金属/分子/金属结的电子输运机理与表征

金属/分子/金属结是分子电子学中的基本单元.根据电子的相位是否发生改变,分子结中的电子输运可以分为相干输运和非相干输运两类.在实验上,分子结的表征方法可以分为电学性质表征和非电学性质表征两类.本文借助能级图,首先对分子结的电子输运机理作了简明解释.在此基础上,结合文献报道和本课题组此前的工作,对分子结的一些常用电学表征方法,包括电流-电压特性曲线、电流-时间曲线、电导统计柱状图、转变电压谱、散粒噪声测试、非弹性电子隧道谱和热电效应法进行了介绍.     

电极距离对4,4’-联苯二硫酚分子器件非弹性电子隧穿谱的影响

基于杂化密度泛函理论和格林函数方法, 计算了4,4’-联苯二硫酚分子器件的非弹性电子隧穿谱, 并研究了电极距离对该非弹性电子隧穿谱的影响. 计算结果表明, 非弹性电子隧穿谱随电极距离的改变呈明显不同的特征, 从而表明了分子的非弹性电子隧穿谱技术能够灵敏地反映出分子器件的微观结构. 研究结果显示, 垂直于电极表面的振动模式对非弹性电子隧穿谱具有较大的贡献.     

非弹性电子隧道谱(Iets)——新技术

电子隧道效应能被用来获得吸附在金属氧化层薄膜表面上有机分子的高分辨振动谱。本文介绍这一最近发展的用于研究被吸附物-吸附物相互作用的新技术,并绘出一些有趣的非弹性电子隧道谱。有重要催化作用氧化物表面特性的许多知识,可从这些系统的谱图和它们表面吸附物谱图的研究来探索。非弹性电子隧道谱为一种新增添的、有价值的技术,它给出有关被吸附材料的详细振动信息。因此,它成为目前广泛应用的其它电子光谱技术的补充,如低能电子衍     

铜-真空-铜金属隧道结转变电压的理论研究

利用非平衡格林函数与密度泛函理论相结合方法研究了电极表面具有原子级突起的铜-真空-铜隧道结的转变电压.计算结果表明,铜电极真空隧道结的转变电压主要决定于电极表面尖端铜原子4p轨道的局域态密度,因而对电极取向和表面局域原子构型非常敏感.对于电极取向沿(111)方向的铜电极真空隧道结,当电极表面原子级突起取为铜吸附原子和金字塔型铜纳米粒子两种构型时,转变电压的计算值分别约为1.40和2.40 V.当电极取向沿(100)方向时,电极表面原子级突起分别为铜吸附原子和金字塔型铜纳米粒子两种构型的铜电极真空隧道结,其转变电压的差异更为显著.具体而言,电极表面有一金字塔型铜纳米粒子的铜电极真空隧道结的转变电压值减小至1.70 V,而电极表面原子级突起为铜吸附原子的铜电极真空隧道结却因铜吸附原子4p轨道的局域态密度过于扩展,即使在偏压超过1.80 V时仍然没有出现转变电压.这些结果表明转变电压谱可用作分析金属电极真空隧道结电子输运特性的有力工具.     

非极性有机溶剂对分子内氢键弱化程度的单分子力谱定量研究

利用单分子力谱技术, 以3种不同醇解度的聚乙烯醇(PVA)为模型体系, 定量研究了非极性有机溶剂对分子内氢键强度的影响. 3种PVA样品在非极性有机溶剂中的分子内氢键强度均约为其在真空中强度的48%. 结果表明, 非极性有机溶剂对分子内氢键的弱化作用十分显著, 可削弱其约50%的本征氢键强度. 本文研究结果提示人们需重新审视液相环境对高分子单链弹性和非共价相互作用的影响.     

基于电化学方法研究以铜和银为电极的对苯二甲酸单分子结电导

利用基于电化学跳跃接触的扫描隧道显微镜裂结法(ECSTM-BJ), 通过现场形成金属电极, 对以Cu和Ag为电极的对苯二甲酸单分子结电导进行了测量. 研究结果表明: 利用该方法对所有数据直接线性统计即可得到很好结果; 两种电极下都存在两套高和低电导值, 其中以Cu为电极的单分子结电导高低值分别为11.5和4.0 nS, 而以Ag为电极的单分子结电导分别为10.3和3.8 nS, 高值都约为低值的3倍, 且以Cu为电极的单分子结电导要略大于以Ag为电极的电导, 可归结于电极和分子的耦合不同造成的. 与同样条件下测量得到的烷基链羧酸单分子结电导只存在一套值相比,对苯二甲酸表现出两套电导值, 反应了分子内主链对分子结电导的影响.     

环戊烷分子的电子动量谱

报导了在高分辨率电子动量谱仪上获得的环戊烷分子的结合能谱和动量谱的实验结果,并用Hartree-Fock方法和密度泛函方法做了理论计算.实验得到的环戊烷分子各电子轨道的电离能值与光电子谱得到的数据一致,动量分布的实验结果也与理论计算基本吻合.     

聚乙二醇在高真空条件下的单链弹性

在过去二十年间，高分子的单链弹性已经得到了广泛的研究.然而由于环境和高分子之间往往有着复杂的相互作用，实验中很难得到高分子在严格无扰状态下的单链弹性（即本征弹性）.为此，利用单分子力谱技术研究了高真空条件下聚乙二醇（PEG）的单链弹性.结果表明，由于高真空条件下溶剂分子的干扰被消除，PEG在这一准无扰状态下呈现其本征弹性.在非极性有机溶剂中，由于溶剂分子和PEG之间只有微弱的范德华力作用，PEG表现出和高真空中基本一致的弹性.然而，在不同环境中，力曲线的低力区（F<100 pN）存在着细微的差异.这一现象可归因于不同条件下基底与PEG链之间的吸附力不同.采用的高真空力谱可用于研究其他高分子单链在准无扰状态下的本征弹性.     

可描述硅橡胶弹性的理论模型

硅橡胶是一类应用广泛的弹性体材料.然而由于其复杂的三维网络结构,难以实现硅橡胶的理性设计.本研究尝试从单分子弹性入手来建立从硅橡胶微观力学性质到宏观性能之间的关联.首先,通过基于原子力显微镜的单分子力谱实验测量了甲基乙烯基硅橡胶中2个主成分(硅氧烷链和碳-碳链)的单链弹性(包含熵弹性和焓弹性).随后,利用量子力学计算得出上述2种分子链的理论单链弹性.硅氧烷链和碳-碳链的实验曲线均能与理论曲线很好地重合,表明已成功获取了这2种分子链在准无扰环境中的基准弹性.然后,2个主成分的基准弹性同时被整合到传统的橡胶统计学模型中.最终,通过参数可调的新橡胶弹性模型(称作TCQMG模型)描述了3种不同硅橡胶在整个形变范围内的力学性能.此外,借助TCQMG模型模拟了多个交联网络参数对于硅橡胶力学性能的影响,且模拟结果符合实验结果.该模型不仅有助于理解硅橡胶的复杂交联网络结构,还能够为新型硅橡胶的理性设计提供指导.考虑到硅橡胶与其他弹性体在交联网络结构上的相似之处,TCQMG模型有望用于描述这些弹性体的宏观力学性能.     

ANiN(A=Li,Na,Mg,Ca)的结构、热力学、弹性和电子性质的第一性原理研究

三元过渡金属氮化物ANiN(A=Li,Na,Mg,Ca)是潜在的可充放电池的电极材料.物理性质,比如热稳定性、电子能隙以及弹性稳定性等,对于这些材料的电池应用都是非常重要的.本文使用第一原理方法,对比研究了 ANiN这些材料的结构、动力学、弹性和电子结构性质.对状态方程和声子谱的计算被用来确定体系的稳定结构.对最稳定结...     

Electronic transportation through asymmetrically substituted oligo(phenylene ethynylene)s: studied by first principles nonequilibrium Green's function formalism

Theoretical investigations of a series of asymmetrically substituted conducting molecular wires [oligo(phenylene ethynylene)s] have been carried out using density functional theory and nonequilibrium Green's function formalism. To get the molecular rectification, the electron-donating group (-NH2) and the electron-withdrawing group (-NO2) are placed on the different positions of the molecular wire. The dependences of spatial distribution and lowest unoccupied molecular orbital (LUMO) energy level on the applied voltage have been found playing dominating but opposite roles in controlling the rectification behavior. In the tested bias range, since the shift LUMO energy level is more important, the electrons transfer more easily from donor to acceptor through the molecular junction in general.     

Inelastic electron tunneling spectroscopy and vibrational coupling

We discuss the relationship between the inelastic electron tunneling spectroscopy (IETS) and vibronic coupling constant within the Green's function formalism at a level of perturbation theory approximation. We also compare our results with experimental measurements. Our results can provide insights into the mechanism of active vibronic modes for IETS.     

Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold

We present results for a simulated inelastic electron-tunneling spectra (IETS) from calculations using the "gDFTB" code. The geometric and electronic structure is obtained from calculations using a local-basis density-functional scheme, and a nonequilibrium Green's function formalism is employed to deal with the transport aspects of the problem. The calculated spectrum of octanedithiol on gold(111) shows good agreement with experimental results and suggests further details in the assignment of such spectra. We show that some low-energy peaks, unassigned in the experimental spectrum, occur in a region where a number of molecular modes are predicted to be active, suggesting that these modes are the cause of the peaks rather than a matrix signal, as previously postulated. The simulations also reveal the qualitative nature of the processes dominating IETS. It is highly sensitive only to the vibrational motions that occur in the regions of the molecule where there is electron density in the low-voltage conduction channel. This result is illustrated with an examination of the predicted variation of IETS with binding site and alkane chain length.     

An efficient nonequilibrium Green's function formalism combined with density functional theory approach for calculating electron transport properties of molecular devices with quasi-one-dimensional electrodes

An efficient self-consistent approach combining the nonequilibrium Green's function formalism with density functional theory is developed to calculate electron transport properties of molecular devices with quasi-one-dimensional (1D) electrodes. Two problems associated with the low dimensionality of the 1D electrodes, i.e., the nonequilibrium state and the uncertain boundary conditions for the electrostatic potential, are circumvented by introducing the reflectionless boundary conditions at the electrode-contact interfaces and the zero electric field boundary conditions at the electrode-molecule interfaces. Three prototypical systems, respectively, an ideal ballistic conductor, a high resistance tunnel junction, and a molecular device, are investigated to illustrate the accuracy and efficiency of our approach.     

Effect of the continuity of the pi conjugation on the conductance of ruthenium-octene-ruthenium molecular junctions

The conductance of a family of ruthenium-octene-ruthenium molecular junctions with different pi conjugation are investigated using a fully self-consistent ab initio approach which combines the nonequilibrium Green's function formalism with density functional theory. Our calculations demonstrate that the continuity of the pi conjugation in the contact region as well as along the molecular backbone affects the junction conductance significantly, showing the advantage of using the ruthenium-carbon double bond as the linkage of conjugated organic molecules.     

First-principles study of length dependence of conductance in alkanedithiols

Electronic transport properties of alkanedithiols are calculated by a first-principles method based on density functional theory and nonequilibrium Green's function formalism. At small bias, the I-V characteristics are linear and the resistances conform to the Magoga's exponential law. The calculated length-dependent decay constant gamma which reflects the effect of internal molecular structure is in accordance with most experiments quantitatively. Also, the calculated effective contact resistance R(0) is in good agreement with the results of repeatedly measuring molecule-electrode junctions [B. Xu and N. Tao, Science 301, 1221 (2003)].     

An efficient molecular orbital approach for self-consistent calculations of molecular junctions

To model electron transport through a molecular junction, we propose an efficient method using an ab initio self-consistent nonequilibrium Green's function theory combined with density functional theory. We have adopted a model close to the extended molecule approach, due to its flexibility, but have improved on the problems relating to molecule-surface couplings and the long-range potential via a systematic procedure for the same ab initio level as that of Green's function. The resulting algorithm involves three main steps: (i) construction of the embedding potential; (ii) perturbation expansion of Green's function in the molecular orbital basis; and (iii) truncation of the molecular orbital space by separating it into inactive, active, and virtual spaces. The above procedures directly reduce the matrix size of Green's function for the self-consistent calculation step, and thus, the algorithm is suitable for application to large molecular systems.     

Analysis on the contribution of molecular orbitals to the conductance of molecular electronic devices

We present a theoretical approach which allows one to extract the orbital contribution to the conductance of molecular electronic devices. This is achieved by calculating the scattering wave functions after the Hamiltonian matrix of the extended molecule is obtained from a self-consistent calculation that combines the nonequilibrium Green's function formalism with density functional theory employing a finite basis of local atomic orbitals. As an example, the contribution of molecular orbitals to the conductance of a model system consisting of a 4,4-bipyridine molecule connected to two semi-infinite gold monatomic chains is explored, illustrating the capability of our approach.     

Functionality in single-molecule devices: model calculations and applications of the inelastic electron tunneling signal in molecular junctions

We analyze how functionality could be obtained within single-molecule devices by using a combination of non-equilibrium Green's functions and ab initio calculations to study the inelastic transport properties of single-molecule junctions. First, we apply a full non-equilibrium Green's function technique to a model system with electron-vibration coupling. We show that the features in the inelastic electron tunneling spectra (IETS) of the molecular junctions are virtually independent of the nature of the molecule-lead contacts. Since the contacts are not easily reproducible from one device to another, this is a very useful property. The IETS signal is much more robust versus modifications at the contacts and hence can be used to build functional nanodevices. Second, we consider a realistic model of a organic conjugated molecule. We use ab initio calculations to study how the vibronic properties of the molecule can be controlled by an external electric field which acts as a gate voltage. The control, through the gate voltage, of the vibron frequencies and (more importantly) of the electron-vibron coupling enables the construction of functionality: nonlinear amplification and/or switching is obtained from the IETS signal within a single-molecule device.     

First-principles calculation on the conductance of a single 1,4-diisocyanatobenzene molecule with single-walled carbon nanotubes as the electrodes

The conductance of a single 1,4-diisocyanatobenzene molecule sandwiched between two single-walled carbon nanotube (SWCNT) electrodes are studied using a fully self-consistent ab initio approach which combines nonequilibrium Green's function formalism with density functional theory calculations. Several metallic zigzag and armchair SWCNTs with different diameters are used as electrodes; dangling bonds at their open ends are terminated with hydrogen atoms. Within the energy range of a few eV of the Fermi energy, all the SWCNT electrodes couple strongly only with the frontier molecular orbitals that are related to nonlocal pi bonds. Although the chirality of SWCNT electrodes has significant influences on this coupling and thus the molecular conductance, the diameter of electrodes, the distance, and the torsion angle between electrodes have only minor influences on the conductance, showing the advantage of using SWCNTs as the electrodes for molecular electronic devices.