Theory

The state of the art in making comparisons between measurements and calculations for the resistance of molecular junctions is generally very poor -- the commonly used calculations, made using the paradigm of mean-field nonequilibrium Greens function (NEGF) theory, yield electrical conductances that are orders of magnitude larger than experimental values. The graphene-molecule-graphene junctions developed at the CCI have the unique virtue that the bonding configuration of the molecule-graphene interface should be accessible for direct experimental detection, thus providing the first system where one might, with confidence, be sure that one is using the same geometry in simulation as is studied in experiments.

Ideal (a) and optimized (b) structures of a graphene-(1,4-diethynylbenzene)-graphene junction. (c) Perspective view of the molecular junction.

The scope of the theoretical efforts includes:

Optical processes in graphitic materials: an absorbed photon creates an exciton pair which may relax back to the ground-state through multiple mechanisms. To understand optical measurements in these materials we have developed a phenomenological Dirac fermion model that takes into account electron-phonon and electron-electron scattering.1. Systematic comparisons between NEGF-DFT theory and experiments for graphene and nanotube based junctions with the aim of calibrating the accuracy of the conventional paradigm, and establishing the relative contributions of different physical processes to electron transport. This effort will also enable us to establish "best practices" and choices of functionals in the NEGF-DFT formalism.

2. Development of improved density functional and many-body wavefunction-based theories that go beyond current NEGF-DFT formulations, guided by our understanding of the importance of various physical mechanisms identified.

3. Extension of existing electronic structure and transport theories to include the effects of electrochemical gating on molecular conductance, and graphene adsorption effects on molecular spectra, to make contact with previously proposed experiments.