Evidence for intrinsic defects and nanopores as hotspots in 2D PdSe2 dendrites for plasmon-free SERS substrate with a high enhancement factor

Abstract Surface-enhanced Raman spectroscopy (SERS), a very powerful tool for the identification of molecular species, has relied mostly on noble metal-based substrates to obtain a high enhancement factor. In this work, we demonstrate that self-driven intrinsic defects in 2D palladium di-selenide (PdSe₂) dendrites grown at low temperature (280 °C) act as hotspots for high SERS enhancement. We grow 2D dendritic PdSe₂ with ample intrinsic defects to exploit it for SERS application. X-ray electron spectroscopy (XPS) analysis reveals 9.3% outer layer and 4.7% interior Se vacancies. A detailed examination of atomic-scale defects revealed Se vacancy (V_Se) coupled with Se–Pd–Se vacancy (V_Se-Pd-Se) in monolayer PdSe₂, and an array of line defects (Se vacancies) and nanopores in bilayer PdSe₂ dendrites. Interestingly, our studies reveal that Se vacancy-rich PdSe₂ gives rise to line defects that act like hotspots for SERS enhancement. Remarkably, the vacancy-rich dendritic PdSe₂ shows a SERS enhancement factor >10⁵ and can detect RhB at a concentration down to 10⁻⁸ M. We speculate that the topological line defects and the edge construction in PdSe₂ dendrites act as metallic wire or edge, which is partly responsible for the high enhancement in the SERS signal. The high SERS sensitivity is explained on the basis of multiple charge transfer processes combined with the predicted metal-like behavior of the defected 2D PdSe₂. Our conclusions are fully supported by the density functional theory calculation of the electronic density of states of the defective bilayer (2L) PdSe₂, which remarkably exhibits metallic character. Being a defect-enabled SERS substrate, dendritic 2D PdSe₂ fills the gap between conventional plasmonic SERS substrate and plasmon-free SERS substrate.