Optically Tracked Reversible Single-Atomic-Monolayer Redox in Plasmonic Nanocavities.

Controlling redox reactions at the atomic scale is key to enabling next-generation low-power memristive and neuromorphic devices. Ultrathin metallic layers, such as atomic monolayer palladium (Pd), offer tunable redox behavior well suited for switchable components in nanoscale electronics. Here, we present an in situ plasmonic-electrochemical platform that probes redox processes within subnanometer gaps using a nanoparticle-on-mirror (NPoM) geometry. Atomically thin Pd layers are grown by underpotential deposition, and their redox is monitored via surface enhanced Raman spectroscopy, revealing reversible transformations between metallic and dielectric Pd redox states. This confined plasmonic cavity enhances sensitivity to vibrational and electronic changes, allowing nondestructive optical readout of redox switching behaviors. By reducing the active switching volume to the monolayer limit while maintaining precise geometric control, this work establishes a powerful platform for probing redox dynamics at the atomic scale and points toward highly energy-efficient nanoswitches based on redox-active ultrathin materials.