First-principles calculations of structural, electronic, optical and elastic properties of magnesite MgCO3 and calcite CaCO3

Detailed ab initio calculations of the structural, electronic, optical and elastic properties of two crystals - magnesite (MgCO₃) and calcite (CaCO₃) - are reported in the present paper. Both compounds are important natural minerals, playing an important role in the carbon dioxide cycling. The optimized crystal structures, band gaps, density of states diagrams, elastic constants, optical absorption spectra and refractive indexes dependence on the wavelength all have been calculated and compared, when available, with literature data. Both crystals are indirect band compounds, with calculated band gaps of 5.08 eV for MgCO₃ and 5.023 eV for CaCO₃. Both values are underestimated by approximately 1.0 eV with respect to the experimental data. Although both crystals have the same structure, substitution of Mg by Ca ions leads to certain differences, which manifest themselves in noticeable change in the electronic bands profiles and widths, shape of the calculated absorption spectra, and values of the elastic constants. Response of both crystals to the applied hydrostatic pressure was analyzed in the pressure range of phase stability, variations of the lattice parameters and characteristic interionic distances were considered. The obtained dependencies of lattice constants and calculated band gap on pressure can be used for prediction of properties of these two hosts at elevated pressures that occur in the Earth's mantle.     

Performance of lead iodide nuclear radiation detectors with the introduction of rare earth elements

Lead iodide has been recognized as a promising material for room temperature radiation detectors. It has a wide band-gap (∼ 2.3 eV), high atomic numbers (82, 53) and it is environmentally very stable compared to mercuric iodide. Electrical and optical properties of lead iodide grown crystals purified under the influence of selected rare earth elements have been investigated. Photo-luminescence and capacitance-voltage measurements have been performed using different rare earth elements.     

Energy band gap engineering in borate ultraviolet nonlinear optical crystals: ab?initio studies

The development of ultraviolet (UV) nonlinear optical (NLO) crystals demands optical materials with wide energy band gaps. Here we report first-principles studies on the electronic structures in several UV NLO borates with representative structures. Combined with model analysis, we find that the oxygen non-bonding 2p orbitals play an important role on the top of valence bands. The energy band gap can be adjusted by modifying the coordination environment around the oxygen atoms. Under ideal conditions the energy band gaps achieve 9?eV if the non-bonding orbitals are totally eliminated, despite the original values varying from 6.6 to 8.3?eV.     

Indirect band gap investigation of orthorhombic single crystals of sulfur

Single crystals of pure and selenium doped orthorhombic sulfur crystal were prepared from carbon disulphide solution. They were investigated at two temperatures (6°C and 85°C) in an optical absorption apparatus. The optical absorption increased with temperature and it was found to be higher for Se doped crystals within the same range of temperatures. The indirect allowed band gap transitions and assisting phonons were observed. Their values were found to shift toward lower energy with increasing temperatures. They are also lower for doped samples. At 6°C, the indirect allowed band gaps are 2.61 eV and 2.56 eV for pure and selenium-doped crystals, respectively. At 85°C, they are 2.44 eV and 2.40 eV for pure and doped crystals, respectively. The phonons associated with the optical transitions could be attributed to the stretching band of S₈ rings.     

The structural,electronic, and optical properties of organic–inorganic mixed halide perovskites CH_3NH_3Pb(I_(1-y)X_y)_3(X = Cl,Br)

Methylammmonium lead iodide perovskites(CH_3NH_3PbI_3) have received wide attention due to their superior optoelectronic properties. We performed first-principles calculations to investigate the structural, electronic, and optical properties of mixed halide perovskites CH_3NH_3Pb(I_(1-y)X_y)_3(X = Cl, Br; y = 0, 0.33, 0.67). Our results reveal the reduction of the lattice constants and dielectric constants and enhancement of band gaps with increasing doping concentration of Cl-/Br-at I-. Electronic structure calculations indicate that the valance band maximum(VBM) is mainly governed by the halide p orbitals and Pb 6 s orbitals, Pb 6 p orbitals contribute the conduction band minimum(CBM) and doping does not change the direct semiconductor material. The organic cation [CH_3NH_3]~+does not take part in the formation of the band and only one electron donates to the considered materials. The increasing trends of the band gap with Cl content from y = 0(0.793 eV) to y = 0.33(0.953 eV) then to y = 0.67(1.126 eV). The optical absorption of the considered structures in the visible spectrum range is decreased but after doping the stability of the material is improving.     

The Urbach tails and optical absorption in layered semiconductor TlGaSe<Subscript>2</Subscript> and TlGaS<Subscript>2</Subscript> single crystals

TlGaSe₂ and TlGaS₂ single crystals were grown by the modified Bridgman-Stockbarger method. We report the result of an experimental study of the optical absorption of TlGaSe₂ and TlGaS₂ crystals. The absorption measurements were performed in steps of 10 K. The direct and indirect band gaps for TlGaSe₂ and TlGaS₂ samples were calculated as a function of temperature. The phonon energies in TlGaSe₂ and TlGaS₂ crystals were calculated as (39±4) and (9±4) meV at 240 K, respectively. At 10 K, direct and indirect band gaps were found as 2.294 and 2.148 eV for TlGaSe₂, 2.547 and 2.521 eV for TlGaS₂ crystals, respectively. The abrupt changes were observed in the direct and indirect band gaps in the some temperature ranges. These changes were interpreted as phase transformation temperatures. The steepness parameters and Urbach energy for TlGaSe₂ and TlGaS₂ samples increased with increasing sample temperature in the range (10–320) K.     

Uncoupled photonic band gaps

We theoretically investigated the symmetry properties of the modes in two-dimensional square lattice photonic crystals in order to study phenomena that would enable new frontiers in the applications of photonic crystals. Using group theory, symmetry analysis of the photonic crystals bands has been done. Particular attention was given to the search for the uncoupled B modes that cannot be excited by the external plane wave because they are symmetry forbidden. The existence of the uncoupled modes enabled to define new physics phenomena: uncoupled photonic band gaps. For the frequency ranges inside the uncoupled photonic band gaps, zero transmission is obtained. Therefore, there are two different types of photonic gaps in the photonic crystals: photonic band gaps and uncoupled photonic band gaps. The appearance of uncoupled photonic band gaps in photonic crystals could at least improve the application of the existing photonic materials and structures or even enable the usage of new ones for devices like waveguides, filters, and lasers.     

Phase transformations in PbI2 crystals with temperature

A systematic study of phase transformation with temperature has been undertaken in single crystals of lead iodide grown in gel. The crystals of the polytype 2H, which is known to be the common modification of PbI₂ at room temperature, have been finally found to transform into polytype 12R after heating at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking or arcing or both, on their X-ray diffraction photographs. The higher polytypes 12H and 16H do not show any change even after prolonged heating at 150°C. If silver iodide is added in a minute quantity during crystallization, the process of phase transformation is appreciably altered. The results have been discussed.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Preparation of CuInGeSe4 thin films by selenization method using the Cu-In-Ge evaporated layer precursors

CuInGeSe₄ quaternary compounds are known to have a chalcopyrite-like structure and have band gaps of about 1.3 eV, suitable for optimum conversion efficiency for solar cells. We have prepared the CuInGeSe₄ thin films by the selenization method using the Cu-In-Ge evaporated layer precursors. The analyses of X-ray diffraction show that the single phase of CuInGeSe₄ is obtained by the selenization of precursors at 450-500 °C. The SEM observation of film surface shows that the grain sizes are in the order of 1-2 μm. The band gaps of selenized films close to 1.6 eV are wider than that of bulk crystals (about 1.3 eV). These films have p-type conduction and higher electrical resistivities than more 10⁵ Ω cm at room temperature.     

Ⅳ族FCC晶体基态结构和电子性质第一性原理研究

采用基于密度泛函理论的第一性原理方法,在广义梯度近似下,计算了Ⅳ族元素晶体的面心立方结构和电子性质.结果表明:Ⅳ族元素晶体的面心立方结构均可存在,面心立方结构Ge晶体的结合能最大,结构最稳定;面心立方结构C、Si、Ge和Sn的晶格常数分别为0.3509nm、0.4322nm、0.4225nm、0.4903nm,不随原子序数的增加而单调增加,是由面心立方锗晶体比面心立方硅晶体中电子云交叠小,产生的排斥较弱所导致的;面心立方结构C晶体是间接能隙为6.5e V的宽禁带半导体,面心立方结构Si晶体的导带和价带存在较小的交叠而呈现出半金属性,面心立方结构Ge和Sn的电子结构相似均表现为金属性,Ⅳ族元素面心立方结构晶体的电学性质由宽禁带半导体向金属转变.     

Influence of different oxidants on the band alignment of HfO₂ films deposited by atomic layer deposition

Based on X-ray photoelectron spectroscopy(XPS),influences of different oxidants on band alignment of HfO2 films deposited by atomic layer deposition(ALD) are investigated in this paper.The measured valence band offset(VBO) value for H2 O-based HfO2 increases from 3.17 eV to 3.32 eV after annealing,whereas the VBO value for O 3-based HfO2 decreases from 3.57 eV to 3.46 eV.The research results indicate that the silicate layer changes in different ways for H2 O-based and O3-based HfO2 films after the annealing process,which plays a key role in generating the internal electric field formed by the dipoles.The variations of the dipoles at the interface between the HfO2 and SiO2 after annealing may lead the VBO values of H2 O-based and O 3-based HfO2 to vary in different ways,which fits with the variation of flat band(VFB) voltage.     

Relaxation of electronic excitations in strontium titanate

The transient absorption spectra and kinetics were studied for undoped, lead doped and high purity SrTiO 3 single crystals. The pulsed electron beam induced transient absorption is studied in all crystals. The strong absorption at 0.8 v eV was observed only in high purity SrTiO 3 . This absorption is suggested to arise from intrinsic electron polaron. The bound electron polarons are likely responsible for absorption band at 1.4 v eV. The main luminescence band under excitation pulse is observed at 2.75 v eV. The luminescence decay is faster than that of transient absorption.     

Tailoring of phononic band structures in colloidal crystals

We report an experimental study of the elastic properties of a two-dimensional (2D) colloidal crystal subjected to light-induced substrate potentials. In agreement with recent theoretical predictions [H. H. von Grünberg and J. Baumgartl, Phys. Rev. E 75, 051406 (2007).10.1103/PhysRevE.75.051406] the phonon band structure of such systems can be tuned depending on the symmetry and depth of the substrate potential. Calculations with binary crystals suggest that phononic band engineering can be also performed by variations of the pair potential and thus opens novel perspectives for the fabrication of phononic crystals with band gaps tunable by external fields.     

Control of photonic band gaps in one-dimensional photonic crystals

The photonic band gaps in one-dimensional photonic crystals (PCs) are theoretically investigated. A new method to broaden the photonic band gaps is introduced. Based on the similar method, a kind of photonic crystals is constructed to generate photonic band gaps with proportioned central frequencies. This technology can be used for designing nonlinear PCs for harmonic generation.     

Theoretical investigation of Cu-containing materials with different valence structure types: BaCu2S2, Li2CuSb,and LiCuS

Optoelectronics research requires cheap materials with a broad spectrum of optical, electronic, and structural properties. The class of Heusler compounds and ternary structures provide many possibilities for finding alternative group IV and III–V semiconductor compounds. This study introduces wider band gap materials for use in solar cells as an alternative to cadmium sulfide buffer layers. The buffer layer is inserted between the absorber layer (p-type) and the transparent window layer (n-type) to enhance the maximum amount of light transmission. Reasonable calculations are reported for the band gaps of copper-containing materials: LiCuS, BaCu₂S₂, and Li₂CuSb. Previous optical analysis measurements of these films determined that the band gaps were 1.8 and 1.9 eV for BaCu₂S₂ and LiCuS, respectively. In general, semiconductor compounds have been studied theoretically, but there are major differences between the experimental and theoretically calculated band gaps. A suitable calculation method for semiconductor compounds is described in this study. For the first time, calculations based on the Engel and Vosko method are introduced for these semiconductor compounds. This method yields band gaps that are comparable to the experimental values, which facilitate the development of microscopic analyses of these compounds. Direct band gaps of 1.15 and 1.7 eV were obtained for BaCu₂S₂ and LiCuS, respectively, whereas the indirect band gap was 0.7 eV for Li₂CuSb.     

第一性原理下铜锰共掺铌酸锂晶体的电子结构和吸收光谱

采用基于密度泛函理论的第一性原理研究了Cu、Mn单掺及共掺LiNbO3晶体的电子结构和光学性质.结果显示,Cu、Mn掺杂LiNbO_3晶体禁带中的杂质能级分别由Cu 3d轨道、Mn 3d轨道贡献;各掺杂体系的带隙均较纯LiNbO_3晶体变窄.共掺晶体中Cu离子形成了较单掺时更浅的能级中心,并在2.87eV处有较强的吸收峰;Mn离子在1.73eV附近的吸收较单掺时减弱且中心略有偏移,在2.24eV处的非光折变峰与Mn~(3+)相关,这对吸收峰的变化被认为与Cu、Mn间电子转移相联系.相对Cu、Fe共掺LiNbO_3晶体,Cu、Mn共掺LiNbO_3晶体可以通过适当提高Cu离子浓度,来改善存储参量中的动态范围和记录灵敏度.由于同一深能级掺杂离子伴以不同浅能级掺离子将呈现出不同的吸收特征并影响存储性能,在共掺离子的配搭选择时对各待选配搭的模拟计算非常必要.     

磁导率对二维蜂窝结构光子晶体带隙的影响

利用平面波展开法对二维六角晶格结构磁性光子晶体的带隙特性进行了研究,给出其能带结构分布图,并与非磁性介质构成的光子晶体进行了比较.结果表明,由磁性材料构成的光子晶体更易出现带隙,磁导率对带隙结构影响很大.空气背景磁性散射子情况,磁导率增加较小时,二个绝对光子带隙宽度逐渐减小,直至消失.继续增加磁导率,在较低频率范围内出现一个绝对光子带隙,占空比逐渐加大,且最佳填充系数基本保持不变.磁性背景空气散射子,类似地在较低频率范围内也出现了一个绝对光子带隙.磁性背景非磁性散射子与空气背景磁性散射子情况相似.     

Photonic band gaps structure properties of two-dimensional function photonic crystals

The tunable two-dimensional photonic crystals band gap, absolute photonic band gap and semi-Dirac point are beneficial to designing the novel optical devices. In this paper, tunable photonic band gaps structure was realized by a new type two-dimensional function photonic crystals, which dielectric constants of medium columns are functions of space coordinates. However for the two-dimensional conventional photonic crystals the dielectric constant does not change with space coordinates. As the parameter adjustment, we found that the photonic band gaps structures are dielectric constant function coefficient, medium columns radius, dielectric constant function form period number and pump light intensity dependent, namely, the photonic band gaps position and width can be tuned. we also obtained absolute photonic band gaps and semi-Dirac point in the photonic band gaps structures of two-dimensional function photonic crystals. These results provide an important theoretical foundation for design novel optical devices.     

Extension of photonic band gaps in one-dimensional photonic crystals

A method is proposed for extending photonic band gaps by constructing periodic structures from two or more consecutively located photonic crystals with different lattice constants or filling factors. The photonic band gaps with a maximum extension are predicted by superposing the photonic band gap maps on one another. It is demonstrated that both the lowest and higher order band gaps can be merged in photonic crystals with a high refractive index contrast.