Optical suppression of energy barriers in single molecule-metal binding

Abstract

Transient bonds between molecules and metal surfaces underpin catalysis, bio/molecular sensing, molecular electronics, and electrochemistry. Techniques aiming to characterize these bonds often yield conflicting conclusions, while single-molecule probes are scarce. A promising prospect confines light inside metal nanogaps to elicit in operando vibrational signatures through surface-enhanced Raman scattering. Here, we show through analysis of more than a million spectra that light irradiation of only a few microwatts on molecules at gold facets is sufficient to overcome the metallic bonds between individual gold atoms and pull them out to form coordination complexes. Depending on the molecule, these light-extracted adatoms persist for minutes under ambient conditions. Tracking their power-dependent formation and decay suggests that tightly trapped light transiently reduces energy barriers at the metal surface. This opens intriguing prospects for photocatalysis and controllable low-energy quantum devices such as single-atom optical switches.

Picocavities: a Primer

“…The picocavity field (Figure b) polarizes the electron cloud around the molecule tip, which induces free electron currents in the metal surface. Solving self-consistently gives strongly enhanced attractive forces . We emphasize theory breakthroughs are required for integrating photons into DFT calculations, not yet realistically feasible.…”

“…This thousand-fold greater force demands new theory. Experiments show a universal power dependence to the picocavity generation rate that scales with the static polarizability of the atom at the molecule tip . The optical forces thus involve not the molecular refractive index but its quantum-mechanical polarizability.…”

“…Picocavities are subsequently created by light (though potentials can also be used). Such optical forces are puzzling since the fields (Figure ) at these laser powers give optical tweezer forces ∝∇ E pico 2 of ∼1pN, while pulling the adatom out by Δ z = 0.3 nm costing Δ U = 1 eV requires F ∼ Δ U /Δ z ∼ 1 nN . This thousand-fold greater force demands new theory.…”

“…The optical forces thus involve not the molecular refractive index but its quantum-mechanical polarizability. Several observations are important:…”

“…Formation times of picocavities are <50 μs but record only when the adatom is within 3 Å, thus amplifying SERS 100-fold. On the other hand, picocavities show two decay rates; the fast rate matches picocavity formation rates, while the slower one may arise from atomic reorganization of the vacated surface pit that prevents adatoms dropping back in (without many surface rearrangements). Picocavities might suppress their subsequent local creation, although analysis suggests not …”