Inorganic Phosphors Research Articles

(Digital Presentation) Fine Nitride Phosphors for Mini-LED Backlights

Mini-LED backlights, combining color conversion materials with blue Mini-LED chips, promise traditional liquid crystal displays (LCDs) with higher luminance, better contrast and wider color gamut. As color conversion materials, Cd- or Pb-based quantum dots are usually toxic and unstable, and commercially available inorganic phosphors are quite stable and cheap, but they have a large size due to their synthesis at high temperature, and their quantum efficiency and reliability are size dependent. To make inorganic phosphors to be well combined with Mini-LED chips (usually 100 x100 μm in size) in the emerging Mini-LED backlights, the particle size of the phosphors needs to be reduced to several micrometers or sub-micrometers while the quantum efficiency remains unchanged. In this work, we prepared fine nitride phosphors by treating commercially available phosphors, including CaAlSiN3:Eu2+ and (Sr, Ba)2Si5N8:Eu2+, with ball milling, centrifuging and acid washing. The particle size of phosphors can be easily controlled by the milling speed, and the phosphors with a size varying from 0.5 to 15 mm were thus obtained. The samples remained the comparable quantum efficiency with the original ones even when their particle size was reduced to 2 μm as they contained less surface defects after acid washing. We demonstrated that red-emitting Mini-LEDs can be fabricated by combining the red phosphors with blue Mini-LED chips, which show higher external quantum efficiency, luminance and stability than those made by using quantum dots. It indicates that fine and high efficiency phosphors can be obtained by the proposed method in this work, and they have great potentials for use in Mini-LED displays.

Photophysical and Primary Self-Referencing Thermometric Properties of Europium Hydrogen-Bonded Triazine Frameworks.

Lanthanide hydrogen-bonded organic frameworks (LnHOFs) are recently emerging as a novel versatile class of multicomponent luminescent materials with promising potential applications in optics and photonics. Trivalent europium (Eu3+) incorporated polymeric hydrogen-bonded triazine frameworks (PHTF:Eu) have been successfully obtained via a facile and low-cost thermal pyrolysis route. The PHTF:Eu material shows a porous frame structure principally composed of isocyanuric acid and ammelide as a minor constituent. Intense red luminescence with high colour-purity from Eu3+ is obtained by exciting over a broad absorption band peaked at 300 nm either at room or low temperature. The triazine-based host works as excellent optical antenna towards Eu3+, yielding ~42% sensitization efficiency (ηsens) and an intrinsic quantum yield of Eu3+ emission (ΦEu) as high as ~46%. Temperature-dependent emission studies show that PHTF:Eu displays relatively high optical stability at elevated temperatures in comparison to traditional inorganic phosphors. The retrieved activation energy of 89 meV indicates that thermal quenching mechanisms are attributed to the intrinsic energy level structure of the metal-triazine assembly, possibly via a thermally activated back transfer to ligand triplet or CT states. Finally, by using an innovative approach based on excitation spectra, we demonstrate that PHTF:Eu can work as a universal primary self-referencing thermometer based on a single-emitting center with excellent relative sensitivity in the cryogenic temperature range.

Prediction of formation energies of UCr4C4-type compounds from Magpie feature descriptor-based machine learning approaches

Lanthanide-doped UCr4C4-type phosphors have become one of the most promising materials for the narrow-band luminescence, due to their high photoluminescence quantum efficiency and good thermal stabilities. In this study, four machine learning regression models (i.e., the gradient boosted regression (GBR), support vector regression (SVR), random forest (RF) and K-nearest neighbor (KNN)) were established in combination with Magpie feature descriptors in order to calculate formation energies and then study thermal stabilities of phosphor hosts. Thereinto, the GBR model had the best performance (R2: 0.945, MAE: 0.106 eV/atom, MSE: 0.032 eV/atom) and then relatively accurately predicted formation energies of UCr4C4-type compounds. Besides, the Shapley additive explanation (SHAP) was implemented to interpret the prediction of the GBR and analyze the importance of feature descriptors. It is expected that the computing framework in the present work would provide a beneficial guidance for the study of the physical and chemical properties of inorganic phosphors.

Повышение извлекаемости алмазов в процессе рентгено-люминесцентной сепарации с применением люминофорсодержащих композиций

Introduction. A promising way to solve the problem of reducing the losses of weakly and abnormally luminescent diamonds in the process of X-ray luminescent separation is to modify their spectral characteristics with special compositions of phosphors. The phosphor-containing compositions used include inorganic and organic phosphors that provide the necessary change in the kinetic characteristics of the diamond X-ray luminescence signal. The purpose of the work. Investigation of the mechanism of the process and selection of compositions of phosphorcontaining compositions that provide the required modification of spectral characteristics and increase the extraction of weakly and abnormally luminescent diamonds in the process of X-ray-luminescent separation. Methodology results. Studies of the mechanism of formation of the X-ray luminescence signal from diamond-phosphor complexes were carried out by determining the degree of coating of the surface of diamonds with phosphors by the visiometric method and the amplitude of the luminescence signal with varying concentrations of phosphor in the emulsion. The effectiveness of the selected compositions of the phosphor-containing composition was evaluated by determining and comparative analysis of the acquired spectral and kinetic characteristics of the diamond-phosphor complexes using the Polyus–M separator. The evaluation of the effectiveness of the selected formulations was determined by the results of a test on industrial X-ray luminescent separators. Research results. The mechanism of formation of the X-ray luminescence signal from a diamond with a phosphorcontaining composition fixed on its surface is determined. It is shown that the total signal is the sum of the diamond and phosphor signals attenuated by screening and scattering of luminescent radiation by 5-15% for the diamond signal and 20-40% for the phosphor signal. The use of inorganic phosphors based on zinc sulfide, or a mixture of phosphor based on zinc orthosilicate and anthracene in compositions is justified, providing an increase in the intensity of the X-ray luminescence signal of a weakly luminescent diamond by 2.5 times, and preserving the shape of the X-ray luminescence signal of natural diamonds. It is proposed to use catalytic cracking as a collector of a diesel fraction compound and heavy gas oil, which ensures effective fixation of the phosphor on the surface of diamonds. The composition of the composition (FL-530, anthracene, diesel fraction, THCC) is proposed, which provides the required modification of spectral characteristics and detection of low-luminous diamonds. Resume. The results of the tests carried out on diamonds of various sizes have established the possibility of increasing the extraction of diamonds by 5-15% due to the detection of diamond crystals with weak or abnormal luminescence. At the same time, the required selectivity of the process with respect to kimberlite minerals is maintained. Conclusions. The results of the research are recommended for practical implementation in X-ray luminescent separation schemes at ALROSA processing plants to reduce losses of weakly and abnormally luminescent diamonds and will also be used in further studies of the possibility of increasing the selectivity of separation of diamonds and rock minerals during kimberlite enrichment.

Preparation of Photochromic and Photoluminescent Nonwoven Fibrous Mat from Recycled Polyester Waste

Photochromic and photoluminescent clothes can be described as smart textiles that alter their color and emission spectra upon exposure to a light stimulus. Recycled nonwoven polyester fabrics screen-printed with rare-earth strontium aluminate nanoparticles were developed to introduce photochromic and photoluminescent properties. Both spinning and preparation of nonwoven fibrous mat was performed industrially starting from recycled polyester waste. Aqueous-based phosphor-binder nanocomposites containing different concentrations of inorganic phosphor with excellent thermal and photostability were applied directly onto nonwoven polyester fabrics. The screen-printing process produced a uniform photochromic and photoluminescent film onto the nonwoven polyester surface that showed strong green emission color (440 nm) under UV light even at lower phosphor concentrations (0.5 wt%) in the printing paste. The excitation wavelength of the printed nonwoven polyester samples was monitored at 382 nm. Long-persistent greenish-yellow phosphorescence was detected in the dark at higher phosphor concentrations. The morphological microscopic data of phosphor nanoparticles and printed nonwoven polyester fabrics were collected using various analytical methods. TEM analysis of phosphor nanoparticles designated diameters of 4–11 nm, whereas XRD analysis indicated a crystal size of 9 nm. The printed cloth exhibited a quick and reversible photochromic emission when exposed to ultraviolet light. The ultraviolet protection, antimicrobial and superhydrophobic properties were improved with increasing the pigment concentration in the printing paste. The static contact and slide angles improved in the ranges of 108.6°–132.6°, and 12°–7°, respectively. The effects of increasing the phosphor concentration in the printing paste on the comfort features and colorfastness were examined.

Ho-SiAlON Ceramics as Green Phosphors under Ultra-Violet Excitations.

In most inorganic phosphors, increasing the concentration of activators inevitably causes the concentration quenching effect, resulting in reduced emission intensity at a high level of activator doping and the conventional practice is to limit the activator concentration to avoid the quenching. In contrast, SiAlON ceramics preserve their chemical composition over a very wide range of doping of activator ions, which favors the adjustment and optimization of the luminescence properties avoiding concentration quenching. Here, we investigate the photoluminescence properties of Ho-doped SiAlON (Ho-SiAlON) ceramics phosphors prepared by the hot-press method. Ho-SiAlON ceramics show strong green visible (554 nm) as well as infrared (2046 nm) broadband downshifting emissions under 348 nm excitation. It is shown that there is no concentration quenching, even at a very high level of Ho doping. The emission intensity of the 554 nm band increased two-fold when the Ho concentration is doubled. The results show that the Ho-SiAlON ceramics can be useful for efficient green phosphors.

Luminescence of Mn4+ in a Zero-Dimensional Organic-Inorganic Hybrid Phosphor [N(CH3)4]2ZrF6 for Dual-Mode Temperature Sensing.

Searching for new low-dimensional organic–inorganic hybrid phosphors is of great significance due to their unique optical properties and wide applications in the optoelectronic field. In this work, we report a Mn4+ doped zero-dimensional organic–inorganic hybrid phosphor [N(CH3)4]2ZrF6, which was synthesized by a wet chemical method. The crystal structure, thermal stability, and optical properties were systemically investigated by means of XRD, SEM, TG-DTA, FTIR, DRS, emission spectra, excitation spectra, as well as decay curves. Narrow red emission with high color purity can be observed from [N(CH3)4]2ZrF6:Mn4+ phosphor, which maintains effective emission intensity even at room temperature, indicating its potential practical application in WLEDs. In the temperature range of 13–295 K, anti-Stokes and Stokes sidebands of Mn4+ ions exhibit different temperature responses. By applying the emission intensity ratio of anti-Stokes vs. Stokes sidebands as temperature readout, an optical thermometer with a maximum absolute sensitivity of 2.13% K−1 and relative sensitivity of 2.47% K−1 can be obtained. Meanwhile, the lifetime Mn4+ ions can also be used for temperature sensing with a maximum relative sensitivity of 0.41% K−1, demonstrating its potential application in optical thermometry.

Photoluminescence and Judd-Ofelt analysis of Eu3+ doped akermanite silicate phosphors for solid state lighting

After aluminates, alkaline-earth silicates are the most researched category of luminescent inorganic phosphors. Eu3+ doped Ca2MgSi2O7 and Sr2MgSi2O7 phosphors synthesized by the combustion process. For the as-prepared phosphors, XRD and Rietveld refinement revealed a single phase with akermanite type structure belonging to tetragonal crystallography. UV–Vis spectroscopy was used to calculate the optical bandgap. Ca2MgSi2O7:x%Eu and Sr2MgSi2O7:x%Eu (x = 0.5 - 2.5) phosphors produced orange-red light with emission peaks at 590 nm, 614 nm, and 702 nm, which corresponded to the Eu3+ transitions 5D0 → 7F1, 5D0 → 7F2, and 5D0 → 7F4, respectively. Using a modified Judd-Ofelt technique based on the emission spectrum, the key spectroscopic properties (transition probabilities, radiative lifetimes, and branching ratios) associated with the 5D0 orange-red emitting level were found. Ω2 < Ω4 reveals that Eu3+ ions occupy high symmetry locations in the generated sample under the doping level studied in this work. There has also been a decrease in the Ω4 parameter. It implies that the 5D0 → 7F2 transition is more efficient than the 5D0 → 7F4. This indicates that the red color emission has been improved. The estimated CIE coordinates corroborated the PL emission in the orange-red region.

Fine-grained phosphors for red-emitting mini-LEDs with high efficiency and super-luminance

Mini-LED backlights, combining color conversion materials with blue mini-LED chips, promise traditional liquid crystal displays (LCDs) with higher luminance, better contrast, and a wider color gamut. However, as color conversion materials, quantum dots (QDs) are toxic and unstable, whereas commercially available inorganic phosphors are too big in size to combine with small mini-LED chips and also have strong size-dependence of quantum efficiency (QE) and reliability. In this work, we prepare fine-grained Sr2Si5N8:Eu2+-based red phosphors with high efficiency and stability by treating commercially available phosphors with ball milling, centrifuging, and acid washing. The particle size of phosphors can be easily controlled by milling speed, and the phosphors with a size varying from 3.5 to 0.7 μm are thus obtained. The samples remain the same QE as the original ones (∼80%) even when their particle size is reduced to 3.2–3.5 μm, because they contain fewer surface suspension bond defects. More importantly, SrBaSi5N8:Eu2+ phosphors show a size-independent thermal quenching behavior and a zero thermal degradation. We demonstrate that red-emitting mini-LEDs can be fabricated by combining the SrBaSi5N8:Eu2+ red phosphor (3.5 μm in size) with blue mini-LED chips, which show a high external quantum efficiency (EQE) of above 31% and a super-high luminance of 34.3 Mnits. It indicates that fine and high efficiency phosphors can be obtained by the proposed method in this work, and they have great potentials for use in mini-LED displays.

Germanium silicon oxide achieves multi-coloured ultra-long phosphorescence and delayed fluorescence at high temperature

Colour-tuned phosphors are promising for advanced security applications such as multi-modal anti-counterfeiting and data encryption. The practical adoption of colour-tuned phosphors requires these materials to be responsive to multiple stimuli (e.g., excitation wavelength, excitation waveform, and temperature) and exhibit excellent materials stability simultaneously. Here we report germanium silicon oxide (GSO) – a heavy-metal-free inorganic phosphor – that exhibits colour-tuned ultra-long phosphorescence and delayed fluorescence across a broad temperature range (300 – 500 K) in air. We developed a sol-gel processing strategy to prepare amorphous oxides containing homogeneously dispersed Si and Ge atoms. The co-existence of Ge and Si luminescent centres (LC) leads to an excitation-dependent luminescence change across the UV-to-visible region. GSO exhibits Si LC-related ultra-long phosphorescence at room-temperature and thermally activated delayed fluorescence at temperatures as high as 573 K. This long-lived PL is sensitized via the energy transfer from Ge defects to Si LCs, which provides PL lifetime tunability for GSO phosphors. The oxide scaffold of GSO offers 500-day materials stability in air; and 1-week stability in strong acidic and basic solutions. Using GSO/polymer hybrids, we demonstrated colour-tuned security tags whose emission wavelength and lifetime can be controlled via the excitation wavelength, and temperature, indicating promise in security applications.

Synthesis of Cs3MnBr5 Green Phosphors Using an Eco-Friendly Evaporative Crystallization Process.

Green (G) and red (R) light-emitting materials, such as quantum dots, perovskite nanocrystals, and inorganic phosphor powders, owing to their excellent optical characteristics, have attracted researchers’ attention as color-conversion materials for lighting and display applications. However, these materials contain environmentally harmful elements, such as Pb or Cd, and/or they are synthesized using environmentally harmful synthetic approaches and conditions, involving the use of organic solvents, high pressure, high temperature, harsh atmosphere, and long reaction time. In this study, as an eco-friendly synthetic approach to synthesize lead-free Cs3MnBr5 G powder phosphor, we suggest an evaporative crystallization process of aqueous reactant solution. This synthetic process does not use toxic elements or solvents and the crystallization process utilizes only low reaction temperature and short reaction time under air atmosphere conditions. We successfully synthesized Cs3MnBr5 green powder phosphor, with excellent optical properties, by evaporative heating of a 200 nm syringe-filtered solution at 150 °C for 2 h. The synthesized Cs3MnBr5 phosphors have a photoluminescence quantum yield of 66.3%, a peak wavelength of 520 nm, a narrow bandwidth of 38 nm, and a photoluminescence decay time of 0.34 ms under blue excitation. This phosphor is expected to be a useful alternative G-emitting material that can compete with commercial green quantum dots, perovskite nanocrystals, or inorganic phosphors.

(Invited) Organic Dyes Derived from Molecules in Cacao Beans for Use in Lighting Applications

LEDs are the foundation of lighting and display products surrounding us. While they have obtained great success in the commercial market, related research and development activities remain highly active aiming to enhance factors such as energy efficiency, stability, and environmental sustainability. Currently, commercial LED products are comprised of two key components: an indium gallium nitride e LED backlight with emission centered at 450 nm to cover the blue region of the visible spectrum, and powder inorganic phosphors on top converting blue light into longer wavelengths (e.g., green and red) to tune the emission of the device. A drawback of inorganic phosphors is that scattering of the emission from the micron-sized phosphor powders leads to substantial backscattering and subsequent absorption of the emission into the LED chip, and reabsorption losses in the phosphor itself, both of which reduce the overall light output of the final LED device. Organic dyes possess environmental advantages over inorganic phosphors because they are pi-conjugated molecules made from abundant elements (C, H, N, O, etc.) and are potentially bio-sourced. In this talk, I will present dyes that have been developed from theobromine, derived from cacao beans. When blended within an industrial polymer, poly(styrene-butadiene-styrene) (SBS), their enhanced solubility enables the formation of highly transparent films, crucial for reducing scattering loss in LEDs. Furthermore, resultant dye-SBS films achieved photoluminescence quantum yields (PLQYs) of around 90% under ambient conditions. Taking advantage of their transparency and solution processability, we fabricated a waveguide with this theobromine-dye-SBS composite, which was subsequentially assembled into an edge-lit LED device of no glare and enhanced aesthetics.

(Digital Presentation) Garnet Structure with UVC and UVB Dual-Band Persistent Luminescence

While persistent luminescence (PersL) at visible and infrared are frequently reported in lanthanide-doped inorganic phosphors for night-vision displays and bio-imaging, an awareness to ultraviolet (UV) PersL remains much less. Here, we have designed Pr3+-doped (Ca1.5Y1.5)(Al3.5Si1.5)O12 (CYAS-Pr) through co-substituting Ca2+-Si4+ into a well-known garnet structure, Y3Al5O12. By the strategy of Ca2+-Si4+ co-substitution, the CYAS-Pr exhibits lattice distortion and subsequently an unprecedent UVC and UVB dual-band PersL with maxima at 266 and 311 nm for 12 hours after charged by UV irradiation due to creation of intrinsic trapping centers. The unique PersL performance is contributed by an existence of two different Pr3+ environments at Ca2+ and Y3+ sites of the CYAS host. The CYAS-Pr shows efficient dual information storage and precise long-distance self-sustained “solar-blind” imaging capabilities in a wide and bright field of view. It is expected that the UVC and UVB dual-band PersL CYAS-Pr will find more applications and trigger further exploration towards persistent sterilization and persistent photocatalysis.

Broadband Yellow Phosphor Discovered by Single-Particle Analysis Based on Fluorescence Microscopy, Scanning Electron Microscopy, Cathodoluminescence, and Energy-Dispersive X-ray Spectroscopy.

The discovery of new, efficient, and stable inorganic phosphors excited by near-ultraviolet (n-UV) light is of great significance for the realization of high-performance healthy lighting and special lighting. The traditional methods to find new phosphors are usually time-consuming, laborious, and inefficient. Here, a simple and efficient method to find new phosphors using fluorescence microscopy, scanning electron microscopy (SEM), cathodoluminescence (CL), and energy-dispersive X-ray spectroscopy (EDS) is proposed. At the same time, a phosphor Ca5BaP4O16 (CBPO): Eu2+ found by this method is reported. Under 365 nm excitation, the optimal phosphor shows an ultrabroad yellow emission band with a peak at 573 nm and a full width at half-maximum of 164 nm. Compared with most phosphate phosphors, CBPO: Eu2+ shows good thermal stability. The emission intensity at 150 °C can maintain 61.4% of the initial value at room temperature. Importantly, the quantum efficiency of the optimal CBPO: Eu2+ sample is 66.2%. In addition, a WLED lamp was obtained by combining CBPO: Eu2+ and BAM: Eu2+ phosphors with a 365 nm n-UV chip, which generated high-performance white light with a correlated color temperature of 7115 K and a color rendering index of 89.4. It is certain that the new method is very effective and useful for the discovery of new phosphors.

Designing Optical Thermometers Using Down/Upconversion Ca14Al10Zn6O35: Ti4+, Eu3+/Yb3+, Er3+ Thermosensitive Phosphors.

Down/upconversion Ca14Al10Zn6O35 inorganic phosphors codoped with Ti4+/Eu3+ or Yb3+/Er3+ were prepared. The crystal structure and downconversion luminescence properties of Ca14Al10Zn6O35:Ti4+, Eu3+ phosphors were studied in detail. Ti4+ and Eu3+ occupied Al3+ and Ca2+ sites in the host lattice, respectively. Under the excitation of 273 nm, the emission peak in the 300-570 nm band originated from the 2T2 → O2- transition of Ti4+. The f-f transition of Eu3+ ions generated multiple peaks in the 570-800 nm range. The emission intensity of Ti4+ and Eu3+ ions can be used as a fluorescence intensity ratio (FIR) signal. Based on the FIR technology, the maximum relative sensitivity (Sr) and the minimum temperature uncertainty (δT) reached 1.41% K-1 and 0.07 K, respectively. Meanwhile, the temperature-sensing behaviors were explored by the temperature-dependent upconversion spectra of Er3+- and Yb3+-codoped Ca14Al10Zn6O35 phosphors. Based on the fluorescence intensity ratio of thermal coupling levels (Er3+:2H11/2/4S3/2), the maximum Sr and minimum δT of upconversion phosphors reached 1.28% K-1 and 0.08 K in the temperature range of 293-473 K, respectively. Ca14Al10Zn6O35:Ti4+/Eu3+ (Yb3+/Er3+) phosphors realize temperature sensors with higher relative sensitivity, and it is a good candidate material for optical temperature measurement.

Preface

The 21st National Conference on Solid State Nuclear Track Detectors and Their Applications (SSNTDs-21), a three-day national conference was jointly organized by the Department of Physics, Ramjas College and Department of Physics and Astrophysics, University of Delhi in collaboration with the Nuclear Track Society of India (NTSI) during January 29-31, 2021. This was the 21st conference into biennial series of the conferences and was inaugurated by Prof. P.C. Joshi, Vice Chancellor, University of Delhi and Prof. Mahesh Verma, Vice Chancellor, GGSIP University, Delhi. The organizing team comprised of Dr. Manoj Kumar Khanna, Principal, Ramjas College (as Patron), Prof. P.D. Sahare, Dr. Rajesh Kumar and Dr. Ashok Kumar as the convener, co-convener and organizing secretory of the conference, respectively. The office bearers of NTSI and advisory committee also played a vital role in organizing the conference. Solid State Nuclear Track Detector (SSNTDs) are a class of passive detectors, developed by R.L. Fleischer, P.B. Price and R.L. Walker in the early 1960 and they have found in numerous applications in various field of science and technology. SSNTDs have been recognize as very potential and effective tools in exploring various research area. Apart from the direct applications of far reaching consequences in nuclear physics, other areas, such as, biomedical sciences, cosmic rays and space physics, environmental sciences geochronology and geophysics, materials sciences, lunar sciences, meteorites and tektites and microanalysis and mine safety, nuclear technology, uranium prospective and most recently nano-/microtechnology, etc. have been greatly influence by SSNTDs. They have a very important role to play in radiation measurements, micro technology and dosimetry and thus they are potent enough in spreading awareness about the radiation environment and its impact on the general public and the academic peers. In order to disseminate the knowledge generated in this fast-growing field, there was a need to bring material science and radiation community on a common platform and discuss various aspects related to it. In view of this, the NTSI along with Ramjas College and Department of Physics and Astrophysics, University of Delhi organized a very comprehensive three-day conference, SSNTDs-21 at the University of Delhi. The focused areas of the conference included, basic studies and methodology; latent tracks in materials and ion track technology; heavy ion nuclear physics and cosmic rays; radon/thoron studies; radiation measurements and dosimetry; trace analysis; Nuclear track filters and their applications; fission track dating and geochronology; instrumentation, devices and software in these areas; SSNTDs in teaching; radiation induced modification of materials; luminescence and it’s applications; synthesis and characterization of materials, etc. The conference was attended by more than 150 scientists, academicians and researchers having 104 oral presentations, 23 invited talks during 12 technical sessions chaired by eminent scientists and scholars. Scientist from different institution and universities all over the country participated in the conference. Young scientist awards were also given under various categories for best papers presentations. The conference was sponsored by UGC, CSIR, INSA and DRDO of Government of India. This special issue contains some ‘Selected and Peer-Reviewed’ full length 11 papers out of the total more than hundred papers presented in the ‘21st National Conference on Solid State Nuclear Track Detectors and Their Applications’ (SSNTD-21). The articles presented in the conference covered a wide range of topics. However, only articles of high quality were selected and the topics include synthesis, characterization and applications of some inorganic luminescent phosphors for their applications in radiation dosimetry of high-energy radiations including swift heavy ions (SHI) using TL/OSL techniques. There are also papers on polymer solid state track detectors. Some papers on nanophosphors have also been included for their related applications. We hope that this special issue of the ‘Journal of Atomic, Molecular, Condensed Matter and Nano Physics’ will be useful to the workers in this field. Lead Guest Editor: Prof. P.D. SahareDepartment of Physics and AstrophysicsUniversity of DelhiDelhi 110007, India Associate Guest Editors: Dr. Rajesh KumarUniversity School of Basic and Applied SciencesGGS Indraprastha UniversityNew Delhi 110078, India Prof. Ashok KumarDepartment of PhysicsRamjas College (University of Delhi)Delhi 110007, India

Luminescent inorganic mixed borate phosphors materials for lighting

The need of new materials with desirable optical properties has become important in recent years. In particular, a need has emerged for compounds having better luminescence properties in various practical applications. The introduction of rare earth ions as activators improves the luminescence properties of the compounds considerably. Boron is one of the most abundant elements in nature. It readily combines with almost all other elements. In combination with oxygen; it forms borates comprised of various anions such as BO3 3- , B2 O5 4- , etc. Apart from these simple borates, several complex compositions exist involving mixed anions as well as double metal borates, due to the three-fold, or four-fold coordination of borate atoms. Borates intrinsically possesses characteristics that are advantageous for optical materials, which include a wide transparency range, large electronic band gap, good thermal and chemical stability, low preparative temperature, optical stability with good nonlinear characteristics, and an exceptionally high optical damage threshold. The unique crystal structure of borates determines their enhanced ultraviolet light transparency, good nonlinearity, and relatively high resistance against laser-induced damage. Some of these complex borates have interesting luminescence properties that are covered in this review. These include double borates containing rare earths RM3 (BO3 )4 , pentaborates LaMgB5 O10 , M3 R2 (BO3 )4 , where M is an alkaline earth, mixed anion borates such as aluminoborate SrAl2 B2 O7 , silicate-borates such as pekovite, SrB2 Si2 O8 , haloborates, M2 B5 O9 X, where M is an alkaline earth and X is a halogen, phosphate borates, and MBPO5 , where M is an alkaline earth. Phosphors based on these compositions find use in various applications such as fluorescence lamps, colour TVs, plasma display panels, high-intensity discharge lamps based on xenon, optically pumped solid-state lasers, eye-safe lasers, and X-ray imaging.

Effect of site occupancy on the reduction of Eu3+ and tunable luminescence of CaLa4Si3O13:Eu phosphor

A series of europium-ion doped CaLa4Si3O13 phosphors were synthesized by using a conventional high-temperature solid-state method. X-ray powder diffraction patterns and photoluminescence spectra were utilized to characterize the as-prepared phosphors. The phosphors show tunable luminescence owing to the existence of Eu2+ and Eu3+ emission. A typical emission maximum of Eu2+ is about 460 nm, and Eu3+ shows a predominant 5D0-7F2 transition emission. The emission of Eu2+ shifts toward the long wavelength side with increasing doping concentration. The site occupancy of Eu2+ and Eu3+ is analyzed according to their luminescence behaviors. The Eu3+ ion at the Ca2+ site can be partially reduced to Eu2+, but at the La3+ site cannot be reduced. The controllable reducing of Eu3+ is explained according to the crystal structure of the host. Utilizing site selection to control the reducing of Eu3+ is considered as an alternative strategy to design novel Eu3+ and Eu2+ codoped inorganic phosphor.

«Ellestadite»-type anionic [PO4]3– → [SO4]2– substitutions in β-Ca3(PO4)2 type compounds: A new route to design the inorganic phosphors

Sulfate anion plays an important role in the development of inorganic phosphors and is considered as the path for for the enhancement of the luminescent properties. The heterovalent [PO4]3– → [SO4]2– anion substitution in the β-Ca3(PO4)2-type compounds was realized using the “ellestadite”-type strategy called by the analogy with the apatite-type structures. The novel compounds belonging to the Ca10.5–0.5x–yMy(PO4)7–x(SO4)x, M = Ca2+, Mg2+ and Zn2+ series have been synthesized using high-temperature solid state reaction. The used isomorphous scheme was [PO4]3– + ½ Ca2+ = [SO4]2– + ½ □ with the formation of the vacancy for the charge compensation. The successful incorporation of [SO4]2– groups was approved by Fourier-transform infrared spectroscopy, EDX method, and powder X-ray diffraction analysis. It was found that the sulfur atoms predominantly occupied T2 sites in the anionic sublattice according to crystal structure refinement based on the analysis of interatomic distances. The photoluminescent properties were studied for the Eu3+-doped samples and it was shown that Zn2+-containing samples exhibit a 1.5 times higher intensity in comparison with Mg2+-containing substituted phosphates. Thus, a new “ellestadite”-type substitution in β-Ca3(PO4)2-type structure significantly expands the crystal chemistry and give more insights into the deeper understanding of the dependence of luminescent properties on the cation distribution. The pathways of the [SO4]2– incorporation into the crystal structures are discussed in details.

Facile synthesis, structure, and properties of Gd2O2Se

Inorganic phosphors based on rare earth elements are commonly used as scintillation materials due to their high chemical and radiation resistance. In this work, we study gadolinium oxyselenide as a new phosphor material. The work describes a facile synthesis method for Gd2O2Se. Single crystal structure of this compound was determined for the first time: sp.gr. P¯3m1 with a ​= ​3.8911(4) Å and c ​= ​6.8829(8) Å. Gd2O2Se is stable in the inert atmosphere and melts at 2075 ​± ​20 ​°C. This material is also stable when heated in air up to 720 ​°C. with the following oxidation and weight increase. This mass increase can be explained by the oxidation of Se2− to Se4+. Further temperature increase above 952°С leads to a sharp mass loss and the final mass of sample corresponds to full oxidation of Gd2O2Se to gadolinium oxide. The band gap of the material was estimated to be 3.6 ​eV. Photoluminescence spectrum for Gd2O2Se:Tb3+ shows the characteristic transitions in Tb3+ ion with the most intense green emission that corresponds 5D4 → 7FJ transitions.