Electronic Transport in Molecular Junction Based on C20 Cages

Choosing closed-ended armchair （5, 5） single-wall carbon nanotubes （CCNTs） as electrodes, we investigate the electron transport properties across an all-carbon molecular junction consisting of C20 molecules suspended between two semi-infinite carbon nanotubes. It is shown that the conductances are quite sensitive to the number of C20 molecules between electrodes for both configuration CF1 and double-bonded models： the conductances of C20 dimers are markedly smaller than those of monomers. The physics is that incident electrons easily pass the C20 molecules and are predominantly scattered at the C20-C20 junctions. Moreover, we study the doping effect of such molecular junction by doping nitrogen atoms substitutionally. The bonding property of the molecular junction with configuration CF1 has been analysed by calculating the Mulliken atomic charges. Our results have revealed that the C atoms in N-doped junctions are more ionic than those in pure-carbon ones, leading to the fact that N-doped junctions have relatively large conductance.     

Classical simulation of atomic beam focusing and deposition for atom lithography

We start from the intensity distribution of a standing wave (SW) laser field and deduce the classical equation of atomic motion.The image distortion is analyzed using transfer function approach. Atomic flux density distribution as a function of propagation distance is calculated based on Monte-Carlo scheme and trajectory tracing method.Simulation results have shown that source imperfection,especially beam spread, plays an important role in broadening the feature width,and the focus depth of atom lens for real atomic source is longer than that for perfect source.The ideal focal plane can be easily determined by the variation of atomic density at the minimal potential of the laser field as a function of traveling distance.     

Simulation of Inelastic Electron Tunnelling Spectroscopy on Different Contact Structures in 4,4＇-Biphenyldithiol Molecular Junctions

A first-principles computational method is developed to study the inelastic electron tunnelling spectroscopy （IETS） of 4,4＇-biphenyldithiol molecular junction with three different contact structures between the molecule and electrodes in the nonresonant regime. The obtained distinct IETS can be used to resolve the geometrical structure of the molecular junction. The computational results demonstrate that the IETS has certain selection rule for vibrational modes, where the longitudinal modes with the same direction as the tunnelling current have greatest contribution to the IETS. The thermal effect on the IETS is also displayed.     

Spatial distribution of electron characteristic in argon rf glow discharges

The spatial distributions of the electron density and the mean electron energy of argon radio frequency （rf） glow discharge plasma in a plasma-enhanced chemical vapour deposition （PECVD） system have been investigated using an established movable Langmuir probe. The results indicate that in the axial direction the electron density tends to peak at midway between the two electrodes while the axial variation trend of mean electron energy is different from that of the electron density, the mean electron energy is high near the electrodes. And the mean electron energy near the cathode is much higher than that near the anode. This article focuses on the radial distribution of electron density and mean electron energy. A proposed theoretical model distribution agrees well with the experimental one： the electron density and the mean electron energy both increase from the centre of the glow to the edge of electrodes. This is useful for better understanding the discharge mechanism and searching for a better deposition condition to improve thin film quality.     

Influence of Interface Charge on Electrical Properties of ZnO／Si Heterojunction

The capacitance-voltage and current-voltage properties of the samples before and after annealing are investigated at 300 K and 195K. It is found that the interface charge plays an important role for the heterojunction properties. After annealing, the samples exhibit typical junction properties. The heterojunction shows a built-in potential 0.62eV consistent with the theoretical result 0.67eV. However, the sample exhibits more complex behaviour before annealing. The experimental results can be explained by heterojunction theory when, introducing interface charge, which suggests that the annealing can reduce interface charge and can improve the junction properties of the samples.     

The effect of electric charge on the mechanical properties of graphene

The effect of electric charge on the mechanical properties of graphene under tensile loading is investigated by using molecular dynamics method.A modified atomistic moment method based on the classical electrostatics theory is proposed to obtain the distribution of extra charges induced by an external electric field and net electric charges stored in graphene.The electrostatic interactions between charged atoms are calculated using the coulomb law.The results show that the Young’s modulus and the critical fracture stress under uniaxial tension decrease with the increase of electric potential and net charges on graphene.The failure of graphene induced by electric charges is found to be controlled by charge level.The results indicate that the carbon-carbon bonds at the edge of graphene will break first.     

Charge and spin-dependent thermal efficiency of polythiophene molecular junction in presence of dephasing

The charge and spin-dependent thermoelectric properties of different lengths of polythiophene in a molecular junction are investigated using the B ¨uttiker probe method within Green function formalism in linear response regime. The coupling of the molecular chain to three-dimensional ferromagnetic electrodes is described by a tight-binding model for both parallel and antiparallel spin configurations. The decrease of height of transmission probability peaks and thermoelectric coefficients are observed in the presence of the B ¨uttiker probes. The reduction is more intensive in the strong dephased chains.Results show that the spin magnetothermopower is bigger than the charge magnetothermopower due to the larger difference between the spin thermopowers with respect to the charge ones. In addition, we observed that the kind of carriers participating in the thermoelectric transport depends on the number of the thiophene rings.     

Study on the mixing of fluid in curved microchannels with heterogeneous surface potentials

In this paper the mixing of a sample in the curved microchannel with heterogeneous surface potentials is analysed numerically by using the control-volume-based finite difference method. The rigorous models for describing the wall potential and external potential are solved to get the distribution of wall potential and external potential, then momentum equation is solved to get the fully developed flow field. Finally the mass transport equation is solved to get the concentration field. The results show that the curved microchannel has an optimized capability of sample mixing and transport when the heterogeneous surface is located at the left conjunction between the curved part and straight part. The variation of heterogeneous surface potential ψn has more influence on the capability of sample mixing than on that of sample transport. The ratio of the curved microchanners radius to width has a comparable effect on the capability of sample mixing and transport. The conclusions above are helpful to the optimization of the design of microfluidic devices for the improvement of the efficiency of sample mixing.     

Electromagnetic dissociation of 3.7 A GeV^16O in nuclear emulsion

The electromagnetic dissociation (ED) of 3.7 A GeV {}^{16}O in nuclear emulsion is investigated with high statistics. It is found that the electromagnetically dissociated cross section increases with increasing beam energy, the charge distribution of projectile fragments is the same as the results at 60 and 200 A GeV, and the production probability of projectile fragments with charge 3≤Z≤5 is less than that of the other projectile fragments. These results can be well explained by use of Weizsacker and Williams method for calculating the ED contributions. The percentile abundance of various decay modes for ED at 3.7 A GeV is close to the result at 60 and 200 A GeV, but it is different from the result at 14.6 A GeV. The ED of 3.7 A GeV is mainly caused by the giant dipole and quadrupole resonance of E1 and E2, which can be qualitatively explained by the multiplicity distribution of projectile proton in ED. The multiplicity distribution of the α fragments in ED and nuclear events have different functional forms. This difference may be a consequence of the different reaction mechanism involved.     

Electric Field Analysis of Space Charge Injection from a Conductive Nano-Filler Electrode

A simulation on the electric field distribution near the electrode is proposed to explain the reason for using nanosized carbon black mixed with ethylene vinyl acetate, as the electrode could lead to more charge injection into the polymer than using a deposited metal electrode. The electrode is simplified to a layer of conductive semi-spheres with fixed size and constant electric potential. By using the finite element method, it is found that both the size of the semi-spheres and the distance between adjacent semi-spheres could dramatically influence the electric field near the surface of the spheres; these are considered to be the two decisive factors for the charge injecting rate at electrodes of various materials.     

Reduced charge transfer exciton recombination in organic semiconductor heterojunctions by molecular doping

We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer-fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ~70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states.     

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.     

On the calculation of the magnetoresistance of tunnel junctions with parallel paths of conduction

At the interfaces between the metallic electrodes and barrier in magnetic tunnel junctions it is possible for localized states to form which are orthogonal to the itinerant states for the junction, as well as resonant states that can form at the interfaces. These states form highly conducting paths across the barrier when their orbitals point directly into the barrier; these paths are in addition to those formed by the itinerant states across the entire junction. Most calculations of transport in magnetic tunnel junctions are made with the assumptions that the transverse momentum of the tunnelling electrons is conserved, in which case the itinerant electron states remain orthogonal to localized states. In principle it is possible to include diffuse scattering in both the bulk of the electrodes and the barrier so that the transverse momentum is not conserved, as well as the processes that couple localized states at the electrode-barrier interface to the itinerant states in the bulk of the electrodes. However, including these effects leads to lengthy calculations. Therefore, to assess the conduction across the barrier through the localized states that exist in parallel to the itinerant states we propose an approximate scheme in which we calculate the conductance of only the barrier region. While we do not take explicit account of either of the effects mentioned above, we do calculate the tunnelling through all the states that exist at the electrode-barrier interfaces by placing reservoirs directly across the barriers. To calculate the current and magnetoresistance for magnetic tunnel junctions (the junction magnetoresistance (JMR)) we have used the lattice model developed by Caroli et al. The propagators, density of states and hopping integrals entering the expressions for the current are determined by using the spin polarized scalar-relativistic screened Korringa-Kohn-Rostoker method that has been adapted to layered structures. By using vacuum as the insulating barrier we have determined with no adjustable parameters the JMR in the linear response region of tunnel junctions with fcc Co(100), fccNi(100) and bcc Fe(100) as electrodes. The JMR ratios that we find for these metal/vacuum/metal junctions are comparable with those measured with alumina as the insulating barrier. For vacuum barriers we find that tunnelling currents have minority- spin polarization whereas the tunnelling currents for th se electrodes have been observed to be positively (majority) polarized for alumina barriers and minority polarized for SrTiO 3 barriers. In addition to determining the JMR ratios in linear response we have also determined how the magnetoresistance of magnetic tunnel junctions varies with a finite voltage bias applied across the junction. In particular we have found how the shape of the potential barrier is altered by the applied bias and how this affects the current. Comparisons with data as they become available will eventually determine whether our approximate scheme or the ballistic Landauer-Büttiker approach is better able to represent the features of the electronic structure that control tunnelling in magnetic tunnel junctions.     

Tunneling through a Metal/Polymer Interface Containing a Thin Oxide Layer: Discussion of the Consequences of Oxide Presence on Charge Injection

The charge injection from oxidized metal electrodes into conjugated polymer thin films is calculated and compared with ideal metal/polymer interfaces. The presence of the oxide layer has a major influence on the magnitude of the tunneling current as well as on its behavior with applied voltage. It was also found that the fitting of the simple Fowler-Nordheim model for triangular barriers to the obtained current-voltage data from the oxidized interfaces leads to incorrect values of the potential barrier height at the metal/polymer interface. Prediction and its consequences on charge injection control in organic devices are discussed.     

Impedance spectrum of a single grain boundary in yttrium stabilized zirconia

Impedance measurements are reported for a bicrystal and single crystals of yttrium-stabilized ZrO₂ (YSZ) over the range from 100 to 10⁷ Hz, and for temperatures from 200 to 500°C in air. In addition to the impedances introduced by the conduction process within the grains and by charge transfer process at the electrodes, the grain-boundary introduced an additional impedance which was observed as an additional arc when the impedance was plotted in the complex plane. These data and an examination by both optical and scanning electron microscopy reveal the grain boundary to be a gap between the adjacent crystals, with occasional bridges of YSZ. These results illustrate the potential of impedance spectroscopy for studying intercrystalline interfaces in solid conductors.     

分子结电学特性的理论研究

在第一性原理的基础上，对共扼分子2-氨基-5-硝基-1，4-二乙炔基-4’，-苯硫醇基苯(2-amino-5-nitro-1,4-diethyny-4’-benzenethiol-benzene)与金表面形成的分子结的电学特性进行了理论研究.利用密度泛函理论计算了该分子及扩展分子的电子结构；讨论了分子与金表面的相互作用，定量地确定了耦合常数，求出了电子的迁移强度；利用弹性散射格林函数法研究了该分子结的伏—安特性.计算结果表明，当外加偏压小于0.9V时分子结存在电流禁区，随着偏压升高，分子结的电导出现平台特 关键词： 化学吸附 分子结 分子电子学     

High probability of single molecule junction formation with Ag electrodes

We use a scanning tunneling microscope-based break-junction technique to compare the probability of the formation of a single-molecule junction for a series of amine-terminated oligophenyl and alkane species using either Ag or Au as electrodes. For all molecules, we find that there is a significantly higher probability of junction formation when using Ag electrodes than with Au electrodes. We also find that longer molecules have a higher probability than shorter molecules to form a junction for both Ag and Au electrodes. For all molecules, the measured molecular junction length that is formed with the Ag electrodes was longer than that formed with Au electrodes. Furthermore, we can make a single atomic oxygen junction and can measure its conductance using Ag electrodes. These observations are attributed to a narrower gap of the Ag electrodes compared to that of the Au electrodes after the metal contact ruptures. Since there is a high probability of a molecular junction forming when using Ag electrodes, we can therefore perform a statistical analysis within the context of the material properties that are suitable for future molecular electronics.     

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     

Voltage induced intensity changes in surface raman bands from a 2-chromophore probe, 4-benzyl pyridine adsorbed on roughened silver electrodes and their variation with excitation frequency

Voltage induced intensity changes in the surface enhanced Raman bands of 4-benzyl pyridine adsorbed on variously roughened silver electrode surfaces peak at different voltages for different excitation wavelengths. This behaviour, which is known to occur for pyridine and the picolines, is shown to occur for both the pyridyl and benzyl residues, though benzene itself does not exhibit enhancement under the same conditions. Results obtained on conventionally prepared, anodised electrodes are compared with those from novel magic array electrodes and results are consistent with the hypothesis that these latter electrodes are more suited for Raman studies of adsorbates in electrochemical systems. This is because they do not have the excess quantities of surface complexes present on them: these are probably responsible for much of the signal from the anodised electrodes. Explanations for the excitation frequency dependence of the voltage/intensity curves are discussed in relation to electrode specific effects as well as to charge transfer.