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Title: SYNTHESIS AND APPLICATION OF ORGANOMETALLIC PRECURSORS FOR TUNGSTEN AND MOLYBDENUM SULFIDE
Creator: Liu, Bo
Date: 2021
Description: Transition metal chalcogenides (TMCs) have unique properties. They are promising materials for the next generation electrical devices due to...
Show moreTransition metal chalcogenides (TMCs) have unique properties. They are promising materials for the next generation electrical devices due to their suitable band gap, outstanding electron mobility, and controllable atomic thickness. In the last few decades, atomic layer deposition (ALD) has been one of the hottest research frontiers for the fabrication of TMCs films. Signification progress has been made on the varieties of material grown by ALD and the improvement of ALD equipment. However, the fast-evolving microelectronic industry set higher requirements for the ALD application. In the potential electronic fabrication process, low-temperature preparation and non-corrosive procedure are critical for the advanced device architecture. Thus the novel precursor development and the investigation of reaction mechanism are necessary. In addition, as the comprehensive research of film deposition, the prevailing crystallographic defects on the as-prepared films are another appealing thing for us to think about and try to eliminate for better film quality. Therefore, this dissertation will describe the precursor ligand design and its effect on the morphology, the development of W/Mo precursors for tungsten/molybdenum disulfide, and the defect passivation of tungsten diselenide films.In chapter 2, a series of heteroleptic tungsten precursors of tetrathiotungstates (WS42-) were prepared through the facile ligand transfer method. Ligand variation has a significant effect on the crystallinity of the resulting tetrathiotungstate products. Crystalline tetrathiotungstates with preferred orientation were prepared from the reaction of synthesized precursors with H2S at room temperature. Results indicated the morphologies and crystallinities of the tetrathiotungstates can be well controlled by their ligand behaviors which give us a better understanding of the growth mechanism. Chapters 3 and 4 focus on the development of W and Mo precursors for W/Mo disulfide and their performance in wet chemistry reactions and ALD. WS2 can be synthesized at the ambient temperature in solution by the non-redox reaction. WS2 film growth can be achieved at the exciting low temperature of 125°C by ALD. Based on the performance of the tungsten precursor, a new molybdenum dimer precursor with improved reactivity was synthesized, and MoSx can be prepared at the ambient temperature in seconds. X-ray absorption spectroscopy (XAS) was also utilized to investigate the interaction between the organometallic precursor and the SiO2 surface. Chapter 5 will focus on the defect passivation of WSe2 films for the improvement of their electrical performance. Precursors were synthesized, and the wet chemistry method was designed for oxidation removal and vacancy healing. Raman spectroscopy was used as the express characterization method to reveal the treatment results. A promising healing reagent was screened out, and the repaired films were fabricated to field-effect transistors (FETs) for electrical measurements. The final results showed the electrical performance of the WSe2 films was improved after the convenient chemical treatment.
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Title: ORGANOFUNCTIONALIZED OXOMETALATES: SYNTHESIS, STRUCTURE, AND PROPERTIES OF A NEW CLASS OF MIXED-METAL TETRAMETALATE CLUSTERS
Creator: Shuaib, Damola Taye
Date: 2022
Description: Oxometalates (OMs) are metal-oxide clusters with addenda mental atom mainly V, Mo, and W and bridged by oxide anions. Prototypical examples...
Show moreOxometalates (OMs) are metal-oxide clusters with addenda mental atom mainly V, Mo, and W and bridged by oxide anions. Prototypical examples like polyoxometalates (POMs) are completely inorganic. While clusters with nuclearities ranging from 6 to 18 are common for purely inorganic examples, those with less than nuclearity 6 are rare. Therefore, functionalization by covalent interaction with organic moiety via self-assembly has been utilized as a viable route for making compact clusters with nuclearity of 4 and below. These compounds constitute the organo-functionalized examples of the purely inorganic structure ([XMaOb]n-) POM. Reports of organo-functionalized tetrametalates (TMs), ([MxOyLz])n- (where M = metal, x = 4 and L represents an organic ligand) are sparse. Mixed metal species are especially interesting as potential redox active materials as they contain energetically distinct potential redox centers. OMs have ability to accept electrons in a chemically reversible manner through the terminal oxo-ligand (M=Ot) leading to dπ–pπ electron transfer. Considering the rich structural and electronic properties of these complexes, four neutral mixed-metal (M-V) tetrametalate clusters, [(CoIICl)2(VIVO)2{((HOCH2CH2)(H)N(CH2CH2O))(HN(CH2CH2O)2}2] (1), [(ZnIICl)2(VIVO)2{((HOCH2CH2)(H)N(CH2CH2O))(HN(CH2CH2O)2}2] (2), [CoII2(VIVOF)2{((HOCH2CH2)(H)N(CH2CH2O))(HN(CH2CH2O)2)}2] (3), and [ZnII2(VIVOF)2{((HOCH2CH2)(H)N(CH2CH2O))(HN(CH2CH2O)2)}2] (4) containing unprecedented oxometallocyclic {M2V2X2N4O8}(M = Co, Zn; X = F, Cl) frameworks decorated with diethanolamine ligand in bidentate and tridentate manners. The type of halo-ligand has direct influence on the geometry of the metal M and UV-Vis reflectance spectra revealed changes in electronic structure consistent with charge transfer processes expected. Computational and magnetic properties studies revealed that the ground state multiplicity of 1 is confirmed as an open-shell singlet with a prediction of an isotropic exchange coupling of -6.6 cm-1 but less clear for 2. The vanadium centers are best described as a V(IV) center and the cobalt centers are high-spin Co(II) centers. Less orbital destabilization was observed due to weaker interaction of Cl- ligand on Co than what was observed for O2- ligand on V centers. In 2, there are four weakly coupled spin centers, where the isotropic exchange couplings are defined as J1, J2’, and J2’’. These couplings are approximated as J1 = 1.5/+11.7 cm-1, J2’ = -22.1/-14.8 cm-1, and J2’’ = +4.2/+4.8 cm-1. Although J2’’ is predicted to be weakly ferromagnetic in nature, whereas the fit suggested a weak antiferromagnetic interaction for each of the V(IV)-Co(II) couplings. The low-temperature magnetic susceptibility suggests a Type III spin frustration present in the system. However, competing magnetic interactions are known to be operative in tetranuclear system which is even observed to be more prominent in the mixed-metal tetranuclear system considering the edge-sharing consequence on magnetic behavior. A new route to metal complex synthesis via in situ ligand transformation from diethanolamine to bicine by disproportionation and oxidation reactions yielded three isostructural mononuclear clusters Bis[N,N-bis(2-hydroxyethyl)glycinato]-Cobalt(II) 5, Bis[N,N-bis(2-hydroxyethyl)glycinato]-Nickel(II) 6, and Bis[N,N-bis(2-hydroxyethyl)glycinato]-Copper(II) 7. The observed transformation is predicted to proceed through nucleophilic substitution (SN2) as expected for substituted ammines. These metal complexes are characterized by various analytical techniques such as, FT-IR and UV-Vis spectroscopies, single crystal and powdered X-ray diffraction analyses, Energy-Dispersed X-ray spectroscopy, magnetic properties measurements, thermal gravimetric analysis, bond valence sum calculations etc. Based on their features and detailed structure-property-application analyses, the clusters showed great potentials for catalysis, materials for digital tools, chemical sensing, molecular magnets and precursors as molecular building blocks for extended open frameworks.
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Title: ELECTROSPUN SILKWORM SILK FIBROIN - INDOCYANINE GREEN BIOCOMPOSITE FIBERS: FABRICATION, CHARACTERIZATION AND APPLICATION TOWARDS HEMORRHAGE CONTROL
Creator: Siddiqua, Ayesha
Date: 2022
Description: Silk fibroin (SF), a structural protein found in the Bombyx mori cocoons has gained attention in several biomedical applications as tissue...
Show moreSilk fibroin (SF), a structural protein found in the Bombyx mori cocoons has gained attention in several biomedical applications as tissue engineering scaffolds and wound dressings owing to its properties such as biocompatibility, water vapor permeability and biodegradability. Indocyanine Green (ICG) is an FDA approved tricarbocyanine dye used in medical diagnostics due to its unique photothermal and fluorescent properties. Electrospinning is a highly efficient, easy, and inexpensive technique used to generate nanometer to micrometer thick fibers. In this study, SF and ICG were co-spun to generate flexible microfibers with high surface area to volume ratios. Pure silk, SF-ICG (0.1%) and SF-ICG (0.4%) were chosen for the purpose of this study. Since, as-spun fibers are unstable in aqueous solutions, post treatment methods were explored to enhance the durability of the fibers and to minimize ICG leaching. It was found that ethanol vapor treatment (EVT) not only induced β-sheet formation in SF but also improved the SF-ICG interaction thereby reducing ICG leaching from the composite fibers. Ethanol vapor treated SF-ICG fibers showed less ICG leaching than liquid ethanol treated (LET) SF-ICG fibers indicating the efficacy of the EVT. The increase in SF solution viscosity with ICG concentration suggested a strong silk-ICG interaction which was further confirmed by DSC. The 1h water uptake and the three-day mass loss experiments indicated that the fibers are stable and highly absorbent material. Heat evolution was evaluated by measuring the temperature change in water of a fixed volume after irradiation with a 500 mW, 808 nm diode laser. The heat evolved by the flat fiber scaffolds was higher than the 3D fiber balls, indicating improved light penetration in the former. Pure silk produced negligible heat and it was used as a control. With 14.9 W/cm2 irradiation, the post-treated SF-ICG (0.4%) 3D fibrous ball of 2-3 mg dry weight, solidified a drop of bovine blood in 40 s. In contrast, a single layer fiber matrix required 3 min. to achieve the same clotting effect. Fibers folded into flat scaffolds were able to solidify a blood drop in 25 s. Pure silk fibers in all the cases showed negligible change after irradiation. The results suggest that a larger contact area of fibers is desirable for faster blood clotting, and EVT prompted better ICG retention in SF fibers. Based on the above results, SF-ICG (0.4%) fibers were utilized in a device developed to mimic blood flowing at a rate of 0.5 mL/h through a damaged blood vessel. It was found that irradiation of SF-ICG locally placed at the “damage” region effectively stopped “bleeding” whereas irradiated pure silk was unable to control the blood flow, which demonstrated the success of our SF-ICG fibers towards hemorrhage control.
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Title: Design and Synthesis of New Sulfur Cathodes Containing Polysulfide Adsorbing Materials
Creator: Suzanowicz, Artur M
Date: 2023
Description: Lithium-sulfur battery (LSB) technology has tremendous prospects to substitute lithium-ion battery (LIB) technology due to its high...
Show moreLithium-sulfur battery (LSB) technology has tremendous prospects to substitute lithium-ion battery (LIB) technology due to its high theoretical specific capacity and energy density. However, escaping polysulfide intermediates (produced during the redox reaction process) from the cathode structure is the primary reason for rapid capacity fading. Suppressing the polysulfide shuttle (PSS) is a viable solution for this technology to move closer to commercialization and supersede the established LIB technology. In this dissertation, I have analyzed the challenges faced by LSBs and selected methods and materials to address these problems. I have concluded that in order to further pioneer LSBs, it is necessary to address these essential features of the sulfur cathode: superior electrical conductivity to ensure faster redox reaction kinetics and high discharge capacity, high pore volume of the cathode host to maximize sulfur loading/utilization, and polar polysulfide-resistive materials to anchor and suppress the migration of lithium polysulfides.Furthermore, a versatile, low-cost, and practical scalable synthesis method is essential for translating bench-level development to large-scale production. This dissertation covers designing and synthesizing new scalable cathode structures for lithium-sulfur batteries that are inexpensive and highly functional. The rationally chosen cathode components accommodate sulfur, suppress the migration of polysulfide intermediates via chemical interactions, enhance redox kinetics, and provide electrical conductivity to sulfur, rendering excellent electrochemical performance in terms of high initial specific capacity and good long-term cycling performance. TiO2, Ni12P5, and g-C3N4 as polysulfide adsorbing materials (PAMs) have been fully studied in this thesis along with three distinct types of host structures for lithium-sulfur batteries: Polymer, Carbon Cloth, and Reduced Graphene Oxide. I have created adaptable bulk synthesis techniques that are inexpensive, easily scalable, and suitable for bench-level research as well as large-scale manufacturing. The exceptional performance and scalability of these materials make my cathodes attractive options for the commercialization of lithium-sulfur batteries.
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Title: Migration of Silver from Silver Zeolite/Low-Density Polyethylene Films into Food Stimulants
Creator: Sayeed, Maryam
Date: 2023
Description: Zeolites are naturally occurring or synthetic crystalline microporous aluminosilicate structures with remarkable catalytic, adsorption, and...
Show moreZeolites are naturally occurring or synthetic crystalline microporous aluminosilicate structures with remarkable catalytic, adsorption, and ion-exchange properties. Their unique framework of pores, channels, and cages with precise dimensions makes them an excellent fit for ion exchange and storage. Silver-exchanged zeolite (Ag/Y) composites may be incorporated into polymer matrices to create antimicrobial packaging materials. The slow release of Ag from nanosilver-enabled polymer nanocomposites (PNCs) may inhibit the growth of bacteria and other pathogens on the film’s surface, improving food quality and reducing food waste. However, the migration of Ag ions from the film into food matrices is of great concern as it could expose humans to high concentrations of a heavy metal from dietary sources. The amount of migration depends on various factors, including the potential form of Ag and its concentration in the film, the film thickness, and the storage conditions.The primary objective of this study is to investigate the effect of the form of Ag bound to the zeolite on the migration behavior of Ag from Ag/Y incorporated low-density polyethylene (LDPE) films. For Ag/Y-incorporated LDPE PNCs with distinct Ag species, the Ag migration into the water and Squirt (a commercial soft drink) was at least four times higher from films containing zeolites exchanged with ionic Ag versus zeolites exchanged with nanoparticulate Ag. Similarly, migration into 9 wt % aqueous Domino sugar (granulated sucrose) solution was seven times higher in the ionic silver-incorporated film than in the nanoparticulate Ag film. This study suggests that it is important to consider the form of Ag in silver-exchanged zeolite while producing packaging materials since the potential form of Ag in the PNCs might significantly affect Ag migration behavior.
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Title: Utilizing Image Processing in Evaluation of Fibroblast Stimulation for Collagen Remodeling
Creator: Yoon, Shin Hae
Date: 2023
Description: This research delves into the realm of image processing as a pivotal component in the evaluation of fibroblast stimulation for collagen...
Show moreThis research delves into the realm of image processing as a pivotal component in the evaluation of fibroblast stimulation for collagen remodeling. The study focuses on unraveling the intricate synergy between electrospun silk fibroin-carbon nanotube (SF-CNT) fibers and electrical stimulation, working in harmony to enhance tissue regeneration. Building upon our previous work, we successfully engineered SF-CNT fibers through the electrospinning process, yielding highly aligned structures reminiscent of natural extracellular matrix proteins. These fibers were fortified with water stability through post-treatment with ethanol vapor, while subtle additions of carbon nanotubes (CNTs) significantly improved fiber alignment, strength, and conductivity without compromising biocompatibility. This innovative platform served as a cell culture matrix for fibroblasts harvested from pelvic organ prolapse (POP) patients, facilitating electrical stimulation that triggered a substantial increase in collagen production. In this study, we harnessed the power of various image-processing software tools, including ImageJ and Python, to analyze immunostained images of fibroblasts obtained from POP patients. Under carefully tailored electrical stimulation conditions, the stimulated cells exhibited an astonishing up to 11.97-fold increase in alpha-smooth muscle actin (α-SMA) expression, unequivocally signifying the successful activation of myofibroblasts. Additionally, in an animal model employing LOX-knockout mice to mimic collagen disorders associated with POP, the application of optimized electrical stimulation conditions for patient 003 led to a remarkable surge in collagen production and structural enhancement, underlining the potential of electrical stimulation in expediting tissue remodeling. Intriguingly, fibroblasts from patient 005 and patient 006 exhibited a distinct response, shedding light on the influence of POP severity on cellular behavior. This study firmly reinforces the imperative of personalized therapeutic approaches, emphasizing the need to customize treatment strategies to align with individual patient characteristics through innovative biological image analysis techniques.
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Title: ATOMIC LAYER DEPOSITION STUDIES OF GOLD AND TUNGSTEN DISULFIDE
Creator: Liu, Pengfei
Date: 2020
Description: In the last few decades, atomic layer deposition (ALD), as a vapor deposition technique and a powerful thin film fabrication method, has...
Show moreIn the last few decades, atomic layer deposition (ALD), as a vapor deposition technique and a powerful thin film fabrication method, has received more and more attention in many fields. A variety of materials can be made by ALD; however, the progress of ALD application is still necessary. Meanwhile, in the process of film fabrication by ALD, the interfacial chemistry is interesting and well worth studying. This dissertation mainly described the process of exploring two materials, gold and tungsten disulfide, fabrication and related content.For the portion of applying ALD in gold thin film deposition, a relatively comprehensive process was explored, studied, analyzed and discussed. Start with the synthesis of the gold precursor, Me2Au(S2CNEt2), the synthetic reaction was explored. By modified the conditions, such as solvent system, twice the yield as previously reported in the literature were achieved. Next, the application of in situ microbalance and infrared spectroscopic technique illuminate the organometallic chemistry during the gold thermal ALD process with Me2Au(S2CNEt2) and ozone. In situ quartz crystal microbalance (QCM) studies give an explanation for the nucleation delay and island growth of gold on a freshly prepared aluminum oxide surface. In situ infrared spectroscopy provides insight to study the surface chemistry during the process, which supports an oxidized gold surface mechanism. The epitaxy of gold thin film was explored by X-ray diffraction. The thermal ALD gold on various substrates reveals out-of-plane orientation, however, in-plane orientation was only existed in the gold film on mica. For the portion of applying ALD in tungsten disulfide fabrication, the early work started with studying the effect of interfaces upon crystallinity. The sulfuration of indium thin film with different interface was explored. Then the idea of “interfaces” was brought into the process of tungsten compounds fabrication. Due to this “indirect” method which made tungsten disulfide by sulfurizing ALD made tungsten compounds (eg. tungsten oxide and tungsten nitride) could not reduce the reaction temperature of tungsten disulfide synthesis to less than 400 °C. Sequently, the “direct” way of tungsten disulfide fabrication which directly utilized tungsten precursor and H2S in ALD system was tested and explored. With the tungsten precursors developed by our group, finally, tungsten disulfide could be fabricated at the temperature as low as 125 °C.
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Title: Development of Metal Oxide-Based Phosphors for Luminescence Thermometry
Creator: Jahanbazi, Forough
Date: 2023
Description: Temperature is both a thermodynamic property and a fundamental unit of measurement; one of the seven base quantities of the international...
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.
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