A quasimolecular approach to the conductance of molecule-metal junctions:: Theory and application to voltage-induced conductance switching

We present a simple methodology to study trends in conductance of molecule-metal junctions based on Density Functional Theory calculations of modified quasimolecular Green functions in a capacitor-like electric field. The approach is based on a series of assumptions about the voltage spatial profile and the molecule-surface chemisorptive coupling in metal-molecule interfaces that seem to be validated for a number of junctions. The method assumes that the voltage drops entirely at the interfaces and that the junction conductance can be approximately factorized as a product of contact and molecular contributions. The value of such severely approximate methodology rests on the fact that it is very simple to use, computationally efficient, and its results can be analyzed in terms of familiar chemical concepts such as molecular orbitals and dipole moments. We have applied this procedure to the study of a series of pi-conjugated oligomers of current interest for device fabrication. Our results correlate well with some recent experimental results, both reported in the literature and presented in this work, that show that for some molecular bridges there is a threshold voltage where there occurs a switching-like effect.