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

Abstract

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.