Upconversion properties in hexagonal-phase NaYF4:Er3+/NaYF4 nanocrystals by off-resonant excitation

Upconversion (UC) emissions in the hexagonal phase of NaYF4:10%Er3+/NaYF4 core/shell nanocrystals have been observed by off-resonant excitation at 1620 nm, whose energy is lower than that corresponding to the lowest f–f transition of Er3+ (4I15/2 → 4I13/2). The UC emission spectrum consists of five main peaks at around 1540 (4I13/2 → 4I15/2), 980 (4I11/2 → 4I15/2), 800 (4I9/2 → 4I15/2), 660 (4F9/2 → 4I15/2) and 540 nm (4S3/2 → 4I15/2). The infrared absorption spectrum shows a weak peak at around 1600 nm which is separated from the 4I15/2 → 4I13/2 absorption peak by 315 cm−1, and phonon bands of NaYF4 has been observed at about 300 cm−1 in the Raman spectrum. From these results, the UC emission is interpreted to involve a phonon-assisted excitation process. The UC emission intensity under off-resonant excitation at 1620 nm exhibits a much stronger temperature dependence than that under resonant excitation at 1550 nm, which is consistent with the phonon-assisted interpretation.