In this study, carbon dots with low content and high content of nitrogen are synthesized. They show differences both in structure and optical emission. The low-content carbon dots are blue-emitting and possess hexagonal graphite crystalline core structure. The high-content are nanoparticles that are both blue- and green-emitting with a β-C3N4 crystalline core and high amount of amide groups grafted on the surface. The photoluminescence mechanism of carbon dots is a hotly debated area, with research effort focusing on the origin of the excitation dependence of photoluminescence. Three main mechanisms have been proposed: (1) quantum confinement or emission from the core, which originates from the conjugated pi-bonds in the carbon core; (2) the surface states, which are related to the presence of surface functional groups connected to the carbon backbone; and (3) the molecular groups, which are free or bonded fluorescent molecules. The surface state theory provides explanation for the pH sensitivity, solvation effect, oxidation effect and surface passivation effect observed on carbon dot emission. Surface states could arise either due to the termination of the carbon lattice at its interface with the environment, or the presence of adsorbed of bonded chemical groups. The latter is more likely to be responsible for surface-state emission, via the hybridization of carbon backbone with the connected chemical groups and the influence on electronic states. This research is devoted to elucidate the role of nitrogen in emission color of carbon dots. It shows that nitrogen is not only a dopant, but determines the structure of carbon dots and directly influences their optical properties. It would be interesting to see the effect of different nitrogen moieties on CD color by using aromatic or non-aromatic nitrogen compounds, such as pyridine, pyrroline, or phenylamine. This could reveal the relationship between CD emission and changes in band gap induced by functional groups. The effect of solvent on carbon dot color should also be established for nitrogen-containing or oxygen containing dopants, as both are commonly used to vary the emission color of carbon dots. The knowledge acquired would allow precise and efficient tuning of CD color by changing the molar ratio of precursors and/or the kind and volume of solvent.
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