## 3 Results and Discussion  
### Material Characterization  
3.1.1. Morphological and Structural Characteristics of N/S-Carbon QDs  
From the diffractogram (Figure 2) of N/S-Carbon QDs, two broad peaks around 30\({}^{\circ}\) and 40\({}^{\circ}\) can be observed, which correspond to the amorphous phase of graphitic carbon structure, according to previous reports [28; 29]. In addition, the peaks located at 27.7\({}^{\circ}\), 31.3\({}^{\circ}\) and 45.3\({}^{\circ}\) correspond to the Fm-3m cubic calcium crystal structure (PDF 23-430) [30], thus indicating the existence of calcium particles in the material after synthesis. SEM-EDS analysis and surface mapping were performed on the low-magnification image of the chromium-sputtered sample in order to elucidate the surface elemental composition of the N/S-Carbon QDs. The results (Figure 3) showed that the surface structure of N/S-Carbon QDs is mainly composed of carbon, oxygen, and nitrogen atoms while sulfur atoms were found to be present although at a low concentration. Interestingly, according to the same results, calcium is also present on the surface of the material in a very low atomic ratio. This finding is in agreement with the aforementioned XRD results, confirming the existence of residual calcium particles in the material after synthesis, which could be attributed to the calcium content of the precursor (crayfish shells). Additionally, the white-colored endogenous particle spots appear to be sodium chloride particles since the elemental mapping showed that both sodium and chlorine atoms are the only ones  
Figure 2: XRD plot of N/S-Carbon QDs. present in these areas while the rest of the aforementioned atoms are absent. Moreover, the fact that their atomic ratios were found to be quite close further confirms the above claim. AFM images (Figure 4) show that N/S-Carbon QDs are spherical, with a diameter varying between 5 and 8 nm (average size = 6.56 nm), thus confirming their \"quantum\" morphology. In particular, N/S-Carbon QDs were found to have thicknesses of 5.362, 7.633, 7.923, and 8.026 nm, which were higher than the average of 3 nm recently reported for N-doped Carbon QDs [13]. The ATR-FT-IR spectrum of N/S-Carbon QDs is presented in Figure 5. As can be observed, the characteristic peak at 2960 cm\({}^{-1}\) is assigned to the stretching vibration of C-H bonds [28; 31], while the broad band between 3000 and 3500 cm\({}^{-1}\) confirms the presence of O-H and/or N-H bonds (bending vibration) [31; 32]. Additionally, the peak at 669 cm\({}^{-1}\) is assigned to the stretching vibration of N-H bonds while the sharp peak at 1397 cm\({}^{-1}\) is related to the stretching vibration of C-N bonds [31; 33]. The peak at 1630 cm\({}^{-1}\) is attributed  
Figure 4: AFM images, AFM section analysis, and size distribution of N/S-Carbon QDs. Figure 3: (**a**) EDS spectrum of the identified elements on the surface of N/S-Carbon QDs and (**b**) EDS surface mapping images for N/S-Carbon QDs. to the double bond vibration of C=O as well as C=C [31; 33; 34] while the peak at 1558 cm\({}^{-1}\) corresponds to the stretching vibration of the N=O double bond [31; 33; 34]. Lastly, the bands at 1050 cm\({}^{-1}\) and 1153 cm\({}^{-1}\) are related to the stretching vibrations of C-O and C-S groups [31] and the band at 2360 cm\({}^{-1}\) is observed as a result of the stretching vibration of the S-H bond thus confirming further the presence of sulfur. Overall, the ATR-FT-IR results are in agreement with the data obtained from EDS analysis as they confirm the presence of carbon, oxygen, nitrogen, and sulfur in the N/S-Carbon QDs structure and are consistent with previous reports [31; 32; 33; 34]. XPS studies (Figure 6a), such as EDS, revealed that the surface of N/S-Carbon QDs is mainly composed of carbon (76.2%), oxygen (9.9%), and nitrogen (13.9%).