Controlling\nStoichiometry in Ultrathin van der Waals\nFilms: PtTe₂, Pt₂Te₃, Pt₃Te₄, and Pt₂Te₂

Abstract

The\nplatinum–tellurium phase diagram exhibits various (meta)­stable\nvan der Waals (vdW) materials that can be constructed by stacking\nPtTe₂ and Pt₂Te₂ layers. Monophase\nPtTe₂, being the thermodynamically most stable compound,\ncan readily be grown as thin films. Obtaining the other phases (Pt₂Te₃, Pt₃Te₄, Pt₂Te₂), especially in their ultimate thin form, is significantly\nmore challenging. We show that PtTe₂ thin films can be\ntransformed by vacuum annealing-induced Te-loss into Pt₃Te₄- and Pt₂Te₂-bilayers. These\ntransformations are characterized by scanning tunneling microscopy\nand X-ray and angle resolved photoemission spectroscopy. Once Pt₃Te₄ is formed, it is thermally stable up to 350°C.\nTo transform Pt₃Te₄ into Pt₂Te₂, a higher annealing temperature of 400°C is required.\nThe experiments combined with density functional theory calculations\nprovide insights into these transformation mechanisms and show that\na combination of the thermodynamic preference of Pt₃Te₄ over a phase segregation into PtTe₂ and Pt₂Te₂ and an increase in the Te-vacancy formation\nenergy 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\nthe temperature. Interestingly, bilayer Pt₂Te₂ can be retellurized by exposure to Te-vapor. This causes the selective\ntransformation of the topmost Pt₂Te₂ layer into\ntwo layers of PtTe₂, and consequently the synthesis of\ne Pt₂Te₃. Thus, all known Pt-telluride vdW compounds\ncan be obtained in their ultrathin form by carefully controlling the\nstoichiometry of the material.