Lu2O3-based storage phosphors. An (in)harmonious family

Electron traps and energy storage: modeling a bright path to the future

By employing time-dependent density functional theory for solid-state chemistry, the research presented by Andrii Shyichuk [Acta Cryst. (2023), B67, 437-449] significantly contributes to the understanding of electron/hole traps in doped materials.

Electron Trap Depths in Cubic Lutetium Oxide Doped with Pr and Ti, Zr or Hf─From Ab Initio Multiconfigurational Calculations

We propose a universal approach to model intervalence charge transfer (IVCT) and metal-to-metal charge transfer (MMCT) transitions between ions in solids. The approach relies on already well-known and reliable ab initio RASSCF/CASPT2/RASSI-SO calculations for a series of emission center coordination geometries (restricted active space self-consistent field, complete active space second-order perturbation theory, and restricted active space state interaction with spin-orbit coupling). Embedding with ab initio model potentials (AIMPs) is used to represent the crystal lattice. We propose a way to construct the geometries via interpolation of the coordinates obtained using solid-state density functional theory (DFT) calculations for the structures where the activator metal is at specific oxidation (charge) states of interest. The approach thus takes the best of two worlds: the precision of the embedded cluster calculations (including localized excited states) and the geometries from DFT, where the effects of ionic radii mismatch (and eventual nearby defects) can be modeled explicitly. The method is applied to the Pr activator and Ti, Zr, Hf codopants in cubic Lu₂O₃, in which the said ions are used to obtain energy storage and thermoluminescence properties. Electron trap charging and discharging mechanisms (not involving a conduction band) are discussed in the context of the IVCT and MMCT role in them. Trap depths and trap quenching pathways are analyzed.

Defects in hafnium-doped lutetium oxide and the corresponding electron traps: a meta-generalized gradient approximation study

A number of Lu₂O₃-based materials were reported to present efficient capability of trapping excited charge carriers in metastable excited states formed either by specific dopants or naturally occurring defects. Over the years, abundant experimental data have been collected, which were taken as a solid ground to treat the problem using computational chemistry. Density functional theory (DFT) calculations with an advanced meta generalized gradient approximation (mGGA) functional were used to analyze electron trapping in cubic Lu₂O₃ doped with Hf. Individual ions of dopant and nearest-neighbor dopant ion pairs were considered. The effects of interstitial anions such as O^2- and Cl^- were analyzed. In most of the analyzed cases the additional electron charge is localized at the dopant site. However, in many of the studied cases, the dopant/defect states overlap with the conduction band and cannot correspond to electron trapping. The Hf³⁺ ion in the Lu site of C_3i local symmetry ({\rm Hf}^{\times}_{{\rm Lu}-C_{\rm 3i}}) corresponds to a moderate trap depth of 0.8-0.9 eV. Several composite defects corresponding to deeper (1.1-1.4 eV) traps also exist. Unambiguous deep traps (1.5-1.8 eV) correspond to systems with Hf dopant in the cationic void, accompanied by two interstitial oxygen atoms. The results thus indicate that basic `Hf-substitutes-Lu' doping is unlikely to correspond to the deep traps observed experimentally in Lu₂O₃:Tb,Hf andLu₂O₃:Pr,Hf and more complex defects must be involved.

Multimodal Tuning of Synaptic Plasticity Using Persistent Luminescent Memitters

Mimicking memory processes, including encoding, storing, and retrieving information, is critical for neuromorphic computing and artificial intelligence. Synaptic behavior simulations through electronic, magnetic, or photonic devices based on metal oxides, 2D materials, molecular complex and phase change materials, represent important strategies for performing computational tasks with enhanced power efficiency. Here, a special class of memristive materials based on persistent luminescent memitters (termed as a portmanteau of "memory" and "emitter") with optical characteristics closely resembling those of biological synapses is reported. The memory process and synaptic plasticity can be successfully emulated using such memitters under precisely controlled excitation frequency, wavelength, pulse number, and power density. The experimental and theoretical data suggest that electron-coupled trap nucleation and propagation through clustering in persistent luminescent memitters can explain experience-dependent plasticity. The use of persistent luminescent memitters for multichannel image memorization that allows direct visualization of subtle changes in luminescence intensity and realization of short-term and long-term memory is also demonstrated. These findings may promote the discovery of new functional materials as artificial synapses and enhance the understanding of memory mechanisms.

Microwave-Assisted Preparation of Luminescent Inorganic Materials: A Fast Route to Light Conversion and Storage Phosphors

Luminescent inorganic materials are used in several technological applications such as light-emitting displays, white LEDs for illumination, bioimaging, and photodynamic therapy. Usually, inorganic phosphors (e.g., complex oxides, silicates) need high temperatures and, in some cases, specific atmospheres to be formed or to obtain a homogeneous composition. Low ionic diffusion and high melting points of the precursors lead to long processing times in these solid-state syntheses with a cost in energy consumption when conventional heating methods are applied. Microwave-assisted synthesis relies on selective, volumetric heating attributed to the electromagnetic radiation interaction with the matter. The microwave heating allows for rapid heating rates and small temperature gradients yielding homogeneous, well-formed materials swiftly. Luminescent inorganic materials can benefit significantly from the microwave-assisted synthesis for high homogeneity, diverse morphology, and rapid screening of different compositions. The rapid screening allows for fast material investigation, whereas the benefits of enhanced homogeneity include improvement in the optical properties such as quantum yields and storage capacity.

Radio-, Thermo- and Photoluminescence Properties of Lu2O3:Eu and Lu2O3:Tb Nanopowder and Film Scintillators

This work is dedicated to the preparation and characterization of the radio-, thermo-, and photoluminescent properties of Lu2O3:Eu and Lu2O3:Tb nanopowder (NPs) scintillators, prepared by means of hydrothermal processing, and their film analogues made of these NPs by the spin coating method. The luminescent properties of NPs and films were characterized by cathodoluminescence (CL), photoluminescence (PL), X-ray excited radioluminescence (RL), and thermoluminescence (TL) at low and high temperatures. In Lu2O3:Eu NPs and films, mostly the luminescence of Eu3+ ions occupying the C2 site of the host, with the most intensive peaks at 611.6 nm and a decay time of 1.5 ms, was observed. On the contrary, two types of Tb3+ centers in the C2 and C3i sites with the main emission lines at 542.4 and 544.0 nm and the corresponding 4f→5d excitation bands at 270 and 305 nm and decay times of t1/e = 2.17 and 3.96 ms were observed in the case of Lu2O3:Tb NPs and films. Indications were noted that Tb3+ in the C3i symmetry position was most active in the CL spectra of Lu2O3:Tb NPs and a respective film. Thermoluminescent peaks at 110 °C and 170 °C for Lu2O3:Eu NPs and at 75 °C and 120 °C in Lu2O3:Tb NPs were observed corresponding to the hole and electron traps, respectively. Significantly different onsets of temperature quenching of Eu3+ and Tb3+ luminescence in Lu2O3:Eu and Lu2O3:Tb NPs were found at ~90 °C and ~320 °C, respectively.

White-Light-Emitting Decoding Sensing for Eight Frequently-Used Antibiotics Based on a Lanthanide Metal-Organic Framework

Developing multi-selective luminescence sensing technology to differentiate serial compounds is very important but challenging. White-light-emitting decoding sensing based on lanthanide metal-organic frameworks (Ln-MOFs) is a promising candidate for multi-selective luminescence sensing application. In this work, three isomorphic Ln-MOFs based on H₃dcpcpt (3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenl)-1H-1,2,4-triazole) ligand, exhibiting red, blue, and green emission, respectively, have been synthesized by solvothermal reactions. The isostructural mixed Eu/Gd/Tb-dcpcpt is fabricated via the in-situ doping of different Ln³⁺ ions into the host framework, which can emit white light upon the excitation at 320 nm. It is noteworthy that this white-light-emitting complex could serve as a convenient luminescent platform for distinguishing eight frequently-used antibiotics: five through luminescence-color-changing processes (tetracycline hydrochloride, yellow; nitrofurazone, orange; nitrofurantoin, orange; sulfadiazine, blue; carbamazepine, blue) and three through luminescence quenching processes (metronidazole, dimetridazole, and ornidazole). Moreover, a novel method, 3D decoding map, has been proposed to realize multi-selective luminescence sensing applications. This triple-readout map features unique characteristics on luminescence color and mechanism. The mechanism has been systematically interpreted on the basis of the structural analysis, energy transfer and allocation process, and peak fitting analysis for photoluminescence spectra. This approach presents a promising strategy to explore luminescent platforms capable of effectively sensing serial compounds.

Rapid and Energy-Saving Microwave-Assisted Solid-State Synthesis of Pr(3+)-, Eu(3+)-, or Tb(3+)-Doped Lu2O3 Persistent Luminescence Materials

Persistent luminescence materials Lu2O3:R(3+),M (Pr,Hf(IV); Eu; or Tb,Ca(2+)) were successfully and rapidly (22 min) prepared by microwave-assisted solid-state synthesis (MASS) using a carbon microwave susceptor and H3BO3 as flux. Reaction times are reduced by up to 93% over previous synthetic methods, without special gases application and using a domestic microwave oven. All materials prepared with H3BO3 flux exhibit LuBO3 impurities that were quantified by Rietveld refinement from synchrotron radiation X-ray powder diffraction patterns. The flux does not considerably affect the crystalline structure of the C-Lu2O3, however. Scanning electron micrographs suggest low surface area when H3BO3 flux is used in the materials' synthesis, decreasing the amount of surface hydroxyl groups in Lu2O3 and improving the luminescence intensity of the phosphors. The carbon used as the susceptor generates CO gas, leading to complete reduction of Tb(IV) to Tb(3+) and partial conversion of Pr(IV) to Pr(3+) present in the Tb4O7 and Pr6O11 precursors, as indicated by X-ray absorption near-edge structure data. Persistent luminescence spectra of the materials show the red/near-IR, reddish orange, and green emission colors assigned to the 4f(n) → 4f(n) transitions characteristics of Pr(3+), Eu(3+), and Tb(3+) ions, respectively. Differences between the UV-excited and persistent luminescence spectra can be explained by the preferential persistent luminescence emission of R(3+) ion in the S6 site rather than R(3+) in the C2 site. In addition, inclusion of Hf(IV) and Ca(2+) codopants in the Lu2O3 host increases the emission intensity and duration of persistent luminescence due to generation of traps caused by charge compensation in the lattice. Photonic materials prepared by MASS with H3BO3 flux show higher persistent luminescence performance than those prepared by the ceramic method or MASS without flux. Color tuning of persistent luminescence in Lu2O3:R(3+),M provides potential applications in bioimaging as well as in solar cell sensitizers.