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 (VZn) 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.