# Synthesis of Nitrogen-Doped Graphene Quantum Dots from Sucrose Carbonization
###### Abstract  
The synthesis of carbon-based quantum dots has been widely explored in the literature in recent years. However, despite the fact that synthesis processes to obtain highly efficient carbon quantum dots (CQDs) and graphene quantum dots (GQDs) with redshifted photoluminescence (PL) have been improved, few works have exploited sucrose in the synthesis of GQDs with high PL efficiency. In this work, sucrose, which is a widely available non-toxic saccharide, was used as a precursor of GQDs. Initially, sucrose was carbonized in sulfuric acid, and thereafter, the material obtained was treated in dimethyl sulfoxide (DMSO). Nitrogen doping was also performed in this work through an additional step involving the treatment of carbonized sucrose in nitric acid reflux. Nitrogen-doped GQDs (N-GQDs) showed tunable PL dependent on the excitation wavelength. It was also verified that the intensity of the emission in the red region was much higher in the N-GQDs in comparison with that in undoped GQDs. X-Ray Diffraction, Raman, FTIR, TEM, and AFM analyzes were also performed to obtain greater structural details of the obtained GQDs. carbon; quantum dots; graphene; luminescence; synthesis; sucrose +
Footnote †: journal: Applied  
predominantly carbons with sp\({}^{2}\) hybridization, and their structure is based on single-layer graphene nanosheets or graphene nanoparticles with few layers [10; 11; 16]. The presence of sp\({}^{3}\) hybridized carbons can give rise to \"cross-links\" between sheets, resulting in CQDs with disordered structures. The size of the sp\({}^{2}\) domains, therefore, has a great influence on the optical properties of GQDs [1; 18]. The quantum confinement effect is generally used to explain the PL of semiconductor quantum dots and occurs when the movement of charge carriers (electrons and holes) is constrained in the three dimensions of space [17; 20]. In the case of GQDs, quantum confinement must be considered when the particle size or the size of the sp\({}^{2}\) domains (also known as conjugated \(\pi\) domains) is smaller than the Bohr radius of the exciton [17; 18]. Quantum confinement results in an increase in the bandgap energy of the material since in these materials, the density of the energy states is much lower compared with the same massive material [17; 18]. It should be noted that colloids obtained experimentally can present CQDs or GQDs with a wide size distribution and different chemical characteristics, which can result in wider absorption and emission bands [1; 21]. In this sense, the use of separation techniques, such as chromatography and centrifugation, can also be used to tune the PL of the CQDs [21; 22]. The PL of the GQDs can also be influenced by the doping of the GQDs with heteroatoms, such as oxygen, nitrogen, boron, sulfur, and phosphorus, among others [18; 23; 24; 25; 26; 27; 28; 29]. In general, electron-accepting impurities act by increasing the energy gap between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) [2]. On the other hand, electron-donating impurities act to reduce the energy gap between HOMOs and LUMOs. The presence of functional groups with oxygen (alcohols, epoxy, carbonyls, carboxyls, and aldehydes) on the surface of GQDs is quite common, especially when the synthesis is carried out in an aqueous medium [1]. Nitrogen doping can be achieved with the introduction of urea [30; 31] or treatment with nitric acid [32] during synthesis. It is noteworthy that studies involving the doping of GQDs with nitrogen have been highlighted in the literature [24; 25; 26] due to the similarity that the nitrogen atom presents in relation to the atomic radius and electronegativity of carbon. In general, the presence of nitrogen causes a redshift of the PL and provides an increase in the quantum yield of the GQDs. Several recent studies involving density functional theory (DFT) have been carried out in order to improve the understanding of the effects of the introduction of dopants on the structure of GQDs [27; 28; 29].