Controlling Stoichiometry in Ultrathin van der Waals Films: PtTe, PtTe, PtTe, and PtTe.

The platinum-tellurium phase diagram exhibits various (meta)stable van der Waals (vdW) materials that can be constructed by stacking PtTe and PtTe layers. Monophase PtTe, being the thermodynamically most stable compound, can readily be grown as thin films. Obtaining the other phases (PtTe, PtTe, PtTe), especially in their ultimate thin form, is significantly more challenging. We show that PtTe thin films can be transformed by vacuum annealing-induced Te-loss into PtTe- and PtTe-bilayers. These transformations are characterized by scanning tunneling microscopy and X-ray and angle resolved photoemission spectroscopy. Once PtTe is formed, it is thermally stable up to 350°C. To transform PtTe into PtTe, a higher annealing temperature of 400°C is required. The experiments combined with density functional theory calculations provide insights into these transformation mechanisms and show that a combination of the thermodynamic preference of PtTe over a phase segregation into PtTe and PtTe and an increase in the Te-vacancy formation energy for PtTe compared to the starting PtTe material is critical to stabilize the PtTe bilayer. To desorb more tellurium from PtTe and transform the material into PtTe, a higher Te-vacancy formation energy has to be overcome by raising the temperature. Interestingly, bilayer PtTe can be retellurized by exposure to Te-vapor. This causes the selective transformation of the topmost PtTe layer into two layers of PtTe, and consequently the synthesis of e PtTe. Thus, all known Pt-telluride vdW compounds can be obtained in their ultrathin form by carefully controlling the stoichiometry of the material.