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.

MBE-grown ultrathin PtTe2 films and their layer-dependent electronic structures

“…2d. Another peak appears around 92 cm −1 at 350 °C and becomes stronger at 400 °C, which is attributed to the E g Raman active mode of PtTe, 6,25 giving further evidence of Te deficiency caused by high growth temperature. Our findings here are consistent with a recent report by Matthias Batzill's group on the phase control of Pt-telluride, in which PtTe and Pt 3 Te 4 were obtained by elevated growth temperature and annealing.…”

“…Our findings here are consistent with a recent report by Matthias Batzill's group on the phase control of Pt-telluride, in which PtTe and Pt 3 Te 4 were obtained by elevated growth temperature and annealing. 25 The XPS and Raman spectra alongside the STM characterization demonstrate that the temperature window for growing PtTe 2 is below 350 °C.…”

“…The spin–orbit-split Pt 4f 5/2 and Pt 4f 7/2 peaks of PtTe 2 (labeled as Pt 4+ ) are located at 75.69 and 72.36 eV, respectively, consistent with previous reports for MBE-grown PtTe 2 . 14,25 From 250 to 300 °C, the line shape of Pt 4+ remains constant, while its peak position shifts to lower binding energy by ≈0.2 eV, which may be because that the film grown at 250 °C is more affected by the substrate due to its larger monolayer fraction. At 350 °C, another pair of peaks located at 74.73 and 71.40 eV appears, which matches well with the Pt 4f peak positions of Pt 2+ in PtTe or Pt 2 Te 2 (labeled as Pt 2+ ).…”

“…At 350 °C, another pair of peaks located at 74.73 and 71.40 eV appears, which matches well with the Pt 4f peak positions of Pt 2+ in PtTe or Pt 2 Te 2 (labeled as Pt 2+ ). 25,36 The Pt 2+ component becomes more pronounced at 400 °C, indicating that high growth temperature causes Te deficiency. In Fig.…”

“…24 The precise layer-dependent electronic structure of PtTe 2 is still controversial. Though MBE-grown PtTe 2 on highly oriented pyrolytic graphite (HOPG) was reported by Matthias Batzill's group, 14,25 its growth behaviour is still not well understood, and the bandgap of monolayer PtTe 2 is different from that obtained by angle-resolved photoemission spectroscopy (ARPES) study and calculations. 13,14 Hence, synthesizing high-quality ultrathin PtTe 2 films, and addressing its precise layer-dependent electronic structure are important for 2D PtTe 2 applications.…”