Preparation and piezoelectric properties of Group IV chalcogenides

The recent advances in the study of the unique capabilities of monolayer crystals had predicted piezoelectricity in two-dimensional (2D) group IV chalcogenides which are not present in their bulk forms. In this study, the predicted piezoelectric properties of was experimentally explored through Piezoelectric Force Microscopy (PFM). Tin Sulfide (SnS), Tin Selenide (SnSe), Germanium Sulfide (GeS), were exfoliated from their bulk forms by the top down methods of liquid sonication, while and Tin Disulfide (SnS2) mechanically exfoliated by tape. These two top-down methods in producing possible monolayers of these layered crystals were chosen for retaining pristine structures. In testing two different solvents for liquid sonication, it was found that Isopropyl Alcohol (IPA) was effective in exfoliating SnS and SnSe into thinner particles. While using ethanol as the solvent proved more effective in exfoliating GeS. Mechanical exfoliation gave varying thicknesses while retaining large lateral dimension, visible under optic microscope. From the PFM analysis, it was found that 1.81 nm of SnS2 had an effective d33 of 7.76 pm/V, and SnS with a thickness of 2.87 nm, had an effective d33 was found to be 16.26 pm/V. While the piezoelectric constant could not be derived for GeS and SnSe, these materials did present piezoelectric responses. The lateral piezoresponse of SnS2 gave the effective d31 to be between 0.61 and 1.55 pm/V from a sample with 3.2 nm thickness. Despite not being in the monolayer form, there were piezoelectric responses from all the studied crystals. This could allow for attaining tailored piezoreponse as per layer. In the process of this study, a more robust methodology for using PFM to mitigate as many factors affecting the accuracy of the results, was able to be developed.