## 4 Results and Discussion  
### Characterization of the Developed Substrates  
SEM image showed in Figure 2 are the hot imprinted nanocone array on the PC substrate. The spacing between the nanocones is about 130 nm, the width of the nanocone at the base is about 90 nm, and the height of the nanocone is about 140 nm. Silver nanospheres were then deposited by sputtering and the size of these nanospheres was controlled by sputtering duration. Figure 3a-f show SEM images of these silver nanospheres with different sputtering durations of 50, 100, 150, 200, 250 and 300 s, respectively. As can be seen from these figures, the silver nanospheres are uniformly decorated on top of the nanocone array substrate. It can be seen that the longer the duration, the larger the diameter of these nanospheres. However, If the sputtering duration is too long, the nanospheres will start to merge. Figure 3d shows that the diameter of the nanospheres is about 120 nm when the sputtering duration is set to 200 s. This results in a gap between the nanospheres in the range of a few nanometers and is therefore suitable for generating many intense hot spots, which is very helpful for enhancing the SERS effect. Figure 3e,f shows that when the sputtering duration is set to 250 and 300 s, the silver nanospheres start to merge together. We show in Figure 4 the relationship between the sputtering time and the size of the obtained nanospheres and the gap between the nanospheres. It can be observed that before the nanospheres merge, there is a linear relationship between the size of the nanospheres and the gap them. Therefore, we can adjust the sputtering time accordingly to change the gap size of the nanospheres to optimize the SERS enhancement. After carefully adjusting the sputtering parameters, we were able to make a uniform array of silver nanospheres with a few nanometers interspacing. These uniform and closely packed silver nanospheres can produce a large number of uniformly distributed plasmonic hotspots, which results in high surface enhancement factors and good uniformity. The advantages of this method are that we can prepare a uniform array of nanocones very quickly and at low cost, and the size of the silver nanoparticles can be easily adjusted to change the gap size between them. EDS was used to analyze the silver nanospheres decorated nanocone array substrate. The EDS spectrum (Figure 5b) shows that the SERS active substrate consisted of 77.5% Ag and 11.48% C. The EDS mapping of Ag of SERS substrate is shown in Figure 5c. From the figure, we can see that the elemental mapping of Ag demonstrates a uniform distribution of the Ag nanosphere on the SERS active substrate. The UV-vis absorption spectrum was used to analyze the surface plasmon resonance (SPR) characteristics of the proposed SERS active substrates. A series of absorption spectra for the SERS substrate with different sputtering durations (different nanospheres diameters) are shown in Figure 6. As the sputtering duration increase from 100 to 200 s, the diameters of the nanosphere increase from 70 to 125 nm, the absorption peak shifts to the red from 450 to 680 nm. We can observe that the SERS substrate with 200 s sputtering duration shows a broad peak at a wavelength of \(\sim\)680 nm, and it is closer to the laser excitation wavelength of 632.8 nm. We believe that this absorption peak is caused by the excitation of surface plasmons. Ideally, it would be optimal to use a laser with a wavelength close to 680 nm. The reason we used the 632.8 nm laser is that our Raman system only has a 632.8 nm wavelength laser. If a 680 nm laser is used as the excitation source, it can be expected that the SERS substrate can have a better performance. Comparing these SERS substrates with different sputtering durations, the SERS substrate with a 200 s sputtering duration also showed the highest SERS signal intensity in subsequent experiments (see Figure 7). Figure 2: (**a,b**) shows the 45 degree view of the hot imprinted nanocone array.