Modulating white light emission temperature in Ho3+/Yb3+/Tm3+ triply doped nanostructured GeO2-Nb2O5 materials for WLEDs applications

Materials based on Ho3+/Yb3+/Tm3+ triply doped with GeO2-Nb2O5 were prepared by the sol-gel methodology followed by thermal annealing at temperatures ranging from 700 to 1100 °C, which affected the crystalline structure and the white light generation of the solids. Trigonal GeO2 was detected in all the samples, whereas a tetragonal GeO2.9Nb2O5 mixed oxide was obtained between 900 and 1100 °C. The spectroscopic properties of the samples depended on the final crystalline structure. When upconversion measurements were performed by using a CW 980 nm laser as excitation source, a cool white light generation was detected for the sample annealed at 900 °C, more specifically; a cool white light has been acquired. In addition, the sample calcined at 1100 °C presented color purity higher than 96% in the yellow-green region where the anti-Stokes process was also being carried out. Excitation with ultraviolet light (245–280 nm) also generated white light. The emitted white light modulation was remarkable. The Cold white light was emitted under shorter wavelengths excitation, whereas warm white light was emitted under longer wavelengths excitation. The outstanding results show the potential of these materials for photonic applications, especially regarding white light emission in solid-state lighting for indoor and outdoor environments.