The NaYF4∶Er3＋, Yb3＋ nanocrystals have been prepared by liquid crystallization from glycerol. Red at 660 nm and green at 550 nm upconversion emissions were observed from NaYF4∶Er3＋, Yb3＋ nanocrystals under 980 nm excitation. The X-ray diffraction patterns confirm the nanocrystals are cubic and hexagonal. The mechanism of nanocrystal upconvertion has been studied. The Stark splits of two excitated states of Er3＋ have been calculated based on theory of crystallization fields. The results confirm the upconverted mechanisms in the NaYF4∶Er3＋, Yb3＋ nanocrystals are due to two processes, one is a consecutive absorption of two 980 nm photon process, and the other process is that electrons transfer to substeady states after absorption of 980 nm photons, then absorb 980 nm photons.

Key words: NaYF4∶Er3＋, Yb3＋, upconversion, Stark split, crystallization from liquid