Developments in advanced manufacturing have promoted the miniaturization of semiconductor electronic devices to a near-atomic scale, which continuously follows the ‘top-down’ construction method. However, huge challenges have been encountered with the exponentially increased cost and inevitably prominent quantum effects. Molecular electronics is a highly interdisciplinary subject that studies the quantum behavior of electrons tunneling in molecules. It aims to assemble electronic devices in a ‘bottom-up’ manner on this scale through a single molecule, thereby shedding light on the future design of logic circuits with new operating principles. The core technologies in this field are based on the rapid development of precise fabrication at a molecular scale, regulation at a quantum scale, and related applications of the basic electronic component of the ‘electrode–molecule–electrode junction’. Therefore, the quantum charge transport properties of the molecule can be controlled to pave the way for the bottom-up construction of single-molecule devices. The review firstly focuses on the collection and classification of the construction methods for molecular junctions. Thereafter, various characterization and regulation methods for molecular junctions are discussed, followed by the properties based on tunneling theory at the quantum scale of the corresponding molecular electronic devices. Finally, a summary and perspective are given to discuss further challenges and opportunities for the future design of electronic devices.

The investigation of electronic excited states in single-molecule junctions not only provides platforms to reveal the photophysical and photochemical processes at the molecular level, but also brings opportunities for the development of single-molecule optoelectronic devices. Understanding the interaction mechanisms between molecules and nanocavities is essential to obtain on-demand properties in devices by artificial design, since molecules in junctions exhibit unique behaviors of excited states benefited from the structures of metallic nanocavities. Here, we review the excitation mechanisms involved in the interplay between molecules and plasmonic nanocavities, and reveal the influence of nanostructures on excited-state properties by demonstrating the differences in excited state decay processes. Furthermore, vibronic transitions of molecules between nanoelectrodes are also discussed, offering a new single-molecule characterization method. Finally, we provide the potential applications and challenges in single-molecule optoelectronic devices and the possible directions in exploring the underlying mechanisms of photophysical and photochemical processes.