Calculations of electronic absorption spectra of polyciclic aromatic hydrocarbon models of graphene quantum dots

Abstract

We report on results of first-principles density-functional theory (DFT) calculations of energy levels of molecular orbitals (MO) and results of electronic excitation spectra calculations performed using Tamm-Dancoff approximation (TDA) to time- dependent density functional theory (TDDFT) of graphene quantum dots (GQDs) with edge functionalization. Polyciclic aromatic hydrocarbon models of GQDs functionalized with oxide and amine groups -OH, -COOH, -COH, NH2 were considered. The calculation results showed that with an increase in the diameter of the GQDs ELUMO-EHOMO decreases, the oxide groups -COOH, -COH shift these levels lower relative to the energy levels of the GQDs passivated by the -H atom due to the withdrawal of electrons from the graphene skeleton. The calculated electronic excitation spectra were compared with the UV- Vis spectra of hydrothermally synthesized carbon quantum dots. Comparison of the calculation results and experimental spectra shows a qualitative agreement, which make it is possible to suggest that the considered disk-shaped models of GQDs (coronene and C54H18 molecules) can be basic absorbing structures of the GQDs or carbon quantum dots.