拓扑绝缘体Bi_2Se_3中层堆垛效应的第一性原理研究

运用第一性原理方法,研究了拓扑绝缘体Bi_2Se_3块体和薄膜中的层堆垛对其结构、电子态、拓扑态和自旋劈裂的影响.发现不同的堆垛会引起Bi_2Se_3层间的相互作用,改变系统的中心对称性.块体的ABC和AAA堆垛都具有中心对称性和相似的能带结构.ABA堆垛破坏了体系的中心对称性,能带发生很大改变,并且产生了很大的能带自旋劈裂.用能带反转的方法判定体系的拓扑相,在不同堆垛的Bi_2Se_3块体中,考虑自旋轨道耦合时都发生了能带反转,因而具有不同堆垛的Bi2Se3仍是拓扑绝缘体.进一步研究了Bi_2Se_3薄膜中的堆垛效应,发现非中心对称的ABA堆垛在Bi_2Se_3薄膜中引起明显的自旋劈裂,并且提出和验证了用应变调控自旋劈裂的方法.     

Tuning band alignment and optical properties of 2D van der Waals heterostructure via ferroelectric polarization switching

Favourable band alignment and excellent visible light response are vital for photochemical water splitting. In this work, we have theoretically investigated how ferroelectric polarization and its reversibility in direction can be utilized to modulate the band alignment and optical absorption properties. For this objective, 2D van der Waals heterostructures (HTSs) are constructed by interfacing monolayer MoS₂ with ferroelectric In2Se3. We find the switch of polarization direction has dramatically changed the band alignment, thus facilitating different type of reactions. In In₂Se₃/MoS₂/In₂Se₃ heterostructures, one polarization direction supports hydrogen evolution reaction and another polarization direction can favour oxygen evolution reaction. These can be used to create tuneable photocatalyst materials where water reduction reactions can be selectively controlled by polarization switching. The modulation of band alignment is attributed to the shift of reaction potential caused by spontaneous polarization. Additionally, the formed type-II van der Waals HTSs also significantly improve charge separation and enhance the optical absorption in the visible and infrared regions. Our results pave a way in the design of van der Waals HTSs for water splitting using ferroelectric materials.     

Reversible doping polarity and ultrahigh carrier density in two-dimensional van der Waals ferroelectric heterostructures

Van der Waals semiconductor heterostructures (VSHs) composed of two or more two-dimensional (2D) materials with different band gaps exhibit huge potential for exploiting high-performance multifunctional devices. The application of 2D VSHs in atomically thin devices highly depends on the control of their carrier type and density. Herein, on the basis of comprehensive first-principles calculations, we report a new strategy to manipulate the doping polarity and carrier density in a class of 2D VSHs consisting of atomically thin transition metal dichalcogenides (TMDs) and α-In₂X₃ (X = S, Se) ferroelectrics via switchable polarization field. Our calculated results indicate that the band bending of In₂X₃ layer driven by the FE polarization can be utilized for engineering the band alignment and doping polarity of TMD/In₂X₃ VSHs, which enables us to control their carrier density and type of the VSHs by the orientation and magnitude of local FE polarization field. Inspired by these findings, we demonstrate that doping-free p−n junctions achieved in MoTe₂/In₂Se₃ VSHs exhibit high carrier density (10¹³−10¹⁴ cm⁻²), and the inversion of the VHSs from n−p junctions to p−i−n junctions has been realized by the polarization switching from upward to downward states. This work provides a nonvolatile and nondestructive doping strategy for obtaining programmable p−n van der Waals (vdW) junctions and opens the possibilities for self-powered and multifunctional device applications.     

Ferroelectric Controlled Spin Texture in Two-Dimensional NbOI_2 Monolayer

The persistent spin helix(PSH) system is considered to have promising applications in energy-conservation spintronics because it supports an extraordinarily long spin lifetime of carriers.Here,we predict that the existence of PSH state in two-dimensional(2 D) ferroelectric NbOI_2 monolayers.Our first-principles calculation results show that there exists Dresselhaus-type spin-orbit coupling(SOC) band splitting near the conduction-band minimum(CBM) of the NbOI_2 monolayer.It is revealed that the spin splitting near CBM merely refers to out-of-plane spin configuration in the wave vector space,which gives rise to a long-lived PSH state that can be controlled by reversible ferroelectric polarization.We believe that the coupling characteristics of ferroelectric polarization and spin texture in NbOI_2 provide a platform for the realization of fully electric controlled spintronic devices.     

硒化锑薄膜太阳电池的模拟与结构优化研究

采用wx-AMPS模拟软件对硒化锑(Sb_2Se_3)薄膜太阳电池进行建模仿真,将CdS, ZnO和Sn02的模型应用到Sb_2Se_3太阳电池的电子传输层中.结果显示,应用CdS和ZnO都能实现较高的器件性能,并发现电子传输层电子亲和势(Xe-ETL)的变化能够调节Sb_2Se_3太阳电池内部的电场分布,是影响器件性能的关键参数之一.过高或者过低的Xe-ETL都会使电池的填充因子降低,导致电池性能劣化.当Xe-ETL为4.2eV时,厚度为0.6μm的Sb_2Se_3太阳电池取得了最优的7.87%的转换效率.应用优化好的器件模型,在不考虑Sb_2Se_3层缺陷态的理想情况下,厚度为3μm的Sb_2Se_3太阳电池的转换效率可以达到16.55%(短路电流密度J_(SC)=34.88 mA/cm~2、开路电压V_(OC)=0.59 V、填充因子FF=80.40%).以上模拟结果表明,Sb_2Se_3薄膜太阳电池在简单的器件结构下就能够获得优异的光电性能,具有较高的应用潜力.     

新型硒化锑材料及其光伏器件研究进展

硒化锑(Sb₂Se₃)是一种二元单相化合物, 原料储量大、毒性低、价格便宜; 同时其禁带宽度合适(～1.15 eV), 吸光系数大(>10⁵ cm^-1), 长晶温度低, 非常适合制作新型低成本低毒的薄膜太阳能电池, 理论光电转换效率可达30%以上. 目前文献报道的Sb₂Se₃薄膜太阳能电池效率已达3.7%, 初步证明了Sb₂Se₃材料在薄膜太阳能电池应用方面的巨大潜力. 本文综述了近年来Sb₂Se₃太阳能电池的研究进展, 着重介绍了Sb₂Se₃的材料特性和薄膜制备及相关理论研究, 阐述了不同结构电池器件的研究进展, 并对其发展趋势进行了展望.     

Na_2Ge_2Se_5电子结构和光学性质的第一性原理研究

Na_2Ge_2Se_5是一种优异的红外非线性晶体材料.采用基于第一性原理的密度泛函理论赝势平面波方法对Na_2Ge_2Se_5进行结构优化,并以此为基础计算研究了Na_2Ge_2Se_5的电子结构和光学性质.结果表明:Na2Ge2Se5是宽禁带间接带隙半导体,价带至导带的电子跃迁主要来自于Ge和Se的48,4p态;Na对光学性质的贡献较小,Ge和Se之间的相互耦合作用决定了Na_2Ge_2Se_5的光学性质;该晶体在紫外区有强烈的反射和吸收,静态折射率为2.133,双折射率值适中,为0.145.理论计算结果表明,Na_2Ge_2Se_5是一种性能优良的红外非线性光学晶体材料.     

Ultrasensitive solar-blind ultraviolet detection and optoelectronic neuromorphic computing using α-In2Se3 phototransistors

Detection of solar-blind ultraviolet (SB-UV) light is important in applications like confidential communication, flame detection, and missile warning system. However, the existing SB-UV photodetectors still show low sensitivities. In this work, we demonstrate the extraordinary SB-UV detection performance of α-In₂Se₃ phototransistors. Benefiting from the coupled semiconductor and ferroelectricity property, the phototransistor has an ultraweak detectable power of 17.85 fW, an ultrahigh gain of 1.2 × 10⁶, a responsivity of 2.6 × 10⁵ A/W, a detectivity of 1.3 × 10¹⁶ Jones and an ultralow noise-equivalent-power of 4.2 × 10⁻²⁰ W/Hz^1/2 for 275 nm light. Its performance exceeds most other UV detectors, even including commercial photomultiplier tubes and avalanche photodiodes. It can be also implemented as an optoelectronic synapse for neuromorphic computing. A 784×300×10 artificial neural network (ANN) based on this optoelectronic synapse is constructed and demonstrated with a high recognition accuracy and good noise-tolerance for the Fashion-MNIST dataset. These extraordinary features endow this phototransistor with the potential for constructing advanced SB-UV detectors and intelligent hardware.     

H2S硫化CuInSe2生成CuIn(SxSe1-x)2薄膜的XRD及Raman分析

将CuInSe₂薄膜在H₂S与Ar的混合气体中硫化是制备CuIn(S_xSe_1-x)₂薄膜的一种常用方法。硫化所用到的CuInSe₂薄膜是用溶剂热法生成的CuInSe₂纳米颗粒旋涂而成。不同于其他真空条件下制备CuInSe₂薄膜的方法.溶剂热法的优点是其相对简单的制备工艺和较低廉的成本。对硫化过程进行研究后发现,硫化温度和时间直接影响CuIn(S_xSe_1-x)₂薄膜的质量,诸如薄膜成分、结晶度、均匀性和带隙宽度都可以通过改变这些实验条件来进行控制。     

Al0.6Ga0.4N/GaN/Al0.3Ga0.7N/Al0.6Ga0.4N量子阱中的Rashba自旋劈裂

正如人们所知, 可以通过电场或者设计非对称的半导体异质结构来调控体系的结构反演不对称性(SIA)和Rashba自旋劈裂. 本文研究了Al_0.6Ga_0.4N/GaN/Al_0.3Ga_0.7N/Al_0.6Ga_0.4N量子阱中第一子带的Rashba 系数和Rashba自旋劈裂随Al_0.3Ga_0.7N插入层(右阱)的厚度w_s以及外加电场的变化关系, 其中GaN层(左阱)的厚度为40-w_s Å. 发现随着w_s的增加, 第一子带的Rashba系数和Rashba自旋劈裂首先增加, 然后在w_s>20 Å 时它们迅速减小, 但是w_s>30 Å时Rashba自旋劈裂减小得更快, 因为此时k_f也迅速减小. 阱层对Rashba系数的贡献最大, 界面的贡献次之且随w_s变化不是太明显, 垒层的贡献相对比较小. 然后, 我们假w_s=20 Å, 发现外加电场可以很大程度上调制该体系的Rashba系数和Rashba自旋劈裂, 当外加电场的方向同极化电场方向相同(相反)时, 它们随着外加电场的增加而增加(减小). 当外加电场从-1.5×10⁸ V·m^-1到1.5×10⁸ V· m^-1变化时, Rashba系数随着外加电场的改变而近似线性变化, Rashba自旋劈裂先增加得很快, 然后近似线性增加, 最后缓慢增加. 研究结果表明可以通过改变GaN层和Al_0.3Ga_0.7N层的相对厚度以及外加电场来调节Al_0.6Ga_0.4N/GaN/Al_0.3Ga_0.7N/Al_0.6Ga_0.4N量子阱中的Rashba 系数和Rashba自旋劈裂, 这对于设计自旋电子学器件有些启示.     

Ionic diffusion and local hopping in copper chalcohalide glasses measured using Cu tracer and I-Mössbauer spectroscopy

For the first time, ⁶⁴Cu tracer measurements of ionic diffusion were performed for several copper-rich glass compositions in the CuI---As₂Se₃, CuI---SbI₃---As₂Se₃, CuI---PbI₂---As₂Se₃, CuI---PbI₂---SbI₃---As₂Se₃ and Cu₂Se---As₂Se₃ systems. In accordance with previous a.c. impedance results and Wagner d.c. polarization measurements, it was found that pure Cu⁺ ion-conducting glasses (50CuI---17PbI₂---33As₂Se₃ and 50CuI---20PbI₂---10SbI₃---20As₂Se₃) exhibit the highest copper tracer diffusion coefficients, D_Cu, and the lowest diffusion activation energies. The values of D_Cu at room temperature are higher by 4.5–5.5 orders of magnitude than those in an As₂Se₃ glass. The Haven ratio, H_R, is found to be 0.52–0.61 (ternary glass) and 0.93–1.00 (quaternary glass). Short-range diffusional displacements of the iodide ions induced by the hopping Cu⁺ ions are also detected in the CuI---PbI₂---SbI₃---As₂Se₃ glassy system using ¹²⁹I-Mössbauer spectroscopy in the temperature range of 4.2 to 305 K. The activation energy of local hopping, E_h ≈ 0.31 eV, is very similar to that of bulk ionic conductivity (0.37 eV) and copper diffusion (≈ 0.33 eV). In contrast to CuI-based vitreous alloys, 50Cu₂Se---50As₂Se₃ glass exhibits D_Cu that are two to five orders of magnitude lower, and the copper ion transport number, t_Cu⁺, is between 10⁻³ and 10⁻⁴ in the temperature range 140–170 °C.     

Topological Dirac surface states in ternary compounds GeBi2Te4, SnBi2Te4 and Sn0.571Bi2.286Se4

Using high-resolution angle-resolved and time-resolved photoemission spectroscopy, we have studied the low-energy band structures in occupied and unoccupied states of three ternary compounds GeBi₂Te₄, SnBi₂Te₄ and Sn_0.571Bi_2.286Se₄ near the Fermi level. In previously confirmed topological insulator GeBi₂Te₄ compounds, we confirmed the existence of the Dirac surface state and found that the bulk energy gap is much larger than that in the first-principles calculations. In SnBi₂Te₄ compounds, the Dirac surface state was observed, consistent with the first-principles calculations, indicating that it is a topological insulator. The experimental detected bulk gap is a little bit larger than that in calculations. In Sn_0.571Bi_2.286Se₄ compounds, our measurements suggest that this nonstoichiometric compound is a topological insulator although the stoichiometric SnBi₂Se₄ compound was proposed to be topological trivial.     

Ge20Sb15Se65薄膜的热致光学特性变化研究

采用磁控溅射法制备了Ge₂₀Sb₁₅Se₆₅薄膜, 研究热处理温度(150—400 ℃)对薄膜光学特性的影响. 通过分光光度计、X射线衍射仪、显微拉曼光谱仪对热处理前后薄膜样品 的光学特性和微观结构进行了表征, 并根据Swanepoel方法以及Tauc公式分别计算了薄膜折射率色散曲线和光学带隙等参数. 结果表明当退火温度(T_a)小于薄膜的玻璃转化温度(T_g)时,薄膜的光学带隙(E_g^opt)随着退火温度的增加由1.845 eV上升至1.932 eV, 而折射率由2.61降至2.54; 当退火温度大于薄膜的玻璃转化温度时,薄膜的光学带隙随退火温度的增加由1.932 eV降至1.822 eV, 折射率则由2.54增至2.71. 最后利用Mott和Davis提出的非晶材料由非晶到晶态的结构转变模型对结果进行了解释, 并通过薄膜XRD和Raman光谱进一步验证了结构变化是薄膜热致变化的重要原因. 关键词： 20Sb₁₅Se₆₅薄膜')" href="#">Ge₂₀Sb₁₅Se₆₅薄膜 热处理 光学带隙 折射率     

First-principles calculations of nitrogen-doped antimony triselenide: A prospective material for solar cells and infrared optoelectronic devices

This study is focused on calculation of the electronic structure and optical properties of non-metal doped Sb2Se3 using the first-principles method. One and two N atoms are introduced to Sb and Se sites in a Sb₂Se₃ crystal. When one and two N atoms are introduced into the Sb₂Se₃ lattice at Sb sites, the electronic structure shows that the doping significantly modifies the bandgap of Sb₂Se₃ from 1.11 eV to 0.787 and 0.685 eV, respectively. When N atoms are introduced to Se sites, the material shows a metallic behavior. The static dielectric constants ε1(0) for Sb₁₆Se₂₄, Sb₁₅N₁Se₂₄, Sb₁₄N₂Se₂₄, Sb₁₆Se₂₃N₁, and Sb₁₆Se₂₂N₂ are 14.84, 15.54, 15.02, 18.9, and 39.29, respectively. The calculated values of the refractive index n(0) for Sb₁₆Se₂₄, Sb₁₅N₁Se₂₄, Sb₁₄N₂Se₂₄, Sb₁₆Se₂₃N₁, and Sb₁₆Se₂₂N₂ are 3.83, 3.92, 3.86, 4.33, and 6.21, respectively. The optical absorbance and optical conductivity curves of the crystal for N-doping at Sb sites show a significant redshift towards the short-wave infrared spectral region as compared to N-doping at Se sites. The modulation of the static refractive index and static dielectric constant is mainly dependent on the doping level. The optical properties and bandgap narrowing effect suggest that the N-doped Sb₂Se₃is a promising new semiconductor and can be a replacement for GaSb due to its very similar bandgap and low cost.     

Photoemission oscillation in epitaxially grown van der Waals β-In2Se3/WS2 heterobilayer bubbles

Thin films of millimeter-scale continuous monolayer WS₂ have been grown on SiO₂/Si substrate, followed by the deposition of β-In₂Se₃ crystals on monolayer WS₂ to prepare In₂Se₃/WS₂ van de Waals heterostructures by a two-step chemical vapor deposition (CVD) method. After the growth of In₂Se₃ at elevated temperatures, high densities of In₂Se₃/WS₂ heterostructure bubbles with monolayer to multilayer β-In₂Se₃ crystals atop are observed. Fluorescence of the resultant β-In₂Se₃/WS₂ heterostructure is greatly enhanced in intensity upon the formation of bubbles, which are evidenced by the Newton's rings in optical image owing to constructive and destructive interference. In photoluminescence (PL) mapping images of monolayer β-In₂Se₃/monolayer WS₂ heterobilayer bubble, significant oscillatory behavior of emission intensity is demonstrated due to constructive and destructive interference. However, oscillatory behaviors of peak position are also observed and come from a local heating effect induced by an excitation laser beam. The oscillatory mechanism of PL is further verified by changing the exterior pressure of bubbles placed in a home-made vacuum chamber. In addition, redshifted in peak position and broadening in peak width are observed due to strain effect during decreasing the exterior pressure of bubbles.     

Electronic and magnetic structures of chain structured iron selenide compounds

Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2 （2212＊） and BaFe2Se3 （123＊） are studied by the first-principles calculations. We find that while all these compounds are composed of one-dimensional （1D） Fe chain （or ladder） structures, their electronic structures are not close to be quasi-lD. The magnetic exchange couplings between two nearest-neighbor （NN） chains in 2212＊ and between two NN two-leg-ladders in 123＊ are both antiferromagnetic （AFM）, which is consistent with the presence of significant third NN AFM coupling, a common feature shared in other iron-chalcogenides, FeTe （11＊） and KyFe2-xSe2 （122＊）. In magnetic ground states, each Fe chain of 2212＊ is ferromagnetic and each two-leg ladder of 123＊ form a block-AFM structure. We suggest that all magnetic structures in iron-selenide compounds can be unified into an extended J1-J2-J3 model. Spin-wave excitations of the model are calculated and can be tested by future experiments on these two systems.     

Energy band alignment of 2D/3D MoS2/4H-SiC heterostructure modulated by multiple interfacial interactions

The interfacial properties of MoS₂/4H-SiC heterostructures were studied by combining first-principles calculations and X-ray photoelectron spectroscopy. Experimental (theoretical) valence band offsets (VBOs) increase from 1.49 (1.46) to 2.19 (2.36) eV with increasing MoS₂ monolayer (1L) up to 4 layers (4L). A strong interlayer interaction was revealed at 1L MoS₂/SiC interface. Fermi level pinning and totally surface passivation were realized for 4H-SiC (0001) surface. About 0.96e per unit cell transferring forms an electric field from SiC to MoS₂. Then, 1L MoS₂/SiC interface exhibits type I band alignment with the asymmetric conduction band offset (CBO) and VBO. For 2L and 4L MoS₂/SiC, Fermi level was just pinning at the lower MoS₂ 1L. The interaction keeps weak vdW interaction between upper and lower MoS₂ layers. They exhibit the type II band alignments and the enlarged CBOs and VBOs, which is attributed to weak vdW interaction and strong interlayer orbital coupling in the multilayer MoS₂. High efficiency of charge separation will emerge due to the asymmetric band alignment and built-in electric field for all the MoS₂/SiC interfaces. The multiple interfacial interactions provide a new modulated perspective for the next-generation electronics and optoelectronics based on the 2D/3D semiconductors heterojunctions.     

Composition and electric field effects on the transport properties of Bi doped chalcogenide glasses thin films

The temperature dependence of the DC electrical conductivity σ_DC was measured in the temperature range from 300–500 K. It was found that there are double activation energies, E_σ, for Ge₂₀Se_80−xBi_x (x=0, 2.5 and 5 at%) films, while there is single activation energy for Ge₂₀Se_72.5Bi_7.5. when incorporation of Bi=7.5 at%, the pre-exponential value σ₀ decreases by about six order of magnitude, the activation energy in the extended states E_σ decreases from 0.96 to 0.09 eV. Also the effect of applied electric field was studied and observed that, activation energy in high temperature region increases with increasing electric field; this behavior can be understood assuming that the contribution to the conductivity activation process is due to conduction in the extended states and also due to hopping in the localized states. With the increasing electric field, as former process, which is having high activation energy, becomes more predominant due to the dumping of the carriers in the extended states, the effective activation energy of the system increases, in spite of the fact that the activation energy of the extended states conduction may remain constant. Finally, the electrical data suggests that the addition of bismuth produces localized states near the conduction band edge so that the electrical transport is due to hopping of electrons after being excited into localized states at the conduction band edge.     

Bi4Se3薄膜的声子结构及电声子相互作用的第一性原理研究

基于密度泛函微扰理论,运用第一性原理研究两种终结面的Bi₄Se₃薄膜的声子结构和电-声子相互作用.结果表明两种终结面的Bi₄Se₃薄膜体系都是动力学稳定的. Bi₄Se₃薄膜中插入的Bi₂双原子层与Bi₂Se₃五原子层的声子投影态密度并不完全匹配,这会阻碍部分声子的输运,降低热导,从而有利于提高材料的热电性能.另外,两种终结面的Bi₄Se₃薄膜的电-声子耦合系数都不太大（约0.278）,有利于制备基于室温工作的电子学器件.     

XPS study of some selected selenium compounds

X-ray photoelectron spectroscopy (XPS) of the Se 3d peak for Se in Se, SeO₂, As₂Se₃, ZnSe, CdSe, CoSe, FeSe, CrSe, VSe₂ and TiSe were studied. It was found that, firstly, in the case of As₂Se₃, ZnSe, CdSe, CoSe and CrSe, as the change in electronegativity of the anion increases, the change in the binding energy value of the Se 3d peak also increases and, secondly, in all of the Se compounds studied, the Se-Se bond in pure Se is stronger than the Se-x bond, where X = As, Zn, Cd, Co, Fe, Cr, V and Ti.