Novel Electrostatic Effects in Single-Molecule Devices

The past decades have witnessed an increasing interest in molecular electronics aiming to assemble functional circuits using single molecules. Researchers from various disciplines have devoted considerable attention in the design and construction of single -molecule junctions and sophisticated functional devices, accompanied by the discovery and utilization of numerous novel quantum phenomena. Many new breakthroughs benefit from the utilization of various stimulus response methods to tune the charge transport in molecular devices, such as light, temperature, magnetic field, pH, and mechanical force. Electrostatic field has superb but distinct abilities to modulate the charge transport in molecular devices. First, like in other electronic devices, electrostatic fields act on single-molecule devices as a noninvasive means. However, unlike in these traditional electronic devices, the voltage applied in the extremely tiny single-molecule devices would generate a large electrostatic field, which could provide the necessary conditions for regulating charge transport and catalyzing single-molecule-scale chemical reactions. This review focuses on the recent advances made in tuning charge transport by electrostatic field in the single-molecule devices. In the second section, we introduce and compare two break junction techniques commonly used to construct molecular junctions: the scanning tunneling microscopy break junction (STMBJ) technique and the mechanically controllable break junction (MCBJ) technique; furthermore, the three-electrode systems based on these two break junction techniques are also introduced. These techniques laid the foundation for various new techniques in tuning charge transport in molecular junctions based on electrostatic field. In the third section, the applications of electrostatic field are introduced, including controlling the molecular-electrode interfaces, varying molecule configurations and conformations, catalyzing single-molecule-scale chemical reactions, switching molecule spin states, changing molecule redox states and shifting the energy levels of the electrodes and molecules. Finally, we discussed the shortcomings of the applications electrostatic field in single-molecule devices. Including the low stability of single-molecule devices under strong electrostatic field, and the introduction of electrostatic field will increase the difficulty of understanding the charge transport mechanism in single-molecule devices. In addition, we point out that electrostatic field modulation of single-molecule charge transport is expected to be further developed in the following aspects: Firstly, multi-stimulus response molecule devices could be built by combining electrostatic field with other stimulus. Secondly, electrostatic field could be used to catalyze more types of chemical reactions, even control the configurations and conformations of products. Thirdly, electrostatic field can be used to design fullerene-based switching molecular diodes that proper for application in random-access memories and memristors.