posted on 2004-12-02, 00:00authored byCarlos Gonzalez, Yamil Simón-Manso, James Batteas, Manuel Marquez, Mark Ratner, Vladimiro Mujica
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 π-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.