Salt-Catalyzed Directed Growth of Bilayer Palladium Diselenide (PdSe2) Dendrites and Pd Nanoparticle-Decorated PdSe2–Pd2Se3 Junction Exhibiting Very High Surface Enhanced Raman Scattering Sensitivity

While the chemical stoichiometry does not change with a reduction in layer numbers for most of the two-dimensional (2D) layered materials, the multilayer Pd₂Se₃ in the noble transition-metal chalcogenides (NTMDs) group changes its stoichiometry to palladium diselenide (PdSe₂) in the bilayer form under Se-rich growth conditions. The experimental realization of PdSe₂–Pd₂Se₃ junctions and their application in various sensing applications are yet to be explored. Herein, we introduce a salt (NaCl) catalyzed chemical vapor deposition growth of bilayer (2L) PdSe₂ dendrites and PdSe₂–Pd₂Se₃ junctions on the mica substrate through the salt solution pretreatment. The pretreated structure triggers the formation of molten Pd–O droplets, which undergo a phase evolution from Pd nanoparticles (NPs) to Pd₂Se₃ (in Se-poor condition) to PdSe₂ (in Se-rich condition). Dendritic 2L PdSe₂ can be transferred from a growth substrate to an arbitrary substrate on a centimeter-scale through a polymer-free water-assisted transfer technique, which results in abundant PdSe₂–Pd₂Se₃ junctions due to the vacancy creation during the transfer process. Remarkably, Pd NPs hotspots on PdSe₂–Pd₂Se₃ junctions enable significant surface-enhanced Raman scattering (SERS) enhancement with an enhancement factor (EF ∼ 3 × 10⁵), which is more than 1 order of magnitude higher than that of 2L PdSe₂ to detect methylene blue molecules due to multiple factors, such as charge transfer and electromagnetic field enhancement. This is confirmed by density functional theory calculations and Finite element method (FEM) simulation, along with Raman imaging. The FEM simulations revealed an electric field enhancement factor of 5.546 × 10³ for Pd NPs decorated bilayer PdSe_2, and the remaining enhancement factor is expected to be contributed by charge transfer mechanisms. This work divulges the controllable protocol for the low-temperature chemical vapor deposition growth of 2L PdSe₂ and PdSe₂–Pd₂Se₃ junctions with facile transfer to arbitrary substrates and is indispensable for unleashing its full potential in a wide range of sensing, electronic, photonic, and biomedical applications.