Non-centrosymmetric features in nanostructured MoS2 and WS2 exfoliated in liquid phase

Exfoliation in liquid phase of Transition Metal Dichalcogenides promotes the industrialization of low dimensional structures in photonic and material science applications, due to its high nonlinear optical and piezoelectric responses on monolayers and few odd layers. Here, we report piezoelectricity and prospects of non-linear optics through Switching Spectroscopy Piezoresponse Force Microscopy (SS-PFM) and Fluorescence spectroscopy on MoS2 - and WS2 - flakes exfoliated in liquid phase. These features are intrinsically related to non-centrosymmetric structures and depend on the number of layers. SS-PFM signal in a small region reveals that the strength of AFM tip deflection was considerable just for an odd number of layers and negligible for even layers. Thin films were evaluated with ellipsometric measurements by constructing models mixing mono and few layers, resulting in a detailed description of the optical and surface properties of the nanostructures studied.