Stable π-π dependent electron conduction band of TPP[M(Pc)L2]2 molecular conductors (TPP = tetraphenylphosphonium; M = Co, Fe; Pc = phthalocyaninato; L = CN, Cl, Br)

The partially-oxidized TPP[M(Pc)L(2)](2) molecular conductors exhibit variable electronic and magnetic transport bulk materials properties due to central metal and axial ligand molecular modifications. The controllable electrical conductivity and giant negative magnetoresistance can be mainly attributable to the varying ligand field energy and physical bulkiness of the axial ligands which cause modulation in the intra-molecular π-d (Pc-M) and inter-molecular π-π (Pc-Pc) interactions in the TPP[M(Pc)L(2)](2) system, respectively. Characterization of the electronic conduction band utilizing one-dimensional (1-D) tight-binding approximation from infrared reflectance and thermoelectric power profile reveal consistent band widths of 0.43 eV-0.62 eV for the Co series (L = Br < Cl < CN) and 0.44-0.56 eV for the Fe series (L = Br < Cl < CN). The fixed band width suggests that stable electron conduction bands (transport pathway) can be constructed which can withstand the molecular π-d interaction modifications that severely alter the bulk electronic and magnetic materials properties of the TPP[M(Pc)L(2)](2) molecular conductors.     

Correlation between HOMO alignment and contact resistance in molecular junctions: aromatic thiols versus aromatic isocyanides

Understanding electron transport in metal-molecule-metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal-molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (-S) or isocyanide (-CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au-CN contact is more resistive than Au-S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyanide-linked molecules. By contrast, the difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance (i.e., the beta value = 0.5 A-1).     

Calculation of direct and indirect excitons in GaAs–Ga1−xAlxAs coupled double quantum wells: Electric and magnetic fields and hydrostatic pressure effects

A study of the electronic and optical properties of coupled double quantum wells is presented. Within the framework of the effective mass and parabolic-band approximations we have calculated the electron–hole and photoluminescence energy transitions under simultaneous effects of electric and magnetic fields. For that purpose, a variational procedure has been used, taking into account the effect of hydrostatic pressure. The electric field is taken to be oriented along the growth direction of the heterostructure whereas for the magnetic field both in-plane and in-growth directions have been considered. The results show that hydrostatic pressure is a useful tool to tune the direct and indirect exciton transitions in such heterostructures. It is shown that the photoluminescence peak energy transitions strongly depend on the external fields and hydrostatic pressure studied here. Furthermore, our numerical outcome is in good agreement with previous experimental findings at zero pressure in double quantum wells under applied electric and magnetic fields.     

Charge and energy dynamics in photo-excited poly(para-phenylenevinylene) systems

We report results from simulations of charge and energy dynamics in poly(para-phenylenevinylene) (PPV) and PPV interacting with C60. The simulations were performed by solving the time-dependent Schrodinger equation and the lattice equation of motion simultaneously and nonadiabatically. The electronic system and the coupling of the electrons to the lattice were described by an extended three-dimensional version of the Su-Schrieffer-Heeger model, which also included an external electric field. Electron and lattice dynamics following electronic excitations at different energies have been simulated. The effect of additional lattice energy was also included in the simulations. Our results show that both exciton diffusion and transitions from high to lower lying excitations are stimulated by increasing the lattice energy. Also field induced charge separation occurs faster if the lattice energy is increased. This separation process is highly nonadiabatic and involves a significant rearrangement of the electron distribution. In the case of PPV coupled to C60, we observe a spontaneous charge separation. The separation time is in this case limited by the local concentration of C60 molecules close to the PPV chain.     

Nanodrop of an Ising magnetic fluid on a solid surface

The density functional theory of inhomogeneous simple fluids is extended to an Ising magnetic fluid in contact with a solid surface, which is subjected to an external uniform or nonuniform magnetic field. The system is described by two coupled integral equations regarding the magnetic moment and fluid density distributions. The dependence of the contact angle that a nanodrop makes with the solid surface on the parameters involved in the magnetic interactions between the molecules of fluid and between the molecules of fluid and an external magnetic field is calculated. For the uniform magnetic field, the contact angle increases with increasing magnetic field, approaching an asymptotic value that depends on the strength of the fluid-fluid magnetic interactions. In the nonuniform field generated by a permanent magnet, the contact angle first increases with increasing magnetic field B(M) and then decreases, with the decrease being almost linear for large values of B(M). The obtained results are in qualitative agreement with the experimental data on the contact angle of magnetic drops on a solid surface available in the literature.     

Excitation of Electronic States of CO in Radio-Frequency Electric Field by Electron Impact

Electron impact excitation rate coefficients for singlet and triplet electronic states of the carbon monoxide molecule have been calculated under non-equilibrium conditions in the presence of radio-frequency electric field. A Monte Carlo simulation of electron transport has been performed in order to determine non-equilibrium electron energy distribution functions within one period of applied electric field. By using these distribution functions and corresponding cross sections, the excitation rate coefficients have been calculated for all electronic states of CO in the frequency range from 13.56 up to 500 MHz, at reduced root mean square electric field values ranging from 200 to 700 Td. We expect these rates to be valuable for modeling radio-frequency CO plasmas since excited neutrals exhibit greater chemical reactivity than neutrals in ground electronic state, hence altering many properties of plasma.     

Triplet-doublet electronic energy transfer from benzophenone to the ketyl radical: viscosity and magnetic field effects

The influence of solvent viscosity and an external magnetic field on the rate constant of electronic energy transfer from triplet bonzophenone to the ketyl radical is studied; it is concluded that the transfer is mediated by electron exchange.     

Inhomogeneous Electron Liquid in a Homogeneous Magnetic Field of Arbitrary Strength

Abstract

Our recent paper [Phys. Rev. A, 60, 2853 (1999)] on the field dependence of the energy of a molecule in an arbitrary magnetic field is extended here by results which can be expressed solely in terms of the total kinetic energy of the electron liquid of a molecule or an atom in a homogeneous magnetic field.     

Magnetoelectrochemistry of gold nanoparticle quantized capacitance charging

Magnetoelectrochemical studies of gold nanoparticle quantized capacitance charging were carried out at ambient conditions. The single electron transfer responses were found to be sensitive to external magnetic fields, reflected in the enhancement of voltammetric peak currents and shifts of peak formal potentials with increasing magnetic field intensities. Additionally, splittings of voltammetric peaks were also observed upon the application of an external magnetic field. These phenomena might be partly attributed to the paramagnetic characters (electron parity) of nanosized gold particles which are contingent upon their charge states. These novel observations suggest that the nanoparticle electronic energy structures can be varied by magnetic fields, leading to molecular manipulations of the nanoscale charge-transfer chemistry.     

Chemisorption-induced spin symmetry breaking in gold clusters and the onset of paramagnetism in capped gold nanoparticles

We present a simple model to describe the induction of magnetic behavior on gold clusters upon chemisorption of one organic molecule with different chemical linkers. In particular, we address the problem of stability of the lowest lying singlet and show that for some linkers there exists a spin symmetry-breaking that lowers the energy and leads to preferential spin density localization on the gold atoms neighboring the chemisorption site. The model is basically an adaptation of the Stoner model for itinerant electron ferromagnetism to finite clusters and it may have important implications for our understanding of surface magnetism in larger nanosystems and its relevance to electronic transport in electrode-molecule interfaces.     

Aligned carbon nanostructures based 3D electrodes for energy storage

Electrochemical energy storage systems with high specific energy and power as well as long cyclic stability attract increasing attention in new energy technologies. The principles for rational design of electrodes are discussed to reduce the activation, concentration, and resistance overpotentials and improve the active material efficiency in order to simultaneously achieve high specific energy and power. Three dimensional(3D)nanocomposites are currently considered as promising electrode materials due to their large surface area,reduced electronic and ionic diffusion distances, and synergistic effects. This paper reviews the most recent progress on the synthesis and application of 3D thin film nanoelectrode arrays based on aligned carbon nanotubes(ACNTs) directly grown on metal foils for energy storages and special attentions are paid on our own representative works. These novel 3D nanoelectrode arrays on metal foil exhibit improved electrochemical performances in terms of specific energy, specific power and cyclic stability due to their unique structures.In this active materials coated ACNTs over conductive substrate structures, each component is tailored to address a different demand. The electrochemical active material is used to store energy, while the ACNTs are employed to provide a large surface area to support the active material and nanocable arrays to facilitate the electron transport. The thin film of active materials can not only reduce ion transport resistance by shortening the diffusion length but also make the film elastic enough to tolerate significant volume changes during charge and discharge cycles. The conductive substrate is used as the current collector and the direct contact of the ACNT arrays with the substrate reduces significantly the contact resistance. The principles obtained from ACNT based electrodes are extended to aligned graphene based electrodes. Similar improvements have been achieved which confirms the reliability of the principles obtained. In addition, we also discuss and view the ongoing trends in development of aligned carbon nanostructures based electrodes for energy storage.     

Geometric Effects on Conductance in Single Molecule Electron Transport Junctions

We studied electron transport properties of a dithiol‐benzene molecule covalently bonded between two gold electrodes by combining ab initio calculations for the central molecule and a green function method to calculate electron transport. Due to the large computational demand, this type of calculations usually involves certain ways of simplification. The simplification commonly used is to fix the contact surface of the electrodes by ignoring the disturbance of the Au contact surface by contacting with the central molecule, i.e. without scattering region relaxation. In this study, we intended to resolve the difference between models with and without the above simplification. The large conductance found in our models without scattering region relaxation is due to the highly symmetric arrangement of the Au contact surface and those layers near the contact. The disturbance of the Au contact surface by the contact of the central molecule is important since the increase of the Au‐S bond and the distortion of the Au atom on the FCC site can lower the transmission coefficient between the two electrodes. In order to obtain better results, the model should include scattering region relaxation. However, when such relaxation is not applicable or demands too much calculation resource, the center molecule of the electronic transport junction should be at least optimized by the calculation level including electronic correlation, i.e. post‐HF methods.     

Theoretical analysis of electron transport through organic molecules

We present a theoretical study of electron transport through a variety of organic molecules. The analysis uses the Landauer formalism in combination with complex bandstructure and projected densities of states calculations to reveal the main aspects of coherent electronic transport through alkanes, benzene-dithiol, and phenylene-ethynylene oligomers. We examine the dependence of the current on molecule length, the effects of molecule-molecule interactions from film packing, differences in contact geometry, and the influence of phenyl ring rotation on the conductances of phenylene-ethynylene oligomers such as 1,4-bis-phenylethynyl-benzene.     

From aromaticity to self-organized criticality in graphene

The unique properties of graphene are rooted in its peculiar electronic structure where effects of electron delocalization are pivotal. We show that the traditional view of delocalization as formation of a local or global aromatic bonding framework has to be expanded in this case. A modification of the π-electron system of a finite-size graphene substrate results in a scale-invariant response in the relaxation of interatomic distances and reveals self-organized criticality as a mode of delocalized bonding. Graphene is shown to belong to a diverse class of finite-size extended systems with simple local interactions where complexity emerges spontaneously under very general conditions that can be a critical factor controlling observable properties such as chemical activity, electron transport, and spin-polarization.     

Substitutional doping of symmetrical small fullerene dimers

Magnetic carbon nano-structures have potential applications in the field of spintronics as they exhibit valuable magnetic properties. Symmetrically sized small fullerene dimers are substitutional doped with nitrogen (electron rich) and boron (electron deficient) atoms to visualize the effect on their magnetic properties. Interaction energies suggests that the resultant dimer structures are energetically favorable and hence can be formed experimentally. There is significant change in the total magnetic moment of dimers of the order of 0.5 μ_B after the substitution of C atoms with N and B, which can also be seen in the change of density of states. The HOMO-LUMO gaps of spin up and spin down electronic states have finite energy difference which confirm their magnetic behaviour, whereas for non-magnetic doped dimers, the HOMO-LUMO gaps for spin up and down states are degenerate. The optical properties show that the dimers behave as optical semiconductors and are useful in optoelectronic devices. The induced magnetism in these dimers makes them fascinating nanocarbon magnetic materials.     

Lagrange multiplier based transport theory for quantum wires

We discuss how the Lagrange multiplier method of nonequilibrium steady state statistical mechanics can be applied to describe the electronic transport in a quantum wire. We describe the theoretical scheme using a tight-binding model. The Hamiltonian of the wire is extended via a Lagrange multiplier to "open" the quantum system and to drive current through it. The diagonalization of the extended Hamiltonian yields the transport properties of wire. We show that the Lagrange multiplier method is equivalent to the Landauer approach within the considered model.     

卟啉-硫醚基共价有机框架材料用于氧还原反应电催化剂

氧还原反应（ORR）是能进行能量存储的核心电化学过程。由于它的动力学速率缓慢,因此亟需制备出高活性的电催化剂来促进这一反应的速率。二维共价有机框架材料（2D COFs）的π-π堆积结构可赋予骨架高导电率,并且一维有序的孔道有利于促进中间反应体传输。因此,其在可再生能源领域中具有良好的应用前景,并有望作为能量存储与转化的强大催化平台。本文通过向2D COFs中引入金属卟啉单元及硫醚单元成功制备了两个2D COFs （JUC-600和JUC-601）。通过多种表征手段证明,这两个2D COFs均具有AA堆积的sql拓扑结构。通过电化学测试表明,Co²⁺配位的JUC-601具有更正的ORR起始电势（0.825 V）和半波电势（0.7 V）、更高的活性表面积（7.8 mF/cm²）,更低的Tafel斜率（58 mV/dec）。这主要是由于JUC-601的高比表面积和高孔隙率使得中间产物能更易在COFs的表面和孔道中接触和传输。此外,Co²⁺-卟啉单元以及硫醚单元的存在使其骨架整体的电子结构发生了变化,更有利于电子转移。这一工作不仅开发了新的二维卟啉-硫醚基COFs材料,同时也拓展了2D COFs材料在电催化领域的应用。     

Electronic structure and conductance of large molecules and DNA

Using a new technique based on embedding in a local orbital formalism, the electronic structure and electron transmission properties of long biological molecules are calculated, in particular DNA. The electronic structure is found by adding one structural unit at a time to the molecule, and calculating an embedding potential for adding the next structural unit. At present, an extended Hückel scheme is used for the Hamiltonian. The transmission is also calculated within the embedding scheme, taking the molecule–metal contacts into account. The results for transmission depend greatly on the orbitals to which contact is made, and also on energy. The implications of these calculations for conductance are discussed.