Molecular-Scale Electronics
Single-molecule junctions provide a unique platform for probing electronic transport through individual molecules connected between nanoscale electrodes. Our group develops new molecular components, junction architectures, and measurement capabilities to move beyond conventional experiments based on gold electrodes and chemically inert molecules. By varying molecular structure, electrode material, chemical environment, and temperature, we seek to uncover how molecular and interfacial properties work together to determine junction conductance, stability, and chemical reactivity.
We measure charge transport through individual molecules using the scanning tunneling microscope-based break-junction (STM-BJ) method. In a typical experiment, a metal tip is repeatedly brought into and out of contact with a metal substrate while conductance (G = I/V) is recorded as a function of tip-substrate displacement (Figure 1a). As the metallic contact is broken, molecules equipped with suitable electrode-binding groups can bridge the resulting nanoscale gap, producing characteristic step features in conductance-displacement traces (Figure 1b). By statistically analyzing thousands of these traces, we can determine the most probable characteristics of the resulting junctions. For example, conductance histograms reveal the most probable single-molecule junction conductance (Figure 1c).
In the coherent tunneling regime, molecular conductance depends strongly on the energetic alignment (ΔE) of molecular orbitals with the electrode Fermi level and the strength of electronic coupling between the molecule and electrodes (Γ). The chemical structure of the junction (including the molecular backbone, electrode-binding groups, and electrode material) therefore provides handles for systematically controlling and understanding charge transport at the single-molecule level with atomic-precision.
Figure 1. Overview of the STM-BJ method.
Beyond gold: understanding the role of the electrodes
Most single-molecule conductance measurements use gold electrodes, but the electrode material itself can strongly influence junction properties. Moving beyond gold is an important step toward understanding and ultimately controlling the behavior of molecular junctions constructed from electrode materials with potentially broader technological relevance. We have introduced methods for studying molecular junctions formed using a much wider range of metals, including reactive materials that must be handled under an inert atmosphere (Figure 2a). Such experiments allow us to investigate how electrode electronic structure, interfacial mechanics, and molecule-metal bonding collectively determine junction properties.
Our measurements have shown, for example, that the conductance of single-molecule junctions does not necessarily follow simple expectations based on metal work function. Studies of atomic-sized contacts also provide experimental evidence that surface-atom mobility exhibits a striking relationship with metal melting point (Figure 2b). By treating the electrodes as a variable component of the molecular junction, rather than simply as electrical contacts, we aim to uncover new principles underpinning charge transport and chemical reactivity at the molecule-metal interface.
Figure 2. Expanding single-molecule measurements beyond gold. (a) Our air-free STM-BJ platform enables conductance measurements using a wide range of electrode metals. (b) Measurements of atomic-scale metal contacts reveal a relationship between junction mechanics and electrode melting point.
Single-molecule chemistry at metal-solution interfaces
It is often assumed that the molecular component introduced into an experiment is the one we measure, and that it remains intact during measurement. However, a growing body of evidence shows that bond-breaking and bond-forming processes can occur spontaneously, or be driven by external stimuli such as electric fields or temperature, during STM-BJ experiments, sometimes producing structures very different from those initially introduced. Interestingly, the undercoordinated metal atoms formed during break-junction experiments are expected to be highly reactive, analogous to edge and corner sites on metal nanoparticles. The STM-BJ method therefore provides a platform that can both promote interfacial reactions and simultaneously probe them through the distinct conductance signatures of reactants, intermediates, and products.
Understanding this reactivity is important for correctly interpreting molecular conductance measurements, developing stable junction components that remain functional at elevated temperatures relevant to future applications, and exploiting interfacial chemistry to generate desired products. More broadly, because many important chemical transformations rely on metal-based heterogeneous catalysis, molecular junction measurements could provide new insight into the elementary reactions occurring at these interfaces. In contrast to many surface-sensitive techniques, STM-BJ measurements can be performed in solution, under controlled atmospheres, and at elevated temperatures, allowing interfacial chemistry to be investigated under conditions that more closely represent real-world processes. We are exploring in situ reactivity through conductance experiments in which we systematically vary molecular structure, electrode material, atmosphere, solvent, and temperature. To date, this approach has revealed unexpected reactions at both gold and silver electrodes, highlighting new opportunities to use molecular junctions as probes of chemistry at metal-solution interfaces.
Controlling charge transport through molecular design
Alongside understanding the role of the electrodes and evaluating potential interfacial reactivity, we must establish how molecular structure controls charge transport. Molecular design not only strongly influences conductance, but also provides opportunities to develop single-molecule components with functions beyond passive, wire-like circuit elements. Accordingly, we systematically investigate how changes to the molecular backbone, electronic structure, electrode-binding groups, and charge state alter junction properties, and explore how external stimuli can be leveraged to induce significant changes in their behavior.
Of particular interest are molecules whose charge-transport characteristics can be switched between two or more states through chemical, electrochemical, or photochemical processes. Such molecular switches could form the basis of electronic components that store information through transitions between distinct states, for example, where "on" and "off" states represent binary information ("1" and "0"), or ultimately control current flow through multicomponent molecule-based circuits. We seek to identify families of molecules that exhibit such behavior and establish structure-property relationships that reveal how specific molecular features influence their properties and behavior. These insights can guide the design of molecular junction components with increasingly complex and controllable functions, while providing fundamental tests of our understanding of through-molecule charge transport at nanometer length scales.
Selected publications
Single‐Molecule Junctions Formed Using Different Electrode Metals Under an Inert Atmosphere
Small, 2025, 21, 2502972
Lewis-acid mediated reactivity in single-molecule junctions
J. Am. Chem. Soc., 2024, 146, 33265
α,ω-Alkanedibromides Form Low Conductance Chemisorbed Junctions with Silver Electrodes
J. Am. Chem. Soc., 2024, 146, 28516