## 3 Results and Discussion
### Material Characterization
#### 3.1.2 Optical Properties of N/S-Carbon QDs  
As can be observed in Figure 7a, the absorbance of N/S-Carbon QDs extended towards the visible region at around 700 nm [35; 36]. This absorption in the visible spectrum is probably related to the presence of calcium in the structure of the material, as revealed by XRD and EDS. Similar results were reported by [PERSON] et al. [36], who prepared calcium-capped carbon QDs with absorbance in the visible light region with a maximum at 405 nm. In the case of the as-prepared N/S-Carbon QDs, the maximum absorbance for N/S-Carbon QDs was displayed at 334 nm and could be attributed to the n \(\rightarrow\)\(\pi^{\star}\) transition of conjugated C=O or C=N. The indirect band-gap energy (E\({}_{\text{g}}\)) for N/S-Carbon QDs was calculated by plotting the transformed Kubelka-Munk function [37] versus the band gap energy as shown in Figure 7b. The calculated E\({}_{\text{g}}\) value (1.118 eV) was found to be much lower than those calculated for carbon QDs obtained from shrimp shell chitin and chitosan [38]. This is related to the aforementioned absorbance in the visible spectrum, which is possibly due to the presence of calcium atoms. Fluorescence spectroscopy results (Figure 8a) revealed that when the N/S-Carbon QDs are excited at \(\lambda_{\text{EX}}\) = 300 nm they emit at \(\lambda_{\text{EM}}\) = 418 nm. From Figure 8b,c, it becomes evident that as the excitation wavelength increases, so does the intensity of the emission peak, with the emission wavelength showing little to no variation. However, after \(\lambda_{\text{EX}}\) = 340 nm, the intensity of the emission peak starts to gradually decrease. Furthermore, as the excitation wavelength increases beyond this point, a shift in the emission peak to higher wavelengths is observed, which is a phenomenon known as red shift [39]. Therefore, for \(\lambda_{\text{EX}}\) > 340 nm, the emission wavelength of N/S-Carbon QDs appears highly dependent on the applied excitation wavelength. These results are in agreement with the findings of other \begin{table}
\begin{tabular}{c c c c c} \hline
**N\%** & **C\%** & **H\%** & **S\%** & **C/N** \\ \hline
10.3 & 53.6 & 7.1 & 6.3 & 5.2 \\ \hline \end{tabular}
\end{table} 
Table 1: Elemental composition of N/S-Carbon QDs. Figure 6: (a) XPS spectrum of N/S-Carbon QDs and high-resolution XPS spectra of (b) C1s, (c) N1s, and (d) O1s. photoluminescence studies on crayfish-derived CQDs, according to which the emission wavelength and the emission intensity depend on the emission wavelength [40]. This photoluminescence behavior of the N/S-Carbon QDs could be the result of the differences between the material's particle sizes and the surface defects caused by the presence of various functional groups [40]. Figure 8: (**a**) Fluorescence emission spectra at specified excitation wavelengths, (**b**) excitation– emission contour plot; (**c**) 3D excitation-emission spectra and of N/S-Cthe sclerbon QDs. Figure 7: (**a**) UV–vis DRS spectra of N/S-Carbon QDs and (**b**) Kubelka–Münk plot. The quantum yield of the as-prepared N/S-Carbon QDs was calculated using the equation [40]: \(\text{QY}_{\text{S}}=\text{QY}_{\text{R}}\)\(\frac{\text{S}_{\text{S}}}{\text{S}_{\text{R}}^{\text{S}}}\times\)\(\frac{\text{A}_{\text{R}}}{\text{A}_{\text{S}}}\)\(\times\)\(\frac{\text{n}_{\text{S}}^{2}}{\text{n}_{\text{S}}^{2}}\)), where \(\text{QY}_{\text{S}}\) and \(\text{QY}_{\text{R}}\) are the quantum yields of the sample and reference compound, respectively; \(\text{A}_{\text{S}}\) and \(\text{A}_{\text{R}}\) are their absorptions at the excitation wavelength (350 nm); \(\text{S}_{\text{S}}\) and \(\text{S}_{\text{R}}\) are the integrated areas of their corresponding emission peaks; and \(\text{n}_{\text{S}}\) and \(\text{n}_{\text{R}}\) are the refractive indices of the solvent, which in both cases is distilled water. N/S-Carbon QDs demonstrated a \(\text{QY}_{\text{S}}\) of \(\sim\)8 %, which is very close to the QY values of other crayfish-derived QDs [40], as well as fruit-derived QDs [40].