Up-conversion in sol–gel derived nano-glass–ceramics comprising NaYF4 nano-crystals doped with Yb3+, Ho3+ and Tm3+

NaYF4 is an excellent host material for rare-earth ions presenting very high efficiencies in up-conversion processes. Thus, nano-glass–ceramics containing NaYF4 nano-crystals emerge as promising candidates for general lighting appliances and integrated optical devices. Here we report highly transparent sol–gel derived nano-glass–ceramics comprising Yb3+–Ho3+ and Yb3+–Ho3+–Tm3+ co-doped NaYF4 nano-crystals. A structural analysis by means of X-ray diffraction measurements confirmed the formation of NaYF4 nano-crystals during thermal treatment. Luminescence features have been related to the crystallinity degree of the samples. Violet, blue, green and red up-conversion emissions were obtained under infrared excitation at 980 nm and corresponding mechanisms involved have been analysed. Additionally, the total visible up-conversion emission has been quantified in terms of the standard chromaticity coordinates. In particular, an overall colour emission, very close to the standard equal energy white-light illumination point of the chromaticity diagram, was obtained in the Yb3+–Ho3+–Tm3+ triply-doped samples.