Mao, Ho-kwang
2016-08-09
The organic-inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to -100% increase) under mild pressures at -0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance.
Pentamode metamaterials with tunable acoustics band gaps and large figures of merit
Wang, Zhaohong; Cai, Chengxin; Li, Qingwei; Li, Jing; Xu, Zhuo
2016-07-01
In this paper, we propose a class of pentamode metamaterials for which the frequency range of the acoustics band gaps can be tuned and large figures of merit can be obtained. The band structures of the pentamode metamaterials are calculated systematically by using the finite element method. The numerical results show that the lower edge frequency of the first acoustics band gaps of pentamode metamaterials can be tuned between 3.72 kHz and 10.6 kHz by changing the diameters of the bottom and top touch cones slightly, and the relative bandwidth of the first acoustics band gaps can also be expanded. In addition, compared with the results seen in the previous research in this area, the volume filling fraction of pentamode metamaterials can be decreased by 15.7%-24.4% and the maximum figure of merit can be increased by 39.2%.
Kong, Lingping; Liu, Gang; Gong, Jue; Hu, Qingyang; Schaller, Richard D; Dera, Przemyslaw; Zhang, Dongzhou; Liu, Zhenxian; Yang, Wenge; Zhu, Kai; Tang, Yuzhao; Wang, Chuanyi; Wei, Su-Huai; Xu, Tao; Mao, Ho-Kwang
2016-08-01
The organic-inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance. PMID:27444014
Tunable Band Gap and Conductivity Type of ZnSe/Si Core-Shell Nanowire Heterostructures
Directory of Open Access Journals (Sweden)
Yijie Zeng
2014-10-01
Full Text Available The electronic properties of zincblende ZnSe/Si core-shell nanowires (NWs with a diameter of 1.1–2.8 nm are calculated by means of the first principle calculation. Band gaps of both ZnSe-core/Si-shell and Si-core/ZnSe-shell NWs are much smaller than those of pure ZnSe or Si NWs. Band alignment analysis reveals that the small band gaps of ZnSe/Si core-shell NWs are caused by the interface state. Fixing the ZnSe core size and enlarging the Si shell would turn the NWs from intrinsic to p-type, then to metallic. However, Fixing the Si core and enlarging the ZnSe shell would not change the band gap significantly. The partial charge distribution diagram shows that the conduction band maximum (CBM is confined in Si, while the valence band maximum (VBM is mainly distributed around the interface. Our findings also show that the band gap and conductivity type of ZnSe/Si core-shell NWs can be tuned by the concentration and diameter of the core-shell material, respectively.
International Nuclear Information System (INIS)
The shape of band-to-band spectrum of graded band-gap semiconductor under conditions of nonequilibrium carrier transfer caused by the built-in quasi-electric field E is calculated. It is shown that deformation of the short wave-length part of the spectrum results from the co-ordinate dependence of the radiation recombination probability in the half-band area of a crystal. The results of calculations are confirmed by experimental measurements of photoluminescence spectra of undoped (n ≤ 1 · 1016 cm-3) graded band-gap AlxGa1-xAs solid solution with E varying in 90-650 V/cm range at 300 K
Strain-Induced Energy Band Gap Opening in Two-Dimensional Bilayered Silicon Film
Ji, Z.; Zhou, R.; Lew Yan Voon, L. C.; Zhuang, Y.
2016-10-01
This work presents a theoretical study of the structural and electronic properties of bilayered silicon film (BiSF) under in-plane biaxial strain/stress using density functional theory (DFT). Atomic structures of the two-dimensional (2-D) silicon films are optimized by using both the local-density approximation (LDA) and generalized gradient approximation (GGA). In the absence of strain/stress, five buckled hexagonal honeycomb structures of the BiSF with triangular lattice have been obtained as local energy minima, and their structural stability has been verified. These structures present a Dirac-cone shaped energy band diagram with zero energy band gaps. Applying a tensile biaxial strain leads to a reduction of the buckling height. Atomically flat structures with zero buckling height have been observed when the AA-stacking structures are under a critical biaxial strain. Increase of the strain between 10.7% and 15.4% results in a band-gap opening with a maximum energy band gap opening of ˜0.17 eV, obtained when a 14.3% strain is applied. Energy band diagrams, electron transmission efficiency, and the charge transport property are calculated. Additionally, an asymmetric energetically favorable atomic structure of BiSF shows a non-zero band gap in the absence of strain/stress and a maximum band gap of 0.15 eV as a -1.71% compressive strain is applied. Both tensile and compressive strain/stress can lead to a band gap opening in the asymmetric structure.
The calculation of the band structure in 3D phononic crystal with hexagonal lattice
Energy Technology Data Exchange (ETDEWEB)
Aryadoust, Mahrokh; Salehi, H. [University of Shahid Chamran, Ahvaz (Iran, Islamic Republic of). Dept. of Physics
2015-07-01
In this article, the propagation of acoustic waves in the phononic crystals (PCs) of three dimensions with the hexagonal (HEX) lattice is studied theoretically. The PCs are constituted of nickel (Ni) spheres embedded in epoxy. The calculations of the band structure and the density of states are performed using the plane wave expansion (PWE) method in the irreducible part of the Brillouin zone (BZ). In this study, we analyse the dependence of the band structures inside (the complete band gap width) on c/a and filling fraction in the irreducible part of the first BZ. Also, we have analysed the band structure of the ALHA and MLHKM planes. The results show that the maximum width of absolute elastic band gap (AEBG) (0.045) in the irreducible part of the BZ of HEX lattice is formed for c/a=6 and filling fraction equal to 0.01. In addition, the maximum of the first and second AEBG widths are 0.0884 and 0.0474, respectively, in the MLHKM plane, and the maximum of the first and second AEBG widths are 0.0851 and 0.0431, respectively, in the ALHA plane.
Wide band-gap materials for high power electronics
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The wide gap semiconductors are the basis for the third generation of microelectronics and specially for the high end of the temperature range. In this presentation we will review the prospects and status of two members of this group: Diamond and Silicon Carbide (SiC). The two are at different stages of technological development and their respective modes of application at present are quite different. SiC devices can operate at up to 105 deg C. High power and high frequency devices have been demonstrated. Diamond is not yet ready for real electronic devices but its many extreme properties find their applications in several cases. The prospects of the future applications will be described in view of the semiconducting characteristics of these materials
International Nuclear Information System (INIS)
The photonic band gap of obliquely incident terahertz electromagnetic waves in a one-dimensional plasma photonic crystal is studied. The periodic structure consists of lossless dielectric and inhomogeneous plasma with a parabolic density profile. The dispersion relation and the THz wave transmittance are analyzed based on the electromagnetic equations and transfer matrix method. The dependence of effective plasma frequency and photonic band gap characteristics on dielectric and plasma thickness, plasma density, and incident angle are discussed in detail. A theoretical calculation for effective plasma frequency is presented and compared with numerical results. Results of these two methods are in good agreement
Specific heat of single crystal MgB2: a two-band superconductor with two different anisotropies.
Bouquet, F; Wang, Y; Sheikin, I; Plackowski, T; Junod, A; Lee, S; Tajima, S
2002-12-16
Heat-capacity measurements of a 39 microg MgB2 single crystal in fields up to 14 T and below 3 K allow the determination of the low-temperature linear term of the specific heat, its field dependence, and its anisotropy. Our results are compatible with two-band superconductivity, the band carrying the smaller gap being isotropic, that carrying the larger gap having an anisotropy of approximately 5. Three different upper critical fields are thus needed to describe the superconducting state of MgB2.
Evanescent waves and deaf bands in sonic crystals
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V. Romero-García
2011-12-01
Full Text Available The properties of sonic crystals (SC are theoretically investigated in this work by solving the inverse problem k(ω using the extended plane wave expansion (EPWE. The solution of the resulting eigenvalue problem gives the complex band structure which takes into account both the propagating and the evanescent modes. In this work we show the complete mathematical formulation of the EPWE for SC and the supercell approximation for its use in both a complete SC and a SC with defects. As an example we show a novel interpretation of the deaf bands in a complete SC in good agreement with multiple scattering simulations.
Evanescent waves and deaf bands in sonic crystals
Romero-García, V.; Garcia-Raffi, L. M.; Sánchez-Pérez, J. V.
2011-12-01
The properties of sonic crystals (SC) are theoretically investigated in this work by solving the inverse problem k(ω) using the extended plane wave expansion (EPWE). The solution of the resulting eigenvalue problem gives the complex band structure which takes into account both the propagating and the evanescent modes. In this work we show the complete mathematical formulation of the EPWE for SC and the supercell approximation for its use in both a complete SC and a SC with defects. As an example we show a novel interpretation of the deaf bands in a complete SC in good agreement with multiple scattering simulations.
Mini-stop bands in single heterojunction photonic crystal waveguides
Directory of Open Access Journals (Sweden)
N. Shahid
2013-03-01
Full Text Available Spectral characteristics of mini-stop bands (MSB in line-defect photonic crystal (PhC waveguides and in heterostructure PhC waveguides having one abrupt interface are investigated. Tunability of the MSB position by air-fill factor heterostructure PhC waveguides is utilized to demonstrate different filter functions, at optical communication wavelengths, ranging from resonance-like to wide band pass filters with high transmission. The narrowest filter realized has a resonance-like transmission peak with a full width at half maximum of 3.4 nm. These devices could be attractive for coarse wavelength selection (pass and drop and for sensing applications.
Directory of Open Access Journals (Sweden)
Arafa H. Aly
2014-03-01
Full Text Available In the present work, we describe an efficient study of the stop-band/pass-band dispersive behavior of 1D phononic crystal. We have treated the propagation and localization of in-plane (P and S/anti-plane (SH shear waves in perfect/defect phononic crystals. Based on the transfer matrix method and Bloch theory, the dispersion relations were calculated and plotted for both SH and in-plane waves. In order to confirm the results, the reflection coefficients were plotted for in-plane waves and compared with dispersion relations results. The effect of several parameters such as type and thickness of defect layer on the waves localization had be taken in account. Moreover, we have studied the effect of temperature on the phononic band gaps for SH and in-plane waves. These results can be useful in using phononic crystals as temperature sensor materials. Also, the presented analysis can be extended to acoustic filters and wave multiplexer.
International Nuclear Information System (INIS)
Mixed crystals of TlInS(2-x)Sex system for 0.0≤x≤1.0 were grown using directional freezing method. The effect of composition and temperature on the optical energy gap for TlInS(2-x)Sex layered crystals were investigated in the temperature range of 10-300 K. The variation of the optical absorption coefficient with energy for the each composition near the band edge was analysed by Elliot's model and compositional dependence of the band gap energies were obtained. For the interpretation of the phase transition (PT) of grown crystals, temperature dependent optical pseudogap E*g was used instead of the optical band gap due to the complex nature of the band edge at the low absorption coefficients. The pseudogap plots having singularities for the compositions of 0.0≤x≤0.2 were associated with PT in the investigated temperature region. However, no singularities were observed for the compositions of 0.4 ≤x≤1.0 , thus confirming the absence of PT in the temperature interval under investigation
Direct Band Gap Gallium Antimony Phosphide (GaSbxP(1-x)) Alloys.
Russell, H B; Andriotis, A N; Menon, M; Jasinski, J B; Martinez-Garcia, A; Sunkara, M K
2016-01-01
Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1-2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSbxP(1-x) alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSbxP(1-x). Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSbxP(1-x) nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields. PMID:26860470
Direct Band Gap Gallium Antimony Phosphide (GaSbxP1-x) Alloys
Russell, H. B.; Andriotis, A. N.; Menon, M.; Jasinski, J. B.; Martinez-Garcia, A.; Sunkara, M. K.
2016-02-01
Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1-2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSbxP1-x alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSbxP1-x. Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSbxP1-x nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields.
Residual stress dependant anisotropic band gap of various (hkl) oriented BaI{sub 2} films
Energy Technology Data Exchange (ETDEWEB)
Kumar, Pradeep; Gulia, Vikash; Vedeshwar, Agnikumar G., E-mail: agni@physics.du.ac.in, E-mail: agvedeshwar@gmail.com [Thin Film Laboratory, Department of Physics and Astrophysics, University of Delhi, Delhi-110007 (India)
2013-11-21
The thermally evaporated layer structured BaI{sub 2} grows in various completely preferred (hkl) film orientations with different growth parameters like film thickness, deposition rate, substrate temperature, etc. which were characterized by structural, morphological, and optical absorption measurements. Structural analysis reveals the strain in the films and the optical absorption shows a direct type band gap. The varying band gaps of these films were found to scale linearly with their strain. The elastic moduli and other constants were also calculated using Density Functional Theory (DFT) formalism implemented in WIEN2K code for converting the strain into residual stress. Films of different six (hkl) orientations show stress free anisotropic band gaps (2.48–3.43 eV) and both positive and negative pressure coefficients. The negative and positive pressure coefficients of band gap are attributed to the strain in I-I (or Ba-Ba or both) and Ba-I distances along [hkl], respectively. The calculated band gaps are also compared with those experimentally determined. The average pressure coefficient of band gap of all six orientations (−0.071 eV/GPa) found to be significantly higher than that calculated (−0.047 eV/GPa) by volumetric pressure dependence. Various these issues have been discussed with consistent arguments. The electron effective mass m{sub e}{sup *}=0.66m{sub 0} and the hole effective mass m{sub h}{sup *}=0.53m{sub 0} have been determined from the calculated band structure.
Microscopic theoretical model study of band gap opening in AA-stacked bi-layer graphene
Sahu, Sivabrata; Parashar, S. K. S.; Rout, G. C.
2016-05-01
We address here a tight-binding theoretical model calculation for AA-stacked bi-layer graphene taking into account of a biased potential between two layers to study the density of states and the band dispersion within the total Brillouin zone. We have calculated the electronic Green's function for electron operator corresponding to A and B sub lattices by Zubarev's Green's function technique from which the electronic density of states and the electron band energy dispersion are calculated. The numerically computed density of states and band energy dispersions are investigated by tuning the biased potential to exhibit the band gap by varying the different physical parameters.
Sorokin, Vladislav S
2016-04-01
The paper concerns determining frequency band-gaps for longitudinal wave motion in a periodic waveguide. The waveguide may be considered either as an elastic layer with variable thickness or as a rod with variable cross section. As a result, widths and locations of all frequency band-gaps are determined by means of the method of varying amplitudes. For the general symmetric corrugation shape, the width of each odd band-gap is controlled only by one harmonic in the corrugation series with its number being equal to the number of the band-gap. Widths of even band-gaps, however, are influenced by all the harmonics involved in the corrugation series, so that the lower frequency band-gaps can emerge. These are band-gaps located below the frequency corresponding to the lowest harmonic in the corrugation series. For the general non-symmetric corrugation shape, the mth band-gap is controlled only by one, the mth, harmonic in the corrugation series. The revealed insights into the mechanism of band-gap formation can be used to predict locations and widths of all frequency band-gaps featured by any corrugation shape. These insights are general and can be valid also for other types of wave motion in periodic structures, e.g., transverse or torsional vibration. PMID:27106336
Energy Technology Data Exchange (ETDEWEB)
Zanatta, G.; Gottfried, C. [Departamento de Bioquímica, Universidade Federal do Rio Grande do Sul, 90035-003 Porto Alegre-RS (Brazil); Silva, A. M. [Universidade Estadual do Piauí, 64260-000 Piripiri-Pi (Brazil); Caetano, E. W. S., E-mail: ewcaetano@gmail.com [Instituto de Educação, Ciência e Tecnologia do Ceará, 60040-531 Fortaleza-CE (Brazil); Sales, F. A. M.; Freire, V. N. [Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, 60455-760 Fortaleza-CE (Brazil)
2014-03-28
Results of optical absorption measurements are presented together with calculated structural, electronic, and optical properties for the anhydrous monoclinic L-asparagine crystal. Density functional theory (DFT) within the generalized gradient approximation (GGA) including dispersion effects (TS, Grimme) was employed to perform the calculations. The optical absorption measurements revealed that the anhydrous monoclinic L-asparagine crystal is a wide band gap material with 4.95 eV main gap energy. DFT-GGA+TS simulations, on the other hand, produced structural parameters in very good agreement with X-ray data. The lattice parameter differences Δa, Δb, Δc between theory and experiment were as small as 0.020, 0.051, and 0.022 Å, respectively. The calculated band gap energy is smaller than the experimental data by about 15%, with a 4.23 eV indirect band gap corresponding to Z → Γ and Z → β transitions. Three other indirect band gaps of 4.30 eV, 4.32 eV, and 4.36 eV are assigned to α3 → Γ, α1 → Γ, and α2 → Γ transitions, respectively. Δ-sol computations, on the other hand, predict a main band gap of 5.00 eV, just 50 meV above the experimental value. Electronic wavefunctions mainly originating from O 2p–carboxyl, C 2p–side chain, and C 2p–carboxyl orbitals contribute most significantly to the highest valence and lowest conduction energy bands, respectively. By varying the lattice parameters from their converged equilibrium values, we show that the unit cell is less stiff along the b direction than for the a and c directions. Effective mass calculations suggest that hole transport behavior is more anisotropic than electron transport, but the mass values allow for some charge mobility except along a direction perpendicular to the molecular layers of L-asparagine which form the crystal, so anhydrous monoclinic L-asparagine crystals could behave as wide gap semiconductors. Finally, the calculations point to a high degree of optical
Band-dropping via coupled photonic crystal waveguides
Bayindir, Mehmet; Ozbay, Ekmel
2002-11-01
We observe the dropping of electromagnetic waves having a specific frequency or a certain frequency band in two-dimensional dielectric photonic crystals. The single frequency is dropped via cavity waveguide coupling. Tunability of the demultiplexing mode can be achieved by modifying the cavity properties. The band-dropping phenomenon is achieved by introducing interaction between an input planar, or coupled-cavity, waveguide and the output coupled-cavity waveguides (CCWs). The dropping band can be tuned by changing the coupling strength between the localized cavity modes of the output CCWs. We also calculate the transmission spectra and the field patterns by using the finite-difference-time-domain (FDTD)method. Calculated results agree well with the microwave measurements.
International Nuclear Information System (INIS)
Calculations relative to the band-gap energy shift and carrier spatial density in cadmium oxide are performed in terms of the oxygen partial pressure and substrate temperature relative to the deposition process in the crystal growth of the above material, starting from the consideration of the Fermi energy of an exciton gas. In particular, the band-gap shift experienced by cadmium oxide in terms of the corresponding partial pressure of oxygen is considered as well as the electron spatial density as a function of the pressure in question. Influence of temperature is discussed by estimating the average rate of variation of the band-gap shift versus temperature. In addition, the sensitivity of the above-mentioned shift to temperature is studied by means of a suitable parameter
International Nuclear Information System (INIS)
New bismuth borophosphate Bi4BPO10 was obtained by spontaneous crystallization from the melt of correspondent composition at 804 °C. Crystal structure with orthorhombic lattice parameters: a = 22.5731(3) Å, b = 14.0523(2) Å, c = 5.5149(1) Å, V = 1749.34(4), Z = 8, SG Pcab was determined by X-ray powder diffraction technique. The [Bi2O2]2+ -layers, which are typical for bismuth oxide compounds, transform into cationic endless strips of 4 bismuth atoms width directed along the c-axis in Bi4BPO10. The strips combining stacks are separated by flat triangle [BO3]3− -anions within stacks. Neighboring stacks are separated by tetrahedral [PO4]3−-anions and shifted relatively to each other. Bismuth atoms are placed in 5–7 vertex oxygen irregular polyhedra. Bi4BPO10 is stable up to 812 °C, then melts according to the peritectic law. The absorption spectrum in the range 350–700 nm was obtained and the width of the forbidden band was estimated as 3.46 eV. The band electronic structure of Bi4BPO10 was modeled using DFT approach. The calculated band gap (3.56 eV) is in good agreement with the experimentally obtained data. - Graphical abstract: Display Omitted - Highlights: • New bismuth borophosphate with composition Bi4BPO10 was synthesized. • The crystal structure was determined by X-ray powder diffraction technique. • Bismuth-oxygen part [Bi4O3]6+ forms endless strips of 4 bismuth atoms width. • Electronic structure was modeled by DFT method. • The calculated band gap (3.56 eV) is very close to the experimental one (3.46 eV)
Synthesis of copper quantum dots by chemical reduction method and tailoring of its band gap
Directory of Open Access Journals (Sweden)
P. G. Prabhash
2016-05-01
Full Text Available Metallic copper nano particles are synthesized with citric acid and CTAB (cetyltrimethylammonium bromide as surfactant and chlorides as precursors. The particle size and surface morphology are analyzed by High Resolution Transmission Electron Microscopy. The average size of the nano particle is found to be 3 - 10 nm. The optical absorption characteristics are done by UV-Visible spectrophotometer. From the Tauc plots, the energy band gaps are calculated and because of their smaller size the particles have much higher band gap than the bulk material. The energy band gap is changed from 3.67 eV to 4.27 eV in citric acid coated copper quantum dots and 4.17 eV to 4.52 eV in CTAB coated copper quantum dots.
Band gap engineering in polymers through chemical doping and applied mechanical strain.
Lanzillo, Nicholas A; Breneman, Curt M
2016-08-17
We report simulations based on density functional theory and many-body perturbation theory exploring the band gaps of common crystalline polymers including polyethylene, polypropylene and polystyrene. Our reported band gaps of 8.6 eV for single-chain polyethylene and 9.1 eV for bulk crystalline polyethylene are in excellent agreement with experiment. The effects of chemical doping along the polymer backbone and side-groups are explored, and the use mechanical strain as a means to modify the band gaps of these polymers over a range of several eV while leaving the dielectric constant unchanged is discussed. This work highlights some of the opportunities available to engineer the electronic properties of polymers with wide-reaching implications for polymeric dielectric materials used for capacitive energy storage. PMID:27324304
Band gap of β-PtO2 from first-principles
Directory of Open Access Journals (Sweden)
Yong Yang
2012-06-01
Full Text Available We studied the band gap of β-PtO2 using first-principles calculations based on density functional theory (DFT. The results are obtained within the framework of the generalized gradient approximation (GGA, GGA+U, GW, and the hybrid functional methods. For the different types of calculations, the calculated band gap increases from ∼0.46 eV to 1.80 eV. In particular, the band gap by GW (conventional and self-consistent calculation shows a tendency of converging to ∼1.25 ± 0.05 eV. The effect of on-site Coulomb interaction on the bonding characteristics is also analyzed.
Mechanical Properties of a Library of Low-Band-Gap Polymers
DEFF Research Database (Denmark)
Roth, Bérenger; Savagatrup, Suchol; de los Santos, Nathaniel V.;
2016-01-01
of low-band-gap polymers to better understand the connection between molecular structures and mechanical properties in order to design conjugated polymers that permit mechanical robustness and even extreme deformability. While one of the principal conclusions of these experiments is that the structure......The mechanical properties of low-band-gap polymers are important for the long-term survivability of roll to-roll processed organic electronic devices. Such devices, e.g., solar cells, displays, and thin-film transistors, must survive the rigors of roll-to-roll coating and also thermal...... and mechanical forces in the outdoor environment and in stretchable and ultraflexible form factors. This paper measures the stiffness (tensile modulus), ductility (crack-onset strain), or both of a combinatorial library of 51 low-band-gap polymers. The purpose of this study is to systematically screen a library...
Band gap engineering in polymers through chemical doping and applied mechanical strain
Lanzillo, Nicholas A.; Breneman, Curt M.
2016-08-01
We report simulations based on density functional theory and many-body perturbation theory exploring the band gaps of common crystalline polymers including polyethylene, polypropylene and polystyrene. Our reported band gaps of 8.6 eV for single-chain polyethylene and 9.1 eV for bulk crystalline polyethylene are in excellent agreement with experiment. The effects of chemical doping along the polymer backbone and side-groups are explored, and the use mechanical strain as a means to modify the band gaps of these polymers over a range of several eV while leaving the dielectric constant unchanged is discussed. This work highlights some of the opportunities available to engineer the electronic properties of polymers with wide-reaching implications for polymeric dielectric materials used for capacitive energy storage.
An efficient method of DFT/LDA band-gap correction
Scharoch, Pawel; Winiarski, Maciej
2013-12-01
It has been shown that the underestimated by DFT/LDA(GGA) band-gap can be efficiently corrected by an averaging procedure of transition energies over a region close to the direct band-gap transition, which we call the Δ(EIG) method (the differences in the Kohn-Sham eigenvalues). For small excitations the averaging appears to be equivalent to the Δ(SCF) approach (differences in the self-consistent energies), which is a consequence of Janak’s theorem and has been confirmed numerically. The Gaussian distribution in k-space for electronic excitation has been used (occupation numbers in the Δ(SCF) or eigenenergy sampling in the Δ(EIG)). A systematic behavior of the k-space localization parameter σk correcting the band-gap has been observed in numerical experiments. On that basis some sampling schemes for band-gap correction have been proposed and tested in the prediction of the band-gap behavior in InxGa(1-x)N semiconducting alloy, and a very good agreement with independent calculations has been obtained. In the context of the work the issue of electron localization in the r-space has been discussed which, as it has been predicted by Mori-Sánchez et al. [P. Mori-Sánchez, A.J. Cohen, W. Yang, Phys. Rev. Lett. 100 (2008) 146401], should reduce the effect of the convex behavior of the LDA/GGA functionals and improve the band-gap prediction within DFT/LDA(GGA). A scheme for electron localization in r-space has been suggested.
International Nuclear Information System (INIS)
The IR emission and the corresponding low energy excitation spectra of the wide band gap semimagnetic semiconductors Zn1-xMnxTe, Cd1-xMnxTe, Zn1-xMnxSe, Cd1-xMnxSe, Zn1-xMnxS, and Cd1-xMnxS are critically discussed in the light of the models proposed until now. The similar shape of these spectra and the comparison with the spectra due to Mn in tetrahedral coordination lead to the conclusion that the longwave emission and excitation bands of these substances can tentatively be interpreted as due to cubically coordinated Mn2+ ions on interstitial sites or in octahedrally coordinated MnTe, MnSe, and MnS cluster. The crystal field parameters B, C, and Dq of these Mn2+ ions (Td or Oh symmetry) are determined on the basis of the experimental data. (author)
Comment on "Quantum electrodynamics near a photonic band gap photon bound states and dressed atoms"
Moroz, A V
1999-01-01
The applicability of the so-called isotropic and anisotropic complete photonic-band-gap (CPBG) models [S. John and J. Wang, Phys. Rev. Lett. {\\bf 64}, 2418 (1990)] to capture essential features of the spontaneous emission (SE) of a fluorescent atom or molecule near a band-gap-edge of a CPBG structure is discussed. It is argued that, depending on the source position within a unit cell, the SE near the same CPBG edge can be either strongly enhanced or strongly depressed.
Role of excited states in Shockley-Read-Hall recombination in wide-band-gap semiconductors
Alkauskas, Audrius; Dreyer, Cyrus E.; Lyons, John L.; Van de Walle, Chris G.
2016-05-01
Defect-assisted recombination is an important limitation on efficiency of optoelectronic devices. However, since nonradiative capture rates decrease exponentially with the energy of the transition, the mechanisms by which such recombination can take place in wide-band-gap materials are unclear. Using electronic structure calculations we uncover the crucial role of electronic excited states in nonradiative recombination processes. The impact is elucidated with examples for the group-III nitrides, for which accumulating experimental evidence indicates that defect-assisted recombination limits efficiency. Our work provides insights into the physics of nonradiative recombination, and the mechanisms are suggested to be ubiquitous in wide-band-gap semiconductors.
Directory of Open Access Journals (Sweden)
S.V. Kryuchkov
2015-03-01
Full Text Available The power of the elliptically polarized electromagnetic radiation absorbed by band-gap graphene in presence of constant magnetic field is calculated. The linewidth of cyclotron absorption is shown to be non-zero even if the scattering is absent. The calculations are performed analytically with the Boltzmann kinetic equation and confirmed numerically with the Monte Carlo method. The dependence of the linewidth of the cyclotron absorption on temperature applicable for a band-gap graphene in the absence of collisions is determined analytically.
Synthesis of Novel Low-band-Gap Thiophene-based Oligomers and Copolymers
Institute of Scientific and Technical Information of China (English)
I.Kmínek; V.Cimrová; D.Vyprachicky; P.Pavlaková
2007-01-01
1 Results In this contribution,the synthesis and characterization of low-band-gap oligothiophenes and copolymers based on fluorene and thiophene units are reported.First,5,5′-dibromo derivative of 1 was synthesised and used as a building block for synthesis of quinquethiophene and septithiophene analogues of 1 and the synthesis of a novel low band-gap polymer - soluble alternating copolymer 3,which was prepared by the Suzuki coupling of 5,5′-dibromo derivative of 1 with bis(propane-1,3-diyl) 9,9-bis(2-...
The scaling of the effective band gaps in indium-arsenide quantum dots and wires.
Wang, Fudong; Yu, Heng; Jeong, Sohee; Pietryga, Jeffrey M; Hollingsworth, Jennifer A; Gibbons, Patrick C; Buhro, William E
2008-09-23
Colloidal InAs quantum wires having diameters in the range of 5-57 nm and narrow diameter distributions are grown from Bi nanoparticles by the solution-liquid-solid (SLS) mechanism. The diameter dependence of the effective band gaps (DeltaE(g)s) in the wires is determined from photoluminescence spectra and compared to the experimental results for InAs quantum dots and rods and to the predictions of various theoretical models. The DeltaE(g) values for InAs quantum dots and wires are found to scale linearly with inverse diameter (d(-1)), whereas the simplest confinement models predict that DeltaE(g) should scale with inverse-square diameter (d(-2)). The difference in the observed and predicted scaling dimension is attributed to conduction-band nonparabolicity induced by strong valence-band-conduction-band coupling in the narrow-gap InAs semiconductor.
Local resonance phononic band gaps in modified two-dimensional lattice materials
Institute of Scientific and Technical Information of China (English)
Wei Liu; Ji-Wei Chen; Xian-Yue Su
2012-01-01
In this paper,modified two-dimensional periodic lattice materials with local resonance phononic band gaps are designed and investigated.The design concept is to introduce some auxiliary structures into conventional periodic lattice materials.Elastic wave propagation in this kind of modified two-dimensional lattice materials is studied using a combination of Bloch's theorem with finite element method.The calculated frequency band structures of illustrative modified square lattice materials reveal the existence of frequency band gaps in the low frequency region due to the introduction of the auxiliary structures.The mechanism underlying the occurrence of these frequency band gaps is thoroughly discussed and natural resonances of the auxiliary structures are validated to be the origin.The effect of geometric parameters of the auxiliary structures on the width of the local resonance phononic band gaps is explored.Finally,a conceptual broadband vibration-insulating structure based on the modified lattice materials is designed and its capability is demonstrated.The present work is anticipated to be useful in designing structures which can insulate mechanical vibrations within desired frequency ranges.
Band Gap Engineering of Cd1-xBexSe Alloys
Djillali Bensaid; Mohammed Ameri; Nadia Benseddik; Ali Mir; Nour Eddine Bouzouira; Fethi Benzoudji
2014-01-01
The structural and electronic properties of the ternary Cd1-xBexSe alloys have been calculated using the full-potential linear muffin-tin-orbital (FP-LMTO) method based on density functional theory within local density approximation (LDA). The calculated equilibrium lattice constants and bulk moduli are compared with previous results. The concentration dependence of the electronic band structure and the direct and indirect band gaps are investigated. Moreover, the refractive index and the opt...
Wang, Qiang; Xiao, Meng; Liu, Hui; Zhu, Shining; Chan, C. T.
2016-01-01
The Zak phase labels the topological property of one-dimensional Bloch bands. Here we propose a scheme and experimentally measure the Zak phase in a photonic system. The Zak phase of a bulk band is related to the topological properties of the two band gaps sandwiching this band, which in turn can be inferred from the existence or absence of an interface state. Using a reflection spectrum measurement, we determined the existence of interface states in the gaps and then obtained the Zak phases. The knowledge of Zak phases can also help us predict the existence of interface states between a metasurface and a photonic crystal. By manipulating the property of the metasurface, we can further tune the excitation frequency and the polarization of the interface state.
Wang, Qiang; Liu, Hui; Zhu, Shining; Chan, C T
2016-01-01
Zak phase labels the topological property of one-dimensional Bloch Bands. Here we propose a scheme and experimentally measure the Zak phase in a photonic system. The Zak phase of a bulk band of is related to the topological properties of the two band gaps sandwiching this band, which in turn can be inferred from the existence or absence of an interface state. Using reflection spectrum measurement, we determined the existence of interface states in the gaps, and then obtained the Zak phases. The knowledge of Zak phases can also help us predict the existence of interface states between a metasurface and a photonic crystal. By manipulating the property of the metasurface, we can further tune the excitation frequency and the polarization of the interface state.
Relationship between band gap and bond length alternation in organic conjugated polymers
Bredas, J. L.
1985-04-01
A description is given of calculations of the evolution of the band gap as a function of geometry in conjugated polymers based on aromatic rings; polyparaphenylene, polypyrrole, polythiophene. The results demonstrate that the gap decreases as a function of increasing quinoid character of the backbone and is thus not minimal in the case of zero bond length alternation, in contrast to the situation found in polyacetylene-like compounds. The consequences of these results are stressed for the understanding of the effects of doping and for the design of new organic polymers with small gaps.
Hofstadter butterflies and magnetically induced band-gap quenching in graphene antidot lattices
DEFF Research Database (Denmark)
Pedersen, Jesper Goor; Pedersen, Thomas Garm
2013-01-01
We study graphene antidot lattices (GALs) in magnetic fields. Using a tight-binding model and a recursive Green's function technique that we extend to deal with periodic structures, we calculate Hofstadter butterflies of GALs. We compare the results to those obtained in a simpler gapped graphene...... model. A crucial difference emerges in the behavior of the lowest Landau level, which in a gapped graphene model is independent of magnetic field. In stark contrast to this picture, we find that in GALs the band gap can be completely closed by applying a magnetic field. While our numerical simulations...
Indian Academy of Sciences (India)
Navinder Singh
2005-01-01
A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons (holes) prepared in the conduction (valence) band of a polar indirect band-gap semiconductor, which has been subjected to homogeneous photoexcitation by a femtosecond laser pulse. The model assumes that the pulsed photoexcitation creates two distinct but spatially interpenetrating electron and hole non-equilibrium subsystems that initially relax non-radiatively through the electron (hole)–phonon processes towards the conduction (valence) band minimum (maximum), and finally radiatively through the phonon-assisted electron–hole recombination across the band-gap, which is a relatively slow process. This leads to an accumulation of electrons (holes) at the conduction (valence) band minimum (maximum). The resulting peaking of the carrier density and the entire evolution of the hot electron (hole) distribution has been calculated. The latter may be time resolved by a pump-probe study. The model is particularly applicable to a divided (nanometric) polar indirect band-gap semiconductor with a low carrier concentration and strong electron–phonon coupling, where the usual two-temperature model [1–4] may not be appropriate.
Band gap engineering of a soft inorganic compound PbI2 by incommensurate van der Waals epitaxy
Wang, Yiping; Sun, Yi-Yang; Zhang, Shengbai; Lu, Toh-Ming; Shi, Jian
2016-01-01
Van der Waals epitaxial growth had been thought to have trivial contribution on inducing substantial epitaxial strain in thin films due to its weak nature of van der Waals interfacial energy. Due to this, electrical and optical structure engineering via van der Waals epitaxial strain has been rarely studied. In this report, we show that significant band structure engineering could be achieved in a soft thin film material PbI2 via van der Waals epitaxy. The thickness dependent photoluminescence of single crystal PbI2 flakes was studied and attributed to the substrate-film coupling effect via incommensurate van der Waals epitaxy. It is proposed that the van der Waals strain is resulted from the soft nature of PbI2 and large van der Waals interaction due to the involvement of heavy elements. Such strain plays vital roles in modifying the band gap of PbI2. The deformation potential theory is used to quantitatively unveil the correlation between thickness, strain, and band gap change. Our hypothesis is confirmed by the subsequent mechanical bending test and Raman characterization.
First-principles study of doping and band gap anomalies in delafossite transparent conductive oxides
Nie, Xiliang; Wei, Su-Huai; Zhang, S. B.
2002-03-01
Despite the success of n-type transparent conductive oxides (TCOs) in flat panel display, solar cell, and touch panel applications, p-type TCOs are rare. Recently, however, several p-type TCO films such as SrCu_2O2 and delafossite CuM^IIIO2 where M^III=Al, Ga, and In have been successfully demonstrated. These materials have some very unusual properties: (i) The band gaps increase with increasing atomic number. This contradicts the trend in normal semiconductors including those with the same group III elements. (ii) Bipolar doping (namely both p- and n-type doping) is observed only in the largest band gap CuInO_2. This contradicts the doping limit rule [1] as no similar trend has ever been observed in any other semiconductors. Here, using first-principles method, we calculate the electronic and optical properties of CuM^IIIO_2. We found that the fundamental direct gap decreases with the increase of the atomic number, following the general trend in conventional semiconductors. But the optical band gap (which has been used in the above experiments to define the band gap) follows an opposite trend. This happens because optical transition at the fundamental direct gap is forbidden as both states have the same parity (even). On the other hand, CuInO2 has exceptionally low conduction band minimum (CBM), 1.48 eV lower than CuAlO_2. According to the doping limit rule [1], low CBM implies good n-type dopability. Our findings explain the puzzling combination of good transparency with bipolar dopability in CuInO_2. This work was supported by the U. S. DOE-SC-BES under contract No. DE-AC36-99GO10337. [1] S. B. Zhang, S. -H. Wei, and A. Zunger, J. Appl. Phys. 83, 3192 (1998).
Investigation of the Band Structure of Graphene-Based Plasmonic Photonic Crystals
Directory of Open Access Journals (Sweden)
Pingping Qiu
2016-09-01
Full Text Available In this paper, one-dimensional (1D and two-dimensional (2D graphene-based plasmonic photonic crystals (PhCs are proposed. The band structures and density of states (DOS have been numerically investigated. Photonic band gaps (PBGs are found in both 1D and 2D PhCs. Meanwhile, graphene-based plasmonic PhC nanocavity with resonant frequency around 175 THz, is realized by introducing point defect, where the chemical potential is from 0.085 to 0.25 eV, in a 2D PhC. Also, the bending wvaguide and the beam splitter are realized by introducing the line defect into the 2D PhC.
Mittal, Mona; Jana, Atanu; Sarkar, Sagar; Mahadevan, Priya; Sapra, Sameer
2016-08-18
A few approaches have been employed to tune the band gap of colloidal organic-inorganic trihalide perovskites (OTPs) nanocrystals by changing the halide anion. However, to date, there is no report of electronic structure tuning of perovskite NCs upon changing the organic cation. We report here, for the first time, the room temperature colloidal synthesis of (EA)x(MA)1-xPbBr3 nanocrystals (NCs) (where, x varies between 0 and 1) to tune the band gap of hybrid organic-inorganic lead perovskite NCs from 2.38 to 2.94 eV by varying the ratio of ethylammonium (EA) and methylammonium (MA) cations. The tuning of band gap is confirmed by electronic structure calculations within density functional theory, which explains the increase in the band gap upon going toward larger "A" site cations in APbBr3 NCs. The photoluminescence quantum yield (PLQY) of these NCs lies between 5% to 85% and the average lifetime falls in the range 1.4 to 215 ns. A mixture of MA cations and its higher analog EA cations provide a versatile tool to tune the structural as well as optoelectronic properties of perovskite NCs. PMID:27494515
A novel benzodipyrrolidone-based low band gap polymer for organic solar cells
DEFF Research Database (Denmark)
Yue, Wei; Huang, Xiaodong; Yuan, Jianyu;
2013-01-01
A low band gap polymer PBDPDP-DTP, with alternating benzodipyrrolidone (BDP) unit and dithienopyrrole, was synthesized and characterized. A PCE of 2.60%and a Voc of up to 0.74 V were realized in PSCs, which demonstrated the strong potential of BDP as the electron deficient unit in the design...... of donor–acceptor conjugated polymers for PSCs....
OPTICAL BAND GAP AND CONDUCTIVITY MEASUREMENTS OF POLYPYRROLE-CHITOSAN COMPOSITE THIN FILMS
Institute of Scientific and Technical Information of China (English)
Mahnaz M.Abdi; H.N.M.Ekramul Mahmud; Luqman Chuah Abdullah; Anuar Kassim; Mohamad Zaki Ab.Rahman; Josephine Liew Ying Chyi
2012-01-01
Electrical conductivity and optical properties of polypyrrole-chitosan (PPy-CHI) conducting polymer composites have been investigated to determine the optical transition characteristics and energy band gap of composite films.The two electrode method and Ⅰ-Ⅴ characteristic technique were used to measure the conductivity of the PPy-CHI thin films,and the optical band gap was obtained from their ultraviolet absorption edges.Depending upon experimental parameter,the optical band gap (Eg) was found within 1.30-2.32 eV as estimated from optical absorption data.The band gap of the composite films decreased as the CHI content increased.The room temperature electrical conductivity of PPy-CHI thin films was found in the range of 5.84 × 10-7-15.25 × 10-7 S.cm-1 depending on the chitosan content.The thermogravimetry analysis (TGA)showed that the CHI can improve the thermal stability of PPy-CHI composite films.
Photonic band gaps in materials with triply periodic surfaces and related tubular structures
Michielsen, K; Kole, JS
2003-01-01
We calculate the photonic band gap of triply periodic bicontinuous cubic structures and of tubular structures constructed from the skeletal graphs of triply periodic minimal surfaces. The effect of the symmetry and topology of the periodic dielectric structures on the existence and the characteristi
Strongly reduced band gap in NiMn2O4 due to cation exchange
International Nuclear Information System (INIS)
NiMn2O4 is extensively used as a basis material for temperature sensors due to its negative temperature coefficient of resistance (NTCR), which is commonly attributed to the hopping mechanism involving coexisting octahedral-site Mn4+ and Mn3+. Using density-functional theory + Hubbard U calculations, we identify a ferrimagnetic inverse spinel phase as the collinear ground state of NiMn2O4. By a 12.5% cation exchange, a mixed phase with slightly higher energy can be constructed, accompanied by the formation of an impurity-like band in the original 1 eV band gap. This impurity-like band reduces the gap to 0.35 eV, suggesting a possible source of NTCR. - Highlights: • Density functional based calculations were used to study collinear phase of NiMn2O4. • The ground-state structure is a ferrimagnetic inverse spinel phase. • The tetrahedral and octahedral Mn cations have ferromagnetic interactions. • A 12.5% cation exchange introduces an impurity-like band in the original 1 eV gap. • The 0.35 eV gap suggests a source of negative temperature coefficient of resistance
DEFF Research Database (Denmark)
Carlé, Jon Eggert; Jørgensen, Mikkel; Manceau, Matthieu;
2011-01-01
We investigate a family of low band-gap polymers based on the common acceptor moiety 2,3-bis-(3-octyloxyphenyl)quinoxaline (Q) combined with thiophene (T) or the fused thiophene systems: benzo[2,1-b:3,4-b′]-dithiophene (BDT) or dithieno[3,2-b,2′,3′-d]-thiophene (DTT). The photochemical stability...
Optical study of the band structure of wurtzite GaP nanowires
Assali, S.
2016-07-25
We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140–2.164–2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature-dependence. The excitonic nature of those peaks is critically discussed, leading to a direct band gap of ∼2.190 eV and to a resonant state associated with the γ-line ∼80 meV above the Γ8C conduction band edge.
Nature of sub-band gap luminescent eigenmodes in a ZnO nanowire
Rühle, S.; van Vugt, L.K.; Li, H.-Y.; Keizer, N.A.; Kuipers, L.; Vanmaekelbergh, D.A.M.
2008-01-01
The emission spectrum of individual high-quality ZnO nanowires consists of a series of Fabry-Pérot-like eigenmodes that extend far below the band gap of ZnO. Spatially resolved luminescence spectroscopy shows that light is emitted predominantly at both wire ends, with identical spectra reflecting st
Effects of weak nonlinearity on dispersion relations and frequency band-gaps of periodic structures
DEFF Research Database (Denmark)
Sorokin, Vladislav; Thomsen, Jon Juel
2015-01-01
The analysis of the behaviour of linear periodic structures can be traced back over 300 years, to Sir Isaac Newton, and still attracts much attention. An essential feature of periodic struc-tures is the presence of frequency band-gaps, i.e. frequency ranges in which waves cannot propagate...
H-shaped oligothiophenes with low band gaps and amphoteric redox properties
Luo, Jing
2010-12-17
H-shaped bridged oligothiophenes HT-1 and HT-2 were synthesized by two different approaches. Different from normal oligothiophenes, HT-1 and HT-2 showed low band gaps and amphoteric redox behaviors due to intramolecular charge transfer, which is further supported by time-dependent DFT calculations. © 2010 American Chemical Society.
Analysis of band-gap formation in squashed arm-chair CNT
Mehrer, H; Anantram, M P; Elstner, M; Frauenheim, T
2005-01-01
The electronic properties of squashed arm-chair carbon nanotubes are modeled using constraint free density functional tight binding molecular dynamics simulations. Independent from CNT diameter, squashing path can be divided into {\\it three} regimes. In the first regime, the nanotube deforms with negligible force. In the second one, there is significantly more resistance to squashing with the force being $\\sim 40-100$ nN/per CNT unit cell. In the last regime, the CNT looses its hexagonal structure resulting in force drop-off followed by substantial force enhancement upon squashing. We compute the change in band-gap as a function of squashing and our main results are: (i) A band-gap initially opens due to interaction between atoms at the top and bottom sides of CNT. The $\\pi-$orbital approximation is successful in modeling the band-gap opening at this stage. (ii) In the second regime of squashing, large $\\pi-\\sigma$ interaction at the edges becomes important, which can lead to band-gap oscillation. (iii) Contr...
Band gap engineering in penta-graphene by substitutional doping: first-principles calculations.
Berdiyorov, G R; Dixit, G; Madjet, M E
2016-11-30
Using density functional theory, we study the structure, electronic properties and partial charges of a new carbon allotrope-penta-graphene (PG)-substitutionally doped by Si, B and N. We found that the electronic bandgap of PG can be tuned down to 0.2 eV due to carbon substitutions. However, the value of the band gap depends on the type and location of the dopants. For example, the strongest reduction of the band gap is obtained for Si substitutions on the top (bottom) plane of PG, whereas the substitution in the middle plane of PG has a smaller effect on the band gap of the material. Surface termination with fluorine and hydroxyl groups results in an increase of the band gap together with considerable changes in electronic and atomic partial charge distribution in the system. Our findings, which are robust against the use of different exchange-correlation functionals, indicate the possibility of tuning the bandgap of the material to make it suitable for optoelectronic and photovoltaic applications. PMID:27633017
Structural characteristic correlated to the electronic band gap in Mo S2
Chu, Shengqi; Park, Changyong; Shen, Guoyin
2016-07-01
The structural evolution with pressure in bulk Mo S2 has been investigated by high-pressure x-ray diffraction using synchrotron radiation. We found that the out-of-plane S-Mo-S bond angle θ increases and that in in-plane angle ϕ decreases linearly with increasing pressure across the known semiconducting-to-metal phase transition, whereas the Mo-S bond length and the S-Mo-S trilayer thickness display only little change. Extrapolating the experimental result along the in-plane lattice parameter with pressure, both S-Mo-S bond angles trend to those found in monolayer Mo S2 , which manifests as a structural characteristic closely correlating the electronic band gap of Mo S2 to its physical forms and phases, e.g., monolayer as direct band gap semiconductor, multilayer or bulk as indirect band gap semiconductor, and high-pressure (>19 GPa ) bulk form as metal. Combined with the effects of bond strength and van der Waals interlayer interactions, the structural correlations between the characteristic bond angle and electronic band gaps are readily extendible to other transition metal dichalcogenide systems (M X2 , where M =Mo , W and X =S , Se, Te).
Temperature dependence of band gaps in Si and Ge in the quasi-ion model
Klenner, M.; Falter, C.; Ludwig, W.
We have calculated the temperature dependence of the direct and indirect band gaps in silicon and germanium. The electron-phonon potential as well as the phonon frequencies and eigenvectors are calculated consistently within the rigid quasi-ion model. Comparison is made with experiment and with the theoretical results of Allen and Cardona and Lautenschlager et al.
Goris, L.; Loi, M.A.; Cravino, A.; Neugebauer, H.; Sariciftci, N.S.; Polec, I.; Lutsen, L.; Andries, E.; Manca, J.; Schepper, L. De; Vanderzande, D.
2003-01-01
To enhance the efficiency of polymer photovoltaics, much effort is put into synthesis of novel compounds which show a better harvesting of solar light. In this respect, a new low band gap polymer, namely, poly(5,6-dithiooctylisothianaphtene), was synthesised. This work focusses on the spectroscopic
Energy Dependence and Scaling Property of Localization Length near a Gapped Flat Band
Ge, Li
2015-01-01
Using a tight-binding model for a one-dimensional Lieb lattice, we show that the localization length near a gapped flat band behaves differently from the typical Urbach tail in a band gap: instead of reducing monotonically as the energy E moves away from the flat band energy E_{FB}, the presence of the flat band causes a nonmonotonic energy dependence of the localization length. This energy dependence follows a scaling property when the energy is within the spread (W) of uniformly distributed diagonal disorder, i.e. the localization length is only a function of (E-E_{FB})/W. Several other lattices are compared to distinguish the effect of the flat band on the localization length, where we eliminate, shift, or duplicate the flat band, without changing the dispersion relations of other bands. Using the top right element of the Green's matrix, we derive an analytical relation between the density of states and the localization length, which shines light on these properties of the latter, including a summation rul...
High-Pressure Crystal Structure, Lattice Vibrations, and Band Structure of BiSbO4.
Errandonea, Daniel; Muñoz, Alfonso; Rodríguez-Hernández, Placida; Gomis, Oscar; Achary, S Nagabhusan; Popescu, Catalin; Patwe, Sadeque J; Tyagi, Avesh K
2016-05-16
The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ∼33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations. PMID:27128858
Can photonic crystals be homogenized in higher bands?
Markel, Vadim A
2015-01-01
We consider the conditions under which photonic crystals (PCs) can be viewed as electromagnetically homogeneous at frequencies in the higher photonic bands and, in particular, near the higher-order $\\Gamma$-points. We show that the observation that a purely real isofrequency line of the PC is close to a mathematical circle is insufficient for establishing homogenizability. Complex dispersion points must be included into consideration even in the case of strictly non-absorbing materials. By applying a more careful analysis to the dispersion relations and complex isofrequency lines, we have found that two-dimensional PCs with $C_4$ and $C_6$ symmetries are not electromagnetically homogeneous in the higher photonic bands in spite of the fact that, at some particular frequencies, the real isofrequency lines of these PCs can be circular with high precision.
Mini-stop bands in single heterojunction photonic crystal waveguides
Shahid, N.
2013-01-01
Spectral characteristics of mini-stop bands (MSB) in line-defect photonic crystal (PhC) waveguides and in heterostructure PhC waveguides having one abrupt interface are investigated. Tunability of the MSB position by air-fill factor heterostructure PhC waveguides is utilized to demonstrate different filter functions, at optical communication wavelengths, ranging from resonance-like to wide band pass filters with high transmission. The narrowest filter realized has a resonance-like transmission peak with a full width at half maximum of 3.4 nm. These devices could be attractive for coarse wavelength selection (pass and drop) and for sensing applications. 2013 Copyright 2013 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License.
Detailed study of flat bands appearing in metallic photonic crystals
Energy Technology Data Exchange (ETDEWEB)
Vala, Ali Soltani [Department of Solid State Physics, Faculty of Physics, University of Tabriz, PO Box 51665-163, Tabriz (Iran, Islamic Republic of); Research Institute for Applied Physics and Astronomy, University of Tabriz, PO Box 51665-163, Tabriz (Iran, Islamic Republic of); Sedghi, Aliasghar; Hosseini, Naser [Department of Solid State Physics, Faculty of Physics, University of Tabriz, PO Box 51665-163, Tabriz (Iran, Islamic Republic of); Kalafi, Manouchehr [Department of Solid State Physics, Faculty of Physics, University of Tabriz, PO Box 51665-163, Tabriz (Iran, Islamic Republic of); Research Institute for Applied Physics and Astronomy, University of Tabriz, PO Box 51665-163, Tabriz (Iran, Islamic Republic of); Excellence Centre for Photonics, University of Tabriz, PO Box 51665-163, Tabriz (Iran, Islamic Republic of)
2011-09-15
It has been difficult to compute the band structures of metallic photonic crystals for H-polarization. The existence of surface plasmon modes is the major reason for difficulty due to the localized nature of these modes. In this study, by virtue of the efficiency of the newly developed Dirichlet-to-Neumann map method, we are able to investigate the details of the flat bands in a two dimensional square lattice with metallic cylinders. We have obtained fine band structure for H polarization around the flat band region which has not been reported to the best of our knowledge. Our numerical results show that for the frequency around the surface plasmon, the modes are highly localized at the interface of the cylindrical metallic rods and air background and also by approaching the modes to the surface plasmon frequency the localization length decreases and the number of field's nodes increases considerably. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Effect of Microstructure of TiO2 Thin Films on Optical Band Gap Energy
Institute of Scientific and Technical Information of China (English)
TIAN Guang-Lei; HE Hong-Bo; SHAO Jian-Da
2005-01-01
@@ TiO2 coatings are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for four hours, the spectra and XRD patterns of TiO2 thin film are obtained. XRD patterns reveal that only anatase phase can be observed in TiO2 coatings regardless of the different annealing temperatures, and with the increasing annealing temperature, the grain size gradually increases. The relationship between the energy gap and microstructure of anatase is determined and discussed. The quantum confinement effect is observed that with the increasing grain size of TiO2 thin film, the band gap energy shifts from 3.4eV to 3.21 eV. Moreover, other possible influence of the TiO2 thin-film microstructure, such as surface roughness and thin film absorption, on band gap energy is also expected.
Light reflector, amplifier, and splitter based on gain-assisted photonic band gaps
Zhang, Yan; Liu, Yi-Mou; Zheng, Tai-Yu; Wu, Jin-Hui
2016-07-01
We study both the steady and the dynamic optical response of cold atoms trapped in an optical lattice and driven to the three-level Λ configuration. These atoms are found to exhibit gain without population inversion when an incoherent pump is applied to activate spontaneously generated coherence. Gain-assisted double photonic band gaps characterized by reflectivities over 100% then grow up near the probe resonance due to the periodic distribution of the atomic density. These band gaps along with the neighboring allowed bands of transmissivities over 100% can be tuned by modulating the control field in amplitude, frequency, and, especially, phase. Consequently it is viable to realize a reflector, an amplifier, or a splitter when a weak incident light pulse is totally reflected in the photonic band gaps, totally transmitted in the allowed bands, or equally reflected and transmitted in the intersecting regions. Our results have potential applications in all-optical networks with respect to fabricating dynamically switchable devices for manipulating photon flows at low-light levels.
Vibration band gaps for elastic metamaterial rods using wave finite element method
Nobrega, E. D.; Gautier, F.; Pelat, A.; Dos Santos, J. M. C.
2016-10-01
Band gaps in elastic metamaterial rods with spatial periodic distribution and periodically attached local resonators are investigated. New techniques to analyze metamaterial systems are using a combination of analytical or numerical method with wave propagation. One of them, called here wave spectral element method (WSEM), consists of combining the spectral element method (SEM) with Floquet-Bloch's theorem. A modern methodology called wave finite element method (WFEM), developed to calculate dynamic behavior in periodic acoustic and structural systems, utilizes a similar approach where SEM is substituted by the conventional finite element method (FEM). In this paper, it is proposed to use WFEM to calculate band gaps in elastic metamaterial rods with spatial periodic distribution and periodically attached local resonators of multi-degree-of-freedom (M-DOF). Simulated examples with band gaps generated by Bragg scattering and local resonators are calculated by WFEM and verified with WSEM, which is used as a reference method. Results are presented in the form of attenuation constant, vibration transmittance and frequency response function (FRF). For all cases, WFEM and WSEM results are in agreement, provided that the number of elements used in WFEM is sufficient to convergence. An experimental test was conducted with a real elastic metamaterial rod, manufactured with plastic in a 3D printer, without local resonance-type effect. The experimental results for the metamaterial rod with band gaps generated by Bragg scattering are compared with the simulated ones. Both numerical methods (WSEM and WFEM) can localize the band gap position and width very close to the experimental results. A hybrid approach combining WFEM with the commercial finite element software ANSYS is proposed to model complex metamaterial systems. Two examples illustrating its efficiency and accuracy to model an elastic metamaterial rod unit-cell using 1D simple rod element and 3D solid element are
Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying
DEFF Research Database (Denmark)
Canulescu, Stela; Rechendorff, K.; Borca, C. N.;
2014-01-01
The band structure of pure and Ti-alloyed anodic aluminum oxide has been examined as a function of Ti concentration varying from 2 to 20 at. %. The band gap energy of Ti-alloyed anodic Al oxide decreases with increasing Ti concentration. X-ray absorption spectroscopy reveals that Ti atoms...... are not located in a TiO2 unit in the oxide layer, but rather in a mixed Ti-Al oxide layer. The optical band gap energy of the anodic oxide layers was determined by vacuum ultraviolet spectroscopy in the energy range from 4.1 to 9.2 eV (300–135 nm). The results indicate that amorphous anodic Al2O3 has a direct...
Directory of Open Access Journals (Sweden)
P. Kovacs
2010-04-01
Full Text Available The paper is focused on the automated design and optimization of electromagnetic band gap structures suppressing the propagation of surface waves. For the optimization, we use different global evolutionary algorithms like the genetic algorithm with the single-point crossover (GAs and the multi-point (GAm one, the differential evolution (DE and particle swarm optimization (PSO. The algorithms are mutually compared in terms of convergence velocity and accuracy. The developed technique is universal (applicable for any unit cell geometry. The method is based on the dispersion diagram calculation in CST Microwave Studio (CST MWS and optimization in Matlab. A design example of a mushroom structure with simultaneous electromagnetic band gap properties (EBG and the artificial magnetic conductor ones (AMC in the required frequency band is presented.
Solid-State Nanopore Confinement for Band Gap Engineering of Metal-Halide Perovskites
Demchyshyn, Stepan; Groiss, Heiko; Heilbrunner, Herwig; Ulbricht, Christoph; Apaydin, Dogukan; Rütt, Uta; Bertram, Florian; Hesser, Günter; Scharber, Markus; Nickel, Bert; Sariciftci, Niyazi Serdar; Bauer, Siegfried; Głowacki, Eric Daniel; Kaltenbrunner, Martin
2016-01-01
Tuning the band gap of semiconductors via quantum size effects launched a technological revolution in optoelectronics, advancing solar cells, quantum dot light-emitting displays, and solid state lasers. Next generation devices seek to employ low-cost, easily processable semiconductors. A promising class of such materials are metal-halide perovskites, currently propelling research on emerging photovoltaics. Their narrow band emission permits very high colour purity in light-emitting devices and vivid life-like displays paired with low-temperature processing through printing-compatible methods. Success of perovskites in light-emitting devices is conditional upon finding reliable strategies to obtain tunability of the band gap. So far, colour can be tuned chemically by mixed halide stoichiometry, or by synthesis of colloidal particles. Here we introduce a general strategy of controlling shape and size of perovskite nanocrystallites (less than 10 nm) in domains that exhibit strong quantum size effects. Without ma...
Inertial amplification of continuous structures: Large band gaps from small masses
DEFF Research Database (Denmark)
Frandsen, Niels Morten Marslev; Bilal, Osama R.; Jensen, Jakob Søndergaard;
2016-01-01
We investigate wave motion in a continuous elastic rod with a periodically attached inertial amplification mechanism. The mechanism has properties similar to an “inerter” typically used in vehicle suspensions, however here it is constructed and utilized in a manner that alters the intrinsic...... properties of a continuous structure. The elastodynamic band structure of the hybridrod-mechanism structure yields band gaps that are exceedingly wide and deep when compared to what can be obtained using standard local resonators, while still being low in frequency. With this concept, a large band gap may...... be realized with as much as twenty times less added mass compared to what is needed in a standard local resonator configuration. The emerging inertially enhanced continuous structure also exhibits unique qualitative features in its dispersion curves. These include the existence of a characteristic double...
DEFF Research Database (Denmark)
Sorokin, Vladislav
2016-01-01
The paper concerns determining frequency band-gaps for longitudinal wave motion in a periodic waveguide. The waveguide may be considered either as an elastic layer with variable thickness or as a rod with variable cross section. As a result, widths and locations of all frequency band-gaps are det......The paper concerns determining frequency band-gaps for longitudinal wave motion in a periodic waveguide. The waveguide may be considered either as an elastic layer with variable thickness or as a rod with variable cross section. As a result, widths and locations of all frequency band......, harmonic in the corrugation series. The revealed insights into the mechanism of band-gap formation can be used to predict locations and widths of all frequency band-gaps featured by any corrugation shape. These insights are general and can be valid also for other types of wave motion in periodic structures......, e.g., transverse or torsional vibration....
Tuning the Band Gap of Cu₂ZnSn(S,Se)₄ Thin Films via Lithium Alloying.
Yang, Yanchun; Kang, Xiaojiao; Huang, Lijian; Pan, Daocheng
2016-03-01
Alkali metal doping plays a crucial role in fabricating high-performance Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this study, we report the first experimental observation and characterizations of the alloyed Li(x)Cu(2-x)ZnSn(S,Se)4 thin films. It is found that Cu(+) ions in Cu2ZnSn(S,Se)4 thin films can be substituted with Li(+) ions, forming homogeneous Li(x)Cu(2-x)ZnSn(S,Se)4 (0 ≤ x ≤ 0.29) alloyed thin films. Consequently, the band gap, conduction band minimum, and valence band maximum of Li(x)Cu(2-x)ZnSn(S,Se)4 thin films are profoundly affected by Li/Cu ratios. The band alignment at the Li(x)Cu(2-x)ZnSn(S,Se)4/CdS interface can be tuned by changing the Li/Cu ratio. We found that the photovoltaic parameters of the Li(x)Cu(2-x)ZnSn(S,Se)4 solar cell devices are strongly influenced by the Li/Cu ratios. Besides, the lattice constant, carrier concentration, and crystal growth of Li(x)Cu(2-x)ZnSn(S,Se)4 thin films were studied in detail. PMID:26837657
Hofstadter butterflies and magnetically induced band-gap quenching in graphene antidot lattices
DEFF Research Database (Denmark)
Pedersen, Jesper Goor; Pedersen, Thomas Garm
2013-01-01
We study graphene antidot lattices (GALs) in magnetic fields. Using a tight-binding model and a recursive Green's function technique that we extend to deal with periodic structures, we calculate Hofstadter butterflies of GALs. We compare the results to those obtained in a simpler gapped graphene...... model. A crucial difference emerges in the behavior of the lowest Landau level, which in a gapped graphene model is independent of magnetic field. In stark contrast to this picture, we find that in GALs the band gap can be completely closed by applying a magnetic field. While our numerical simulations...... can only be performed on structures much smaller than can be experimentally realized, we find that the critical magnetic field for which the gap closes can be directly related to the ratio between the cyclotron radius and the neck width of the GAL. In this way, we obtain a simple scaling law for...
Photonic band gap of a graphene-embedded quarter-wave stack
Fan, Yuancheng; Li, Hongqiang; Chen, Hong; Soukoulis, Costas M
2013-01-01
Here, we present a mechanism for tailoring the photonic band structure of a quarter-wave stack without changing its physical periods by embedding conductive sheets. Graphene is utilized and studied as a realistic, two-dimensional conductive sheet. In a graphene-embedded quarter-wave stack, the synergic actions of Bragg scattering and graphene conductance contributions open photonic gaps at the center of the reduced Brillouin zone, that nonexistent in conventional quarter-wave stacks. Such photonic gaps show giant, loss-independent density of optical states at the fixed lower-gap-edges, of even-multiple characteristic frequency of the quarter-wave stack. The novel conductive sheets induced photonic gaps provide a new platform for the enhancement of light-matter interactions.
Band gap modulation of transition-metal dichalcogenide MX2 nanosheets by in-plane strain
Su, Xiangying; Ju, Weiwei; Zhang, Ruizhi; Guo, Chongfeng; Yong, Yongliang; Cui, Hongling; Li, Xiaohong
2016-10-01
The electronic properties of quasi-two-dimensional honeycomb structures of MX2 nanosheets (M=Mo, W and X=S, Se) subjected to in-plane biaxial strain have been investigated using first-principles calculations. We demonstrate that the band gap of MX2 nanosheets can be widely tuned by applying tensile or compressive strain, and these ultrathin materials undergo a universal reversible semiconductor-metal transition at a critical strain. Compared to WX2, MoX2 need a smaller critical tensile strain for the band gap close, and MSe2 need a smaller critical compressive strain than MS2. Taking bilayer MoS2 as an example, the variation of the band structures was studied and the semiconductor-metal transition involves a slightly different physical mechanism between tensile and compressive strain. The ability to tune the band gap of MX2 nanosheets in a controlled fashion over a wide range of energy opens up the possibility for its usage in a range of application.
Enhanced thermoelectric performance in the Rashba semiconductor BiTeI through band gap engineering
International Nuclear Information System (INIS)
Rashba semiconductors are of great interest in spintronics, superconducting electronics and thermoelectrics. Bulk BiTeI is a new Rashba system with a giant spin–split band structure. 2D-like thermoelectric response has been found in BiTeI. However, as optimizing the carrier concentration, the bipolar effect occurs at elevated temperature and deteriorates the thermoelectric performance of BiTeI. In this paper, band gap engineering in Rashba semiconductor BiTeI through Br-substitution successfully reduces the bipolar effect and improves the thermoelectric properties. By utilizing the optical absorption and Burstein–Moss-effect analysis, we find that the band gap in Rashba semiconductor BiTeI increases upon bromine substitution, which is consistent with theoretical predictions. Bipolar transport is mitigated due to the larger band gap, as the thermally-activated minority carriers diminish. Consequently, the Seebeck coefficient keeps increasing with a corresponding rise in temperature, and thermoelectric performance can thus be enhanced with a ZT = 0.5 at 570 K for BiTeI0.88Br0.12. (paper)
Enhanced thermoelectric performance in the Rashba semiconductor BiTeI through band gap engineering.
Wu, Lihua; Yang, Jiong; Zhang, Tiansong; Wang, Shanyu; Wei, Ping; Zhang, Wenqing; Chen, Lidong; Yang, Jihui
2016-03-01
Rashba semiconductors are of great interest in spintronics, superconducting electronics and thermoelectrics. Bulk BiTeI is a new Rashba system with a giant spin-split band structure. 2D-like thermoelectric response has been found in BiTeI. However, as optimizing the carrier concentration, the bipolar effect occurs at elevated temperature and deteriorates the thermoelectric performance of BiTeI. In this paper, band gap engineering in Rashba semiconductor BiTeI through Br-substitution successfully reduces the bipolar effect and improves the thermoelectric properties. By utilizing the optical absorption and Burstein-Moss-effect analysis, we find that the band gap in Rashba semiconductor BiTeI increases upon bromine substitution, which is consistent with theoretical predictions. Bipolar transport is mitigated due to the larger band gap, as the thermally-activated minority carriers diminish. Consequently, the Seebeck coefficient keeps increasing with a corresponding rise in temperature, and thermoelectric performance can thus be enhanced with a ZT = 0.5 at 570 K for BiTeI0.88Br0.12.
Energy Technology Data Exchange (ETDEWEB)
Vos, M. [Atomic and Molecular Physics Laboratories, Research School of Physics and Engineering, Australian National University, Canberra ACT (Australia); Marmitt, G. G. [Atomic and Molecular Physics Laboratories, Research School of Physics and Engineering, Australian National University, Canberra ACT (Australia); Instituto de Fisica da Universidade Federal do Rio Grande do Sul, Avenida Bento Goncalves 9500, 91501-970 Porto Alegre, RS (Brazil); Finkelstein, Y. [Nuclear Research Center — Negev, Beer-Sheva 84190 (Israel); Moreh, R. [Physics Department, Ben-Gurion University of the Negev, Beer-Sheva 84105 (Israel)
2015-09-14
Reflection electron energy loss spectra from some insulating materials (CaCO{sub 3}, Li{sub 2}CO{sub 3}, and SiO{sub 2}) taken at relatively high incoming electron energies (5–40 keV) are analyzed. Here, one is bulk sensitive and a well-defined onset of inelastic excitations is observed from which one can infer the value of the band gap. An estimate of the band gap was obtained by fitting the spectra with a procedure that includes the recoil shift and recoil broadening affecting these measurements. The width of the elastic peak is directly connected to the mean kinetic energy of the atom in the material (Doppler broadening). The experimentally obtained mean kinetic energies of the O, C, Li, Ca, and Si atoms are compared with the calculated ones, and good agreement is found, especially if the effect of multiple scattering is taken into account. It is demonstrated experimentally that the onset of the inelastic excitation is also affected by Doppler broadening. Aided by this understanding, we can obtain a good fit of the elastic peak and the onset of inelastic excitations. For SiO{sub 2}, good agreement is obtained with the well-established value of the band gap (8.9 eV) only if it is assumed that the intensity near the edge scales as (E − E{sub gap}){sup 1.5}. For CaCO{sub 3}, the band gap obtained here (7 eV) is about 1 eV larger than the previous experimental value, whereas the value for Li{sub 2}CO{sub 3} (7.5 eV) is the first experimental estimate.
DEFF Research Database (Denmark)
Bundgaard, Eva; Hagemann, Ole; Jørgensen, Mikkel;
2011-01-01
In this paper we present the design and synthesis of 25 new low band gap polymers. The polymers were characterized by UV-vis spectroscopy which showed optical band gaps of 2.0–0.9 eV. The polymers which were soluble enough were applied in organic photovoltaics, both small area devices with a spin...
Band-gap shrinkage calculations and analytic model for strained bulk InGaAsP
Connelly, Michael J.
2015-02-01
Band-gap shrinkage is an important effect in semiconductor lasers and optical amplifiers. In the former it leads to an increase in the lasing wavelength and in the latter an increase in the gain peak wavelength as the bias current is increased. The most common model used for carrier-density dependent band-gap shrinkage is a cube root dependency on carrier density, which is strictly only true for high carrier densities and low temperatures. This simple model, involves a material constant which is treated as a fitting parameter. Strained InGaAsP material is commonly used to fabricate polarization insensitive semiconductor optical amplifiers (SOAs). Most mathematical models for SOAs use the cube root bandgap shrinkage model. However, because SOAs are often operated over a wide range of drive currents and input optical powers leading to large variations in carrier density along the amplifier length, for improved model accuracy it is preferable to use band-gap shrinkage calculated from knowledge of the material bandstructure. In this letter the carrier density dependent band-gap shrinkage for strained InGaAsP is calculated by using detailed non-parabolic conduction and valence band models. The shrinkage dependency on temperature and both tensile and compressive strain is investigated and compared to the cube root model, for which it shows significant deviation. A simple power model, showing an almost square-root dependency, is derived for carrier densities in the range usually encountered in InGaAsP laser diodes and SOAs.
Band-gap shrinkage calculations and analytic model for strained bulk InGaAsP
International Nuclear Information System (INIS)
Band-gap shrinkage is an important effect in semiconductor lasers and optical amplifiers. In the former it leads to an increase in the lasing wavelength and in the latter an increase in the gain peak wavelength as the bias current is increased. The most common model used for carrier-density dependent band-gap shrinkage is a cube root dependency on carrier density, which is strictly only true for high carrier densities and low temperatures. This simple model, involves a material constant which is treated as a fitting parameter. Strained InGaAsP material is commonly used to fabricate polarization insensitive semiconductor optical amplifiers (SOAs). Most mathematical models for SOAs use the cube root bandgap shrinkage model. However, because SOAs are often operated over a wide range of drive currents and input optical powers leading to large variations in carrier density along the amplifier length, for improved model accuracy it is preferable to use band-gap shrinkage calculated from knowledge of the material bandstructure. In this letter the carrier density dependent band-gap shrinkage for strained InGaAsP is calculated by using detailed non-parabolic conduction and valence band models. The shrinkage dependency on temperature and both tensile and compressive strain is investigated and compared to the cube root model, for which it shows significant deviation. A simple power model, showing an almost square-root dependency, is derived for carrier densities in the range usually encountered in InGaAsP laser diodes and SOAs. (paper)
Development of ultra-narrow gap welding with constrained arc by flux band
Institute of Scientific and Technical Information of China (English)
Zhu Liang; Zheng Shaoxian; Chen Jianhong
2006-01-01
Narrow gap welding has merits of lower residual stress and distortion, and superior mechanical properties of joints.A major problem of this process is the lack of fusion in sidewalls, hence many methods of weaving arc have been developed to increase heating effect of arc to the sidewalls. In this work, a new approach without weaving arc is attempted to ensure the penetration of sidewall, and ultra-narrow gap welding with the gap of less than 5 mm was executed successfully. In this approach, the width of gap is decreased further, so that the sidewalls are made within range of arc heating to obtain the enough heat. Inorder to prevent the arc from being attracted by sidewall and going up alongthe sidewalls, two pieces of flux bands consisting of the specified aggregates are adhered to the sidewalls to constrain the arc. In addition, when flux band being heated by the arc, slag and gases are formed to shield the arc and the weld pool. This technique was tested on the welding experiment of pipeline steel with thickness of 20 mm. The involved welding parameters were obtained, that is, the width of gap is 4 mm, the welding current 250 A, and the heat input 0. 5 k J/mm, the width of heat-affected zone is 1 - 2mm.
Zhukovsky, Sergei V; Babicheva, Viktoriia E; Lavrinenko, Andrei V; Sipe, J E
2013-01-01
We theoretically study the propagation of large-wavevector waves (volume plasmon polaritons) in multilayer hyperbolic metamaterials with two levels of structuring. We show that when the parameters of a subwavelength metal-dielectric multilayer ("substructure") are modulated ("superstructured") on a larger, wavelength scale, the propagation of volume plasmon polaritons in the resulting multiscale hyperbolic metamaterials is subject to photonic band gap phenomena. A great degree of control over such plasmons can be exerted by varying the superstructure geometry. When this geometry is periodic, stop bands due to Bragg reflection are shown to form within the volume plasmonic band. When a cavity layer is introduced in an otherwise periodic superstructure, resonance peaks of the Fabry-P\\'erot nature are shown to be present within the stop bands. More complicated superstructure geometries are also considered. For example, fractal Cantor-like multiscale metamaterials are found to exhibit characteristic self-similar s...
Molla, Aniruddha; Sahu, Meenakshi; Hussain, Sahid
2016-01-01
Controlled shape and size with tuneable band gap (1.92–2.41 eV), nickel sulphide NPs was achieved in presence of thiourea or thioacetamide as sulphur sources with the variations of temperature and capping agents. Synthesized NPs were fully characterized by powder XRD, IR, UV-vis, DRS, FE-SEM, TEM, EDX, XPS, TGA and BET. Capping agent, temperature and sulphur sources have significant role in controlling the band gaps, morphology and surface area of NPs. The catalytic activities of NPs were tested for round the clock (light and dark) decomposition of crystal violet (CV), rhodamine B (RhB), methylene blue (MB), nile blue (NB) and eriochrome black T (EBT). Agitation speed, temperature, pH and ionic strength have significant role on its catalytic activities. The catalyst was found to generate reactive oxygen species (ROS) both in presence and absence of light which is responsible for the decomposition of dyes into small fractions, identified with ESI-mass spectra. PMID:27185051
Molla, Aniruddha; Sahu, Meenakshi; Hussain, Sahid
2016-05-01
Controlled shape and size with tuneable band gap (1.92-2.41 eV), nickel sulphide NPs was achieved in presence of thiourea or thioacetamide as sulphur sources with the variations of temperature and capping agents. Synthesized NPs were fully characterized by powder XRD, IR, UV-vis, DRS, FE-SEM, TEM, EDX, XPS, TGA and BET. Capping agent, temperature and sulphur sources have significant role in controlling the band gaps, morphology and surface area of NPs. The catalytic activities of NPs were tested for round the clock (light and dark) decomposition of crystal violet (CV), rhodamine B (RhB), methylene blue (MB), nile blue (NB) and eriochrome black T (EBT). Agitation speed, temperature, pH and ionic strength have significant role on its catalytic activities. The catalyst was found to generate reactive oxygen species (ROS) both in presence and absence of light which is responsible for the decomposition of dyes into small fractions, identified with ESI-mass spectra.
Strong interaction of a transmon qubit with 1D band-gap medium
Liu, Yanbing; Sadri, Darius; Houck, Andrew; Bronn, Nicholas; Chow, Jerry; Gambetta, Jay
2015-03-01
The spontaneous emission of an atom will be enhanced or suppressed in a structured vacuum, commonly known as Purcell effect. Moreover, in a frequency gap medium, an atom-photon bound state is predicted to exist in the band gap, causing the localization of light. Here using the technology of circuit quantum electrodynamics, we experimentally explore this mechanism by fabricating a microwave step-impedance filter strongly coupled to a transmon qubit. Standard transmission and spectroscopy measurements support the existence of atom-photon bound states in the system. Correlation measurement shows that the atom-photon interaction induces strong correlation of the transmitted light through the system. Thanks support from IARPA
Low-frequency band gaps in chains with attached non-linear oscillators
DEFF Research Database (Denmark)
Lazarov, Boyan Stefanov; Jensen, Jakob Søndergaard
2007-01-01
in structures with periodic or random inclusions are located mainly in the high frequency range, as the wavelength has to be comparable with the distance between the alternating parts. Band gaps may also exist in structures with locally attached oscillators. In the linear case the gap is located around......The aim of this article is to investigate the wave propagation in one-dimensional chains with attached non-linear local oscillators by using analytical and numerical models. The focus is on the influence of non-linearities on the filtering properties of the chain in the low frequency range...
Two-dimensional microwave band-gap structures of different dielectric materials
Indian Academy of Sciences (India)
E D V Nagesh; G Santosh Babu; V Subramanian; V Sivasubramanian; V R K Murthy
2005-12-01
We report the use of low dielectric constant materials to form two-dimensional microwave band-gap structures for achieving high gap-to-midgap ratio. The variable parameters chosen are the lattice spacing and the geometric structure. The selected geometries are square and triangular and the materials chosen are PTFE ( = 2.1), PVC ( = 2.38) and glass ( = 5.5). Using the plane-wave expansion method, proper lattice spacing is selected for each structure and material. The observed experimental results are analyzed with the help of the theoretical prediction.
Energy Technology Data Exchange (ETDEWEB)
Pietzsch, A., E-mail: annette.pietzsch@helmholtz-berlin.de [Institute for Methods and Instrumentation in Synchrotron Radiation Research G-ISRR, Helmholtz-Zentrum für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489 Berlin (Germany); Nisar, J. [Pakistan Atomic Energy Commission (PAEC), P.O. Box 2151, Islamabad (Pakistan); Jämstorp, E. [Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI (Switzerland); Gråsjö, J. [Department of Pharmacy, Uppsala University, Box 580, 75123 Uppsala (Sweden); Århammar, C. [Coromant R& D, S-126 80 Stockholm (Sweden); Ahuja, R.; Rubensson, J.-E. [Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala (Sweden)
2015-07-15
Highlights: • The respective electronic structure of synthetic and natural kaolinite is compared. • The size of the band gap and thus many important material properties are defined by defect states in the band gap. • The oxygen-based defect states are identified and analyzed. • The band gap of kaolinite decreases significantly due to the forming of defects. - Abstract: By combining X-ray absorption spectroscopy and first principles calculations we have determined the electronic structure of synthetic and natural kaolinite as a model system for engineered and natural clay materials. We have analyzed defect states in the band gap and find that both natural and synthetic kaolinite contain defects where oxygen replaces hydrogen in one of the Al (0 0 1)-hydroxyl groups of the kaolinite clay sheets. The band gap of both synthetic and natural kaolinite is found to decrease by about 3.2 eV as this defect is formed.
Theoretical aspects of photonic band gap in 1D nano structure of LN: MgLN periodic layer
Energy Technology Data Exchange (ETDEWEB)
Sisodia, Namita, E-mail: namitasisodiya@gmail.com [Department of Physics, Holkar Science Collage, Indore-452017 (India)
2015-06-24
By using the transfer matrix method, we have analyzed the photonic band gap properties in a periodic layer of LN:MgLN medium. The Width of alternate layers of LN and MgLN is in the range of hundred nanometers. The birefringent and ferroelectric properties of the medium (i.e ordinary, extraordinary refractive indices and electric dipole moment) is given due considerations in the formulation of photonic band gap. Effect of electronic transition dipole moment of the medium on photonic band gap is also taken into account. We find that photonic band gap can be modified by the variation in the ratio of the width of two medium. We explain our findings by obtaining numerical values and the effect on the photonic band gap due to variation in the ratio of alternate medium is shown graphically.
Theoretical aspects of photonic band gap in 1D nano structure of LN: MgLN periodic layer
International Nuclear Information System (INIS)
By using the transfer matrix method, we have analyzed the photonic band gap properties in a periodic layer of LN:MgLN medium. The Width of alternate layers of LN and MgLN is in the range of hundred nanometers. The birefringent and ferroelectric properties of the medium (i.e ordinary, extraordinary refractive indices and electric dipole moment) is given due considerations in the formulation of photonic band gap. Effect of electronic transition dipole moment of the medium on photonic band gap is also taken into account. We find that photonic band gap can be modified by the variation in the ratio of the width of two medium. We explain our findings by obtaining numerical values and the effect on the photonic band gap due to variation in the ratio of alternate medium is shown graphically
Birefringence and band structure of CdP{sub 2} crystals
Energy Technology Data Exchange (ETDEWEB)
Beril, S.I.; Stamov, I.G. [Tiraspol State Corporative University, Yablocikin Street 5, 2069 Tiraspol, Republic of Moldova (Moldova, Republic of); Syrbu, N.N., E-mail: sirbunn@yahoo.com [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of); Zalamai, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of)
2013-08-01
The spatial dispersion in CdP{sub 2} crystals was investigated. The dispersion is positive (n{sup k||c}>n{sup k||y}) at λ>λ{sub 0} and negative (n{sup k||c}
Vijayakumar, Vishnuvardhanan
Development of low cost, high efficiency tandem solar cells is essential for large scale adoption of solar energy especially in densely populated regions of the world. In this thesis four-terminal mechanical (stack like) tandem solar cells were evaluated using detailed simulation models and design criteria for selecting candidate materials were established. Since silicon solar cells are low cost and have a multi-giga watt global manufacturing and supply chain capacity already in place then only tandem stacks incorporating silicon as one of the layers in the device was investigated. Two candidate materials which have high band gaps that could be used as top cells in the mechanical tandem device were explored as part of the thesis. Dye-sensitized solar cells (DSSC) sensitized with N719 dye (one of the candidates for the top cell) were fabricated with the goal of enabling a flexible processing path to lower cost. Stainless steel (SS) mesh substrates were used to fabricate anodes for flexible DSSC in order to evaluate them as replacements for more expensive Transparent Conducting Oxides (TCO's). Loss mechanisms in DSSC's due to SS mesh oxidation were quantified and protective coatings to prevent oxidation of SS mesh were developed. The second material which was evaluated for use as the top cell was copper zinc tin sulfide (CZTS). CZTS was deposited through a solution deposition route. Detailed investigations were done on the deposited films to understand the chemistry, crystal structure and its opto-electronic properties. Deposited CZTS films were found to be highly crystalline in direction. The films had a direct band gap of 1.5 eV with absorption coefficient greater than 104 cm -1 in agreement with published values. In the second part of the thesis detailed electrical and optical simulation models of the mechanical tandem solar cells were developed based on the most up-to-date materials physical constants available for each layer. The modeling was used to quantify
Strain- and electric field-induced band gap modulation in nitride nanomembranes.
Amorim, Rodrigo G; Zhong, Xiaoliang; Mukhopadhyay, Saikat; Pandey, Ravindra; Rocha, Alexandre R; Karna, Shashi P
2013-05-15
The hexagonal nanomembranes of the group III-nitrides are a subject of interest due to their novel technological applications. In this paper, we investigate the strain- and electric field-induced modulation of their band gaps in the framework of density functional theory. For AlN, the field-dependent modulation of the bandgap is found to be significant whereas the strain-induced semiconductor-metal transition is predicted for GaN. A relatively flat conduction band in AlN and GaN nanomembranes leads to an enhancement of their electronic mobility compared to that of their bulk counterparts. PMID:23604312
Directory of Open Access Journals (Sweden)
Jia Li
2015-11-01
Full Text Available The Heusler alloy Ti2CrGe is a stable L21 phase with antiferromagnetic ordering. With band-gap energy (∼ 0.18 eV obtained from a first-principles calculation, it belongs to the group of narrow band gap semiconductor. The band-gap energy decreases with increasing lattice compression and disappears until a strain of −5%; moreover, gap contraction only occurs in the spin-down states, leading to half-metallic character at the −5% strain. The Ti1, Ti2, and Cr moments all exhibit linear changes in behavior within strains of −5%– +5%. Nevertheless, the total zero moment is robust for these strains. The imaginary part of the dielectric function for both up and down spin states shows a clear onset energy, indicating a corresponding electronic gap for the two spin channels.
Energy Technology Data Exchange (ETDEWEB)
Li, Jia, E-mail: jiali@hebut.edu.cn [School of Science, Hebei University of Technology, Tianjin 300401 (China); Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300401 (China); Zhang, Zhidong [School of Science, Hebei University of Technology, Tianjin 300401 (China); Lu, Zunming; Xie, Hongxian; Fang, Wei; Li, Shaomin; Liang, Chunyong; Yin, Fuxing [Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300401 (China)
2015-11-15
The Heusler alloy Ti{sub 2}CrGe is a stable L2{sub 1} phase with antiferromagnetic ordering. With band-gap energy (∼ 0.18 eV) obtained from a first-principles calculation, it belongs to the group of narrow band gap semiconductor. The band-gap energy decreases with increasing lattice compression and disappears until a strain of −5%; moreover, gap contraction only occurs in the spin-down states, leading to half-metallic character at the −5% strain. The Ti{sub 1}, Ti{sub 2}, and Cr moments all exhibit linear changes in behavior within strains of −5%– +5%. Nevertheless, the total zero moment is robust for these strains. The imaginary part of the dielectric function for both up and down spin states shows a clear onset energy, indicating a corresponding electronic gap for the two spin channels.
Liang, Yunye; Wang, Vei; Mizuseki, Hiroshi; Kawazoe, Yoshiyuki
2012-11-14
The electronic properties of silicene zigzag nanoribbons with the presence of perpendicular fields are studied by using first-principles calculations and the generalized nearest neighboring approximation method. In contrast to the planar graphene, in silicene the Si atoms are not coplanar. As a result, by applying perpendicular fields to the two-dimensional silicene sheet, the on-site energy can be modulated and the band gap at the Dirac point is open. The buckled structure also creates a height difference between the two edges of the silicene zigzag nanoribbons. We find that the external fields can modulate the energies of spin-polarized edge states and their corresponding band gaps. Due to the polarization in the plane, the modulation effect is width dependent and becomes much more significant for narrow ribbons.
Electrostatic tuning of Kondo effect in a rare-earth-doped wide-band-gap oxide
Li, Yongfeng
2013-04-29
As a long-lived theme in solid-state physics, the Kondo effect reflects the many-body physics involving the short-range Coulomb interactions between itinerant electrons and localized spins in metallic materials. Here we show that the Kondo effect is present in ZnO, a prototypical wide-band-gap oxide, doped with a rare-earth element (Gd). The localized 4f electrons of Gd ions do not produce remanent magnetism, but interact strongly with the host electrons, giving rise to a saturating resistance upturn and negative magnetoresistance at low temperatures. Furthermore, the Kondo temperature and resistance can be electrostatically modulated using electric-double-layer gating with liquid ionic electrolyte. Our experiments provide the experimental evidence of tunable Kondo effect in ZnO, underscoring the magnetic interactions between localized and itinerant electrons and the emergent transport behaviors in such doped wide-band-gap oxides.
Light-gated single CdSe nanowire transistor: photocurrent saturation and band gap extraction
Energy Technology Data Exchange (ETDEWEB)
Zhang, Yang, E-mail: yangzh08@gmail.com; Chakraborty, Ritun; Kudera, Stefan; Krahne, Roman, E-mail: roman.krahne@iit.it [Istituto Italiano di Tecnologia, Nanochemistry department (Italy)
2015-11-15
CdSe nanowires are popular building blocks for many optoelectronic devices mainly owing to their direct band gap in the visible range of the spectrum. Here we investigate the optoelectronic properties of single CdSe nanowires fabricated by colloidal synthesis, in terms of their photocurrent–voltage characteristics and photoconductivity spectra recorded at 300 and 18 K. The photocurrent is identified as the secondary photocurrent, which gives rise to a photoconductive gain of ∼35. We observe a saturation of the photocurrent beyond a certain voltage bias that can be related to the finite drift velocity of electrons. From the photoconductivity spectra, we determine the band gap energy of the nanowires as ∼1.728 eV, and we resolve low-energy peaks that can be associated with sub-bandgap states.Graphical Abstract.
Energy Band Gap Study of Semiconducting Single Walled Carbon Nanotube Bundle
Elkadi, Asmaa; Decrossas, Emmanuel; El-Ghazaly, Samir
2013-01-01
The electronic properties of multiple semiconducting single walled carbon nanotubes (s-SWCNTs) considering various distribution inside a bundle are studied. The model derived from the proposed analytical potential function of the electron density for an individual s-SWCNT is general and can be easily applied to multiple nanotubes. This work demonstrates that regardless the number of carbon nanotubes, the strong coupling occurring between the closest neighbours reduces the energy band gap of the bundle by 10%. As expected, the coupling is strongly dependent on the distance separating the s-SWCNTs. In addition, based on the developed model, it is proposed to enhance this coupling effect by applying an electric field across the bundle to significantly reduce the energy band gap of the bundle by 20%.
Niedermeier, Christian A.; Råsander, Mikael; Rhode, Sneha; Kachkanov, Vyacheslav; Zou, Bin; Alford, Neil; Moram, Michelle A.
2016-08-01
Epitaxial transparent oxide NixMg1‑xO (0 ≤ x ≤ 1) thin films were grown on MgO(100) substrates by pulsed laser deposition. High-resolution synchrotron X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the thin films are compositionally and structurally homogeneous, forming a completely miscible solid solution. Nevertheless, the composition dependence of the NixMg1‑xO optical band gap shows a strong non-parabolic bowing with a discontinuity at dilute NiO concentrations of x 0.074 and account for the anomalously large band gap narrowing in the NixMg1‑xO solid solution system.
Microwave irradiation induced band gap tuning of MoS2-TiO2 nanocomposites
Shakya, Jyoti; Mohanty, T.
2016-05-01
The MoS2-TiO2 nanocomposites have been synthesized by sol-gel method and characterized by different microscopic and spectroscopic techniques. The crystallinity of these nanocomposites has been confirmed by X-ray diffraction (XRD) analysis. The Raman spectrum of MoS2-TiO2 nanocomposites consists of three distinct peaks (E1 g, E1 2g and A1g) which are associated with TiO2 and MoS2. The morphological study is carried out by scanning electron microscope. The effect of microwave irradiation on the band gap of MoS2-TiO2 nanocomposites has been investigated; it is observed that the microwave irradiation causes decrease in the band gap of MoS2-TiO2 nanocomposites. The microwave treated MoS2-TiO2 thin films offers a novel process route in treating thin films for commercial applications.
Very low band gap thiadiazoloquinoxaline donor-acceptor polymers as multi-tool conjugated polymers.
Steckler, Timothy T; Henriksson, Patrik; Mollinger, Sonya; Lundin, Angelica; Salleo, Alberto; Andersson, Mats R
2014-01-29
Here we report on the synthesis of two novel very low band gap (VLG) donor-acceptor polymers (Eg ≤ 1 eV) and an oligomer based on the thiadiazoloquinoxaline acceptor. Both polymers demonstrate decent ambipolar mobilities, with P1 showing the best performance of ∼10(-2) cm(2) V(-1) s(-1) for p- and n-type operation. These polymers are among the lowest band gap polymers (≲0.7 eV) reported, with a neutral λmax = 1476 nm (P2), which is the farthest red-shifted λmax reported to date for a soluble processable polymer. Very little has been done to characterize the electrochromic aspects of VLG polymers; interestingly, these polymers actually show a bleaching of their neutral absorptions in the near-infrared region and have an electrochromic contrast up to 30% at a switching speed of 3 s.
Band gap and chemically ordered domain structure of a graphene analogue BCN
Venu, K.; Kanuri, S.; Raidongia, K.; Hembram, K. P. S. S.; Waghmare, U. V.; Datta, R.
2010-12-01
Chemically synthesized few layer graphene analogues of B xC yN z are characterized by aberration corrected transmission electron microscopy and high resolution electron energy loss spectroscopy (HREELS) to determine the local phase, electronic structure and band gap. HREELS band gap studies of a B xC yN z composition reveal absorption edges at 2.08, 3.43 and 6.01 eV, indicating that the B xC yN z structure may consist of domains of different compositions. The K-absorption edge energy position of the individual elements in B xC yN z is determined and compared with h-BN and graphite. An understanding of these experimental findings is developed with complementary first-principles based calculations of the various ordered configurations of B xC yN z.
Tuning the Refractive Index and Optical Band Gap of Silk Fibroin Films by Electron Irradiation
Directory of Open Access Journals (Sweden)
S. Asha
2015-01-01
Full Text Available The Bombyx mori silk fibroin (SF films were prepared by solution casting method and effects of electron beam on the optical properties and optical constants of the films have been studied by using UV-Visible spectrophotometer. Optical properties like optical band gap Eg, refractive index n, extinction coefficient k, optical conductivity σopt, and dielectric constants ε∗ of virgin and electron irradiated films were determined by using UV-Visible absorption and transmission spectra. It was found that the reduction in optical band gap and increase in refractive index with increasing radiation dosage was observed. It is also observed from results that there is increase in dielectric constants with increasing photon energy. The observed optical changes have been tried to be correlated with the structural changes, revealed through FT-IR spectroscopy. The present study is quite important for tailoring the optical responses of SF films as per specific requirements.
Directory of Open Access Journals (Sweden)
Amir Reza Sadrolhosseini
2016-01-01
Full Text Available A polypyrrole-nanoparticles reduced graphene oxide nanocomposite layer was prepared using electrochemical method. The prepared samples were characterized using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and UV-visible spectroscopy. The band gap of nanocomposite layers was calculated from UV-visible spectra and the thermal diffusivity of layers was measured using a photoacoustic technique. As experimental results, the optical band gap was in the range between 3.580 eV and 3.853 eV, and thermal diffusivity was increased with increasing the layer thickness from 2.873 cm2/s to 12.446 cm2/s.
Band gap tuning of armchair silicene nanoribbons using periodic hexagonal holes
International Nuclear Information System (INIS)
The popularity of graphene owing to its unique and exotic properties has triggered a great deal of interest in other two-dimensional nanomaterials. Among them silicene shows considerable promise for electronic devices with a carrier mobility comparable to graphene, flexible buckled structure, and expected compatibility with silicon electronics. Using first-principle calculations based on density functional theory, the electronic properties of armchair silicene nanoribbons perforated with periodic nanoholes (ASiNRPNHs) are investigated. Two different configurations of mono-hydrogenated (:H) and di-hydrogenated (:2H) silicene edges are considered. Pristine armchair silicene nanoribbons (ASiNRs) can be categorized into three branches with width W = 3P − 1, 3P, and 3P + 1, P is an integer. The order of their energy gaps change from “EG (3P − 1) < EG (3P) < EG (3P + 1)” for W-ASiNRs:H to “EG (3P + 1) < EG (3P − 1) < EG (3P)” for W-ASiNRs:2H. We found the band gaps of W-ASiNRs:H and (W + 2)-ASiNRs:2H are slightly different, giving larger band gaps for wider ASiNRs:2H. ASiNRPNHs' band gaps changed based on the nanoribbon's width, nanohole's repeat periodicity and position relative to the nanoribbon's edge compared to pristine ASiNRs because of changes in quantum confinement strength. ASiNRPNHs:2H are more stable than ASiNRPNHs:H and their band gaps are noticeably greater than ASiNRPNHs:H. We found that the value of energy band gap for 12-ASiNRPNHs:2H with repeat periodicity of 2 is 0.923 eV. This value is about 2.2 times greater than pristine ASiNR:2H and double that of the 12-ASiNRPNHs:H with repeat periodicity of 2
Band gap tuning of armchair silicene nanoribbons using periodic hexagonal holes
Energy Technology Data Exchange (ETDEWEB)
Mehdi Aghaei, Sadegh; Calizo, Irene, E-mail: icalizo@fiu.edu [Department of Electrical and Computer Engineering, Florida International University, Miami, Florida 33174 (United States)
2015-09-14
The popularity of graphene owing to its unique and exotic properties has triggered a great deal of interest in other two-dimensional nanomaterials. Among them silicene shows considerable promise for electronic devices with a carrier mobility comparable to graphene, flexible buckled structure, and expected compatibility with silicon electronics. Using first-principle calculations based on density functional theory, the electronic properties of armchair silicene nanoribbons perforated with periodic nanoholes (ASiNRPNHs) are investigated. Two different configurations of mono-hydrogenated (:H) and di-hydrogenated (:2H) silicene edges are considered. Pristine armchair silicene nanoribbons (ASiNRs) can be categorized into three branches with width W = 3P − 1, 3P, and 3P + 1, P is an integer. The order of their energy gaps change from “E{sub G} (3P − 1) < E{sub G} (3P) < E{sub G} (3P + 1)” for W-ASiNRs:H to “E{sub G} (3P + 1) < E{sub G} (3P − 1) < E{sub G} (3P)” for W-ASiNRs:2H. We found the band gaps of W-ASiNRs:H and (W + 2)-ASiNRs:2H are slightly different, giving larger band gaps for wider ASiNRs:2H. ASiNRPNHs' band gaps changed based on the nanoribbon's width, nanohole's repeat periodicity and position relative to the nanoribbon's edge compared to pristine ASiNRs because of changes in quantum confinement strength. ASiNRPNHs:2H are more stable than ASiNRPNHs:H and their band gaps are noticeably greater than ASiNRPNHs:H. We found that the value of energy band gap for 12-ASiNRPNHs:2H with repeat periodicity of 2 is 0.923 eV. This value is about 2.2 times greater than pristine ASiNR:2H and double that of the 12-ASiNRPNHs:H with repeat periodicity of 2.
Energy Technology Data Exchange (ETDEWEB)
Morales-Acevedo, Arturo [CINVESTAV del IPN, Electrical Engineering Department, Avenida IPN No. 2508, 07360 Mexico, D. F. (Mexico)
2009-01-15
A simple model for the generation of carriers by photons incident on a (linearly) decreasing band-gap material, such as has been described in recent CIGS solar cells, is developed. The model can be generalized for different cases such as increasing band-gap grading or for having a more complex band-gap profile. The model developed for direct band semiconductors such as CIGS or AlGaAs allows us to define an effective absorption coefficient, so that the ideal photocurrent density can be calculated in a similar manner as for solar cells with non-graded band-gap materials. We show that this model gives completely different results as those expected from intuitive approaches for calculating this ideal photocurrent density. We also show that grading of the band-gap of the absorbing material in solar cells makes the photocurrent less sensitive to the total band-gap change, in such a way that the design of the band-gap variation can be more flexible in order to have other advantages such as higher built-in voltage or higher back surface field in the device structure. (author)
Band gap tunning in BN-doped graphene systems with high carrier mobility
Kaloni, T. P.
2014-02-17
Using density functional theory, we present a comparative study of the electronic properties of BN-doped graphene monolayer, bilayer, trilayer, and multilayer systems. In addition, we address a superlattice of pristine and BN-doped graphene. Five doping levels between 12.5% and 75% are considered, for which we obtain band gaps from 0.02 eV to 2.43 eV. We demonstrate a low effective mass of the charge carriers.
Optimizing the band gap of effective mass negativity in acoustic metamaterials
Tan, K. T.; Huang, H. H.; Sun, C. T.
2012-12-01
A dual-resonator microstructure design is proposed for acoustic metamaterials to achieve broadband effective mass negativity. We demonstrate the advantage of acoustic wave attenuation over a wider frequency spectrum as compared to the narrow band gap of a single-resonator design. We explicitly confirm the effect of negative effective mass density by analysis of wave propagation using finite element simulations. Examples of practical application like vibration isolation and blast wave mitigation are presented and discussed.
Band gap tunning in BN-doped graphene systems with high carrier mobility
International Nuclear Information System (INIS)
Using density functional theory, we present a comparative study of the electronic properties of BN-doped graphene monolayer, bilayer, trilayer, and multilayer systems. In addition, we address a superlattice of pristine and BN-doped graphene. Five doping levels between 12.5% and 75% are considered, for which we obtain band gaps from 0.02 eV to 2.43 eV. We demonstrate a low effective mass of the charge carriers
Zhang, Haichang
2013-01-01
In this work new monomers and polymers with donor-acceptor ability and polymers containing diketopyrrolo[3,2-b]pyrrole (isoDPP), benzodione and naphthodione chromophores in the main chain with deep colour, broad absorption and low band gap are prepared and investigated. The compounds might be suitable for electronic applications, especially in solar cell devices. Characteristic properties are studied using spectroscopic methods (UV/vis, fluorescecne, NMR spectroscopy), gel permeation chromato...
Short pulse equations and localized structures in frequency band gaps of nonlinear metamaterials
Energy Technology Data Exchange (ETDEWEB)
Tsitsas, N.L. [School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Zografos, Athens 15773 (Greece); Horikis, T.P. [Department of Mathematics, University of Ioannina, Ioannina 45110 (Greece); Shen, Y.; Kevrekidis, P.G.; Whitaker, N. [Department of Mathematics and Statistics, University of Massachusetts, Amherst, MA 01003-4515 (United States); Frantzeskakis, D.J., E-mail: dfrantz@phys.uoa.g [Department of Physics, University of Athens, Panepistimiopolis, Zografos, Athens 157 84 (Greece)
2010-03-01
We consider short pulse propagation in nonlinear metamaterials characterized by a weak Kerr-type nonlinearity in their dielectric response. Two short-pulse equations (SPEs) are derived for the high- and low-frequency 'band gaps' (where linear electromagnetic waves are evanescent) with linear effective permittivity epsilon<0 and permeability mu>0. The structure of the solutions of the SPEs is also briefly discussed, and connections with the soliton solutions of the nonlinear Schroedinger equation are made.
Thiophene-fused tetracene diimide with low band gap and ambipolar behavior
Ye, Qun
2011-11-18
The first tetracene diimide derivative fused with four thiophene rings, TT-TDI, was synthesized by an FeCl3 mediated oxidative cyclodehydrogenation reaction. TT-TDI exhibited a low band gap of 1.52 eV and amphoteric redox behavior. TT-TDI also showed a liquid crystalline property and ambipolar charge transport in thin film field-effect transistors. © 2011 American Chemical Society.
Tuning the electronic band-gap of fluorinated 3C-silicon carbide nanowires
Miranda Durán, Álvaro; Trejo Baños, Alejandro; Pérez, Luis Antonio; Cruz Irisson, Miguel
The possibility of control and modulation of the electronic properties of silicon carbide nanowires (SiCNWs) by varying the wire diameter is well known. SiCNWs are particularly interesting and technologically important, due to its electrical and mechanical properties, allowing the development of materials with specific electronic features for the design of stable and robust electronic devices. Tuning the band gap by chemical surface passivation constitutes a way for the modification of the electronic band gap of these nanowires. We present, the structural and electronic properties of fluorinated SiCNWs, grown along the [111] crystallographic direction, which are investigated by first principles. We consider nanowires with six diameters, varying from 0.35 nm to 2.13 nm, and eight random covering schemes including fully hydrogen- and fluorine terminated ones. Gibbs free energy of formation and electronic properties were calculated for the different surface functionalization schemes and diameters considered. The results indicate that the stability and band gap of SiCNWs can be tuned by surface passivation with fluorine atoms This work was supported by CONACYT infrastructure project 252749 and UNAM-DGAPA-PAPIIT IN106714. A.M. would like to thank for financial support from CONACyT-Retención. Computing resources from proyect SC15-1-IR-27 of DGTIC-UNAM are acknowledged.
Band gap calculation and photo catalytic activity of rare earths doped rutile TiO2
Institute of Scientific and Technical Information of China (English)
BIAN Liang; SONG Mianxin; ZHOU Tianliang; ZHAO Xiaoyong; DAI Qingqing
2009-01-01
The density of states (DOS) of 17 kinds of rare earths (RE) doped futile TiO2 was by using fast-principles density functional the-ory (DFF) calculation. The band gap widths of RE doped rutile TiO2 were important factors for altering their absorbing wavelengths. The results show that RE ions could obviously reduce the band gap widths and form of energy of rutile TiO2 except Lu, Y, Yb and Sc, and the order of absorbing wavelengths of RE doped rutile TiO2 were the same as that of the results of calculation. The ratio of RE dopant was an-other important factor for the photo catalytic activity of RE doped rutile TiO2, and there was an optimal ratio of dopant. There was a constant for predigesting the calculation difficulty, respectively, which were 0.5mol.% and 100 mol-1 under supposition. The band gap widths of RE doped rutile TiO2 by DFT calculation were much larger than that by experiment. Finally, by transferring the calculation values to experiment values, it could be found and predicted that RE enlarged obviously the absorbing wavelengh of futile TiO2. In addition, the degree of RE ions edging out the Ti atom using the parameters of RE elements was computed.
Band-gap tunable dielectric elastomer filter for low frequency noise
Jia, Kun; Wang, Mian; Lu, Tongqing; Zhang, Jinhua; Wang, Tiejun
2016-05-01
In the last decades, diverse materials and technologies for sound insulation have been widely applied in engineering. However, suppressing the noise radiation at low frequency still remains a challenge. In this work, a novel membrane-type smart filter, consisting of a pre-stretched dielectric elastomer membrane with two compliant electrodes coated on the both sides, is presented to control the low frequency noise. Since the stiffness of membrane dominates its acoustic properties, sound transmission band-gap of the membrane filter can be tuned by adjusting the voltage applied to the membrane. The impedance tube experiments have been carried out to measure the sound transmission loss (STL) of the filters with different electrodes, membrane thickness and pre-stretch conditions. The experimental results show that the center frequency of sound transmission band-gap mainly depends on the stress in the dielectric elastomer, and a large band-gap shift (more than 60 Hz) can be achieved by tuning the voltage applied to the 85 mm diameter VHB4910 specimen with pre-stretch {λ }0=3. Based on the experimental results and the assumption that applied electric field is independent of the membrane behavior, 3D finite element analysis has also been conducted to calculate the membrane stress variation. The sound filter proposed herein may provide a promising facility to control low frequency noise source with tonal characteristics.
Band gap engineered nano perforated graphene microstructures for field effect transistor
Palla, Penchalaiah; Tiwari, Durgesh Laxman; Ansari, Hasan Raza; Babu, Taraprasanna Saha; Ethiraj, Anita Sagadevan; Raina, J. P.
2016-05-01
To make use of exceptional properties of graphene in Field effect Transistor (FETs) for switching devices a band gap must be introduced in order to switch -off the device. Through periodic nano perforations a semi-metallic graphene is converted into semiconducting graphene. To understand the device physics behind the reported experiments theoretical simulations has been carried out. The present paper illustrates nano perforated semiconducting graphene Field effect Transistor (FETs) with micron scale dimensions. The simulation has been performed using drift-diffusion semi-classical and tight-binding based non-equilibrium green's function (NEGF) methods. The obtained simulation results are compared with previously reported experimental work. The device dimensions considered for simulations and the experiment are similar with neck width, hole periodicity and channel length of 6.3 nm, 16.3 nm and 1 µm respectively. The interesting and new finding in this work is the p-type I-V characteristics for small band gap devices and n-type behavior for large band gap devices.
Quantum confinement induced band gaps in MgB2 nanosheets
Xu, Bo Z.; Beckman, Scott P.
2016-09-01
The discovery of two-dimensional semiconducting materials, a decade ago, spawned an entire sub-field within solid-state physics that is focused on the development of nanoelectronics. Here we present a new class of semiconducting two-dimensional material based on hexagonal MgB2. Although MgB2 is a semimetal, similar to the other well-studied transition metal diborides, we demonstrate that, unlike the transition metal diborides, thinning MgB2, to create nanosheets, opens a band gap in the density of states. We predict that a 7 Å thick MgB2 nanosheet will have a band gap of 0.51 eV. MgB2 nanosheets differ from other two-dimensional semiconductors in that the band gap is introduced by (001) surfaces and is opened by the quantum confinement effect. The implications of these findings are that nanostructured MgB2 is not merely a new composition, but also has intrinsic mechanisms for tuning its electronic properties, which may facilitate the development of nanoelectronics.
Numerical modelling of Mars supersonic disk-gap-band parachute inflation
Gao, Xinglong; Zhang, Qingbin; Tang, Qiangang
2016-06-01
The transient dynamic behaviour of supersonic disk-gap-band parachutes in a Mars entry environment involving fluid structure interactions is studied. Based on the multi-material Arbitrary Lagrange-Euler method, the coupling dynamic model between a viscous compressible fluid and a flexible large deformation structure of the parachute is solved. The inflation performance of a parachute with a fixed forebody under different flow conditions is analysed. The decelerating parameters of the parachute, including drag area, opening loads, and coefficients, are obtained from the supersonic wind tunnel test data from NASA. Meanwhile, the evolution of the three-dimensional shape of the disk-gap-band parachute during supersonic inflation is presented, and the structural dynamic behaviour of the parachute is predicted. Then, the influence of the presence of the capsule on the flow field of the parachute is investigated, and the wake of unsteady fluid and the distribution of shock wave around the supersonic parachute are presented. Finally, the structural dynamic response of the canopy fabric under high-pressure conditions is comparatively analysed. The results show that the disk-gap-band parachute is well inflated without serious collapse. As the Mach numbers increase from 2.0 to 2.5, the drag coefficients gradually decrease, along with a small decrease in inflation time, which corresponds with test results, and proves the validity of the method proposed in this paper.
Tunable band gaps in acoustic metamaterials with periodic arrays of resonant shunted piezos
Institute of Scientific and Technical Information of China (English)
Chen Sheng-Bing; Wen Ji-Hong; Wang Gang; Wen Xi-Sen
2013-01-01
Periodic arrays of resonant shunted piezoelectric patches are employed to control the wave propagation in a twodimensional (2D) acoustic metamaterial.The performance is characterized by the finite element method.More importantly,we propose an approach to solving the conventional issue of the nonlinear eigenvalue problem,and give a convenient solution to the dispersion properties of 2D metamaterials with periodic arrays of resonant shunts in this article.Based on this modeling method,the dispersion relations of a 2D metamaterial with periodic arrays of resonant shunted piezos are calculated.The results show that the internal resonances of the shunting system split the dispersion curves,thereby forming a locally resonant band gap.However,unlike the conventional locally resonant gap,the vibrations in this locally resonant gap are unable to be completely localized in oscillators consisting of shunting inductors and piezo-patches.
A generation/recombination model assisted with two trap centers in wide band-gap semiconductors
Yamaguchi, Ken; Kuwabara, Takuhito; Uda, Tsuyoshi
2013-03-01
A generation/recombination (GR) model assisted with two trap centers has been proposed for studying reverse current on pn junctions in wide band-gap semiconductors. A level (Et1) has been assumed to be located near the bottom of the conduction band and the other (Et2) to be near the top of the valence band. The GR model has been developed by assuming (1) a high-electric field; F, (2) a short distance; d, between trap centers, (3) reduction in an energy-difference; Δeff = |Et1 - Et2| - eFd, and (4) hopping or tunneling conductions between trap centers with the same energy-level (Δeff ≈ 0). The GR rate has been modeled by trap levels, capture cross-sections, trap densities, and transition rate between trap centers. The GR rate, about 1010 greater than that estimated from the single-level model, has been predicted on pn junctions in a material with band-gap of 3.1 eV. Device simulations using the proposed GR model have been demonstrated for SiC diodes with and without a guard ring. A reasonable range for reverse current at room temperature has been simulated and stable convergence has been obtained in a numerical scheme for analyzing diodes with an electrically floating region.
Aziz, Hany; Wang, Qi
2015-10-01
The limited performance stability and gradual loss in the electroluminescence efficiency of OLEDs utilizing wide band-gap materials, such as blue-emitting phosphorescent and fluorescent devices, continues to be a challenge for wider technology adoption. We recently found that interactions between excitons and polarons play an important role in the aging behavior of electroluminescent materials, and that a correlation exists between the susceptibility of these materials to this aging mode and their band-gap. This degradation mode is also found to be often associated with the emergence of new bands - at longer wavelength - in the electroluminescence spectra of the materials, that can often be detected after prolonged electrical driving. Such bands contribute to the increased spectral broadening and color purity loss often observed in these devices over time. Exciton-polaron interactions, and the associated degradation, are also found to occur most significantly in the vicinity of device inter-layer interfaces such as at the interface between the emitter layer and the electron or hole transport layers. New results obtained from investigations of these phenomena in a wide range of commonly used host and guest OLED materials will be presented.
Research on the band-gap of InN grown on silicon substrates
Energy Technology Data Exchange (ETDEWEB)
Xiao, Hongling; Wang, Xiaoliang; Wang, Junxi; Zhang, Nanhong; Liu, Hongxin; Zeng, Yiping; Li, Jinmin [Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China)
2006-03-15
Photoluminescence (PL) and absorption experiments were carried out to examine the fundamental band-gap of InN films grown on silicon substrates. A strong PL peak at 0.78 eV was observed at room temperature, which is much lower than the commonly accepted value of 1.9 eV. The integrated PL intensity was found to depend linearly on the excitation laser intensity over a wide intensity range. These results strongly suggest that the observed PL is related to the emission of the fundamental inter-band transitions of InN rather than to deep defect or impurity levels. Due to the effect of band-filling with increasing free electron concentration, the absorption edge shifts to higher energy. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Research on the band-gap of InN grown on silicon substrates
International Nuclear Information System (INIS)
Photoluminescence (PL) and absorption experiments were carried out to examine the fundamental band-gap of InN films grown on silicon substrates. A strong PL peak at 0.78 eV was observed at room temperature, which is much lower than the commonly accepted value of 1.9 eV. The integrated PL intensity was found to depend linearly on the excitation laser intensity over a wide intensity range. These results strongly suggest that the observed PL is related to the emission of the fundamental inter-band transitions of InN rather than to deep defect or impurity levels. Due to the effect of band-filling with increasing free electron concentration, the absorption edge shifts to higher energy. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Increased visible-light photocatalytic activity of TiO2 via band gap manipulation
Pennington, Ashley Marie
Hydrogen gas is a clean burning fuel that has potential applications in stationary and mobile power generation and energy storage, but is commercially produced from non-renewable fossil natural gas. Using renewable biomass as the hydrocarbon feed instead could provide sustainable and carbon-neutral hydrogen. We focus on photocatalytic oxidation and reforming of methanol over modified titanium dioxide (TiO2) nanoparticles to produce hydrogen gas. Methanol is used as a model for biomass sugars. By using a photocatalyst, we aim to circumvent the high energy cost of carrying out endothermic reactions at commercial scale. TiO2 is a semiconductor metal oxide of particular interest in photocatalysis due to its photoactivity under ultraviolet illumination and its stability under catalytic reaction conditions. However, TiO2 primarily absorbs ultraviolet light, with little absorption of visible light. While an effective band gap for absorbance of photons from visible light is 1.7 eV, TiO2 polymorphs rutile and anatase, have band gaps of 3.03 eV and 3.20 eV respectively, which indicate ultraviolet light. As most of incident solar radiation is visible light, we hypothesize that decreasing the band gap of TiO2 will increase the efficiency of TiO2 as a visible-light active photocatalyst. We propose to modify the band gap of TiO2 by manipulating the catalyst structure and composition via metal nanoparticle deposition and heteroatom doping in order to more efficiently utilize solar radiation. Of the metal-modified Degussa P25 TiO2 samples (P25), the copper and nickel modified samples, 1%Cu/P25 and 1%Ni/P25 yielded the lowest band gap of 3.05 eV each. A difference of 0.22 eV from the unmodified P25. Under visible light illumination 1%Ni/P25 and 1%Pt/P25 had the highest conversion of methanol of 9.9% and 9.6%, respectively.
A model describing the pressure dependence of the band gap energy for the group III-V semiconductors
Zhao, Chuan-Zhen; Wei, Tong; Sun, Xiao-Dong; Wang, Sha-Sha; Lu, Ke-Qing
2016-08-01
A model describing the pressure dependence of the band gap energy for the group III-V semiconductors has been developed. It is found that the model describes the pressure dependence of the band gap energy very well. It is also found that, although the pressure dependence of the band gap energy for both the conventional III-V semiconductors and the dilute nitride alloys can be described well by the model in this work, the physical mechanisms for them are different. In addition, the influence of the nonlinear compression of the lattice on the band gap energy is smaller than that of the coupling interaction between the N level and the conduction band minimum of the host material.
Wave propagation in ordered, disordered, and nonlinear photonic band gap materials
Energy Technology Data Exchange (ETDEWEB)
Lidorikis, Elefterios
1999-12-10
Photonic band gap materials are artificial dielectric structures that give the promise of molding and controlling the flow of optical light the same way semiconductors mold and control the electric current flow. In this dissertation the author studied two areas of photonic band gap materials. The first area is focused on the properties of one-dimensional PBG materials doped with Kerr-type nonlinear material, while, the second area is focused on the mechanisms responsible for the gap formation as well as other properties of two-dimensional PBG materials. He first studied, in Chapter 2, the general adequacy of an approximate structure model in which the nonlinearity is assumed to be concentrated in equally-spaced very thin layers, or 6-functions, while the rest of the space is linear. This model had been used before, but its range of validity and the physical reasons for its limitations were not quite clear yet. He performed an extensive examination of many aspects of the model's nonlinear response and comparison against more realistic models with finite-width nonlinear layers, and found that the d-function model is quite adequate, capturing the essential features in the transmission characteristics. The author found one exception, coming from the deficiency of processing a rigid bottom band edge, i.e. the upper edge of the gaps is always independent of the refraction index contrast. This causes the model to miss-predict that there are no soliton solutions for a positive Kerr-coefficient, something known to be untrue.
Band gap opening in strongly compressed diamond observed by x-ray energy loss spectroscopy
Energy Technology Data Exchange (ETDEWEB)
Gamboa, E. J. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Fletcher, L. B. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Lee, H. J. [SLAC National Accelerator Lab., Menlo Park, CA (United States); MacDonald, M. J. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Univ. of Michigan, Ann Arbor, MI (United States); Zastrau, U. [High-Energy Density Science Group, Hamburg (Germany); Gauthier, M. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Gericke, D. O. [Univ. of Warwick (United Kingdom); Vorberger, J. [Helmholtz Association of German Research Centres, Dresden (Germany); Granados, E. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Hastings, J. B. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Glenzer, S. H. [SLAC National Accelerator Lab., Menlo Park, CA (United States)
2016-01-25
The extraordinary mechanical and optical properties of diamond are the basis of numerous technical applications and make diamond anvil cells a premier device to explore the high-pressure behavior of materials. However, at applied pressures above a few hundred GPa, optical probing through the anvils becomes difficult because of the pressure-induced changes of the transmission and the excitation of a strong optical emission. Such features have been interpreted as the onset of a closure of the optical gap in diamond, and can significantly impair spectroscopy of the material inside the cell. In contrast, a comparable widening has been predicted for purely hydrostatic compressions, forming a basis for the presumed pressure stiffening of diamond and resilience to the eventual phase change to BC8. We here present the first experimental evidence of this effect at geo-planetary pressures, exceeding the highest ever reported hydrostatic compression of diamond by more than 200 GPa and any other measurement of the band gap by more than 350 GPa. We here apply laser driven-ablation to create a dynamic, high pressure state in a thin, synthetic diamond foil together with frequency-resolved x-ray scattering as a probe. The frequency shift of the inelastically scattered x-rays encodes the optical properties and, thus, the behavior of the band gap in the sample. Using the ultra-bright x-ray beam from the Linac Coherent Light Source (LCLS), we observe an increasing direct band gap in diamond up to a pressure of 370 GPa. This finding points to the enormous strains in the anvils and the impurities in natural Type Ia diamonds as the source of the observed closure of the optical window. Our results demonstrate that diamond remains an insulating solid to pressures approaching its limit strength.
Froufe-Pérez, Luis S; Damasceno, Pablo F; Muller, Nicolas; Haberko, Jakub; Glotzer, Sharon C; Scheffold, Frank
2016-01-01
We study photonic band gap formation in two-dimensional high refractive index disordered ma- terials where the dielectric structure is derived from packing disks in real and reciprocal space. Numerical calculations of the photonic density of states demonstrate the presence of a band gap for all polarizations in both cases. We find that the band gap width is controlled by the increase in positional correlation inducing short-range order and hyperuniformity concurrently. Our findings suggest that the optimization of short-range order, in particular the tailoring of Bragg scattering at the isotropic Brillouin zone, are of key importance for designing disordered PBG materials.
Study of the band--gap shift in CdS films: Influence of thermal annealing in different atmospheres
S.A. Tomas
1995-01-01
We study by photoacoustic spectroscopy the band--gap shift effect of CdS films. The CdS films were grown by chemical bath deposition and exposed to different annealing atmospheres over a range of temperature in which the sample structure is observed to change. We show the band--gap evolution as a function of temperature of thermal annealing and determine the process which produces the best combination of high band--gap energy and low resistivity. It allows us to know a possible procedure to o...
Controllable Synthesis of Band Gap-Tunable and Monolayer Transition Metal Dichalcogenide Alloys
Directory of Open Access Journals (Sweden)
Sheng-Han eSu
2014-07-01
Full Text Available The electronic and optical properties of transition metal dichalcogenide (TMD materials are directly governed by their energy gap; thus, the band gap engineering has become an important topic recently. Theoretical and some experimental results have indicated that these monolayer TMD alloys exhibit direct-gap properties and remain stable at room temperature, making them attractive for optoelectronic applications. Here we systematically compared the two approaches of forming MoS2xSe2(1-x monolayer alloys: selenization of MoS2 and sulfurization of MoSe2. The optical energy gap of as-grown CVD MoS2 can be continuously modulated from 1.86 eV (667 nm to 1.57 eV (790 nm controllable by the reaction temperature. Spectroscopic and microscopic evidences show that the Mo-S bonds can be replaced by the Mo-Se bonds in a random and homogeneous manner. By contrast, the replacement of Mo-Se by Mo-S does not randomly occur in the MoSe2 lattice, where the reaction preferentially occurs along the crystalline orientation of MoSe2 and thus the MoSe2/MoS2 biphases are easily observed in the alloys, which makes the optical band gap of these alloys distinctly different. Therefore, the selenization of metal disulfide is preferred and the proposed synthetic strategy opens up a simple route to control the atomic structure as well as optical properties of monolayer TMD alloys.
Comparison of the mechanism of gap formation for tri- and bi-component phononic crystal
Institute of Scientific and Technical Information of China (English)
Zhao Hong-Gang; Wen Ji-Hong; Liu Yao-Zong; Yu Dian-Long; Wang Gang; Wen Xi-Sen
2008-01-01
Using an exact Mie scattering solution, this paper investigates the mode conversions during the Mie scattering of a single bi- or one-component sphere in unbounded epoxy. Then the formation mechanism of the first complete gap in the corresponding tri- or bi-component phononic crystal is investigated by the multiple-scattering method. It is shown that the heavy density of the scatterer plays an essential role in the Mie resonance and the formation of the gaps for both types of the phononic crystals. For the tri-component phononic crystal, the gap is mainly induced by the Mie resonance of the single scatterer. For the bi-component phononic crystal, the transverse wave (by mode-conversion during the Mie scattering under a longitudinal wave incidence) is modulated by the periodicity and governed by the Bloch theory,which induces the gap cooperatively.
Nanoscale Imaging of Band Gap and Defects in Polycrystalline CdTe Photovoltaic Devices
Zhitenev, Nikolai; Yoon, Yohan; Chae, Jungseok; Katzenmeyer, Aaron; Yoon, Heayoung; An, Sangmin; Shumacher, Joshua; Centrone, Andrea
To further increase the power efficiency of polycrystalline thin film photovoltaic (PV) technology, a detailed understanding of microstructural properties of the devices is required. In this work, we investigate the microstructure of CdTe PV devices using two optical spectroscopies. Sub-micron thickness lamella samples were cut out from a PV device, either in cross-section or in-plane, by focused ion beam. The first technique is the photothermal induced resonance (PTIR) used to obtain absorption spectra over a broad range of wavelengths. In PTIR, a wavelength tunable pulsed laser is combined with an atomic force microscope to detect the local thermal expansion of lamella CdTe sample induced by light absorption. The second technique based on a near-field scanning optical microscope maps the local absorption at fixed near-IR wavelengths with energies at or below CdTe band-gap energy. The variation of the band gap throughout the CdTe absorber determined from PTIR spectra is ~ 20 meV. Both techniques detect strong spatial variation of shallow defects over different grains. The spatial distribution of mid-gap defects appears to be more uniform. The resolution, the sensitivity and the applicability of these two approaches are compared.
Simplification of QPLDA: A practical method to the correction for the LDA band gap problem
Kikuchi, Akihito
2012-01-01
It is necessary to employ quasi-particle calculations to correct band gap problems in LDA. As an expedient way for the reliable but massive GWA, Quasi-Particle-LDA (QPLDA) is proposed by several authors, where the total computational cost scales with N. Historically, Sham and Kohn introduced the idea of the local mass operator based on local wavenumber similar to WKB, but did not execute actual numerical calculations. They took into account exchange interaction only, from which we could not expect precise treatments anyway. Later, Pickett and Wang had proposed a more qualitative method and had shown its potentiality in examples of semiconductors, such as silicon, diamond and GaP. They used a model analytic formula for the dialectic function and adopted a model energy dispersion, which is free-electron like one, except that, being accompanied with an artificial band-gap discontinuity. The latter method has two shortcomings. First, to execute the calculation, several parameters are needed, such as the macroscop...
Band gap engineering and \\vec{k}\\cdot \\vec{\\pi } electronic structure of lead and tin tellurides
Behera, S. S.; Tripathi, G. S.
2016-06-01
We study the effect of the variation of energy gap on the k\\cdot π electronic structure of PbTe and SnTe, using a six-level basis at the L point. The basis functions in both the systems have the same transformation properties. However, the basis functions of the band edge states in SnTe are reversed with respect to the same in PbTe. Band dispersions are obtained analytically for a two band model. As the band gap decreases, the bands become linear. Far bands are included in the electronic dispersion, using perturbation theory. Fermi energy and the Density of States at the Fermi energy, { D }({\\varepsilon }F), are calculated for different carrier concentrations and energy gaps through a self-consistent approach. Interesting results are seen when the energy gap is reduced from the respective equilibrium values. For both the systems, the Fermi energy increases as the gap is decreased. The behavior of { D }({\\varepsilon }F) is, however, different. It decreases with the gap. It is also on expected lines. Calculated values of the electronic effective mass, as a function of temperature, energy gap and carrier concentration, are compared with previously published data. As distinguished from a first principles calculation, the work has focused on the carrier dependent electronic parameters for use both by theorists and experimenters as well.
Chun-Ying Kang; Shu Lin; Hua Zong; Zhi-Hua Zhao; Xue-Ying Zhang
2015-01-01
A wide-band circularly polarized wide-gap antenna loaded with a Y-shaped metal strip applied to L-band is proposed in this paper. The Y-shaped metal strip coupling motivates the wide gap to achieve wide-band circularly polarized radiation. Both the simulated results by CST Microwave Studio and the measured results indicate that the antenna impedance bandwidth (reflection coefficient less than −10 dB) and axial ratio bandwidth (AR < 3 dB) are 35.9% (1.1–1.71 GHz). The antenna produces a dual c...
Flexible band gap tuning of hexagonal boron nitride sheets interconnected by acetylenic bonds.
Zhang, Hongyu; Luo, Youhua; Feng, Xiaojuan; Zhao, Lixia; Zhang, Meng
2015-08-21
The energetic and electronic properties of acetylenic-bond-interconnected hexagonal boron nitride sheets (BNyne), in which the number of rows of BN hexagonal rings (denoted as BN width) between neighboring arrays of acetylenic linkages increases consecutively, have been explored using first-principles calculations. Depending on the spatial position of B/N atoms with respect to the acetylenic linkages, there are two different types of configurations. The band structure features and band gap evolutions of BNyne structures as a function of the BN width can be categorized into two families, corresponding to two distinct types of configurations. In particular, for both types of BNyne structures, the band gap variations exhibit odd-even oscillating behavior depending on the BN width, which is related to the different symmetries of acetylenic chains in the unit cell. These results suggest that the embedded linear acetylenic chains can provide more flexibility for manipulation of the atomic and electronic properties of hexagonal boron nitride. These sp-sp(2) hybrid structures might promise importantly potential applications for developing nanoscale electronic and optoelectronic devices. PMID:26194068
A model for the direct-to-indirect band-gap transition in monolayer MoSe2 under strain
Indian Academy of Sciences (India)
Ruma Das; Priya Mahadevan
2015-06-01
A monolayer of MoSe2 is found to be a direct band-gap semiconductor. We show, within ab-initio electronic structure calculations, that a modest biaxial tensile strain of 3% can drive it into an indirect band-gap semiconductor with the valence band maximum (VBM) shifting from point to point. An analysis of the charge density reveals that while Mo–Mo interactions contribute to the VBM at 0% strain, Mo–Se interactions contribute to the highest occupied band at point. A scaling of the hopping interaction strengths within an appropriate tight binding model can capture the transition.
International Nuclear Information System (INIS)
By definition, a temporal photonic crystal (TPC) has a permittivity ε(t) that varies periodically with time. We prove that, in the long wavelength limit, a TPC is accurately mimicked by a dynamic transmission line (DTL) having a capacitance (inductance) per unit length equal to ε(t) (μ). Employing a DTL in the microwave region, we measured the photonic band structure, which results to display a genuine wave vector (k or β) gap, in very good agreement with our theoretical model and the equivalent TPC
Energy Technology Data Exchange (ETDEWEB)
Reyes-Ayona, J. R.; Halevi, P., E-mail: halevi@inaoep.mx [Electronics Department, Instituto Nacional de Astrofísica, Óptica y Electrónica, Tonantzintla, Puebla 72840 (Mexico)
2015-08-17
By definition, a temporal photonic crystal (TPC) has a permittivity ε(t) that varies periodically with time. We prove that, in the long wavelength limit, a TPC is accurately mimicked by a dynamic transmission line (DTL) having a capacitance (inductance) per unit length equal to ε(t) (μ). Employing a DTL in the microwave region, we measured the photonic band structure, which results to display a genuine wave vector (k or β) gap, in very good agreement with our theoretical model and the equivalent TPC.
Reyes-Ayona, J. R.; Halevi, P.
2015-08-01
By definition, a temporal photonic crystal (TPC) has a permittivity ɛ(t) that varies periodically with time. We prove that, in the long wavelength limit, a TPC is accurately mimicked by a dynamic transmission line (DTL) having a capacitance (inductance) per unit length equal to ɛ(t) (μ). Employing a DTL in the microwave region, we measured the photonic band structure, which results to display a genuine wave vector (k or β) gap, in very good agreement with our theoretical model and the equivalent TPC.
International Nuclear Information System (INIS)
We propose a multiscale spoof–insulator–spoof (SIS) waveguide by introducing periodic geometry modulation in the wavelength scale to a SIS waveguide made of a perfect electric conductor. The MSIS consists of multiple SIS subcells. The dispersion relationship of the fundamental guided mode of the spoof surface plasmon polaritons (SSPPs) is studied analytically within the small gap approximation. It is shown that the multiscale SIS possesses microwave band gap (MBG) due to the Bragg scattering. The ‘gap maps’ in the design parameter space are provided. We demonstrate that the geometry of the subcells can efficiently adjust the effective refraction index of the elementary SIS and therefore further control the width and the position of the MBG. The results are in good agreement with numerical calculations by the finite element method (FEM). For finite-sized MSIS of given geometry in the millimeter scale, FEM calculations show that the first-order symmetric SSPP mode has zero transmission in the MBG within frequency range from 4.29 to 5.1 GHz. A cavity mode is observed inside the gap at 4.58 GHz, which comes from a designer ‘point defect’ in the multiscale SIS waveguide. Furthermore, ultrathin MSIS waveguides are shown to have both symmetric and antisymmetric modes with their own MBGs, respectively. The deep-subwavelength confinement and the great degree of control of the propagation of SSPPs in such structures promise potential applications in miniaturized microwave device. (paper)
Strain-modulated ferromagnetism and band gap of Mn doped Bi2Se3
Qi, Shifei; Yang, Hualing; Chen, Juan; Zhang, Xiaoyang; Yang, Yingping; Xu, Xiaohong
2016-07-01
The quantized anomalous Hall effect (QAHE) have been theoretically predicted and experimentally confirmed in magnetic topological insulators (TI), but dissipative channels resulted by small-size band gap and weak ferromagnetism make QAHE be measured only at extremely low temperature (theory calculations, we systemically study of the magnetic properties and electronic structures of Mn doped Bi2Se3 with in-plane and out-of-plane strains. It is found that out-of-plane tensile strain not only improve ferromagnetism, but also enlarge Dirac-mass gap (up to 65.6 meV under 6% strain, which is higher than the thermal motion energy at room temperature ~26 meV) in the Mn doped Bi2Se3. Furthermore, the underlying mechanisms of these tunable properties are also discussed. This work provides a new route to realize high-temperature QAHE and paves the way towards novel quantum electronic device applications.
Effects of Controlling the AZO Thin Film's Optical Band Gap on AZO/MEH-PPV Devices with Buffer Layer
Directory of Open Access Journals (Sweden)
Jaehyoung Park
2012-01-01
Full Text Available Organic/inorganic hybrid solar cells were fabricated incorporating aluminum-doped zinc oxide (AZO thin films of varying optical band gap in AZO/poly(2-methoxy-5-(2′-ethyl-hexyloxy-p-phenylene vinylene structures. The band gaps were controlled by varying the flow rates of Ar and O2 used to deposit the AZO. Devices with CdS buffer layer were also fabricated for improved efficiency. The effects of AZO optical band gap were assessed by testing the I–V characteristics of devices with structures of glass/ITO/AZO/MEH-PPV/Ag under AM1.5 illumination (100 mW/cm2. Efficiency was improved about 30 times by decreasing the AZO optical band gap, except in devices deposited without oxygen. A power conversion efficiency of 0.102% was obtained with the incorporation of a CdS buffer layer.
Band gap opening in silicene on MgBr2(0001) induced by Li and Na
Zhu, Jiajie
2014-11-12
Silicene consists of a monolayer of Si atoms in a buckled honeycomb structure and is expected to be well compatible with the current Si-based technology. However, the band gap is strongly influenced by the substrate. In this context, the structural and electronic properties of silicene on MgBr2(0001) modified by Li and Na are investigated by first-principles calculations. Charge transfer from silicene (substrate) to substrate (silicene) is found for substitutional doping (intercalation). As compared to a band gap of 0.01 eV on the pristine substrate, strongly enhanced band gaps of 0.65 eV (substitutional doping) and 0.24 eV (intercalation) are achieved. The band gap increases with the dopant concentration.
Suppressing band gap of MoS{sub 2} by the incorporation of four- and eight-membered rings
Energy Technology Data Exchange (ETDEWEB)
Zhu, Liyan; Zhang, Tingting, E-mail: ttzhang@hytc.edu.cn [Huaiyin Normal University, School of Physics and Electronic & Electrical Engineering, and Jiangsu Key Laboratory of Modern Measurement Technology and Intelligent Systems (China)
2015-05-15
A stable planar allotrope of MoS{sub 2}, formed by introducing four- and eight-membered rings into its hexagonal network (H468), is identified to be a narrow direct-band-gap semiconductor by first principle calculations, which is remarkably different from the large band gap semiconductor of conventional MoS{sub 2} and also the zero band gap allotrope consisting of four- and eight-membered rings (H48) only. The medium-sized direct band gap indicates that H468 would find applications in nanoelectronics and near-infrared optoelectronic devices. Furthermore, the distinctive simulated scanning tunneling microscope images under positive and negative biases might be a unique characteristic for the experimental identification of such an allotrope of MoS{sub 2}.
Oxygen pressure-dependent band gap modification in Cu-doped and -undoped ZnO films
International Nuclear Information System (INIS)
A study of the effect of oxygen flow rate on the energy band gap of the c-axis-oriented ZnO and Zn0.95Cu0.05O films, deposited on glass substrates by RF magnetron sputtering, is reported. An increase of the oxygen flow rate (partial pressure) during deposition results in an increase in the band gap (Eg) of Zn0.95Cu0.05O films from 2.80 to 3.10 eV. The maximum observed enhancement is 10.5%. For the same oxygen flow rate (partial pressure), the band gap of a Zn0.95Cu0.05O film is found to be lower than that of ZnO as predicted by Ferhat et al (2009 Appl. Phys. Lett. 94 142502). A correlation has been observed between the residual strain and the band gap of doped and undoped ZnO films. (paper)
Institute of Scientific and Technical Information of China (English)
Yuan Jin-Hui; Yu Chong-Xiu; Sang Xin-Zhu; Zhang Jin-Long; Zhou Gui-Yao; Li Shu-Guang; Hou Lan-Tian
2011-01-01
This paper investigates the guided-mode characteristics of hollow-core photonic band-gap fibre (HC-PBGF) with interstitial holes fabricated by an improved twice stack-and-draw technique at visible wavelengths. Based on the simulation model with interstitial holes, the influence of glass interstitial apexes on photonic band-gaps is discussed.The existing forms of guided-mode in part band gaps are shown by using the full-vector plane-wave method. In the experiment, the observed transmission spectrum corresponds to the part band gaps obtained by simulation. The fundamental and second-order guided-modes with mixture of yellow and green light are observed through choosing appropriate fibre length and adjusting coupling device. The loss mechanism of guided-modes in HC-PBGF is also discussed.
Effect of ZnO on the Physical Properties and Optical Band Gap of Soda Lime Silicate Glass
Directory of Open Access Journals (Sweden)
Mohd Sabri Mohd Ghazali
2012-06-01
Full Text Available This manuscript reports on the physical properties and optical band gap of five samples of soda lime silicate (SLS glass combined with zinc oxide (ZnO that were prepared by a melting and quenching process. To understand the role of ZnO in this glass structure, the density, molar volume and optical band gaps were investigated. The density and absorption spectra in the Ultra-Violet-Visible (UV-Visible region were recorded at room temperature. The results show that the densities of the glass samples increased as the ZnO weight percentage increased. The molar volume of the glasses shows the same trend as the density: the molar volume increased as the ZnO content increased. The optical band gaps were calculated from the absorption edge, and it was found that the optical band gap decreased from 3.20 to 2.32 eV as the ZnO concentration increased.
The study of the change in the optical band-gap of titania nanoparticles supported on silica
Directory of Open Access Journals (Sweden)
SH Nasirian
2013-09-01
Full Text Available In this letter, titania nanopowder and titania-silica nanocomposite were prepared using sol-gel method. Although the size of nanocrystallites and the mass fraction percent age of rutile phase ( after phase transformation were increased by increasing calcination in the two samples , their size in titania-silica nanocomposite was smaller than that in pure titania . Moreover, the calculations of the indirect optical band gap in the two samples show ed that by increasing temperature up to the transition onset point a nd above, the optical band gap had a steady growth and the corresponding wavelength reduced. However , a remarkable red shift in the indirect optical band gap was noticed in both samples around the transition onset point . The red shift was observed more in silica-titania nano composite than in titania nanopowder . In addition, the indirect band gap of silica-titania nano composite was less than that of pure titania in the same temperature conditions.
Melting and band gap-dynamics of shock-compressed graphite diagnosed by x-ray scattering at the LCLS
Zastrau, Ulf; Lee, Hae Ja
2015-11-01
The diversity of the electronic properties of carbon makes it of key interest to the material science community; By contrast, at the high pressures typical of planetary and stellar interiors, the behavior of carbon is poorly understood with large uncertainties in the conductivity and even the material phase. Tremendous efforts have been made to measure properties of warm dense matter (WDM) in extreme conditions, e.g. temperatures in excess of 1000 K of temperature and pressures in the Mbar regime. In laboratory experiments, practical issues with gradients in the temperature and density of shock compressed matter have hindered accurate measurement and further from distinguishing theoretical models. Here, we present measurements of melting of graphite upon coalescence of two counter-propagating shocks using combinations of spatially and spectrally resolved x-ray scattering methods at the LCLS free electron laser. The MEC nanosecond lasers launch counter-propagating shock waves into graphite. At shock coalescence, pressures in excess of 1 Mbar are reached. At given time delay, we measure scattering from the sample using 5070 eV x-ray pulses. We employed curved mosaic and perfect imaging crystals for spatially resolved x-ray scattering. Compared with hydrodynamics simulations, we present data on plasmon dispersion, axial compression gradients and finally carbon melting at shock coalescence. We have indication for a widening of the band gap during compression of the solid, while the band gab fully closes in the melt. UZ was supported by the German Volkswagen Foundation.
Photonic Band Gaps in 3D Network Structures with Short-range Order
Liew, Seng Fatt; Noh, Heeso; Schreck, Carl F; Dufresne, Eric R; O'Hern, Corey S; Cao, Hui
2011-01-01
We present a systematic study of photonic band gaps (PBGs) in three-dimensional (3D) photonic amorphous structures (PAS) with short-range order. From calculations of the density of optical states (DOS) for PAS with different topologies, we find that tetrahedrally connected dielectric networks produce the largest isotropic PBGs. Local uniformity and tetrahedral order are essential to the formation of PBGs in PAS, in addition to short-range geometric order. This work demonstrates that it is possible to create broad, isotropic PBGs for vector light fields in 3D PAS without long-range order.
Surface Plasmon-Induced Band Gap in the Photocurrent Response of Organic Solar Cells
Directory of Open Access Journals (Sweden)
Ribal Georges Sabat
2010-01-01
Full Text Available A 260 nm layer of organic bulk heterojunction blend of the polymer poly(3-hexylthiophene (P3HT and the fullerene [6,6]-phenyl C61-butyric (PCBM was spin-coated in between aluminum and gold electrodes, respectively, on top of a laser inscribed azo polymer surface-relief diffraction grating. Angle-dependent surface plasmons (SPs with a large band gap were observed in the normalized photocurrent by the P3HT-PCBM layer as a function of wavelength. The SP-induced photocurrents were also investigated as a function of the grating depth and spacing.
Wide Band Gap poluvodički sklopivi za učinsku elektroniku
Millán, José; Godignon, Philippe; Pérez-Tomás, Amador
2012-01-01
U današnje vrijeme napredak u polju učinske elektronike prvenstveno dolazi razvojem i uporabom Wide Band Gap (WGB) poluvodičkih uređaja. WBG poluvodi či kao Sic, GaN i dijamant pokazuju iznimna svojstva materijala, što omogućuje korištenje pri brzim promjenama stanja, visokim naponima i visokim temperaturama. Ova jedinstvena svojstva osiguravaju kvalitativne promjene njihovom primjenon u obradi energije. Od početnih energenata (ugljen, ulje, plin ili obnovljivi izvori) do završne faze korište...
Incorporation of Furan into Low Band-Gap Polymers for Efficient Solar Cells
Woo, Claire H.
2010-11-10
The design, synthesis, and characterization of the first examples of furan-containing low band-gap polymers, PDPP2FT and PDPP3F, with substantial power conversion efficiencies in organic solar cells are reported. Inserting furan moieties in the backbone of the conjugated polymers enables the use of relatively small solubilizing side chains because of the significant contribution of the furan rings to overall polymer solubility in common organic solvents. Bulk heterojunction solar cells fabricated from furan-containing polymers and PC71BM as the acceptor showed power conversion efficiencies reaching 5.0%. © 2010 American Chemical Society.
Anharmonic stabilization and band gap renormalization in the perovskite CsSnI_{3}
DEFF Research Database (Denmark)
Patrick, Christopher; Jacobsen, Karsten Wedel; Thygesen, Kristian Sommer
2015-01-01
Amongst the X(Sn,Pb)Y3 perovskites currently under scrutiny for their photovoltaic applications, the cubic B−α phase of CsSnI3 is arguably the best characterized experimentally. Yet, according to the standard harmonic theory of phonons, this deceptively simple phase should not exist at all due to...... of the electronic energies due to vibrations and find an unusual opening of the band gap, estimated as 0.24 and 0.11 eV at 500 and 300 K, which we attribute to the stretching of Sn-I bonds. Our work demonstrates the important role of temperature in accurately describing these materials....
Fock exchange in the augmented spherical waves method and the optical band gap
Bott, Erika
1999-01-01
It is investigated, if and how we can use the Fock exchange instead of the usual local exchange in the augmented spherical waves (ASW) program which is based on the Kohn-Sham formalism. In this way we want to obtain a density functional program which calculates optical band gap values which are closer to experimental values than those from the usual density functional approach or from Hartree-Fock calculations. We use as exchange-correlation energy functional the exact Fock exchange energy fu...
Continuous generation of Rubidium vapor in hollow-core photonic band-gap fibers
Donvalkar, Prathamesh S; Clemmen, Stephane; Gaeta, Alexander L
2015-01-01
We demonstrate high optical depths (50+/-5), lasting for hours in Rubidium-filled hollow-core photonic band-gap fibers, which represents a 1000X improvement over operation times previously reported. We investigate the vapor generation mechanism using both a continuous-wave and a pulsed light source and find that the mechanism for generating the Rubidium atoms is primarily due to thermal vaporization. Continuous generation of large vapor densities should enable measurements at the single-photon level by averaging over longer time scales.
A Large-Area Transferable Wide Band Gap 2D Silicon Dioxide Layer.
Büchner, Christin; Wang, Zhu-Jun; Burson, Kristen M; Willinger, Marc-Georg; Heyde, Markus; Schlögl, Robert; Freund, Hans-Joachim
2016-08-23
An atomically smooth silica bilayer is transferred from the growth substrate to a new support via mechanical exfoliation at millimeter scale. The atomic structure and morphology are maintained perfectly throughout the process. A simple heating treatment results in complete removal of the transfer medium. Low-energy electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and environmental scanning electron microscopy show the success of the transfer steps. Excellent chemical and thermal stability result from the absence of dangling bonds in the film structure. By adding this wide band gap oxide to the toolbox of 2D materials, possibilities for van der Waals heterostructures will be broadened significantly.
Mode multiplexing at 2×20Gbps over 19-cell hollow-core photonic band gap fibre
DEFF Research Database (Denmark)
Carpenter, Joel; Xu, Jing; Peucheret, Christophe;
2012-01-01
This paper demonstrates the first mode-multiplexed system over 19-cell hollow-core photonic band gap fibre, at 2×20Gbps using the LP0,1 and LP2,1-like modes.......This paper demonstrates the first mode-multiplexed system over 19-cell hollow-core photonic band gap fibre, at 2×20Gbps using the LP0,1 and LP2,1-like modes....
Composition dependence of the band gap of CuInSsub(2x)Sesub(2(1-x))
International Nuclear Information System (INIS)
The composition dependence of the band gap of CuInSsub(2x)Sesub(2(1-x)) solid solutions is studied by aid of infrared transmission spectra recorded at 77 K and 293 K. In the wavelength-modulated experiments the band gap was determined by the position of the maximum in the dI/dlambda spectrum and was found to change linearly with composition
Coupled flexural-torsional vibration band gap in periodic beam including warping effect
Institute of Scientific and Technical Information of China (English)
Fang Jian-Yu; Yu Dian-Long; Han Xiao-Yun; Cai Li
2009-01-01
The propagation of coupled flexural-torsional vibration in the periodic beam including warping effect is investigated with the transfer matrix theory.The band structures of the periodic beam,both including warping effect and ignoring warping effect,are obtained.The frequency response function of the finite periodic beams is simulated with finite element method,which shows large vibration attenuation in the frequency range of the gap as expected.The effect of warping stiffness on the band structure is studied and it is concluded that substantial error can be produced in high frequency range if the effect is ignored.The result including warping effect agrees quite well with the simulated result.
Pseudomorphic GeSn/Ge(001) quantum wells: Examining indirect band gap bowing
Energy Technology Data Exchange (ETDEWEB)
Tonkikh, Alexander A. [Max Planck Institute of Microstructure Physics, Weinberg 2 D-06120, Halle (Saale) (Germany); Institute for Physics of Microstructures RAS, GSP-105, Nizhniy Novgorod (Russian Federation); Eisenschmidt, Christian; Schmidt, Georg [Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 3 D-01620, Halle (Saale) (Germany); Talalaev, Vadim G. [Max Planck Institute of Microstructure Physics, Weinberg 2 D-06120, Halle (Saale) (Germany); ZIK SiLi-Nano, Martin Luther University Halle-Wittenberg, Karl-Freiherr-von-Fritsch-Str. 3 D-06120, Halle (Saale) (Germany); Zakharov, Nikolay D.; Werner, Peter [Max Planck Institute of Microstructure Physics, Weinberg 2 D-06120, Halle (Saale) (Germany); Schilling, Joerg [ZIK SiLi-Nano, Martin Luther University Halle-Wittenberg, Karl-Freiherr-von-Fritsch-Str. 3 D-06120, Halle (Saale) (Germany)
2013-07-15
A study of the bandgap character of compressively strained GeSn{sub 0.060-0.091}/Ge(001) quantum wells grown by molecular beam epitaxy is reported. The built-in strain in GeSn wells leads to an increased separation between L and {Gamma} conduction band minima. The prevalent indirect interband transitions in GeSn were probed by photoluminescence spectroscopy. As a result we could simulate the L-valley bowing parameter in GeSn alloys, b{sub L} = 0.80 {+-} 0.06 eV at 10 K. From this we conclude that even compressively strained GeSn/Ge(001) alloys could become direct band gap semiconductors at the Sn-fraction higher than 17.0 at. %.
Band gap narrowing of TiO2 by compensated codoping for enhanced photocatalytic activity
Institute of Scientific and Technical Information of China (English)
Jindou Huang; Shuhao Wen; Jianyong Liu; Guozhong He
2012-01-01
In this study,we have performed first-principles screened exchanged hybrid density function theory with the HSE06 function calculations of the C-Mo,C-W,N-Nb and N-Ta codoped anatase TiO2 systems to investigate the effect of codoping on the electronic structure of TiO2.The calculated results demonstrate that (W(s)+C(s)) codoped TiO2 narrows the band gap significantly,and have little influence on the position of conduction band edges,therefore,enhances the efficiency of the photocatalytic hydrogen generation from water and the photodegradation of organic pollutants.Moreover,the proper oxygen pressure and temperature are two key factors during synthesis which should be carefully under control so that the desired (W(s)+C(s)) codoped TiO2 can be obtained.
Katiyar, Ajit K.; Grimm, Andreas; Bar, R.; Schmidt, Jan; Wietler, Tobias; Joerg Osten, H.; Ray, Samit K.
2016-10-01
Compressively strained Ge films have been grown on relaxed Si0.45Ge0.55 virtual substrates using molecular beam epitaxy in the presence of Sb as a surfactant. Structural characterization has shown that films grown in the presence of surfactant exhibit very smooth surfaces with a relatively higher strain value in comparison to those grown without any surfactant. The variation of strain with increasing Ge layer thickness was analyzed using Raman spectroscopy. The strain is found to be reduced with increasing film thickness due to the onset of island nucleation following Stranski-Krastanov growth mechanism. No phonon assisted direct band gap photoluminescence from compressively strained Ge films grown on relaxed Si0.45Ge0.55 has been achieved up to room temperature. Excitation power and temperature dependent photoluminescence have been studied in details to investigate the origin of different emission sub-bands.
DEFF Research Database (Denmark)
Zhukovsky, Sergei; Orlov, Alexey A.; Babicheva, Viktoriia E.;
2014-01-01
We study theoretically the propagation of large-wave-vector waves (volume plasmon polaritons) in multilayer hyperbolic metamaterials with two levels of structuring. We show that when the parameters of a subwavelength metal-dielectric multilayer (substructure) are modulated (superstructured......) on a larger, wavelength scale, the propagation of volume plasmon polaritons in the resulting multiscale hyperbolic metamaterials is subject to photonic-band-gap phenomena. A great degree of control over such plasmons can be exerted by varying the superstructure geometry. When this geometry is periodic, stop......, fractal Cantor-like multiscale metamaterials are found to exhibit characteristic self-similar spectral signatures in the volume plasmonic band. Multiscale hyperbolic metamaterials are shown to be a promising platform for large-wave-vector bulk plasmonic waves, whether they are considered for use as a kind...
Modifications of band gap in Si/Ge multilayers through vacuum annealing
Sharma, A.; Tripathi, S.; Tripathi, J.; Shripathi, T.
2016-05-01
In the present paper, important experimental findings and their impact on physical properties of Si/Ge systems are presented with the focus on structural and electronic properties investigations also covering band gap engineering. The use of synchrotron radiation based valence band photoemission spectroscopy technique has been demonstrated which provides the variation in bandgap values as a function of annealing on [Si/Ge]x10 multilayers structure. For this purpose, the required VB offsets are obtained by considering the corresponding VB maximum of as prepared sample as a reference. The bandgap values thus obtained show a gradually decreasing pattern with increasing temperature, which is discussed in terms of the effect of various factors such as: (i) quantum confinement effect normally observed in confined systems (ii) change in the annealing induced intermixing leading to the formation of SiGe alloy and (iii) roughness at the surface/interface.
Gap states in the electronic structure of SnO2 single crystals and amorphous SnOx thin films
Haeberle, J.; Machulik, S.; Janowitz, C.; Manzke, R.; Gaspar, D.; Barquinha, P.; Schmeißer, D.
2016-09-01
The electronic structure of a SnO2 single crystal is determined by employing resonant photoelectron spectroscopy. We determine the core level, valence band, and X-ray absorption (XAS) data and compare these with those of amorphous SnOx thin films. We find similar properties concerning the data of the core levels, the valence band features, and the absorption data at the O1s edge. We find strong signals arising from intrinsic in-gap states and discuss their origin in terms of polaronic and charge-transfer defects. We deduce from the XAS data recorded at the Sn3d edge that the Sn4d10 ground state has contributions of 4d9 and 4d8 states due to configuration interaction. We identify localized electronic states depending on the strength of the 4d-5s5p interaction and of the O2p-to-Sn4d charge-transfer processes, both appear separated from the extended band-like states of the conduction band. For the amorphous SnOx thin films, significant differences are found only in the absorption data at the Sn3d-edge due to a stronger localization of the in-gap states.
Energy Technology Data Exchange (ETDEWEB)
Malek, M.F., E-mail: firz_solarzelle@yahoo.com [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); NANO-SciTech Centre (NST), Institute of Science (IOS), Universiti Teknologi MARA UiTM, 40450 Shah Alam, Selangor (Malaysia); Mamat, M.H. [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); Musa, M.Z. [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM) Pulau Pinang, Jalan Permatang Pauh, 13500 Permatang Pauh, Pulau Pinang (Malaysia); Soga, T. [Department of Frontier Materials, Nagoya Institute of Technology (NITech), Nagoya 466-8555 (Japan); Rahman, S.A. [Low Dimensional Materials Research Centre (LDMRC), Department of Physics, Faculty of Science, Universiti Malaya (UM), 50603 Kuala Lumpur (Malaysia); Alrokayan, Salman A.H.; Khan, Haseeb A. [Department of Biochemistry, College of Science, King Saud University (KSU), Riyadh 11451 (Saudi Arabia); Rusop, M. [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); NANO-SciTech Centre (NST), Institute of Science (IOS), Universiti Teknologi MARA UiTM, 40450 Shah Alam, Selangor (Malaysia)
2015-04-15
We report on the growth of Al-doped ZnO (ZAO) thin films prepared by the sol–gel technique associated with dip-coating onto Corning 7740 glass substrates. The influence of varying thermal annealing (T{sub a}) temperature on crystallisation behaviour, optical and electrical properties of ZAO films has been systematically investigated. All films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction 〈0 0 2〉. The metamorphosis of strain/stress effects in ZAO thin films has been investigated using X-ray diffraction. The as growth films have a large compressive stress of 0.55 GPa, which relaxed to 0.25 GPa as the T{sub a} was increased to 500 °C. Optical parameters such as optical transmittance, absorption coefficient, refractive index and optical band gap energy have been studied and discussed with respect to T{sub a}. All films exhibit a transmittance above 80–90% along the visible–NIR range up to 1500 nm and a sharp absorption onset below 400 nm corresponding to the fundamental absorption edge of ZnO. Experimental results show that the tensile stress in the films reveals an incline pattern with the optical band gap energy, while the compressive stress shows opposite relation. - Highlights: • Minimum stress of highly c-axis oriented ZAO was grown at suitable T{sub a} temperature. • The ZAO crystal orientation was influenced by strain/stress of the film. • Minimum stress/strain of ZAO film leads to lower defects. • Bandgap and defects were closely intertwined with strain/stress. • We report additional optical and electrical properties based on T{sub a} temperature.
Energy Technology Data Exchange (ETDEWEB)
Ribeiro, M. [Centro de Pesquisas Avancadas Wernher von Braun, Av. Alice de Castro P.N. Mattosinho 301, CEP 13098-392 Campinas, SP (Brazil); Ferreira, L.G. [Departamento de Fisica dos Materiais e Mecanica, Instituto de Fisica, Universidade de Sao Paulo, 05315-970 Sao Paulo, SP (Brazil); Fonseca, L.R.C. [Center for Semiconductor Components, State University of Campinas, R. Pandia Calogeras 90, 13083-870 Campinas, SP (Brazil); Ramprasad, R. [Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269 (United States)
2012-09-20
We performed ab initio calculations of the electronic structures of bulk CdSe and CdTe, and their interface band alignments on the CdSe in-plane lattice parameters. For this, we employed the LDA-1/2 self-energy correction scheme to obtain corrected band gaps and band offsets. Our calculations include the spin-orbit effects for the bulk cases, which have shown to be of importance for the equilibrium systems and are possibly degraded in these strained semiconductors. Therefore, the SO showed reduced importance for the band alignment of this particular system. Moreover, the electronic structure calculated along the transition region across the CdSe/CdTe interface shows an interesting non-monotonic variation of the band gap in the range 0.8-1.8 eV, which may enhance the absorption of light for corresponding frequencies at the interface between these two materials in photovoltaic applications.
Institute of Scientific and Technical Information of China (English)
Yang Ping; Li Pei; Zhang Li-Qiang; Wang Xiao-Liang; Wang Huan; Song Xi-Fu; Xie Fang-Wei
2012-01-01
The lattice,the band gap and the optical properties of n-type ZnO under uniaxial stress are investigated by firstprinciples calculations.The results show that the lattice constants change linearly with stress.Band gaps are broadened linearly as the uniaxial compressive stress increases.The change of band gap for n-type ZnO comes mainly from the contribution of stress in the c-axis direction,and the reason for band gap of n-type ZnO changing with stress is also explained.The calculated results of optical properties reveal that the imaginary part of the dielectric function decreases with the increase of uniaxial compressive stress at low energy.However,when the energy is higher than 4.0 eV,the imaginary part of the dielectric function increases with the increase of stress and a blueshift appears.There are two peaks in the absorption spectrum in an energy range of 4.0-13.0 eV.The stress coefficient of the band gap of n-type ZnO is larger than that of pure ZnO,which supplies the theoretical reference value for the modulation of the band gap of doped ZnO.
Lee, Joohwi; Seko, Atsuto; Shitara, Kazuki; Nakayama, Keita; Tanaka, Isao
2016-03-01
Machine learning techniques are applied to make prediction models of the G0W0 band gaps for 270 inorganic compounds using Kohn-Sham (KS) band gaps, cohesive energy, crystalline volume per atom, and other fundamental information of constituent elements as predictors. Ordinary least squares regression (OLSR), least absolute shrinkage and selection operator, and nonlinear support vector regression (SVR) methods are applied with two levels of predictor sets. When the KS band gap by generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) or modified Becke-Johnson (mBJ) is used as a single predictor, the OLSR model predicts the G0W0 band gap of randomly selected test data with the root-mean-square error (RMSE) of 0.59 eV. When KS band gap by PBE and mBJ methods are used together with a set of predictors representing constituent elements and compounds, the RMSE decreases significantly. The best model by SVR yields the RMSE of 0.24 eV. Band gaps estimated in this way should be useful as predictors for virtual screening of a large set of materials.
A Honeycomb BeN2 Sheet with a Desirable Direct Band Gap and High Carrier Mobility.
Zhang, Cunzhi; Sun, Qiang
2016-07-21
Using global particle-swarm optimization method, we report, for the first time, a BeN2 sheet (h-BeN2) with a graphene-like honeycomb lattice but displaying a direct band gap. Symmetry group analysis indicates that the dipole transition is allowed between the conduction band minimum and the valence band maximum. Although the direct band gap of 2.23 eV is close to that (2.14 eV) of MoS2 sheet, the h-BeN2 sheet has additional advantages: the direct band gap feature of the h-BeN2 sheet is quite insensitive to the layer stacking pattern and layer number, in contrast to the well-known direct-to-indirect band gap transition observed in TMDs and h-BN sheets. When rolled up, all the resulting h-BeN2 nanotubes have direct band gaps independent of chirality and diameter. Furthermore, the intrinsic acoustic-phonon-limited carrier mobility of the h-BeN2 sheet can reach ∼10(5) cm(2) V(-1) s(-1) for electron and ∼10(4) cm(2) V(-1) s(-1) for hole, which are higher than that of MoS2 and black phosphorus. PMID:27338078
International Nuclear Information System (INIS)
Photonic crystal waveguides (PCWs), which usually exhibit the slow-light effect near the band edge frequency, may in fact be used to induce higher group velocity (vg). The presence of the band edge, due to periodically carving a strip waveguide (SW) into a PCW of a comb-shape dielectric wire, results in the band-pulling effect. The band-pulling effect, which lifts the fundamental photonic band toward the band gap, not only enhances vg by reducing the effective index but also prevents the band from bending toward the light cone of the core material, leading to additional vg enhancement. The maximum PCW vg (0.43c; c = vacuum light speed) found from numerical modeling was higher than SW vg at all widths and thicknesses, except for extremely low thickness below 80 nm. We found by numerical modeling and experiments that up to 75% enhancement at the 1550 nm wavelength may be achieved in a fabricated single-mode PCW (0.38c) compared with an SW (0.21c) of the same width and thickness. The higher vg was confirmed by measuring the PCW’s group index using ring resonators. The PCW propagation loss of 2.1 dB mm−1 and the bandwidth-distance product of 100 Gb/s-cm were deduced from experimental results. Our findings seemed encouraging for employing PCWs in optical interconnect applications. (paper)
Numerical investigation of band gaps in 3D printed cantilever-in-mass metamaterials.
Qureshi, Awais; Li, Bing; Tan, K T
2016-01-01
In this research, the negative effective mass behavior of elastic/mechanical metamaterials is exhibited by a cantilever-in-mass structure as a proposed design for creating frequency stopping band gaps, based on local resonance of the internal structure. The mass-in-mass unit cell model is transformed into a cantilever-in-mass model using the Bernoulli-Euler beam theory. An analytical model of the cantilever-in-mass structure is derived and the effects of geometrical dimensions and material parameters to create frequency band gaps are examined. A two-dimensional finite element model is created to validate the analytical results, and excellent agreement is achieved. The analytical model establishes an easily tunable metamaterial design to realize wave attenuation based on locally resonant frequency. To demonstrate feasibility for 3D printing, the analytical model is employed to design and fabricate 3D printable mechanical metamaterial. A three-dimensional numerical experiment is performed using COMSOL Multiphysics to validate the wave attenuation performance. Results show that the cantilever-in-mass metamaterial is capable of mitigating stress waves at the desired resonance frequency. Our study successfully presents the use of one constituent material to create a 3D printed cantilever-in-mass metamaterial with negative effective mass density for stress wave mitigation purposes. PMID:27329828
Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment.
Yan, Rusen; Fathipour, Sara; Han, Yimo; Song, Bo; Xiao, Shudong; Li, Mingda; Ma, Nan; Protasenko, Vladimir; Muller, David A; Jena, Debdeep; Xing, Huili Grace
2015-09-01
van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials. PMID:26226296
Energy Technology Data Exchange (ETDEWEB)
Li, Jianwei, E-mail: jwl189@163.com; Zhao, Xinsheng [Laboratory for Quantum Design of Functional Material, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116 (China); Liu, Xinjuan [Center for Coordination Bond and Electronic Engineering, College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018 (China); Zheng, Xuejun [School of Mechanical Engineering, Xiangtan University, Xiangtan, Hunan 411105 (China); Yang, Xuexian [Department of Physics, Jishou University, Jishou 416000, Hunan (China); Zhu, Zhe [School of Materials Science and Engineering, Xiangtan University, Xiangtan, Hunan 411105 (China)
2015-09-28
The band gap and melting temperature of a semiconductor are tunable with the size and shape of the specimen at the nanometer scale, and related mechanisms remain as yet unclear. In order to understand the common origin of the size and shape effect on these two seemingly irrelevant properties, we clarify, correlate, formulate, and quantify these two properties of GaAs, GaN, InP, and InN nanocrystals from the perspectives of bond order-length-strength correlation using the core-shell configuration. The consistency in the theoretical predictions, experimental observations, and numerical calculations verify that the broken-bond-induced local bond contraction and strength gain dictates the band gap expansion, while the atomic cohesive energy loss due to bond number reduction depresses the melting point. The fraction of the under-coordinated atoms in the skin shell quantitatively determines the shape and size dependency. The atomic under-coordination in the skin down to a depth of two atomic layers inducing a change in the local chemical bond is the common physical origin.
All-optical dynamical Casimir effect in a three-dimensional terahertz photonic band gap
Hagenmüller, David
2016-06-01
We identify an architecture for the observation of all-optical dynamical Casimir effect in realistic experimental conditions. We suggest that by integrating quantum wells in a three-dimensional (3D) photonic band-gap material made out of large-scale (˜200 -μ m ) germanium logs, it is possible to achieve ultrastrong light-matter coupling at terahertz frequencies for the cyclotron transition of a two-dimensional electron gas interacting with long-lived optical modes, in which vacuum Rabi splitting is comparable to the Landau level spacing. When a short, intense electromagnetic transient of duration ˜250 fs and carrying a peak magnetic field ˜5 T is applied to the structure, the cyclotron transition can be suddenly tuned on resonance with a desired photon mode, switching on the light-matter interaction and leading to a Casimir radiation emitted parallel to the quantum well plane. The radiation spectrum consists of sharp peaks with frequencies coinciding with engineered optical modes within the 3D photonic band gap, and its characteristics are extremely robust to the nonradiative damping which can be large in our system. Furthermore, the absence of continuum with associated low-energy excitations for both electromagnetic and electronic quantum states can prevent the rapid absorption of the photon flux which is likely to occur in other proposals for all-optical dynamical Casimir effect.
Measurements of band gap structure in diamond compressed to 370 GPa
Gamboa, Eliseo; Fletcher, Luke; Lee, Hae-Ja; Zastrau, Ulf; Gauthier, Maxence; Gericke, Dirk; Vorberger, Jan; Granados, Eduardo; Heimann, Phillip; Hastings, Jerome; Glenzer, Siegfried
2015-06-01
We present the first measurements of the electronic structure of dynamically compressed diamond demonstrating a widening of the band gap to pressures of up to 370 +/- 25 GPa. The 8 keV free electron laser x-ray beam from the Linac Coherently Light Source (LCLS) has been focussed onto a diamond foil compressed by two counter-propagating laser pulses to densities of up to 5.3 g/cm3 and temperatures of up to 3000 +/- 400 K. The x-ray pulse excites a collective interband transition of the valence electrons, leading to a plasmon-like loss. We find good agreement with the observed plasmon shift by including the pressure dependence of the band gap as determined from density functional theory simulations. This work was performed at the Matter at Extreme Conditions (MEC) instrument of LCLS, supported by the DOE Office of Science, Fusion Energy Science under Contract No. SF00515. This work was supported by DOE Office of Science, Fusion Energy Science under F.
Numerical investigation of band gaps in 3D printed cantilever-in-mass metamaterials
Qureshi, Awais; Li, Bing; Tan, K. T.
2016-06-01
In this research, the negative effective mass behavior of elastic/mechanical metamaterials is exhibited by a cantilever-in-mass structure as a proposed design for creating frequency stopping band gaps, based on local resonance of the internal structure. The mass-in-mass unit cell model is transformed into a cantilever-in-mass model using the Bernoulli-Euler beam theory. An analytical model of the cantilever-in-mass structure is derived and the effects of geometrical dimensions and material parameters to create frequency band gaps are examined. A two-dimensional finite element model is created to validate the analytical results, and excellent agreement is achieved. The analytical model establishes an easily tunable metamaterial design to realize wave attenuation based on locally resonant frequency. To demonstrate feasibility for 3D printing, the analytical model is employed to design and fabricate 3D printable mechanical metamaterial. A three-dimensional numerical experiment is performed using COMSOL Multiphysics to validate the wave attenuation performance. Results show that the cantilever-in-mass metamaterial is capable of mitigating stress waves at the desired resonance frequency. Our study successfully presents the use of one constituent material to create a 3D printed cantilever-in-mass metamaterial with negative effective mass density for stress wave mitigation purposes.
Structural-crossover-induced optical band gap variation of Hf-doped ZnO films
Energy Technology Data Exchange (ETDEWEB)
Bae, Jong-Seong [Busan Center, Korea Basic Science Institute, Busan 618-230 (Korea, Republic of); Jeong, Young-Eui [Department of Physics, Pusan National University, Busan 609-735 (Korea, Republic of); Park, Sungkyun, E-mail: psk@pusan.ac.kr [Department of Physics, Pusan National University, Busan 609-735 (Korea, Republic of)
2014-12-01
Highlights: • Stress changed from compressive to tensile as deposition temperature increased. • Optical band gap energy shifted to the lower energy for the films under tensile stress. • As deposition temperature increased, the oxygen vacancies decreased. - Abstract: The deposition-temperature-dependent physical properties of Hf-doped ZnO thin films grown on Al{sub 2}O{sub 3}(0 0 0 1) by pulsed laser deposition were examined. X-ray diffraction measurements showed that all the samples had the (0 0 2) orientation except for the sample grown at room temperature, which showed amorphous characteristics because of the lack of kinetic energy. The in-plane strain changed from compressive to tensile as the deposition temperature increased above 200 °C. Optical transmission data revealed that all the samples exhibited >90% transmittance regardless of the deposition temperature. However, the optical band gap decreased with increasing deposition temperature, which was related to variation in stress in the films. X-ray photoelectron spectroscopy also revealed an increase in Zn–O bonding but decreases in oxygen vacancies with increasing deposition temperature.
Tunable Lattice Constant and Band Gap of Single- and Few-Layer ZnO.
Lee, Junseok; Sorescu, Dan C; Deng, Xingyi
2016-04-01
Single and few-layer ZnO(0001) (ZnO(nL), n = 1-4) grown on Au(111) have been characterized via scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. We find that the in-plane lattice constants of the ZnO(nL, n ≤ 3) are expanded compared to that of the bulk wurtzite ZnO(0001). The lattice constant reaches a maximum expansion of 3% in the ZnO(2L) and decreases to the bulk wurtzite ZnO value in the ZnO(4L). The band gap decreases monotonically with increasing number of ZnO layers from 4.48 eV (ZnO(1L)) to 3.42 eV (ZnO(4L)). These results suggest that a transition from a planar to the bulk-like ZnO structure occurs around the thickness of ZnO(4L). The work also demonstrates that the lattice constant and the band gap in ultrathin ZnO can be tuned by controlling the number of layers, providing a basis for further investigation of this material. PMID:27003692
Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment.
Yan, Rusen; Fathipour, Sara; Han, Yimo; Song, Bo; Xiao, Shudong; Li, Mingda; Ma, Nan; Protasenko, Vladimir; Muller, David A; Jena, Debdeep; Xing, Huili Grace
2015-09-01
van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.
Bahk, Je-Hyeong; Shakouri, Ali
2016-04-01
We present detailed theoretical predictions on the enhancement of the thermoelectric figure of merit by minority carrier blocking with heterostructure barriers in bulk narrow-band-gap semiconductors. Bipolar carrier transport, which is often significant in a narrow-band-gap material, is detrimental to the thermoelectric energy conversion efficiency as it suppresses the Seebeck coefficient and increases the thermal conductivity. When the minority carriers are selectively prevented from participating in conduction while the transport of majority carriers is relatively unaffected by one-sided heterobarriers, the thermoelectric figure of merit can be drastically enhanced. Thermoelectric transport properties such as Seebeck coefficient, electrical conductivity, and electronic thermal conductivity including the bipolar term are calculated with and without the barriers based on the near-equilibrium Boltzmann transport equations under the relaxation time approximation to investigate the effects of minority carrier barriers on the thermoelectric figure of merit. For this, we provide details of carrier transport modeling and fitting results of experimental data for three important material systems, B i2T e3 -based alloys, M g2S i1 -xS nx , and S i1 -xG ex , that represent, respectively, near-room-temperature (300 K-500 K), midtemperature (600 K-900 K), and high-temperature (>1000 K ) applications. Theoretical maximum enhancement of thermoelectric figure of merit that can be achieved by minority carrier blocking is quantified and discussed for each of these semiconductors.
The study of response of wide band gap semiconductor detectors using the Geant4
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Hussain Riaz
2014-01-01
Full Text Available The energy dependence on the intrinsic efficiency, absolute efficiency, full energy peak absolute efficiency and peak-to-total ratio have been studied for various wide band gap semiconductor detectors using the Geant4 based Monte Carlo simulations. The detector thickness of 1-4 mm and the area in 16-100 mm2 range were considered in this work. In excellent agreement with earlier work (Rybka et al., [20], the Geant4 simulated values of detector efficiencies have been found to decrease with incident g-ray energy. Both for the detector thickness and the detector area, the increasing trends have been observed for total efficiency as well as for full-energy peak efficiency in 0.1 MeV-50 MeV range. For Cd1-xZnxTe, the detector response remained insensitive to changes in relative proportions of Zn. For various wide band gap detectors studied in this work, the detection efficiency of TlBr was found highest over the entire range of energy, followed by the HgI2, CdTe, and then by CZT.
Numerical investigation of band gaps in 3D printed cantilever-in-mass metamaterials
Qureshi, Awais; Li, Bing; Tan, K. T.
2016-01-01
In this research, the negative effective mass behavior of elastic/mechanical metamaterials is exhibited by a cantilever-in-mass structure as a proposed design for creating frequency stopping band gaps, based on local resonance of the internal structure. The mass-in-mass unit cell model is transformed into a cantilever-in-mass model using the Bernoulli-Euler beam theory. An analytical model of the cantilever-in-mass structure is derived and the effects of geometrical dimensions and material parameters to create frequency band gaps are examined. A two-dimensional finite element model is created to validate the analytical results, and excellent agreement is achieved. The analytical model establishes an easily tunable metamaterial design to realize wave attenuation based on locally resonant frequency. To demonstrate feasibility for 3D printing, the analytical model is employed to design and fabricate 3D printable mechanical metamaterial. A three-dimensional numerical experiment is performed using COMSOL Multiphysics to validate the wave attenuation performance. Results show that the cantilever-in-mass metamaterial is capable of mitigating stress waves at the desired resonance frequency. Our study successfully presents the use of one constituent material to create a 3D printed cantilever-in-mass metamaterial with negative effective mass density for stress wave mitigation purposes. PMID:27329828
Accurate prediction of band gaps and optical properties of HfO2
Ondračka, Pavel; Holec, David; Nečas, David; Zajíčková, Lenka
2016-10-01
We report on optical properties of various polymorphs of hafnia predicted within the framework of density functional theory. The full potential linearised augmented plane wave method was employed together with the Tran-Blaha modified Becke-Johnson potential (TB-mBJ) for exchange and local density approximation for correlation. Unit cells of monoclinic, cubic and tetragonal crystalline, and a simulated annealing-based model of amorphous hafnia were fully relaxed with respect to internal positions and lattice parameters. Electronic structures and band gaps for monoclinic, cubic, tetragonal and amorphous hafnia were calculated using three different TB-mBJ parametrisations and the results were critically compared with the available experimental and theoretical reports. Conceptual differences between a straightforward comparison of experimental measurements to a calculated band gap on the one hand and to a whole electronic structure (density of electronic states) on the other hand, were pointed out, suggesting the latter should be used whenever possible. Finally, dielectric functions were calculated at two levels, using the random phase approximation without local field effects and with a more accurate Bethe-Salpether equation (BSE) to account for excitonic effects. We conclude that a satisfactory agreement with experimental data for HfO2 was obtained only in the latter case.
Band gap engineering in finite elongated graphene nanoribbon heterojunctions: Tight-binding model
Energy Technology Data Exchange (ETDEWEB)
Tayo, Benjamin O. [Physics Department, Pittsburg State University, Pittsburg, KS 66762 (United States)
2015-08-15
A simple model based on the divide and conquer rule and tight-binding (TB) approximation is employed for studying the role of finite size effect on the electronic properties of elongated graphene nanoribbon (GNR) heterojunctions. In our model, the GNR heterojunction is divided into three parts: a left (L) part, middle (M) part, and right (R) part. The left part is a GNR of width W{sub L}, the middle part is a GNR of width W{sub M}, and the right part is a GNR of width W{sub R}. We assume that the left and right parts of the GNR heterojunction interact with the middle part only. Under this approximation, the Hamiltonian of the system can be expressed as a block tridiagonal matrix. The matrix elements of the tridiagonal matrix are computed using real space nearest neighbor orthogonal TB approximation. The electronic structure of the GNR heterojunction is analyzed by computing the density of states. We demonstrate that for heterojunctions for which W{sub L} = W{sub R}, the band gap of the system can be tuned continuously by varying the length of the middle part, thus providing a new approach to band gap engineering in GNRs. Our TB results were compared with calculations employing divide and conquer rule in combination with density functional theory (DFT) and were found to agree nicely.
Band gap and FTIR studies for copper-zinc sol-gel glasses
Kaur, G.; Kaur, Navneet; Rawat, Mohit; Singh, K.; Kumar, Vishal
2016-05-01
Sol-gel technique was used for synthesis of Calcium phosphorous Borosilicate (CaO-SiO2-B2O3-P2O5-CuO-ZnO) glasses by varying composition of Copper oxide and Zinc oxide. Sol-gel route uses organic precursors which provide better homogeneity and uniform particle size compared to melt quenched glass. Four different glass stoichiometries were characterised using UV-visible spectroscopy and Fourier transforms infra-red spectroscopy (FTIR). Infra-red spectrum of transmittance of powdered glass samples is obtained by FTIR which measure the transmittance of wavelength in them and it also determines the presence of different functional group. Band gap has been obtained using UV-visible spectroscopy for all the glasses so as to study the effect of increasing ZnO content in glass composition. The change in band gap with ZnO content is indication of the change in number of non-bridging oxygen's (NBO).
Transparent with wide band gap InZnO nano thin film: Preparation and characterizations
Sugumaran, Sathish; Ahmad, Mohd Noor Bin; Jamlos, Mohd Faizal; Bellan, Chandar Shekar; Pattiyappan, Sagadevan; Rajamani, Ranjithkumar; Sivaraman, Rathish Kumar
2015-11-01
Novel indium zinc oxide (InZnO) thin film of 100 nm thickness was prepared onto pre-cleaned glass plate by thermal evaporation technique from InZnO nanoparticles. The metal oxide (In-O and Zn-O) bond and In, Zn and O elements present in the films were confirmed by Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy. The X-ray diffraction patterns revealed the mixed phase of cubic In2O3 and wurzite-hexagonal ZnO structure. SEM images showed smooth surface with uniform distribution of grains (201-240 nm) over the entire film surface. High transparency and low absorption obtained from optical study. The band gap energy was evaluated to be about 3.46-3.55 eV by Tauc's plot. The structure, smooth surface and high transparency with wide band gap energy lead the thermally evaporated InZnO nano thin film to be used for transparent layer in optoelectronic devices in the future.
Institute of Scientific and Technical Information of China (English)
无
2006-01-01
With the idea of the phononic crystals, the beams with periodic structure are designed.Flexural vibration through such periodic beams composed of two kinds of materials is studied. The emphasis is laid on the effects of rotary inertia and shear deformation. Based on the vibration equation, plane wave expansion method is provided. The acceleration frequency responses of such beams with finite structure are simulated by the finite element method. The frequency ranges of sharp drops in the calculated acceleration frequency response curves are in good agreement with those in the band structures. The findings will be significant in the application of the periodic beams.
International Nuclear Information System (INIS)
We studied the effects of carbon and aluminum substitutions on the gaps of the two-band superconductor MgB2 by means of point-contact measurements in Mg(B1-xCx)2 and Mg1-yAlyB2 single crystals with 0≤x≤0.132 and 0≤y≤0.21. The gap amplitudes, Δω and Δπ, were determined by fitting the conductance curves of the point contacts with the standard Blonder-Tinkham-Klapwijk (BTK) model generalized to the two-band case. Whenever possible, their values were confirmed by the independent fit (with a single-band BTK model) of the partial contribution of the two bands to the conductance, separated by means of a suitable magnetic field B*. In C-substituted crystals, the two gaps remain clearly distinct up to x∝0.10, but at x=0.132 we observed for the first time their merging into a single gap Δ≅3 meV with a gap ratio 2Δ=kBTc close to the standard BCS value. In Al-substituted crystals, we found no evidence of this gap merging. Instead, Δπ reaches the value 0.4 meV at y=0.21, where Δπ saturates at about 4 meV. These results are compared with other recent experimental findings in polycrystals and with the predictions of the models for multiband superconductivity. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Junction-type photonic crystal waveguides for notch- and pass-band filtering
Shahid, Naeem
2011-01-01
Evolution of the mode gap and the associated transmission mini stop-band (MSB) as a function of photonic crystal (PhC) waveguide width is theoretically and experimentally investigated. The change of line-defect width is identified to be the most appropriate way since it offers a wide MSB wavelength tuning range. A high transmission narrow-band filter is experimentally demonstrated in a junction-type waveguide composed of two PhC waveguides with slightly different widths. The full width at half maximum is 5.6 nm; the peak transmission is attenuated by only ∼5 dB and is ∼20 dB above the MSBs. Additionally, temperature tuning of the filter were also performed. The results show red-shift of the transmission peak and the MSB edges with a gradient of dλ/dT = 0.1 nm/°C. It is proposed that the transmission MSBs in such junction-type cascaded PhC waveguides can be used to obtain different types of filters. © 2011 Optical Society of America.
Phononic band gap design in honeycomb lattice with combinations of auxetic and conventional core
Mukherjee, Sushovan; Scarpa, Fabrizio; Gopalakrishnan, S.
2016-05-01
We present a novel design of a honeycomb lattice geometry that uses a seamless combination of conventional and auxetic cores, i.e. elements showing positive and negative Poisson’s ratio. The design is aimed at tuning and improving the band structure of periodic cellular structures. The proposed cellular configurations show a significantly wide band gap at much lower frequencies compared to their pure counterparts, while still retaining their major dynamic features. Different topologies involving both auxetic inclusions in a conventional lattice and conversely hexagonal cellular inclusions in auxetic butterfly lattices are presented. For all these cases the impact of the varying degree of auxeticity on the band structure is evaluated. The proposed cellular designs may offer significant advantages in tuning high-frequency bandgap behaviour, which is relevant to phononics applications. The configurations shown in this paper may be made iso-volumetric and iso-weight to a given regular hexagonal topology, making possible to adapt the hybrid lattices to existing sandwich structures with fixed dimensions and weights. This work also features a comparative study of the wave speeds corresponding to different configurations vis-a vis those of a regular honeycomb to highlight the superior behaviour of the combined hybrid lattice.
Theoretical study of time-resolved luminescence in semiconductors. III. Trap states in the band gap
Energy Technology Data Exchange (ETDEWEB)
Maiberg, Matthias, E-mail: matthias.maiberg@physik.uni-halle.de; Hölscher, Torsten; Zahedi-Azad, Setareh; Scheer, Roland [Institute of Physics, Martin-Luther-University Halle-Wittenberg, 06120 Halle (Germany)
2015-09-14
In the third part of this series, we study the influence of trap states in the band gap of semiconductors on the time-resolved luminescence decay (TRL) after a pulsed excitation. The results based on simulations with Synopsys TCAD{sup ®} and analytical approximations are given for p-doped Cu(In,Ga)Se{sub 2} as a working example. We show that a single trap can be mostly described by two parameters which are assigned to minority carrier capture and emission. We analyze their influence on the luminescence decay and study the difference between a single trap and an energetic Gaussian trap distribution. It is found that trap states artificially increase the TRL decay and obscure the recombination dynamics. Thus, there is a demand for experimental methods which can reveal the recombination of minority carriers in a TRL experiment without trapping effect. In this regard, a variation of the device temperature, the excitation frequency, the injection level, as well as a bias illumination may be promising approaches. We study these methods, discuss advantages and disadvantages, and show experimental TRL for prove of concept. At the end, we validate our approach of simulating only band-to-band radiative recombination although photoluminescence spectra often exhibit free-to-bound radiative recombination of charge carriers.
Energy Technology Data Exchange (ETDEWEB)
Pistor, P., E-mail: paul.pistor@physik.uni-halle.de [Martin-Luther-Universität Halle, Von-Danckelmann-Platz 3, 06120 Halle (Germany); Greiner, D.; Kaufmann, C. A.; Brunken, S.; Gorgoi, M.; Steigert, A.; Calvet, W.; Lauermann, I.; Klenk, R.; Unold, T.; Lux-Steiner, M.-C. [Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin (Germany)
2014-08-11
The implementation of potassium fluoride treatments as a doping and surface modification procedure in chalcopyrite absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se{sub 2} absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se{sub 2} absorber surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the absorbers after the KF PDT which is limited to the very near surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu deficient compounds. The KF PDT increased the open circuit voltage by 60–70 mV compared to the untreated absorbers, while the fill factor deteriorated.
Directory of Open Access Journals (Sweden)
Sanjeev Sharma
2012-06-01
Full Text Available Omni-directional reflection (ODR property of one-dimensional photonic crystal (PC structures consisting of alternate layers of Germanium (Ge, and calcium fluoride (CaF2. The existence of total Omni-directional reﬂection band gap in 1D-PC is predicted theoretically. Reﬂectivity of one dimensional periodic structure for TE and TM-modes at diﬀerent angles of incidence has been calculated by using Transfer Matrix Method (TMM. It is found that the mirror designed with these materials has a ODR range which covers the window of 1550nm used in optical communications.
Lang, Mario; Renz, Tobias; Mathes, Niklas; Neuwirth, Markus; Schnabel, Thomas; Kalt, Heinz; Hetterich, Michael
2016-10-01
We investigate the electronic structure and the radiative recombination in wet-chemically fabricated Cu2ZnSn(S,Se)4 solar cell absorbers utilizing photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy, focusing especially on the effects of varying Cu content. This includes the impact of the latter on the band gap energy and the change in band gap energy related to the order-disorder transition. Characteristic PL and PLE parameters like the energetic position of the PL maximum and the PL yield as a function of the excitation power as well as the PLE tailing parameter do not depend on composition indicating that the nature of the radiative transition is not altered by the Cu content. However, the band gap energy Eg significantly increases as a function of decreasing Cu content. This increase is more pronounced in the disordered than in the ordered atomic arrangement of Cu and Zn atoms in the Cu-Zn planes of the kesterite crystal structure.
Tolerance of Photonic Crystal Impurity Bands to Disorder of Defects in Coupled Cavity Waveguides
Institute of Scientific and Technical Information of China (English)
LIN Xu-Sheng; HUANG Chong; OUYANG Yan-Dong; LAN Sheng
2004-01-01
@@ We investigate the tolerance of photonic crystal impurity bands to the disorder of defects in one-dimensional coupled cavity waveguides. Although impurity bands formed by defect modes close to the air band are quasiflat in the absence of disorder, they are easily deteriorated when disorders are introduced into defects. In contrast,impurity bands created by defect modes near the dielectric band are less sensitive to disorder in the defect size.It is found that the sensitivity of defect mode frequency to defect size and the quality factor of defect modes are two crucial factors in determining the tolerance of impurity bands to the disorder of defects.
Forbidden energy band gap in diluted a-Ge1−xSix:N films
International Nuclear Information System (INIS)
By means of electron gun evaporation Ge1−xSix:N thin films, in the entire range 0 ≤ x ≤ 1, were prepared on Si (100) and glass substrates. The initial vacuum reached was 6.6 × 10−4 Pa, then a pressure of 2.7 × 10−2 Pa of high purity N2 was introduced into the chamber. The deposition time was 4 min. Crucible-substrate distance was 18 cm. X-ray diffraction patterns indicate that all the films were amorphous (a-Ge1−xSix:N). The nitrogen concentration was of the order of 1 at% for all the films. From optical absorption spectra data and by using the Tauc method the energy band gap (Eg) was calculated. The Raman spectra only reveal the presence of Si-Si, Ge-Ge, and Si-Ge bonds. Nevertheless, infrared spectra demonstrate the existence of Si-N and Ge-N bonds. The forbidden energy band gap (Eg) as a function of x in the entire range 0 ≤ x ≤ 1 shows two well defined regions: 0 ≤ x ≤ 0.67 and 0.67 ≤ x ≤ 1, due to two different behaviors of the band gap, where for x > 0.67 exists an abruptly change of Eg(x). In this case Eg(x) versus x is different to the variation of Eg in a-Ge1−xSix and a-Ge1−xSix:H. This fact can be related to the formation of Ge3N4 and GeSi2N4 when x ≤ 0.67, and to the formation of Si3N4 and GeSi2N4 for 0.67 ≤ x. - Highlights: ► Nitrogen doped amorphous Ge1-xSix thin films are grown by electron gun technique. ► Nitrogen atoms on Eg of the a-Ge1-xSix films in the 0 £ x £ 1 range are analyzed. ► Variation in 0 £ x £ 1 range shows a warped change of Eg in 1.0 – 3.6 eV range. ► The change in Eg(x) behavior when x ∼ 0.67 was associated with Ge2SiN4 presence.
The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique
Energy Technology Data Exchange (ETDEWEB)
Kevin Jerome Sutherland
2001-06-27
Over the last ten years, photonic band gap (PBG) theory and technology have become an important area of research because of the numerous possible applications ranging from high-efficiency laser diodes to optical circuitry. This research concentrates on reducing the length scale in the fabrication of layered photonic band gap structures and developing procedures to improve processing consistency. Various procedures and materials have been used in the fabrication of layered PBG structures. This research focused on an economical micro transfer molding approach to create the final PBG structure. A poly dimethylsiloxane (PDMS) rubber mold was created from a silicon substrate. It was filled with epoxy and built layer-by-layer to create a 3-D epoxy structure. This structure was infiltrated with nanoparticle titania or a titania sol-gel, then fired to remove the polymer mold, leaving a monolithic ceramic inverse of the epoxy structure. The final result was a lattice of titania rolds that resembles a face-centered tetragonal structure. The original intent of this research was to miniaturize this process to a bar size small enough to create a photonic band gap for wavelengths of visible electro-magnetic radiation. The factor limiting progress was the absence of a silicon master mold of small enough dimensions. The Iowa State Microelectronics Research Center fabricated samples with periodicities of 2.5 and 1.0 microns with the existing technology, but a sample was needed on the order of 0.3 microns or less. A 0.4 micron sample was received from Sandia National Laboratory, which was made through an electron beam lithography process, but it contained several defects. The results of the work are primarily from the 2.5 and 1.0 micron samples. Most of the work focused on changing processing variables in order to optimize the infiltration procedure for the best results. Several critical parameters were identified, ranging from the ambient conditions to the specifics of the
Energy Technology Data Exchange (ETDEWEB)
Xiao Sanshui; He Sailing
2002-12-01
An FDTD numerical method for computing the off-plane band structure of a two-dimensional photonic crystal consisting of nearly free-electron metals is presented. The method requires only a two-dimensional discretization mesh for a given off-plane wave number k{sub z} although the off-plane propagation is a three-dimensional problem. The off-plane band structures of a square lattice of metallic rods with the high-frequency metallic model in the air are studied, and a complete band gap for some nonzero off-plane wave number k{sub z} is founded.