Phase transition, size control and color tuning of NaREF4:Yb3+, Er3+ (RE = Y, Lu) nanocrystals.

It is important to finely tune the upconversion (UC) properties through controlling size and structure of upconversion nanocrystals (UCNCs) and improve their efficiency. Recently, it was reported that the UC luminescence (UCL) intensity of NaLuF4:Yb(3+), Er(3+) UCNCs could be tremendously improved compared to the most efficient NaYF4:Yb(3+), Er(3+) under 980 nm excitation. In this paper, we first studied the varied size control and phase transition properties of NaREF4:Yb(3+), Er(3+) (RE = Y, Lu) UCNCs prepared by the solvothermal method. Then, we discussed if the UCL intensity of NaLuF4:Yb(3+), Er(3+) could be universally improved over that of NaYF4:Yb(3+), Er(3+) UCNCs. Furthermore, the color tuning properties of the NaREF4:Yb(3+), Er(3+) family on crystalline phase, particle size, doping concentration and phonon energy were systemically discussed. It is very interesting to observe that in heavily doped cubic NaREF4:Yb(3+), Er(3+), the color of UCL changed from red to green with decreasing particle size, while on the contrary, in hexagonal phase the color of UCL changed from green to red. A unit model was proposed to explain the phase-dependent, size-dependent color tuning UCL behavior of NaREF4:Yb(3+), Er(3+).