Electronic transport properties of phenylacetylene molecular junctions

Electronic transport properties of a kind of phenylacetylene compound— (4-mercaptophenyl)-phenylacetylene are calculated by the first-principles method in the framework of density functional theory and the nonequilibrium Green's function formalism. The molecular junction shows an obvious rectifying behaviour at a bias voltage larger than 1.0 V. The rectification effect is attributed to the asymmetry of the interface contacts. Moreover, at a bias voltage larger than 2.0 V, which is not referred to in a relevant experiment [Fang L, Park J Y, Ma H, Jen A K Y and Salmeron M 2007 Langmuir 23 11522], we find a negative differential resistance phenomenon. The negative differential resistance effect may originate from the change of the delocalization degree of the molecular orbitals induced by the bias.     

Recursive approach to study transport properties of atomic wire

In this study, we propose a recursive approach to study the transport properties of atomic wires. It is based upon a real-space block-recursion technique with Landauer's formula being used to express the conductance as a scattering problem. To illustrate the method, we have applied it on a model system described by a single band tight-binding Hamiltonian. Results of our calculation therefore may be compared with the reported results on Na-atom wire. Upon tuning the tight-binding parameters, we can distinctly identify the controlling parameters responsible to decide the width as well as the phase of odd-even oscillations in the conductance.     

Percolation approach for atomic and molecular cluster formation

We apply a percolation approach for the theoretical analysis of mass spectra of molecular microclusters obtained by adiabatic expansion technique. The evolution of the shape of the experimental size distributions as function of stagnation pressure and stagnation temperature are theoretically reproduced by varying the percolation parameter. Remaining discrepancies between theory and experiment are discussed. In addition, the even-odd alternation as well as the “magic” shell structure within metallic, secondary ion mass spectra are investigated by introducing statistical weights for the cluster formation probabilities. Shell correction energies of atomic clusters as function of cluster size are deduced from the experimental data.     

Application of the time-dependent close-coupling approach to few-body atomic and molecular ionizing collisions

We review the recent progress made in applying the time-dependent close-coupling approach to ionizing collisions of electrons, photons, and ions with small atoms and molecules. The last twenty years have seen a proliferation of non-perturbative approaches applied to fundamental atomic and molecular scattering processes. Such processes form the building blocks of describing the dynamics of plasmas over a wide range of temperatures and densities, and also provide insight into the long-range Coulomb interactions between charged particles. Studies of the few-body Coulomb problem presented in electron, photon, or ion-impact ionization of small atoms and molecules, by direct solution of the time-dependent Schr?dinger equation, are particularly useful because the complicated three-body boundary conditions of more than one continuum particle in a Coulomb potential are not required. With the continuing growth and increasing availability of high-performance computing resources, such methods can now be applied to a wide variety of scattering processes. The recent progress made using such a time-dependent approach is described in this colloquium. In this paper, we focus on the recent results obtained for one-, two-, and three-electron systems, thus building on a previous review of the time-dependent close-coupling method [M.S. Pindzola et?al., J. Phys. B 40, R39 (2007)], which also described the application to multi-electron targets.     

An empirical approach to clustering based on atomic energies

We present an empirical scheme that relates the Ω parameter, a quantity important in the context of clustering, to atomic p-energies. This scheme allows the determination of hitherto experimentally unknown Ω values by extrapolating from the known ones.     

Electron transport properties of single molecular junctions under mechanical modulations

Electron transport behaviors of single molecular junctions are very sensitive to the atomic scale molecule-metal electrode contact interfaces, which have been difficult to control. We used a modified scanning probe microscope-break junction technique (SPM-BJT) to control the dynamics of the contacts and simultaneously monitor both the conductance and force. First, by fitting the measured data into a modified multiple tunneling barrier model, the static contact resistances, corresponding to the different contact conformations of single alkanedithiol and alkanediamine molecular junctions, were identified. Second, the changes of contact decay constant were measured under mechanical extensions of the molecular junctions, which helped to classify the different single molecular conductance sets into specific microscopic conformations of the molecule-electrode contacts. Third, by monitoring the changes of force and contact decay constant with the mechanical extensions, the changes of conductance were found to be caused by the changes of contact bond length and by the atomic reorganizations near the contact bond. This study provides a new insight into the understanding of the influences of contact conformations, especially the effect of changes of dynamic contact conformation on electron transport through single molecular junctions.     

溶液酸碱性对低聚苯亚乙炔基分子结电输运性质的影响

结合密度泛函理论和非平衡格林函数方法计算了溶液酸碱性对低聚苯亚乙炔基分子结电输运性质的影响,此低聚苯亚乙炔基分子中两个不同位置的H原子被氨基和羧基取代.通过质子化和去质子化模拟酸性溶液和碱性溶液对分子结构的影响.计算结果表明:中性环境下分子器件具有良好的导电性和微弱的整流效应;碱性溶液中羧基去质子化后,分子器件电流值增长近一倍,但整流效应变化不明显;酸性溶液中氨基质子化后,分子器件正向偏压导电性能略微降低,但整流方向发生明显反转,且与中性环境下的情况相比,整流比提高了近三倍.提出了一种利用化学手段控制分子结导电能力和整流性能的方法.     

An overview of atomic and molecular processes in critical velocityionization

The authors present an overview of the time development of some atomic and molecular processes in critical ionization velocity (CIV). In the preonset stage, metastable states play an important role: they provide an energy pooling mechanism allowing low-energy electrons to participate in the ionization process; they may explain the low energy threshold was well as the fast time scale in the onset of CIV. For a sustaining CIV to occur, Townsend's criterion has to be satisfied. The kinetic energies of the neutrals are transformed to plasma wave energies via beam-plasma instabilities, and the plasma waves that heat the electron result in a tail formation. Excitation of neutrals with subsequent radiation is an important energy loss mechanism. Finite size also limits the instability growth rate. In the propagation of CIV, ion-molecule reactions and molecular dissociative recombination are important     

Relativistic effects on atomic and molecular properties of the heaviest elements

Interaction of superheavy element 112 and its homolog Hg with inert and gold surfaces was studied on the basis of atomic and molecular fully-relativistic (4-component) DFT electronic structure calculations. Performance of additional non-relativistic calculations allowed one to demonstrate the role and magnitude of relativistic effects on adsorption energies and bond distances of the studied systems. For example, on quartz, element 112 will be stronger adsorbed than Hg by about 5 kJ/mol (or at 5 degrees higher temperatures) due to the stronger van der Waals interaction. This is caused by the relativistically contracted smallest atomic radius of element 112. Non-relativistically, the trend would be opposite. On surface of gold, element 112 will be about 20 kJ/mol weaker adsorbed than Hg (i.e., it will be deposited at about 100 degrees lower temperatures than Hg). Such a decrease in ΔH_ads comes at the account of the weaker interaction of the relativistically stabilized 7s_1/2(112) orbital with valence orbitals of gold. Still, the relatively large adsorption energy of element 112 is indicative that it is a transition metal forming intermetallic compounds with Au and other metals due to the involvement of the relativistically destabilized 6d orbitals. The influence of relativistic effects on the adsorption energy depends, however, on the adsorption position.     

An atomic diffraction theory of molecular photoionization cross sections

A theory of molecular photoionization cross sections is developed on the basis of locally atomic character of the one-electron final state in the Golden Rule expression for the molecular orbital cross section. Ionization amplitudes from several atomic centres are added and rotationally averaged to produce molecular orbital cross sections displaying a sum of pseudo-atomic cross sections weighted according to the LCAO composition of the orbital and also two-centre products reflecting interference effects. The atomic ionization amplitudes are obtained by use of an atomic central potential constructed by an inversion procedure from the form of the ground state orbital. The theory is of a simple chemical nature but usually of at least semi-quantitative accuracy. In this work we illustrate the nature of the two-centre interference effects in small diatomic molecules (H₂, HF, N₂).     

B_(80)分子结电子输运的第一性原理研究

我们采用第一性原理方法研究了由单个和两个B80分子构成的分子结的电子输运性质.研究发现,单个B80分子是一个很好的导体,平衡电导为2.28G0(G0为电导量子,G0=2e2/h),而由两个B80分子构成的二聚体则是一个绝缘体,电导仅为0.062G0.进一步的研究发现,二聚体的导电性可以通过电子掺杂来显著提高,如通过在每一个B80分子中用C替换一个B,电导可以提高到0.26G0.     

B80分子结电子输运的第一性原理研究

我们采用第一性原理方法研究了由单个和两个B80分子构成的分子结的电子输运性质。研究发现,单个B80分子是一个很好的导体,平衡电导为2.28G0（G0为电导量子,2e^2/h ）,而由两个B80分子构成的二聚体则是一个绝缘体,电导仅为0.062G0。进一步的研究发现,二聚体的导电性可以通过电子掺杂来显著提高,如通过在每一个B80分子中用C替换一个B,电导可以提高到0.26G0。     

低维原子/分子晶体材料的可控生长、物性调控和原理性应用

本文介绍了高鸿钧课题组在物理所20年来的部分代表性工作.研究的主要方向为低维纳米功能材料的分子束外延可控制备、生长机制、物性调控及其在未来信息技术中的原理性应用.从材料的可控制备入手,结合第一性原理的理论计算,阐明材料生长机制和结构与物性的关系,进而实现物性调控和原理性应用.主要内容有:1)纳米尺度"海马"分形结构的形成及其生长机制;2)STM分辨率的提高及最高分辨Si(111)-7×7原子图像的获得;3)固体表面上功能分子的吸附、组装及其机制;4)稳定、重复、可逆的纳米尺度电导转变与超高密度信息存储;5)固体表面上单分子自旋态的量子调控及其原理性应用;6)原子尺度上朗德g因子的空间分辨及其空间分布不均匀性的发现;7)晶圆尺寸、高质量、单晶石墨烯的制备及原位硅插层绝缘化;8)几种新型二维原子晶体材料的可控构筑及其物性调控;9)"自然图案化"的新型二维原子晶体材料及其功能化.这些工作为低维量子结构的构造、物性调控及其原理性应用奠定了基础.     

Switching properties of bi-OPE-monothiol molecular junctions: Substituent effects and improvement of open-close ratio

The electronic transport properties of a new kind of molecular switches-bi-OPE-monothiol molecular switches-were studied by applying first-principles calculations and generalized elastic scattering Green’s function.The numerical results show that,for a bi-OPE-molecule junction,the offset face-to-face configuration induces more delocalized molecular orbitals,and results in higher conductivity than the parallel face-to-face configuration,so it can be used as a molecular switch.The side substituent groups containing more delocalized electrons can strengthen the intermolecular coupling and raise the conductivities of bi-OPE-monothiol molecular devices.On the basis of the investigations,we find a scheme to enhance the open-close ratios of bimolecular switches.     

Feynman diagram approach to atomic collisions

A quantum field theoretic formulation of atomic collision phenomena involving non-relativistic free and bound systems is developed and a calculational procedure in terms of Feynman diagrams is prescribed. Matrix elements of several atomic collision processes have been calculated. In most cases standard quantum mechanical results are reproduced. But in some cases new terms appear in the scattering matrix whose contribution though negligibly small in the low energy region, become important at higher energies. A large portion of this work formed material for an invited talk delivered by one of us (T.P.) at the Second National Workshop on Atomic and Molecular Physics at Visva-Bharati, Santiniketan (India) held from 18–23 November 1979.     

Hyperspherical approach to three-electron atomic systems

A computational procedure for carrying out calculations on three-electron atomic systems in hyperspherical coordinates is presented. By adopting an adiabatic approximation, the solution of the Schrödinger equation at a fixed hyperradius is reduced to the solution of a set of coupled partial differential equations involving the two hyperangles. The derivation of such a set of coupled equations is explicitly illustrated.     

Conductance of small molecular junctions

A new method of fabricating small metal-molecule-metal junctions is developed, approaching the single-molecule limit. The conductance of different conjugated molecules in a broad temperature, source-drain, and gate voltage regime is reported. At low temperature, all investigated molecules display sharp conductance steps periodic in source-drain voltage. The position of these steps can be controlled by a gate potential. The spacing corresponds to the energy of the lowest molecular vibrations. These results show that the low-bias conductance of molecules is dominated by resonant tunneling through coupled electronic and vibration levels.     

A molecular quantum mechanics approach to evaluate molecular properties in liquid phase using statistical mechanics

A method for the calculation of molecular properties in liquid phase is proposed. The theoretical development links in a natural fashion Molecular Quantum Mechanics and Statistical Mechanics. The method can be applied to molecular liquids in thermodynamical equilibrium without strong interactions between molecules. After the Introduction (in which some fundamental ideas are discussed) the model is developed in its simplest form, giving a numerical application of interest for IR spectroscopy in liquid phase. Immediatly, this preliminar version is generalized for pure liquids taking into account the polarization effects on all the molecules of the sample. A numerical example on carbon disulphide shows the convergence of this method and its sensitivity to the structural changes in the liquid. In the last section, some future improvements are suggested and several topics related with the study of molecular liquids are considered.