Development of Metal Oxide-Based Phosphors for Luminescence Thermometry
Description
Temperature is both a thermodynamic property and a fundamental unit of measurement; one of the seven base quantities of the international system of units (SI). It can be seen simply as the degree of hotness or coldness, a qualitative definition built on the bodily sensation of heat and cold.... Show moreTemperature is both a thermodynamic property and a fundamental unit of measurement; one of the seven base quantities of the international system of units (SI). It can be seen simply as the degree of hotness or coldness, a qualitative definition built on the bodily sensation of heat and cold. Today it is readily defined from the principles of classical thermodynamics as the parameter of state that has the same value for any systems which are in thermal equilibrium, and from statistical mechanics as a direct measure of the average kinetic energy of noninteracting particles. Temperature is an intensive quantity, meaning that its value does not depend on the amount of the substance for which it is measured. It is important because it is something we feel and because it influences the smallest aspects of our daily life, from how to adjust our housing and clothing to what we eat for supper. It affects the life cycles of plants and animals, governs rates of chemical reactions, influences tides and so on. For these reasons, it is by far the most measured physical quantity; sensors of temperature account for 80% of all sensors worldwide at present and they are used across a broad spectrum of human activities, such as in medicine, home appliances, meteorology, agriculture, and industrial and military contexts, to mention some of the most significant areas. Thus, the market demand for temperature sensors is increasing due to their extending applications in human activities. Traditional “contact” temperature measurements, which are mainly based on the expansion and contraction of an employed material, encounter difficulties when used in some emerging technologies and environments, such as nanotechnology and biomedicine. Today, an immediate need exists for the “non-contact” thermometry of moving or contact-sensitive objects, difficult to access pieces, bodies in hazardous locations, objects of nano-size dimensions, or living cells and organisms. However, the properties of existing thermometers and sensor platforms limit their use in such environments. Non-contact sensors measure object temperature without the need for physical contact between sensors and objects. Therefore, they have been considered as a great interest for hardly accessible objects. As non-contact thermometry methods, besides pyrometers and radiation thermometers, optical thermometers have drawn extensive attention nowadays. Specifically, among all the optical based thermometry methods, including Raman scattering, optical interferometry, and near field optical scanning microscopy, the one having drawn the most attention is luminescence thermometry in which the temperature detection is based on the luminescent signal accompanied with acceptable spatial resolution.In luminescence thermometry method, temperature can be determined from different features of luminescence using luminescence thermometers. Depending on the temporal nature of these features, the principles of their measurements are classified as either time-integrated (steady-state) or time-resolved ones. The temperature measurement based on the excitation and emission band positions and bandwidths, emission band intensities, luminescence/fluorescent intensity ratio (LIR or FIR, the ratio of the intensities of two emission bands) are classified as time-integrated methods. The temperature measurements based on the emission decay- or rise-times are classified as time-resolved ones. Temperature readouts from LIR and emission lifetime are by far the most exploited methods. Both readouts are self-referenced, so they are not affected by fluctuations in excitation and signal detection. Moreover, thermal sensing ability of many lanthanide-based luminescent materials is not limited to only one read-out method. Some of them can be used as dual/multiple modes via utilizing a combination of two or more read-out methods for temperature measurement. Non-contact luminescence thermometry based on LIR read-out method has attracted much attention due to its excellent accuracy and sensitivity. The intensity ratio is independent of undesirable factors that makes this luminescence thermometry more appropriate. Moreover, the method is self- referencing which removes the need for a temperature standard. In principle, it can be realized with any combinations of the emission lines from lanthanides and transition metallic ions with different temperature dependencies, either from single or multiple luminescent centers. It is the most reported luminescence thermometric read-out method in the past few years. In the past years, researchers have done a lot of work on developing high-efficient LIR thermometers by employing a single center emitting. This ratiometric method is mainly performed based on the principle that governs thermally coupled energy level of the luminescent ions. The electronic distribution between electronic states of closely separated excited levels of the doped element follows the Boltzmann equation. The two excited levels of ions are thermally coupled with a maximum energy gap of 2000 cm-1, which is sufficiently small to allow electrons to transit to high energy level upon thermal excitation and at the same time large enough to have different electronic populations and high sensitivity value. In this case, both high and low excited states share the electronic population according to Boltzmann’s distribution. Therefore, the ratio of the number of electrons between the high and the low excited levels can be defined as follows for LIR-based thermometry utilizing single emitting centers. In addition to LIR between two thermally coupled energy levels of the luminescent ion, in some ions LIR between two other energy levels which are not coupled thermally were employed to reach to a high-sensitive thermometry. The quantitative evaluation of the thermometric performance of a temperature probe is defined by its absolute and relative thermal sensitivities, temperature resolution, and repeatability. The rate of change in thermometric parameters (indicated by Δ) over a temperature changing process (∂T) is defined as absolute thermal sensitivity (Sa). However, absolute sensitivity is not appropriate to compare the performance among thermometers with different employed materials or physical principles. The relative thermal sensitivity (Sr) is defined to eliminate the problem associated with comparison between the performance of thermometers with different natures. Sr of a luminescent thermometer is one of the most important factors which determine its temperature readout accuracy. The smallest temperature change resolvable by a thermometer is defined as temperature resolution or temperature uncertainty (indicated by δT) which is expressed in Kelvin and depends on the characteristic of measuring systems such as the experimental detection setup and the signal-to noise ratio: The reproducibility is defined as the change of the same measurement performed under different conditions such as different methods or devices. The repeatability (indicated by R) is the ability of a thermometer to provide the same result under different conditions.
Regarding temperature resolution, most light detection systems, including thermometry systems, suffer from low resolution because of the scattering at both excitation and emission wavelengths. Light scattering of thermometric phosphors is induced by their grain size, shape, and surface roughness. This is a problem particularly associated with conventional phosphors which typically have micrometer grain size. On the other hand, the light scattering by nanoparticles (NPs) is close to zero, which leads to better resolution of luminescence thermometers using NPs. Consequently, nanothermometry has emerged as a hot research area of thermometers for new technological applications with high resolution. Accordingly, below in chapter 1, we discussed a host material, pyrochlore compound of La2Zr2O7, doped with Tb3+ and Eu3+, synthesized in nanoscale (~15 nm) that showed a great potential for LIR temperature sensing with a high resolution based on dual emitting centers. In chapter 2, another sample of this nano powder host, La2Zr2O7 doped with Pr3+, is discovered and discussed for LIR temperature sensing based on single emitting center. Beside the high-resolution thermometry by La2Zr2O7: Pr3+ nano powder, a broad-temperature sensing range was achieved using it. The broad temperature sensing range obtained only by using one LIR-read out mode originated from high-lying charge transfer states with slow thermal-quenching that will be elaborated in chapter 2. Multiple materials employed for luminescence thermometry application, such as organic dyes, quantum dots, metal–organic complexes and frameworks, among which lanthanide or transition metal ion-based phosphors, are most promising. The electronic states of lanthanides are characterized by partially filled 4f orbitals as they are gradually filling up from 4f0 for La3+ to 4f14 for Lu3+. Their luminescence emission occurs due to interconfigurational f-f transitions except some ions like Eu2+ and Ce3+ which have f-d allowed transition emissions. The partially filled 4f orbitals of lanthanide ions are shielded by 5s and 5p subshells from surrounding environment that leads to long lifetime and narrowband emission characteristics. Once excited with UV light, lanthanide-doped materials mostly emit light in visible/near infrared (NIR) range in a downshift (DS) photoluminescence (PL) mechanism. In DS emission, high energy photons are converted into phonons with lower energy. Overall, having excellent repeatability, reproducibility and photostability with thermally and chemically stable structures makes the lanthanide-based materials the most favorite choices for luminescent thermometry applications. Their luminescence is easy to identify and differentiate from other materials. Multiplexing is possible due to their narrow emission bands which are easily identifiable.
Host materials also play a crucial role in thermal sensing properties of thermometric phosphors. Various hosts such as fluorides, ceramic oxides, nitrides, chalcogenides, and phosphides have been employed for luminescence thermometry. Ceramic hosts are composed of different elements, thus often require complex synthesis processes which would limit their applicability. Fluoride hosts have a level of toxicity which is harmful for living systems, so they are not environmentally friendly. Nitride compounds are commonly prepared in oxygen/water-free glove boxes and synthesized in harsh synthesis conditions under high pressure/temperature which restrict their large-scale production. Chalcogenides and phosphides may not be sufficiently stable. On the other hand, metal oxide phosphors possess the advantages of convenient preparation, non-toxicity, excellent chemical stability (capable of withstanding sustained exposure to high temperature), and low cost. Moreover, they are preferable in biomedical luminescence thermometry as applications for measuring long-wavelength emissions where tissues are optically transparent and are less affected by scattering and background luminescence. Considering all these aspects, metal oxide-based phosphors are more favorable for luminescent thermometry.
One of the goals of research in luminescence thermometry field has been to push the limit of temperature measurement capability to higher temperatures. However, the development of luminescent phosphors with high thermal stability of emission and high sensing efficiency still is a paramount challenge. Thermal stability of photoluminescence (PL) is a property related to the chemical composition, electronic structure, and crystal structure rigidity of phosphors. It is commonly referred to as positive thermal quenching (TQ), that is, the loss of light emission with rising temperature. Most phosphors indicate positive TQ which stems from high non-radiative transition probability at elevating temperatures. This phenomenon severely limits the applications of luminescent phosphors and degrades their devices’ performance.
To compensate for the thermally induced emission loss of phosphors, several strategies have been reported, while as will be discussed in chapter 3, mostly have negative impacts on their inherent luminescence properties. From the structural perspective, TQ caused by nonradiative relaxations is closely related to the crystal structure stability. A rigid structural framework with high lattice symmetry has reduced nonradiative transitions at elevated temperatures. As one of the rigid-type hosts, materials possessing a negative thermal expansion (NTE) property have been explored as suitable hosts for anti-TQ phosphors doped with lanthanides. NTE refers to the unique property of some unique and rare materials with their volume abnormally contracting with increasing temperature. Among various reported NTE families, compounds with the general formula of A2M3O12, where A is a trivalent rare earth ion and M stands for W6+ or Mo6+, are well-known with a broad range of compositions and have been explored for anti-TQ in the recent years. Some earlier works reported employing A2M3O12 host to obtain thermally enhanced upconversion (UC) emission. However, the upconversion emission is not the type of widely used emission as they produce weaker emissions mostly limited to a higher wavelength range than most-applicable visible range. Thus, NTE phosphors and thermally enhanced stronger downshift (DS) emissions on visible range are not yet high enough to fulfill their practical application. To explore the applicability of NTE idea for down-shift (DS) emitting phosphors, we reported the anti-TQ performance of single and co-doped samples of Sc2Mo3O12: Eu3+ and Sc2Mo3O12: Tb3+, Eu3+ in chapter 3 and 4, respectively. Specifically, we took advantage of the existence of interionic energy transfer in our NTE host, to achieve superior anti-TQ performance for DS luminescence that can be employed for efficient thermometry at high temperatures range. The structural shrinkage with rising temperature shortens the distance between the host and activator dopant ions, which enhances the host to activator ET and consequently the final emission intensity as will be elaborated in two last chapters. As a highly promising strategy, there is an urgent need to obtain more evidence on how NTE property, associates with the anti-TQ of luminescence that we tried to discover in our works. We explored these compound’s potential for high temperature luminescence thermometry. We tested both LIR and lifetime-based temperature sensing and revealed their great potential for an efficient temperature sensing at high temperature ranges. This study opens a new design strategy and perspective to obtain phosphors with thermally boosted luminescence based on NTE host materials to meet the serious demands for their broad applications at elevated temperatures and harsh conditions. Show less
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