Nature of Absorption Bands in Oxygen-Functionalized Graphitic Carbon Dots

Carbon dots (CDs) belong to a class of materials considered technologically important for their tunable absorption and emission properties and a huge application potential in cell labeling, theranostics, and optoelectronic technologies including LED diodes. Although improvement of their properties relies on a fundamental understanding of the underlying photophysical processes, this is currently far from complete. Here, we analyze the absorption spectra of nontrivial multilayer graphitic oxygen-functionalized CD models. The results suggested that the experimentally observed broad bands around 300 nm originated from n -> pi* and pi -> pi* charge transfer transitions, whereas the effects of structural/energetic disorder, water environment, deprotonation, and excitonic coupling only weakly contributed to the spectra when compared to their monolayer counterparts. Owing to their weak interlayer interactions and thermal accessibility of low-energy conformations, the graphitic CDs are prone to structural disorder and consequent spectral-line broadening.