二维MoSi2N4WSe2异质结的第一性原理研究

实验上新合成的MoSi2N4（MSN）由于其独特的七原子层结构和电子特性引起了人们的广泛关注。本文搭建了一种由二维MSN与二维WSe2（WS）垂直堆垛而成的二维MSN/WS异质结，其表现出直接间隙半导体和I型能带排列的特性，具有1.46 eV的带隙。在异质结界面处存在一个由电荷耗尽层MSN指向电荷积累层WS微弱的内建电场。最后，通过施加双轴应变对二维MSN/WS异质结进行调控。发现在正双轴应变的作用下，MSN/WS异质结保持了原来直接带隙半导体和I型能带排列特性；在负双轴应变作用下，MSN/WS异质结由原来的直接带隙半导体转变为间接带隙半导体，当施加的负双轴应变达到-6%与-8%时，I型能带排列转变为Ⅱ型能带排列。     

应变和电场对Ga2SeTe/In2Se3异质结电子结构和光学性质的影响

异质结构的构筑与堆垛是新型二维材料物性调控及应用的有效策略.基于密度泛函理论的第一性原理计算,本文研究了4种不同堆叠构型的新型二维Janus Ga₂SeTe/In₂Se₃范德瓦耳斯异质结的电子结构和光学性质. 4种异质结构型均为Ⅱ型能带结构的间接带隙半导体,光致电子的供体和受体材料由二维In₂Se₃的极化方向决定.光吸收度在可见光区域高达25%,有利于太阳可见光的有效利用.双轴应变可诱导直接-间接带隙转变,外加电场能有效调控异质结构带隙,使AA2叠加构型的带隙从0.195 eV单调增大到0.714 eV,AB2叠加构型的带隙从0.859 eV单调减小到0.058 eV,两种调控作用下异质结的能带始终保持Ⅱ型结构.压缩应变作用下的异质结在波长较短的可见光区域表现出更优异的光吸收能力.这些研究结果揭示了Janus Ga₂SeTe/In₂Se₃范德瓦耳斯异质结电子结构的调控机理,为新型光电器件的设计提供理论指导.     

剪切形变下掺杂及缺陷对MoS2电子结构的影响

本文利用密度泛函理论,研究剪切形变下掺杂改性及不同类型缺陷对MoS2电子结构的影响。发现：剪切形变下,MoS2+P体系为相对最稳定的结构,掺杂改性相较于缺陷对模型稳定性影响更小;模型MoS2+P+Se中P-Mo键易形成共价键,而其中的Se-Mo键和MoS2+P-Mo-S模型中的P-Mo键,易形成离子键;掺杂使MoS2模型能隙变大,而缺陷使能隙减小,且S和Mo原子共缺陷的模型带隙为0;缺陷相较于掺杂改性模型,更能使Mo原子周围增加电荷聚集度,带隙值更低,更能影响或调控模型的电子结构。     

电场对graphene/InSe范德瓦耳斯异质结肖特基势垒的调控

半导体与金属接触是制作纳电子和光电子器件时非常重要的问题,接触类型对器件的功能实现和性能影响很大.为了制备高性能多功能化器件,就必须对界面处的势垒高度和接触类型进行调控.采用基于密度泛函理论的第一性原理计算研究了外电场作用下graphene/In Se范德瓦耳斯异质结的电子结构.计算结果表明异质结中的graphene和In Se保留了各自的本征电子性质,在界面处形成了欧姆接触.外电场可以有效调控graphene/In Se异质结中的肖特基势垒,不但可以调控肖特基势垒的高度,而且可以调控界面接触类型.外电场还可以有效调控graphene和In Se界面电荷转移的数量和方向.     

点缺陷对单层MoS2电子结构及光学性质的影响研究

采用基于第一性原理的贋势平面波方法,对不同类型点缺陷单层MoS2电子结构、能带结构、态密度和光学性质进行计算。计算结果表明：单层MoS2属于直接带隙半导体,禁带宽度为1.749ev,V-Mo缺陷的存在使得MoS2转化为间接带隙Eg=0.671eV的p型半导体,V-S缺陷MoS2的带隙变窄为Eg=0.974eV,S-Mo缺陷的存在使得MoS2转化为间接带隙Eg=0.482eV; Mo-S缺陷形成Eg=0.969eV直接带隙半导体,费米能级上移靠近价带。 费米能级附近的电子态密度主要由Mo的4d态和s的3p态电子贡献。光学性质计算表明：空位缺陷对MoS2的光学性质影响最为显著,可以增大MoS2的静态介电常数、折射率n0和反射率,降低吸收系数和能量损失。     

应力调控BlueP/XTe2(X=Mo,W)范德瓦耳斯异质结电子结构及光学性质理论研究

通过第一性原理计算探讨了蓝磷烯与过渡金属硫化物MoTe₂/WTe₂形成范德瓦耳斯异质结的电子结构和光学性质,以及施加双轴应力对相关性质的影响.计算结果表明,形成BlueP/XTe₂(X=Mo,W)异质结,二者能带排列为间接带隙type-Ⅱ并有较强的红外光吸收,同时屏蔽特性增强.随压缩应力增加,BlueP/XTe₂转变为直接带隙type-Ⅱ能带排列最后转变为金属性;随拉伸应力增加,异质结转变为间接带隙type-Ⅰ能带排列.外加应力也能有效调控异质结的光吸收性质,随压缩应力增加吸收边红移,光吸收响应拓展至中红外光谱区且吸收系数增大;BlueP/MoTe₂较BlueP/WTe₂在中红外至红外光区间表现出更强的光吸收响应;静态介电常数ε1(0)大幅增加.结果表明,压缩应力对BlueP/MoTe₂和BlueP/WTe₂能带排列、光吸收特性均有显著的调控作用,其中BlueP/MoTe₂对调控更敏感,这些特性也使BlueP/XTe₂异质结在窄禁带中红外半导体材料及光电器件具有令人期待的应用价值.     

类石墨烯硼-磷单层材料的电子与光学性能的第一性原理计算研究

基于第一性原理计算,对硼-磷单层材料的电子结构和光学性质进行系统地理论研究. 全局结构搜索和第一性原理分子动力学模拟现实二维硼-磷单层材料能量最低的结构与石墨烯类似,具有很高的稳定性. 类石墨烯二维硼-磷单层是直接带隙半导体,带隙宽度1.37 eV,其带隙宽度随层数增加而减少. 硼-磷单层的带隙宽度受外界应力影响.硼-磷单层的载流子迁移率达到106 cm2/V. MoS2/BP二维异质结可用于光电器件,其理论光电转换效率为17.7%?19.7%. 表明类石墨烯硼-磷二维材料在纳米电子器件与光电子器件的潜在应用价值.     

强激光照射对2H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一原理赝势法, 计算了纤锌矿结构2H-SiC晶体在强激光照射下的电子特性, 分析了其能带结构和电子态分布. 计算结果表明: 2H-SiC平衡晶格参数a 和c随电子温度T_e的升高逐渐增大; 电子温度在0–2.25 eV范围内时, 2H-SiC仍然是间接带隙的半导体晶体, 当T_e超过2.25 eV达到2.5 eV以上时, 2H-SiC变为直接带隙的半导体晶体; 随着电子温度升高, 导带底和价带顶向高能量或低能量方向发生了移动, 当电子温度T_e大于3.5 eV以后, 价带顶穿越费米能级; 电子温度T_e在0–2.0 eV变化时, 带隙随电子温度升高而增大; T_e在2.0–3.5 eV范围变化时, 带隙随电子温度升高而快速地减少, 表明2H-SiC晶体的金属性随电子温度T_e的继续升高而增强. 在T_e =0, 5.0 eV 处, 计算了2H-SiC晶体总的电子态密度和分波态密度. 电子结构表明T_e =0 eV 时, 2H-SiC 是一个带隙为2.3 eV的半导体; 在T_e =5.0 eV时, 带隙已经消失而呈现出金属特性, 表明当电子温度升高时晶体的共价键变弱、金属键增强, 晶体经历了一个熔化过程, 过渡到金属状态.     

二维BC2 N薄片的结构稳定性和电子性质

采用基于密度泛函理论的第一性原理方法,对二维BC₂N薄片的结构稳定性和电子性质进行了系统的研究.计算了BC₂N化合物16种可能的二维单层结构.对它们的能带结构分析发现,对称性最高的构型与石墨烯一样是一种半金属,而其他二维结构则为有不同带隙的半导体,其中最稳定的构型是带隙值为1.63 eV的直接带隙半导体.对最稳定构型的差分电荷密度分析和Bader分析发现:在最稳定的构型中,C–C键、C–N键、C–B键和B–N键主要以共价键的形式呈现,也具有比较明显的离子性.在应力作用下最稳定构型的单层BC₂N的带隙宽度会发生变化,压缩时带隙变宽,而拉伸时带隙变窄,但仍然为直接带隙半导体. 关键词： 2N')" href="#">BC₂N 单层原子薄片 电子结构 从头计算     

TiO$lt;sub$gt;2$lt;/sub$gt;纳米管电子结构和光学性质的第一性原理研究

运用基于密度泛函理论的第一性原理方法, 系统研究了小尺寸锐钛矿相(n,0)型TiO₂纳米管(D<16 Å)的几何构型、电子结构和光学性质. 结果表明: 随着管径增大, 体系单位TiO₂分子的形成能降低, 体系趋于稳定; 在管径14 Å左右, (n,0)型TiO₂纳米管会发生一次构型的转变. 能带分析显示, TiO₂纳米管的电子态比较局域化, 小管径下(D<14 Å)其导电性更好; 随着构型的转变, TiO₂纳米管由直接带隙转变为间接带隙, 并且带隙值随着管径的增大而增大, 这是由于π轨道重叠效应的影响大于量子限域效应所导致的结果. 两种效应的竞争, 使得TiO₂纳米管的介电函数虚部ε₂ (ω)谱的峰值位置随管径增大既可能红移也可能蓝移, 管径大于9 Å (即(8, 0)管)之后, TiO₂纳米管的光吸收会出现明显的增强. 关键词： 2纳米管')" href="#">TiO₂纳米管 第一性原理 电子结构 光学性质     

Modulation of band gap by normal strain and an applied electric field in SiC-based heterostructures

The structure and electronic properties of the WS₂/SiC van der Waals (vdW) heterostructures under the influence of normal strain and an external electric field have been investigated by the ab initio method. Our results reveal that the compressive strain has much influence on the band gap of the vdW heterostructures and the band gap monotonically increases from 1.330 to 1.629 eV. The results also imply that electrons are likely to transfer from WS₂ to SiC monolayer due to the deeper potential of SiC monolayer. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the E-field changes from to ?0.50 +0.20 V/Å, the band gap first increases from zero to a maximum of about 1.90 eV and then decreases to zero. The significant variations of band gap are owing to different states of W, S, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the WS₂/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

Band gap tuning of Ge/SiC bilayers under an electric field: a density functional study

The structure and electronic properties of Ge/SiC van der Waals (vdW) bilayer under the influence of an electric field have been investigated by the first-principles method. Without an electric field, the system shows a small band gap of 126 meV at the equilibrium state. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the negative E-field changes from 0.0 to ?0.40 V/Å, the band gap first increases to a maximum of about 378 meV and then decreases to zero. A similar trend is exhibited for the positive E-field, ranging from 0.0 to +0.40 V/Å. The band gap reaches a maximum of about 315 meV at +0.10 V/Å. The significant variations of band gap are owing to different states of Ge, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the Ge/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

Thermally induced band hybridization in bilayer-bilayer MoS_2/WS_2 heterostructure

Transition metal dichalcogenides(TMDs), being valley selectively, are an ideal system hosting excitons. Stacking TMDs together to form heterostructure offers an exciting platform to engineer new optical and electronic properties in solid-state systems. However, due to the limited accuracy and repetitiveness of sample preparation, the effects of interlayer coupling on the electronic and excitonic properties have not been systematically investigated. In this report, we study the photoluminescence spectra of bilayer-bilayer MoS_2/WS_2 heterostructure with a type Ⅱ band alignment. We demonstrate that thermal annealing can increase interlayer coupling in the van der Waals heterostructures, and after thermally induced band hybridization such heterostructure behaves more like an artificial new solid, rather than just the combination of two individual TMD components. We also carry out experimental and theoretical studies of the electric controllable direct and indirect infrared interlayer excitons in such system. Our study reveals the impact of interlayer coupling on interlayer excitons and will shed light on the understanding and engineering of layer-controlled spin-valley configuration in twisted van der Waals heterostructures.     

Computational screening of vdWs heterostructures of BSe with MoSe2 and WSe2 as sustainable hydrogen production materials

Recently, fabricating type-II vertical van der Waals (vdWs) heterostructure is a promising material for hydrogen production. The absorption capability, charge density distributions, band alignments and electronic properties of the monolayers and heterostructures are systematically investigated using computational studies. Using ab initio molecular dynamics, binding energy and phonon calculations, the stability of the heterostructures are verified. Both heterostructures are type-II materials, which can increase the separation of charge carriers. Moreover, the charge density difference and the potential drop across the interface of MSe₂/BSe creates a high built-in electric field that can prevent the recombination of charge carriers. We found that the visible-light optical properties of both heterostructures are much enhanced with suitable bandgap energy for water splitting. The band alignment suggests that the heterostructures straddle water redox potentials in acid solutions (0 < pH < 7). Our study predicted that MSe₂/BSe vdW heterostructures have great potential for photocatalytic hydrogen production.     

Efficient band structure engineering and visible-light response in ZrS2/GaS heterobilayer by electrical field or external strains

Via first-principle methods, the electronic structures and optical properties of 2D ZrS₂/GaS van der Waals heterostructure (vdWH) are studied. It is found that the band alignment changes from type-II to type-I under negative electrical field, and compressive strains. The transition points are -0.2 V/Å and -1%, respectively. The band gap changes efficiently under positive electrical field and compressive strains. The tensile strains increase the optical adsorption coefficients in ultraviolet regions, while the compressive strains increase the optical adsorption coefficients in visible region significantly.     

Strain and electric-field induced tunable electronic properties of blue phosphorus-GeS/SnS/SnSe (orthorhombic) vdW heterostructures

In this work, we composite blue phosphorous (blueP) and monolayer GeS/SnS/SnSe through van der Waals (vdW) force interaction. It is found that blueP-GeS/SnS heterostructures are stable and form type-II band alignments, which can effectively promote the separation of photoinduced carriers. We perform a systematic theoretical study of interlayer coupling effects and band realignment of blueP-GeS/SnS/SnSe heterostructures after the strain and electric-field are imposed. BlueP and GeS/SnS/SnSe are twisted with different angles, and the theoretical framework of bands alignment and carriers' separation are established. The results show that the electronic properties of independent blueP and GeS/SnS/SnSe can be roughly maintained. When strain is applied, the band alignment shows significant adjustability by changing the external strain. Besides, the blueP-SnSe heterostructure show type-II characteristic in the range from -0.25 V/Å to -0.1 V/Å. Our theoretical calculation proves that strain and electric field engineering are two useful methods to design novel electronic devices.     

二维MoS2/MoSe2异质材料的电子结构和热电性质研究

利用密度泛函理论结合玻尔兹曼输运理论计算体相和双层二维MoS₂/MoSe₂异质材料的热电性质. 计算表明,体相MoS₂/MoSe₂异质材料的热电性质比之于MoSe₂会有较大程度的提高. 该异质材料热电性质的提高主要源于异质材料本身带隙的减小以及层间的范德瓦尔斯相互作用. 二维MoS₂/MoSe₂异质材料存在热电应用的可能性.     

Tunable electronic and optical properties of GaS/GaSe van der Waals heterostructure

We have used first-principles calculations to investigate the electronic and optical properties of GaS/GaSe van der Waals heterostructures formed by stacking two-dimensional GaSe and GaSe monolayers. Our findings confirm that the GaS/GaSe heterostructures transform from an indirect to a direct band gap material for the two stackings considered in this study. In addition, we found that the direct band gaps are 1.780 eV and 1.736 eV for AA and AB stacking, respectively. It is observed that the behavior of the optical properties of AA stacking is similar to AB stacking with some differences in details and both heterostructures located in UV range. The refractive index values are 2.21 (AA pattern) and 2.18 (AB pattern) at zero photon energy limit and increase to 2.937 for AA and 2.18 AB patterns and both located in the visible region. More importantly, the GaS/GaSe heterostructures have a variety of extraordinary electronic and optical properties. Accordingly, these heterostructures can be useful for the solar cell, nanoelectronics, and optoelectronic applications.     

First-principles study of metal-semiconductor contact between MX2 (M = Nb,Pt; X = S,Se) monolayers

First principles calculations are performed to investigate the structural and electronic properties of MX₂ (M = Nb, Pt; X = S, Se) monolayers and their van der Waals (vdW) heterostructures. The dynamical stability of monolayers and vdW heterostructures is confirmed by binding energy and phonon spectra. An indirect band gap nature is found for PtS₂ and PtSe₂ monolayers while NbS₂, NbSe₂ and all vdW heterostructures are metals. The intrinsic electronic properties of both NbX₂ and PtX₂ are well preserved due to weak vdW contact. It is demonstrated that a p-type Schottky contact with a small barrier height is formed at NbX₂-PtX₂ interface. The zero tunnel barrier and higher potential drop across the interface in these contacts imply large transfer of charge carriers across the interface, making them potential candidates in nanoelectronic device applications.