Measurement of the surface susceptibility and the surface conductivity of atomically thin MoS₂ by spectroscopic ellipsometry

We show how to correctly extract from the ellipsometric data the surface susceptibility and the surface conductivity that describe the optical properties of monolayer MoS₂. Theoretically, these parameters stem from modelling a single-layer two-dimensional crystal as a surface current, a truly two-dimensional model. Current experimental practice is to consider this model equivalent to a homogeneous slab with an effective thickness given by the interlayer spacing of the exfoliating bulk material. We prove that the error in the evaluation of the surface susceptibility of monolayer MoS₂, owing to the use of the slab model, is at least 10% or greater, a significant discrepancy in the determination of the optical properties of this material.