Introduction

The term ‘luminescent thermometry’ covers application of all luminescent materials for temperature sensing. The general idea of this kind of thermometry is based on temperature dependent luminescent features of phosphors. Depending on the type of change which is caused by the temperature we can distinguish few types of luminescent thermometers like: band-shape thermometry, spectral luminescent thermometry, bandwidth thermometry, intensity luminescence thermometry, lifetimes thermometry [1].

 

In case of intensity luminescent thermometry the change of the temperature results in the change of one of emitting band intensity (see below). However the main advantage of such thermometer is simplicity its application is rather not common due to the fact of its low accuracy. Since the intensity of emission band may depend on many factors like: amount of phosphor used, absorption or scattering etc. the measurement of temperature is rather risky and its accuracy is very low.

Intensity luminescent thermometer

The spectral luminescent thermometers are characterized by the changes of the position of emission bands with temperature. This kind of luminescent thermometer is well known in case of quantum dots. However in case of f-f interconfigurational transition observed for lanthanide ions (Ln3+) their spectral position dependency of the temperature can be neglected. Meaningless shifting of the emitting bands results in extremely low sensitivity of such thermometer.

Spectral luminescence thermoemetr

The idea of bandwidth thermometry is associated with the electron-phonon coupling which is temperature dependent resulting in broadening of emission bands at higher temperatures.

Bandwith luminescent thermometer

The influence of the temperature on lifetimes of the excited states is used for temperature determination in lifetime thermometers. Since the probability of the energy transfer between optically active ions depends on the temperature, the value of its excited state decay times may be taken into account as a temperature sensor. However in many examples the interionic energy transfer results in nonexponential decay profiles what implies the necessity of determination of average decay time.

Lifetime luminescent thermometer

The band-shape thermometry based on the changes of the shape of emission spectra (in contrary to intensity luminescence-whole or part of the spectra is analyzed not only one peak) affected on the temperature. Usually relative intensities of two bands are analyzed. Since the determination of temperature is possible by comparing the intensities of two bands the temperature measurement in general is not affected by the scattering or the amount of phosphor used. Actually this is one of the most often used mechanism for temperature determination.

Band-shape luminescent thermometer

 

Depending on the type of interaction analyzed this thermometry technique can be divided into two groups: luminescent thermometry based on thermalization and temperature dependent of energy transfer probability.

First of them is ruled by the change of the relative states population via thermal population of higher laying state. If the thermal energy (kBT) is high enough for population of state 2 in cost of state 1, its emission intensity which in general can analyzed as a proportional to the emitting state population, increases according to the Boltzmann population.

Prezentacja2

Thermalizaion effect and proportion between luminescence intensities ratio (LIR) and Boltzmann distribution

Therefore relative emission intensities of bands 2 and 1 is a linear function of the 1/T parameter in double logarithmic scale. Typical example of this kind of luminescent thermometry is emission of Er3+ (2H11/24I15/2 and 4S3/24I15/2)

The band-shape luminescent thermometry based on energy transfer rely on the fact that some of the energy transfer due to the lack of resonance between emitting and absorbing energies can be realized only with assistance of phonon. Depending on the type of nonresonance the transition can take place with emission or absorption of phonons (See Fig. below).

 

rys phonon assistedPhonon-assisted energy transfers

Two the most important factors which characterize the luminescent thermometer are its sensitivity and usable range of temperature.

The sensitivity of thermometer is defined as follow:

sensitivity

where R is measured feature and Δ R is its change in  ΔT change of temperature.

Usable range of the temperature defines temperature limitation for which described termometer can be applied. This range can be determined by the limitation of the phosphor stability but very often R changes only in some limited temperature range and above or below undergo saturation etc.

***

[1] Jaque, D.; Vetrone, F., Luminescence nanothermometry. Nanoscale 2012, 4 (15), 4301-4326.