Quantum Interference Effects in Charge Transport through Single-Molecule Junctions: Detection, Manipulation, and Application.

Quantum interference effects (QIEs), which offer unique opportunities for the fine-tuning of charge transport through molecular building blocks by constructive or destructive quantum interference, have become an emerging area in single-molecule electronics. Benefiting from the QIEs, charge transport through molecular systems can be controlled through minor structural and environmental variations, which cause various charge transport states to be significantly changed from conductive to insulative states and offer promising applications in future functional single-molecule devices. Although QIEs were predicted by theoreticians more than two decades ago, only since 2011 have the challenges in ultralow conductance detection originating from destructive quantum interference been overcome experimentally. Currently, a series of single-molecule conductance investigations have been carried out experimentally to detect constructive and destructive QIEs in charge transport through various types of molecular junctions by altering molecular patterns and connectivities. Furthermore, the use of QIEs to tune the properties of charge transport through single-molecule junctions using external gating shows vital potential in future molecular electronic devices. The experimental and theoretical investigations of QIEs offer new fundamental understanding of the structural-electronic relationships in molecular devices and materials at the nanoscale. In this Account, we discuss our progress toward the experimental detection, manipulation, and further application of QIEs in charge transport through single-molecule junctions. These experiments were carried out continuously in our previous group at the University of Bern and in our lab at Xiamen University. As a result of the development of mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques, we could detect ultralow charge transport through the cross-conjugated anthraquinone center, which was one of the earliest experimental studies of QIEs. In close cooperation with organic chemists and theoretical physicists, we systematically investigated charge transport through single-molecule junctions originating from QIEs in conjugated centers ranging from simple single benzene to polycyclic aromatic hydrocarbons (PAHs), heteroaromatics, and even complicated metalla-aromatics at room temperature. Then we further investigated the quantitative correlation between molecular structure and quantum interference by altering different molecular patterns and connectivities in homologous series of PAHs and heteroatom systems. Additionally, external chemical and electrochemical gating of single-molecule devices can be used for direct QIE manipulation via not only tuning molecular conjugation but also shifting the electrode Fermi level. Our study further suggested that distinguishable differences in conductance resulting from QIEs offer opportunities to detect photothermal reaction kinetics and to recognize isomers at the single-molecule scale. These investigations demonstrate the universality of QIEs in charge transport through various molecular building blocks. Moreover, effective manipulation of QIEs leads to various novel phenomena and promising applications in molecular electronic devices.