Size dependent sensitivity of Yb3+, Er3+ up-converting luminescent nano-thermometers

The non-contact temperature sensing using the luminescence intensity ratio (LIR) between H-2(11/2) -> I-4(15/2) and S-4(3/2) -> I-4(15/2) electronic transition in upconverting Yb3+ and Er3+ co-doped nanocrystals has been known for two decades. This phenomenon is governed by the simple Boltzmann formula, which however does not consider material's properties other than energy gap between the respective energy states. For example, the temperature sensitivity has never been shown to depend on the size of nanoparticles, however our studies, performed on two different NaYF4:Er3+, Yb3+ and LiLaP4O12:Er3+, Yb3+ nanocrystals clearly show evidences of such a relationship. Upon constant excitation intensity, the changes in the LIR emission intensity ratio in response to rising external temperature become consistently more rapid for smaller nanocrystals. It was found that the sensitivity of an up-converting luminescent thermometer changes proportionally to the nanoparticle size, i.e. raised from 1.1% K-1 @ 240 nm to 1.8% K-1 @ 20 nm for tetraphosphate nanocrystals and similarly raised from 1.1% K-1 @ 200 nm to 2.1% K-1 @ 8 nm for fluoride nanocrystals at 200 K. These phenomena were manifested mainly at low temperatures and can be neglected at temperatures above 250 K. The observed changes were discussed in terms of ratio of superficial to bulk ions. Phenomenological models have been proposed to describe both the size dependent luminescence lifetimes and temperature sensitivity.