尾流效应对快速双原子分子离子在固体中电荷态及分子轴取向的影响

研究了快速双原子分子离子在固体中穿行时,尾流效应对各离子电荷态以及库仑爆炸过程的影响.借助于线性介电响应理论和局域介电函数,离子之间的动力学相互作用势可以表示成对称的屏蔽库仑势和非对称的尾势.通过对分子离子上所有束缚电子的总能量进行变分和求解单个离子的运动方程,自洽地确定出分子离子中每个离子的电荷态.数值结果表明,由于尾流效应的影响,在初始穿行阶段,分子离子中导航离子的电荷数随穿行深度的增加而单调递增,而尾随离子的电荷数则随穿行深度的增加而振荡.但当穿行深度很大时,两个离子的电荷数都趋于具有相同速度的孤立离子的电荷数.此外,还发现分子轴的取向朝入射速度方向偏转     

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.     

Effect of finite Coulomb interaction on full counting statistics of electronic transport through single-molecule magnet

We study the full counting statistics (FCS) in a single-molecule magnet (SMM) with finite Coulomb interaction U. For finite U the FCS, differing from U→∞, shows a symmetric gate-voltage-dependence when the coupling strengths with two electrodes are interchanged, which can be observed experimentally just by reversing the bias-voltage. Moreover, we find that the effect of finite U on shot noise depends on the internal level structure of the SMM and the coupling asymmetry of the SMM with two electrodes as well. When the coupling of the SMM with the incident-electrode is stronger than that with the outgoing-electrode, the super-Poissonian shot noise in the sequential tunneling regime appears under relatively small gate-voltage and relatively large finite U, and dose not for U→∞; while it occurs at relatively large gate-voltage for the opposite coupling case. The formation mechanism of super-Poissonian shot noise can be qualitatively attributed to the competition between fast and slow transport channels.     

Effect of molecular structure on fluid transport through carbon nanotubes

We perform molecular dynamics simulations to study the transport of geometrically modified water models through channels of carbon nanotube (CNT) membranes. We use two modifications to an existing water model (extended simple point charge SPC/E) as representative surrogates of molecular fluids: (1) bent model (model B) in which the HOH angle is varied while keeping the dipole moment constant by adjusting the OH bond length and (2) modified bent model (model MB) in which the HOH angle changes without any change in OH bond length thereby changing the dipole moment. Interestingly, we find that the fluid transport is a nonmonotonic function of the bond angle for both fluid models. This observed trend is not anticipated based on the fluid density as a function of the bond angle inside and outside of the nanotube channel. However, the average residence time of transmitted molecules through the channel provides an approximately inverse linear correlation with the observed flux, independent of the fluid model. Based on these correlations, we have developed an empirical design parameter connecting fluid transport through CNTs as a function of average occupancy (number of fluid molecules inside the nanotube) and the average residence time. Our results suggest that transport through carbon nanotubes can be sensitive to small changes in the structure of fluid molecules that can potentially be utilised for mixture separation.     

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.     

Effect of torsion angle on electronic transport through different anchoring groups in molecular junction

By applying nonequilibrium Green's function formalism combined with first-principles density functional theory, we investigate effect of torsion angle on electronic transport properties of 4,^4′-biphenyl molecule connected with different anchoring groups (dithiocarboxylate and thiol group) to Au(111) electrodes. The influence of the HOMO-LUMO gaps and the spatial distributions of molecular orbitals on the quantum transport through the molecular device are discussed. Theoretical results show that the torsion angle plays important role in conducting behavior of molecular devices. By changing the torsion angle between two phenyl rings, namely changing the magnitude of the intermolecular coupling effect, a different transport behavior can be observed in these two systems.     

Influences of space charge and electrode on the electrical transport through (Ba,Sr)TiO3 thin film capacitors

The combined model of thermionic emission and carrier drift-diffusion is derived to simulate the electrical transport through BST thin film capacitors. In the model the field-dependent permittivity is obtained from the derivative of the polarization distinguished with the traditional characterization. The simulated currents show the hysteresis. The influences of space charges and electrode materials on the current density-applied voltage characteristics have been studied. The simulation results suggest that the current densities can be greatly influenced by the space charges at the cathode interface and the barrier height at the electrode/BST interface. It is expected that this work can provide some useful guidelines to the design and performance improvement of BST thin film capacitors and other BST thin film devices.