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。     

Strong interlayer coupling in phosphorene/graphene van der Waals heterostructure: A first-principles investigation

Based on first-principles calculations within the framework of density functional theory, we study the electronic properties of phosphorene/graphene heterostructures. Band gaps with different sizes are observed in the heterostructure, and charges transfer from graphene to phosphorene, causing the Fermi level of the heterostructure to shift downward with respect to the Dirac point of graphene. Significantly, strong coupling between two layers is discovered in the band spectrum even though it has a van der Waals heterostructure. A tight-binding Hamiltonian model is used to reveal that the resonance of the Bloch states between the phosphorene and graphene layers in certain K points combines with the symmetry matching between band states, which explains the reason for the strong coupling in such heterostructures. This work may enhance the understanding of interlayer interaction and composition mechanisms in van der Waals heterostructures consisting of two-dimensional layered nanomaterials, and may indicate potential reference information for nanoelectronic and optoelectronic applications.     

二硫化钨/石墨烯异质结的界面相互作用及其肖特基调控的理论研究

采用第一性原理方法研究了二硫化钨/石墨烯异质结的界面结合作用以及电子性质,结果表明在二硫化钨/石墨烯异质结中,其界面相互作用是微弱的范德瓦耳斯力.能带计算结果显示异质结中二硫化钨和石墨烯各自的电子性质得到了保留,同时,由于石墨烯的结合作用,二硫化钨呈现出n型半导体.通过改变界面的层间距可以调控二硫化钼/石墨烯异质结的肖特基势垒类型,层间距增大,肖特基将从p型转变为n型接触.三维电荷密度差分图表明,负电荷聚集在二硫化钨附近,正电荷聚集在石墨烯附近,从而在界面处形成内建电场.肖特基势垒变化与界面电荷流动密切相关,平面平均电荷密度差分图显示,随着层间距逐渐增大,界面电荷转移越来越弱,且空间电荷聚集区位置向石墨烯层方向靠近,导致费米能级向上平移,证实了肖特基势垒随着层间距的增加由p型接触向n型转变.本文的研究结果将为二维范德瓦耳斯场效应管的设计与制作提供指导.     

Electronic and optical properties of 3N-doped graphdiyne/MoS2 heterostructures tuned by biaxial strain and external electric field

The construction of van der Waals (vdW) heterostructures by stacking different two-dimensional layered materials have been recognised as an effective strategy to obtain the desired properties. The 3N-doped graphdiyne (N-GY) has been successfully synthesized in the laboratory. It could be assembled into a supercapacitor and can be used for tensile energy storage. However, the flat band and wide forbidden bands could hinder its application of N-GY layer in optoelectronic and nanoelectronic devices. In order to extend the application of N-GY layer in electronic devices, MoS₂ was selected to construct an N-GY/MoS₂ heterostructure due to its good electronic and optical properties. The N-GY/MoS₂ heterostructure has an optical absorption range from the visible to ultraviolet with a absorption coefficient of 10⁵ cm^-1. The N-GY/MoS₂ heterostructure exhibits a type-Ⅱ band alignment allows the electron-hole to be located on N-GY and MoS₂ respectively, which can further reduce the electron-hole complexation to increase exciton lifetime. The power conversion efficiency of N-GY/MoS₂ heterostructure is up to 17.77%, indicating it is a promising candidate material for solar cells. In addition, the external electric field and biaxial strain could effectively tune the electronic structure. Our results provide a theoretical support for the design and application of N-GY/MoS₂ vdW heterostructures in semiconductor sensors and photovoltaic devices.     

Strain drived band aligment transition of the ferromagnetic VS_2/C_3N van der Waals heterostructure

Exploring two-dimensional(2 D) magnetic heterostructures is essential for future spintronic and optoelectronic devices.Herein,using first-principle calculations,stable ferromagnetic ordering and colorful electronic properties are established by constructing the VS_2/C_3 N van der Waals(vdW) heterostructure.Unlike the semiconductive properties with indirect band gaps in both the VS_2 and C_3 N monolayers,our results indicate that a direct band gap with type-Ⅱ band alignment and p-doping characters are realized in the spin-up channel of the VS_2/C_3 N heterostructure,and a typical type-Ⅲband alignment with a broken-gap in the spin-down channel.Furthermore,the band alignments in the two spin channels can be effectively tuned by applying tensile strain.An interchangement between the type-Ⅱ and type-Ⅲ band alignments occurs in the two spin channels,as the tensile strain increases to 4%.The attractive magnetic properties and the unique band alignments could be useful for prospective applications in the next-generation tunneling devices and spintronic devices.     

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.     

Magnetic proximity effect induced spin splitting in two-dimensional antimonene/Fe3GeTe2 van der Waals heterostructures

Recently, two-dimensional van der Waals (vdW) magnetic heterostructures have attracted intensive attention since they can show remarkable properties due to the magnetic proximity effect. In this work, the spin-polarized electronic structures of antimonene/Fe₃GeTe₂ vdW heterostructures were investigated through the first-principles calculations. Owing to the magnetic proximity effect, the spin splitting appears at the conduction-band minimum (CBM) and the valence-band maximum (VBM) of the antimonene. A low-energy effective Hamiltonian was proposed to depict the spin splitting. It was found that the spin splitting can be modulated by means of applying an external electric field, changing interlayer distance or changing stacking configuration. The spin splitting energy at the CBM monotonously increases as the external electric field changes from -5 V/nm to 5 V/nm, while the spin splitting energy at the VBM almost remains the same. Meanwhile, as the interlayer distance increases, the spin splitting energies at the CBM and VBM both decrease. The different stacking configurations can also induce different spin splitting energies at the CBM and VBM. Our work demonstrates that the spin splitting of antimonene in this heterostructure is not singly dependent on the nearest Sb—Fe distance, which indicates that magnetic proximity effect in heterostructures may be modulated by multiple factors, such as hybridization of electronic states and the local electronic environment. The results enrich the fundamental understanding of the magnetic proximity effect in two-dimensional vdW heterostructures.     

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 properties of silicene in BN/silicene van der Waals heterostructures

Silicene is a promising 2D Dirac material as a building block for van der Waals heterostructures(vd WHs). Here we investigate the electronic properties of hexagonal boron nitride/silicene(BN/Si) vd WHs using first-principles calculations.We calculate the energy band structures of BN/Si/BN heterostructures with different rotation angles and find that the electronic properties of silicene are retained and protected robustly by the BN layers. In BN/Si/BN/Si/BN heterostructure, we find that the band structure near the Fermi energy is sensitive to the stacking configurations of the silicene layers due to interlayer coupling. The coupling is reduced by increasing the number of BN layers between the silicene layers and becomes negligible in BN/Si/(BN)_3/Si/BN. In(BN)_n/Si superlattices, the band structure undergoes a conversion from Dirac lines to Dirac points by increasing the number of BN layers between the silicene layers. Calculations of silicene sandwiched by other 2D materials reveal that silicene sandwiched by low-carbon-doped boron nitride or HfO_2 is semiconducting.     

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.     

Prediction of quantum anomalous Hall effect in CrI3/ScCl2 bilayer heterostructure

Based on first-principles calculations, a two-dimensional (2D) van der Waals (vdW) bilayer heterostructure consisting of two topologically trivial ferromagnetic (FM) monolayers CrI₃ and ScCl₂ is proposed to realize the quantum anomalous Hall effect (QAHE) with a sizable topologically nontrivial band gap of 4.5 meV. Its topological nature is attributed to an interlayer band inversion between the monolayers and critically depends on the symmetry of the stacking configuration. We further demonstrate that the topologically nontrivial band gap can be increased nearly linearly by the application of a perpendicular external pressure and reaches 8.1 meV at 2.7 GPa, and the application of an external out-of-plane electric field can also modulate the band gap and convert the system back to topologically trivial via eliminating the band inversion. An effective model is developed to describe the topological phase evolution in this bilayer heterostructure. This work provides a new candidate system based on 2D vdW materials for realization of potential high-temperature QAHE with considerable controllability.     

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.     

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.     

TiS2-graphene heterostructures enabling polysulfide anchoring and fast electrocatalyst for lithium-sulfur batteries: A first-principles calculation

Lithium-sulfur batteries have attracted attention because of their high energy density. However, the "shuttle effect" caused by the dissolving of polysulfide in the electrolyte has greatly hindered the widespread commercial use of lithium-sulfur batteries. In this paper, a novel two-dimensional TiS₂/graphene heterostructure is theoretically designed as the anchoring material for lithium-sulfur batteries to suppress the shuttle effect. This heterostructure formed by the stacking of graphene and TiS₂ monolayer is the van der Waals type, which retains the intrinsic metallic electronic structure of graphene and TiS₂ monolayer. Graphene improves the electronic conductivity of the sulfur cathode, and the transferred electrons from graphene enhance the polarity of the TiS₂ monolayer. Simulations of the polysulfide adsorption show that the TiS₂/graphene heterostructure can maintain good metallic properties and the appropriate adsorption energies of 0.98-3.72 eV, which can effectively anchor polysulfides. Charge transfer analysis suggests that further enhancement of polarity is beneficial to reduce the high proportion of van der Waals (vdW) force in the adsorption energy, thereby further enhancing the anchoring ability. Low Li₂S decomposition barrier and Li-ion migration barrier imply that the heterostructure has the ability to catalyze fast electrochemical kinetic processes. Therefore, TiS₂/graphene heterostructure could be an important candidate for ideal anchoring materials of lithium-sulfur batteries.     

钙钛矿CsPbX3(X=Cl,Br,I)与五环石墨烯范德瓦耳斯异质结的界面相互作用和光电性能的第一性原理研究

异质结工程是一种提高半导体材料光电性能的有效方法.本文构建了全无机钙钛矿CsPbX₃(X=Cl,Br,I)和二维五环石墨烯penta-graphene(PG)的新型范德瓦耳斯(vdW)异质结,利用第一性原理研究了CsPbX₃-PG异质结不同界面接触的稳定性,进而计算了稳定性较好的Pb-X接触界面异质结的电子结构和光电性能.研究结果表明,CsPbX₃-PG(X=Cl,Br,I)异质结具有II型能带排列特征,能级差距由Cl向I逐渐缩小,具有良好的光生载流子分离能力和电荷输运性质.此外,研究发现CsPbX₃-PG异质结能有效拓宽材料的光吸收谱范围,并能显著提高其光吸收能力,尤其是CsPbI₃具有最优的光吸收性能.经理论估算,CsPbX₃-PG的光电功率转换效率(PCE)可高达21%.这些结果表明,全无机金属卤化物钙钛矿CsPbX₃-PG异质结可以有效地提高半导体材料的光电性能,预期在光电转换器件中具有重要的应用潜力.     

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.     

High-performance self-powered photodetector based on organic/inorganic hybrid van der Waals heterojunction of rubrene/silicon

Organic/inorganic hybrid van der Waals heterostructure with an atomically abrupt interface has attracted great research interests within the field of multifunctional electronic and optoelectronic devices. The integration of organic rubrene films with inorganic Si semiconductors can avoid the atomic mutual-diffusion at the interface, and provide the possibility of forming two-dimensional van der Waals heterojunction accompanied with the type-II energy band alignment, due to the transfer behaviors of majority carriers at the interface. In this study, the high-quality rubrene/Si van der Waals heterostructure with an electronically abrupt junction was prepared, and a self-powered photodetector was then constructed based on this hybrid heterojunction. The photodetector demonstrated an excellent switching response to the 1064 nm monochromatic light with large on/off current ratio of 7.0×10~3, the maximum photocurrent of 14.62 m A, the maximum responsivity of 2.07 A/W, the maximum detectivity of 2.9×10~(11) Jones, and a fast response time of 13.0 μs. This study offers important guidance for preparing high-quality rubrene/Si hybrid van der Waals heterostructure with desirable band alignment, and the designed heterojunction photodetector has an important application prospect in the field of multifunctional optoelectronics.     

Electronic and optical properties of an intrinsic type-I band alignment ZrS_2/SnS_2 van der Waals heterostructure for optoelectronic devices

The electronic and optical properties of the ZrS_2/SnS_2 van der Waals heterostructure have been investigated.We find out that the formed heterostructure has an intrinsic type-I band alignment. Moreover, the characteristics of optical absorption in the heterostructure can be enhanced to the amount of 106 in the ultraviolet light region. In addition, the tuning electronic properties of ZrS_2/SnS_2 heterostructure are very interesting, due to the transitions from type-I to type-II band alignment that can occur by applying an external electric field.These results suggest that the atomically thin materials ZrS_2/SnS_2 heterostructure will be utilized for flexible optoelectronic applications.     

基于磁圆二向色谱的单层MoS_2激子能量和线宽温度依赖特性

以二硫化钼(MoS_2)为代表的过渡金属硫属化物属于二维层状材料,样品可以薄至单层.单层MoS_2是一种直接带隙半导体,在纳米逻辑器件、高速光电探测、纳米激光等领域具有广阔的应用前景.在实际应用中,温度是影响半导体材料能带结构和性质的主要因素之一.因此研究单层二维材料能带的温度依赖特性对理解其物理机理以及开展器件应用具有重要的意义.目前,在广泛采用的测量单层MoS_2反射谱的研究中,激子峰往往叠加在一个很强的光谱背底上,难以准确分辨激子的峰位和线宽.基于自行搭建的显微磁圆二向色谱系统,研究了单层MoS_2在65—300 K温度范围内的反射谱和磁圆二向色谱,结果表明磁圆二向色谱在研究单层材料激子能量和线宽方面具有明显的优势.通过分析变温的磁圆二向色谱,得到了不同温度下的A,B激子的跃迁能量和线宽.通过对激子能量和线宽的温度依赖关系进行拟合,进一步讨论了声子散射对激子线宽的影响.     

Valley-polarized quantum anomalous Hall effect in van der Waals heterostructures based on monolayer jacutingaite family materials

We numerically study the general valley polarization and anomalous Hall effect in van der Waals (vdW) heterostructures based on monolayer jacutingaite family materials Pt₂AX₃ (A = Hg, Cd, Zn; X = S, Se, Te). We perform a systematic study on the atomic, electronic, and topological properties of vdW heterostructures composed of monolayer Pt₂AX₃ and two-dimensional ferromagnetic insulators. We show that four kinds of vdW heterostructures exhibit valley-polarized quantum anomalous Hall phase, i.e., Pt₂HgS₃/NiBr₂, Pt₂HgSe₃/CoBr₂, Pt₂HgSe₃/NiBr₂, and Pt₂ZnS₃/CoBr₂, with a maximum valley splitting of 134.2 meV in Pt₂HgSe₃/NiBr₂ and sizable global band gap of 58.8 meV in Pt₂HgS₃/NiBr₂. Our findings demonstrate an ideal platform to implement applications on topological valleytronics.