关键词： Nb3Cl8单层   磁性材料   磁各向异性   自旋电子输运  

摘要： 具有本征磁性的二维半导体材料在实现低功耗、小尺寸和高效率的自旋电子学器件方面具有重要应用前景.一些拥有特殊晶格结构的二维材料,如笼目晶格结构,凭借其在磁性和电学方面的新颖性质而受到研究者的青睐.最近,一种新的具有笼目晶格结构的二维磁性半导体材料Nb3Cl8单层被成功制备出来,为探索具有笼目结构的二维磁性半导体器件提供了新机会.本文通过第一性原理方法研究了 Nb3Cl8单层的电子结构、磁各向异性,构造了其p-n结二极管结构,并研究了其自旋输运性质.结果表明,Nb3Cl8单层易磁化轴在平面内,沿x轴方向,Nb原子对磁各向异性起主要贡献,且相关磁性可通过应力应变进行调控.此外,基于Nb3Cl8单层的p-n结二极管纳米器件表现出整流效应、自旋过滤效应以及负微分电阻现象.这些结果表明了 Nb3Cl8单层在下一代高性能自旋电子器件方面具有较大的应用潜力.

关键词： Janus   2H-VSeS   monolayers   valleytronic   semiconductors  

摘要： Valleytronic as a hot topic in recent years focuses on electrons' valley degree of freedom as a quantum information carrier. Here, by combining two-band k.p model with high-throughput density functional theory (DFT) calculations, the valley states of Janus 2H-VSSe monolayer are studied which have spontaneous polarization. Nonvolatile valley polarization state is mainly arises from intrinsic ferromagnetism contributed by V-3d electronic configuration and not the spontaneous out-of-plane dipole moment of VSSe monolayer. The effective Hamiltonian model and DFT calculations both showed that the valley splitting mainly originates from the smaller spin splitting coming from the spin-orbit coupling effect rather than the spin splitting of magnetic exchange field. By using the effective Dirac Hamiltonian and Kubo formula, we further calculated the longitudinal and transversal conductivities and absorption spectra of VSSe monolayer which exhibits an anomalous valley Hall effect and clear valley-selective circular dichroism. Our calculations indicate that the modification of valley and spin splitting related to Berry curvature by applying an external strain is more noticeable than by the change of the magnetic moment orientation and electric field. We found that carriers accumulation with particular spin and valley label can be manipulated by tuning effective Hamiltonian parameters. The coexistence of robust in-plane magnetic ordering and spontaneous valley polarization of 2H-VSSe monolayer supports the possibility of applications in spintronics, valleytronics and optoelectronics devices.

关键词： CVD   MoS2   单层   光电特性  

摘要： 二维半导体材料是一类新兴的新材料,具有广泛的电学性能和潜在的应用价值。本文以S和MoO3粉末为原料,利用化学气相沉积法(Chemical Vapor Deposition, CVD),在SiO2/Si片上制备了高质量、大面积的单层MoS2单晶样品。利用XRD、OM、EDS、TEM、AFM及半导体光电特性分析设备,对不同制备样品的表面形貌、晶体结构、组成成份、样品厚度及光电特性进行了测试分析。结果发现,制备的三角形的单层及少层MoS2单晶主要呈现六角晶系结构,且边界清晰,表面光滑,有较好的透光性,最大尺度可达250 μm,MoS2单晶生长中的硫化反应过程符合VLS生长机制。在不同生长条件中,反应物中的S/MoO3的质量比和反应温度是影响单层MoS2单晶样品生长类型的主要影响因素。分析发现,制备单层MoS2单晶的最佳S/MoO3质量比为1000/30,而最佳沉积温度为900℃。测试发现,由单层MoS2单晶样品制备的光电器件具有很高的光电灵敏度和较好的光电响应特性。

关键词： first-principles   puckered pentagonal   VTe_(2)   ferromagnetic semiconductor   ferroelasticity  

摘要： Two-dimensional(2D)magnetic crystals have been extensively explored thanks to their potential applications in spintronics,valleytronics,and topological *** we report a novel monolayer magnet,namely puckered pentagonal VTe_(2)(PP-VTe_(2)),intriguing atomic and electronic structures of which were firmly validated from first-principles *** PP-VTe_(2) exhibits strong intrinsic ferromagnetism and semiconducting property distinct from the half-metallic bulk pyrite VTe_(2)(BP-VTe_(2))*** unusual magnetic anisotropy with large magnetic exchange energies is *** interestingly,the multiferroic coupling between its 2D ferroelasticity and in-plane magnetization is further identified in PP-VTe_(2),lending it unprecedented controllability with external strains and electric *** as an emergent 2D ferromagnetic semiconductor with a novel crystal structure,monolayer PP-VTe_(2) provides an ideal platform for exploring exotic crystalline and spin configurations in low-dimensional systems.

关键词： 二维过渡金属硫属化物   化学气相沉积   外延生长   异质结   晶圆级  

摘要： 二维过渡金属硫属化物以其优异的性能近年来得到了科学和产业界的广泛关注和研究兴趣。其中,单层二硫化钼和二硒化钼是最具代表性的半导体材料,在电子器件和光电器件方面具有巨大的应用潜力。过渡金属硫属化物不仅可以形成原子级薄的单层,并且可以通过不同单层材料的组合,形成异质结构,包括横向异质结和垂直异质结,具有新奇的可调控物理性能,为实现功能化的器件应用拓宽了道路。为了实现这些新型二维半导体的实际应用,大面积高质量的过渡金属硫属化物单层薄膜及其异质结的可控制备是一个重要的研究课题。本文采用化学气相沉积的方法进行晶圆级二维材料生长并进行了器件的制备和表征,主要取得的研究结果如下:1)实现了单层二硒化钼连续薄膜的4英寸晶圆外延生长。以蓝宝石衬底作为外延生长的衬底,利用三温区化学气相沉积系统,采用多源控制的生长方法,得到了4英寸晶圆级单层二硒化钼的连续薄膜。晶圆尺寸薄膜的结构和光谱学表征表明薄膜在大面积范围具有很好的均匀性。基于生长的单层二硒化钼制备的场效应晶体管器件的电流开关比可达10～7。此外,通过化学气相沉积法也成功制备了二硒化钼的横向异质结。2)实现了过渡金属硫属化物双层垂直异质结的2英寸晶圆外延生长。采用两步化学气相沉积的生长方法在蓝宝石衬底上得到了2英寸晶圆尺寸的双层垂直异质结的连续薄膜。异质结由两种材料的单层薄膜组成,具有干净的界面以及很好的大面积均匀性。由异质结制备的器件具有良好的光电性能,光响应时间可达几十微秒。3)构筑了基于单层二硫化钼薄膜的可拉伸场效应晶体管器件。采用化学气相沉积方法生长得到的单层二硫化钼薄膜作为场效应晶体管器件的沟道材料,通过预拉伸的方法制备了弹性衬底上的可拉伸器件。器件具有褶皱的结构使其能够承受较大应力,并且可以在拉伸前后重复稳定地工作。对可拉伸的二硫化钼场效应晶体管器件进行了在光电突触方面的应用探索,展示了良好的图像识别准确率。

关键词： two-dimensional materials   density-functional theory   carrier mobility   direct band-gap  

摘要： Atomically two-dimensional materials with direct band-gap and high carrier mobility are highly desirable due to their promising applications in electronic devices. Here, on the basis of ab initio calculations and global particle-swarm optimization method, we predict the B2S3 monolayer as a new semiconductor with favorable functional properties. The B2S3 monolayer possesses a high electron mobility of 553 cm(2) V-1 s(-1) and a direct band-gap of 1.85 eV. The direct band-gap can be manipulated under biaxial strain. Furthermore, B2S3 monolayer can absorb sunlight efficiently in the entire range of the visible light spectrum. Besides, this monolayer holds good dynamical, thermal, and mechanical stabilities. All the desired properties render B2S3 monolayer a promising candidate for future applications in high-speed (opto)electronic devices.

关键词： Calculations  

摘要： Motivated by the recent successful synthesis of highly crystalline ultrathin BiTeCl and BiTeBr layered sheets [Debarati Hajra et al., ACS Nano, 2020, 14, 15626], herein for the first time, we carry out a comprehensive study on the structural and electronic properties of BiTeCl and BiTeBr Janus monolayers using density functional theory (DFT) calculations. Different structural and electronic parameters including the lattice constant, bond lengths, layer thickness in the z-direction, different interatomic angles, work function, charge density difference, cohesive energy and Rashba coefficients are determined to acquire a deep understanding of these monolayers. The calculations show good stability of the studied single layers. BiTeCl and BiTeBr monolayers are semiconductors with electronic bandgaps of 0.83 and 0.80 eV, respectively. The results also show that the semiconductor-metal transformation can be induced by increasing the number of layers. In addition, the engineering of the electronic structure is also studied by applying an electric field, and mechanical uniaxial and biaxial strain. The results show a significant change of the bandgaps and that an indirect-direct band-gap transition can be induced. This study highlights the positive prospect for the application of BiTeCl and BiTeBr layered sheets in novel electronic and energy conversion systems.

关键词： 2D material   perovskite oxide   semiconductor   electronic structure   phonon spectrum  

摘要： It is highly desirable to search for promising two-dimensional (2D) monolayer materials for obtaining deep insight of 2D materials and developing device applications. We use first-principles method to investigate tetragonal perovskite oxide monolayers as 2D materials, and find three stable freestanding 2D monolayer materials from important perovskite oxides (ABO(3)), namely SrTiO3, LaAlO3, and KTaO3, denoting them as STO-ML, LAO-ML, and KTO-ML. Such an oxide monolayer consists of one AO and one BO2 atomic layers. Further study shows that the three monolayers are 2D wide-gap semiconducotors, and there is a large electrostatic potential energy difference between the two sides, reflecting a large out-of-plane dipole, in each of the monolayers. We also investigate optical properties of the three monolayer semiconductors and compare them with graphene and MoS2 monolayer. These make a series of 2D monolayer materials, and should be useful in novel electronic and optoelectronic devices considering emerging phenomena in perovskite oxide heterostructures.

关键词： MoTeI monolayer   Ferrimagnetic semiconductor   Tunable magnetic phase  

摘要： We find through density functional theory that the two-dimensional MoTeI monolayer is intrinsically a ferromagnetic semiconductor. The magnetic states of MoTeI monolayer are sensitive and can be significantly changed by biaxial strain. It is fully spin polarized and half-metallic intrinsically or with tensile strain, while becomes antiferromagnetic when over 0.4% compressive strain is applied. After the magnetic phase transition, the semiconducting characteristics are always maintained and little charge transfer is observed. We believe that the magnetic phase transition is attributed to the competition of the direct interaction and indirect superexchange interaction between magnetic centers of Mo atoms. These findings suggest that the MoTeI monolayer can be a potential candidate for spintronics applications.