Controlling Stoichiometry in Ultrathin van der Waals Films: PtTe2, Pt2Te3, Pt3Te4, and Pt2Te2

The platinum-tellurium phase diagram exhibits various (meta)stable van der Waals (vdW) materials that can be constructed by stacking PtTe2 and Pt2Te2 layers. Monophase PtTe2, being the thermodynamically most stable compound, can readily be grown as thin films. Obtaining the other phases (Pt2Te3, Pt3Te4, Pt2Te2), especially in their ultimate thin form, is significantly more challenging. We show that PtTe2 thin films can be transformed by vacuum annealing-induced Te-loss into Pt3Te4- and Pt2Te2-bilayers. These transformations are characterized by scanning tunneling microscopy and X-ray and angle resolved photoemission spectroscopy. Once Pt3Te4 is formed, it is thermally stable up to 350°C. To transform Pt3Te4 into Pt2Te2, 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 Pt3Te4 over a phase segregation into PtTe2 and Pt2Te2 and an increase in the Te-vacancy formation energy for Pt3Te4 compared to the starting PtTe2 material is critical to stabilize the Pt3Te4 bilayer. To desorb more tellurium from Pt3Te4 and transform the material into Pt2Te2, a higher Te-vacancy formation energy has to be overcome by raising the temperature. Interestingly, bilayer Pt2Te2 can be retellurized by exposure to Te-vapor. This causes the selective transformation of the topmost Pt2Te2 layer into two layers of PtTe2, and consequently the synthesis of e Pt2Te3. Thus, all known Pt-telluride vdW compounds can be obtained in their ultrathin form by carefully controlling the stoichiometry of the material.