Stability of sp³ Carbons in Hydrogenated Graphene Quantum Dots and Their Electronic and Optical Properties Studied Using Density Functional Theory

Graphene quantum dots (GQDs) are zero-dimensional nanomaterials composed of sp²-hybridized carbon atoms, which are widely researched because of their tunable optical properties. GQDs contain defects, such as sp³-hybridized carbon atoms, which may be introduced during synthesis and can affect these materials' properties. In this study, we use hydrogenated polycyclic aromatic hydrocarbons as models for GQDs containing sp³-hybridized carbon atoms. We analyze the effect of sp³ carbons on the stabilities and electronic and optical properties of GQDs using density functional theory (DFT) and time-dependent DFT calculations. We find that sp³ carbons can form stable arrangements as dimers or continuous chains along the edges of GQDs. Our results reveal that the presence of sp³ carbons can tune the HOMO-LUMO gap, dependent on the position of sp³ carbons within the GQD. Calculated optical absorption spectra show a reduction in intensity and a blue shift of the main absorption peak for most of the investigated sp³-containing structures; additionally, the presence of sp³ carbons can extend the optical absorption of these structures into the red and infrared regions of the solar spectrum (600 to 900 nm), depending on the concentration and arrangement of sp³ carbons. These results provide insight into structural factors responsible for the variation of the electronic and optical properties of GQD nanomaterials and suggest that controlling the amount of residual sp³ carbon atoms introduced during synthesis can be used to tailor the properties of GQDs.