Interfacial Reaction and Diffusion at the One-Dimensional Interface of Two-Dimensional PtSe 2 .

Two-dimensional (2D) PtSe 2 has potential applications in near-infrared optoelectronics because its band gap can be tuned by varying the layer thickness. There are several different platinum-selenide phases with different stoichiometries that result from high-temperature processing. In this report, we use in situ scanning/transmission electron microscopy (STEM) to investigate high-temperature phase transitions in 2D PtSe 2 and observe interfacial reactions as well as the Kirkendall effect. The 2D nature of PtSe 2 plays a key role in the unique one-dimensional interfaces that result during the formation of Se-poor phases (PtSe and PtSe 1- x ) at the edges of the PtSe 2 crystals. The activation energy extracted for this formation suggests that the process is mediated by Se vacancies, as evidenced by the large strain variations in the material made via 4D STEM measurements. The observation of the Kirkendall effect in a 2D material suggests routes to engineer 1D edge chemistry for contact engineering in device applications.