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

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 F-2(5/2) 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.