First-principles study on current through a single π conjugate molecule for analysis of carrier injection through an organic/metal interface

We have studied current flow through a single π conjugate molecule weakly adsorbed to, and sandwiched between, two electrodes, focusing specifically on carrier injection through an organic/metal interface. This is the first calculation to investigate the effects of the orientation of the molecule and the electrode material on current using a first-principles method: in the past, most calculations of current were based on the assumption of covalent bonding of a molecule to electrodes. We modeled two systems in which a naphthalene molecule is sandwiched between gold (Au) or aluminum (Al) electrodes. First, in both systems, the current through the molecule depends on the orientation of the molecule. This indicates that electrons mainly transfer through the π channel, which is the overlap between the molecular π orbital and the electrode orbital. Next, the current in the Au-naphthalene-Au system is higher than that in the Al-naphthalene-Al system. This shows that Au is more suitable as an electrode material than Al. Therefore, the orientation of the molecule at an organic/metal interface and the materials comprising the electrodes play a key role in carrier injection through the interface.     

Giant switch-on and switch-off currents in device “electrode-molecule-electrode”

The transient current formation in a device “electrode-single molecule-electrode” is studied theoretically. It is demonstrated that a giant transient current is formed if the two transfer couplings of the molecule to the electrodes differ strongly and if the applied voltage is instantaneously switched on or off. This behavior is mainly caused by a recharge of the molecule due to nonequilibrium electron hopping processes from or into the electrodes.     

电场对1,4-苯二甲氰分子电输运特性的影响

用从头算理论和弹性散射格林函数的方法,计算电场作用下的1,4-苯二甲氰分子体系的电子结构及其电导和电流.结果表明:电场使体系在分子和电极间存在电荷转移和重新分布,分子与电极接触面附近出现电子积聚区和电子耗散区,从而产生附加电偶极子,对分子的电导和电流产生抑制.此外,对分子轨道能级和分子与电极的耦合系数的影响也比较明显,导致分子的伏-安特性在考虑电场作用前后有一定的差异.     

Peak and steady-state photocurrents in a molecular diode

It has been shown that the time evolution of a photocurrent under the conditions of the irradiation of a photochromic molecule by moderate-intensity light includes the fast and slow stages of the kinetic process. The fast stage corresponds to an increase in the current and is associated with the charging of the molecule, which is in a singlet excited state after a phototransition. The slow stage includes electron transfer between electrodes involving both charged and excited (singlet and triplet) states of the molecule. When the exchange interaction of unpaired electrons on the HOMO and LUMO levels of the photoexcited molecule is weak, the steady-state photocurrent is close in magnitude to the maximum transient current, whereas the steady-state current is suppressed when the exchange interaction is strong. The reason is that a molecule in the triplet state can block electron transfer between electrodes. The conditions have been found under which such blocking is a manifestation of the peak transient photocurrent that is formed immediately after the beginning of the irradiation of the molecule.     

Phonon effects in tunnelling through a double quantum dot molecule

Phonon effects in tunnelling through a double quantum dot molecule are investigated by use of a recently developed technique, which is based on an exact mapping of a many-body electron-phonon interaction problem onto a multichannel one-body problem. The molecule is sandwiched between two ideal electrodes and the electron at each dot of the molecule interacts independently with Einstein phonons. Single-electron transmission rates through the molecule are computed and the nonlinear spectrum obtained shows a structure with many more satellite peaks due to the excitations of phonons. The strength of resonant peaks is found to be strongly dependent on the number of excited phonons. The effects of electron-phonon interaction on the current and shot noise, depending on the voltage bias applied at the two electrodes as well as the potential energy of the molecule, are discussed.     

接触构型及电极距离对4,4’-二巯基二苯醚分子电输运性质的影响

利用从头计算方法和弹性散射格林函数的方法,对4,4'-二巯基二苯醚分子电输运特性的研究结果显示,分子与电极之间接触点的构型以及两电极之间的距离对4,4'-二巯基二苯醚分子的电输运性质都有很大影响.电流随电极距离的变化与耦合系数的变化存在着密切关系.分子末端硫原子处于金原子的顶位上时电流的开启电压很小,而处在金(111)面的空位上时约有1.0V左右电流禁区.与实验结果相比,硫原子更可能处在金(111)面的空位上方.     

Electron transport through single Mn12 molecular magnets

We report transport measurements through a single-molecule magnet, the Mn12 derivative [Mn12O12(O2C-C6H4-SAc)16(H2O)4], in a single-molecule transistor geometry. Thiol groups connect the molecule to gold electrodes that are fabricated by electromigration. Striking observations are regions of complete current suppression and excitations of negative differential conductance on the energy scale of the anisotropy barrier of the molecule. Transport calculations, taking into account the high-spin ground state and magnetic excitations of the molecule, reveal a blocking mechanism of the current involving nondegenerate spin multiplets.     

Driving current through single organic molecules

We investigate electronic transport through two types of conjugated molecules. Mechanically controlled break junctions are used to couple thiol end groups of single molecules to two gold electrodes. Current-voltage characteristics ( IVs) of the metal-molecule-metal system are observed. These IVs reproduce the spatial symmetry of the molecules with respect to the direction of current flow. We hereby unambiguously detect an intrinsic property of the molecule and are able to distinguish the influence of both the molecule and the contact to the metal electrodes on the transport properties of the compound system.     

基于石墨烯电极的Co-Salophene分子器件的自旋输运

采用基于非平衡格林函数结合第一性原理的密度泛函理论的计算方法,研究了基于锯齿型石墨纳米带电极的Co-Salophene分子器件的自旋极化输运性质.计算结果表明,当左右电极为平行自旋结构时,自旋向上的电流明显大于自旋向下的电流,自旋向下的电流在[-1V,1V]偏压下接近零,分子器件表现出优异的自旋过滤效应.与此同时,在自旋向上电流中发现负微分电阻效应.当左右电极为反平行自旋结构时,器件表现出双自旋过滤和双自旋分子整流效应.除此之外,整个分子器件还表现出较高的巨磁阻效应.通过分析器件的自旋极化透射谱、局域态密度、电极的能带结构和分子自洽投影哈密顿量,详细解释该分子器件表现出众多特性的内在机理.研究结果对设计多功能分子器件具有重要的借鉴意义.     

Quantum loop current in a C(60) molecular bridge

The existence of a quantum loop current in a C(60) molecular bridge is predicted using the Green's function method. The model for the molecular bridge consists of a C(60) molecule attached to one-dimensional conductive electrodes. It is shown that the loop current is related to the degeneracy of the energy levels of the C(60) molecule. Specific to this loop current is its magnitude which is much larger than that of the source-drain current. The associated magnetic moment also shows certain remarkable features such as its inversion with the energy across the molecular levels and the restriction of its direction onto a single plane.     

Automatic microcircuit formation based on gold-coated SU-8 microrods via dielectrophoresis

To explore the application of the characteristics of metallic microparticles, alternating current electric trapping of the SU-8 microrods coated with a thin gold layer by the chemical approach is investigated. Positive dielectrophoresis is used to absorb the gold-coated SU-8 microrods at the edge of the parallel electrodes, thereby forming chains to connect the electrodes. This is a fast automatic microcircuit formation process. Moreover, a non-charged molecule is modified on the surface of the gold-coated SU-8 microrod, and the modified microrods are controlled by the alternating electric field to form a number of chains. The different chains between the parallel electrodes consist of various parallel circuits. In order to compare these chains with different electric surfaces, the impedances of the metallic and modified microrods are measured and compared, and the results show that the gold-coated microrods act as pure resistors, while the microrods functionalized by a non-charged molecule behave as good capacitors.     

Vibrational excitation of a molecule by a resonance current

Correct expressions are obtained for calculating a tunnel-resonance current through molecules. The participation of molecular vibrations in the resonance charge transfer through a molecule and vibrational excitation of the molecule are determined by the reorganization energy E _r of the vibrational system depending on the displacement of the equilibrium position of vibrational modes in passing from the neutral molecule to the resonance state of a molecular ion. The mean excitation energy of the molecule during the propagation of an elementary charge changes from E _r at the voltage across electrodes close to the threshold up to 2E _r at voltages considerably exceeding the threshold voltage. An expression is obtained for the stationary vibrational temperature of the molecule, which is proportional to the resonance current.     

C20分子的一个电子输运模型研究

本文构建了Li链为电极的富勒烯C20分子的电子输运模型, 使用非平衡格林函数方法(Non-equilibrium Green's function, NEGF)对构建的Li电极和C20分子构成的分子器件进行了电子输运性质的计算. 计算得出了电子透射谱和电流电压曲线, 分析了产生这个分子器件电子输运性质的原因. 研究计算结果发现,大部分能量的电子是不会穿过器件的, 即使考虑不同偏压,能穿过器件的电子都集中在几个固定的能量上, 且在这几个固定能量上透过率比较高. 计算得出的伏安曲线说明在偏压为1.925V时电流达到最大, 只有在这个偏压周围才有相对明显的电流.     

Spin-dependent rectification in the C 59 N molecule

Coherent spin-dependent electron transport is investigated in three conditions: (1) a C₆₀ molecule is connected to two ferromagnetic (FM) electrodes symmetrically, (2) a C₅₉N molecule is connected to two FM electrodes symmetrically and (3) a C₅₉N molecule is connected to two FM electrodes asymmetrically. This work is based on a single-band tight-binding model Hamiltonian and the Green’s function approach with the Landauer–Buttiker formalism. Electrodes used in this study are semi-infinite FM electrodes with finite cross-section. Obvious rectification effect is observed in the C₅₉N molecule which is connected to the FMelectrodes asymmetrically. This effect is more in the P alignment of FM electrodes than in AP alignment of FM electrodes. This study indicates that the rectification behaviour is due to the asymmetry in molecule and junctions. Also in this investigation tunnel magnetoresistance (TMR) is calculated for these molecules. Asymmetry is observed in TMR of C₅₉N which is coupled to the electrodes asymmetrically due to asymmetric junctions, but TMR of C₆₀ is symmetric.     

Theory of inelastic electric current through single molecules

An electronic structure theory has been developed for an inelastic electric current of electron-intramolecular vibration coupling origin in terms of the Keldysh Green function method and the self-consistent Born approximation. Numerical calculations were made for the benzenedithiol molecule linking the two Au(111) electrodes. The calculations successfully reproduce typical features commonly observed in inelastic tunneling spectroscopy. The vibrational excitation energy due to the inelastic current was estimated. The inelastic electric current is quite important for the structural stability and the switching possibility of the molecular device.     

Au电极连接富勒烯C32分子的电子结构与传输特性

采用基于密度泛函理论(DFT)和非平衡格林函数(NEGF)的第一性原理方法对富勒烯C₃₂分子及在C₃₂分子的距离最远的两个碳原子处连接Au(1,1,1)电极的分子器件进行了电子结构和电子输运性质的研究.考虑到中间分子与Au电极间距离变化的情况,通过计算得出了在不同距离下分子器件的电子传输谱和I-V特性,分析了各器件的电子结构和电子输运特性产生的原因,并分析了电极与中间分子的连接距离及门电压对分子器件电子输运的影响.得出了电极与所连接的中间分子之     

Electrofluorescence polarity in a molecular diode

The kinetic equations describing the transmission of an electron in the molecular compound “electrode 1–molecule–electrode 2” (1M2 system) are derived using the method of a nonequilibrium density matrix. The steady-state transmission regime is considered, for which detailed analysis of the kinetics of electrofluorescence formation in systems with symmetric and asymmetric couplings between the molecule and the electrodes is carried out. It is shown that the optically active state of the molecule is formed as a result of electron hops between the molecule and each of the electrodes, as well as due to inelastic interelectrode tunneling of the electron. The electrofluorescence power for a molecular diode (asymmetric 1M2 system) depends on the polarity of the voltage bias applied to the electrodes. The polarity is explained using a model in which the optically active part of the molecule (chromophore group) is represented by the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Two mechanisms of the emergence of polarity are revealed. One mechanism is associated with nonidentical Stark shifts of the HOMO and LUMO levels relative to the Fermi levels of the electrodes. The second mechanism is associated with the fact that the rates of an electron hopping between HOMO (LUMO) and one of the electrodes are much higher than the rates of such a hopping with the other electrode. The conditions in which each mechanism can be implemented experimentally are indicated.     

Superexchange interaction enhancement of the quantum transport in a DNA-type molecule

We use the transfer matrix method and the Green function technique to theoretically study the quantum tunnelling through a DNA-type molecule. Ferromagnetic electrodes are used to produce the spin-polarized transmission probability and therefore the spin current. The distance-dependent crossover comes from the topological variation from the one-dimensional to the two-dimensional model transform as we switch on the interstrand coupling; a new base pair will present N-1 extrachannels for the charge and spin as N being the total base pairs. This will restrain the decay of the transmission and improve the stability of the quantum transport. The spin and charge transfer through the DNA-type molecule is consistent with the quantum tunneling barrier.     

Contact interference in molecular junctions

We highlight a new manifestation of quantum interference in transport through single-molecule junctions, owing to different charge transport pathways from the molecule through the contacts into the three-dimensional electrode. Such contact interference can be identified with characteristic features of the current–voltage curve, as dramatic as closing an otherwise open transport channel. Based on analysis of the necessary conditions for observation of this phenomenon, we conclude that this effect should be detectable in relatively small molecules, where a molecular site shares coupling to two electrodes, and an electrode surface site shares coupling to different molecular sites. The importance of the surface-molecule connectivity scheme implies that commonly used reduced models which couple different molecular sites to effectively independent electrodes, fail to account for this effect and must be corrected in order to properly capture contact interference.     

Electronic and thermoelectric properties of a single pyrene molecule

We propose a system containing a single pyrene molecule sandwiched between two metallic electrodes. The transport properties of the single pyrene molecule with four configurations are investigated using a steady-state theoretical model. We calculate the transmission probability and the electric current for the structure (1, 8), the structure (1, 7), the structure (1, 5) and the structure (1, 4). By applying a gate voltage on the pyrene molecule, we calculate the thermoelectric properties. The thermoelectric and electron transport properties can be controlled by quantum interference, the contact geometry and the gate voltage. The asymmetric behavior and the splitting of resonances in the transmission spectrum occur due to applying a gate voltage on the pyrene molecule. As a result, the structures (1, 5) and (1, 7) have a maximum value of the figure of merit reaching to 0.8 at the Fermi level. According to the results, the structures (1, 5) and (1, 7) can be act as promising thermoelectric applications in molecular electronics.