## 3 Results and Discussion  
### Characterization of the Developed Substrates  
The SEM image showed in Figure 2a is the anisotropic wet etched silicon micro-pyramid array structure. From the image, one can also see that the produced micro-pyramid array is a randomly sized pyramid array surface. This feature may be an advantage, as mentioned in [15], a random-sized pyramid array surface can have better light collection efficiency than a planar and single-sized pyramid array surface over a wide wavelength range. The average spacing between the micro-pyramids is about 2.8 \(\upmu\)m, and the average width of the micro-pyramids at the base is about 1.4 \(\upmu\)m. The AFM image in Figure 2b indicates that the average height of these micro-pyramids is about 2 \(\upmu\)m.  
A gold film with a thickness of 100 \(\upmu\)m was then deposited on the micro-pyramid array by sputtering and then electrochemical treated to form nanoparticles on the gold film surface. The size of these nanoparticles was controlled by electrochemical redox cycles. Figure 3a-d show SEM images of these gold nanoparticles with different electrochemical redox cycles of 0, 5, 10, and 15, respectively. As can be seen from these figures, the morphology of the gold film on the surface of the micro-pyramid structure after electrochemical treatment clearly shows the formation of many tiny nano-grain structures. It can be seen that the longer the duration, the larger the diameter of these nanoparticles. In the first 10 redox cycles, the diameter of these nanoparticles has relatively faster increasement, and the nanoparticles become larger and larger. After 10 redox cycles, the nanoparticles' changes are not obvious. However, if too many redox cycles are performed, the gold film will become too thin and expose the silicon pyramid (as shown in Figure 4). Figure 2: (a) The SEM image of pyramid microstructure formed by anisotropic wet etching. (b) The pyramid size measured by an atomic force microscope (AFM), the overall pyramid average height is 2 \(\upmu\)m, and the average spacing is 2.8 \(\upmu\)m.  
Figure 4: The gold film on the surface of the pyramid is oxidized and corroded. After too many oxidization-reduction cycles, the gold film became too thin and exposed the pyramid. Figure 3: (**a**) The gold film morphology on the surface of the micro-pyramid structure before electrochemical treatment, the surface is relatively smooth (compared with the electrochemically treated surface). (**b**) The gold film morphology after 5 redox cycles. (**c**) The gold film morphology after 10 redox cycles. (**d**) The gold film morphology after 15 redox cycles. Figure 3c,d shows that the diameter of the nanoparticles is about 20 nm when the redox cycle is 10-15 cycles. The gap between the nanoparticles is in the range of a few nanometers and is suitable for producing many intense hot spots. Therefore, it is very helpful to enhance the SERS effect. Figure 4 shows that when the redox cycle was set to 30 cycles, the gold film became too thin and exposed the underlying pyramid. When the gold film on the surface of the pyramid is subjected to cyclic voltammetry redox scanning, the gold on the surface is first oxidized into the solution and then reduced to the surface of the gold film. During this process, the gold surface is granulated to form many gold nano-gaps. In the experiment, we repeated the cyclic voltammetry redox scanning cycle multiple times. Figure 5 shows the cyclic voltammetry curves from the 1 st scan to the 15 th scan cycle in a 0.2 M KCl solution at a scan rate of 50 mV s\({}^{-1}\). The position of the blue arrow in the figure shows the position where the oxidation peak occurs, which is about 530 mV. The oxidation peak currents were further plotted against the scanning cycles in Figure 6. As can be seen from Figures 5 and 6, when the number of scanning cycles increased, the peak current of the oxidation also increased. When it reaches a certain amount of cycles (in this case around 10 cycles), the oxidation peaks reach a plateau state.