Phase Variations and Layer Epitaxy of 2D PdSe 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 and PdSe 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 and PdSe with inherently asymmetric structure on honeycomb lattice 2D monolayer substrates of graphene and MoS. 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 PdSe phase, to continuous few-layer PdSe films. Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations, and heteroepitaxy at the atomic level. PdSe with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe and monolayer PdSe are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe 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.