GeTe和GeSe 分子在外电场下的特性研究

对Ge原子采用6-311++G^**基函数,Te和Se原子采用SDB-cc-pVTZ基函数,利用密度泛函理论的局域自旋密度近似方法优化得到了GeTe和GeSe分子的稳定构型,并计算了外电场作用下GeTe和GeSe基态分子的平衡核间距、总能量、最高已占据分子轨道能量E_H、最低未占分子轨道能量E_L、能隙、谐振频率和红外谱强度. 在上述计算的基础上利用单激发组态相互作用-局域自旋密度近似方法研究了GeTe和GeSe分子在外电场下的激发特性. 结果表明:随着正向电场强度的增大,分子核间距逐渐增大,分子总能量逐渐降低,谐振频率逐渐减小,红外谱强度则逐渐增大. 在0-2.0569×10¹⁰ V·m^-1的电场范围内,GeTe分子的E_H 均高于GeSe分子的E_H;随着正向电场的增大,GeTe与GeSe的E_H差逐渐变大,GeTe的E_L低于GeSe的E_L,它们的E_L均随正向电场的增大而增大. 无外场时,GeTe分子的能隙比GeSe分子的能隙要小;在外电场反向增大的过程中, GeTe和GeSe的分子能隙始终减小. 外电场的大小和方向对GeTe和GeSe分子的激发能、振子强度及跃迁的波长均有较大影响. 关键词： GeTe GeSe 外电场 激发态     

The He(I) photoelectron spectroscopy of heavy group IV—VI diatomics

The He(I) photoelectron spectra of the Group IV—VI diatomics GeO, GeS, GeSe, GeTe, SnS, SnSe, and SnTe are presented. The outermost valence structure of these molecules is similar to that observed in the lighter series CO, CS, etc. of this valence isoelectronic group; in each case a relatively sharp peak is assigned to ionization from the nominally non-bonding 3σ orbital and a broader band to ionization from the bonding 1π orbital. At higher binding energies the spectra exhibit several peaks where only a single peak is expected, from the (2σ)^?1 hole state. This structure is assigned to correlation peaks resulting from configuration interaction among hole states of ²Σ⁺ (Ω = $\text{1}\text{2}$) symmetry. Semi-empirical CNDO—MO calculations have been performed for these molecules, and the results are used to interpret the observed trends. In addition, a simple molecular orbital model is employed to estimate the importance of spin—orbit coupling in the valence electronic structure of the heavy IV—VI ions.     

Far infrared absorption of a-GeSe

Far infrared absorption measurements have been made on a series of a-Ge_xSe_1?x films with near equiatomic compositions. The only sharp feature is an absorption line at 260cm^-1, and the compositional dependance of this feature is correctly described by a model in which the Ge(Se) atoms are 4(2) fold co-ordinated. There is no evidence in our amorphous samples of any absorption band in the frequency range (160?190cm^?1) of the major IR active modes of 3:3 fold co-ordinated crystalline GeSe.     

H,F修饰单层GeTe的电子结构与光催化性质

采用基于密度泛函理论的第一性原理平面波赝势方法,计算了单层GeTe、表面氢化及氟化单层GeTe的晶体结构、稳定性、电子结构和光学性质.计算结果表明,经过修饰后, GeTe的晶格常数、键角、键长增大,且均具有较好的稳定性.电子结构分析表明,单层GeTe为间接带隙半导体,全氢化修饰、全氟化修饰以及氢氟共修饰(F, Ge同侧;H, Te同侧)则转变为直接带隙半导体,且修饰后的能隙均不同程度减小.载流子有效质量表明,全氢化、全氟化以及氢氟共修饰GeTe (F, Ge同侧;H, Te同侧)的有效质量减小,其载流子迁移率增强.带边势分析结果显示,单层GeTe能够光裂解水制氢和析氧,而修饰后的GeTe的价带带边势明显下移,其氧化性明显增强,能够光裂解水析O2, H2O2, O3以及OH·等产物.光学性质表明,修饰后的GeTe对可见光区和红、紫外区的光谱吸收效果明显增强,表明其在光催化领域有着广阔的应用前景.     

Infrared spectra of some matrix-isolated germanium,tin, and lead chalcogenides

Infrared spectra are reported for GeS, GeSe, GeTe, SnS, SnSe, PbS and PbSe in argon and nitrogen matrices at 12°K. The absorptions due to the matrix-isolated diatomic IV–VI molecules show considerable isotopic structure. The dimer absorptions are treated in terms of a D_2h planar model and indicate that the dimers have nearly square geometries and that the bond stretching force constants in the dimers are about half those of the diatomic molecules.     

Photoemission (UPS and XPS) study of crystallization of amorphous GeTe film

In situ photoemission (UPS and XPS) measurements have been performed for amorphous GeTe films. The photoemission spectra exhibit a drastic change upon thermal annealing and/or the crystallization of the film. It has been found that an amorphous GeTe film deposited onto a room temperature substrate has a 4-2 coordinated local structure, while a highly disordered amorphous GeTe film evaporated onto a cooled (77 K) substrate is largely 3-3 coordinated, and relaxes into the 4-2 coordinated structure upon thermal annealing within amorphous phase.     

提高单层GeSe对H2O气体传感性能的第一性原理研究

采用第一性原理计算方法,通过在单层GeSe上施加双轴向应变、外加电场、掺Ag等途径来探索提高单层GeSe对H₂O分子传感性能的有效方法,并从微观角度阐明内在机理.计算结果表明,-1.0 V/?的外加电场能有效降低H₂O分子在单层GeSe的吸附能并使二者之间的电荷转移量增加11倍,显著提高了单层GeSe对H₂O分子的响应速度和敏感性.研究结果为进一步设计并制成二维GeSe基湿度传感器提供了理论依据.     

Investigation of the band structure of germanium chalcogenides by means of photoelectron spectroscopy

The valence band photoelectron spectra of amorphous GeS, GeSe and GeTe have been measured. The ultraviolet and X-ray excited spectra show three bands associated with bonding p-states, anti-bonding and bonding s-states, respectively. In the ultraviolet photoemission spectra the p-band shows a pronounced fine structure. Features of the valence band density of states estimated on the basis of a tightbinding model are in reasonable agreement with experiment. Some problems connected with the extension of the ionicity concept of Phillips and van Vechten to the IV–VI compounds are discussed.     

利用晶体结构工程提升GeSe化合物热电性能的研究

在热电研究领域, Ge Se是一种二维层状结构具有较大带隙的半导体,本征载流子浓度低,热电性能差.在本工作中,采用熔融淬火结合放电等离子活化烧结工艺制备了一系列的Ge Se_1–x Te_x (x=0, 0.05, 0.15, 0.25,0.35, 0.45)多晶样品,研究了Te含量对Ge Se化合物物相结构和热电输运性能的影响规律.结果表明:随着Te含量的增加, Ge Se的晶体结构逐渐由正交相向菱方相转变,使得材料的带隙降低,载流子浓度和迁移率同步增加;同时,晶体对称性的提高增加了化合物的能带简并度,有效提高了载流子有效质量.在这些因素的共同作用下,菱方相Ge Se的功率因子比正交相Ge Se提高约2—3个数量级.此外,菱方相Ge Se具有丰富的阳离子空位缺陷以及铁电特性所导致的声子软化现象,这导致其晶格热导率比正交相Ge Se降低近60%.当Te含量为0.45时,样品在573 K取得最大热电优值ZT为0.75,是本征Ge Se样品的19倍.晶体结构工程是提升Ge Se化合物热电性能的有效途径.     

Amorphous Ge??Te??: density functional, high-energy x-ray and neutron diffraction study

The structure and electronic properties of amorphous Ge(15)Te(85) have been studied by combining density functional (DF) simulations with high-energy x-ray and neutron diffraction measurements. Three models with 560 atoms have been constructed using reverse Monte Carlo methods constrained to (1) agree with the experimental structure factors S(Q), and have (2) energies close to the DF minimum and (3) a semiconducting band structure. The best structure is based on the melt-quenched DF structure and has a small number of Ge-Ge bonds. It shows interlocking networks of Te and GeTe with a significant fraction (22-24%) of voids (cavities). Ge occurs with both tetrahedral and 3 + 3 defective octahedral configurations, and the coordination of Te is slightly higher than indicated by the '8 - N rule' (N is the number of valence electrons). The GeTe network includes clusters of ABAB squares (A = Ge, B = Te), and the bonding is characterized by the chemical bond orders.     

Magneto-electronic properties and spin-resolved I–V curves of a Co/GeSe heterojunction diode: an ab initio study

We present ab initio calculations of magnetoelectronic and transport properties of the interface of hcp Cobalt (001) and the intrinsic narrow-gap semiconductor germanium selenide (GeSe). Using a norm-conserving pseudopotentials scheme within DFT, we first model the interface with a supercell approach and focus on the spin-resolved densities of states and the magnetic moment (spin and orbital components) at the different atomic layers that form the device. We also report a series of cuts (perpendicular to the plane of the heterojunction) of the electronic and spin densities showing a slight magnetization of the first layers of the semiconductor. Finally, we model the device with a different scheme: using semiinfinite electrodes connected to the heterojunction. These latter calculations are based upon a nonequilibrium Green’s function approach that allows us to explore the spin-resolved electronic transport under a bias voltage (spin-resolved I–V curves), revealing features of potential applicability in spintronics.     

A minimal basis semi-ab initio approach to the band structures of semiconductors

The band structures of 32 of the most important semiconductor crystals are calculated using an efficient, minimal basis, orthogonalized LCAO method. These include the diamond structure of C, Si, Ge, α-Sn; the zinc blende structure of β-SiC, BN, BP, AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, β-ZnS, ZnSe, ZnTe, CdS, CdTe; the wurtzite structure of AlN, GaN, ZnO, α-ZnS, CdS, CdSe; the sodium chloride structure of CdO, GeTe, SnTe and trigonal Se and Te. The calculations, which involve diagonalizations of small size matrix equations yield results having the following characteristics: (1) satisfactory valence bands and lower conduction bands and bulk densities of states; (2) the gap sizes and the locations of valence band maximum and conduction band minimum in agreement with experiment; (3) reasonable values of fractional ionicity and electron and hole effective masses. These are achieved by fine-tuning the exchange parameters in the construction of the potentials. Application of this approach to the study of the electronic structures of disordered and other complex semiconductor systems is also discussed.     

Mg0.3Fe2.7GeTe2 单晶的磁性及霍尔效应研究

Fe3GeTe2 是一种具有稳定长程磁有序的准二维范德瓦尔斯磁性材料, 范德瓦尔斯材料的稳定性和可调性使其在自旋电子器件的应用方面具有巨大潜力. 本文用助熔剂法生长了 Mg 原子掺杂Fe2 位的 Mg0.3Fe2.7GeTe2单晶样品, 并对 Mg 掺杂Fe3GeTe2 的结构、磁性和输运性质的影响进行了研究. 磁性数据表明 Mg 掺杂后铁磁转变温度不变, 但样品的饱和磁矩减小. 输运性质的测量中观察到各向异性的反常霍尔效应, 与Fe3GeTe2 相比, Mg掺杂后的反常霍尔电阻率减小, 同时各向异性发生了变化.     

Defect physics of the quasi-two-dimensional photovoltaic semiconductor GeSe

GeSe has recently emerged as a photovoltaic absorber material due to its attractive optical and electrical properties as well as earth abundancy and low toxicity. However, the efficiency of GeSe thin-film solar cells (TFSCs) is still low compared to the Shockley-Queisser limit. Point defects are believed to play important roles in the electrical and optical properties of GeSe thin films. Here, we perform first-principles calculations to study the defect characteristics of GeSe. Our results demonstrate that no matter under the Ge-rich or Se-rich condition, the Fermi level is always located near the valence band edge, leading to the p-type conductivity of undoped samples. Under Se-rich condition, the Ge vacancy (V_Ge) has the lowest formation energy, with a (0/2-) charge-state transition level at 0.22 eV above the valence band edge. The high density (above 10¹⁷ cm^-3) and shallow level of V_Ge imply that it is the p-type origin of GeSe. Under Se-rich growth condition, Se_i has a low formation energy in the neutral state, but it does not introduce any defect level in the band gap, suggesting that it neither contributes to electrical conductivity nor induces non-radiative recombination. In addition, Ge_i introduces a deep charge-state transition level, making it a possible recombination center. Therefore, we propose that the Se-rich condition should be adopted to fabricate high-efficiency GeSe solar cells.     

新型GeSe2-In2Se3-AgI玻璃性能研究

采用传统熔融-淬冷法制备了一系列新型(100-x)(4GeSe2-In2Se3)-xAgI(x=20,30,40mol%)硫系玻璃样品.利用X射线衍射分析、差热分析、可见-近红外吸收光谱、红外透过光谱、喇曼分析等技术手段研究了该玻璃系统的组成、结构、热稳定性和光学特性等.利用Tauc方程计算出了样品的间接带隙;测试了部分样品在不同升温速率下的差示扫描量热曲线,并采用Kissinger法计算了玻璃样品的析晶活化能.X射线衍射数据表明,该玻璃体系在较宽的组分范围内有良好的非晶特性,成玻范围较宽;差热分析和析晶动力学研究表明,玻璃样品70(4GeSe2-In2Se3)-30AgI具有较好的热稳定性(ΔT=114℃)和较高的活化能(Ea=320.4kJ/mol).随着AgI含量的增加,玻璃的短波吸收限蓝移,并且光学带隙有增大的趋势.此外,红外透过光谱分析表明该玻璃体系具有良好的红外透过性能,其红外截止波长不会随着AgI含量的增加而发生明显变化,皆为16μm左右.     

块状NiSe2的电学，光学和热力学性质的理论研究

在密度泛函理论框架下通过平面波赝势的第一原理计算,研究了块体材料NiSe2的电学,光学和热力学性质.根据计算得到的立方晶体（空间群：PM3,No：200）的能量-体积曲线,获得了最低能量的最优结构.基于能带结构和电子态密度结构计算的结果表明,NiSe2为半金属.当有电磁波穿过块体NiSe2时,通过非自洽计算,分析介电函数的实部和虚部得到了能量损失函数谱,反射率,吸收谱,折射系数和消光系数.在准谐波德拜模型的基础上,我们还研究了NiSe2的热力学性质.     

Magnetic properties of Pd atomic clusters from different theoretical approaches

We report a comparative study of the magnetic properties of free-standing Pd_N clusters (2 ≤N ≤21) obtained through two different theoretical approaches that are extensively employed in electronic structure calculations: a semi-empirical Tight-Binding (TB) model and an ab-initio DFT pseudopotential model. Conclusions are drawn about the reliability of the TB model for the investigation of the electronic structure and magnetic properties of such complex 4d Transition Metals (TM) systems and we compare the results with previous systematic DFT calculations and comment on some available experiments in the literature.     

物理截断与电子局域函数结合法研究非晶态结构中的原子成键

本文将常用的键长物理截断方法与电子局域函数方法相结合,用于分析共价系统的非晶态结构,得到了更合理的原子成键信息,提高了非晶态结构模型分析的可靠性.以相变存储材料GeTe合金为例,通过采用上述方法详细研究了GeTe合金非晶态中原子间成键及结构信息,确定了其合理的成键物理截断长度为3.05?,电子局域函数阈值为0.63.研究结果显示,当电子局域函数的数值由0.58逐渐增大至0.63时,结构中所截断的化学键大部分是强度较弱的非同质键(即Ge—Te键),而强度较强的Ge—Ge键的数量几乎不变.对Ge原子配位数和其键角分布等结构信息的分析表明,Ge原子以3配位和4配位为主,其中3配位的Ge原子主要是以缺陷八面体形式存在,而4配位的Ge原子则主要以四面体的形式存在.此外,在本研究工作中所建立的确定成键物理截断长度及电子局域函数阈值的方法也可以应用于其他具有共价键性质的非晶态材料研究.     

二维GeSe纳米片五族和七族原子掺杂的受主和施主杂质态

采用第一性原理的计算方法研究GeSe纳米片结构掺杂V和VII族元素对其电子结构、形成能和跃迁能级的影响.结果表明：无论是掺杂V族还是VII族元素,体系的形成能均随杂质半径的增加而增加.V族元素掺杂体系的跃迁能级随杂质原子半径的增加而降低,而VII元素掺杂的体系却随杂质原子半径的增加而增加.其中,F、Cl、Br和I的掺杂为n型施主浅能级杂质,而N、P和As掺杂为p型受体深能级杂质.为相关的实验研究提供了理论参考.     

First-principles study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallics

First-principles calculations were performed to study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallic compounds (AlCu₃, AlCu₂Zr, and AlZr₃). The calculated results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The calculation of cohesive energies indicated that the structure stability of these Al-based intermetallics will become higher with increasing Zr element in crystal. The calculations of formation energies showed that AlCu₂Zr has the strongest alloying ability, followed by AlZr₃ and finally the AlCu₃. The further analysis find out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on the ratio of bulk modulus to shear modulus reveal that AlCu₂Zr can exhibit a good ductility, followed by AlCu₃, whereas AlZr₃ can have a poor ductility; however, for stiffness, these intermetallics show a converse order. The calculations on Poisson's ratio show that AlCu₃ is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicate that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu₃, Cu 3d, and Zr 4d states for AlCu₂Zr, and Al 3s, Zr 5s and 4d states for AlZr₃, respectively; in particular, the electronic structure of the AlZr₃ shows the strongest hybridization, leading to the worst ductility.