Engineering Optoelectronic and Excitonic Properties in Carbon Quantum Dots via Defect Encoding: A First-Principles Study

Carbon quantum dots (CQDs) are emerging nanomaterials with broadly tunable optoelectronic properties, yet the atomistic mechanisms by which structural defects govern their electronic structure, excitonic behavior, and optical response remain poorly understood. Here, we present a systematic first-principles investigation of eight defectencoded CQD configurations pristine, core vacancy, edge vacancy, graphitic nitrogen, pyridinic nitrogen, pyrrolic nitrogen (2.38% defect density), surface functionalization (C=O/CONH/NH₂/COOH), and edge vacancy-graphitic nitrogen (4.76% defect density) combining density functional theory (DFT), time-dependent DFT (TD-DFT), and Hirshfeld population analysis to establish a unified structure-property framework. DFT calculations reveal that defect type and position selectively reshape the electronic density of states and frontier orbital topology. Graphitic and edgegraphitic nitrogen act as n-type dopants that narrow the HOMO-LUMO gap, pyridinic and pyrrolic nitrogen localize frontier orbitals at edge sites producing wider gaps, vacancy defects introduce mid-gap dangling bond states, and surface oxygen functionalization generates heterogeneous charge distributions with pronounced C=O trap states. TD-DFT calculations show systematic optical tunability spanning 313-1193 nm, an 880 nm spectral window, with oscillator strengths ranging from f = 0.003-0.743 and transition dipole moments up to ~7.03 D, indicating large variations in optical brightness. Exciton binding energies vary from −0.19 to 1.88 eV, revealing a transition from weakly bound or charge-transfer excitations in vacancy structures to strongly localized excitons in nitrogen-doped CQDs. Three optical regimes emerge: UV-bright emitters, visible absorbers, and near-infrared active systems, corresponding to distinct charge redistribution modes confirmed by Hirshfeld analysis. These findings provide atomistic design principles for defect-engineered CQDs across fluorescence sensing, photocatalysis, bioimaging, and photothermal applications.