苯基分子器件量子相干输运第一性原理研究

分子器件在纳米尺度下,电子的相干性将对体系的电导产生重大影响.本文基于第一性原理计算研究了苯分子连接于一维金属电极下的电荷输运性质.发现一维金电极连接下,不同的连接方式(para与meta)体系下的电导将会有显著差别,而一维铂电极连接下,体系的电导差别不大.我们通过计算电极的能带,发现金电极与铂电极在费米面处的散射态数目有差别.当量子相干效应导致散射态局域化发生改变时,由于铂电极的通道数较多,电子依然可以通过扩展的通道输运,因此不同连接方式下的电导变化不明显.     

(CH3)2和(NH2)2基团修饰的齐聚苯乙炔分子电子输运性质研究

采用密度泛函理论和非平衡格林函数相结合的方法研究了S原子作为单、双端基的(CH₃)₂-OPE (齐聚苯乙炔)和(NH₂)₂-OPE分子在金电极间的电子输运性质. 通过第一性原理优化计算获得分子部分稳定结构, 再置于Au电极之间构成两极系统, 然后再优化整个两极系统获得稳定结构. 另外, 通过非平衡格林函数方法计算了两极系统的电子输运性质. 计算结果表明, 不同的修饰基团和桥接方式可以导致两极系统的开关效应、负微分电阻行为和整流行为等不同的电子输运性质. 通过计算不同偏压下的分子体系投影轨道电子分布、透射谱、态密度, 对这些新异的电输运性质出现的机理进行了解释.     

电极位置和截面尺寸对分子器件输运性质的调控

研究表明分子器件的性能受器件结构搭建精度影响,分子与电极接触构型的微弱变化可能引起电输运特性较大差异.本文运用密度泛函理论和非平衡格林函数相结合的方法,研究了由金纳米线与benzene-1,4-dithiol（BDT）形成的分子结的电输运性质.通过对不同的Au-BDT接触构型输运性质的研究,发现当两电极处于对位构型时,有较好的电荷输运行为,而且比较符合制备工艺要求;当电极偏离轴线的角度不大于5°,且电极散射截面尺寸不小于4×4时,该分子结体系的电导和透射谱均比较稳定.电极截面尺寸小于4×4或者电极偏离轴线的夹角大于5°时,透射谱在费米能级附近出现不连续现象,导致体系电导降低.较小电极截面尺寸或者电极以较大角度偏离轴线将导致该分子结体系电导降低和透射谱连续性降低,主要是组成电极的金原子轨道与苯基分子轨道耦合缺失造成的.该研究为Au-BDT-Au体系设计和制备过程中电极的位置及电极截面尺寸做了科学的界定.     

光致异构体开关特性的理论研究

利用弹性散射格林函数方法,对4,4-二羧基1,2-二苯乙烯分子的两种异构体与金电极构成的单分子结进行了研究. 研究表明,该类分子是通过末端羧基化学吸附于金表面的,两种分子结电导特性的差异主要是因为分子与电极的相互作用所致. 对每一种分子来说,都存在三种不同的稳定电导值,分别对应着分子末端与金表面的不同接触方式. 分子与金表面的相互作用导致分子结电子结构的变化是其电导差异的主要原因. 理论结果与实验测量结果符合得较好. 关键词： 光致异构体 伏安特性 电子输运 分子电子学     

4,4’-对苯巯基醚分子器件的几何结构及其电学性质的研究（英文）

从第一性原理出发,用弹性散射格林函数的方法计算了4,4-对苯巯基醚分子器件几何结构的变化对其电子结构及其电学性质的影响.计算结果表明,选择不同的电极间距会引起电子输运峰的高度和分子与电极的耦合程度的变化,同时也对分子电流和电导的开启位置有较小的影响,进而使分子体系的电输运性质表现各异     

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

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。     

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.     

拓扑石墨烯电极分子器件输运特性

利用密度泛函理论结合非平衡格林函数方法,研究了不同拓扑能带结构的石墨烯电极分子器件输运特性.结果表明器件导通电压与电极禁带宽度正相关,同时器件在输运过程中表现出负微分电阻特性,峰谷电流比可达2697.分析认为器件导通源自于偏压升高过程中两电极能带匹配.器件负微分电阻特性源自于偏压升高过程中两电极能带交错.散射态分析表明,能带匹配后散射态分布较为离域,有利于电子通过器件.能带交错后散射态局域于电极处,表明电子输运受到抑制.     

1,4-丁二硫醇分子器件电输运性质的力敏特性研究

基于杂化密度泛函理论,研究了1,4-丁二硫醇分子体系的结构随电极作用力的变化及拉断过程;并利用弹性散射格林函数方法进一步计算了不同电极作用力下分子体系的电输运特性.结果显示,界面结构不同,拉断分子体系所用的拉力也不同:分子末端硫原子处于Au(111)面的空位上方时,拉断分子体系需约1.75 nN的拉力;若金电极表面存在孤立金原子与1,4-丁二硫醇分子末端的硫原子相连,拉断分子体系只需约1.0 nN的力,且伴有孤立金原子被拉出.两种情况分别与不同实验测量相符合.分子在压缩过程中发生扭曲并引起表面金原子滑移,然而压缩扭曲过程与拉伸回复过程不可逆.电极拉力约为0.7—0.8 nN时,分子体系在不同界面构型下以及在不同扭转状态下,电导都出现极小值,这与实验结论一致.分子的末端原子与电极间耦合强度随电极作用力的变化是引起分子体系电导变化的主要因素.实验在0.8 nN附近同时测得较小概率的高电导值与双分子导电有关.     

基于碳纳米管分子封装的一维电子波函数扫描

利用第一性原理,设计并研究了一类基于单臂碳纳米管的分子封装的分子体系．计算表明,半环葫芦脲类化合物可有效封装碳纳米管,引入微弱的分子间相互作用,对碳纳米管的电子态能级结构分布 仅带来微弱影响．半环葫芦脲分子与碳纳米管在管径方向的一维电子态波函数充分耦合,进而有效改变了一些前沿分子轨道的波函数在管径两头的分布以及相应的电子布居浓度．基于电子输运的模拟,发现半环葫芦脲分子在碳纳米管一维方向滑动时的某个电压下的电导变化可准确反映电子态波函数在相应分子导电通道上的一维分布信息．     

Effect of crystallographic orientations on transport properties of methylthiol-terminated permethyloligosilane molecular junction

The understanding of the influence of electrode characteristics on charge transport is essential in the field of molecular electronics. In this work, we investigate the electronic transport properties of molecular junctions comprising methylthiol-terminated permethyloligosilanes and face-centered crystal Au/Ag electrodes with crystallographic orientations of (111) and (100), based on the ab initio quantum transport simulations. The calculations reveal that the molecular junction conductance is dominated by the electronic coupling between two interfacial metal-S bonding states, which can be tuned by varying the molecular length, metal material of the electrodes, and crystallographic orientation. As the permethyloligosilane backbone elongates, although the σ conjugation increases, the decreasing of coupling induced by the increasing number of central Si atoms reduces the junction conductance. The molecular junction conductance of methylthiol-terminated permethyloligosilanes with Au electrodes is higher than that with Ag electrodes with a crystallographic orientation of (111). However, the conductance trend is reversed when the electrode crystallographic orientation varies from (111) to (100), which can be ascribed to the reversal of interfacial coupling between two metal-S interfacial states. These findings are conducive to elucidating the mechanism of molecular junctions and improving the transport properties of molecular devices by adjusting the electrode characteristics.     

点间耦合强度对三耦合量子点系统微分电导的影响

本文利用非平衡格林函数运动方程方法,研究了与两个电极耦合在一起的三耦合量子点系统的微分电导及量子干涉的AB振荡问题.通过理论计算发现,由于量子点上的局域态密度的不同从而导致系统电导或隧穿性质的不同,而且量子点间耦合强度、量子点能级等都会对输运性质产生影响. 关键词： 量子点 非平衡格林函数 运动方程 局域态密度     

耦合形貌对Si4团簇电子输运影响的第一性原理计算

运用密度泛函理论与非平衡格林函数相结合的方法,对Si₄团簇与Au （100）-3×3两电极以顶位-顶位、顶位-空位、空位-空位三种形貌相连构成的Au-Si₄-Au纳米结点的拉伸过程进行第一性原理模拟,计算不同构型纳米结点在不同距离的电导和结合能.讨论耦合形貌、距离对结点电导的影响,结合能的计算表明三种不同耦合形貌结点存在稳定平衡结构,其平衡电导分别为0.71 G₀、0.96 G₀和2.44 G₀,且在-1.2 V~1.2 V的电压范围内,三种不同耦合形貌结点稳定结构表现出类似金属的导电特性,其I-V关系都近似为直线.计算结果表明Si₄团簇与电极的耦合形貌、两极距离对纳米结点电子输运有重要影响.     

Effective interactions in molecular dynamics simulations of lysozyme solutions

On the basis of ab-initio calculations, we predict the effect of conformation and molecule-electrode distance on transport properties of asymmetric molecular junctions for different electrode materials M (M = Au, Ag, Cu, and Pt). The asymmetry in these junctions is created by connecting one end of the biphenyl molecule to conjugated double thiol (model A) and single thiol (model B) groups, while the other end to Cu atom. A variety of phenomena viz. rectification, negative differential resistance (NDR), switching has been observed that can be controlled by tailoring the interface state properties through molecular conformation and molecule-electrode distance for various M. These properties are further analyzed by calculating transmission spectra, molecular orbitals, and orbital energy. It is found that Cu electrode shows significantly enhanced rectifying performance with change in torsion angles, as well as with increase in molecule-electrode distances than Au and Ag electrodes. Moreover, Pt electrode manifests distinctive multifunctional behavior combining switch, diode, and NDR. Thus, the Pt electrode is suggested to be a good potential candidate for a novel multifunctional electronic device. Our findings are compared with available experimental and theoretical results.     

Current-voltage curves of atomic-sized transition metal contacts: an explanation of why Au is Ohmic and Pt is not

We present an experimental study of current-voltage (I-V) curves on atomic-sized Au and Pt contacts formed under cryogenic vacuum (4.2 K). Whereas I-V curves for Au are almost Ohmic, the conductance G=I/V for Pt decreases with increasing voltage, resulting in distinct nonlinear I-V behavior. The experimental results are compared with first principles density functional theory calculations for Au and Pt, and good agreement is found. The difference in conductance properties for Pt vs Au can be explained by the underlying electron valence structure: Pt has an open d shell while Au has not.     

Conduction mechanism in a molecular hydrogen contact

We present first principles calculations for the conductance of a hydrogen molecule bridging a pair of Pt electrodes. The transmission function has a wide plateau with T approximately 1 which extends across the Fermi level and indicates the existence of a single, robust conductance channel with nearly perfect transmission. Through a detailed Wannier function analysis we show that the H(2) bonding state is not involved in the transport and that the plateau forms due to strong hybridization between the H(2) antibonding state and states on the adjacent Pt atoms. The Wannier functions furthermore allow us to derive a resonant-level model for the system with all parameters determined from the fully self-consistent Kohn-Sham Hamiltonian.     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

Interplay between weak localization and quantized conductance

The interplay between weak localization and conductance quantization through a narrow channel is considered. The “electrodes” are essentially two half planes with slight disorder. In this system weak localization results from a coherent echo in the electrodes towards the channel. Therefore one has an interesting mixture of dimensions, the channel being quasi-one dimensional and the electrodes being two dimensional. As a consequence the quantum interference correction to the conductance is non-universal.     

Correlation of interfacial bonding mechanism and equilibrium conductance of molecular junctions

We report theoretical investigations on the role of interfacial bonding mechanism and its resulting structures to quantum transport in molecular wires. Two bonding mechanisms for the Au-S bond in an Au(111)/1,4-benzenedithiol(BDT)/Au(111) junction were identified by ab initio calculation, confirmed by a recent experiment, which, we showed, critically control charge conduction. It was found, for Au/BDT/Aujunctions, the hydrogen atom, bound by a dative bond to the Sulfur, is energetically non-dissociativeafter the interface formation. The calculated conductance and junction breakdown forces of H-non-dissociative Au/BDT/Au devices are consistent with the experimental values, while the H-dissociated devices, with the interface governed by typical covalent bonding, give conductance more than an order of magnitude larger. By examining the scattering states that traverse the junctions, we have revealed that mechanical and electric properties of a junction have strong correlation with the bonding configuration. This work clearly demonstrates that the interfacial details, rather than previously believed many-body effects, is of vital importance for correctly predicting equilibrium conductance of molecular junctions; and manifests that the interfacial contact must be carefully understood for investigating quantum transport properties of molecular nanoelectronics.