Vibrational Strong Coupling of Thin Water Layers Using Plasmonic Cavities

Abstract Strong coupling of molecular vibrations with resonant optical cavities creates vibro‐polaritonic states, which can alter chemical reaction rates and product distributions without altering molecular structure. However, so far vibrational strong coupling has only been demonstrated in films of material so thick that the effects have been limited to bulk chemical reactions. Demonstrating vibrational strong coupling in nanometer‐scale surface layers of molecules facilitates applying the results of vibro‐polaritonic chemistry to the vast majority of industrial chemistry: catalytic reactions occurring at surfaces. Here, highly confined plasmonic cavities are designed and fabricated that are tunable over the entire mid‐infrared region. Vibrational strong coupling of water layers that are only 44 nm thick, in both the O─H stretching and bending modes is demonstrated, with Rabi splitting energy of 468 and 282 cm −1 , respectively. Detuning experiments show the dispersive behavior of the polaritonic states, and changing the oscillator strength of water molecules by diluting with D 2 O follows the theoretically predicted change in Rabi splitting energy, assuming a strongly bound layer of water at the surface. We confirm that dry samples annealed in vacuum and measured under nitrogen purging still display a substantial Rabi splitting of 202 cm −1 coming from surface‐bound water with an estimated thickness from literature of a few monolayers to a few nanometers, closely approaching the 242 cm −1 threshold for vibrational strong coupling. Such strongly perturbed water layers directly at the surface of metal electrodes broaden the relevant range of reactions where vibro‐polaritonic chemistry can be applied and point toward the exciting possibility of a totally new way of altering aqueous electrocatalytic activity for important reactions such as water‐splitting and CO 2 reduction.