Unconventional two-dimensional germanium dichalcogenidesElectronic supplementary information (ESI) available. See DOI: 10.1039/c8nr01747f

The recently discovered two-dimensional (2D) group IV chalcogenides attract much attention owing to their novel electronic and photonic properties. All the reported materials of this class favor (distorted) octahedral coordination via p bonding; by contrast, in the dichalcogenides where the bonding tendency approaches sp 3 , no corresponding 2D phase has been realized so far. Here, by engineering the composition of a chalcogenide heterostructure, the hitherto elusive GeTe 2 is experimentally observed in a confined 2D environment. Density functional theory simulations predict the existence of a freestanding monolayer of octahedrally coordinated GeTe 2 under tensile strain, and the existence of GeSe 2 and GeS 2 in the same form under equilibrium conditions. These 2D germanium dichalcogenides are either metallic or narrow gap semiconducting, and may lead to new applications in nanoscale electronics. A novel 2D phase of germanium dichalcogenides (GeTe 2 ) is achieved by engineering the composition of a GeSbTe heterostructure.