Photocurrent Experiments as Probes and Prods in Large-Area Molecular Electronic Junctions

Since its experimental realization in the late 1990s, molecular electronics (ME) has received significant attention due to its unique charge transport behavior that occurs over nanoscale dimensions. The molecular junction (MJ), wherein single molecules or arrays of parallel molecules are oriented between two metallic electrodes is the fundamental building block of ME. Specifically, temperature-independent quantum mechanical tunneling across a few nanometers in MJs is distinct from transport in organic or inorganic semiconductors and may be valuable for next-generation electronics. Although molecular tunneling junctions have been introduced commercially, fundamental questions remain about the control of charge transport in various MJs, including those containing proteins and DNA. Examples include: (i) Does molecular structure play an important role in charge transport? (ii) What is the effective barrier (tunneling or any other) for transport in MJs? (iii) Does the molecular structure remain intact during junction fabrication and operation? (iv) What charge transport mechanisms are present beyond the range of quantum tunneling? We propose a simple yet effective approach to resolve the important questions mentioned above through photocurrent experiments. In this perspective, we are showcasing various capabilities of photocurrent experiments to probe, prod, and investigate various features in MEs including internal energy barriers, in situ monitoring of the molecular structure, activationless transport over long distances, and biological charge transport.