Electronic and Optical Properties of Graphene Oxide Quantum Dots: DFT Insights of Mixed Vacancy and Dopants

In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are used to understand the effects of monovacancy defects and heteroatom doping (O, B, N, and P) on the structural, electronic, and optical properties of graphene oxide quantum dots (GOQDs). Structurally, a single vacancy breaks three sp² bonds, while heteroatom substitution at the vacancy site promotes further structural relaxation, most notably in phosphorus-doped GOQDs. DFT-based chemical stability descriptors indicate that pristine GOQD possesses the highest hardness and lowest electrophilicity. Among heteroatom-doped GOQDs, O-N₂-SV-GOQD has improved electronic stability with high hardness and relatively low electrophilicity. In addition, TDOS and PDOS analyses show that vacancy defects and heteroatom doping significantly modify the electronic structure of GOQDs by introducing new states near the Fermi level and altering the band gap, which can influence their optical response. TD-DFT calculations have been performed to study the optical absorption spectra of different GOQDs. Our results show that vacancy defects and heteroatom doping modify the GOQD spectra, varying their intensity and shifting the bands across the visible region. The LHE results further confirm that vacancy defects and heteroatom doping can affect the light-harvesting performance of GOQDs.