Effect of electron confinement on the optical properties in transdimensional plasmonic TiN

Similar to 2D materials, plasmonic transdimensional materials (TDMs) - atomically thin metal films spanning a few atomic layers - are expected to exhibit strong dependences on structural parameters and high sensitivity to external optical and electrical perturbations. The small atomic thicknesses also lead to strongly confined surface plasmons and quantum phenomena. With all the promising properties of metallic TDMs, it has become increasingly necessary to understand the thickness dependent properties of atomically thin metals. In the current study, we characterize the evolution of the permittivity of passivated trandimensional TiN using spectroscopic ellipsometry. The influence of oxidation and thickness on the optical properties is decoupled by measuring passivated TiN. A decrease in the Drude plasma frequency is observed in thinner films because of spatial confinement. We explain the experimental trends of the plasma frequency using a nonlocal Drude dielectric response model theory based on the Keldysh-Rytova (KR) potential that predicts the thickness dependent optical properties of metal TDMs caused by electron confinement. Our experimental findings are consistent with the KR model, indicating quantum confinement induced optical properties in plasmonic transdimensional TiN.