Isolation and characterisation of monolayer phosphorene analogues.

Atomically thin group IV monochalcogenides or phosphorene analogues are a promising family of materials. Theoretical calculations predict that monolayers (MLs) should be semiconducting, ferroelectric and ferroelastic at room temperature, exhibit large charge mobilities and large non-linear optical response. Yet, experimental studies of these systems are scarce because of the difficulty to produce such MLs. Here we focus on two members of this family: GeSe and SnS. We demonstrate a simple mechanical exfoliation method to produce ML samples on gold substrates. We observe the evolution of the Raman scattering as a function of layers and the anisotropic optical response from reflectance contrast measurements. To the best of our knowledge this is the first report of mechanical exfoliation down to the ML of these materials and the first realisation of ML GeSe.