Quantum many-body effects on the electric and thermoelectric response of molecular heterojunctions

A semi-empirical $\pi$-electron Hamiltonian (extended Hubbard model) is used to model the electronic degrees of freedom most relevant for transport in a heterojunction consisting of a conjugated organic molecule coupled to two (or more) metallic electrodes. With an appropriate choice of parameters, the {\em complete spectrum of electronic excitations} of the molecule up to 8--10eV can be accurately described,$^1$ which is essential to accurately model transport far from equilibrium. The electric and thermoelectric response of the junction is calculated within a many-body theory of transport based on nonequilibrium Green's functions. For benzenedithiol-Au junctions, the parameters characterizing the lead-molecule coupling (tunneling width and chemical potential offset) are determined by comparison to linear-response measurements of conductance and thermopower. The nonlinear transport can then be predicted: the differential conductance as a function of gate and bias voltages exhibits clear signatures of charge quantization and resonant tunneling through excited states, with an irregular ``molecular diamond'' structure analogous to the regular Coulomb diamonds observed in quantum dot transport experiments. Several other small conjugated organic molecules are also investigated. $^1$C.\ W.\ M.\ Castelton and W.\ Barford, J.\ Chem.\ Phys.\ {\bf 117}, 3570 (2002).