Thermal Phase Control of Two-Dimensional Pt-Chalcogenide (Se and Te) Ultrathin Epitaxial Films and Nanocrystals

Abstract

The controlled synthesis and modification of the composition of layered compounds are essential prerequisites for their deployment in electronic or chemical applications. Pt-chalcogenides exhibit various compositional phases. Here, we investigate how Pt-selenide and Pt-telluride phases can be obtained as ultrathin films or as supported nanocrystals by physical vapor deposition and thermal treatment. The films are characterized by scanning tunneling microscopy and spectroscopy, scanning transmission electron microscopy, and photoemission and Raman spectroscopy. In all cases, Pt-dichalcogenides are obtained by Pt and chalcogen codeposition at growth temperatures below 300 °C. These films can be grown by van der Waals epitaxy in a layer-by-layer fashion, enabling the characterization of the pronounced layer-dependent electronic properties of these compounds. Pt-telluride growth at elevated temperatures (above 400 °C) results in the formation of Pt-monotelluride. Interestingly, the thin film Pt-dichalcogenides can also be transformed into different phases with lower chalcogen concentration by post-growth vacuum annealing. Annealing-induced loss of chalcogen results in new composites. With this thermal process, an intermittent layered compound of Pt3Te4 is synthesized, which consists of alternating PtTe2 and PtTe van der Waals layers. By thermal treatment of PtSe2, we obtain a non-layered Pt-monoselenide in nanocrystalline form. PtSe is not reported in the bulk Pt-Se phase diagram, but its structure is analogue to the known Pt-monosulfide with a tetragonal unit cell. This PtSe phase is semiconducting with a band gap of ∼0.9 eV. The nanocrystalline PtSe phase is, however, unstable and easily loses more Se and eventually converts into Pt. Thus, it is demonstrated that post-growth thermally induced transformation of Pt-dichalcogenides films enables the synthesis of new Pt-chalcogenide phases as ultrathin films or nanocrystals.