Fig. 1. (a) TEM image and HR-TEM image (inset), (b) SAED pattern, (c) size distribution, and (d) XRD pattern of the NaYF4:Yb,Tm NCs.

Fig. 2. (a) Schematic diagram of energy levels and transitions of Yb3+ and Tm3+ ions by 980 nm pumping. (b) Upconversion luminescence spectra of NaYF4:20%Yb,2%Tm by 980 nm excitation at room temperature. The inset of (b) is the plot of peak intensity ratios, I345/I364, I346/I450, and I450/I474 versus Ip.

Fig. 3. Emission spectra versus different excitation power at (a) 345 nm and (b) 474 nm. The insets of (a) and (b) are the FWHM of the emission spectra of NCs lasers. (c) and (d) Output intensity and FWHM of the emission spectra at 345 nm and 474 nm versus different pump power density. Emission spectra at different observation angles at (e) 345 nm and (f) 474 nm. The left inset of (e) is the optical microscope image of the NaYF4:20%Yb,2%Tm NCs film. The inset of (f) is the sandwich structure of the proposed NaYF4:Yb,Tm NCs lasers.