Implication of Current-Voltage Curve Shape in Molecular Electronics

The transmission function, T(E), widely used as a toy model in molecular electronics, relies exclusively on a Lorentzian-shaped energy level. The shape of the energy level may be sensitive to the inhomogeneity of the active monolayer in a tunnel junction, yet it is usually ignored or underestimated in explaining the charge tunneling behavior. This article describes the interplay between the supramolecular packing feature of a self-assembled monolayer (SAM) and the shape of an energy level in T(E). Using a T(E) based on the Gaussian-Lorentzian product (GLP), line-fitting analysis was conducted over experimentally obtained current density-voltage curves of n-alkanethiolate SAMs to determine the mixing ratio between Gaussian and Lorentzian functions. It was revealed that the contribution of the Gaussian to the shape of the energy level in T(E) was dominant for solid-like SAMs whereas that of the Lorentzian was dominant for liquid-like SAMs. The energy-level shape also responded to defects induced by the surface roughness of the bottom electrode. We further demonstrated that our approach can be applied to rectifying junctions, using ferrocenyl-terminated n-alkanethiolate SAM. These findings indicate that shape analysis over the current-voltage curve, intimately related to the shape of the energy level of T(E), may provide implications for the packing features of SAMs reminiscent of spectral line fitting in spectroscopy.