The (re-)discovery of graphene by A.K. Geim and K.S. Novoselov1 in 2004 has led to renewed interest in two-dimensional (2D) materials. Beyond graphene, hundreds of other 2D materials (also called van der Waals materials) exist in bulk form that can be thinned down to atomically thin layers, in a similar manner as graphene can be obtained from graphite.2 One important class of 2D materials is layered transition metal dichalcogenides (TMDs) that show a wide range of physical properties, depending on material and thickness. TMDs have been studied for decades, and, already back in 1963, the pioneering work by Frindt and Yoe3 demonstrated that these materials can be exfoliated into ultrathin crystals by mechanical cleavage.