Spectroscopic properties and tunable visible emission of Ho3+ doped CdS nanoparticles in silica glass

Abstract

CdS nanoparticles doped with Ho³⁺ were synthesized in a silica glass matrix via the in situ sol–gel method, and their structural and optical properties were systematically investigated. Structural characterization including FTIR at 100 °C and 350 °C confirms annealing-induced modification of the silica network. TEM analysis revealed an average particle size of 29.12 nm. The CdS emission band exhibited clear spectral overlap with Ho³⁺ excitation, suggesting a non-radiative sensitization pathway from CdS to Ho³⁺ ions. Absorption spectra displayed broad CdS band-edge absorption along with sharp f–f transitions of Ho³⁺, and the optical bandgap was estimated as 2.9 eV using Tauc analysis. Photoluminescence studies showed both CdS host emission and characteristic Ho³⁺ transitions, with maximum intensity achieved at 3 mol% Ho³⁺ doping. CIE chromaticity analysis indicated color purity exceeding 70 % for all samples, peaking at 91.47 % for 2 mol% Ho³⁺. Annealing studies revealed optimal PL emission at 250 °C, attributed to hydroxyl removal and defect-state formation, while higher annealing temperatures led to ion clustering and reduced defect density. Judd–Ofelt analysis yielded Ω₂ = 2.51 × 10⁻²⁰ cm², Ω₄ = 1.45 × 10⁻²⁰ cm², and Ω₆ = 1.31 × 10⁻²⁰ cm², with a spectroscopic quality factor of 1.10. Radiative parameters and optical gain analysis identified the ⁵F₄ → ⁵I₈ transition at 529 nm as the dominant green emission with high branching ratio. These findings highlight the tunable visible luminescence and favourable gain properties of Ho³⁺ doped CdS nanocomposites, demonstrating their potential for photonic and display applications.