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+ (I-4(15/2) -> I-4(13/2)). The UC emission spectrum consists of five main peaks at around 1540 (I-4(13/2) -> I-4(15/2)), 980 (I-4(11/2) -> I-4(15/2)), 800 (I-4(9/2) -> I-4(15/2)), 660 (F-4(9/2) -> I-4(15/2)) and 540 nm (S-4(3/2) -> I-4(15/2)). 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. (C) 2013 Elsevier B.V. All rights reserved.