First‐principle investigation of strain effects on the electronic properties of germanium nanowires

Using the GGA functional in density functional theory, the effects of axial strain on the band structure and effective mass of narrow [110] and [100] germanium nanowires are investigated. It is observed that both compressive and tensile strain cause indirect‐to‐direct bandgap transitions. One percent of tensile strain can cause a 40 meV change in the bandgap of [110] nanowires. Effective masses of electrons and holes are subject to a change of 3–4 times in the strain‐induced transition point. This change translates into a density of state modulation which opens new possibilities for the construction of Ge nanowire‐based sensors. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First‐principles X‐ray absorption dose calculation for time‐dependent mass and optical density

A dose integral of time‐dependent X‐ray absorption under conditions of variable photon energy and changing sample mass is derived from first principles starting with the Beer–Lambert (BL) absorption model. For a given photon energy the BL dose integral D(e, t) reduces to the product of an effective time integral T(t) and a dose rate R(e). Two approximations of the time‐dependent optical density, i.e. exponential A(t) = c + aexp(?bt) for first‐order kinetics and hyperbolic A(t) = c + a/(b + t) for second‐order kinetics, were considered for BL dose evaluation. For both models three methods of evaluating the effective time integral are considered: analytical integration, approximation by a function, and calculation of the asymptotic behaviour at large times. Data for poly(methyl methacrylate) and perfluorosulfonic acid polymers measured by scanning transmission soft X‐ray microscopy were used to test the BL dose calculation. It was found that a previous method to calculate time‐dependent dose underestimates the dose in mass loss situations, depending on the applied exposure time. All these methods here show that the BL dose is proportional to the exposure time D(e, t) ? K(e)t.     

First‐principle investigations of the peculiar magnetic behavior of Sr2FeOsO6

Structural, electronic and magnetic properties of Sr₂FeOsO₆ have been revisited by using the first‐principle calculations. Semiconducting behavior is reproduced. The band gap is 0.09 eV from generalized gradient approximation (GGA) and 0.30 eV by considering both SOC and U, a bit larger than the experimental observed 0.125 eV. In the C‐type antiferromagnetic configuration, spin frustration is found by analysing the magnetic exchange parameters, explaining the experimental observed magnetic complexity.

     

Cr1-xNbxSi2固溶体弹性性质、电子结构和光学性质的理论研究

采用基于密度泛函理论(DFT)的第一性原理计算,对Nb掺杂CrSi2的晶格结构、弹性性质,电子结构和光学性质进行了系统的研究. 研究结果表明：随着Nb掺杂浓度增加,弹性常数、体变模量、剪切模量、杨氏模量均减小,而且能带间隙也逐渐减小,表现为p型掺杂特点. 基于电子结构计算结果以及已知的实验结果, 讨论了Nb掺杂CrSi2后对其复介电函数、折射率、消光系数、反射率和吸收谱等光学性质的影响.     

Highly anisotropic and tunable charge carrier of monolayer phosphorus allotropes by bi-axial strain

The complex variation of electronic properties and carrier mobility of four typical allotropes of phosphorus are investigated using first-principles calculations of bi-axial strain. Our study shows that the electronic properties and carrier mobility of single-layer α-, β-, γ- and δ-P are sensitive to bi-axial strain, and that the carrier mobility can even be increased by several orders of magnitude under specific tensile or compressive strain. Moreover, the anisotropy of their mobility in the two-dimensional plane shows different changes according to the variation of the bi-axial strain. In particular, the electron mobility of α-P (γ-P) along the two main axis directions is reversed when the bi-axial strain increased by 5% (−4%). This transformation is mainly caused by changes in the effective mass anisotropy due to alterations in band dispersion under external strain, as illustrated by the distribution of effective mass anisotropy and the corresponding changes of electronic properties under strain.     

Tailoring electronic properties of two-dimensional antimonene with isoelectronic counterparts

Using ab initio density functional theory calculations, we explore the three most stable structural phases, namely, α,β, and cubic(c) phases, of two-dimensional(2D) antimonene, as well as its isoelectronic counterparts SnTe and InI. We find that the band gap increases monotonically from Sb to SnTe to InI along with an increase in ionicity, independent of the structural phases. The band gaps of this material family cover the entire visible-light energy spectrum, ranging from 0.26 eV to 3.37 eV, rendering them promising candidates for optoelectronic applications. Meanwhile, band-edge positions of these materials are explored and all three types of band alignments can be achieved through properly combining antimonene with its isoelectronic counterparts to form heterostructures. The richness in electronic properties for this isoelectronic material family sheds light on possibilities to tailor the fundamental band gap of antimonene via lateral alloying or forming vertical heterostructures.     

Alkali-metal-embedded in monolayer MoS<Subscript>2</Subscript>: optical properties and work functions

Embedding alkali-metal in monolayer MoS₂ has been investigated by using first principles with density functional theory. The calculation of the electronic and optical properties indicates that alkali-metal was embedded in monolayer MoS₂ appearing almost metallic behavior, and the MoS₂ layer shows clear p-type doping behavior. The covalent bonding appears between the alkali-metal atoms and defective MoS₂. More importantly, embedding alkali-metal can increase the work function for monolayer MoS₂. Furthermore, the absorption spectrum of monolayer MoS₂ is red shifted because of alkali metal embedding. Accordingly, this study will provide the theoretical basis for producing the alkali-metal-doped monolayer MoS₂ radiation shielding and photoelectric devices.     

SrHfO3和SrTiO3光学特性的第一性原理研究

用全电势线性缀加平面波法(FLAPW)计算了SrTiO3和SrHfO3的光学特性,即介电函数虚部ε2(ω)、光学吸收系数I(ω)和反射率R(ω).对它们光学特性进行了对比分析,给出了它们光学特性的差别,并进行了解释.计算的SrTiO3光学谱分别在4.4,7.4,8.3和23.6eV处出现峰值,且其在4.4eV处的峰值比较高而尖.计算结果与实验值符合得很好.     

CuInSe2电子结构与光学性质的第一性原理计算

从头计算了CuInSe2(CIS)体相的性质,参数设定和性质计算都基于密度泛函理论,交换相关能采用GGA,泛函形式为PBE,原子间相互作用的描述采用超软赝势.计算发现CIS中存在共价键,是一种非典型的离子型晶体,在整个晶体内存在共用电子对,Cu原子和Se原子的作用大于Se原子和In原子.CIS是一种典型的直接带隙半导体,计算得到了光学性质的各项参数,包括折射指数和反射率,吸收系数以及介电函数与光子能量的关系,发现CIS的主要光吸收峰有6个,分别为:3.1,7.6,10.0,16.1,19.0,21.0 eV,理论上最强吸收峰在紫外光区.     

A study on monolayer MoS2 doping at the S site via the first principle calculations

Through the first principle calculation, electronic properties of monolayer MoS₂ doped with single, double, triple and tetra-atoms of P, Cl, O, Se at the surface S site are discussed. Among the substitutional dopant, our calculation results show that when P atoms are doped on a monolayer MoS₂, a shift in the Fermi energy into the valence band is observed, making the system p-type. Meanwhile, band gap gradually decreases as increasing the number of P atoms. On the contrary, Cl is identified as a suitable n-type dopant. It is observed that Cl for initial three dopant behaved as magnetic and afterwards returned to non-magnetic behavior. The band gap of the Cl doped system is also dwindling gradually. Finally, O and Se doped systems have little effect on electronic properties near band gap. Such doping method at the S site, and the TDOS and PDOSs of each doping system provide a detailed of understanding toward working mechanism of the doped and the intrinsic semiconductors. This doping model opens up an avenue for further clarification in the doping systems as well as other dopant using this method.     

First-principles calculations of optical properties of Mg2Pb

Based on the density functional theory(DFT),the electronic structures and optical properties of Mg2Pb are calculated by using the local density approximation(LDA)and plane wave pseudo-potential method.The calculation results show that the indirect band gap width of Mg2Pb is 0.02796 eV.The optical properties of Mg2Pb have isotropic characteristics,the static dielectric function of Mg2Pb is1(0)=10.33 and the refractive index is n0=3.5075.The maximum absorption coefficient is 4.8060×105cm 1.The absorption in the photon energy range of 25–40 eV approaches to zero,shows the optical colorless and transparent behaviors.     

First-principles calculations on the electronic structure and optical properties of BaSi<Subscript>2</Subscript>

The electronic structure, densities of states and optical properties of the stable orthorhombic BaSi₂ have been calculated using the first-principle density function theory and pseudopotential method. The results show that BaSi₂ is an indirect semiconductor with the band gap of 1.086 eV, the valence bands of BaSi₂ are mainly composed of Si 3p, 3s and Ba 5d, and the conduction bands are mainly composed of Ba 6s, 5d as well as Si 3p. The static dielectric function ɛ ₁(0) is 11.17, the reflectivity n ₀ is 3.35, and the biggest peak of the absorption coefficient is 2.15×10₅ cm⁻¹. Supported by the National Natural Science Foundation of China (Grant Nos. 60566001 and 60766002), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050657003), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China (Grant No. (2005)383), the Specialized Fund of Nomarch for Excellent Talent of Science and Technology of Guizhou Province (Grant No. Z053114), the Scientific and Technological Projects for the Returned Overseas of Guizhou Province (Grant No. (2004)03), and the Top Talent’s Scientific Research Project of Organization Department of Guizhou Province (Grant No. Z053123)     

First-principles calculations for electronic, optical and thermodynamic properties of ZnS

The electronic, optical and thermodynamic properties of ZnS in the zinc-blende （ZB） and wurtzite （WZ） structures are investigated by using the plane-wave pseudopotential density functional theory （DFT）. The results obtained are consistent with other theoretical results and the available experimental data. When the pressures are above 20.5 and 27 GPa, the ZB-ZnS and the WZ-ZnS are converted into indirect gap semiconductors, respectively. The critical point structure of the frequency-dependent complex dielectric function is investigated and analysed to identify the optical transitions. Moreover, the values of heat capacity Cv and Debye temperature θ at different pressures and different temperatures are also obtained successfully.     

Ab-initio calculations of luminescence and optical gain properties in silicon nanostructures

Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si–O–Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO₂ matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc. To cite this article: E. Degoli et al., C. R. Physique 10 (2009).     

Magnetic and optical properties of Cu-doped ZnO nanosheet: First-principles calculations

We performed first-principles calculations within density-functional theory to study the magnetic and optical properties of Cu-doped ZnO nanosheet (NS). We found that Cu atom prefers to substitute for Zn site and can induce a local magnetic moment of 1.00 μ_B per unit in ZnO NS. When two Zn atoms are substituted by two Cu dopants, they tend to form a cluster and ferromagnetic (FM) ordering becomes energetically more favorable. In addition, localized states appear within the band gap due to the introduction of Cu dopant to ZnO NS. With increasing Cu concentrations, both the imaginary part of dielectric function and the absorption spectrum exhibit a red-shift behavior, which are in good agreement with the recent experimental results. The ferromagnetic coupling can be attributed to the p–d hybridization mechanism. The intriguing properties of Cu-doped ZnO NS may be promising for designing novel multifunctional nanodevice.     

Electronic structure and optical properties of LaNiO3: First-principles calculations

Using a first-principles method, we investigate the electronic structure and optical properties of rhombohedral LaNiO₃. The total density of states shows that there is no band gap and bulk LaNiO₃ is metallic. There is a strong hybridization between Ni and O orbits near the Fermi level, suggesting that the metallic nature of LaNiO₃ mainly originates from Ni 3d states and La atoms have no noticeable contribution to this. The absorption coefficient of LaNiO₃ is one order of magnitude less than that of nickel in the lower energy region (0–5 eV), and the interband optical transitions are mainly derived from O 2p and Ni 3d states. In reflectivity spectrum of LaNiO₃, there are three main reflectance peaks located at 0 eV, 15.6 eV and 22.9 eV, respectively. In the visible–ultraviolet energy range, the reflectivity of LaNiO₃ remarkably decreases with the increasing photon energy and the value is always smaller than that of nickel in the region.     

Tunable band gap and optical properties of surface functionalized Sc_2C monolayer

Using the density functional theory, we have investigated the electronic and optical properties of two-dimensional Sc_2C monolayer with OH, F, or O chemical groups. The electronic structures reveal that the functionalized Sc_2C monolayers are semiconductors with a band gap of 0.44–1.55 eV. The band gap dependent optical parameters, like dielectric function, absorption coefficients, reflectivity, loss function, and refraction index were also calculated for photon energy up to 20 eV. At the low-energy region, each optical parameter shifts to red, and the peak increases obviously with the increase of the energy gap. Consequently, Sc_2C monolayer with a tunable band gap by changing the type of surface chemical groups is a promising 2D material for optoelectronic devices.     

First‐principles identification of defect levels in Er‐doped GaN

Erbium (Er) doped GaN has been studied extensively for optoelectronic applications, yet its defect physics is still not well understood. In this work, we report a first‐principles hybrid density functional study of the structure, energetics, and thermodynamic transition levels of Er‐related defect complexes in GaN. We discover for the first time that Er_Ga–C_N–V_N, a defect complex of Er, a C impurity, and an N vacancy, and Er_Ga–O_N–V_N, a complex of Er, an O impurity, and an N vacancy, form defect levels at 0.18 eV and 0.46 eV below the conduction band, respectively. Together with Er_Ga–V_N, a complex of Er and an N vacancy which has recently been found to produce a donor level at 0.61 eV, these defect complexes provide explanation for the Er‐related defect levels observed in experiments. The role of these defects in optical excitation of the luminescent Er center is also discussed.     

Stress and strain effects on the electronic structure and optical properties of ScN monolayer

Based on the density functional theory, electronic and optical properties of a monolayer scandium nitride structure have been studied under different strain conditions. Our results indicate that both biaxial compressive and tensile strain effects lead to change the band gap of this structure with different rates. Also, optical absorption spectrum peaks experience an obvious red and blue shifts with the exerting of tensile and compressive strains, respectively. Our results express that ScN monolayer can be the promising candidate for the future nano-base electrical and optical devices.