# Synthesis of Nitrogen-Doped Graphene Quantum Dots from Sucrose Carbonization
## 3 Results and Discussion
### Photoluminescence (PL)  
Figure 9 shows the PL contour plots of the GQDs for different excitation wavelengths, where the y-axis shows the excitation wavelengths, the x-axis shows the emission wavelengths, and the color bar scale indicates the PL intensity. The GQDs presented maximum PL intensity for excitation at \(\sim\)350 nm. In this case, the emission band of the sample extended from approximately 400 to 650 nm. It is noteworthy that the excitation at 350 nm (\(\sim\)3.54 eV) was higher than the bandgap energy determined by the Tauc analysis (see Figure 7). Figure 8: Direct bandgap Tauc plots for GQDs and N-GQDs. The bandgap energies are determined by extrapolating the linear part of the absorption edge (dashed lines) to the x-axis at y = 0. Figure 9: Emission–excitation–intensity contour plots of (**a**) undoped GQDs and (**b**) N-GQDs. In N-GQDs, it was observed that PL was clearly dependent on the excitation wavelength; when the sample was excited at \(\sim\)350 nm, the photoluminescence behavior was similar to that of undoped GQDs samples, in which the luminescent emission extended from 450 to 650 nm, indicating that for excitations in the UV region, the luminescence mechanism occurs through an interband transition. However, the N-GQD nitrogen-doped sample also showed luminescent emission for a wide range of excitation wavelengths, from \(\sim\)320 to 600 nm. It was also verified that the PL was redshifted when the sample was pumped at longer wavelengths of excitation. The presence of heteroatoms, such as nitrogen and oxygen, is expected to establish new energy states in GQDs. In the literature, these states are called surface states [4,23] or defect states [12,48,55]. Theoretical studies involving density functional theory (DFT) have indicated that in quantum dots based on graphene monolayers, the presence of functional groups with oxygen can change the bandgap of the material, especially when they are present inside the quantum dot [28]. The effect of the bandgap change is, however, much smaller when the functional groups with oxygen are bound at the edges of the graphene particles [28]. In general, electron-donating functional groups, such as epoxy groups (C-O-C), hydroxyl groups (-OH), and ethers (-OCH3), act by decreasing the bandgap of the material, and electron accepting groups, such as aldehydes (-CHO) and carboxyls (-COOH), result in an increase in the quantum dot bandgap [28]. Nitrogen can be found in graphene in three configurations: graphitic nitrogen, pyridine nitrogen, and pyrrole nitrogen [27,29]. In all configurations, a decrease in the bandgap is observed in DFT analyses [30,31,56]. However, graphitic nitrogen allows the greatest reduction in bandgap [27,29]. In general, theoretical DFT studies concerning analyses of GQDs generally involve simulations of isolated particles with a known number of atoms in a specific solvent (generally water) [29]. On the other hand, experimental GQD suspensions have a wide variety of particles with distinct characteristics. In this way, the size distribution of the GQDs, the number of layers, and the presence of intrinsic defects (such as carbon vacancies) and extrinsic ones (such as the presence of heteroatoms) can also act to modify the densities of the energy states of the system [1,27]. The entire set of characteristics mentioned above can generate a complex mechanism for the photoluminescence of GQDs [1]. To better illustrate the influence of doping on GQDs, Figure 10 shows an illustration of some of the different possible paths for observing photoluminescence in doped and non-doped GQDs. The photoluminescence presented by undoped GQDs occurs primarily through an interband transition, excited by photons in the UV region, followed by non-radiative relaxations to energy states associated with oxygen functional groups. From heteroatoms energy states, radiative relaxations may occur with the emission of photons, mainly with wavelengths between 400 and 500 nm.