Single-Molecule Functional Chips: Unveiling the Full Potential of Molecular Electronics and Optoelectronics

An ideal methodology for miniaturizing the physical size, enhancing the operational frequency, and building the multifunctional capability of functional chips is to use opto- or electroactive single molecules as their central elements; such devices are generally termed single-molecule electronics and optoelectronics. The exploration of the electronic and optoelectronic properties of materials at the single-molecule level also allows the complete elucidation of the correlation between molecular structure and function, which in turn aids technological advances that can help to address the challenge raised by Moore’s Law. In this Account, we present our ongoing investigative pursuits in the realm of single-molecule electronics and optoelectronics, with a particular emphasis on studies using graphene-molecule-graphene single-molecule junctions as the primary framework. To date, we have established a diverse range of single-molecule multifunctional devices, including photoswitches, field-effect transistors, rectifiers, light-emitting diodes, spin electronic devices, memristors, and molecular wires. These types of devices possess stable graphene electrodes and robust covalent molecule-electrode interfaces.

The main focuses of this account are our proposed molecular/interface engineering strategy including interface design using particular linkers, spacers, insulation, and functional centers and our device engineering strategy that covers the design of the device structure and electrode materials. These strategies adequately consider the coupling between functional centers and their external environment, thus affording the ability to evaluate and manipulate the intrinsic behaviors of target molecules. Specifically, a covalent molecule-electrode interface enables high device stability at a high bias voltage. Three nonconjugated methylene groups are inserted at the electrode-molecule interface to prevent the quenching of the excited state of the central molecule (e.g., diarylethene) by the graphene electrode, thereby achieving robust and reversible photoswitches. Cyclodextrins are introduced as insulating groups around molecular bridges to weaken the coupling of the bridges with the environment, which increases the quantum yield of light-emitting diodes. Additional reactive sites are introduced on the sides of the molecular bridges, providing the ability to add new functional centers. We show that using materials with a high dielectric constant as the dielectric layer enables efficient electrical manipulations of single-molecule electronics and optoelectronics by the gate voltage. We reveal that the use of ferromagnetic metal electrodes in single-molecule electronics and optoelectronics can meet the requirements for spin injection. In particular, the two-dimensional structure of graphene electrodes that can be tailored by etching enables high-density integration of molecules, paving the way for future logical manipulation and real-time communication.

These systematic investigations emphasize the importance of single-molecule electronics and optoelectronics for miniaturized device fabrication, intrinsic mechanism exploration, and advanced chip applications. Further interdisciplinary cooperative efforts, including micro- and nanoprocessing, organic synthesis, and theoretical calculation, will contribute to the rapid development of single-molecule electronics and optoelectronics that are suitable for practical applications.