Optical and Structural Properties of Co²⁺-Doped CsPbI₃ Nanocrystals Embedded in Borosilicate Glass

Co²⁺-doped CsPbI₃ nanocrystals (NCs) (CsPbI₃:xCo, x = 0, 5, and 10 mol%) were synthesized in situ within a borosilicate glass matrix by the fusion method followed by controlled thermal treatment at 500 °C for 6-24 h. Transmission electron microscopy images showed quasi-spherical NCs with mean diameters of 4.9-7.1 nm. Energy-dispersive X-ray spectroscopy suggested cobalt incorporation within the nanocrystalline regions. X-ray diffraction patterns confirmed the exclusive stabilization of the cubic α-phase across all compositions, with systematic lattice contraction from a = 6.321 Å to a = 6.301 Å with increasing Co content, consistent with preferential B-site substitution of Pb²⁺ by Co²⁺. Transmittance measurements confirmed macroscopic optical transparency of all glass-NC composites after thermal treatment. The crystal field theory and Tanabe-Sugano analysis for d⁷ ions in tetrahedral (Td) symmetry yielded Δ = 5032 cm^-1 and B = 725 cm^-1 in the as-prepared state, evolving to Δ = 4428 cm^-1 and B = 805 cm^-1 after thermal treatment, confirming Td Co²⁺ coordination and significant metal-iodide covalency. CIE 1931 chromaticity analysis revealed tunable emission from deep-red coordinates to near-white-light regions, demonstrating potential for LED and single-material WLED phosphor applications. Long-term photoluminescence measurements demonstrated full preservation of α-phase excitonic emission after approximately 365 days under ambient conditions, establishing the robust phase stability of CsPbI₃:xCo NCs embedded in borosilicate glass.