Controllable Phase Transformation by Van der Waals Encapsulation in Electrochemically Exfoliated PdSe₂ Nanosheets

Abstract 2D orthorhombic palladium diselenide is attracting rapidly increased interest by virtue of its fascinating physical properties and feasibility of phase transformation. However, it remains a major challenge to produce ultrathin PdSe 2 through a facile chemical route and control phase transformation because of its anisotropic structure with strong interlayer coupling. Here, the efficient synthesis of few‐layer PdSe 2 nanosheets with large sizes using an electrochemical exfoliation approach is reported. Upon thermal annealing at 300–350 °C, the as‐exfoliated PdSe 2 nanosheets transform into metallic phase PdSe 2‐ x , as verified by scanning transmission electron microscopy, Raman spectroscopy, and electrical characterizations. Simple encapsulation using hexagonal boron nitride (h‐BN) can effectively suppress the Se‐loss triggered phase transformation, so that a metal‐semiconductor junction is formed by local phase modification. The fabricated PdSe 2 field‐effect transistors exhibit p‐type transport property, which is in stark contrast to electron‐dominated ambipolar transport of pristine PdSe 2 devices. The combination of high‐resolution X‐ray photoelectron spectroscopy and cross‐sectional transmission electron microscopy analysis reveals that the modulation of carrier polarity in h‐BN encapsulated PdSe 2 should arise from the p‐doping effect associated with the impact of interfacial condition. The study opens up a new route for future phase‐engineered electronics in PdSe 2 and other 2D noble metal dichalcogenides materials.