Measurements of the surface susceptibility and conductivity of atomically thin MoS2 and WS2 by spectroscopic ellipsometry

The optical properties of a monolayer MoS2 or WS2 are described in literature by a bulk dielectric function. The material is modelled as a homogeneous medium with an effective thickness given by the interlayer spacing of the exfoliating bulk material. This model has been considered equivalent to the surface current model where the monolayer material is described as a truly two-dimensional system. Here we prove experimentally that for a monolayer MoS2 and WS2 the two models are not equivalent. We show how to correctly extract from the experimental data the surface susceptibility and the surface conductivity that describe the optical properties of a monolayer MoS2