In-Situ Atomic-Scale Dynamics of Thermally Driven Phase Transition of 2D Few-Layered 1T PtSe₂ into Ultrathin 2D Nonlayered PtSe Crystals

Atomic resolution studies of 2D layered PtSe2 crystals under in situ heating conditions are performed with an aberration corrected scanning transmission electron microscope (STEM). We reveal the temperature driven phase change in PtSe2 driven by Se loss and stoichiometry modification. Annular dark field STEM (ADF-STEM) shows that the 1T PtSe2 layered structure transforms into nonlayered PtSe crystals that are 2D and ultrathin. Above 500 °C, PtSe2 rapidly transforms within minutes to this new phase. The detailed mechanism of this phase transformation is captured, revealing lateral diffusion and atomic digestion to yield PtSe crystals that are thicker than the original PtSe2 layered precursor but shorter in width to conserve total atom number. A minimum thickness of the PtSe crystals is observed. The behavior of PtSe2 is distinctly different from the phase transformations in the other Nobel metal chalcogenide PdSe2 and in transition metal dichalcogenides, such as MoS2 due to the presence of a stable 1:1 metal:chalcogen intermediate phase before pure metal formation during continuous vacancy addition. These results show how 2D layered precursor films can be transformed into stable nonlayered ultrathin crystals by depleting the chalcogen stoichiometry combined with thermally driven atomic migration.