# Synthesis of Nitrogen-Doped Graphene Quantum Dots from Sucrose Carbonization
## 3 Results and Discussion  
### Structural Characterization  
The particle size distribution and morphology of the GQDs were evaluated by AFM (Figure 1). The undoped GQDs presented heights between approximately 9 and 21 nm and widths between approximately 344 and 396 nm. The aspect ratios (RA = width/height) of the undoped GQDs were, therefore, around RA = 29.15, which indicates that the particles had lamellar morphology, as expected for graphene and oxidized graphene nanoparticles. Clusters with heights higher than 100 nm were also found, which reflects the material's tendency to form clusters, especially after drying. On the other hand, the N-GQDs presented heights in the range of 0.8 and 5.0 nm and widths between 197 and 311 nm. In this case, the aspect ratio was RA = 106.53, once again reflecting lamellar morphology, as expected for graphene nanoclusters. It should be noted that the measurements for the widths of the GQDs may be overestimated due to the convolution effect of the AFM tip [42]. The heights obtained by AFM already indicate that undoped GQDs were slightly larger (1-20 nm) in comparison with N-GQDs (1-6 nm). Figure 1: AFM results of (**a**) undoped GQDs and (**b**) N-GQDs. The images on the left show AFM images of the graphene dots, and the curves on the right show profiles of particles that were highlighted in the AFM images. TEM images and size distribution of the GQDs are shown in Figure 2. The lattice-resolved high-magnification images of single quantum dots revealed d-spacing of 0.336 and 0.328 nm for GQDs and N-GQDs, respectively. These distances are in accordance with the (002) lattice planes of multilayered graphene. The average sizes of undoped GQDs were slightly larger in comparison with those of N-GQDs, as was observed in the AFM analysis. The diffractograms obtained for the different precursors used in the syntheses are shown in Figure 3. The carbonized precursor (Precursor A) showed only two broad peaks, located at 10\({}^{\circ}\) and 23\({}^{\circ}\). The broad peak near 10\({}^{\circ}\) can be associated with the presence of graphene oxide nanosheets [24; 43], and the peak centered at 23\({}^{\circ}\) could be caused by the presence of graphene nanosheets [24; 43]. It is interesting to note that in the doped precursor (Precursor B), in addition to the broad peaks, a much narrower peak was observed at approximately 10.8\({}^{\circ}\), corresponding to an interplanar distance of 0.818 nm. This peak can be associated with the crystalline plane (100) of graphene oxide [43; 44]. These results indicate, therefore, that the doping process carried out under reflux in nitric acid must also act on the condensation of graphene oxide nanosheets in order to generate larger sheets with fewer defects. Figure 3: XRD results of Precursor A and Precursor B used in the synthesis of the GQDs. Figure 2: TEM images of (**a**) undoped GQDs and (**b**) N-GQDs. The insets show size distribution of the particles. Figure 4 presents the diffractograms of the GQDs produced in this work. The curves obtained for the GQDs and N-GQDs were very similar, showing a broad peak centered at approximately 25\({}^{\circ}\) and a much narrower peak located around 26\({}^{\circ}\). The broad peak located around 25\({}^{\circ}\) was also observed in XRD analyses of pure graphene samples and graphene nanosheets [24; 43; 44]. The presence of the narrowest peak can be related to the crystalline planes (002) of graphite (d = 3.36 A) [24; 43] and is in accordance with TEM results. The presence of the peak associated with the graphite planes (002) shows that the treatment of the precursors under reflux in DMSO was responsible for both the condensation process of the graphene oxide nanosheets and the graphene oxide reduction process. This conclusion is supported by the fact that the broad peak at 10\({}^{\circ}\) (present in the precursors) was no longer present in the GQDs diffractograms after the DMSO treatment.