Emission Centers Research Articles

Tunable luminescence and oxygen defects of the spinel MgAl2O4: Eu3+/Eu2+ for photonic application

In this study, we have reported the luminescent property of 0, 0.05, 0.1, and 0.5 mol. % Eu2O3 doped in bulk MgAl2O4 spinel excited at 266 nm. The samples were prepared by slip-cast method and sintered at 1650 °C for 2 h in the air. The sintered samples were subjected to hot isostatic pressing (HIP) in argon atmosphere to achieve theoretical density and transparency. X-ray diffraction patterns for all samples revealed the spinel phase of the face-centered cubic structure. The luminescence of the sintered and HIPed samples was recorded using photoluminescence spectrometers. The sintered samples exhibited characteristic red light emission of Eu3+ at 613 nm. Surprisingly, this Eu3+ emission does not appear for the HIPed samples and shows a characteristic blue emission for the Eu2+ in the 445–480 nm range. Interestingly, further heat treatment of the HIPed samples at 1300 °C for 1 h in air revealed the re-appearance of the characteristic Eu3+ red emission at 613 nm without the Eu2+ blue emission. It is concluded that the Eu3+/Eu2+ emission centers are tunable depending on the processing conditions. Here a characteristic emission center at 482 nm was found for the sintered samples that indicate the presence of oxygen vacancy.

A highly sensitive blue-LED-excitable self-calibrated luminescent thermometer based on Cr3+/Eu3+ Co-doped Al2W3O12

In this work, Al2W3O12:1%Cr3+,2%Eu3+ with two discriminable dual emissive centers is synthesized. Both Eu3+ and Cr3+ ions can be excited efficiently by 465 nm. Photoluminescence excitation (PLE) spectra and photoluminescence (PL) lifetime indicate that there is no energy transfer (ET) between Eu3+ and Cr3+ ions. It is worth noting that the photothermal stability of Eu3+ ions is much better than that of Cr3+ ions. Specifically, the ratio of ICr/IEu decreases linearly with increasing temperature. Therefore, a blue-LED-excitable luminescent thermometer based on Cr3+/Eu3+ co-doped Al2W3O12 is proposed. This developed self-calibrated luminescent temperature sensor possesses the relatively high absolute and relative sensitivity (0.38 K-1 and 2.9% K−1 respectively) as well as good cycle stability among the previously reported TM/Ln-based luminescent; thermometers. These superior features of the developed self-calibrated luminescent thermometer (SCLT) make it a very promising candidate for temperature sensing applications.

Photoluminescence properties and energy transfer of double perovskite Ca2LaTaO6:Bi3+,Tb3+ phosphor

The improvement of solid-state luminescence technology is an important way to cope with the current severe energy consumption. For the efficient utilization of rare earth ions, Bi3+ and Tb3+ activated Ca2LaTaO6 (CLTO) phosphors were designed and prepared using a solid phase reaction technique. The structural properties of the samples have been studied in detail through XRD, EDS and XPS techniques. Combined with UV–vis reflectance spectroscopy, excitation and emission spectra, the photoluminescence characteristics of CLTO phosphors were analyzed in detail. Bi3+ ions can occupy distinct sites in the CLTO host to create two emission centers. The adjustable multicolor emission is realized in the CLTO:Bi3+,Tb3+ compounds depending on the approximately 100% energy transfer from Bi3+ to Tb3+ ions. By exploring the fluorescence spectra, it was demonstrated that the energy transfer process is contributed by the dipole-dipole interaction mechanism. Besides, the CLTO:Bi3+,Tb3+ phosphors have good thermal stability, suggesting it is a potential luminescent material for UV LEDs.

Atomic-scale study on the dopant distribution in phosphorus and boron-doped Si nanocrystals/SiO2 multilayers

Understanding the distributions and behaviors of dopants in Si nanocrystal are the primary and necessary issues to realize the controllable doping at nanoscale and develop the next generation of optoelectronic devices. This work reports the atomic-scale distributions of phosphorus and boron dopants in Si nanocrystals multilayers. The phosphorus-doped and boron-doped Si nanocrystals/SiO2 multilayers are fabricated by PECVD and subsequently annealed at 1000 °C. It is found that the locations of phosphorus are redistributed after the formation of Si nanocrystals due to the combined effects of formation energy and self-purification. Phosphorus dopants are mainly distributed at the Si nanocrystals surfaces to passivate the dangling bonds, while part of them incorporate into Si nanocrystals lattice sites to provide free electrons. However, boron dopants exhibit different distributions in contrast to phosphorus. The concentration of boron on Si nanocrystals surfaces can reach as high as 40.0 at. %, which forms a dopant-shell covering on Si nanocrystals. Meanwhile, the boron dopant-shell can modify the surface states of Si nanocrystals like Si-oxide related emission centers and dangling bonds, which is responsible for the luminescence properties. Moreover, the boron-aggregations with concentration near 74.8 at. % are appeared inside Si nanocrystals and led to the damage of crystalline lattice.

Multi behaviors of Dy3+ for improving the luminescence properties of Pr3+-activated Sr3Al2O6 orange-reddish phosphors

Pr3+/Dy3+ co-doped Sr3Al2O6 phosphors were synthesized by a solid-state reaction. The powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and thermoluminescence (TL) were analyzed to investigate the luminescence and structure properties of the samples. The introduction of Dy3+ ions as multi behaviors significantly improve the luminescence properties of the phosphors. Dy3+ ions also behave as an emission center, showing the strong emission peaks at 617 nm and 645 nm, and work together with the activator of Pr3+ to responsible for the red color of the phosphors. The dopant of Dy3+ can significantly increase the trap concentrations with a suitable trap depth of 0.79 eV to extend the afterglow time of the materials. There exists some energy level overlaps among some excitation peaks of Dy3+ and some emission peaks of Pr3+, showing that Dy3+ ions still behave as sensitizers to improve the luminescence properties of the materials. Based on the energy band theory and defect theory, the luminescence mechanism of Pr3+/Dy3+ co-doped Sr3Al2O6 phosphors was proposed.

Improved scintillation intensity of CsPbBr3 perovskite film via introducing BaLuF5: Gd/Tb nanoparticles

Metal halide perovskites nanoparticles (PNPs) show promising application in X-ray detection and imaging. However, they exhibit poor stability, especially, the emission peak might be evidently shifted with increasing the PNPs concentration. In this work, the composites of CsPbBr3 PNPs and BaLuF5: 15Gd/15Tb NPs are used to prepare scintillation film for the first time. When fixing the PNPs content in the film, its emission center is not changed after the addition of fluoride NPs. The uniform film containing both BaLuF5: 15Gd/15Tb to CsPbBr3 is prepared. The scintillation intensity of the CsPbBr3 PNPs is enhanced owing to the radiative energy transfer process from BaLuF5: 15Gd/15Tb to CsPbBr3. Our results provide a novel route to improve the scintillation intensity, which might promote the exploration of new scintillators with high performance.

Laser-modified luminescence for optical data storage

The yearly growing quantities of dataflow create a desired requirement for advanced data storage methods. Luminescent materials, which possess adjustable parameters such as intensity, emission center, lifetime, polarization, etc., can be used to enable multi-dimensional optical data storage (ODS) with higher capacity, longer lifetime and lower energy consumption. Multiplexed storage based on luminescent materials can be easily manipulated by lasers, and has been considered as a feasible option to break through the limits of ODS density. Substantial progresses in laser-modified luminescence based ODS have been made during the past decade. In this review, we recapitulated recent advancements in laser-modified luminescence based ODS, focusing on the defect-related regulation, nucleation, dissociation, photoreduction, ablation, etc. We conclude by discussing the current challenges in laser-modified luminescence based ODS and proposing the perspectives for future development.

Efficient Quasi-2D Perovskite Light-Emitting Diodes Enabled by Regulating Phase Distribution with a Fluorinated Organic Cation.

Metal halide perovskites have become a research highlight in the optoelectronic field due to their excellent properties. The perovskite light-emitting diodes (PeLEDs) have achieved great improvement in performance in recent years, and the construction of quasi-2D perovskites by incorporating large-size organic cations is an effective strategy for fabricating efficient PeLEDs. Here, we incorporate the fluorine meta-substituted phenethylammonium bromide (m-FPEABr) into CsPbBr3 to prepare quasi-2D perovskite films for efficient PeLEDs, and study the effect of fluorine substitution on regulating the crystallization kinetics and phase distribution of the quasi-2D perovskites. It is found that m-FPEABr allows the transformation of low-n phases to high-n phases during the annealing process, leading to the suppression of n = 1 phase and increasing higher-n phases with improved crystallinity. The rational phase distribution results in the formation of multiple quantum wells (MQWs) in the m-FPEABr based films. The carrier dynamics study reveals that the resultant MQWs enable rapid energy funneling from low-n phases to emission centers. As a result, the green PeLEDs achieve a peak external quantum efficiency of 16.66% at the luminance of 1279 cd m−2. Our study demonstrates that the fluorinated organic cations would provide a facile and effective approach to developing high-performance PeLEDs.

Luminescence of MgF2-WO3 ceramics synthesized in the flux of 1.5 MeV electron beam

The results of comparative analysis of optical properties of MgF2 single crystals and MgF2 ceramics, non-doped and doped with tungsten trioxide obtained in the process of high-speed radiation synthesis in air under a flux of 1.5 МеV electrons, are presented.The synthesized ceramics are found to have a high degree of crystallinity, their elements composition is close to stoichiometric, and the phase composition is a rutile lattice.Using time-resolved spectroscopy, it was shown that the initial defectiveness of as-synthesized ceramics samples is represented by emission centers belonging to WO3 complexes and radiation defects of F2- types with spectral-kinetics parameters similar to those in irradiated by ionizing radiation MgF2 single crystals. The ratio between these types of defects is determined by the temperature and luminescence excitation method and the ratios between dopant and co-dopant (LiOH).In MgF2 ceramics samples, being exposed to ionizing radiation, two radiative channels of relaxation of electronic excitations were found: T-So (triplet-singlet) emission transition in self-trapped excitons (STE) at 3.2 eV and recombination of F–H Frenkel pairs accompanied with the emission at 4.0 eV. The temperature dependences of these channel efficiency in the range of 5–300 K are similar to those ones observed in MgF2 single crystals.

Luminescence and photoconversion properties of micro-powder phosphors based on the Ce3+ and Mn2+ doped Ca2YMgScSi3O12 silicate garnets

This work is dedicated to study the synthesis processes and luminescence properties of micro-crystalline powder phosphors of Ce3+ and Mn2+ doped Ca2YMgScSi3O12 garnets (CYMSSG:Ce and CYMSSG:Ce,Mn) The photoluminescence emission spectra and decay kinetics confirm the formation of Ce3+ multicenters in the CYMSSG:Ce powder phosphor due to local inhomogeneity of the different dodecahedral positions and presence of the energy transfer between high-energy and low energy Ce3+ emission centers in this garnet. The effective Ce3+ to Mn2+ energy transfer process is observed in CYMSSG:Ce, Mn phosphors resulting in the redshift of the emission spectra in comparison with CYMSSG:Ce counterpart. We have shown that CYMSSG:Ce and CYMSSG:Ce, Mn powder phosphors can be used as an efficient photoconverter (PC) for creation of the high-power white LEDs (WLEDs). The prototypes of PC-WLEDs, based on 450 nm emitting blue LED and planar layers of CYMSSG:Ce and CYMSSG:Ce, Mn microcrystalline powder phosphors, embedded in the epoxy resin, were prepared and their photoconversion properties were investigated as well.

A lanthanide-doped glass-ceramic fiber for stress sensing

Stress sensors that can recognize the location where a force is applied and propagate strain information are desirable for future integrated intelligent devices. However, developing sensors free from electromagnetic interference and complicated detector setups is challenging. Here, a self-driven optical fiber is developed and implemented as a remote stress sensor. Terbium-doped fluoride nanocrystals (Ba 2 LaF 7 : Tb 3+ ) are precipitated in situ from a transparent glass and have notable mechanoluminescence (ML) because of designed defect traps. We propose that the encapsulated nature of the nanocrystals (NCs) within the amorphous matrix facilitates efficient energy transfer from traps to emission centers, which is critical for achieving the ML behavior. The optical waveguide effect of the as-developed transparent ML sensor facilitates capture and transmission of the signals. The explored self-driven glass ceramic fiber opens a door for detecting and recording mechanical information within integrated devices. • Bright green mechanoluminescence is observed from terbium-doped fluoride nanocrystals in a transparent medium • An optical fiber is developed and implemented as a remote stress sensor • The explored transparent mechanoluminescent stress sensor is self-driven, free from external power Tang et al. demonstrate bright green mechanoluminescence from a transparent glass ceramic. The transparent medium developed in this work not only accurately records stress signals but also provides a promising strategy for transmission of the recorded signals.

Resonance Energy Transfer Reveals the Presence of Multiple Luminescence Emission Centers within a Carbon Nanodot

Resonance Energy Transfer Reveals the Presence of Multiple Luminescence Emission Centers within a Carbon Nanodot

Theoretical Model of the Retrograde Motion of Cathode Spots of Vacuum Arc

The phenomenon of cathode spot (CS) retrograde motion has been attracting extensive research in the field of vacuum arc. Although a number of theoretical models have been proposed to explain this phenomenon, up to now, the first-principle understanding is still missing. In this work, based on the generalized Ohm’s law, it is proposed that there is a transport current in the explosive emission center when a transverse magnetic field (TMF) is applied. The investigation of the influence of the transport current on the strength of the electric field in the cathode sheath suggests that the transport current affects the ignition of new CS in the retrograde direction. In the framework of the ecton model, combined with the dynamics of explosive emission plasma, the relationship between the strength of applied TMF and the ignition probability of new CS in different directions is established, and the retrograde motion characteristics of CS are simulated using a statistical model. The simulation results show that the directionality of retrograde motion of CS is enhanced with an increase in TMF, and the retrograde motion speed of CS increases with the increase of the applied TMF. When the strength of TMF is small, the motion speed of a single CS has a linear relationship with the strength of TMF, but with the increase of the strength of TMF, the motion speed of a single CS is gradually saturated, which is consistent with experimental facts. The feasibility of the model is verified by comparing the simulation results with relevant experimental results.

Large‐scale mammal monitoring: The potential of a citizen science camera‐trapping project in the United Kingdom

Abstract In light of global biodiversity loss, there is an increasing need for large‐scale wildlife monitoring. This is difficult for mammals, since they can be elusive and nocturnal. In the United Kingdom, there is a lack of systematic, widespread mammal monitoring, and a recognized deficiency of data. Innovative new approaches are required. We developed MammalWeb, a portal to enable UK‐wide camera trapping by a network of citizen scientists and partner organizations. MammalWeb citizen scientists contribute to both the collection and classification of camera trap data. Following trials in 2013–2017, MammalWeb has grown organically to increase its geographic reach (e.g. ∼2000 sites in Britain). It has so far provided the equivalent of over 340 camera trap‐years of wild mammal monitoring, and produced nearly 440,000 classified image sequences and videos, of which, over 180,000 are mammal detections. We describe MammalWeb, its background, its development and the novel approaches we have for participation. We consider the data collected by MammalWeb participants, especially in light of their relevance to the main goals of wildlife monitoring: to provide spatial data, abundance data and temporal behavioural data. MammalWeb can complement existing approaches to mammal monitoring. Explicit accounting for spatial and temporal patterns in animal activity enables accounting of bias relative to ad hoc observational data. Estimating abundance presents challenges, as for many camera‐trapping studies, but we discuss the potential of the data as they stand, and opportunities to advance their value for abundance estimation. Challenges remain to MammalWeb's central missions of enhancing engagement with and connection to nature, and delivering policy‐relevant data on Britain's wild mammals. We discuss these challenges and the opportunities they provide for advances in respect of engagement, science and financial security. Our approach reduces administrative burden and increases spatial coverage and, as such, MammalWeb provides a useful addition to existing case studies of citizen science camera‐trapping program design. We believe MammalWeb is an important step towards fulfilling calls for UK‐wide mammal monitoring and our description of challenges identifies an agenda for fulfilling that purpose.

Undoing Extinction: The Role of Zoos in Breeding Back the Tarpan Wild Horse, 1922–1945

Although episodes of captive breeding for display and acclimatization purposes date back to the 19th century, systematic breeding for species conservation first became the central mission for European zoological gardens in the interwar period. While most scholars explain this shift as a result of a decline in the global trade of exotic animals, my analysis points to the simultaneous renewed interest in native endangered and extinct species as a catalyst for captive breeding experiments. This article considers the role of zoos in one of the earliest de-extinction projects, namely the breeding back of the Eurasian wild horse. By investigating two breeding programs, those initiated by Tadeusz Vetulani in Poland and the Heck brothers in Germany, my analysis reveals not only the political stakes, but also the different methods employed in these attempts to resurrect the tarpan. More specifically, I explore how scientific ideas about heredity and hybridization, as well as understandings of the porous boundary between domestic and wild horses, paved the way for selective breeding to be mobilized as a means of bringing back the extinct species. This ambitious goal reinvigorated the scientific function of the zoo by making it an experimental ground for undoing extinction, and laid the groundwork for its future role in the emergent wildlife conservation movement. In addition to mapping out knowledge exchange between zoologists, geneticists, and paleontologists, this article traces the circulation of specimens between natural history museums, zoos, and wildlife reserves to underscore the material dimension of ex situ conservation practices.

A water-stable Eu-MOF as multi-responsive luminescent sensor for high-efficiency detection of Fe3+, MnO4− ions and nicosulfuron in aqueous solution

Effective detection of contaminants in water, such as pesticides, excess Fe3+ and MnO4−, is critical to human health and environmental protection. They can be quantitatively detected by a new luminescent sensor based on a water stable europium metal-organic framework (Eu-MOF). The dual nucleus Eu-MOF is connected by L6−ligands through μ6: η1, η2, η3, η3, η4, η5 coordination to form a 2D wavy network, and then expands into a three-dimensional structure through ligand L6− connection. Due to its high water stability and multiple emission centers, the Eu-MOF can be used as luminescence sensors for pollutants in water. Eu-MOF exhibits high selectivity for Fe3+, MnO4−, and nicosulfuron (NSF) in water through luminescence quenching with low detection limit, and fast response, indicating that Eu-MOF is a reliable analysis platform for multi-responsive sensing.

Dye-based dual-emission Eu-MOF synthesized by Post-modification for the sensitive ratio fluorescence visualization sensing of ClO-

As we all know, excessive hypochlorite will be transformed into highly toxic substances, while insufficient hypochlorite can not completely kill bacteria and viruses in water. Therefore, it is desirable to develop a new analytical method to detect ClO- in environmental water. Here, a novel and simple fluorescence sensor was constructed for monitoring ClO- by an effective strategy. An Acriflavine@lanthanide metal-organic framework (Acr@Eu(BTEC)) was designed by covalently integrating amino-rich dye (Acr) and carboxyl-rich Eu(BTEC) via post-synthesis method. The created fluorescence sensor has two emission centers originating from Acr and Eu(BTEC), respectively. In the presence of ClO-, the strong green fluorescence derived from Acr was significantly quenched, while the invariant red emission from Eu3+ acted as the reference signal. Thus, Acr@Eu(BTEC) with two emissions was developed as a ratiometric fluorescence sensor for highly sensitive and selective detection of ClO-. The limit of detection (LOD) was as low as 10.75 nM. Moreover, visual detection of ClO- by the naked eyes is feasible with obvious fluorescent color changes from green to orange and then red. This method shows excellent performance in practical application, which suggests that it has great potential in water quality monitoring.

Near-Infrared Carbonized Polymer Dots for NIR-II Bioimaging.

Carbon dots (CDs) or carbonized polymer dots (CPDs) are an emerging class of optical materials that have exceptional applications in optoelectronic devices, catalysis, detection, and bioimaging. Although cell studies of CPDs have produced impressive results, in vivo imaging requires available CPDs to fluoresce in the near‐infrared‐II (NIR‐II) window (1000−1700 nm). Here, a two‐step bottom‐up strategy is developed to synthesize NIR‐CPDs that provide bright emissions in both NIR‐I and NIR‐II transparent imaging windows. The designed strategy includes a hydrothermal reaction to form a stable carbon core with aldehyde groups, followed by the Knoevenagel reaction to tether the molecular emission centers. This procedure is labor‐saving, cost‐efficient, and produces a high yield. The NIR‐CPDs enable high‐performance NIR‐II angiography and real‐time imaging of the disease degree of colitis noninvasively. This technology may therefore provide a next‐generation synthesis strategy for CPDs with rational molecular engineering that can accurately tune the absorption/emission properties of NIR‐emissive CPDs.

Energy Transfer Processes in the Excited States of an {[Ir(N C)2(N N)]+‐Rhodamine} Dyad: An Experimental and Theoretical Study

AbstractHerein we report the synthesis, characterization, photophysical and theoretical study of the luminescent dyad Ir‐RhB containing two emissive centers: The first fluorescent (RhB=rhodamine B) and the second phosphorescent (Ir=orthometalated iridium complex), bound with an aliphatic bridge. These nearly independent chromophores have triplet excited states which are very close in terms of energy, allowing for reversible energy transfer between them. The observed transient absorption spectra and their time dependent variations were interpreted in terms of a relatively simple kinetic model, where selective excitation of the iridium‐based chromophore affords the corresponding emissive triplet state, followed by its equilibration with the rhodamine based triplet state and simultaneous emissive relaxation to the dyad ground state. This model allows for analytical solution to give key characteristics of the dyad excited‐state dynamics, including the energy gap between these triplets (ΔE=0.017 eV) and the rate constant of energy transfer (k(Ir‐RhB)=6.0 ⋅ 108 s−1). These data were also verified by quantum chemical DFT and TD‐DFT calculations.

Laser activated room-temperature excimer delayed fluorescence of difluoroboron β-diketonate complexes in polymer matrix

Great potential has been discovered in the development of materials with a long afterglow (phosphorescence at room temperature and delayed fluorescence) for the purposes of luminescent sensing, bioimaging, information encryption and OLED. Here, we report the long-lived (lifetime up to 1.5 s) laser-induced room-temperature excimer delayed fluorescence (RTEDF) of difluoroboron β-diketonate complexes (BF2bdks) already at concentrations of 0.05% in solution cast polymer films. The emission centers of RTEDF of BF2bdks can retain for a long time in the polycarbonate matrix (>3 months) and can be repeatedly re-excited with a low-intensity ultraviolet light source. These results provide a foundation for the design of materials with applications in optical information storage and data encryption and decryption.