Resolving Coordination-Dependent Bonding Selectivity at Gold-Sulfur Interfaces

Gold-sulfur interfaces underpin surface chemistry, molecular sensing and nanoelectronics, yet the atomic-scale factors governing whether thiols bind as intact Au-thiol (Au-SH) species or deprotonate to form Au-thiolate (Au-S) contacts remain unclear. Here we show at the single-molecule level that this binding selectivity is controlled by the local coordination environment of Au atoms. Using scanning tunneling microscope-break junction measurements in both solution and self-assembled monolayer systems, we construct molecular junctions with distinct Au coordination environments and resolve their interfacial bonding configurations through single-molecule conductance and flicker noise signatures. Highly coordinated Au sites favor proton-retaining Au-SH bond, whereas undercoordinated sites promote S-H bond cleavage and the formation of deprotonated Au-S bond. Density functional theory calculations and a d-band model attribute this selectivity to an upward shift of the gold d-band center at undercoordinated sites, which strengthens gold-sulfur interactions and lowers the barrier for thiol deprotonation. These results establish local Au coordination as a key descriptor of thiol binding and molecule-electrode electronic coupling, providing a molecular-level framework for tailoring gold-sulfur interfaces.