表面氢化双层硅烯的结构和电子性质

采用密度泛函理论(DFT)广义梯度近似GGA和HSB06方法研究了氢化双层硅烯(silicene)的结构和电子性质, 结果表明: 氢化后的双层硅烯可能存在三种稳定的构型, AA椅型、AB椅型和AA船型, 其中AA椅型和AB椅型结构最为稳定, 氢化后这三种稳定构型材料的性质由零带隙的半金属(semimetal)转变为禁带宽度分别为1.208, 1.437和1.111 eV 的间接带隙的半导体, 采用混合泛函HSB06计算修正得到的带隙分别为1.595, 1.785 和1.592 eV. 进一步分析了在双轴应变下氢化双层硅烯的带隙随应变的关系, 得到应变可以连续的调节材料的带隙宽度, 这些性质有可能应用于未来的纳米电子器件.     

Magnetic and topological properties in hydrogenated transition metal dichalcogenide monolayers

Two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers have currently been of immense interest in materials research because of their versatility, and tunable electronic and magnetic properties. In this study, we systematically studied the electronic and magnetic properties in pristine and hydrogenated 1T, 1T’, and 2H TMD monolayers. We found Group IV (Ti, Zr, and Hf), VI (Cr, Mo, and W), and X (Ni, Pd, and Pt) pristine TMD monolayers, respectively, mostly adopted 1T, 2H, and 1T as their stable structures, except for WTe₂ which exhibits 1T’. The stable 1T’ structure only exists for pristine WTe₂ and it had been identified as a topological insulator with a band gap of 0.11 eV. Upon hydrogenation, a structural phase transition occurred from 1T to 2H in Group IV, while for Group X, the stable structure remained 1T. For Group VI, the stable phase transitioned from 1T to 2H or 1T’ phases. Moreover, we found nineteen 2D magnetic materials through hydrogenation. Finally, further exploration of band topologies under hybrid functional calculations revealed that four of these identified magnetic monolayer structures exhibit quantum anomalous Hall effect. Our findings show that hydrogenated TMDs provide a new ground in searching for materials which have the potential for spintronics applications.     

Band gap engineering in silicene: A theoretical study of density functional tight-binding theory

In this work, we performed first principles calculations based on self-consistent charge density functional tight-binding to investigate different mechanisms of band gap tuning of silicene. We optimized structures of silicene sheet, functionalized silicene with H, CH₃ and F groups and nanoribbons with the edge of zigzag and armchair. Then we calculated electronic properties of silicene, functionalized silicene under uniaxial elastic strain, silicene nanoribbons and silicene under external electrical fields. It is found that the bond length and buckling value for relaxed silicene is agreeable with experimental and other theoretical values. Our results show that the band gap opens by functionalization of silicene. Also, we found that the direct band gap at K point for silicene changed to the direct band gap at the gamma point. Also, the functionalized silicene band gap decrease with increasing of the strain. For all sizes of the zigzag silicene nanoribbons, the band gap is near zero, while an oscillating decay occurs for the band gap of the armchair nanoribbons with increasing the nanoribbons width. At finally, it can be seen that the external electric field can open the band gap of silicene. We found that by increasing the electric field magnitude the band gap increases.     

First-principles modeling hydrogenation of bilayered boron nitride

We have investigated the structural and electronic characteristics of hydrogenated boron-nitride bilayer(H–BNBN–H) using first-principles calculations. The results show that hydrogenation can significantly reduce the energy gap of the BN–BN into the visible-light region. Interestingly, the electric field induced by the interface dipoles helps to promote the formation of well-separated electron–hole pairs, as demonstrated by the charge distribution of the VBM and CBM.Moreover, the applied bias voltage on the vertical direction of the bilayer could modulate the band gap, resulting in transition from semiconductor to metal. We conclude that H–BNBN–H could improve the solar energy conversion efficiency, which may provide a new way for tuning the electronic devices to meet different environments and demands.     

CO2 capture by Li‐functionalized silicene

CO₂ capture and storage technology is of key importance to reduce the greenhouse effect. By its large surface area and sp³ hybridization, Li‐functionalized silicene is demonstrated to be a promising CO₂ absorbent that is stable up to at least 500 K and has a very high storage capacity of 28.6 mol/kg (55.7 wt%). The adsorption energy of CO₂ on Li‐functionalized silicene is enhanced as compared to pristine silicene, to attain an almost ideal value that still facilitates easy release. In addition, the band gap is found to change sensitively with the CO₂ coverage. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

硅烯的化学功能化

硅烯是一种零能隙的狄拉克费米子材料,对其能带结构的有效调控进而打开带隙是硅烯进一步器件化的基础.而化学功能化是调控二维材料的结构和电子性质的一种有效方法.本文简要介绍了近几年在硅烯的化学功能化方面取得的进展,主要包括硅烯的氢化、氧化、氯化以及其他几种可能的化学修饰方法.     

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.     

Impact of Phosphorus superlattices on charge and spin dependent transport properties of zigzag silicene nanoribbons

To investigate charge and spin dependent conductance properties of Phosphorus doped zigzag silicene nanoribbons (ZSiNRs), we utilize recursive Green's function method and Landauer-Büttiker formalism. Our calculations are performed in the absence and presence of exchange magnetic fields with both parallel and antiparallel configurations. Considering a supperlattice of Phosphorus substituents in a periodic distribution at the edge of nanoribbon, the effect of increasing number of dopants and period of the distribution on transport properties are studied. It is found that transport properties of doped ZSiNRs vary with doping concentration according to being odd or even of number of dopants. For parallel configuration, doped ZSiNR with various concentrations works as a controllable spin filter with Fermi energy. Increasing doping concentration leads to increasing size of conductance gap and improvement of controlling quality of spin-filtering property while increasing period of Phosphorus atomic distribution has destructive effect on size of conductance gap and destroys spin-filtering property. Moreover, we show that although the same results are obtained for transport properties of doped ZSiNR with various concentrations of Phosphorus atoms in presence of antiparallel exchange magnetic fields, a completely controllable spin-filtering property cannot be achieved by Fermi energy changes.     

应变对两层半氢化氮化镓薄膜电磁学性质的调控机理研究

采用基于密度泛函理论的第一性原理模拟计算,研究了在应变作用下两层半氢化氮化镓纳米薄膜的电学和磁学性质.没有表面修饰的两层氮化镓纳米薄膜的原子结构为类石墨结构,并具有间接能隙.然而,当两层氮化镓纳米薄膜的一侧表面镓原子被氢化时,该纳米薄膜却依然保持纤锌矿结构,并且展示出铁磁性半导体特性.在应变作用下,两层半氢化氮化镓纳米薄膜的能隙可进行有效调控,并且它将会由半导体性质可转变为半金属性质或金属性质.这主要是由于应变对表面氮原子的键间交互影响和p-p轨道直接交互影响之间协调作用的结果.该研究成果为实现低维半导体纳米材料的多样化提供了有效的调控手段,为其应用于新型电子纳米器件和自旋电子器件提供重要的理论指导.     

First-principles studies of the hydrogenation effects in silicene sheets

Using density functional theory (DFT) with both the generalized gradient approximation (GGA) and hybrid functionals, we have investigated the structural, electronic and magnetic properties of a two-dimensional hydrogenated silicon-based material. The compounds, i.e. silicene, full- and half-hydrogenated silicene, are studied and their properties are compared. Our results suggest that silicene is a gapless semimetal. The coverage and arrangement of the absorbed hydrogen atoms on silicene influence significantly the characteristics of the resulting band structures, such as the direct/indirect band gaps or metallic/semiconducting features. Moreover, it is interesting to see that half-hydrogenated silicene with chair-like structure is shown to be a ferromagnetic semiconductor.     

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.     

Influence of strain and electric field on the properties of silicane

We investigate the influence of strain and electric field on the properties of a silicane sheet. Some elastic parameters of silicane, such as an in-plane stiffness of 52.55 N/m and a Poisson’s ratio of 0.24, are obtained by calculating the strain energy. Compared with silicene, silicane is softer because of its relatively weaker Si-Si bonds. The band structure of silicane is tunable by a uniform tensile strain, with the increase of which the band gap decreases monotonously. Moreover, silicane undergoes an indirect-direct gap transition under a small strain, and a semiconductor-metal transition under a large strain. The electric field can change the Si-H bond length of silicane significantly. When a strong field is applied, the H atom at the high potential side becomes desorbed, while the H atom at the low potential side keeps bonded. So an external electric field can help to produce single-side hydrogenated silicene from silicane. We believe this study will be helpful for the application of silicane in the future.     

Picosecond spectroscopy of hydrogenated MBE-GaAs

Picosecond-resolved and steady-state photoluminescence at LHe temperature in low-energy ion-gun hydrogenated GaAs/GaAlAs heterostructures are reported. The exciton in the GaAs layer shows an increase in lifetime — up to a factor of 3 — for moderate hydrogenation, followed by a sharp decrease below the value for the untreated sample, for higher H doses. Luminescence efficiency shows a consistent behavior. Incorporation of H generates a strong D-A band falling 64 meV below the gap energy. The behavior for heavy hydrogenation indicates the formation of a new type of deep defect, not ascribed to surface damage, because of the protective GaAlAs layer, plus the fact that the excitonic emission of the latter shows no variation.     

渐变带隙氢化非晶硅锗薄膜太阳能电池的优化设计

利用一维微电子-光电子结构分析软件(AMPS-1D)在AM1.5G(100 mW/cm2)、室温条件下模拟和比较了有、无渐变带隙氢化非晶硅锗(a-SiGe:H)薄膜太阳能电池的各项性能.计算结果表明:渐变带隙结构电池具有较高的开路电压(V oc)和较好的填充因子(FF),转换效率(E ff)比非渐变带隙电池提高了0.477%.研究了氢化非晶硅(a-Si:H)、氢化非晶碳化硅(a-SiC:H)和氢化纳米晶硅(nc-Si:H)三种不同材料的窗口层对a-SiGe:H薄膜太阳能电池性能的影响.结果显示:在以nc-Si:H为窗口层的电池能带中,费米能级E F已经进入价带,使得窗口层电导率及电池开路电压有所提高,又由于ITO与p-nc-Si:H的接触势垒较低,使得接触处的电场降低,更有利于载流子的收集.另一方面,窗口层与a-SiGe:H薄膜之间存在较大的带隙差,在p/i界面由于能带补偿作用形成了价带势垒(带阶)?E v,阻碍了空穴的迁移,因此我们在p/i界面引入缓冲层,使得能带补偿作用得到释放,更有利于空穴的迁移和收集,得到优化后单结渐变带隙a-SiGe:H薄膜结构太阳能电池的转换效率达到了9.104%.     

Ab initio studies of isolated hydrogen vacancies in graphane

We present a density functional study of various hydrogen vacancies located on a single hexagonal ring of graphane (fully hydrogenated graphene) considering the effects of charge states and the position of the Fermi level. We find that uncharged vacancies that lead to a carbon sublattice balance are energetically favorable and are wide band gap systems just like pristine graphane. Vacancies that do create a sublattice imbalance introduce spin polarized states into the band gap, and exhibit a half-metallic behavior with a magnetic moment of 1.00 μ_B per vacancy. The results show the possibility of using vacancies in graphane for novel spin-based applications. When charging such vacancy configurations, the deep donor (+1/0) and deep acceptor (0/−1) transition levels within the band gap are noted. We also note a half-metallic to metallic transition and a significant reduction of the induced magnetic moment due to both negative and positive charge doping.     

Quantum spin Hall effect in silicene and two-dimensional germanium

We investigate the spin-orbit opened energy gap and the band topology in recently synthesized silicene as well as two-dimensional low-buckled honeycomb structures of germanium using first-principles calculations. We demonstrate that silicene with topologically nontrivial electronic structures can realize the quantum spin Hall effect (QSHE) by exploiting adiabatic continuity and the direct calculation of the Z(2) topological invariant. We predict that the QSHE can be observed in an experimentally accessible low temperature regime in silicene with the spin-orbit band gap of 1.55 meV, much higher than that of graphene. Furthermore, we find that the gap will increase to 2.9 meV under certain pressure strain. Finally, we also study germanium with a similar low-buckled stable structure, and predict that spin-orbit coupling opens a band gap of 23.9 meV, much higher than the liquid nitrogen temperature.     

非晶硅界面缓冲层对非晶硅锗电池性能的影响

针对非晶硅锗电池本征层高锗含量时界面带隙失配以及高界面缺陷密度造成电池开路电压和填充因子下降的问题,通过在PI界面插入具有合适带隙的非晶硅缓冲层,不仅有效缓和了带隙失配,降低界面复合,同时也通过降低界面缺陷密度改善内建电场分布,从而提高了电池的收集效率. 进一步引入IN界面缓冲层以及对非晶硅锗本征层进行能带梯度设计,在仅采用Al背电极时,单结非晶硅锗电池转换效率达8.72%. 关键词： 非晶硅缓冲层 非晶硅锗薄膜太阳电池 带隙 界面     

Tuning spin-filtering,rectifying, and negative differential resistance by hydrogenation on topological edge defects of zigzag silicene nanoribbons

By first-principles calculations, we propose three heterojunction nanodevices based on zigzag silicene nanoribbons with different edge-hydrogenated topological line defects. The devices all present excellent spin-filtering properties with 100% spin polarization as well as remarkable rectifying effect (with rectification ratio around 10²) and negative differential resistance behaviors. Our findings shed new light on the design of silicon-based nanodevices with intriguing spintronic applications.     

Spin transport properties for B-doped zigzag silicene nanoribbons with different edge hydrogenations

Exploring silicon-based spin modulating junction is one of the most promising areas of spintronics.Using nonequilibrium Green's function combined with density functional theory,a set of spin filters of hydrogenated zigzag silicene nanoribbons is designed by substituting a silicon atom with a boron one and the spin-correlated transport properties are studied.The results show that the spin polarization can be realized by structural symmetry breaking induced by boron doping.Remarkably,by tuning the edge hydrogenation,it is found that the spin filter efficiency can be varied from 30%to 58%.Moreover,it is also found and explained that the asymmetric hydrogenation can give rise to an obvious negative differential resistance which usually appears at weakly coupled junction.These findings indicate that the boron-doped ZSiNR is a promising material for spintronic applications.