Modulation of the emission intensity and color output of NaYF4 : Yb3+,Er3+ nanocrystals by OH−

Oleic acid together with sodium hydroxide (NaOH) was widely used to control the size, phase and morphology of NaYF4 UCNCs. However, reports are scarce about the modulation of the luminescence intensity and color output of NaYF4 UCNCs by using different amounts of NaOH. Here, we synthesized NaYF4:Yb3+, Er3+ nanocrystals by a hydro/solvent thermal method using oleic acid as ligand with different amounts of NaOH and systematically investigated their upconverted fluorescence spectra. The results indicated that the emission intensity of NaYF4 : Yb3+,Er3+, especially the green emission at 540 nm was dramatically increased as NaOH decreased. The enhancement factors of 540 nm emissions were 48, 42 and 32 for samples prepared with 0 g, 0.2 g and 0.4 g NaOH (the corresponding pH of the reaction solution were 5.9, 6.9 and 7.4), respectively, compared with that of the product prepared with the usually used 0.7 g NaOH (the corresponding pH was 8.3). Importantly, the ratio of green to red (RGR) increased from 0.4 to 5.9 when the pH decreased from 8.3 to 5.9. That is to say, the color output was tuned from orange yellow to nearly pure green. This resulted from the high energy vibration of OH−, which led to nonradiative relaxation from green emission levels (2H11/2 and 4S3/2) to the red emission level (4F9/2) and nonradiative relaxation from the red emission level (4F9/2) to the no emission level (4I11/2) of Er3+.