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.     

Band-Gap Sensitived Seebeck Effect in Heavy Group-IV Monolayers

Physics of the Solid State - We have systematically investigated the spin- and valley-dependent Seebeck effect in irradiated heavy group-IV monolayers including silicene, germanene, and stanene by...     

Semimetal behavior of bilayer stanene

Stanene is a two-dimensional (2D) buckled honeycomb structure which has been studied recently owing to its promising electronic properties for potential electronic and spintronic applications in nanodevices. In this article we present a first-principles study of electronic properties of fluorinated bilayer stanene. The effect of tensile strain, intrinsic spin-orbit and van der Waals interactions are considered within the framework of density functional theory. The electronic band structure shows a very small overlap between valence and conduction bands at the Γ point which is a characteristic of semimetal in fluorinated bilayer stanene. A relatively high value of tensile strain is needed to open an energy band gap in the electronic band structure and the parity analysis reveals that the strained nanostructure is a trivial insulator. According to our results, despite the monolayer fluorinated stanene, the bilayer one is not an appropriate candidate for topological insulator.     

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)     

Electronic properties and STM images of vacancy clusters and chains in functionalized silicene and germanene

Electronic properties and STM topographical images of X (=F, H, O) functionalized silicene and germanene have been investigated by introducing various kind of vacancy clusters and chain patterns in monolayers within density functional theory (DFT) framework. The relative ease of formation of vacancy clusters and chain patterns is found to be energetically most favorable in hydrogenated silicene and germanene. F- and H-functionalized silicene and germanene are direct bandgap semiconducting with bandgap ranging between 0.1–1.9 eV, while O-functionalized monolayers are metallic in nature. By introducing various vacancy clusters and chain patterns in both silicene and germanene, the electronic and magnetic properties get modified in significant manner e.g. F- and H-functionalized silicene and germanene with hexagonal and rectangle vacancy clusters are non-magnetic semiconductors with modified bandgap values while pentagonal and triangle vacancy clusters induce metallicity and magnetic character in monolayers; hexagonal vacancy chain patterns induce direct-to-indirect gap transition while zigzag vacancy chain patterns retain direct bandgap nature of monolayers. Calculated STM topographical images show distinctly different characteristics for various type of vacancy clusters and chain patterns which may be used as electronic fingerprints to identify various vacancy patterns in silicene and germanene created during the process of functionalization.     

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.     

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)     

A first principle calculation of electronic and dielectric properties of electrically gated low-buckled mono and bilayer silicene

The structural, electronic and dielectric properties of mono and bilayer buckled silicene sheets are investigated using density functional theory. A comparison of stabilities, electronic structure and effect of external electric field are investigated for AA and AB-stacked bilayer silicene. It has been found that there are no excitations of electrons i.e. plasmons at low energies for out-of-plane polarization. While for AB-stacked bilayer silicene 1.48 eV plasmons for in-plane polarization is found, a lower value compared to 2.16 eV plasmons for monolayer silicene. Inter-band transitions and plasmons in both bilayer and monolayer silicene are found relatively at lower energies than graphene. The calculations suggest that the band gap can be opened up and varied over a wide range by applying external electric field for bilayer silicene. In infra-red region imaginary part of dielectric function for AB-stacked buckled bilayer silicene shows a broad structure peak in the range of 75–270 meV compared to a short structure peak at 70 meV for monolayer silicene and no structure peaks for AA-stacked bilayer silicene. On application of external electric field the peaks are found to be blue-shifted in infra-red region. With the help of imaginary part of dielectric function and electron energy loss function effort has been made to understand possible interband transitions in both buckled bilayer silicene and monolayer silicene.     

Silicene transistors——A review

Free standing silicene is a two-dimensional silicon monolayer with a buckled honeycomb lattice and a Dirac band structure. Ever since its first successful synthesis in the laboratory, silicene has been considered as an option for post-silicon electronics, as an alternative to graphene and other two-dimensional materials. Despite its theoretical high carrier mobility,the zero band gap characteristic makes pure silicene impossible to use directly as a field effect transistor(FET) operating at room temperature. Here, we first review the theoretical approaches to open a band gap in silicene without diminishing its excellent electronic properties and the corresponding simulations of silicene transistors based on an opened band gap.An all-metallic silicene FET without an opened band gap is also introduced. The two chief obstacles for realization of a silicene transistor are silicene's strong interaction with a metal template and its instability in air. In the final part, we briefly describe a recent experimental advance in fabrication of a proof-of-concept silicene device with Dirac ambipolar charge transport resembling a graphene FET, fabricated via a growth-transfer technique.     

Electronic structure of silicene

In this topical review, we discuss the electronic structure of free-standing silicene by comparing results obtained using different theoretical methods. Silicene is a single atomic layer of silicon similar to graphene. The interest in silicene is the same as for graphene, in being two-dimensional and possessing a Dirac cone. One advantage of silicene is due to its compatibility with current silicon electronics. Both empirical and first-principles techniques have been used to study the electronic properties of silicene. We will provide a brief overview of the parameter space for first-principles calculations.However, since the theory is standard, no extensive discussion will be included. Instead, we will emphasize what empirical methods can provide to such investigations and the current state of these theories. Finally, we will review the properties computed using both types of theories for free-standing silicene, with emphasis on areas where we have contributed.Comparisons to graphene is provided throughout.     

Fast Car Physics,by Chuck Edmondson

An introduction to the theory of modular symmetries in two-dimensional materials, and its application to ‘relativistic’ group IV materials like graphene, silicene, germanene and stanene, is given. Universal properties of the magneto-electric Hall effect are extracted by projecting experimental transport data directly onto the phase diagram. When families of data depending on the dominant scale parameter (usually temperature) are available, we can extract flow lines that chart the geometry of the phase diagram, including the location of quantum critical points and phase boundaries connecting these. The universal data are used to identify emergent modular symmetries, which are infinite discrete groups of fractional linear (Möbius) transformations. Such symmetries are extremely rigid, and therefore spawn a host of sharp predictions that are easy to falsify, but so far they have failed to fail. The unique topology of the Fermi surface in the graphene family gives a robust gapless mode with linear dispersion (relativistic Dirac cones) that shifts the spectrum of Landau levels that appear when the material is placed in a strong magnetic field. The modular analysis can be extended to this case, and where reliable data are available, there appears to be agreement. A convincing case for the ‘relativistic’ quantum Hall group is hampered by the paucity of fractional quantum Hall data, the absence of scaling data and the crossover between different scaling regimes. This is likely to change in the near future, as scaling data for graphene are just now becoming available.     

First-principles investigation of armchair stanene nanoribbons

In this study, we systematically investigated the structural, electronic and optical properties of armchair stanene nanoribbons (ASNRs) by using the first-principles calculations. First, we performed full geometry optimization calculations on various finite width ASNRs where all the edge Sn atoms are saturated by hydrogen atoms. The buckled honeycomb structure of two dimensional (2D) stanene is preserved, however the bond length between the edge Sn atoms is shortened to 2.77 Å compared to the remaining bonds with 2.82 Å length. The electronic properties of these nanoribbons strongly depend on their ribbon width. In general, band gap opens and increases with decreasing nanoribbon width indicating the quantum confinement effect. Consequently, the band gap values vary from a few meV exhibiting low-gap semiconductor (quasi-metallic) behavior to ～0.4–0.5 eV showing moderate semiconductor character. Furthermore, the band gap values are categorized into three groups according to modulo 3 of integer ribbon width N which is the number of Sn atoms along the width. In order to investigate the optical properties, we calculated the complex dielectric function and absorption spectra of ASNRs, they are similar to the one of 2D stanene. For light polarized along ASNRs, in general, largest peaks appear around 0.5 eV and 4.0 eV in the imaginary part of dielectric functions, and there are several smaller peaks between them. These major peaks redshifts, slightly to the lower energies of incident light with increasing nanoribbon width. On the other hand, for light polarized perpendicular to the ribbon, there is a small peak around 1.6 eV, then, there is a band formed from several peaks from 5 eV to ～7.5 eV, and the second one from 8 eV to ～9.5 eV. Moreover, the peak positions hardly move with varying nanoribbon width, which indicates that quantum confinement effect is not playing an essential role on the optical properties of armchair stanene nanoribbons. In addition, our calculations of the optical properties indicate the anisotropy with respect to the type of light polarization. This anisotropy is due to the quasi-2D nature of the nanoribbons.     

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)     

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)     

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.     

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.     

Superconductivity in electron-doped arsenene

Based on the first-principles density functional theory electronic structure calculation,we investigate the possible phonon-mediated superconductivity in arsenene,a two-dimensional buckled arsenic atomic sheet,under electron doping.We find that the strong superconducting pairing interaction results mainly from the pz-like electrons of arsenic atoms and the A1 phonon mode around the K point,and the superconducting transition temperature can be as high as 30.8 K in the arsenene with 0.2 doped electrons per unit cell and 12%-applied biaxial tensile strain.This transition temperature is about ten times higher than that in the bulk arsenic under high pressure.It is also the highest transition temperature that is predicted for electron-doped two-dimensional elemental superconductors,including graphene,silicene,phosphorene,and borophene.     

Strain and chirality effects on the mechanical and electronic properties of silicene and silicane under uniaxial tension

We present first principles theory calculations on the mechanical and electronic properties of silicene and silicane structure under uniaxial tensile strain along different directions. Chirality effect is more significant in the mechanical properties of silicene than those of silicane. Different failure mechanisms are identified. A small band gap (up to 0.8 eV) is developed from zero with silicene structure under uniaxial tension and vanishes before the structure reaches its in-plane ultimate strength. However, a pre-existing band gap (2.39 eV) exists with silicane structure and decreases to zero with the increasing tensile strain without chirality effects.     

硅烯双层纳米管自旋劈裂及半金属特性研究

在低维材料体系中寻找半金属,对实现纳米自旋电子器件具有重要的研究意义.基于第一性原理密度泛函理论计算方法,研究了AB堆栈的双层硅烯结构及其自旋极化的电子结构间的映射关系,发现其导带底和价带顶都具有负的变形势.基于此,我们预测硅烯双层在弯曲应力作用下,原本简并的空间自旋分布对称性打破,其自旋简并的电子态会出现自旋劈裂,因此双层硅烯纳米管会出现我们预期的半金属性.计算结果表明,AB堆栈结构的硅烯双层纳米管(55, 0)出现了半金属态,并且具有较好的磁稳定性.该结果对低维材料体系实现半金属性提供理论借鉴.     

Valley-dependent electron transport in ferromagnetic/normal/ferromagnetic silicene junction

The electron transport through ferromagnetic/normal/ferromagnetic silicene junction with an induced energy gap is investigated in this work. The energy gap can be tuned by applying electric field or exchange fields due to the buckled structure of silicene. We analyze the local electric field, exchange field, length of normal region-dependence transmission probabilities of four groups and valley conductance. These transmission probabilities and valley conductance can be turned on or off by adjusting the local electric field and exchange field. In particular, a fully valley polarized conductance with 80% transmission is found in this junction, which can be caused by the interplay of valley-dependent massive Dirac electron, the exchange potential and the on-site potential difference of sublattices. Our findings will benefit applications in silicene-based high performance nano-electronics.