Density functional theory (DFT) is
applied to three models for
carbon-based molecular junctions containing fragments of graphene
with covalent edge-bonding to aromatic and aliphatic molecules, with
the graphene representing a sp<sup>2</sup> hybridized carbon electrode
and the molecule representing a molecular layer between two electrodes.
The DFT results agree well with experimental work functions and transport
barriers, including the electronic coupling between molecular layers
and graphitic contacts, and predict the compression of tunnel barriers
observed for both ultraviolet photoelectron spectroscopy (UPS) and
experimental tunneling currents. The results reveal the strong effect
of the dihedral angle between the planes of the graphene electrode
and the aromatic molecule and imply that the molecules with the smallest
dihedral angle are responsible for the largest local current densities.
In addition, the results are consistent with the proposal that the
orbitals which mediate tunneling are those with significant electron
density in the molecular layer. These conclusions should prove valuable
for understanding the relationships between molecular structure and
electronic transport as an important step toward rational design of
carbon-based molecular electronic devices