Na在B掺杂的空位石墨烯上吸附性能的第一性原理研究

采用基于密度泛函理论的第一性原理方法,研究了本征石墨烯和B掺杂的空位石墨烯吸附Na原子的电荷密度、吸附能、态密度、储存量以及电极电压.结果表明,两种石墨烯中,Na原子的最佳吸附位置都是H位.B掺杂的空位石墨烯对Na原子的吸附能是-2.08 eV,比本征石墨烯对Na原子的吸附能(-0.71eV)低很多.B掺杂的空位石墨烯中Na原子与B原子发生轨道杂化,本征石墨烯中没有杂化现象.B掺杂的空位石墨烯能够吸附12个Na原子,较本征石墨烯多.因此,B掺杂的空位石墨烯更适合储钠.     

Enhanced multiple exciton generation in amorphous silicon nanowires and films

ABSTRACT

Multiple exciton generation (MEG) in nanometer-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms depends strongly on the degree of the core structural disorder as shown by the perturbative many-body quantum mechanics calculations based on the density functional theory simulations. Working to the second order in the electron–photon coupling and in the screened Coulomb interaction, we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, in the Si₂₉H₃₆ quantum dots (QDs) with crystalline and amorphous core structures, simple cubic three-dimensional arrays constructed from these QDs, crystalline and amorphous NWs, and quasi two-dimensional silicon nanofilms, also both crystalline and amorphous. Efficient MEG with QE ranging from 1.3 up to 1.8 at the photon energy of about 3E_g, where E_g is the electronic gap, is predicted in these nanoparticles except for the crystalline NW and crystalline film where QE ? 1. MEG in the amorphous nanoparticles is enhanced by the electron localisation due to structural disorder. Combined with the lower gaps, the nanometer-sized amorphous silicon NWs and films are predicted to have effective carrier multiplication within the solar spectrum range.     

Electronic and structural properties of N-vacancy in AlN nanowires: A first-principles study

The stability and electronic structures of AlN nanowires with and without N-vacancy are investigated using firstprinciples calculations.We find that there is an inverse correlation between formation energy and diameter in ideal AlN nanowires.After calculating the formation energies of N-vacancy at different sites in AlN nanowires with different diameters,we find that the N-vacancy prefers to stay at the surface of the nanowires and it is easier to fabricate them under Al-rich conditions.Through studying the electronic properties of AlN nanowires with N-vacancies,we further find that there are two isolated bands in the deep part of the band gap,one of them is fully occupied and the other is half occupied.The charge density indicates that the half-fully occupied band arises from the Al at the surface,and this atom becomes an active centre.     

DFT方法研究掺杂氮化硅对SONOS器件保持性能的作用

可通过对氮化硅层掺杂来改变俘获电荷的缺陷种类和数量的方法，改善SONOS非挥发性存储器件的保持性能.建立无定形氮化硅和氧、硫、磷、氟或氯掺杂氮化硅中缺陷的簇模型；根据第一性原理的密度泛函理论(DFT)，对缺陷的簇模型结构优化并计算能量，得到缺陷俘获电荷过程的能量变化.发现缺陷俘获电子的能力比俘获空穴的能力好，电子释放过程应对温度敏感，而空穴释放过程主要由隧穿机理控制.预测与氧氮化硅一样，硫或磷掺杂氮化硅代替氮化硅作为SONOS器件的电荷储存层，可改善器件的保持性能.     

Surface passivation effects in silicon nanowires

We report first-principles simulation results for the electronic band structure of Si nanowires (SiNWs) aligned along the ?100? and ?110? directions with H, OH, and CH₃ substituents passivating the surfaces. The ?100? wires studied have {110} faces and square cross-sections with diameters up to 1.73 nm, while the ?110? wires have {111} faces and diamond cross-sections with diameters up to 1.46 nm. We found that passivation using OH or CH₃ groups reduced the band gaps compared to H-terminated ?100? SiNWs, and passivation using CH₃ groups produced systems with indirect gaps for all ?100? SiNWs studied. All band gaps were direct in the ?110? SiNWs independent of passivation. The near-gap orbitals are greatly affected by the different substituents. We also found that the carrier effective masses of ?100? SiNWs are sensitive to the diameter and passivation, while those of ?110? SiNWs are not.     

Laser doping: bipolar structures in silicon

Ultra-violet laser doping of silicon with boron and phosphorus, formed by the dissociation of trimethyl boron and phosphorus trichloride, has been achieved with sufficient control over concentration and depth to yield npn bipolar structures.     

Ultra-violet laser doping of silicon

In situ generation of boron from triethyl boron has been used in the ultra-violet laser doping of silicon, over a range of dopant concentrations. The quality of the doped material has been investigated using Auger electron analysis and electrical probes, and indicates that high activated dopant concentrations are readily achieved, although the purity of the material is degraded by unwanted alkyl derivatives.     

掺杂浓度对硅锥阴极特性的影响

基于漂移扩散模型和量子理论中的WKB方法,用数值模拟方法分析了材料掺杂浓度对硅锥阴极场致发射特性及工作状态的影响,结果表明,硅锥阴极单纯的场致发射IemitE特性受硅材料掺杂浓度的影响很小.但低掺杂硅锥阴极顶端的电位随发射电流增大而明显上升.锥体上电位变化可以等效为一个与锥体形状与掺杂相关的串联电阻的作用,这一电阻对单尖发射电流有负反馈作用.另外,在常规的工作状态下,硅锥阴级的温升并不严重.这些结果可以作为硅锥阴极设计的参考. 关键词： 硅 掺杂 场致发射     

镍掺杂硅纳米线电子结构和光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理计算,对镍掺杂硅纳米线的结构稳定性、电子与光学性质进行了研究.结果表明:Ni容易占据硅纳米线表面的替代位置.镍掺杂后的硅纳米线引入了杂质能级,杂质能级主要来源于Ni的3d电子的贡献.由于Ni的3d态和Si的3p态的耦合作用,使禁带宽度变窄.掺杂后的硅纳米线在低能区出现了一个较强的吸收峰,且吸收带出现宽化现象. 关键词： 硅纳米线 掺杂 电子结构 光学性质     

3d过渡金属Co掺杂核壳结构硅纳米线的第一性原理研究

基于密度泛函理论的第一性原理计算,研究了横截面为五边形和六边形的核壳结构硅纳米线的过渡金属Co原子替代掺杂.通过比较形成能发现,核心位置掺杂、壳层单链掺杂以及外壳层全替代掺杂的硅纳米线都具有稳定性,其中核心位置掺杂结构的稳定性最高.掺杂体系均呈现金属性,随着掺杂浓度的增加,电导通道数增加.Co原子掺杂的硅纳米线呈现铁磁性,具有磁矩.Bader电荷分析表明,电荷从Si原子转移至过渡金属Co原子.与自由态时过渡金属Co原子的磁矩相比,体系中Co原子的磁矩有所降低,这主要是由Co原子4s轨道向3d/4p轨道的电荷转移以及4s,3d,4p的上自旋电子转移至下自旋导致的.     

Computing Raman and infrared wavenumbers of nanostructures: application to silicon nanowires

The characterization of nanostructures with spectroscopic methods is a fundamental tool in nanoscience. For novel nanostructures, the interpretation of spectral features is a challenging task. To address this issue, we present the “Symmetry‐Filtered Molecular Dynamics (SFMD)” method to calculate Raman and infrared wavenumbers from molecular dynamics (MD) simulations, employing only the symmetry of the atomic structure. Explicit and expensive calculations of the electric polarizability or the dipole moment are not required. Therefore, our method can be easily used with any standard MD software. On the basis of the density functional tight‐binding method for the MD simulations, we apply our method to bulk silicon and small‐diameter hydrogen‐passivated silicon nanowires. For bulk silicon, we study the wavenumber shift of the Raman peak with temperature and obtain results that are in good agreement with experiments. We further show that thermal lattice expansion is a minor effect (22%) and that temperature‐driven anharmonic effects (78%) are the main contributions to that wavenumber shift. By analyzing the bond lengths of different silicon nanowires, we found that surface stress manifests as a 0.37% shortening of bonds only in the outermost silicon layer. We further analyzed the diameter‐dependent wavenumber shift of a Raman peak in silicon nanowires. We found that the main contribution to the wavenumber shift comes from the phonon confinement effect and surface stress leads to an additional shift of 9–22%. Our results indicate that our method is able to produce quantitative results that can be compared with experiments. We propose our method to be used for the understanding of Raman and infrared spectra of novel bulk and nanostructures. Copyright © 2012 John Wiley & Sons, Ltd.     

Exploration the p-type doping mechanism of GaAs nanowires from first-principles study

Using first-principle calculations, we present a systematic investigation upon the influence of p-type doping on the structural and electronic properties of H-passivated GaAs nanowires with wurtzite structure. The GaAs nanowire models of different doping types, different doping elements, different doping positions and different doping concentrations are established. The calculated formation energies show that Zn element becomes more competitive or even slightly favored in realizing p-type doping compared to Be element. For an individual Zn incorporation model, Zn atom tends to substitute the subsurface Ga atom. As increasing Zn doping concentration, the p-type doping process becomes more and more difficult. Besides, both interstitial and substitutional doping lead to the distortion of atomic structure near impurity atoms and cause the ionicity of GaAs nanowires enhanced. The p-type doped GaAs nanowires models are all direct band gap semiconductors. After substitutional doping, the total density of state curves shift toward higher energy sides and the Fermi level entering valence bands. Our calculations provide a significant reference for the preparation of p-type doping GaAs nanowire, which has a promising potential application in the field of photocathodes.     

尺寸相关的弹性常数对应变硅纳米线电学性质的影响

从Keating模型出发,基于离散化思想建立了计算单晶硅纳米线弹性常数和杨氏模量的半连续原子晶格力学模型. 从微扰理论和形变势理论出发,采用有限差分方法建立了计算不同晶向应变硅纳米线价带结构的数值模型. 结合上述的两个计算模型,进而应用经典弹道传输模型研究了轴向应力和弹性常数对p型硅纳米线弹道晶体管电学特性的影响. 研究结果表明,硅纳米线的弹性常数和杨氏模量呈现尺寸效应,该结果与分子动力学的模拟结果具有很好的一致性. 同时发现尺寸相关的弹性常数对硅纳米线晶体管输运电流的影响强烈依赖于单轴应力对输运电流的影 关键词： 应变硅纳米线 弹性常数 弹道电流 价带结构模型     

空位的第一性原理及经验势函数的对比研究

利用第一性原理及Stillinger-Weber（SW）,EDIP和Tersoff经验势函数对比研究了硅中单空位(V₁)、双空位(V₂)和六边形空位环(V₆)的结构特性及形成能.讨论了经验势函数描述空位时的优点和缺点.结果发现,第一性原理方法可以精确描述空位的原子结构及能量特性,而短程有效的经验势函数无法描述空位所固有的量子效应,如Jahn-Teller变形等.另外,由于经验势函数自身的缺陷,EDIP和T3无法应用于空位结构特性的计算.虽然 关键词： 空位 第一性原理 经验势函数     

Electronic transport properties of silicon chains sandwiched between graphene electrodes: first-principles study

The effects of orientation and silicon chain length on the electronic transport properties for linear silicon chains sandwiched between two graphene electrodes are investigated by using non-equilibrium Green’s functions combined with density functional theory. Our results demonstrate that the conductance of single silicon chains can hardly be affected by its orientation, as there is negligible difference between the conductance of tilted and un-tilted chains, and the conductance is impacted greatly by the length of chains, i.e. the transmission coefficient is doubled for double chains. The equilibrium conductance of single silicon chains shows even-odd oscillating behavior, and its tendency decreases with the increase of the chain length. The non-equilibrium electronic transport properties for all types of chains are also calculated, and all current–voltage curves of silicon chains show a linear character. The frontier molecular orbitals, the total and projected density of states are used to analyse the electronic transport properties for all types of chains.     

Ga和Sb纳米线声子结构和电子-声子相互作用的第一性原理研究

基于密度泛函理论的第一性原理方法,系统研究了Ga和Sb纳米线的电子能带结构和声子结构以及电子-声子耦合(EPC)作用.通过对声子的完整Brillouin区分析来研究纳米线的结构稳定性.结果表明,所考察的纳米线显示出不稳定性,不稳定声子波矢远离Brillouin区中心.与通常的Peierls变形机理相比,不稳定的横向声子模会导致一种无开口带隙的相变.Sb比Ga纳米线的EPC要强很多,并且横向变形导致的锯齿形结构使纳米线中的电子-声子相互作用增加了几个数量级.     

Enhanced gas-sensing performance of graphene by doping transition metal atoms: A first-principles study

By performing the first-principles calculations, we investigated the sensitivity and selectivity of transitional metal (TM, TMSc, Ti, V, Cr and Mn) atoms doped graphene toward NO molecule. We firstly calculated the atomic structures, electronic structures and magnetic properties of TM-doped graphene, then studied the adsorptions of NO, N₂ and O₂ molecules on the TM-doped graphene. By comparing the change of electrical conductivity and magnetic moments after the adsorption of these molecules, we found that the Sc-, Ti- and Mn-doped graphene are the potential candidates in the applications of gas sensor for detection NO molecule.     

High-efficient Schottky-junction silicon solar cell using silver nanowires covering nitrogen-doped amorphous carbon

Synthesized graphene (Gr) on metal substrates that requires additional surface-to-surface transfer procedure to form Gr-on-silicon (Gr-Si) Schottky-junction configuration, which in turn results in the photovoltaic degradation caused by both mechanical damages and chemical contaminations during several wet chemical steps. This current issue has motivated us to develop alternative Schottky-junction configuration using silver nanowires (AgNWs) covering nitrogen (N)-doped amorphous carbon (a-C) films annealed in the temperature range 750–900 °C. Compared to the Schottky-junction Si solar cell based on 900 °C annealed N-doped a-C films (C_N-900-Si) with only Ag grid, all of AgNWs-C_N-900-Si solar cells exhibit the significant enhancement of photovoltaic characteristics. Consequently, the remarkable power conversion efficiency (PCE) of 6.17% is achieved on 0.2 wt% AgNWs-C_N-900-Si solar cell, which is far superior to that of the C_N-900-Si solar cell with only Ag grid (~0.13%). Furthermore, the 0.2 wt% AgNWs-C_N-900-SiNWs solar cell shows the highest short-circuit current density (J_SC) of 23.42 mA/cm² and PCE of 7.67%, which is a PCE enhancement of ~24% when compared to the 0.2 wt% AgNWs-C_N-900-Si solar cell. This study demonstrates that AgNWs network can accelerate the charge carrier extraction from Schottky-contact between C_N-900 and n-Si substrate, leading to greatly reduced series resistance that results in significantly enhanced photovoltaic characteristics.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.