TMDs are a class of layered materials in which, similarly to graphite, there are covalently bonded monolayers with van der Waals interactions between layers to form layered bulk structures. Unlike graphite, where all atoms are in a single plane, TMD monolayers consist of a covalent X-M-X "sandwich structure", with X and M being the chalcogen and metal species respectively . These materials have a wide range of optical , electrical , and mechanical properties .