Direct Observation of the Thickness-Dependent Dielectric Response of MoS2 and WSe2

We report a direct observation of the thickness-dependent dielectric response of MoS2 and WSe2 via mobile charge carrier measurements based on second-harmonic electrostatic force microscopy (EFM). The measured second harmonic (2ω) signals in the EFM are related to the dielectric polarization of MoS2 and WSe2 due to their charge carrier concentration and mobility. We find that the 2ω signals increase with the thickness in thin layers and saturate to bulk values for MoS2 and WSe2 flakes. These thickness-dependent dielectric behaviors are consistent with previous first-principles calculations for layered transition metal dichalcogenide (TMDC) materials. Our work provides useful information about enhancing the performance of the optoelectrical devices based on layered two-dimensional materials.