Controlled synthesis and tunable upconversion luminescence of NaYW2O8:Yb3+/Ho3+ nanocrystals

Tetragonal phase NaYW2O8:Yb3+/Ho3+ nanocrystals were successfully synthesized using a hydrothermal method with a subsequent calcination treatment. Under 980 nm excitation, the 5S2/5F4 --> 5I8 and 5F5 --> 5I8 emissions were observed. The relative intensity of the 5F5 --> 5I8 to the 5S2/5F4 -->5I8 in the samples increased with increasing the pH values of the resultant solutions. The CIE coordinates of the upconversion luminescence were (0.43, 0.54), (0.41, 0.55), (0.40, 0.56), (0.39, 0.54), (0.37, 0.49), (0.35, 0.44), and (0.33, 0.37) for the samples prepared at pH = 7, 8, 9, 10, 11, 13, and 14, respectively. In addition, the upconversion mechanism was also investigated.