# Synthesis of Nitrogen-Doped Graphene Quantum Dots from Sucrose Carbonization
## 3 Results and Discussion
### Photoluminescence (PL)
In the case of N-GQDs, the increase in the density of states caused by doping with nitrogen opens the possibility of excitation with photons in the blue/green region. In this case, the energy transition occurs from the valence band to an energy state originated by the presence of heteroatoms; phonon relaxations can occur, shifting the emission spectrum to longer wavelengths. Thus, nitrogen doping makes it possible to increase the probability of radiative transitions in the green and red regions, while it allows the realization of PL tuning through the control of the excitation wavelength. In general, this work showed that nitrogen doping was able to introduce energy states in the bandgap region of N-GQDs, allowing the observation of a highly tunable PL according to the excitation wavelength. On the other hand, the reflux treatment of DMSO may have contributed to the reduction of GQDs in order to minimize the presence of energy states associated with oxygen. We thus saw a much more intense PL in the red region for the N-GQDs since the energy states associated with nitrogen may have given rise to new mechanisms of lower-energy photon emission, as illustrated in Figure 10.  
Figure 11 presents the normalized emission curves of the GQDs for different excitation wavelengths. As already observed in the graphs in Figure 10, the undoped GQDs presented an emission band that varies between 400 and 650 nm. The peak emission wavelength  
underwent a small shift to longer wavelengths as the excitation wavelength increased, which may be associated with the influence of the size distribution of the GQDs and the presence of oxygen heteroatoms [28]. In N-GQDs, the emission bands extended from 450 to 700 nm. However, the dependence of PL on the excitation wavelength was remarkable; the sample excited at 420 nm had an emission peak at approximately 550 nm, and the same sample, when excited at 570 nm, had an emission peak centered at 640 nm. To better visualize the possibility of tuning the PL of the GQDs, Figure 12 presents the chromaticity diagrams obtained for the GQDs dispersed in DMSO. Figure 12a shows the chromaticity diagram for the undoped GQDs. In this work, chromaticity diagrams of the CIE 1931 color space [57] were used. The points marked on the diagram indicate how the color of the luminescent emission is physiologically perceived by human vision, and its x and y coordinates were calculated from the emission spectra, according to the methodology indicated by the international light commission [57]. For the GQDs without doping under excitation in the UV region, the occurrence of blue luminescent emission could be seen. As the wavelength increased, a shift in the color of the emission to the green region (for -440 nm excitations), then to yellow (under -500 nm excitation) and orange (under -500 nm excitation), was observed. Emission in the blue  
Figure 11: PL of (**a**) GQDs and (**b**) N-GQDs under different excitation wavelengths. Figure 10: Energy diagrams showing possible mechanisms for the PL of (**a**) GQDs and (**b**) N-GQDs. The energy states between the valence and conduction bands are inherent to the presence of heteroatoms in the structure of GQDs. region occurred due to interband transitions, as discussed earlier. The photos in Figure 12c illustrate the PL of the GQDs under excitation with cw lasers. N-GQDs presented emissions closer to the center of the chromaticity diagram under UV light excitation, indicating the possibility of white light generation. The emission of green light was already obtained for excitations in the range of approximately 400 to 420 nm. PL in the yellow region could be obtained by exciting the N-GQDs between 460 and 480 nm. Luminescent emission in the orange region was observed for excitations between 480 and 560 nm. Finally, emission in the red region was observed for pumping wavelengths greater than 560 nm. The luminescence tuning range in the N-GQDs was superior to that of the undoped GQDs since, in this sample, it was possible to verify luminescent emission up to the red region.