GaAs(111)表面硅烯、锗烯的几何及电子性质研究

基于密度泛函理论的第一性原理计算方法, 系统研究了硅烯、锗烯在GaAs(111) 表面的几何及电子结构. 研究发现, 硅烯、锗烯均可在As-中断和Ga-中断的GaAs(111) 表面稳定存在, 并呈现蜂窝状六角几何构型. 形成能计算结果证明了其实验制备的可行性. 同时发现硅烯、锗烯与GaAs表面存在共价键作用, 这破坏了其Dirac电子性质. 进一步探索了利用氢插层恢复硅烯、锗烯Dirac电子性质的方法. 发现该方法可使As-中断面上硅烯、锗烯的Dirac电子性质得到很好恢复, 而在Ga-中断面上的效果不够理想. 此外, 基于原子轨道成键和杂化理论揭示了GaAs表面硅烯、锗烯能带变化的物理机理. 研究结果为硅烯、锗烯在半导体基底上的制备及应用奠定了理论基础.     

类石墨烯锗烯研究进展

近年来,伴随石墨烯研究的深入开展,考虑到兼容半导体工业,构筑类石墨烯锗烯并探究其奇特电学性质已成为凝聚态物理领域的研究前沿.本文首先简要介绍了锗烯这一全新二维体系的理论研究进展,包括锗烯的几何结构、电子结构及其调控以及它们之间的关系.理论研究表明,因最近邻原子间距大,锗烯比硅烯更难构筑,实验上构筑锗烯颇具挑战性.针对这一问题,介绍了实验上制备锗烯的一些进展,重点介绍了金属表面外延制备锗烯,并对本征锗烯的制备及其在未来纳电子学器件的潜在应用做出了展望.     

First-principles investigation of mechanical properties of silicene,germanene and stanene

Two-dimensional allotropes of group-IV substrates including silicene, germanene and stanene have recently attracted considerable attention in nanodevice fabrication industry. These materials involving the buckled structure have been experimentally fabricated lately. In this study, first-principles density functional theory calculations were utilized to investigate the mechanical properties of single-layer and free-standing silicene, germanene and stanene. Uniaxial tensile and compressive simulations were carried out to probe and compare stress-strain properties; such as the Young’s modulus, Poisson’s ratio and ultimate strength. We evaluated the chirality effect on the mechanical response and bond structure of the 2D substrates. Our first-principles simulations suggest that in all studied samples application of uniaxial loading can alter the electronic nature of the buckled structures into the metallic character. Our investigation provides a general but also useful viewpoint with respect to the mechanical properties of silicene, germanene and stanene.     

Topological phases of silicene and germanene in an external magnetic field: Quantitative results

We investigate the topological phases of silicene and germanene that arise due to the strong spin–orbit interaction in an external perpendicular magnetic field. Below and above a critical field of 10 T, respectively, we demonstrate for silicene under 3% tensile strain quantum spin Hall and quantum anomalous Hall phases. Not far above the critical field, and therefore in the experimentally accessible regime, we obtain an energy gap in the meV range, which shows that the quantum anomalous Hall phase can be realized experimentally in silicene, in contrast to graphene (tiny energy gap) and germanene (enormous field required). (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

掺杂硅烯吸附CO分子的第一性原理研究

硅烯具有独特的电子、光学、热学、力学以及量子特性,在电子器件、电极材料、储氢材料、催化剂和气体传感器等领域有巨大的潜在应用价值.本文采用基于密度泛函理论的第一性原理计算方法,利用Materials Studio软件中的CASTEP程序包对硅烯与CO分子之间的吸附行为进行了研究.重点研究了硅烯掺杂方式、CO分子吸附构型及硅烯空位缺陷浓度对CO分子吸附的影响,研究结果表明：1)空位缺陷硅烯对CO分子的吸附能力最强;2)碳原子垂直朝向空位缺陷硅烯更有利于CO分子的吸附;3)硅烯对CO分子的吸附能力随其空位浓度的增加显著增强;4)空位硅烯向CO分子转移电荷,电荷转移量与二者的吸附作用强弱呈正相关.该研究可为硅烯基CO气体传感器的设计提供理论指导.     

Mechanical and electronic properties of stoichiometric silicene and germanene oxides from first‐principles

Using first‐principles calculations, we investigate the fully oxidized silicene and germanene with stoichiometric ratio Si:O/Ge:O = 1:1. For both compounds, the zigzag ether‐like conformation (z‐sSiO/z‐sGeO) is found to be the most energetically favorable structure. These z‐sSiO and z‐sGeO nanosheets have prominent elastic characteristics, which even exhibit an unconventional auxetic behavior with negative Poisson ratios. After oxidation, the semi‐metallic nanosheets are transformed into semiconductors with narrow direct band gaps. Due to the anisotropic mechanical and electronic properties, the z‐sSiO and z‐sGeO possess an axially high intrinsic charge mobility up to the order of 10⁴ cm²/Vs, which is comparable to that of graphene nanoribbons. Our studies demonstrate that the silicene and germanene oxides have peculiar mechanical and electronic properties, which endow these nanostructures for potential applications in nanoelectronics and devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

原子模拟钛中微孔洞的结构及其失效行为

六角金属由于其各向异性等特点,在塑性变形等过程中容易产生形状和构型都相对复杂的点缺陷团簇.这些团簇之间及其与运动位错等缺陷的相互作用直接影响材料的物理和力学性能.然而对相关问题的原子尺度、尤其是空位团簇的演化和微孔洞的形成乃至裂纹形核扩展等的理解还不全面.本文采用激发弛豫算法结合第一原理及原子间作用势,系统考察了钛中的空位团簇构型及不同构型间的相互转变,给出了不同尺寸空位团簇的稳定和亚稳构型、空位团簇合并分解和迁移的激发能垒等关键参数,发现较小的空位团簇形成稳定构型,较大的空位团簇呈现出空间对称分布趋势进而形成微孔洞;采用高通量分子动力学模拟系统研究了不同尺寸的空位团簇在拉应力作用下对变形过程的影响,发现这些空位团簇可以形成层错,并对微裂纹的形核产生影响.     

第一性原理计算研究立方氮化硼空位的电学和光学特性

对立方氮化硼的空位进行了基于密度泛函理论框架下的第一性原理平面波超软赝势方法的研究. 通过对总能量、能带结构、态密度及电子密度分布图的分析发现, B空位相比起N空位更加稳定. 并且空位仅影响最近邻原子的电子分布, 空位浓度的增加使禁带宽度逐渐变窄. 从复介电函数和光学吸收谱分析中发现, 随着空位浓度的增加, 立方氮化硼在深紫外区的吸收逐渐减弱. 并且B空位还导致在可见光区域出现明显的吸收带. 关键词： 立方氮化硼 空位 第一性原理 电光学特性     

Current–voltage characteristics of armchair Sn nanoribbons

Two‐dimensional group‐IV lattices silicene and germanene are known to share many of graphene's remarkable mechanical and electronic properties. Due to the out‐of‐plane buckling of the former materials, there are more means of electronic funtionalization, e.g. by applying uniaxial strain or an out‐of‐plane electric field. We consider monolayer hexagonal Sn (stanene) as an ideal candidate to feasibly implement and exploit graphene physics for nanoelectronic applications: with increased out‐of‐plane buckling and sizable spin–orbit coupling it lends itself to improved Dirac cone engineering. We investigate the ballistic charge transport regime of armchair Sn nanoribbons, classified according to the ribbon width W = {3m – 1, 3m, 3m + 1} with integer m. We study transport through (non‐magnetic) armchair ribbons using a combination of density functional theory and non‐equilibrium Green's functions. Sn ribbons have earlier current onsets and carry currents 20% larger than C/Si/Ge‐nanoribbons as the contact resistance of these ribbons is found to be comparable. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of impurities on the surface morphology of Fe(111)

The surface composition and morphology of Fe(111) have been examined through a combined analysis that includes low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The preferential segregation of sulfur has been clearly identified by AES upon annealing. The STM images exhibit numerous triangular pits of various sizes, and the LEED patterns show diffused n × 1 spots. The triangular pits reveal a Sierpinski gasket fractal. For sulfur-free Fe(111), nitrogen segregates to the surface upon annealing, forming a 4√3 × 4√3 superstructure that is identified by LEED patterns and STM images. The STM images show nanoscale triangular clusters regularly aligned in a hexagonal 4√3 × 4√3 configuration. Ultra-thin chromium film deposited on a nitrogen-segregated Fe(111) surface with post-annealing induces further nitrogen segregation, resulting in the formation of triangular pyramid-shaped CrN nanoclusters.     

Electronic and magnetic properties of silicene nanoflakes by first-principles calculations

The geometric, electronic, and magnetic properties of silicene nanoflakes (SiNFs) and corresponding two-dimensional (2D) framework assembled by SiNFs are studied by first-principles calculations. We find that the hexagonal SiNFs exhibit semiconducting behavior, while the triangular SiNFs is magnetic. Although the triangular SiNFs linked directly is antiferromagnetic, the system linked with an odd-number Si chains can exhibit ferromagnetic (FM) behavior, which is ascribed to anti-parallel spin rule on Si atoms, consistent with the Lieb–Mattis criterion. More interestingly, the 2D framework composed of triangular SiNFs linked by a Si atom shows a half-metallic character with an integer magnetic moment. These results provide a better understanding for silicene-based nanoflakes, and expect to pave an avenue to assemble FM silicon materials in spintronics.     

Scanning tunneling microscopy chemical signature of point defects on the MoS2(0001) surface

Using ab initio calculations, we have studied the modification of the electronic structure of the MoS2(0001) surface by several point defects: a surface S vacancy and different transition metal atoms substituting a S atom (Pd, Au, Fe, and V). With a S vacancy, a gap state appears with weight mostly on the Mo and S atoms surrounding the vacancy. The substitutional atoms of complete d band (Pd and Au) do not present magnetic polarization and slightly modify the DOS near the Fermi energy. On the other hand, the incomplete d band atoms (Fe and V) present spin polarization and modify significantly the states near the band edges. From calculated STM images and STS curves, we show that this chemical signature can be measured and used to characterize the surface defects of the substrate which are suitable nucleation centers for nanocluster growth.     

Moiré superstructures of silicene on hexagonal boron nitride: A first-principles study

Using first-principles calculations, we predicted hexagonal boron nitride (h-BN) with flat surface is an ideal substrate for silicene. Van der Waals interactions hold silicene and h-BN together, forming silicene/BN moiré superstructures. The moiré superstructures open a band gap of about 30 meV at the Dirac point of silicene at equilibrium distance. The band gap is almost independent of the rotation angle between the two lattices, but can be effectively tuned by changing the interlayer spacing. The high Fermi velocity of silicene is well preserved in these superstructures. These features are helpful in achieving applications of silicene in nanoscale electronic devices.     

Observation of coexistence of 1D and 2D nanostructures in cobalt dipyrromethene trimer complexes adsorbed on a graphite surface

We report the direct observation of 1D and 2D nanostructures of cobalt dipyrromethene trimer complexes adsorbed on a highly oriented pyrolytic graphite surface using scanning tunneling microscopy (STM). STM images showed two types of ordered structures coexisting on the surface: long 1D molecular chains isolated on the terraces, and 2D hexagonal patterns confined by a 1D chain and/or a graphite step edge. These 1D and 2D structures are attributed to ‘edge-on’ and ‘face-on’ complex alignments on the surface, respectively. In both configurations, substrate-mediated molecule-molecule interactions may play a significant role in stabilizing the nanostructures.     

Current developments in silicene and germanene

Exploration of the unusual properties of the two‐dimensional materials silicene and germanene is a very active research field in recent years. This paper therefore reviews the latest developments, focusing both on the fundamental materials properties and on possible applications. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

First-principle study of magnetism induced by vacancies in graphene

Spin-polarized density functional theory has been used to study the effects of vacancy defects on the magnetic properties of graphene. Structural optimization shows that introducing a carbon vacancy cluster into a graphene sheet changes the spatial distribution of the neighbor atoms, particularly those located around the vacancy. From spin-polarized DOS and LPDOS calculations, we find that only vacancies containing unpaired electrons show magnetism. These results lead us to formulate a relation between the vacancy-induced magnetic moment and the size and shape of the vacancy clusters in graphene sheet.     

表面吸附对单层黑磷砷结构、电子性质和磁性调控的第一性原理研究

基于第一性原理，系统研究了11种不同原子吸附在单层AsP上的几何结构、吸附能、磁矩和电子结构性质. 使用的吸附原子包括轻质非金属(C、N、O)原子，第三周期金属原子(Na、Mg、Al)和过渡金属原子(Ti、V、Cr、Mn和Fe). 研究结果表明，吸附原子引起了AsP多样的结构、磁性和电子性质改变. AsP与所研究的吸附原子都能紧密结合，并且所有系统的吸附能都比吸附原子在石墨烯、SiC、BN以及MoS₂上的吸附能强得多. AsP的半导体特性受到吸附原子的影响，其可以诱导产生中间能隙态或引起n型掺杂. 此外，表面吸附产生了不同的自旋电子特性，具体而言，吸附N、Ti和Fe的AsP成为双极半导体；Mn修饰的AsP成为双极自旋无间隙半导体.     

Growth of silicene layers on Ag(111): unexpected effect of the substrate temperature

The deposition of one silicon monolayer on the silver (111) substrate in the temperature range 150-300 °C gives rise to a mix of (4 × 4), (2√3 × 2√3)R30° and (√13 × √13)R13.9° superstructures which strongly depend on the substrate temperature. We deduced from a detailed analysis of the LEED patterns and the STM images that all these superstructures are given by a quasi-identical silicon single layer with a honeycomb structure (i.e. a silicene-like layer) with different rotations relative to the silver substrate. The morphologies of the STM images are explained from the position of the silicon atoms relative to the silver atoms. A complete analysis of all possible rotations of the silicene layer predicts also a (√7 × √7)R19.1° superstructure which has not been observed so far.     

卤化硅烯结构、电子和光学性质的第一性原理研究

二维硅烯的商业用途通常受到其零带隙的抑制,限制了其在纳米电子和光电器件中的应用.利用基于密度泛函理论的第一性原理计算,单层硅烯的带隙通过卤原子的化学官能化被成功打开了,并综合分析了卤化对单层硅烯的结构,电子和光学性质的影响.研究结果表明卤化使结构变得扭曲,但保持了良好的稳定性.通过HSE06泛函,全功能化赋予硅烯1.390至2.123 eV的直接带隙.键合机理分析表明,卤原子与主体硅原子之间的键合主要是离子键.最后,光学性质计算表明,I-Si-I单层在光子频率为10.9 eV时达到最大光吸收,吸收值为122000 cm^-1,使其成为设计新型纳米电子和光电器件的有希望的候选材料.