Fig. 1. Preparation process and SEM characterization of CNTs/Ag₁/Ag₂. (a) Sample preparation process; SEM images of CNTs/Ag1/Ag2 substrates under different volume ratios of CNTs/Ag1 and Ag2: (b) 1∶0, (c) 1∶0.5, and (d) 1∶2

Fig. 2. Schematic of CNTs/Ag₁/Ag₂ and FDTD simulation results. (a) Test schematic of CNTs/Ag₁/Ag₂ composite structure; (b1) Raman spectra of CNTs and CNTs/Ag₁/Ag₂ composite structure, (b2) Raman spectra of analyte to test; (c1)-(c3) Schematic of simulation structure of AgNPs solution with different volumes; (d1)-(d3) corresponding FDTD simulation results

Fig. 3. Raman measurements of R6G. (a) Average Raman spectra of R6G with different concentrations; (b) Raman mapping data of R6G; (c)(d) relationship between k, Raman intensity, and concentration at 613 cm^-1 and 773 cm^-1 respectively

Fig. 4. Calculated RSD values at different Raman frequency shifts. (a) 613 cm^-1; (b)773 cm^-1

Fig. 5. Raman measurement results of CV. (a) Averaged Raman spectra of CV molecules with different concentrations; (b) Calculated RSD values of Raman intensities and k at Raman frequency shift of 803 cm^-1 and 915 cm^-1