Temperature-dependent upconversion luminescence and dynamics of NaYF4:Yb3+/Er3+ nanocrystals: influence of particle size and crystalline phase.

Oleic acid-capped NaYF4:Yb(3+)/Er(3+) upconversion nanocrystals (UCNCs) with different sizes and crystalline phases were prepared, and their temperature-dependent upconversion luminescence (UCL) and dynamics were studied. It is interesting to observe that the temperature-dependent behavior of UCL for the β-phase (25 nm, 45 nm and bulk) and α-phase (<10 nm) UCNCs is quite different. The UCL intensity of Er(3+) ions of the β-phase NaYF4 demonstrates a maximum around 100 K, while the intensity of the α-phase quenches monotonously with elevated temperature (10-400 K). The intensity ratio of (2)H11/2-(4)I15/2 to (4)S3/2-(4)I15/2, RHS, increases solely with temperature for β-phase NaYF4, while for the α-phase sample, it demonstrates a complex and indeterminate variation as a function of temperature. In the β-phase samples, rising processes were observed in the dynamics of Er(3+) ions, while in the α-phase sample, no rising process was observed and the decay processes of Er(3+) ions were bi-exponential. The rationale for these different temperature-dependent UCL properties was explained carefully.