What makes β-NaYF4:Er3+, Yb3+ such a successful luminescent thermometer?

Abstract

Luminescence thermometry has emerged as a promising approach for remote, non-invasive temperature sensing at the nanoscale. One of the simplest approaches in that regard is single-ion luminescence Boltzmann thermometry that exploits thermal coupling between two radiatively emitting levels. The working horse example for this type of luminescence thermometry is undoubtedly the green-emitting upconversion phosphor β-NaYF4:Er3+, Yb3+ exploiting the thermal coupling between the two excited 2H11/2 and 4S3/2 levels of Er3+ for this purpose. Within this tutorial article, I would like to give a theoretically motivated account on the underlying reasons for the experimentally recorded success of this material for Boltzmann thermometry referring to time-resolved data on both the bulk and nanocrystalline material. Guidelines are established and both advantages and potential pitfalls in β-NaYF4:Er3+, Yb3+ for luminescence thermometry are given.