Color tunable luminescence in ThO2:Er3+,Yb3+ nanocrystals: a promising new platform for upconversion

Lanthanide-doped luminescent nanoparticles are an appealing system for many applications in the area of biomedical, solar cell, thermometry, anti-counterfeiting, etc. due to their sensitivity, reliability, high photochemical stability, and high optical transparency in the visible-NIR range. A color-tunable upconversion-luminescence (UCL) in a new low phonon energy ThO2 host based on modulating sensitizer concentration has been realized in this work and it may work as a potential candidate to replace corrosive and toxic fluoride based hosts in the future. Er3+-Yb3+ co-doped thoria nanoparticles were prepared using a gel combustion route and their structural and luminescence properties were determined as a function of the Yb3+ concentration. Phonon dispersion measurements have established the dynamic structural stability of the thoria nanoparticles. Density functional theory (DFT) was used to calculate the defect formation energy, highlighting the feasibility of dual ion (Er3+ and Yb3+) doping in thoria. The morphology and average size of the doped thoria was studied using high resolution transmission electron microscopy (HRTEM), and any defects evolving as a result of aliovalent doping were probed using positron annihilation lifetime spectroscopy (PALS). With 980 nm laser excitation, the nanothoria emits green and near-red light. A significant enhancement of the red-to-green intensity ratio of Er3+ ions in nanothoria was observed with an increase in Yb3+ concentration which resulted in beautiful color tunability from green to yellow light in going from lower (up to ∼5 mol%) to higher (10 and 15 mol%) Yb3+ concentration. The power dependence and the dynamics of the upconverted emission confirm the existence of two-photon upconversion processes for the green and red emissions.

Understanding the excited state dynamics and redox behavior of highly luminescent and electrochemically active Eu(III)-DES complex

Deep eutectic solvents (DES) are considered a novel class of environmentally benign molecular solvents that are considered as potential solvents for nuclear fuel reprocessing, material recycling, and many other technological applications in both research and industry. However, there is a complete dearth of understanding pertaining to the behavior of metal ions in DES. Herein, we have investigated the speciation, complexation behavior, photochemistry, and redox properties and tried to obtain insight into the chemical aspects of the europium ion in DES (synthesized from heptyltriphenylphosphonium bromide and decanoic acid). The same has been probed using time-resolved photoluminescence (TRPL), cyclic voltammetry (CV), synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations. TRPL indicated the stabilization of europium in the +3 oxidation state, favoring the potential of the Eu(III)-DES complex to emit red light under near UV excitation and the existence of inefficient energy transfer between DES and Eu3+. EXAFS analysis revealed the presence of Eu-O and Eu-Br, which represent the local surroundings of Eu3+ in the Eu(III)-DES complex. TRPL measurement has also suggested two distinct local environments of europium ions in the complex. DFT calculations supported the EXAFS findings, confirming that the Eu(III)-DES structure involves not only the oxygen atom of decanoic acid but also the oxygen atoms from the nitrate ions, contributing to the local coordination of Eu(III). Electrochemical studies demonstrated that the redox reaction of Eu(III)/Eu(II) in DES displays quasi-reversible behavior. The reaction rate was observed to increase with higher temperatures. The findings of this study can contribute to the understanding of the fundamental properties and potential applications of this luminescent and electrochemically active complex and pave the way for further studies and the development of novel materials with enhanced luminescent and electrochemical properties.

Harvesting Light from BaHfO3/Eu3+ through Ultraviolet, X-ray, and Heat Stimulation: An Optically Multifunctional Perovskite

Materials with optical multifunctionality such as photoluminescence (PL), radioluminescence, and thermoluminescence (TL) are a boon for a sustainable society. BaHfO₃ (barium hafnium oxide [BHO]) under UV irradiation demonstrated visible PL endowed by oxygen vacancies (OVs). Eu³⁺ doping in BHO (BHOE) introduces f-state impurity levels just below the conduction band for both Eu@Ba and Eu@Hf sites, causing efficient host-to-dopant energy transfer, generating intense ⁵D₀ → ⁷F₁ magnetic dipole transitions (MDT) with internal quantum yield of ∼70%. X-ray photoelectron spectroscopy and electron paramagnetic resonance showed the formation of OVs in both BHO and BHOE samples with more vacancies in the doped sample. The positron lifetime measurements suggested that Eu³⁺ ions are distributed at both Ba²⁺ and Hf⁴⁺ sites. The association of OVs with Hf⁴⁺ and Eu³⁺ ions due to high charge/radius ratio is considered to be responsible for lowering the symmetry around Eu³⁺ ions to C _4v in BHOE. Density functional theory studies of defect formation energy justified the same. Time-resolved emission spectroscopy showed distinct spectra for Eu@Ba and Eu@Hf sites corresponding to symmetric and asymmetric environments, respectively. This could be highly relevant in designing color tunable phosphor by forcing dopant ions at one specific site because Eu@Ba displayed orange emission whereas Eu@Hf displayed red emission. We could further harness BHOE for X-ray scintillator application by designing a thin film, which showed efficient conversion of high-energy X-ray into visible light. Under beta irradiation; both BHO and BHOE showed distinct TL glow curves as shallow traps were formed in the former and deep traps in the latter, which could have long-term implications in harnessing this material for persistent luminescence. We believe that BHO/BHOE demonstrated an extraordinary credential as a perovskite for multifunctional applications in the area of defect-induced light emission, UV phosphor, X-ray scintillator, and TL crystals.

Correlation among photoluminescence and the electronic and atomic structures of Sr2SiO4:xEu3+ phosphors: X-ray absorption and emission studies

A series of Eu³⁺-activated strontium silicate phosphors, Sr₂SiO₄:xEu³⁺ (SSO:xEu³⁺, x = 1.0, 2.0 and 5.0%), were synthesized by a sol–gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic α'-SSO and monoclinic β-SSO phases of the SSO:xEu³⁺ phosphors. When SSO:xEu³⁺ phosphors are excited under ultraviolet (UV) light (λ = 250 nm, ~ 4.96 eV), they emit yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 and 703 nm) PL bands. These PL emissions typically correspond to 4f–4f electronic transitions that involve the multiple excited ⁵D₀ → ⁷F_J levels (J = 1, 2, 3 and 4) of Eu³⁺ activators in the host matrix. This mechanism of PL in the SSO:xEu³⁺ phosphors is strongly related to the local electronic/atomic structures of the Eu³⁺–O²⁻ associations and the bandgap of the host lattice, as verified by Sr K-edge and Eu L₃-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K-edge XANES and K_α X-ray emission spectroscopy. In the synthesis of SSO:xEu³⁺ phosphors, interstitial Eu₂O₃-like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5d and/or O 2p–Eu 4f/5d states (mostly the O 2p–Eu 5d states) to the ⁵D₀ levels, facilitating the recombination of electrons that have transitioned from the ⁵D₀ level to the ⁷F_J level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu³⁺ phosphors. This PL-related behavior indicates that SSO:xEu³⁺ phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.

Multicolor tunable luminescence and energy transfer of core-shell structured SiO2@Gd2O3 microspheres co-activated with Dy3+/Eu3+ under single UV excitation

Optimizing structure and varying doped ions are two main strategies to obtain excellent luminescence performance. Spherical morphology is considered to be the most ideal phosphor structure due to the least surface defects. Herein, a series of spherical and monodispersed Dy³⁺/Eu³⁺ co-activated SiO₂@Gd₂O₃ core-shell phosphors with multicolor tunable luminescence were successfully prepared via a facile urea assisted precipitation method. Related chemical reactions and the possible growth mechanism of Gd₂O₃:Ln³⁺ directional deposition on the surface of SiO₂ microspheres were put forward. Upon 273 nm UV radiation excitation, SiO₂@Gd₂O₃:Dy³⁺ and SiO₂@Gd₂O₃:Eu³⁺ samples exhibited characteristic yellow (⁴F_9/2-⁶H_13/2) and blue (⁴F_9/2-⁶H_15/2) emissions of Dy³⁺ and red (⁵D₀-⁷F₂) emission of Eu³⁺, respectively. Meanwhile, multicolor emissions (warm white, yellow and orange) could be easily obtained by modulating the relative content of Dy³⁺ and Eu³⁺ in SiO₂@Gd₂O₃:Ln³⁺ samples under a single excitation wavelength. Moreover, it was confirmed that Dy³⁺ could transfer energy to Eu³⁺ in the form of quadrupole-quadrupole interaction and further improved the luminescence intensity of Eu³⁺ by comparing experimental data with theoretical calculations. These results imply that tunable luminescence Dy³⁺,Eu³⁺ co-doped SiO₂@Gd₂O₃ microspheres have great potential applications in multicolor displays and biolabeling fields.

Controlling the luminescence in K2Th(PO4)2:Eu3+ by energy transfer and excitation photon: a multicolor emitting phosphor

This work highlighted green, red, and white light emission from a single K₂Th(PO₄)₂ compound consisting of actinide and an alkali ion through defect, doping, excitation, and energy transfer manipulation.

Ultraviolet emission and electron spin characteristics of Th(C2O4)2·xH2O:Gd3+

UV emitting Gd³⁺ doped phosphors have recently attracted significant attention among materials scientists owing to their important applications in the areas of photothermal therapy, transilluminators and sensitizer based luminescent phosphors.

Sol-Gel Synthesis of High-Purity Actinide Oxide ThO2 and Its Solid Solutions with Technologically Important Tin and Zinc Ions

The applicability of epoxide-based sol-gel synthesis for actinide oxide (thoria) starting from air-stable salt, Th(NO₃)₄, has been examined. The homogeneous gel formed from Th(NO₃)₄ when calcined at 400 °C yielded nanostructured thoria, and with increasing tempeartures (600, 700, and 800 °C), the average crystallite size increased. Successful Rietveld refinement of the powder X-ray diffraction pattern of ThO₂ in Fm3̅m space group was carried out with a = 5.6030(35) Å. The fingerprint vibrational mode of the fluorite structure of ThO₂ was noticed as a sharp band in the Raman spectrum at 457 cm^-1. In the SEM image, a near spherical morphology of thoria was noticed. Samples showed blue emission on exciting with λ = 380 nm in the photoluminescence spectrum indicative of the presence of defects. Following this approach, 50 mol % of Sn⁴⁺ could be substituted for Th⁴⁺, retaining the fluorite structure as evidenced by the PXRD, Raman spectroscopy, electron microscopy, EDAX, and XPS measurements. Randomization of the lattice was observed for the tin-substituted samples. A significant blue shift in the absorption threshold along with a persistent blue emission in the photoluminesence spectra were evident for the tin-substituted samples. The concentration of Zn²⁺ ion in thoria was limited to 15 mol % as revealed by PXRD and XPS measurements. The Raman peak shifted to higher values for Zn²⁺-substituted samples. A change in the optical absorbance characteristics was observed for the zinc-substituted thoria. A 50 mol % Sn⁴⁺-substituted thoria degraded aqueous Rhodamine 6G dye solutions in the presence of UV-vis radiation following pseudo-first-order kinetics.

Why host to dopant energy transfer is absent in the MgAl2O4:Eu3+ spinel? And exploring Eu3+ site distribution and local symmetry through its photoluminescence: interplay of experiment and theory

Local site occupancy of Eu³⁺ in combustion synthesized MgAl₂O₄ spinel and host → Eu³⁺ energy transfer dynamics is investigated using photoluminescence and DFT calculations.

Nature of defects in blue light emitting CaZrO3: spectroscopic and theoretical study

Defects responsible for intense blue emission in CaZrO₃ are investigated using PL and EPR spectroscopy and complimented by DFT calculations.

Energy transfer dynamics and luminescence properties of Eu3+ in CaMoO4 and SrMoO4

Undoped and europium doped CaMoO₄ and SrMoO₄ scheelites are synthesized using a complex polymerization method.