Fig. 1. Schematic diagram of the SERS substrate preparation device

Fig. 2. SEM characterization of the SERS substrate.（a）Fabricated SERS substrate；（b）Position 1；（c）Position 2；（d）Position 3；（e）Position 4；（f）Position 5；（g）Single silver nanoflower structure.

Fig. 3. Schematic representation of Raman spectrometer.（a）Optical setup for excitation and collection light；（b）Focused at different positions.

Fig. 4. Pictures of the surface coverage measured by a Raman spectrometer.（a）Laser is turned on；（b）Laser is turned off；（c）Binarized image；（d）Testing area.

Fig. 5. SEM pictures of different SERS substrates.（a）Substrate A；（b）Substrate B；（c）Substrate C；（d）Substrate D.

Fig. 6. Pictures of different SERS substrates under the Raman spectrometer.（a）Substrate A；（b）Substrate B；（c）Substrate C；（d）Substrate D.

Fig. 7. Raman spectra of the SERS substrate at different surface coverage levels.（a）Substrate A；（b）Substrate B；（c）Substrate C；（d）Substrate D.

Fig. 8. Relationship between the surface coverage and the normalized Raman spectral intensity.（a）Substrate A；（b）Substrate B；（c）Substrate C；（d）Substrate D.

Fig. 9. Coefficient of variation for different substrates

Fig. 10. Raman spectral intensities measured on the same substrate with multiple times.（a）10 μm spot；（b）20 μm spot；（c）40 μm spot.

Fig. 11. Effect of spot size on the spectral coefficient of variation