Optical Nanothermometry Based on the Luminescence of Rare-Earth Ion-Doped Phosphors

Abstract

Temperature, as one of the fundamental physical parameters, plays an important role in living organisms, ecosystems and many fields of science and technology, demanding accurate thermal sensing. The currently employed temperature sensors have some limitations, such as their inability to measure higher temperatures from far-away objects in harsh environments, like detection of building fires, temperatures in coal fields, oil refineries, biomedical imaging systems, detection at micro/nano-meter scales, etc. As a remedy to this, luminescence based non-contact optical sensors have been developed with unique advantages such as easy detection, fast response, high spatial resolution, non-invasiveness, and good accuracy etc. This kind of temperature sensing is based on measuring the temperature-dependent optical parameters such as emission intensity, spectral line position, bandwidth, lifetime, polarization, etc. Mostly, luminescence intensity ratio (LIR) and fluorescence lifetime (FL) based temperature monitoring are very popular and useful for temperature sensing purposes. Among the aforementioned methods, LIR methods involve the measurement of temperature via measuring the LIR between two designated thermal coupling energy levels associated with temperatures which are unaffected by light source fluctuation or the transmission of optical signal loss. In this case, rare-earth doped materials are considered the most suitable materials for temperature sensing purposes due to the presence of large number of thermally-coupled levels, high transition probabilities of non-radiative relaxations, large Stokes shift, sharp emission profiles, and long fluorescence lifetime etc. Since thermal sensitivity is affected by many factors like crystal field environment around rare-earth ion, crystallinity, physical and chemical stability, phonon energy, luminescence intensity, choices of host material and dopant, etc., are very important. Present article focuses on the characteristics and implications of rare-earth-doped nanophosphors in the field of optical temperature sensing. A brief overview of the concept of phosphor-based temperature sensors, as well as various types of contact and non-contact temperature sensing technologies are presented. Considering the cruciality of thermally coupled energy levels of rare-earth elements in the luminescence-based optical temperature measurement and thermal sensing, relevant properties of rare-earth elements like energy levels, emission wavelengths, thermally coupled energy levels, etc., have been discussed. Mechanisms of LIR, life time and band shape methods in various rare-earth based compounds have also been explained.