Band alignment in SiC-based one-dimensional van der Waals homojunctions

The density functional theory method is utilized to verify the electronic structures of SiC nanotubes (SiCNTs) and SiC nanoribbons (SiCNRs) one-dimensional (1D) van der Waals homojunctions (vdWh) under an applied axial strain and an external electric field. According to the calculated results, the SiCNTs/SiCNRs 1D vdWhs are direct semiconductors with a type-II band alignment and robust electronic structures with different diameters or widths. Furthermore, the SiCNTs/SiCNRs 1D vdWhs are direct semiconductors with a type-I band alignment, respectively, in a range of[-0.3, -0.1] V/Å and[0.1, 0.3] V/Å and change into metal when the electric field intensity is equal to or higher than 0.4 V/Å. Interestingly, the SiCNTs/SiCNRs 1D vdWhs have robust electronic structures under axial strain. These findings demonstrate theoretically that the SiCNTs/SiCNRs 1D vdWhs can be employed in nanoelectronics devices.     

二维InSe/SnSe2范德华异质结的电子结构和光学特性研究

本文构建了三种不同堆叠形式下的二维InSe/SnSe₂范德华异质结模型,利用基于密度泛函理论的第一性原理方法综合考察了二维InSe/SnSe₂三种不同堆叠情况下的几何构型及稳定性,在此基础上选取具有最稳定性能的构型.该异质结呈现出Ⅱ型能带对齐特征,带隙值为1.118 eV,可以实现电子-空穴的有效分离.另外,相比与单层二维InSe/SnSe₂范德华异质结的光吸收能力达到明显提升,在紫外光范围内吸收系数达到10~6 cm^-1.研究结果将为相关物理实验及机理研究提供理论基础,对二维InSe/SnSe₂范德华异质结在光电器件中的应用具有重要的物理意义.     

Two dimensional GeO2/MoSi2N4 van der Waals heterostructures with robust type-II band alignment

Constructing two-dimensional (2D) van der Waals heterostructures (vdWHs) can expand the electronic and optoelectronic applications of 2D semiconductors. However, the work on the 2D vdWHs with robust band alignment is still scarce. Here, we employ a global structure search approach to construct the vdWHs with monolayer MoSi₂N₄ and wide-bandgap GeO₂. The studies show that the GeO₂/MoSi₂N₄ vdWHs have the characteristics of direct structures with the band gap of 0.946 eV and type-II band alignment with GeO₂ and MoSi₂N₄ layers as the conduction band minimum (CBM) and valence band maximum (VBM), respectively. Also, the direct-to-indirect band gap transition can be achieved by applying biaxial strain. In particular, the 2D GeO₂/MoSi₂N₄ vdWHs show a robust type-II band alignment under the effects of biaxial strain, interlayer distance and external electric field. The results provide a route to realize the robust type-II band alignment vdWHs, which is helpful for the implementation of optoelectronic nanodevices with stable characteristics.     

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.     

Z型InN/SnS2范德华异质结增强光催化水分解

寻找高效的光催化剂分解水制氢是解决能源危机和环境问题的有效途径之一.基于第一性原理,对InN/SnS₂异质结的几何结构、电子结构和光催化水分解性能进行研究.结果表明InN/SnS₂异质结是具有的Ⅱ型能带排列半导体材料可以有效地分离电子空穴对.在光激发下,较小的带隙以及合适的内建电场使得光生载流子迁移路径成“Z”字型,这保留了InN/SnS₂异质结强氧化还原能力.光生电子在InN的导带底累积并发生析氢反应,而积累在SnS₂上的光生空穴使析氧反应自发发生.它们的带边位置都跨越了水的氧化还原电位,证明能够实现水的完全分解.因此,InN/SnS₂异质结有希望成为高效光解水催化剂.     

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.     

电场对PtO2/MoS2范德瓦尔斯异质结电子结构调控

本文利用基于密度泛函理论的第一性原理计算方法研究了外电场对PtO2/MoS2范德瓦尔斯异质结电子结构的调控,发现当层间距d=2.83Å时异质结结构最稳定,且表现为Ⅱ型间接带隙半导体,其带隙为0.68 eV。通过施加垂直平面方向电场可有效调控PtO2/MoS2异质结电子结构,当外电场为-1 V/Å时,发生半导体-金属相变。这些研究结果表明PtO2/MoS2异质结在新型二维材料光电纳米器件方面具有广泛应用前景。     

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.     

Interfacial defect engineering and photocatalysis properties of hBN/MX2 (M = Mo,W, and X = S,Se heterostructures

Van der Waals (VDW) heterostructures have attracted significant research interest due to their tunable interfacial properties and potential applications in many areas such as electronics, optoelectronic, and heterocatalysis. In this work, the influences of interfacial defects on the electronic structures and photocatalytic properties of hBN/MX₂ (M = Mo, W, and X=S, Se) are studied using density functional theory calculations. The results reveal that the band alignment of hBN/MX₂ can be adjusted by introducing vacancies and atomic doping. The type-I band alignment of the host structure is maintained in the heterostructure with n-type doping in the hBN sublayer. Interestingly, the band alignment changed into the type-II heterostructrue due to V_B defect and p-type doping is introduced into the hBN sublayer. This can conduce to the separation of photo-generated electron-hole pairs at the interfaces, which is highly desired for heterostructure photocatalysis. In addition, two Z-type heterostructures including hBN(Be_B)/MoS₂, hBN(Be_B)/MoSe₂, and hBN(V_N)/MoSe₂ are achieved, showing the decreasing of band gap and ideal redox potential for water splitting. Our results reveal the possibility of engineering the interfacial and photocatalysis properties of hBN/MX₂ heterostructures via interfacial defects.     

Electronic and optical properties of van der Waals vertical heterostructures based on two-dimensional transition metal dichalcogenides: First-principles calculations

Four vertical heterostructures based on two-dimensional transition-metal dichalcogenides (TMDs) – MoS₂/GeC, MoSe₂/GeC, WS₂/GeC, and WSe₂/GeC, were studied by density functional theory calculations to investigate their structure, electronic characteristics, principle of photogenerated electron–hole separation, and optical-absorption capability. The optimized heterostructures were formed by van der Waals (vdW) forces and without covalent bonding. Their most stable geometric configurations and band structures display type-II band alignment, which allows them to spontaneously separate photogenerated electrons and holes. The charge difference and built-in electric field across the interface of these vdW heterostructures also contribute to preventing the photogenerated electron–hole recombination. Finally, the high optical absorption of the four TMD-based vdW heterostructures in the visible and near-infrared regions indicates their suitability for photocatalytic, photovoltaic, and optical devices.     

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.     

Raman scattering in SnSe

Zone center optical phonons in SnSe were investigated using Raman scattering data obtained here and previously reported far infrared reflectivity measurements. The values of the ratio of the pairs of interlayer force constants were calculated te be between 3·3 and 4·5. Thus it can be concluded that SnS is a layer like semiconductor but less layer like than GeS, GeSe or SnS.     

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.     

Coexistence of type-I and type-II band alignment in Ga(Sb,P)/GaP heterostructures with pseudomorphic self-assembled quantum dots

Band alignment of heterostructures with pseudomorphic GaSb_{1 ? x} P _x /GaP self-assembled quantum dots (SAQDs) lying on a wetting layer was studied. Coexistence of type-I and type-II band alignment was found within the same heterostructure. Wetting layer has band alignment of type-I with the lowest electronic state belonging to the X _XY valley of GaSb_{1 ? x} P _x conduction band, in contrast to SAQDs, which have band alignment of type-II, independently of the ternary alloy composition x. It is shown that type-I-type-II transition is a result of GaP matrix deformation around the SAQD.     

Tunable electronic and optical properties of SnC/BAs heterostructure by external electric field and vertical strain

Based on the first-principles method, we investigate the electronic structure of SnC/BAs van der Waals (vdW) heterostructure and find that it has an intrinsic type-II band alignment with a direct band gap of 0.22 eV, which favors the separation of photogenerated electron–hole pairs. The band gap can be effectively modulated by applying vertical strain and external electric field, displaying a large alteration of band gap via the strain and experiencing an indirect-to-direct band gap transition. Moreover, the band gap of the heterostructure varies almost linearly with external electric field, and the semiconductor-to-metal transition can be realized in the presence of a strong electric field. The calculated band alignment and the optical absorption reveal that the SnC/BAs heterostructure could present an excellent light-harvesting performance. Our designed heterostructure is expected to have great potential applications in nanoelectronic devices and photovoltaics and optical properties.     

Tunable electronic properties of GaS-SnS2 heterostructure by strain and electric field

Vertically stacked heterostructures have received extensive attention because of their tunable electronic structures and outstanding optical properties. In this work, we study the structural, electronic, and optical properties of vertically stacked GaS-SnS₂ heterostructure under the frame of density functional theory. We find that the stacked GaS-SnS₂ heterostructure is a semiconductor with a suitable indirect band gap of 1.82 eV, exhibiting a type-II band alignment for easily separating the photo-generated carriers. The electronic properties of GaS-SnS₂ heterostructure can be effectively tuned by an external strain and electric field. The optical absorption of GaS-SnS₂ heterostructure is more enhanced than those of the GaS monolayer and SnS₂ monolayer in the visible light region. Our results suggest that the GaS-SnS₂ heterostructure is a promising candidate for the photocatalyst and photoelectronic devices in the visible light region.     

Interfacial properties of black phosphorus/transition metal carbide van der Waals heterostructures

Owing to its outstanding electronic properties, black phosphorus (BP) is considered as a promising material for next-generation optoelectronic devices. In this work, devices based on BP/MXene (Zr_n+1C _n T₂, T = O, F, OH, n = 1, 2) van der Waals (vdW) heterostructures are designed via first-principles calculations. Zr_n+1C _n T₂ compositions with appropriate work functions lead to the formation of Ohmic contact with BP in the vertical direction. Low Schottky barriers are found along the lateral direction in BP/Zr₂CF₂, BP/Zr₂CO₂H₂, BP/Zr₃C₂F₂, and BP/Zr₃C₂O₂H₂ bilayers, and BP/Zr₃C₂O₂ even exhibits Ohmic contact behavior. BP/Zr₂CO₂ is a semiconducting heterostructure with type-II band alignment, which facilitates the separation of electron-hole pairs. The band structure of BP/Zr₂CO₂ can be effectively tuned via a perpendicular electric field, and BP is predicted to undergo a transition from donor to acceptor at a 0.4 V/Å electric field. The versatile electronic properties of the BP/MXene heterostructures examined in this work highlight their promising potential for applications in electronics.     

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.     

Optical anisotropy of surface-emitting ZnSe/BeTe LEDs

The electric field-induced in-plane optical anisotropy (Pockels effect) of ZnSe/BeTe multi-layered heterostructures has been studied by analysing the linear polarization of the spatially indirect photoluminescence. A pronounced quantum-confined Pockels effect was found in ZnSe/BeTe double-barrier structures. A tight-binding model is proposed which consistently interprets the experimental findings and suggests a new mechanism for the effect as realized in type-II heterostructures. The model takes into account that the ZnSe/BeTe heterosystem exhibits a type-II band alignment with large band offsets and that in the zinc-blende lattice the chemical bonds are oriented in 〈111〉 directions and, when shifted along the [001] principal axis, alternatively change their orientation in the (001) plane from [1 1 0] to [110] and vice versa. A light-emitting diode based on a single ZnSe/BeTe interface is demonstrated.     

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.