Exploring Defect-Induced Emission in ZnAl2O4: An Exceptional Color-Tunable Phosphor Material with Diverse Lifetimes

Heterogenous Core-Shell Persistent Luminescent Nanoparticles with Enhanced Afterglow Luminescence

Persistent luminescent nanoparticles (PLNPs) are promising for many bioapplications due to their unique afterglow luminescence following the stoppage of light excitation. However, PLNPs are prone to surface quenching that results in weak afterglow luminescence. Although some efforts have been made to reduce surface quenching through designing homogeneous core-shell PLNPs, the enhancement in afterglow luminescence was insignificant. We hypothesize that the independent absorption and emission of the shell caused less energy to reach the activator ions in the core. Hence, a heterogeneous core-shell PLNP where the shell has a higher band gap than the core would reduce the absorption and emission of the shell. In this work, ZnGa2O4 and Zn2GeO4 were coated on Zn1.2Ga1.6Ge0.2O4:Cr and Zn3Ga2Ge2O10:Eu nanocrystals, respectively, to form heterogeneous core-shell PLNPs and significant luminescence enhancement was achieved compared to their traditional homogeneous core-shell nanostructures.

Zinc-aluminum layered double hydroxide and double oxide for room-temperature oxidation of sulfur dioxide gas

Sulfur dioxide (SO2) gas at trace levels challenges the consumption of fuel gases and cleaning of flue gases originating from diverse anthropogenic sources. We have demonstrated Zn-Al layered double hydroxide (LDH) and layered double oxide (LDO) as low-cost and effective adsorbents in removing lowly concentrated SO2 gas at room temperature. Water in the adsorbent bed significantly improved the performance, where the maximum adsorption capacity of 38.0 mg g-1 was achieved for LDO. Based on the spectroscopic findings, the adsorbed gas molecules were oxidized to surface-bound sulfate/bisulfate species, showing complete mineralization of SO2 molecules. By employing an inexpensive NaOH-H2O2 solution-based regeneration strategy, we successfully regenerated the spent LDO, significantly restoring its gas uptake capacity. The regenerated oxide exhibited an increased gas uptake capacity ranging from 38.0 to 98.5 mg g-1, highlighting the practicality and economic feasibility of our approach. LDH/LDO materials are promising regenerable adsorbents for removing low concentrations of SO2 gas in ambient conditions.

Utilizing Energy Transfer in Mn2+/Ho3+/Yb3+ Tri-doped ZnAl2O4 Nanophosphors for Tunable Luminescence and Highly Sensitive Visual Cryogenic Thermometry

Lanthanide (Ln3+)-doped upconversion (UC) phosphors converting near-infrared (NIR) light to visible light hold very high promise toward biomedical applications. The scientific findings on luminescent thermometers revealed their superiority for noninvasive thermal sensing. However, only few reports showcase their potential for applications in extreme conditions (temperatures below -70 °C) restricted by low thermal sensitivity. Here, we demonstrate the tailoring of luminescence properties via introducing Ho3+-Mn2+ energy transfer (ET) routes with judicious codoping of Mn2+ ions in ZnAl2O4/Ho3+,Yb3+ phosphor. Preferentially, a singular red UC emission is required to improve the bioimaging sensitivity and minimize tissue damage. We could attain UC emission with 94% red component by a two-photon UC process. Higher temperature annealing brings the color coordinates to the green domain, highlighting the potential for color-tunable luminescence switch. Moreover, this work investigates the thermometric properties of ZnAl2O4/Yb3+, Ho3+ in the range of 80-300 K and influence of inducing extra ET pathways by Mn2+ codoping. Interestingly, the luminescence intensities for nonthermally coupled (5F4,5S2) and the 5F5 radiative transitions of Ho3+ ions display opposite behavior at 80 and 300 K, which revealed competition between temperature-sensitive decay pathways. The codoping of Mn2+ ions is fruitful in causing a fourfold increase of absolute sensitivity. Notably, the color tunability from green through yellow to red is helpful in rough temperature estimation by naked eyes. The maximum relative (Sr) and absolute sensitivities (Sa) were estimated to be 1.89% K-1 (140 K) and 0.0734 K-1 (300 K), respectively. Even at 80 K, a Sa of 0.00447 K-1 and Sr of 0.6025% K-1 were achievable in our case, which are higher than most of the other Ln3+-based systems. The above-mentioned results demonstrate the potential of ZnAl2O4/Yb3+,Ho3+ for cryogenic optical thermometry and a strategy to design new Ln3+-based UC thermometers by taking advantage of ET routes.

Achieving Persistent Luminescence Performance Based on the Cation-Tunable Trap Distribution

Deep-red persistent luminescence (PersL) materials have promising applications in fluorescence labeling and tracking. PersL spectral range and PersL duration are considered to be the key factors driving the development of high-performance deep-red PersL materials. To address these two key issues, the performance of PersL materials was continually optimized by doping with cations (Si⁴⁺ and Al³⁺ ions), relying on the material of Li₂ZnGe₃O₈:Cr³⁺ from the previous work of our group, and a 4.8-fold increase in PersL radiation spectrum intensity and more than twice the PersL duration was achieved (PersL duration up to 47 h). Ultimately, the obtained PersL materials are used to demonstrate their potential use in multi-level anti-counterfeiting, tracking and localization, respectively. This study provides a unique and novel entry point for achieving high-performance PersL materials by optimizing the PersL material host to modulate the electronic structure.

Design of need-based phosphors and scintillators by compositional modulation in the ZnGa2-xAlxO4:Cr3+ spinel: pure compound versus solid solutions

Materials that can depict persistent deep red light under both ultraviolet (UV) and X-ray illumination can be a boon to sustainable economy, particularly for optical imaging, solid state lighting, and anticounterfeiting applications. Herein, we have made a series of compounds starting from ZnGa₂O₄:Cr³⁺ to ZnAl₂O₄:Cr³⁺ (individual spinel) by substituting the varied concentration of Al³⁺ in place of Ga³⁺ in ZnGa_2-xAl_xO₄:Cr³⁺ (solid solution). By virtue of the structural and defect engineering doping strategy, the photo and radioluminescence are expected to be improved. Both Cr and Al doping was found to be energetically favorable in ZnGa₂O₄, where the same does not hold true for Ga doping in ZnAl₂O₄, as indicated by the DFT-calculated defect formation energies. There seems to be ordering around the dopant ion in the solid solutions compared to either ZnGa₂O₄ or ZnAl₂O₄ and is also reflected to as lower persistent luminescence (PerL) lifetimes. PerL under UV, in general. was found to be lower with the enhancement in the Al³⁺ content endowed by the formation of Cr-Cr ion pair, lower probability of antisite formation, and widening band gap. On the other hand, X-ray excited emission enhances in the solid solution due to the decrease in cation inversion and associated defects. Confocal Microscopy showed that larger particles depicted much brighter deep red emission but failed to percolate to the human cells to a detectable limit; hence, future work is needed for the functionalization of the ZnGa_2-xAl_xO₄:Cr³⁺ spinel. This work could be of great implication in designing need-based materials, where UV and X-ray excitation is required, for deep red emission with persistent characteristics from chromium-doped spinels.

Metal ion induced dual switchable dielectric and luminescent properties in hybrid halides

A new multi-functional organic-inorganic hybrid compound was successfully obtained by regulating metal halides. Apart from excellent luminescence properties, in particular, the introduction of a Mn halide successfully achieved a dual-switchable dielectric property, which could lead to very interesting exploration in sensors.

Deep-Ultraviolet Emitter: Rare-Earth-Free ZnAl2O4 Nanofibers via a Simple Wet Chemical Route

Deep-UV (180-280 nm) phosphors have attracted tremendous interest in tri-band-based white light-emitting diode (LED) technology, bio- and photochemistry, as well as various medical fields. However, the application of many UV-emitting materials has been hindered due to their poor thermal or chemical stability, complex synthesis, and environmental harmfulness. A particular concern is posed by the utilization of rare earths affected by rising price and depletion of natural resources. As a consequence, the development of phosphors without rare-earth elements represents an important challenge. In this work, as a potential UV-C phosphor, undoped ZnAl₂O₄ fibers have been synthesized by a cost-efficient wet chemical route. The rare-earth-free ZnAl₂O₄ nanofibers exhibit a strong UV emission with two bands peaking at 5.4 eV (230 nm) and 4.75 eV (261 nm). The emission intensity can be controlled by tuning the Zn/Al ratio. A structure-property relationship has been thoroughly studied to understand the origin of the UV emission. For this reason, ZnAl₂O₄ nanofibers have been analyzed by X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and Raman spectroscopy techniques showing that a normal spinel structure of the synthesized material is preserved within a wide range of Zn/Al ratios. The experimental evidence of a strong and narrow band at 7.04 eV in the excitation spectrum of the 5.4 eV emission suggests its excitonic nature. Moreover, the 4.75 eV emission is shown to be related to excitons perturbed by lattice defects, presumably oxygen or cation vacancies. These findings shed light on the design of UV-C emission devices for sterilization based on a rare-earth-free phosphor, providing a feasible alternative to the conventional phosphors doped with rare-earth elements.

Tailoring defect structure and dopant composition and the generation of various color characteristics in Eu3+ and Tb3+ doped MgF2 phosphors

A novel approach to generate a wide range of color characteristics such as near white, yellow, orange and red in MgF₂, by proper tailoring of the defect structure and varying the composition of Eu³⁺ and Tb³⁺ dopant ions have been presented here. It has been observed from positron annihilation lifetime spectroscopy (PALS) study that various defect centers such as mono vacancies and their cluster forms exist in the system, whose amount varies upon varying the dopant ion's composition. The experimentally observed positron lifetime values of the defect centers also matched well with the theoretically calculated lifetime values using the MIKA-DOPPLER package. It has been found that a few vacancies or defect centers act as color centers, while the cluster vacancies change the local symmetry of the rare earth ion by inducing more distortion surrounding them thereby resulting in different emission characteristics in the photoluminescence (PL) study. The defect-related host emission in combination with the green and red emission from Tb³⁺ and Eu³⁺ ions generated near-white-light in some of the compounds, while other compounds showed a variety of other color characteristics due to the Tb³⁺ → Eu³⁺energy transfer dynamics. The various defect-related emissions, the role of the defect-related trap state in the decay kinetics and the energy-transfer dynamics were also understood by analyzing the electronic structure using HSE06 hybrid functional calculation.

Unraveling U6+, Am3+&Eu3+ ion's distribution in Ca10(PO4)6F2for radioactive waste immobilization and the associated U6+→ Eu3+energy transfer dynamics for tunable emission characteristics

A combined photoluminescence (PL) and theoretical study has been performed on Ca₁₀(PO₄)₆F₂:U⁶⁺ and Ca₁₀(PO₄)₆F₂:U⁶⁺,Eu³⁺ compounds in order to explore Ca₁₀(PO₄)₆F₂ as potential host for radioactive waste immobilization by understanding the distribution U⁶⁺, Eu³⁺ and Am³⁺ ions among the lattice sites and the related radiation stability. DFT based calculations on various structures with different distribution of U⁶⁺, Eu³⁺ and Am³⁺ ions showed that Eu³⁺ and Am³⁺ ions prefer to occupy the Ca2 sites while the highly charged U⁶⁺ ions prefer Ca1 site. This is also supported by the PL lifetime study, which provided two lifetime components with different contribution for both U⁶⁺ and Eu³⁺ ions present at two different lattice sites. The PL study of U⁶⁺ doped compounds confirmed the existence of U in the UO₂²⁺ form, which makes it as a pure green emitter. Upon co-doping Eu³⁺ ion, the compounds were transformed to red emitter. Further, there is an energy transfer process from U⁶⁺to Eu³⁺, which shifted the CIE color coordinates towards pure red region while increasing doping level of U⁶⁺. This proves U⁶⁺ as a good sensitizer for Eu³⁺ ion. PL study on gamma irradiated U⁶⁺ doped Ca₁₀(PO₄)₆F₂ compound also showed excellent radiation stability at Ca2 site.

Structural, morphological, and electrical properties of silver-substituted ZnAl2O4 nanoparticles

In this paper, nanoparticles of (x = 0.05 and x = 0.1) were synthesized by the sol–gel auto-combustion method and characterized by various techniques.

Enhancing the activity of Pd/Zn–Al–O catalysts for esterification of CO to dimethyl oxalate via increasing oxygen defects by tuning the Zn/Al ratio

The enhancement of oxygen defects in the spinel support is the essential reason for the improvement of catalytic activity, which reveals the support effect of catalyst for CO direct esterification to dimethyl oxalate.

Rapid annealing: minutes to enhance the green emission of the Tb3+-doped ZnGa2O4 nanophosphor with restricted grain growth

Rapid annealing boosted the green emission of the Tb3+:ZnGa2O4 nanophosphor within minutes.

Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu3+ and La3+: accessing the role of defects and local structures

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO₃ exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO₃ in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu³⁺ ions stabilize at both Ba²⁺ and Sn⁴⁺ sites consistent with the highly negative formation energy of around -6.26 eV. Eu³⁺ doping leads to an intense ⁵D₀→⁷F₁ orange emission and a feeble ⁵D₀→⁷F₂ red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of Eu_Ba and Eu_Sn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn²⁺ in the PL of BaSnO₃ or Eu²⁺ in the PL of BaSnO3:Eu³⁺. Interestingly, when co-doped, Eu³⁺ stabilizes at Sn⁴⁺ sites whereas La³⁺ stabilizes at Ba²⁺ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La³⁺ ions lead to lowering of symmetry around Eu³⁺ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO₃ in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO₃ and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO₃ is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

Turning-on persistent luminescence out of chromium-doped zinc aluminate nanoparticles by instilling antisite defects under mild conditions

Spinel oxide nanocrystals are appealing hosts for Cr³⁺ for forming persistent luminescent nanomaterials due to their suitable fundamental bandgaps. Benefiting from their antisite defect-tolerant nature, zinc gallate doped with Cr³⁺ ions has become the most studied near-infrared (NIR) persistent luminescent material. However, it remains challenging to achieve persistent luminescence from its inexpensive analogs, e.g., zinc aluminate (ZnAl₂O₄). Because the radius difference of the cations in the latter system is bigger, it is intrinsically unfavorable for ZnAl₂O₄ to form Zn-Al antisite defects under mild conditions. Herein, we report a wet-chemical synthetic route for preparing Cr³⁺-doped ZnAl₂O₄ nanoparticles with long NIR persistent luminescence. It was demonstrated that methanol (MeOH) as an important component of the mixed solvent played a critical role in tailoring the morphology of the resulting ZnAl₂O₄:Cr nanocrystals. It could particularly drive the formation of antisite defects in the resulting coral-like nanoparticles bearing zinc-rich cores and zinc gradient peripheries. To disclose the effects of MeOH on the formation of antisite defects as well as particle morphologies, small molecules released during the pyrolysis of metal acetylacetonate precursors were analyzed by using gas chromatography-mass spectrometry. In combination with density functional theory (DFT) calculations, it was found that MeOH can effectively catalyze the thermolysis of metal acetylacetonate precursors, in particular Zn(acac)₂. Therefore, MeOH exhibits remarkable effects on the formation of antisite defects by balancing the decomposition rates of Zn(acac)₂ and Al(acac)₃ through its volume fraction in the reaction system. This work thus constitutes a hitherto less common strategy for achieving NIR persistent luminescence from Cr³⁺-doped ZnAl₂O₄ nanoparticles by engineering the cation defects under mild conditions.

Engineering defect clusters in distorted NaMgF3 perovskite and their important roles in tuning the emission characteristics of Eu3+ dopant ion

An attempt has been made to explore various new defect clusters in distorted NaMgF3 perovskite and their important role in tuning optical properties. We have tried to tailor the defect clusters and to understand the impact on the luminescence of the lanthanide, for example the Eu3+ ion. Defect engineering has been carried out by doping aliovalent dopant ions to create a charge imbalance in the matrix, which in turn led to the creation of various mono-, di- and new cluster vacancies. Such vacancies have been characterized by Electron Para-magnetic Resonance (EPR), Positron Annihilation Lifetime Spectroscopy (PALS) and Photoluminescence (PL) studies. The PALS data of both undoped and Eu3+ doped compounds confirmed that in addition to Mg mono vacancies, cluster vacancies with different configurations comprising Mg, Na and F atom vacancies also exist in the matrix. The PL study revealed that depending on the surrounding defect structure, three different types of Eu3+ components can be created. The position of the Eu3+ ion with respect to these cluster vacancies determines the respective emission profiles and the decay kinetics. It has been found that when Li+ ions are co-doped with Eu3+, there is a sudden change in the decay kinetics and the emission profiles. The PALS study revealed that Li+ co-doping modified the configuration of the vacancy clusters, which in turn changes the emission characteristics. The EPR study confirmed the presence of different types of F-centers (F, F2, etc.) which are responsible for the host emission. Overall, this new study will be very helpful for a detailed understanding of the defect structures, in particular the cluster vacancies in distorted NaMgF3 perovskite, which have a direct or indirect impact on many physical properties.

Probing the validity of the spinel inversion model: a combined SPXRD, PDF, EXAFS and NMR study of ZnAl2O4

Spinels are of essential interest in the solid-state sciences with numerous important materials adopting this crystal structure. One defining feature of spinel compounds is their ability to accommodate a high degree of tailorable point defects, and this significantly influences their physical properties. Standard defect models of spinels often only consider metal atom inversion between octahedral and tetrahedral sites, thereby neglecting other defects such as interstitial atoms. In addition, most studies rely on a single structural characterization technique, and this may bias the result and give uncertainty about the correct crystal structure. Here we explore the virtues of multi-technique investigations to limit method and model bias. We have used Pair Distribution Function analysis, Rietveld refinement and Maximum Entropy Method analysis of Powder X-ray Diffraction data, Zn edge Extended X-ray Absorption Fine Structure, and solid-state 27Al Nuclear Magnetic Resonance to study the structural defects in ZnAl2O4 spinel samples prepared by either microwave hydrothermal synthesis, supercritical flow synthesis, or spark plasma sintering. In addition, the samples were subjected to thermal post treatments. The study demonstrates that numerous synthesis dependent defects are present and that the different synthesis pathways allow for defect tailoring within the ZnAl2O4 structure. This suggests a pathway forward for optimization of the physical properties of spinel materials.

Mechanistic insights into defect generation and tuning of optical properties in Zn1-xFexAl2O4(0.01 ≤ x ≤ 0.40) nanocrystals

The correlation of several defects and optical and magnetic properties with Fe content in Zn_1-xFe_xAl₂O₄ (0.01 ≤ x ≤ 0.40) nanocrystals has been scrutinized through X-ray diffraction, O K-edge X-ray absorption near-edge structure, FT-IR, diffuse reflectance, photoluminescence and electron spin-resonance spectroscopies, and vibrating sample magnetometry. Increasing Fe content causes elongation in the octahedral units of the lattice, accompanied by distortion in the octahedral coordination. Fe introduces non-radiative centres in the forbidden gap, thereby tuning the band gap from 4.37 to 3.88 eV and eliminating emission in the visible region. Zn vacancies are found to tail off, while {\rm Fe}_i^{\bullet \bullet \bullet}, {\rm Al}_{\rm Zn}^\bullet and Fe_Al^× antisite defects increase in concentration with increasing Fe content. Inhomogeneous broadening of spin-resonance signals infers strong spin-lattice interactions of Fe³⁺ ions at distorted octahedral and non-symmetric tetrahedral sites. A transition is observed from paramagnetism to superparamagnetism at higher Fe concentrations. A visual colour change from pearly white to orange-brown is observed in Zn_1-xFe_xAl₂O₄ nanocrystals with increasing Fe content, revealing its potential candidature for pigments in the paint and dye industries.

Color modulation by selective excitation activated defects and complex cation distribution in Zn1-xMgxAl2O4 nanocrystals

Complex cation distribution in spinel solid solution, fosters defect generation and permutates the optical properties. To scrutinize the effect on structural properties, viz. the cation distribution and defect states upon substitution of Zn with Mg; and to tune the emission properties, Zn1-xMgxAl2O4 (0.01 ≤ x ≤ 0.30) nanocrystals are synthesized. The nanocrystals show increased inversion and generation of multiple defects, namely zinc vacancies, zinc interstitials, oxygen vacancies and antisite defects with increasing Mg content, which thereby impacts the optical band gap. Pentahedral coordination in addition to tetra- and octahedral coordination of Al has been observed, which infers the presence of oxygen vacancies and dangling bonds. Moire fringes formation has intimated the presence of two or more crystal lattices with higher Mg substitution. Band-to-band and defect-assisted photoluminescence shows the role of multiple defects, especially defect clusters, in deciphering the properties of the resulting crystals. Color change from bluish-white to pink has been achieved depending upon the excitation wavelength and emission mechanism, as proposed through a band model schematic. The presented study may be beneficial for designing the Zn1-xMgxAl2O4 nanocrystals with optimized emission properties.

Crystal Structure and Site Symmetry of Undoped and Eu3+ Doped Ba2 LaSbO6 and BaLaMSbO6 Compounds (M=Mg,Ca): Tuning Europium Site Occupancy to Develop Orange and Red Phosphor

Double perovskite antimonates of the type BaLaMSbO₆ (M=Mg, Ca) were synthesized by a standard solid-state route. The compounds were characterized by X-ray crystallography and the structures were refined using Rietveld method. BaLaMgSbO₆ and BaLaCaSbO₆ crystallized in monoclinic space groups (I2/m) and (P2₁ /n), respectively. In both compounds, La occupied the A-site of perovskite, which is 12-coordinated as compared to Ba₂ LaSbO₆ where La ion shifts to the B-site octahedral coordination due to the larger size of Ba as compared with Mg and Ca. The samples were further characterized using FTIR and the frequency of the octahedral vibration is correlated to the electronegativity of the B-site ions. Photoluminescence study of the title compounds and Ba₂ LaSbO₆ was carried out upon doping with 2 atom% Eu³⁺ ion, which confirmed that Eu³⁺ occupies distorted 12-coordinated A-site in BaLaMSbO₆ (M=Mg, Ca) and symmetrical octahedral B-site in Ba₂ LaSbO₆ . Furthermore, the emission spectrum corresponding to each Eu³⁺ ion at different crystal site was successfully isolated through a TRES study. This site selective occupancy of Eu³⁺ ion also has a direct impact on the light emission, which was found to change from orange to red in a dark room in the order Ba₂ LaSbO₆  : Eu→BaLaCaSbO₆  : Eu→BaLaMgSbO₆  : Eu. Such an outcome will have high impact in designing new commercial Eu³⁺ ion doped phosphor materials and tailoring of their optical properties.

Defect states and kinetic parameter analysis of ZnAl2O4 nanocrystals by X-ray photoelectron spectroscopy and thermoluminescence

Defect states in ZnAl₂O₄ have a significant role in its applicability as a luminescent material. To understand the nature and distribution of defects in its crystal lattice, thermoluminescence (TL) study has been carried out. Excellent TL response is observed from γ- and ultraviolet-irradiated samples at different doses and exposure durations, respectively. Different type of fuels employed in combustion synthesis show a remarkable effect on the trap distribution and hence luminescence properties. Shallow and deep traps are observed in crystals attributed to O⁻ vacancies and F⁺ centers. The mechanism of trapping, retrapping and recombination have been depicted through schematic band model diagram. X-ray photoelectron spectroscopy indicated the presence of various types of defects specifically Al_Zn antisite defect, oxygen and zinc vacancies which are further upheld by photoluminescence and Raman spectroscopy. All results when summed up, predict ZnAl₂O₄ to be a quality material for dosimetry.

Simultaneous tuning of optical and electrical properties in a multifunctional LiNbO3 matrix upon doping with Eu3+ ions

Combined photoluminescence (PL) and dielectric studies have been carried out on both undoped and Eu³⁺ doped LiNbO₃ compounds for their potential application in optical–electrical integration for the first time. Special focus has been given to simultaneously tuning both these physical properties. A PL study reveals that the blank compound is a blue emitting material, while upon doping with Eu³⁺ ions, the emitting color can be tuned from blue to red upon changing the excitation wavelength. Interestingly, the electrical property measurement of this ferroelectric compound showed that upon doping with Eu³⁺ ions, the remnant polarization was increased significantly. Density Functional Theory (DFT) based calculations were carried out to explain both the optical and electrical properties. It has been found that different defect centers are responsible for the bluish host emission while Eu³⁺ ions are energetically preferred to occupy the Nb site and gives rise to red emission. The DFT based results also showed that Eu³⁺ ions induced more distortion into the nearby Nb-site, which is responsible for enhancement of the remnant polarization. Stark-splitting patterns in the PL study also showed that the point symmetry of LiNbO₃ upon Eu³⁺ doping changes from C_6v to D₃, which indicates that the structure becomes less symmetric. Overall, the study presents a novel approach to designing multifunctional materials for optical–electrical integration application and to tuning their physical properties simultaneously in the desired range.

PL and dielectric studies have been carried out on LiNbO₃ and Eu³⁺:LiNbO₃ compounds with a special focus on simultaneous tuning of optical and electrical properties for their potential application in optical–electrical integration.

Unlocking the key to persistent luminescence with X-ray absorption spectroscopy: a local structure investigation of Cr-substituted spinel-type phosphors

Developing new persistent luminescent phosphors, a unique class of inorganic materials that can produce a visible light emission lasting minutes to hours requires improving our understanding of their fundamental structure-property relationships. Research has shown that one of the most critical components governing persistent luminescence is the existence of lattice defects in a material. Specifically, vacancies and anti-site defects that coincide with substitution of the luminescent center, e.g., Eu2+ or Cr3+, are generally considered essential to generate the ultra-long luminescent lifetimes. This research solidifies the connection between defects and the remarkable optical properties. The persistent luminescent compound Zn(Ga1-xAlx)2O4 (x = 0-1), which adopts a spinel-type structure, is investigated by examining the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) at the Cr K and Zn K edges. This investigation reveals a structural distortion of the octahedrally coordinated main group metal site concurrent with increasing Al3+ content. Moreover, these results suggest there is a dependence between the local crystallographic distortions, the presence of defects, and a material's persistent luminescence. In combination, this work provides an avenue to understand the connection between the structure-defect-property relationships that govern the properties of many functional inorganic materials.