Effect of Molten Salt Synthesis Processing Duration on the Photo- and Radioluminescence of UV-, Visible-, and X-ray-Excitable La2Hf2O7:Eu3+ Nanoparticles

Strong green upconversion emission from submicron spindle-shaped SrMoO4:Yb3+,Er3

Upconversion luminescence (UCL) is a fluorescence process where two or more lower-energy photons convert into a higher-energy photon. Lanthanide (Ln3+)-doped UCL materials often suffer from weak luminescence, especially when directly synthesized by a hydrothermal (HT) process due to the existing hydroxyl group and undesirable arrangement of dopants within host lattices which quench luminescence and limit energy transfer. Therefore, additional heat treatment processes are required to enhance their UCL emission, even though direct hydrothermal synthesis without further heat treatment has the advantages of low energy consumption, fast synthesis, and wide applicability to generate UCL materials. In this study, via a HT process without annealing, we have produced Yb3+ and Er3+ co-doped SrMoO4 submicron spindles with a strong green UCL emission which can be seen with the naked eye, which HT produced oxide-based UCL materials often fail to demonstrate. We have investigated different HT synthesis conditions, such as temperature, time, pH and dopant composition, which control the nucleation, growth, lattice structure arrangement, and ultimately their UCL properties through XRD, SEM, EDS and UCL measurements. The bright green UCL from the SrMoO4:Yb,Er submicron spindles is further enhanced by post-synthesis annealing within a molten NaNO3/KNO3 system to prevent particle size growth. The green UCL intensity from the annealed SrMoO4:Yb,Er submicron spindles surpasses samples produced by the solid-state method and is comparable to that from the commercial NaYF4:Yb,Er sample. We have further studied the temperature-dependent luminescence of both the HT-prepared and molten-salt annealed SrMoO4:Yb,Er submicron spindle samples. The strong UCL from our SrMoO4:Yb,Er submicron spindles could warrant their candidacy for bioimaging and anticounterfeiting applications.

Rose Bengal Decorated NaYF4:Tb Nanoparticles for Low Dose X-ray-Induced Photodynamic Therapy in Cancer Cells

The use of scintillating nanoparticles (ScNPs) in X-ray-induced photodynamic therapy (X-PDT) is a technique for deep tissue-localized tumor therapy with few side effects. ScNPs transfer X-ray-induced energy to photosensitizers, which generate massive amounts of reactive oxygen species (ROS) and kill cancer cells. Here we fabricated rose bengal (RB)-installed, Tb³⁺-rich NaYF₄ nanocrystals (NaYF₄:Tb@RB), in which optically inert Y³⁺ enables highly efficient energy transfer via high amounts of Tb³⁺ doping. NaYF₄:Tb was prepared via solvothermal synthesis to have an average size of 7.6 nm, followed by coating with poly(maleic anhydride-alt-1-octedecene)-poly(ethylene glycol) with a molecular weight of 2000 (C₁₈PMH-PEG_2k). Further, RB was covalently conjugated to carboxyl groups generated from PMH on NaYF₄:Tb using an ethylenediamine linker. NaYF₄:Tb@RB exhibited a hydrodynamic diameter of ∼75 nm with a ζ-potential of -12 mV. NaYF₄:Tb@RB efficiently generated ROS in cultured luciferase-expressing murine epithelial breast cancer (4T1-luc) cells under low dose X-ray irradiation (0.5 Gy). The ROS generation amounts of NaYF₄:Tb@RB were 1.5-2-fold higher than those of NaGdF₄:Tb@RB, in which host nanocrystals were prepared with optically active Gd³⁺. Flow cytometric and confocal microscopic analyses showed higher intracellular ROS production of NaYF₄:Tb@RB, compared to NaYF₄:Tb and RB, resulting in higher X-ray-induced DNA damage in cultured 4T1-luc cells. Ultimately, NaYF₄:Tb@RB elicited significant cytotoxicity after X-ray irradiation (0.5 Gy), while inducing marginal cytotoxicity without X-ray irradiation. Altogether, this research proposes a promising ScNP design for efficient X-PDT agents that make the better use of incident X-ray energy while causing the fewest side effects.

Defect engineering in trivalent ion doped ceria through vanadium assisted charge compensation: insight using photoluminescence, positron annihilation and electron spin resonance spectroscopy

Pair matching charge compensation with trivalent and pentavalent dopants in ceria was found to be an attractive strategy in engineering defects with minimal distortions in the lattice and obtaining enhanced catalytic properties. In the present study, charge compensation with a vanadium codopant in trivalent ion doped ceria is studied. Defect evolution in the trivalent ion doped ceria with vanadium codoping has been studied in CeO₂:Eu³⁺, CeO₂:La³⁺,Eu³⁺ and CeO₂:Y³⁺,Eu³⁺ systems and the choices of the dopant and co-dopant are triggered by their ionic radius. Eu³⁺ photoluminescence (PL) is used as a spectroscopic probe to monitor local structural changes around the dopants. Positron lifetime studies showed that oxygen vacancies formed due to trivalent ion doping are weakly associated when larger ions are doped and result in the formation of vacancy aggregates. Positron lifetime studies along with XRD studies show that vanadium codoping effectively removes the vacancies but the distortions are significant when the size mismatch between the pair match used for charge compensation is higher. Photoluminescence demonstrated that the oxygen vacancies associated with Eu are more effectively removed in the case of Y codoped samples. Electron Spin Resonance (ESR) studies suggested that vanadium in excess over the stoichiometric concentration of the trivalent ion can lead to additional defects. These studies are expected to help in tuning the vacancy concentrations as well as controlling the lattice distortions for technological applications such as catalysis, ionic conductivity, etc.

White light emission from co-doped La2Hf2O7 nanoparticles with suppressed host → Eu3+ energy transfer via a U6+ co-dopant

This work highlights white light emission from La₂Hf₂O₇:Eu³⁺ nanoparticles assisted by uranium co-doping to restrict energy transfer from the host oxygen vacancy to the Eu³⁺ dopant.

Heterometallic Group 4-Lanthanide Oxo-alkoxide Precursors for Synthesis of Binary Oxide Nanomaterials

In this study, an efficient procedure for the synthesis of uncommon group 4-lanthanide oxo-alkoxide derivatives was developed. Heterometallic clusters with the structures [La₂Ti₄(μ₄-O)₂(μ₃-OEt)₂(μ-OEt)₈(OEt)₆(Cl)₂(HOEt)₂] (1), [La₂Zr₂(μ₃-O)(μ-OEt)₅(μ-Cl)(OEt)₂(HOEt)₄(Cl)₄]_n (2), [La₂Hf₂(μ₃-O)(μ-OEt)₅(μ-Cl)(OEt)₂(HOEt)₄(Cl)₄]_n (3), [Nd₂Ti₄(μ₄-O)₂(μ₃-OEt)₂(μ-OEt)₈(OEt)₆(HOEt)₂(Cl)₂] (4), [Nd₄Zr₄(μ₃-O)₂(μ-OEt)₁₀(μ-Cl)₄(OEt)₈(HOEt)₁₀(Cl)₂] (5), and [Nd₄Hf₄(μ₃-O)₂(μ-OEt)₁₀(μ-Cl)₄(OEt)₈(HOEt)₁₀(Cl)₂] (6) were synthesized via the reaction of a metallocene dichloride, Cp₂M'Cl₂ (where M' = Ti, Zr, and Hf), and metallic lanthanum or neodymium in the presence of excess ethanol. This procedure gave crystalline precursors with molecular stoichiometries suitable for obtaining group 4-lanthanide oxide materials. Compounds 1-6 were examined by analytical and spectroscopic techniques and single-crystal X-ray diffraction. The magnetic properties of 5 and 6 were investigated by using direct and alternating current (dc and ac) susceptibility measurements. The results indicated weak antiferromagnetic interactions between Nd^III ions and a field-supported slow magnetic relaxation. Lanthanum-titanium compound 1 decomposed at 950 °C to give the perovskite compound La_0.66TiO₃ and small amounts of rutile TiO₂. Under the same conditions, 4 decomposed to give a mixture of Nd₄Ti₉O₂₄ and Nd_0.66TiO₃. When 4 was calcined at 1300 °C, decomposition of Nd₄Ti₉O₂₄ to Nd_0.66TiO₃ and TiO₂ was observed. Calcination of 2, 3, 5, and 6 at 950-1500 °C led to the selective formation of heterometallic La₂Zr₂O₇, La₂Hf₂O₇, Nd₂Zr₂O₇, and Nd₂Hf₂O₇ phases, respectively.

Luminescence properties and energy transfer of Tb3+, Eu3+ co-doped YTaO4 phosphors obtained via sol-gel combustion process

Tantalate is considered as a valuable and efficient luminescence host because of its intense absorption in the ultraviolet area and excellent chemical properties. In this work, a series of pure YTaO4:Eu3+ and/or Tb3+ crystals were prepared via a sol-gel combustion method. The morphology, structure, and optical properties of the samples were discussed in detail. The Eu3+, Tb3+ co-doped YTaO4 samples are consisted of small spherical particles of around 18 nm. The prepared YTaO4:Tb3+ and/or Eu3+ samples exhibit the characteristic wide excitation band around 210-300 nm, the characteristic narrow red emission of Eu3+ (5D0 → 7F2) transitions and green emission of the Tb3+ (5D4 → 7F5) transitions when excited by UV light. It is focused on the energy transfer processes from the YTaO4 to Tb3+ as well as Eu3+ ions and from Tb3+ to Eu3+ ions of YTaO4:Eu3+/Tb3+ phosphors. Color-tunable emissions are realized through adjusting the types of rare earth ion (Eu3+ and Tb3+) and relative doping concentrations excited by a single wavelength. That is to say, the obtained Tb3+ and Eu3+ co-doped YTaO4 phosphors have a promising prospect in lasers, white light diodes (WLED), fluorescent lamp, and field emission display devices, etc.

Recent advances, challenges, and opportunities of inorganic nanoscintillators

This review article highlights the exploration of inorganic nanoscintillators for various scientific and technological applications in the fields of radiation detection, bioimaging, and medical theranostics. Various aspects of nanoscintillators pertaining to their fundamental principles, mechanism, structure, applications are briefly discussed. The mechanisms of inorganic nanoscintillators are explained based on the fundamental principles, instrumentation involved, and associated physical and chemical phenomena, etc. Subsequently, the promise of nanoscintillators over the existing single-crystal scintillators and other types of scintillators is presented, enabling their development for multifunctional applications. The processes governing the scintillation mechanisms in nanodomains, such as surface, structure, quantum, and dielectric confinement, are explained to reveal the underlying nanoscale scintillation phenomena. Additionally, suitable examples are provided to explain these processes based on the published data. Furthermore, we attempt to explain the different types of inorganic nanoscintillators in terms of the powder nanoparticles, thin films, nanoceramics, and glasses to ensure that the effect of nanoscience in different nanoscintillator domains can be appreciated. The limitations of nanoscintillators are also highlighted in this review article. The advantages of nanostructured scintillators, including their property-driven applications, are also explained. This review article presents the considerable application potential of nanostructured scintillators with respect to important aspects as well as their physical and application significance in a concise manner.

High pressure induced local ordering and tunable luminescence of La2Hf2O7:Eu3+ nanoparticles

This work highlights the high-pressure induced site swapping and improved ordering of Eu³⁺ in La₂Hf₂O₇:Eu³⁺ nanocrystals which leads to red-orange-yellow tunable emission at low-moderate-high pressure regime and enhanced correlated color temperature.

Samarium-Activated La2Hf2O7 Nanoparticles as Multifunctional Phosphors

Recent developments in the field of designing novel nanostructures with various functionalities have pushed the scientific world to design and develop high-quality nanomaterials with multifunctional applications. Here, we propose a new kind of doped metal oxide pyrochlore nanostructure for solid-state phosphor, X-ray scintillator, and optical thermometry. The developed samarium-activated La₂Hf₂O₇ (LHOS) nanoparticles (NPs) emit a narrow and stable red emission with lower color temperature and adequate critical distance under near-UV and X-ray excitations. When the LHOS NPs are exposed to an energetic X-ray beam, the Sm³⁺ ions situated at the symmetric environment get excited along with those located at the asymmetric environment, which results in a low asymmetry ratio of Sm³⁺ under radioluminescence compared to photoluminescence. High activation energy and adequate thermal sensitivity of the LHOS NPs highlight their potential as a thermal sensor. Our results indicate that these Sm³⁺-activated La₂Hf₂O₇ NPs can serve as a multifunctional UV, X-ray, and thermographic phosphor.

Thermally Induced Disorder-Order Phase Transition of Gd2Hf2O7:Eu3+ Nanoparticles and Its Implication on Photo- and Radioluminescence

Crystal structure has a strong influence on the luminescence properties of lanthanide-doped materials. In this work, we have investigated the thermally induced structural transition in Gd₂Hf₂O₇ (GHO) using Eu³⁺ ions as the spectroscopic probe. It was found that complete phase transition from the disordered fluorite phase (DFP) to the ordered pyrochlore phase (OPP) can be achieved in GHO with the increase of annealing temperature from 650 → 1100 → 1300 °C. OPP is the more stable structural form for the GHOE nanoparticles (NPs) annealed at a higher temperature based on the energy calculation by density functional theory (DFT). The asymmetry ratio of the GHOE-650 NPs was the highest, whereas the quantum yield, luminescence intensity, and lifetime values of the GHOE-1300 NPs were the highest. Emission intensity of Eu³⁺ ions increases significantly with the phase transition from the DFP to OPP phase and is attributed to the higher radiative transition rate (281 s^-1) of the ⁵D₀ level of the Eu³⁺ ion in the environment with relatively lower symmetry (C _2v ) because of the increase of crystal size. As the structure changes from DFP to OPP, radioluminescence showed tunable color change from red to orange. The Eu³⁺ local structure obtained from DFT calculation confirmed the absence of inversion symmetry in the DFP structure, which is consistent with the experimental emission spectra and Stark components. We also elucidated the host to dopant optical energy transfer through density of states calculations. Overall, our current studies present important observations for the GHOE NPs: (i) thermally induced order-disorder phase transition, (ii) change of point group symmetry around Eu³⁺ ions in the two phases, (iii) high thermal stability, and (iv) tunability of radioluminescent color. This work provides fundamental understanding of the relationship between the crystal structure and photophysical properties of lanthanide-doped materials and helps design a strategy for advanced optoelectronic materials.