Synthesis, structure and optical studies of ZnO:Eu3+,Er3+,Yb3+ thin films: Enhanced up-conversion emission

Abstract

ZnO:Eu3+/Er3+/Yb3+thin films were deposited on Si (100) substrates using the sol–gel spin coating technique. The effect of Yb3+ concentration on the structure, particle morphology and optical properties of the films was examined. X-ray diffraction data showed that all the thin films crystallized in a hexagonal wurtzite structure of ZnO. The particle morphology and the distribution of atomic and molecular ionic species on the surface were investigated using scanning electron microscopy and time‐of‐flight secondary ion mass spectrometry, respectively. Atomic force microscopy was used to determine the surface roughness and particle sizes. Photoluminescence (PL) properties of the samples were measured using a mono-chromatized xenon lamp (conventional PL) and a 980 nm laser (up converted PL) as excitation sources. The conventional PL consisted of the normal defect emission. Yb3+ co-doping was found to improve the up converted (UC) PL intensity of both the RE3+ (Eu3+ and Er3+) ion emissions. This was ascribed to two possible energy transfer mechanisms from the 2F5/2 level of the Yb3+ ions to both the excited states of Er3+ and Eu3+ ions through Yb3+ → Er3+ & Er3+ → Eu3+ and Yb3+ → Eu3+ & Yb3+ → Er3+ on an individual behavior. The mechanism of energy transfer from Yb3+ to Eu3+ is explained by simplified energy level diagrams. The diagrams illustrate various defect levels and f-f transitions of the dopant ions. The emission colour was quantified by the standard international commission on illumination (CIE) chromaticity diagram.