过渡金属硼碳化物TM_3B_3C和TM_4B_3C_2稳定性和性能的理论计算

在过渡金属轻元素化合物中,寻找新的硬质或者超硬材料是当前的一个研究热点.目前寻找范围多集中在过渡金属硼化物、碳化物和氮化物等二元体系,三元相的研究则相对较少.本文以已知Nb_3B_3C和Nb_4B_3C_2结构为模板,采用元素替代法构建了29种TM_3B_3C (TM为过渡金属元素)结构和29种TM_4B_3C_2结构,采用基于密度泛函理论的第一性原理计算方法,成功找到了热力学、动力学以及力学都稳定的Ta_3B_3C和Ta4B_3C2两种新相.结构搜索计算确认了这两相为全局能量最优结构.能带结构和态密度的计算显示这两相均为导体,导电性主要源于Ta原子的d电子.这两种新相的硬度大约为26 GPa,说明Ta_3B_3C和Ta_4B_3C_2属于高硬度材料,但不是超硬材料.     

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.     

A First-Principles Study on the Vibrational and Electronic Properties of Zr-C MXenes

Within the framework of density functional theory calculations,the structural,vibrational,and electronic properties of Zr_(n )C_(n-1)(n=2,3,and 4)and their functionalized MXenes have been investigated.We find that the most stable configurations for Zr-C MXene are the ones that the terminal groups F,O,and OH locate on the common hollow site of the superficial Zr layer and its adjacent C layer.F and OH-terminated Zr_3C_(2 )and Zr_4C_(3 )have small imaginary acoustic phonon branches aroundΓpoint while the others have no negative phonon modes.The pristine MXenes(Zr_2C,Zr_3C_(2 )and Zr_4C_3)are all metallic with large DOS contributed by the Zr atom at the Fermi energy.When functionalized by F,O and OH,new hybridization states appear and the DOS at the Fermi level are reduced.Moreover,we find that their metallic characteristic increases with an increase in n.For(Zr_(n )C_(n-1))O_2,Zr_2CO_(2 )is a semiconductor,Zr_3C_2O_(2 )is a semimetal,and Zr_4C_3O_(2 )becomes a metal.     

Theoretical optoelectronic analysis of intermediate-band photovoltaic material based on ZnY1-xOx （Y = S,Se, Te） semiconductors by first-principles calculations

The structural, energetic, and electronic properties of lattice highly mismatched ZnY1-xOx （Y = S, Se, Te） ternary alloys with dilute O concentrations are calculated from first principles within the density functional theory. We demonstrate the formation of an isolated intermediate electronic band structure through diluted O-substitute in zinc-blende ZnY （Y = S, Se, Te） at octahedral sites in a semiconductor by the calculations of density of states （DOS）, leading to a significant absorption below the band gap of the parent semiconductor and an enhancement of the optical absorption in the whole energy range of the solar spectrum. It is found that the intermediate band states should be described as a result of the coupling between impurity O 2p states with the conduction band states. Moreover, the intermediate bands （IBs） in ZnTeO show high stabilization with the change of O concentration resulting from the largest electronegativity difference between O and Te compared with in the other ZnSO and ZnSeO.     

氢化二维过渡金属硫化物的稳定性和电子特性:第一性原理研究

用氢对单层二维过渡金属硫化物(TMDs)进行功能化是调节单层TMDs电子性质的既有效又经济的方法.采用密度泛函理论,对单层TMDs (MX_2 (M=Mo, W; X=S, Se, Te))的稳定性和电子性质进行理论研究,发现在单层MX_2 的层间有一个比其表面更稳定的氢吸附位点.当同阳离子时,随着阴离子原子序数的增加, H原子与MX_2 层的结合越强,氢化单层MX_2 结构越稳定;相反,同阴离子时,随着阳离子原子序数的增加, H原子与MX_2 层的结合越弱.氢原子从MoS_2的表面经层间穿越到另一表面的扩散势垒约为0.9 eV.氢化对单层MX_2 的电子特性也会产生极大的影响,主要表现在氢化实现了MX_2 体系从无磁性到磁性体系的过渡.表面氢化会使MX_2 层的带隙急剧减小,而层间氢化使MX_2 的电子结构从半导体转变为金属能带.     

NiAs、PbO型FeX(X=S,Se,Te)结构的稳定性与电子特征

利用基于密度泛函理论的第一性原理方法,Ni As与Pb O型Fe X(X=S,Se,Te)结构的稳定性与电子特征得到了研究.计算结果显示Fe的内聚能与X-Fe元素之间的相互作用是影响Fe X结构稳定性的重要因素.当X原子半径较小、电负性较大时(X=S),Fe X趋向于形成Fe-X相互作用较强、密度较大的Ni As型结构;当X原子半径较大、电负性较小时(X=Se,Te),Fe X趋向于形成Fe-Fe相互作用较强、密度较小的Pb O型结构.此外,压强使得Pb O型Fe X结构的稳定性降低.当压强分别大于5、9 GPa时,Fe Te、Fe Se趋向于形成Ni As型结构.Pb O型Fe Se中Fe原子周围的电子密度随压强的增大而增大.     

高压下ErNi2B2C弹性性质、电子结构和热力学性质的第一性原理研究

采用密度泛函理论中的赝势平面波方法研究了高压下超导材料 ErNi₂B₂C 的弹性性质、电子结构和热力学性质.分析表明, 弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显. 电子态密度(DOS)的计算结果显示, 在费米能级(E_F)处的 DOS 峰随外界压强的增大显著降低, 由于 ErNi₂B₂C 相对较高的超导温度(T_c)起因于E_F处的 DOS 峰, 因此推测压强增大可能会降低 ErNi₂B₂C 的 T_c.类似的现象在超导材料 MgB₂和 SrAlSi 中已被发现.此外, 基于准谐德拜模型, 对 ErNi₂B₂C 在高温高压下的热力学性质的研究表明, 在一定范围内, 温度和压强将对其热膨胀系数和热容产生明显的影响. 关键词： 高压 弹性性质 电子结构 热力学性质     

Electronic structures of vacancies in Co_3Sn_2S_2

Co_3Sn_2S_2 has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co_3Sn_2S_2 with exchange–correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co_3Sn_2S_2. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di-and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co_3Sn_2S_2.     

First principle investigation of the electronic and thermoelectric properties of Mg_2C

In this paper, electronic and thermoelectric properties of Mg_2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters,bulk modulus, band gap and thermoelectric properties(Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg_2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor(PF) increases with temperature. The thermoelectric properties of Mg_2C have been calculated in a temperature range of 100 K–1200 K.     

LiGaX2(X=S,Se, Te)的电子结构,光学和弹性性质的第一性原理计算

采用基于第一性原理的密度泛函理论赝势平面波方法,对LiGaX₂(X=S, Se, Te)的能带结构、态密度、光学以及弹性性质进行了理论计算. 能带结构计算表明LiGaS₂ 的禁带宽度为4.146 eV, LiGaSe₂ 的禁带宽度为3.301 eV, LiGaTe₂ 的禁带宽度为2.306 eV; 其价带主要由Ga-4p 层电子和X- np 层电子的能态密度决定; 同时也对LiGaX< 关键词： 电子结构 光学性质 弹性性质 LGX     

Surface properties of clean and Au or Pd covered hematite (α-Fe(2)O(3)) (0001)

The structure and electronic properties of different terminations of the α-Fe(2)O(3)(0001) surface were studied with spin-polarized density functional theory (DFT) and the DFT + U method accounting for the effect of strong on-site Coulomb correlations. The results for lattice relaxation, electronic and magnetic properties are presented and discussed. Though the DFT and DFT + U methods provide qualitatively very similar surface geometries they differ very much in the prediction of the electronic and magnetic properties, and the surface energetics of the clean α-Fe(2)O(3)(0001). The most stable single iron terminated (0001) surface and the oxygen-rich termination were chosen to study Au and Pd atom adsorption. The results show that both Au and Pd bind strongly to hematite surfaces and induce large changes in their geometry. The DFT + U bonding is weaker by 0.3-0.6 eV than DFT on the iron terminated surface and about 2 eV stronger on the oxygen terminated one. The binding is stronger for Pd than Au and for both adsorbates is distinctly stronger at the oxygen than at the iron terminated surface. On the iron terminated surface the adsorption binding energy per adatom increases both with Au and Pd coverage, whereas for the oxygen terminated one the opposite trend is observed.     

Observation of large in-plane anisotropic transport in van der Waals semiconductor Nb2SiTe4

Two-dimensional (2D) van der Waals material is a focus of research for its widespread application in optoelectronics, memories, and spintronics. The ternary compound Nb₂SiTe₄ is a van der Waals semiconductor with excellent air stability and small cleavage energy, which is suitable for preparing a few layers counterpart to explore novel properties. Here, properties of bulk Nb₂SiTe₄ with large in-plane electrical anisotropy are demonstrated. It is found that hole carriers dominate at a temperature above 45 K with a carrier active energy of 31.3 meV. The carrier mobility measured at 100 K is about 213 cm²·V^-1·s^-1 in bulk Nb₂SiTe₄, higher than the reported results. In a thin flake Nb₂SiTe₄, the resistivity ratio between the crystalline axes of a and b is reaching about 47.3 at 2.5 K, indicating that there exists a large anisotropic transport behavior in their basal plane. These novel transport properties provide accurate information for modulating or utilizing Nb₂SiTe₄ for electronic device applications.     

Nb‐based MXenes for Li‐ion battery applications

Li‐ion batteries depend critically on the stability and capacity of the electrodes. In this respect the recently synthesized two‐dimensional MXenes are promising materials, as they combine an excellent Li‐ion capacity with very high charging rates. We employ density functional theory to investigate the impact of Li adsorption on the structural and electronic properties of monolayer Nb₂C and Nb₂CX₂. The Li ions are predicted to migrate easily on the pristine MXene due to a diffusion barrier of only 36 meV, whereas larger diffusion barriers are obtained for the functionalized MXenes. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Robust surface state of intrinsic topological insulator Bi2Te2Se thin films: a first-principles study

Bi(2)Te(2)Se, a ternary tetradymite compound, has recently been identified to be a three-dimensional topological insulator. In this paper, we theoretically study the electronic structures of bulk and thin films of Bi(2)Te(2)Se employing spin-orbit coupling (SOC) self-consistently with density-functional theory. It is found that SOC plays an important role in determining the electronic properties of Bi(2)Te(2)Se. A finite bandgap opens up in the surface states of Bi(2)Te(2)Se thin films due to the hybridization of the top and bottom surface states of films. The intrinsic Bi(2)Te(2)Se thin films of three or more quintuple layers exhibit a robust topological nature of electronic structure with the Fermi energy intersecting the Dirac cone of the surface states only once between time-reversal-invariant momenta. These characteristics of Bi(2)Te(2)Se are similar to the topological behavior of Bi(2)Te(3), promising a variety of potential applications in nanoelectronics and spintronics.     

密度泛函理论研究硫族元素取代对单层Ag2S结构和电子性质的影响

本文利用第一性原理计算讨论了硫族元素掺杂单层Ag₂S的缺陷形成能和电子性质.缺陷形成能反映了在富Ag条件下的掺杂更容易.计算得到的带隙、Mulliken布居和态密度展示出了其相应结构的电子性质.与纯单层的Ag₂S相比,Se/Te掺杂Ag₂S后的带隙显示出其电导率变化不大.基于Mulliken原子和键布居,研究了硫族元素掺杂后Ag₂S中的共价性.此外,通过讨论态密度,分析了能级的移动和电子的贡献.     

Preserving the half-metallicity at the Heusler alloy Co2MnSi(001) surface: a density functional theory study

We have studied the stability, the electronic, and the magnetic properties of Co2MnSi(001) thin films for 15 different terminations using density functional theory calculations. The phase diagram obtained by ab initio atomistic thermodynamics shows that in practice the MnSi, pure Mn, or pure Si terminated surfaces can be stabilized under suitable conditions. Analyzing the surface band structure, we find that the pure Mn termination, due to its strong surface-subsurface coupling, preserves the half-metallicity of the system, while surface states appear for the other terminations.     

A First-Principles Study of TiX2 (X = S,Se, and Te) Compounds Optical Properties under the Effect of Externally Applied Electric Field and Strain

Physics of the Solid State - Optical properties of titanium dichalcogenide compounds, TiX2 (X = S, Se, and Te) have been calculated by first-principles calculations using density functional theory...     

杂质对Nb2GeC性质影响的第一性原理研究

为进一步研究Nb₂GeC在辐照环境中的稳定性, 本文研究了O, H和He杂质在Nb₂GeC中的稳定情况. 所有杂质的研究都是从替代和间隙两个方面来进行的, 计算得到了替代和间隙的形成能, 存在替代和间隙时Nb₂GeC的晶格常数, 以及单胞体积, 并且与完美的晶胞进行了比较. 此外, 通过电荷密度分布和Mulliken 布居, 分析了O, H, He杂质对Nb₂GeC 的电子性质的影响.     

Ni与钇稳定的氧化锆(111)表面相互作用以及界面活性的第一性原理研究

采用基于密度泛函理论的第一性原理方法, 系统研究了Ni原子在钇稳定的氧化锆(YSZ)(111)和富氧的YSZ(YSZ+O)(111)表面不同位置的吸附, 以及CO和O₂分子在Ni₁(单个镍原子)/YSZ和Ni₁/YSZ+O表面吸附的几何与电子结构特征. 结果表明: 1) 单个Ni原子倾向于吸附在O原子周围, 几乎不吸附在Y原子周围, 且Ni原子在氧空位上吸附最稳定; 2)和YSZ相比, 单个Ni原子在YSZ+O表面易发生氧化现象, Ni原子失去1.06 e电子, 被氧化成了Ni⁺, 吸附能力更强; 3)被氧化的Ni催化活性大幅下降, 大大减弱了表面对O₂和CO等燃料气体的吸附作用.     

天然杂质对黄铁矿的电子结构及催化活性的影响

采用密度泛函理论和平面波赝势方法对含天然杂质黄铁矿的电子结构和光学性质进行了计算,并讨论了二十种天然杂质:钴、镍、砷、硒、碲、铜、金、银、钼、锌、铊、锡、钌、钯、铂、汞、镉、铋、铅和锑,对黄铁矿催化活性的影响.结果表明在过渡金属杂质中,杂质能级主要由它们的d轨道产生,而在主族金属及非金属杂质中,杂质能级主要由它们的s或p轨道产生.含铜、钼、砷、金、银或镍的黄铁矿对氧的还原的电催化能力增强.除锌、钼、钌、砷、锑、硒和碲外,其余杂质能增强黄铁矿表面俘获电子的能力,使光生电子和空穴复合的概率减小.光学性质计算表 关键词： 天然黄铁矿 杂质 电子结构 光学性质