Orbital Control of Long-Range Transport in Conjugated and Metal-
Centered Molecular Electronic Junctions

Large area molecular junctions consisting of covalently bonded
molecular layers between conducting carbon electrodes were compared for
Co and Ru complexes as well as nitroazobenzene and anthraquinone to
investigate the effect of molecular structures and orbital energies on electronic
behavior. A wide range of molecular layer thickness (d) from 1.5−28 nm was
examined and three distinct transport regimes in attenuation plots of current
density (J) vs thickness were revealed. For d < 5 nm, the four molecular
structures had comparable current densities and thickness dependence
despite significant differences in orbital energies, consistent with coherent
tunneling and strong electronic coupling between the molecules and contacts.
For d > 12 nm, transport depends on the electric field rather than bias, with
the slope of ln J vs d near-zero when plotted at a constant electric field. At low
temperature (T < 150 K), transport is nearly activationless and likely occurs
by sequential tunneling and/or field-induced ionization. For d = 5−10 nm,
transport correlates with the energy gap between the highest occupied and lowest unoccupied molecular orbitals, and ln J is
linear with the square root of the bias or electric field. Such linearity occurs for all three transport regimes and is consistent with
the energy barrier lowering by the applied electric field. The results clearly indicate a strong dependence of charge transport on
molecular orbital energies provided d > 5 nm, with a variation of 7 orders of magnitude of J for different molecules and d = 10
nm. The results provide insights into charge transport mechanisms as well as a basis for rational design of molecular electronic
devices.