Analytical model of molecular wire performance: A comparison of π and σ electron systems

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Abstract
The exact closed expression of zero-temperature Ohmic conductance for heterojunctions of the type three-dimensional (3D) metal/molecule/3D metal is obtained. It is specified further in explicit forms to examine electrical transport across self-assembled monolayers of linear molecules with conjugated and saturated carbon-carbon bonds. The focus is at the role of thiol head groups in functioning of polyenes and alkanes as charge transmitters between metal pads. Both tunneling and resonance tunneling across linear hydrocarbons without and with one and two end groups have been studied as a function of the Fermi energy, strength of the metal-molecule coupling, and the molecular length. Regarding the linear-response ballistic conductance of the given type of heterostructures the predictions are: pseudoresonances in the through-molecule transmission spectrum, which are generated by sulfur end groups in the band gap of the carbon chain, and suppression of the resonance and pseudoresonance tunneling by the asymmetry of the electronic structure of the system. These and other results of the paper provide a guidance for the electrical-transport spectroscopy experiments.