Concepts in the design and engineering of single-molecule electronic devices

Over the past two decades, various techniques for fabricating nano-gapped electrodes have emerged, promoting rapid development in the field of single-molecule electronics, on both the experimental and theoretical sides. To investigate intrinsic quantum phenomena and achieve desired functionalities, it is important to fully understand the charge transport characteristics of single-molecule devices. In this Review, we present the principles that have been developed for fabricating reliable molecular junctions and tuning their intrinsic properties from an engineering perspective. Through holistic consideration of the device structure, we divide single-molecule junctions into three intercorrelated components: the electrode, the contact (spacer–linker) interface and the molecular backbone or functional centre. We systematically discuss the selection of the electrode material and the design of the molecular components from the point of view of the materials, the interface and molecular engineering. The influence of the properties of these elements on the molecule–electrode interface coupling and on the relative energy gap between the Fermi level of the electrode and the orbital energy levels of the molecule, which directly influence the charge transport behaviour of single-molecule devices, is also a focus of our analysis. On the basis of these considerations, we examine various functionalities demonstrated in molecular junctions through molecular design and engineering.In this Review, the principles developed for fabricating reliable molecular junctions and tuning their intrinsic properties are examined from the point of view of the electrode, the interface and molecular engineering. The various functionalities demonstrated in molecular junctions through molecular design are discussed, along with the open challenges in the field.Key pointsSingle-molecule electronics has become a burgeoning subfield of nanoscience and has begun to develop beyond the basic description of carrier transport, expanding in different research directions.A single-molecule junction can be divided into three intercorrelated components: the electrode, the contact interface and the molecular backbone or functional centre.Both the mechanical stability and electronic coupling of the molecule–electrode interface increase with the binding energy of the electrode–anchoring moiety interaction. A compromise between these factors can be achieved by inserting suitable spacers between the molecular kernel and anchoring groups.To select suitable electrode materials, chemical inertness to air, good processability, suitable work function and good compatibility with molecules should be taken into consideration.The structures of molecular bridges can be tuned by the molecular length, the geometry of the main chains, the responsivity of the functional centres and the types of side groups, offering opportunities to probe intrinsic physical properties and realize various functionalities.Challenges in the field of single-molecule electronics include improving device-to-device uniformity, stability, integration capability and accuracy of theoretical models.