金电极下二元化合物氮化镓链结构演化和电导的从头计算（英文）

基于从头密度泛函理论结合非平衡格林函数方法,首次研究了二元化合物GaN链介于两个金电极之间的电子输运性质.模拟了Au-(GaN)_2-Au节点断裂过程,计算了相应的结合能,获得了平衡态电导以及平衡态下节点的投影态密度.此外,计算了平衡态下节点在微小偏压下的电流和电导.结果发现,随着偏压增强,GaN分子链的电导也随之下降.最后给出的伏安特性曲线呈现一个非线性关系,表明节点具有类似半导体的特征.     

钽硅团簇电子输运性质的第一性原理研究

利用基于密度泛函理论和非平衡格林函数的第一性原理方法,对位于两段半无限长铝导线之间的TaSi₃团簇的电子输运性质进行了研究.研究表明：该团簇分子投影自洽哈密顿量的8态和9态决定着系统在小偏压下的输运特性.TaSi₃团簇的平衡态电导对团簇与电极间距离的变化十分敏感,当距离小于0.35 nm时,平衡态电导随之剧烈的振荡;当距离大于0.35 nm后,平衡态电导迅速减小.在-1—1 V偏压下,团簇表现出一定的电压-电流非对称整流特性,在0.3—0.4 V的偏压下,观察到了该团簇的负微分电导特性. 关键词： 铝/钽硅混合团簇/铝分子结 电子输运 密度泛函理论 非平衡格林函数     

异构二芳基乙烯分子结低偏压电导机制的第一性原理研究

本文用结合密度泛函理论的非平衡格林函数方法研究了具有开环和闭环两种同分异构体的二芳基乙烯衍生物分子的结构-特性关系. 该功能分子通过末端的吡啶基团连接到取向沿(111)方向的金电极. 计算结果表明，异构体分子结的不同低偏压电导主要是由于它们具有不同的电子结构. 两种构型分子结的低偏压导电都主要来自于电子隧穿最低未占据分子轨道(LUMO). 由于具有扩展的单双键交替共轭结构，闭环构型分子具有更好的导电通道. 通过计算分子结在平衡状态下的电子转移，可发现更多电子转移到闭环构型分子，导致了其LUMO能量更靠近费米能级，从而有利于低偏压下的导电性能. 结果有助于理解二芳基乙烯分子及其衍生物分子的低偏压电导机制，也为设计更高性能的同类分子开关提供理论依据.     

准二维无序系统的电子结构

对形如N_t×N_l型准二维无序系统，只考虑格点之间的最近邻跳跃积分，采用特殊的格点编号方案，在单电子近似下，系统的哈密顿量可表示为简明对称矩阵，借助豪斯荷尔德变换将其约化为对称三对角矩阵，再利用负本征值理论及传输矩阵等方法,对系统态密度、局域长度及电导等电子结构特性进行数值计算. 重点研究了准一维四平行链和五平行链无序系统， 将结果与一维单链、准一维双链及三链系统进行对比，发现随维度的增加，系统的能带有所展宽，能态密度分布发生很大的变化，其峰值数量呈偶数规律增加. 并且在能带中心处存在有局域长度大于系统大小的扩展态，处于这些态下的系统具有较大电导. 从单链到多链，相当于扩大了系统的关联范围，使系统出现了类似非对角长程关联的行为. 关键词： 准二维无序系统 态密度 局域长度 电导     

拓扑缺陷对Armchair型小管径多壁碳纳米管输运性质的影响

采用密度泛函理论和非平衡格林函数方法研究了纯净的及带有不同数目的 Stone-Wale拓扑缺陷下的扶手椅型单壁,双壁和三壁小管径碳纳米管的能带结构和电子输运性质,通过计算并分析不同偏压下的微分电导和非弹性电子隧穿谱(IETS),计算结果表明单壁,双壁和三壁碳纳米管的特征偏压区间分别为[-1.0 V,1.0 V],[-0.5 V,0.5 V]和[-0.25 V,0.25 V],特征偏压区间内SW拓扑缺陷所产生的电导波动平缓,而特征偏压区间外因缺陷的数目变化所带来的电导波动显著,通过IETS谱线的分析得到单壁,双壁和三壁碳纳米管的特征峰偏压分别为±1.25 V,±0.625 V和±0.125 V.碳纳米管的特征偏压区间和IETS特征峰偏压可为较小管径碳纳米管单壁,双壁和多壁类型的区分提供一种新的途径,同时也为小管径多壁碳纳米管的输运性质在出现拓扑缺陷时的响应提供参考依据.     

拓扑缺陷对Armchair型小管径多壁碳纳米管输运性质的影响

采用密度泛函理论和非平衡格林函数方法研究了纯净的及带有不同数目的Stone-Wale拓扑缺陷下的扶手椅型单壁, 双壁和三壁小管径碳纳米管的能带结构和电子输运性质, 通过计算并分析不同偏压下的微分电导和非弹性电子隧穿谱(IETS), 计算结果表明单壁, 双壁和三壁碳纳米管的特征偏压区间分别为[-1.0V, 1.0V], [-0.5V, 0.5V] 和[-0.25V, 0.25V], 特征偏压区间内SW拓扑缺陷所产生的电导波动平缓, 而特征偏压区间外因缺陷的数目变化所带来的电导波动显著, 通过IETS谱线的分析得到单壁, 双壁和三壁碳纳米管的特征峰偏压分别为 1.25V, 0.625V和 0.125V. 碳纳米管的特征偏压区间和IETS特征峰偏压可为较小管径碳纳米管单壁, 双壁和多壁类型的区分提供一种新的途径, 同时也为小管径多壁碳纳米管的输运性质在出现拓扑缺陷时的响应提供参考依据.     

基于电容和电导特性分析GaN蓝光发光二极管老化机理

采用正向交流小信号方法测试和分析老化前后GaN发光二极管（LED）的电容-电压特性,结合串联电阻、理想因子、隧穿电流参数讨论负电容以及电导变化情况.基于L-V曲线定性分析老化前后负电容的阈值电压,老化之后样管的受主浓度降低,辐射复合概率下降,大量缺陷以及非辐射复合中心出现,对载流子俘获作用增强,造成负电容降低.反向偏压以及小正向偏压下,隧穿效应导致老化之后样管的电导增大;正向偏压大于2.2 V区域,考虑串联电阻效应,老化后样管电导减小.在分析LED电容-电压、光输出、电学特性曲线与老化机理基础上,通过实验论证以及理论解释表明,负电容以及电导特性可作为分析LED老化特性的参考依据. 关键词： GaN发光二极管 负电容 电导 老化机理     

方阱链流体在固液界面分布的密度泛函理论研究

应用Yethiraj的加权密度近似泛函理论研究平板狭缝中方阱链流体的密度分布，系统的Helm holtz自由能泛函分为理想气体的贡献利剩余贡献两部分，其中剩余贡献部分分别采用刘洪 来等人建立的基于空穴相关函数的方阱链流体状态方程和Gil-Villegas等人提出的统计缔合 流体理论状态方程(SAFT-VR)结合简单加权密度近似计算.考察了不同链长、温度、系统密度 和壁面吸引强度下平板狭缝中方阱链流体的密度分布，并与Monte Carlo(MC)模拟结果进行 了比较.结果表明采用不同的状态方程对密度分布的计算有明显的影响，对于受限于硬壁狭 缝中的方阱链流体，温度和密度比较高时，两种状态方程计算的结果均与MC模拟符合得比较 好，在低温和低密度下效果变差，SAFT-VR方程的计算结果更接近于MC模拟结果.对于受限于 方阱壁狭缝中的方阱链流体，由于系统密度分布的非均匀性加强，采用两种状态方程计算的 结果均与MC模拟结果有一定偏差，寻找更合适的权重函数是进一步改进的关键. 关键词： 密度泛函理论 非均匀流体 密度分布 固液界面 方阱链     

碳链输运对基团吸附的敏感性分析

基于局域原子轨道的密度泛函理论和非平衡格林函数方法,研究了碳链输运特性对分别吸附7种常见官能团NO₂, CN, CHO, Br, C₆H₅, C₅H₄N和NH₂时的敏感性. 计算表明,电流对C₆H₅和CHO的吸附最为敏感,其次是对CN和C₅H₄N的吸附,在某些偏压下电流有大幅度的下降,其值 关键词： 碳链 输运特性 密度泛函理论 非平衡格林函数     

电场对低聚呋喃的几何构型、电子结构和输运行为的影响

采用密度泛函方法在6-31G*水平上,对一系列不同链长的低聚呋喃分子在外电场作用下的性质进行了理论计算研究。结果表明电场的引入提高了呋喃分子共轭性,电场的诱导使偶极增加,SCF能量降低和HOMO-LUMO能隙变窄.进一步通过硫原子将呋喃分子与金电极相连接,利用非平衡格林函数方法对其在0.0~2.0V偏压下电子输运特征进行了研究.讨论链长效应对这些性质的影响.     

Effect of gold electrode crystallographic orientations on charge transport through aromatic molecular junctions

ABSTRACT

We examined the electrical conduction through single-molecular junctions comprising of anthracenedithiol molecule coupled to two gold electrodes having ?1,0,1?, ?1,1,0? and ?1,1,1? crystallographic orientations. Owing to this jellium model, we evaluated the values of current and conductance using non-equilibrium Green's functions combined with extended Huckel theory. This data was further interpreted in terms of transmission spectra, density of states and their molecular orbital analysis for zero bias. We evinced the oscillating conductance in all three cases, due to the oscillation of orbital energy relative to Fermi level. Our detailed analysis suggested that electrode orientation can tune the molecule–electrode coupling and hence conduction. Anthracene molecular junction with ?1,1,0? orientation displayed favourable conduction, when compared to the other two orientations, thus can provide us an insight while designing futuristic molecular electronic devices.     

High-efficiency spin filtering in salophen-based molecular junctions modulated with different transition metal atoms

Based on the density functional theory and nonequilibrium Green's function methods, we investigate the spin transport properties of the molecular junctions constructed by a homologous series of 3d transition metal(II) salophens (TM-salophens, TM = Co, Fe, Ni and Mn) sandwiched between two gold electrodes. It is found that among the four molecular junctions only Co-salophen junction can act as an efficient spin filter distinctively. The conductance through Co-salophen molecular junction is dominated by spin-down electrons. The mechanism is proposed for these phenomena.     

Unusual conductance of polyyne-based molecular wires

We report a full self-consistent ab initio calculation of the current-voltage curve and the conductance of thiolate capped polyynes in contact with gold electrodes. We find the conductance of polyynes an order of magnitude larger compared with other conjugated oligomers. The reason lies in the position of the Fermi level deep in the highest occupied molecular orbital related resonance. With the conductance weakly dependent on the applied bias and almost independent of the length of the molecular chain, polyynes appear as nearly perfect molecular wires.     

An ab initio study on the effect of junctions on transport characteristics of nitrogen and aluminum mixed chain

We have studied the effect of junctions on transport characteristics of Nitrogen and Aluminum (AlN)_n (n represents the number of “AlN” units) mixed chain between Al (100) electrodes by using the first-principle based on density functional theory and non-equilibrium Green's function in this paper. From the results of the study, the longer the atomic chain, the weaker the coupling between the atom chain and the electrodes. When n?>?2, the equilibrium conductance in Fermi level is very small and tends to zero, in a non-conducting state. In the case of bias, when the bias voltage increases, the resonance transmission peaks move from the Fermi level. When the bias is increased to 4?V, the device is in the cut-off in the energy region of -2–0?eV, there are several resonant transmittance peaks at the same time in the energy region of 0–2?eV, showing a good diode behavior.     

First-principles calculation on the conductance of ruthenium-quasi cumulene-ruthenium molecular junctions

The conductance of a family of ruthenium-quasi cumulene-ruthenium molecular junctions including different numbers of carbon atoms, both in even numbers and odd numbers, are investigated using a fully self-consistent ab initio approach which combines the non-equilibrium Green’s function formalism with density functional theory. Our calculations demonstrate that although the overall transport properties of the Ru-quasi cumulene-Ru junctions with an even number of carbon atoms are different from those of the junctions with an odd number of carbon atoms, the difference between the corresponding current-voltage (I–V) characteristics of these molecular junctions declines to lesser than 16% when the voltage goes up. In each group, the molecular junctions give a large transmission around the Fermi level since the Ru-C π bonds can extend the π conjugation of the carbon chains into the Ru electrodes, and their I–V characteristics are almost linear and independent of the chain length, illustrating potential applications as conducting molecular wires in future molecular electronic devices and circuits.      

压缩应变载荷下氮化镓隧道结微观压电特性及其巨压电电阻效应

电子器件可控性研究在日益追求器件智能化和可控化的当今社会至关重要. 基于第一性原理和量子输运计算, 本文研究了压缩应变载荷对氮化镓(GaN)隧道结基态电学性质和电流输运的影响, 在原子尺度上窥视了氮化镓隧道结的微观压电性, 验证了其内在的巨压电电阻(GPR)效应. 计算结果表明, 压缩应变载荷可以调节隧道结内氮化镓势垒层的电势能降、内建电场、电荷密度和极化强度, 进而实现对隧道结电流输运和隧穿电阻的调控. 在-1.0 V的偏置电压下, -5%的压缩应变载荷将使氮化镓隧道结的隧穿电阻增至4倍. 本研究展现了氮化镓隧道结在可控电子器件中的应用潜力, 也展现了应变工程在调控电子器件性能方面的光明前景.     

Molecular rectification of thiol-linked Au|PTCDI-[CH2]n|Au junctions

The electronic transport properties of the PTCDI-[CH₂]_n(0≤n≤6) molecular junctions with different molecular lengths are theoretically investigated via the first-principles density functional theory (DFT) and non-equilibrium Green's function (NEGF) method. Our results show that the transport properties depend on molecular lengths. The equilibrium conductance of the probed systems decreases exponentially with the increasing number n of the CH₂ unit. With n≥1, the rectifying effect has been found. In the n=6 case, a significant rectification ratio of 72.6 is achieved at the bias of ±2.1 V in our probed voltage range. The rectification effect arises from asymmetric molecular structures. Our results suggest these molecules have great potential application in the molecular-scale device.     

硅纳米结点电子输运性质的计算

运用第一性原理密度泛函理论结合非平衡格林函数方法,对3个Si原子构成的直线链耦合在Au(100)面形成的三明治结构的纳米结点的电子输运进行计算.结果得到结点电导随距离的变化,当d_z=1.584 nm时,结合能最小,结构最稳定,此时Si-Si键长为0.216 nm,Si-Au键长为0.227 nm,电导为0.729 G₀(G₀=2e²/h),其电子传输通道主要由Si原子的p_x、p_y轨道电子构成;随着外电压的增大,结点的电导减小,而其I-V曲线表现出线性特征.     

Voltage-dependent conductance states of a single-molecule junction

Ag-Sn-phthalocyanine-Ag junctions are shown to exhibit three conductance states. While the junctions are conductive at low bias, their impedance drastically increases above a critical bias. Two-level fluctuations occur at intermediate bias. These characteristics may be used to protect a nanoscale circuit. Further experiments along with calculations reveal that the self-limiting conductance of the junctions is due to reversible changes of the junction geometry.     

Au-Si_(60)-Au分子结电子输运性质的理论计算

运用密度泛函理论对Si60团簇的结构进行几何优化,得到基态结构是一个直径为1.131 nm,平均键长为0.239 nm,分子最低未占据轨道与最高占据轨道能量差即能隙值为0.72 eV,具有C1点群的空心笼状结构.然后把它与两半无限的Au(100)-4×4电极相连构成Au-Si60-Au三明治结构分子结点,运用密度泛函理论结合非平衡格林函数的方法对其电子输运性质进行了第一性原理计算.当两电极的距离为1.74 nm时,分子结点的平衡电导为1.93G0(G0=2e2/h),然后在-2.0—2.0 V的电压范围内,计算了不同电压下的电导与电流,得到其I-V曲线成近线性关系,从分子前线轨道与透射谱分析了Si60分子的电子输运特性,讨论了电荷转移量与电导之间的关系.