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The electron transport property of alkanedithiol molecules was investigated using conducting atomic force microscopy and first-principles calculations. The measured Current-voltage (I-V) curves show sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, β, is found to be approximately 1.15 per carbon atom (C−1) and is independent of applied bias (over a voltage range of ±1.0 V). The measured I-V characteristics of alkanedithiols are in excellent qualitative agreement with the results from first-principles calculations. The β value (0.97 per carbon atom) obtained from first-principles calculations is also independent of applied bias but slightly smaller than that from AFM measurement. The electron transmission spectra of alkanedithiol junctions are also analyzed to gain more insight into the length-dependent I-V curves. [ABSTRACT FROM AUTHOR]

Abstract: The nonequilibrium Green’s function approach in combination with density-functional theory is used to perform ab initio quantum-mechanical calculations of the electron transport properties of polyacetylene, polythiophene, poly(phenylene vinylene), poly(p-phenylene ethynylene), and poly(p-phenylene) molecules sandwiched between two gold electrodes. The results demonstrate that the conjugation path has a profound effect on the electron transport property of the molecular wires. Among the five molecular wires, polyacetylene is the most conductive one. The conductivities of the five molecular wires decrease with an order of polyacetylene>polythiophene>poly(phenylene vinylene)>poly(p-phenylene ethynylene)>poly(p-phenylene). The conductivities of polyacetylene and polythiophene are much higher than those of poly(phenylene vinylene), poly(p-phenylene ethynylene), and poly(p-phenylene). The difference of electron transport behaviors of these molecular wires are analyzed in terms of the electronic structures, the transmission spectra, and the spatial distributions of molecular orbitals. [Copyright &y& Elsevier]

Abstract: The effect of molecular orientation on the electron transport behavior of single porphyrin sandwiched between two gold (111) electrodes is investigated by density functional theory calculations combined with non-equilibrium Green’s function method. The results show that the porphyrin with parallel connection to gold (111) electrodes is more conductive than the porphyrin with diagonal connection to gold (111) electrodes. The mechanism of the difference of electron transport for these two molecular junctions is analyzed from the transmission spectra and the molecular projected self-consistent Hamiltonian states. It is found that the intrinsic nature of the molecule, such as the π-conjugated framework and the strength of molecule–electrode coupling, are the essential reason for generating this difference of electron transport for the two molecular systems. [Copyright &y& Elsevier]

Abstract: The electron transport properties of cis-polyacetylene and cis-polyacetylene based molecular wires (oligo(cyclopentadiene), oligo(pyrrole), and oligo(furan)) have been studied theoretically using a combination of density-functional theory and non-equilibrium Green′s functions method. The results demonstrate that the introduction of bridging group X (X=CH2, NH, and O) in cis-polyacetylene has a profound effect on the electron transport behavior of the molecules. The conductance of the four molecular wires decreases in the order of polyacetylene>oligo(cyclopentadiene)>oligo(furan)>oligo(pyrrole). In particular, the conductances of oligo(furan) and oligo(pyrrole) are much lower than those of polyacetylene and oligo(cyclopentadiene). The mechanism of this difference of electron transport properties of these four molecular systems is analyzed in terms of their geometric structures, electronic structures, transmission spectra, and spatial distribution of frontier orbitals. It is found that the energy levels of frontier molecular orbitals and the evolution of spatial distribution of frontier molecular orbitals with the applied bias are the essential reason for generating this difference of electron transport behaviors of the four molecular systems. [Copyright &y& Elsevier]

The electron transport behaviors of oxygen and carbon monoxide complexes of Fe-porphyrin (FeP) are investigated by nonequilibrium Green's function techniques combined with Density Functional Theory calculations. The results show that the molecular current of FeP decreases dramatically after adsorptions of oxygen and carbon monoxide. The molecular current decreases with a order of FeP > FeP+O2 complex > FeP+CO complex. This change of the molecular current after adsorption of oxygen or carbon monoxide can be potentially used to design a molecular sensor or a molecular switch. [ABSTRACT FROM AUTHOR]

Abstract: The nonequilibrium Green’s function (NEGF) approach in combination with density functional theory (DFT) was used to investigate the electron transport properties of a single diode molecule that consists of two weakly coupled electron donating π-system and electron withdrawing π-system, resembling the conventional p–n junctions. The results demonstrate rectification behavior of the diode molecule and the maximum rectification ratio is up to 7.7. The mechanism of the rectification behavior was analyzed in terms of the evolution of molecular energy levels, the change of spatial distribution of molecular orbitals, and the electron transmission spectra. The asymmetric evolutions of the energy levels and the alignment of the states localized on the electro donating and electron withdrawing segments with the applied voltage were found to be essential in generating this rectification behavior for the molecular system. [Copyright &y& Elsevier]

Abstract: The nonequilibrium Green’s function technique and density functional theory were used to perform ab initio quantum-mechanical calculations of electronic transport of single polyacetylene (PA) molecular wire sandwiched between two gold electrodes. To inspect the substituents effect on the electronic transport properties, both the electron-donating (–NH2) and electron-withdrawing (–NO2) groups were asymmetrically introduced into the conjugated PA molecular wire, resembling the semiconductor p–n junction. The results demonstrated the rectification behavior of the substituted PA. The asymmetric evolutions of the energy levels and spatial distributions of the frontier molecular orbitals with the applied voltage are found to be essential in generating this current–voltage asymmetry. [Copyright &y& Elsevier]