Lead-free halide double perovskite nanoflakes as high-performance SERS substrates for detection of trace organic pollutants: chemical enhancement <i>versus</i> electromagnetic enhancement

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic method known for its ultrasensitive characterization capabilities. Recently, lead-based halide perovskites incorporated with noble metals have gained much attention as SERS substrates. However, their practical applications are often hindered by the toxicity of the lead content and low stability. Herein, for the first time, we have synthesized lead-free halide double perovskite (DP) Cs₂AgBiBr₆ to overcome the toxicity, stability, and uniformity issues and studied the performance of lead-based perovskite SERS substrates. The self-trapped exciton (STE) defects were controlled by post-growth annealing of the sample under an argon (Ar) atmosphere, minimizing the Ag_Bi and Bi_Ag anti-site disorder. The sample with the highest STE defects demonstrates the highest SERS performance owing to the defect-assisted charge transfer process. We successfully identified methylene blue (MB) and rhodamine 6G (R6G) at concentrations as low as ∼10^-10 M, achieving a remarkable SERS enhancement factor (EF) of 5.04 × 10⁷ and 1.37 × 10⁷, respectively, which is highly significant for a semiconductor-based SERS substrate. Additionally, notable amplification was observed for other cationic dyes, including crystal violet (CV), rhodamine B (RhB), and malachite green (MG). By varying the annealing temperature and the deconvolution of the photoluminescence spectra, we demonstrate a direct correlation between the defect density and the SERS signal intensity. To further understand the underlying enhancement mechanisms, we analyzed the individual contributions of chemical and electromagnetic enhancements to the overall SERS amplification. This analysis was conducted using finite element method (FEM) simulations and density functional theory (DFT) computations. These insights provide a foundational basis for designing highly efficient metal-free SERS substrates, opening new possibilities for advanced detection technologies. Additionally, the Cs₂AgBiBr₆ substrate exhibited excellent stability, retaining performance after four months of storage under ambient conditions, highlighting its potential for environmental monitoring.