DFT and TD-DFT Investigation of Mg Adsorption, Electronic Structure, and Optical Properties of Finite M₂B₂ MBene Quantum Dots (M = Cr, Fe, Mn, Sc, Ti, and V).

The electronic structure, Mg adsorption behavior, and optical response of M₂B₂ MBene quantum dots (M = Cr, Fe, Mn, Sc, Ti, and V) were systematically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to evaluate their potential as anode materials for magnesium-ion batteries. The calculated adsorption energies demonstrate that the transition metal species strongly influences Mg binding, with V₂B₂ and Fe₂B₂ exhibiting the most favourable adsorption energies of - 1.44 and - 1.31 eV, respectively, accompanied by open-circuit voltages of 0.72 and 0.65 V. Among the investigated systems, V₂B₂ delivers the highest theoretical specific capacity of 780.93 mAh g⁻¹ while maintaining stable Mg adsorption energies ranging from - 0.94 to - 1.09 eV during successive Mg loading, indicating favorable electrochemical reversibility. Frontier molecular orbital, density of states, and charge-transfer analyses reveal pronounced Mg-V orbital hybridization and reduced HOMO-LUMO energy gaps upon Mg adsorption, leading to enhanced electronic conductivity. Furthermore, TD-DFT calculations predict tunable optical absorption extending from the near-infrared to the mid-infrared region with progressive red shifts as the Mg concentration increases. These findings provide molecular-level insight into the structure-property relationships governing Mg storage in MBene quantum dots and identify V₂B₂ as a promising candidate for next-generation magnesium-ion battery anodes.