Fundamentals, Measurement and Regulation of the Conductance of Single Molecule Junctions

The miniaturization of conventional silicon-based devices is one of the pinnacles of achievement of the 20^th century which evolved according to Moore's prediction, demanding a higher number and smaller size of electronic components each year. One path forward is the incorporation of atoms and molecules as small, low-cost, and stable structures in electronic circuits and their integration into complex architectures which have been a desire of the nanoscale community for many decades. At the quantum physics scale, the unique physical and chemical properties of single molecules could lead to numerous new and interesting phenomena that are not accessible using conventional approaches, resulting in the emergence of a wide variety of device functionalities and applications, for example, nano-switches, single-molecule sensors, and spin filters. Although single-molecule electronics is still at an early phase, the investigation of charge transport through molecules and their dynamics at the nanoscale is fundamentally important to understand the relevant scientific concepts and technological applications. We briefly review the history of molecular electronics as well as the fundamentals and theories required to understand charge transport through molecules. We provide an overview of methods to fabricate single-molecule junctions with a focus on STM-based approaches, their advantages, and limitations. The review highlights new insights and the latest progress on the structure-property relationship of single-molecule junctions that includes the effect of anchoring groups, molecular orientation in the junction (anisotropy of conductance), molecule-electrode binding (denticity), and the role of solvent on charge transport at the nanoscale. We also highlight how advances in machine learning and molecular dynamics techniques have impacted theoretical and computation-based approaches to studying molecular electronics. We then summarize the contribution of advanced statistical analysis and machine-based approaches to the analysis of single-molecule conductance data. We wrap up the review with a discussion on new materials for molecular electronics, as well as current challenges and the outlook in the development of practical molecular electronics.