Phase Variations and Layer Epitaxy of 2D PdSe2 Grown on 2D Monolayers by Direct Selenization of Molecular Pd Precursors

Two-dimensional (2D) materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe 2 and Pd 2 Se 3 are two of the noble transition-metal chalcogenides with excellent physical properties that have recently emerged as promising materials for electronics, optoelectronics, catalyst, and sensors. This research presents a feasible approach to synthesize PdSe 2 and Pd 2 Se 3 with inherently asymmetric structure on honeycomb lattice 2D monolayer substrates of graphene and MoS 2 . We directly deposit a molecular transition-metal precursor complex on the surface of the 2D substrates, followed by low-temperature selenization by chemical vapor flow. Parameter control leads to tuning of the material from monolayer nanocrystals with Pd 2 Se 3 phase, to continuous few-layer PdSe 2 films. Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations, and heteroepitaxy at the atomic level. PdSe 2 with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe 2 and monolayer Pd 2 Se 3 are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe 2 and underlying 2D substrate was also revealed by ADF-STEM. These results offer both nanoscale and atomic-level insights into direct growth of van der Waals heterostructures, as well as an innovative method for 2D synthesis by predetermined nucleation.