Thermomchromic Reaction-Induced Reversible Upconversion Emission Modulation for Switching Devices and Tunable Upconversion Emission Based on Defect Engineering of WO3:Yb3+,Er3+ Phosphor

The Role of WO3 Nanoparticles on the Properties of Gelatin Films

Gelatin films are very versatile materials whose properties can be tuned through functionalization with different systems. This work investigates the influence of WO3 nanoparticles on the swelling, barrier, mechanical, and photochromic properties of gelatin films. To this purpose, polyvinylpirrolidone (PVP)-stabilized WO3 nanoparticles were loaded on gelatin films at two different pH values, namely, 4 and 7. The values of swelling and solubility of functionalized films displayed a reduction of around 50% in comparison to those of pristine, unloaded films. In agreement, WO3 nanoparticles provoked a significant decrease in water vapor permeability, whereas the decrease in the values of elastic modulus (from about 2.0 to 0.7 MPa) and stress at break (from about 2.5 to 1.4 MPa) can be ascribed to the discontinuity created by the nanoparticles inside the films. The results of differential scanning calorimetry and X-ray diffraction analysis suggest that interaction of PVP with gelatin reduce gelatin renaturation. No significant differences were found between the samples prepared at pH 4 and 7, whereas crosslinking with glutaraldehyde greatly influenced the properties of gelatin films. Moreover, the incorporation of WO3 nanoparticles in gelatin films, especially in the absence of glutaraldehyde, conferred excellent photochromic properties, inducing the appearance of an intense blue color after a few seconds of light irradiation and providing good resistance to several irradiation cycles.

Reversible Multi-Mode Optical Modification in Inverse-Opal-Structured WO3: Yb3+, Er3+ Photonic Crystal

Reversible optical regulation has potential applications in optical anti-counterfeiting, storage, and catalysis. Compared to common power materials, the reverse opal structure has a larger specific surface area and an increased contact area for optical regulation, which is expected to achieve higher regulation rates. However, it is difficult to achieve reversible and repeatable regulation of the luminescent properties of photonic crystals, especially with the current research on the structural collapse of photonic crystals. In this work, WO3: Yb3+, Er3+ inverse photonic crystals were prepared by the template approach, and reversible multi-mode optical modification was investigated. Upon heat treatment in a reducing atmosphere or air, the color of the photonic crystals can reversibly change from light yellow to dark green, accompanied by changes in absorption and upconversion of luminescence intensity. The stability and fatigue resistance of this reversible optical modification ability were explored through cyclic experiments, providing potential practical applications for photocatalysis, optical information storage, and electrochromism.

ZnAl2O4:Er3+ Upconversion Nanophosphor for SPECT Imaging and Luminescence Modulation via Defect Engineering

This work reports an "all-in-one" theranostic upconversion luminescence (UCL) system having potential for both diagnostic and therapeutic applications. Despite considerable efforts in designing upconversion nanoparticles (UCNPs) for multimodal imaging and tumor therapy, there are few reports investigating dual modality SPECT/optical imaging for theranostics. Especially, research focusing on in vivo biodistribution studies of intrinsically radiolabeled UCNPs after intravenous injection is of utmost importance for the potential clinical translation of such formulations. Here, we utilized the gamma emission from 169Er and 171Er radionuclides for the demonstration of radiolabeled ZnAl2O4:171/169Er3+ as a potent agent for dual-modality SPECT/optical imaging. No uptake of radio nanoformulation was detected in the skeleton after 4 h of administration, which evidenced the robust integrity of ZnAl2O4:169/171Er3+. Combining the therapeutics using the emission of β- particulates from 169Er and 171Er will be promising for the radio-theranostic application of the synthesized ZnAl2O4:169/171Er3+ nanoformulation. Cell toxicity studies of ZnAl2O4:1%Er3+ nanoparticles were examined by an MTT assay in B16F10 mouse melanoma cell lines, which demonstrated good biocompatibility. In addition, we explored the mechanism of UCL modulation via defect engineering by Bi3+ codoping in the ZnAl2O4:Er3+ upconversion nanophosphor. The UCL color tuning was successfully achieved from the red to the green region as a function of Bi3+ codoping concentrations. Further, we tried to establish a correlation of UCL tuning with the intrinsic oxygen and cation vacancy defects as a function of Bi3+ codoping concentrations with the help of electron paramagnetic resonance (EPR) and positron annihilation lifetime spectroscopy (PALS) studies. This study contributes to building a bridge between nature of defects and UC luminescence that is crucial for the design of advanced UCNPs for theranostics.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Reversible 3D optical data storage and information encryption in photo-modulated transparent glass medium

Transparent glass has been identified as a vital medium for three-dimensional (3D) optical information storage and multi-level encryption. However, it has remained a challenge for directly writing 3D patterning inside a transparent glass using semiconductor blue laser instead of high-cost femtosecond laser. Here, we demonstrate that rare earth ions doped transparent glass can be used as 3D optical information storage and data encryption medium based on their reversible transmittance and photoluminescence manipulation. The color of tungsten phosphate glass doped with rare earth ions change reversibly from light yellow to blue upon alternating 473 nm laser illumination and temperature stimulation, resulting in the reversible luminescence modulation. The information data could be repeatedly written and erased in arbitrary 3D space of transparent glass, not only showing the ability of the excellent reproducibility and storage capacity, but also opening opportunities in information security. The present work expands the application fields of luminescent glass, and it is conducive to develop a novel 3D data storage and information encryption media.

Novel Strategy for Designing Photochromic Ceramic: Reversible Upconversion Luminescence Modification and Optical Information Storage Application in the PbWO4:Yb3+, Er3+ Photochromic Ceramic

Inorganic photochromic material is an available medium to obtain optical information storage. The photochromic property of the inorganic material is mainly from the defects of the host. However, the formation of defects in the host is uncontrollable, in particular, the revisable formation and removement of defects are difficult. Thus, there are few inorganic materials with the revisable photochromism upon the entire light stimulation. Therefore, it is an urgent need to find a suitable approach to design inorganic photochromic materials. Here, the photochromic PbWO₄:Yb³⁺, Er³⁺ ceramic was designed with the help of valence state change of W⁶⁺ → W⁵⁺ and Pb²⁺ → Pb⁴⁺. Upon the 532 nm laser stimulation, the photochromism of the PbWO₄:Yb³⁺, Er³⁺ ceramic was obtained based on the Pb²⁺ + hν (532 nm) → Pb⁴⁺ + 2e^- and W⁶⁺ + e^- + hν (532 nm) → W⁵⁺ reaction, resulting in the optical information writing. Under the stimulation of an 808 nm laser, the written optical information was erased based on the W⁵⁺ + hν (808 nm) → W⁶⁺ + e^- and Pb⁴⁺ + 2e^- + hν (808 nm) → Pb²⁺ reaction. In addition, the photochromism-induced upconversion emission modification was obtained in the PbWO₄:Yb³⁺, Er³⁺ ceramic, realizing the effective and nondestructive reading out of optical information. The cyclic experiment demonstrated a good reproducibility of both photochromism and upconversion emission modification, exhibiting the potential application of the PbWO₄:Yb³⁺, Er³⁺ ceramic as the optical data storage medium.

"Roller coaster"-like thermal evolution of the Er3+ ion's red photoluminescence in CaWO4:Yb3+/Er3+ phosphors

The abnormal "roller coaster"-like thermal evolution of the Er³⁺ ion's red photoluminescence (corresponding to the F_9/24-I4_15/2 transition) in CaWO₄:Yb³⁺/Er³⁺ phosphors is observed. This red emission suffers from a strong thermal quenching in the 293-573 K temperature range, followed by a sharp increase on further increasing the temperature. The mechanism behind this phenomenon is confirmed to be from the dynamic temperature-dependent multiple mechanisms imposed on the F_9/24 state. At relatively low temperatures, the two-photon upconversion mechanism plays a leading role while, with the increasing of temperature, the one-photon channel, ascribed to the thermal population from the lower I_9/24 state, gradually takes a dominant place.

Reversible Modulated Upconversion Luminescence of MoO3:Yb3+,Er3+ Thermochromic Phosphor for Switching Devices

Reversible modulation of upconversion luminescence induced by the external field stimuli exhibits potential applications in various fields, such as photoswitches, optical sensing, and optical memory devices. Herein, a new MoO₃:Yb³⁺,Er³⁺ thermochromic phosphor was synthesized via a high-temperature solid-state method, and the reversible color modification of the MoO₃:Yb³⁺,Er³⁺ phosphor was obtained by alternating the sintering conditions either in a reducing atmosphere or in air. The color of the MoO₃:Yb³⁺,Er³⁺ phosphor changed from light-yellow to blue under sintering in the reducing atmosphere and returned back after sintering again in air. The influence of reversible thermochromism on the upconversion luminescence of MoO₃:Yb³⁺,Er³⁺ phosphor was investigated. The MoO₃:Yb³⁺,Er³⁺ phosphor prepared in air exhibited visible upconversion luminescence, while the MoO₃:Yb³⁺,Er³⁺ phosphor has no upconversion luminescence after sintering in the reducing atmosphere. This up-conversion luminescence modulation shows excellent reproducibility after several cycles. The thermochromic MoO₃:Yb³⁺,Er³⁺ phosphor with reversible modulated upconversion luminescence shows great potential for practical applications in optical switches and optoelectronic multifunctional devices.

Hydrothermal Synthesis and Upconversion Properties of About 19 nm Sc2O3: Er3+, Yb3+ Nanoparticles with Detailed Investigation of the Energy Transfer Mechanism

The Sc₂O₃: Er³⁺, Yb³⁺ nanoparticles (NPs) with the size of about 19 nm were synthesized by a simple oleic acid-mediated hydrothermal (HT) process. X-ray diffraction (XRD), transmission electron microscopy (TEM), upconversion luminescence (UCL) spectra, and decay curves were used to characterize the resulting samples. The Sc₂O₃: Er³⁺, Yb³⁺ NPs made by HT method exhibit the stronger UCL, of which the red UCL are enhanced by a factor of 4, in comparison with those samples prepared by solvothermal (ST) method at the same optimized lanthanide ion concentrations. The UCL enhancement can be attributed to the reduced surface groups and longer lifetimes. Under 980 nm wavelength excitation, the decay curves of Er³⁺: (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 emissions for Sc₂O₃: Er³⁺, Yb³⁺ NPs samples are both close to each other, resulting from the cross relaxation energy transfer from Er³⁺ to Yb³⁺, followed by an energy back transfer within the same Er³⁺-Yb³⁺ pair. Also, under the relatively low-power density, the slopes of the linear plots of log(I) vs. log(P) for red and green emissions are 2.5 and 2.1, implying the existence of three-photon processes. Our results indicate that Sc₂O₃: Er³⁺, Yb³⁺ NPs is an excellent material for achieving intense UCL with small size in the biological fields.

Tailoring Trap Depth and Emission Wavelength in Y3Al5- xGa xO12:Ce3+,V3+ Phosphor-in-Glass Films for Optical Information Storage

Deep-trap persistent luminescent materials, due to their exceptional ability of energy storage and controllable photon release under external stimulation, have attracted considerable attention in the field of optical information storage. Currently, the lack of suitable materials is still the bottleneck that restrains their practical applications. Herein, we successfully synthesized a series of deep-trap persistent luminescent materials Y₃Al_{5- x}Ga _xO₁₂:Ce³⁺,V³⁺ ( x = 0-3) with a garnet structure and developed novel phosphor-in-glass (PiG) films containing these phosphors. The synthesized PiG films exhibited sufficiently deep traps, narrow trap depth distributions, high trap density, high quantum efficiency, and excellent chemical stability, which solved the problem of chemical stability at high temperatures in the reported phosphor-in-silicone films. Moreover, the trap depth in the phosphors and PiG films could be tailored from 1.2 to 1.6 eV, thanks to the bandgap engineering effect, and the emission color was simultaneously changed from green to yellow due to the variation of crystal field strength. Image information was recorded on the PiG films by using a 450 nm blue-light laser in a laser direct writing mode and the recorded information was retrieved under high-temperature thermal stimulation or photostimulation. The Y₃Al_{5- x}Ga _xO₁₂:Ce³⁺,V³⁺ PiG films as presented in this work are very promising in the applications of multidimensional and rewritable optical information storage.

Upconversion from fluorophosphate glasses prepared with NaYF4:Er3+,Yb3+ nanocrystals

The direct doping method was applied to fabricate upconverter fluorophosphate glasses in the system (90NaPO₃-(10-x)Na₂O-xNaF) (mol%) by adding NaYF₄:Er³⁺,Yb³⁺ nanocrystals. An increase in the network connectivity, a red shift of the optical band gap and a decrease in the thermal properties occur when Na₂O is progressively replaced by NaF. To ensure the survival and the dispersion of the nanocrystals in the glasses with x = 0 and 10, three doping temperatures (T_doping) (525, 550 and 575 °C) at which the nanocrystals were added in the glass melt after melting and 2 dwell times (3 and 5 minutes) before quenching the glasses were tested. Using 5 wt% of the NaYF₄:Er³⁺,Yb³⁺ nanocrystals, green emission from the NaYF₄:Er³⁺,Yb³⁺ nanocrystals-containing glasses was observed using a 980 nm pumping, the intensity of which depends on the glass composition and on the direct doping parameters (T_doping and dwell time). The strongest upconversion was obtained from the glass with x = 10 prepared using a T_doping of 550 °C and a 3 min dwell time. Finally, we showed that the upconversion, the emission at 1.5 μm and of the transmittance spectra of the nanocrystals-containing glasses could be measured to verify if decomposition of the nanocrystals occurred in glass melts during the preparation of the glasses.

The direct doping method was applied to fabricate upconverter fluorophosphate glasses in the system (90NaPO₃-(10-x)Na₂O-xNaF) (mol%) by adding NaYF₄:Er³⁺,Yb³⁺ nanocrystals.