Molecule–Electrode Electronic Coupling Modulates Optoelectronics of (Bio)Molecular Junctions

Abstract

The charge transport across a molecular junction formed by sandwiching molecules between two electrodes in testbed architectures depends not only on the work function of the metal electrodes and energy gap of the molecules but also on the efficacy of the molecule–electrode electronic coupling. Insights into such molecule–electrode coupling would help to understand the relation between the coupling strength and electron transport processes. With this aim, the optoelectronic modulation across bacteriorhodopsin-based molecular junctions has been studied using experimental current–voltage traces obtained by conducting-probe atomic force microscopy under various illuminations. The energy barrier $\left( {\varepsilon_{0} } \right)$, molecule–electrode coupling (Γ_g), and other transport parameters were determined utilizing the Landauer model with a single-Lorentzian transmission function, transition voltage spectroscopy, and the law of corresponding states in the universal tunneling model approach. The findings reveal that the optoelectronic modulation of bacteriorhodopsin molecular junctions originate from alteration of the molecule–electrode coupling, which could originate from modulation of electronic states and the electrostatic environment of retinal chromophores made of the protein under dark and green or green–blue illumination conditions.