Wide-range and highly-sensitive optical thermometers based on the temperature-dependent energy transfer from Er to Nd in Er/Yb/Nd codoped NaYF4 upconversion nanocrystals

In this paper, we report the temperature-dependent energy transfer (ET) process between Er3+ and Nd3+ ions and optical temperature sensing properties based on the upconversion (UC) luminescence of Er3+ and Nd3+ in Er/Yb/Nd codoped NaYF4 nanocrystals synthesized via a hydrothermal method. The decrease of green and red UC emissions of Er3+ and the increase of infrared UC emissions of Nd3+ at elevated temperatures indicated an effective ET from the Er3+ to Nd3+ ions with increasing temperature, which was investigated both experimentally and theoretically. Based on the fluorescence intensity ratio technique, the optical temperature sensing behaviors from the thermally coupled (TC) energy levels (Er3+: 2H11/2/4S3/2 and Nd3+: 4F7/2/4F5/2/4F3/2) and inter-ion energy levels between 4F9/2 of Er3+ and 4F7/2/4F5/2/4F3/2 of Nd3+ were studied in the cryogenic region and high-temperature range. The sensing sensitivity of an optical thermal sensor based on the inter-ion energy levels between Er3+ and Nd3+ was as high as 0.83 K−1, which was an approximate increase of three orders of magnitude compared to that based on the TC energy levels of Er3 and Nd3+. It is anticipated that the Er3+/Nd3+ codoped phosphors could be used to construct wide-range and highly sensitive luminescent thermometers.