Experimental and DFT investigation of electronic structure, defect states, and visible luminescence in Co-doped ZnO nanocrystals

We report on surface defects, electronic structure, and visible luminescence in pristine and Co-doped ZnO nanocrystals (NCs) by combining experimental characterization with density functional theory (DFT) calculations. Co doping notably reduced the crystallite size (25-18 nm) and nearly doubled the dislocation density (δ), indicating a decline in crystal quality. X-ray photoelectron spectroscopy (XPS) confirmed the incorporation of Co ions in a high-spin Co²⁺ (3d⁷, e₄t₂³) configuration, with no detectable traces of Co³⁺ or metallic Co⁰ clusters. Photoluminescence (PL) spectra exhibited a dominant blue emission (2.7-2.8 eV), primarily due to electron transitions from the conduction band to zinc vacancy (V_Zn) acceptor states, further enhanced by Co-induced defects. A strong correlation between the experiment and DFT (Perdew-Burke-Ernzerhof generalized gradients approximation + U with mBJ corrections) elucidates the defect states responsible for visible emission. Our findings show how local atomic defects tune the optical properties and highlight the potential of Co-doped ZnO nanocrystals for blue light-emitting diode applications.