Colour Tuneability in Sol–Gel Nano-Glass-Ceramics Comprising Yb3+-Er3+-Tm3+ Co-Doped NaYF4 Nanocrystals

White light can be obtained by up-conversion processes in a cheap and efficient way, being of great interest for lighting, displays, and photonic integrated devices. Here we report a generation of white light under 980 nm laser infrared excitation in Y(b3+)-Er(3+)-Tm3+ triply doped sol-gel derived nano-glass-ceramics containing NaYF4 nanocrystals where co-dopants ions are partitioned. Efficient simultaneous blue, green, and red up-conversion emissions give rise to a balanced white overall emission and corresponding up-conversion mechanisms have been analyzed. Changes in pump power intensity or heat treatment temperature widely vary the ratio of emission bands, resulting in a colour tuneability which has been analyzed and quantified in terms of CIE standard chromaticity diagram.