Charge Transport In Systems Research Articles

Impact of molecular conformation on transport and transport-related properties at the nanoscale

Among the various factors influencing the charge transport through molecular electronic devices, changes in molecular conformation are of interest also because they do not have counterpart in traditional electronics. Out of the multitude of issues that are relevant from this perspective, in the present work we discuss aspects relevant for molecules forming self-assembled monolayers (SAMs) adsorbed on metallic electrodes in the limiting cases of low coverage and high coverage, respectively. The molecules to be examined, which are very common work-horses of molecular electronics, are in both cases easily deformable (“floppy”) species. In the former case, the specific molecule to be considered is azurin, an archetype of soluble redox proteins containing metal ions. Biphenylene monothiol and 4-(4-pyridinyl)benzenethiol, the specific molecules chosen for the analysis of the second case, are representative for the class of molecules consisting of two aromatic (benzene or benzene derivative) rings that can easily rotate relative to each other. The possibility to quantitatively describe the charge transport as a one-step coherent tunneling mechanism is the main aspect that will be emphasized in the former case. This is important because the charge transport in systems where molecular conformation substantially changes is commonly assumed to be a two-step hopping process and described within flavors of Marcus type formulas, which turned out to be invalidated by the azurin transport data examined. For high coverage situations, we present new results providing further support to our recent finding on the possibility to strongly enhance the twisting with respect to the situation of isolated molecules. Furthermore, we demonstrate that this twisting enhancement is not caused by the charge transfer at the metal-molecule interface. Rather, it represents an unusual effect of spatial confinement; although driven by the substrate metal, it can be theoretically modeled within a metal-free approach.

Microscopic length scale of charge transport and structural properties of cobalt doped Ni–Zn ferrite nanocrystals: A structure property correlation study

Cobalt doped Ni–Zn ferrite nanoparticle (NZFC) were synthesized via citrate auto-ignition method to investigate the structural correlations with microscopic length scale of charge transport in system. Impedance spectroscopy revealed that both grain and grain boundary have combined effect on the conductivity relaxation process. Temperature dependent dc conductivity showed Arrhenius behavior for both grain interiors and grain boundaries. Dielectric relaxation mechanism has been elucidated via Harviliak-Negami (H.N.) formalism. Two characteristic length scales i.e. mean square displacement and spatial extent of the charge carriers inside the system were calculated via standard methods. Microstructural parameters such as bond length, bond angles, bond valence sums (BVS) etc. were estimated by Rietveld refinement of the X-Ray Diffraction (XRD) data. Changes in microstructural parameters followed the same trend as the characteristic length scales with the variation of doping content. Optimum transport properties were observed for 10 mol % of doping which were correlated with electron density plotting and BVS of different samples.

Theoretical study of the bridging effect on the charge carrier transport properties of cyclooctatetrathiophene and its derivatives

The bridging effect on the charge transport properties of cyclooctatetrathiophene and its derivatives (systems 1–4) was investigated at the level of density functional theory (DFT). Insights into their geometries, frontier molecular orbitals, reorganization energies, transfer integrals and band structures were provided in detail. Increasing charge mobilities for both holes and electrons were predicted in cyclooctatetrathiophene derivatives as the number of bridging sulfur atoms increased. The improved charge transport from system 1 to 4 can be interpreted from two contributions: (i) decreased reorganization energy with improved molecular planarity under the consideration of intermolecular interactions; and (ii) enhanced transfer integral derived from the π-stacking arrangement for 2 and 3, and multidimensional S⋯S interactions are found to contribute to charge transport in system 4 besides π⋯π interactions. The charge transport properties were also analyzed with a band-like model and the results were in agreement with those gained from the hopping model in the fact that the paths with large transfer integrals are all along the directions with large dispersions in the valence band or conduction band.

Transport in carbon nanotubes: Two-level SU(2) regime reveals subtle competition between Kondo and intermediate valence states

In this work, we use three different numerical techniques to study the charge transport properties of a system in the two-level SU(2) (2LSU2) regime, obtained from an SU(4) model Hamiltonian by introducing orbital mixing of the degenerate orbitals via coupling to the leads. SU(4) Kondo physics has been experimentally observed, and studied in detail, in Carbon Nanotube Quantum Dots. Adopting a two molecular orbital basis, the Hamiltonian is recast into a form where one of the molecular orbitals decouples from the charge reservoir, although still interacting capacitively with the other molecular orbital. This basis transformation explains in a clear way how the charge transport in this system turns from double- to single-channel when it transitions from the SU(4) to the 2LSU2 regime. The charge occupancy of these molecular orbitals displays gate-potential-dependent occupancy oscillations that arise from a competition between the Kondo and Intermediate Valence states. The determination of whether the Kondo or the Intermediate Valence state is more favorable, for a specific value of gate potential, is assessed by the definition of an energy scale $T_0$, which is calculated through DMRG. We speculate that the calculation of $T_0$ may provide experimentalists with a useful tool to analyze correlated charge transport in many other systems. For that, a current work is underway to improve the numerical accuracy of its DRMG calculation and explore different definitions.

Effect of Electrostatic Interactions and Dynamic Disorder on the Distance Dependence of Charge Transfer in Donor−Bridge−Acceptor Systems

Using a tight-binding model of charge transport in systems with static and dynamic disorder, we present a theoretical study of the positive charge transfer in molecular assemblies that involve a hole donor and an acceptor connected by fluorene and phenyl bridges. Two parameters that determine the rate of charge transfer within the proposed model are the charge transfer integral between neighboring units and the site energies. Fluctuations in the values of the charge transfer integral and the energy landscape for hole transport were calculated by taking into account variations of the dihedral angle between neighboring units and electrostatic interaction of positive charge moving along the bridge and the negative charge that remains on the hole donor. Analysis of the dynamics of hole transfer and the distribution of the positive charge during this process allows the conclusion that the rapid fall of the hole transfer rate coefficient observed in experiments with short bridges (three and four structural units for systems with fluorene and phenyl bridges, respectively) can be attributed to the electrostatic interaction. This interaction is responsible for the formation of the effective barrier between donor and acceptor with the height that increases as the number of structural bridge units remains less than 3 (fluorene bridge) or 4 (phenyl bridge). For longer bridges, however, the effective barrier changes only weakly and now the charge transport is mostly dominated by the fluctuation-assisted incoherent hole migration along the bridge. The latter mechanism exhibits much weaker dependence of the rate coefficient on the bridge length in agreement with the available experimental results.

Electrical, optical, and structural characterization of polymer blend (PVC/PMMA) electrolyte films

Ion-conducting solid polymer blend electrolytes based on polyvinyl chloride (PVC)/poly methyl methacrylate (PMMA) complexed with sodium perchlorate (NaClO4) were prepared in various concentrations by solution cast technique. The features of complexation of the electrolytes were studied by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. DC conductivity of the films was measured in the temperature range 303–398 K. Transference number measurements were carried out to investigate the nature of charge transport in the polymer blend electrolyte system. The electrical conductivity increased with increasing dopant concentration, which is attributed to the formation of charge transfer complexes. The polymer complexes exhibited Arrhenius type dependence of conductivity with temperature. In the temperature range studied, two regions with different activation energies were observed. Transference number data showed that the charge transport in this system is predominantly due to ions. Optical properties like absorption edge, direct band gap, and indirect band gap were estimated for pure and doped films from their optical absorption spectra in the wavelength region 200–600 nm. It was found that the energy gap and band edge values shifted to lower energies on doping with NaClO4 salt.

Ion transport and battery discharge characteristics of polymer electrolyte based on PEO complexed with NaFeF4 salt

Ion-conducting solvent-free solid polymer electrolytes based on polyethylene oxide (PEO) complexed with sodium ferric tetrafluoride (NaFeF4) were prepared using a solution casting technique. The complexation of the films was investigated through X-ray diffraction and Fourier transform infrared spectroscopic studies. Measurements of DC conductivity in the temperature range 300–370 K and the transference numbers were carried out to investigate the nature of charge transport in the polymer electrolyte system. Transference number data shows that the charge transport in this system is found to be predominantly due to ions. Using these polymer electrolytes, electrochemical cells were fabricated with the configuration of Na/(PEO+NaFeF4)/(I2+C+electrolyte). Various cell parameters, such as open circuit voltage, short circuit current, power density, and energy density of the device were evaluated and reported.