Unravelling the Role of Structural Factors in the Luminescence Properties of Persulfurated Benzenes.

Room temperature phosphorescence rarely occurs from pure organic molecules, especially in the solid-state. A strategy for the design of highly emissive organic phosphors is still hard to predict, thus impeding the development of new functional materials with the desired optical properties. Herein, we analyze a family of alkyl and aryl-substituted persulfurated benzenes, the latter representing a class of organic solid-state triplet emitters able to show very high emission quantum yield at room temperature. In this work, we correlate structural parameters with the photophysical properties observed in different experimental conditions (diluted solution, crystalline and amorphous phase at RT and low temperature). Our results, corroborated by a detailed computational analysis, indicate a close relationship between the luminescence properties and i) the nature of the substituents on the persulfurated core, ii) the adopted conformations in the solid state, both in crystalline and amorphous phases. These factors contribute to characterize the lowest-energy lying excited-state, its deactivation pathways, the phosphorescence lifetime and quantum yield. These findings provide a useful roadmap for the development of highly performing purely organic solid-state emitters based on the persulfurated benzene platform.

Luminescent nanomaterials for developing high contrast latent fingerprints.

Fingerprint pattern (or epidermal ridges) is by far one of the most reliable techniques for individual identification. Fingerprint patterns get deposited on any kind of solid surfaces due to human transudation or exudation process. Bodily fluids through sweat glands contain moisture, natural oils and proteins. Since latent fingerprint patterns are not readily recognisable thus they are collected from a crime scene and are further processed physically or chemically. Fingerprints obtained using conventional black and white powders; face severe drawbacks including low sensitivity, high background interference from the substrates, involvement of toxic materials, and poor stability. To overcome the above listed issues, especially for coloured and transparent substrates, luminescent materials have emerged as potential agents for rapid visualization of high contrast latent fingerprints. This review article covers the recent advancements in luminescent nanomaterials of both kinds (up and down conversion) and persistent nanophosphors for developing latent fingerprints. Special emphasis has been given to unusual class of luminescent materials known as persistent nanophosphors, which does not need a constant excitation thereby completely eradicating background noise. Review also covers different approaches of gathering fingerprints such as powder dusting, cyanoacrylate fuming, ninhydrin fuming and vacuum metal deposition.

Investigation on Luminescence Properties of Dy3+ Ions Doped NaBaB9O15 Phosphors for Optoelectronic Applications.

A series of Dy3+ ions doped NaBaB9O15 phosphors with different dopant concentration was synthesized by solid state reaction. The phase purity was checked by X-ray diffraction analysis and the functional groups present in as prepared phosphors was investigated with the help of FTIR spectral analysis. Under 386 nm excitation, the photoluminescence spectra exhibit three emission bands around 482 nm, 574 nm and 664 nm due to 4F9/2→6HJ/2 (J = 15, 13, 11) transions respectively. The optimum doping concentration of Dy3+ ions was found to be 5 mol%. The color coordinates are estimated from the emission spectra to check the dominant emission color and the color coordinates of the studied phosphors are fall on white region of CIE 1931 diagram. The decay curve analysis was made to determine the lifetime of excited state of Dy3+ ions and the decay curves are exhibited bi-exponential behavior irrespective of Dy3+ ion concentration. All these measurents are done in room temperature and the results obtained from these studies are discussed in detail to claim their usage in light emitting devices.

The long rod-shaped Sr2 MgSi2 O7 :Eu2+ ,Dy3+ with ultrahigh afterglow performance.

The afterglow properties of long afterglow luminescent materials are greatly affected by their defects, which are distributed on the grain surface. Increasing the exposed surface area is an important method to improve the afterglow performance. In this research, long rod-shaped long afterglow materials Sr2 MgSi2 O7 :Eu2+ ,Dy3+ were prepared using the hydrothermal-coprecipitation method. When the reaction time reached 96 h, the length of the afterglow materials could grow to 2 mm, and the sintering temperature was just 1150°C. The emission spectra of all obtained samples upon excitation at 397 nm had a maximum of 465 nm, which belonged to the representative transition of Eu2+ . The initial brightness was 1.35 cd/m2 . The afterglow time could reach 19 h, giving a good afterglow performance. The research on this kind of material has essential significance in the exploration of luminescence mechanisms and their applications.

Optical analysis of RE3+ (RE = Eu,Tb):MgLa2 V2 O9 nano-phosphors.

Red and green rare-earth ion (RE3+ ) (RE = Eu, Tb):MgLa2 V2 O9 micro-powder phosphors were produced utilizing a standard solid-state chemical process. The X-ray diffraction examination performed on the phosphors showed that they were crystalline and had a monoclinic structure. The particles grouped together, as shown in the scanning electron microscopy (SEM) images. Powder phosphors were examined using a variety of spectroscopic techniques, including photoluminescence (PL), Fourier-transform infrared, and energy dispersive X-ray spectroscopy. Brilliant red emission at 615 nm (5 D0  â†’ 7 F2 ) having an excitation wavelength (λexci ) of 396 nm (7 F0  â†’ 5 L6 ) and green emission at 545 nm (5 D4  â†’ 7 F5 ) having an λexci  = 316 nm (5 D4  â†’ 7 F2 ) have both been seen in the emission spectra of Tb3+ :MgLa2 V2 O9 nano-phosphors. The emission mechanism that is raised in Eu3+ :MgLa2 V2 O9 and Tb3+ :MgLa2 V2 O9 powder phosphors has been explained in an energy level diagram.

Broadband sensitized near-infrared emission in Eu2+-Nd3+ co-doped BaAl2O4 as a potential spectral modulator for silicon solar cells.

The near-infrared (NIR) down-conversion process for broadband sensitization has been studied in Eu2+-Nd3+ co-doped BaAl2O4. This material has a broad absorption band of 200-480 nm and can convert photons in the visible region into NIR photons. The NIR emission at 1064 nm, attributed to the Nd3+:4F3/2 → 4I11/2 transition, matches the bandgap of Si, allowing Si solar cells to utilize the solar spectrum better. The energy transfer (ET) process between Eu2+ and Nd3+ was demonstrated using photoluminescence spectra and luminescence decay curves, and Eu2+ may transfer energy to Nd3+ through the cooperative energy transfer (CET) to achieve the down-conversion process. The energy transfer efficiency (ETE) and theoretical quantum efficiency (QE) were 68.61% and 156.34%, respectively, when 4 mol% Nd3+ was introduced. The results indicate that BaAl2O4:Eu2+-Nd3+ can serve as a potential modulator of the solar spectrum and is expected to be applied to Si solar cells.

Robust and orange-yellow-emitting Sr-rich polytypoid α-SiAlON (Sr3Si24Al6N40:Eu2+) phosphor for white LEDs.

Nitrides and oxynitrides isostructural to α-Si3N4 (M-α-SiAlON, M = Sr, Ca, Li) possess superb thermally stable photoluminescence (PL) properties, making them reliable phosphors for high-power solid-state lighting. However, the synthesis of phase-pure Sr-α-SiAlON still remains a great challenge and has only been reported for Sr below 1.35 at.% as the large size of Sr2+ ions tends to destabilize the α-SiAlON structure. Here, we succeeded to synthesize the single-phase powders of a unique 'Sr-rich' polytypoid α-SiAlON (Sr3Si24Al6N40:Eu2+) phosphor with three distinctive Sr/Eu luminescence sites using a solid-state remixing-reannealing process. The Sr content of this polytypoid structure exceeds those of a few previously reported structures by over 200%. The phase purity, composition, structure, and PL properties of this phosphor were investigated. A single phase can be obtained by firing the stoichiometric mixtures of all-nitride precursors at 2050°C under a 0.92 MPa N2 atmosphere. The Sr3Si24Al6N40:Eu2+ shows an intense orange-yellow emission, with the emission maximum of 590 nm and internal/external quantum efficiency of 66%/52% under 400 nm excitation. It also has a quite small thermal quenching, maintaining 93% emission intensity at 150°C. In comparison to Ca-α-SiAlON:Eu2+, this Sr counterpart shows superior quantum efficiency and thermal stability, enabling it to be an interesting orange-yellow down-conversion luminescent material for white LEDs. The experimental confirmation of the existence of such 'Sr-rich' SiAlON systems, in a single-phase powder form, paves the way for the design and synthesis of novel 'Sr-rich' SiAlON-based phosphor powders with unparalleled properties.

A distinctive orange-yellow-emitting 'Sr-rich' α-SiAlON-based phosphor with quite small thermal quenching (93% PL intensity at 150°C) that can surprisingly be synthesized in a single-phase powder form for white LEDs.

Size-Dependent Optical Properties of InP Colloidal Quantum Dots.

Indium phosphide colloidal quantum dots (CQDs) are the main alternative for toxic and restricted Cd based CQDs for lighting and display applications. Here we systematically report on the size-dependent optical absorption, ensemble, and single particle photoluminescence (PL) and biexciton lifetimes of core-only InP CQDs. This systematic study is enabled by improvements in the synthesis of InP CQDs to yield a broad size series of monodisperse core-only InP CQDs with narrow absorption and PL line width and significant PL quantum yield.

Efficient and Near-Zero Thermal Quenching Cr3+-Doped Garnet-Type Phosphor for High-Performance Near-Infrared Light-Emitting Diode Applications.

In recent years, near-infrared (NIR) phosphors have attracted great research interest due to their unique physical properties and broad application prospects. However, obtaining NIR phosphors with both high quantum efficiency and excellent thermal stability remains a great challenge. In this study, novel NIR Ca3Mg2ZrGe3O12:Cr3+ phosphors were successfully prepared using a high-temperature solid-phase method, and the structure and luminescent properties of the material were systematically investigated. Ca3Mg2ZrGe3O12:0.01Cr3+ emits NIR light in the range of 600 to 900 nm with a peak at 758 nm and a half-height width of 89 nm under the excitation of 457 nm blue light. NIR luminescence shows considerable quantum efficiency, and the internal quantum efficiency of the optimized sample is up to 68.7%. Remarkably, the Ca3Mg2ZrGe3O12:0.01Cr3+ phosphor exhibits a near-zero thermal quenching behavior, and the luminescence intensity of the sample at 250 °C maintains 92% of its intensity at room temperature. The mechanism of high thermal stability has been elucidated by calculating the Huang Kun factor and activation energy. Finally, NIR pc-LED devices prepared from Ca3Mg2ZrGe3O12:0.01Cr3+ phosphor with commercial blue LED chips have good performance, proving that this Ca3Mg2ZrGe3O12:0.01Cr3+ NIR phosphor has potential applications in night vision and biomedical imaging.

Oriented Growth of Highly Emissive Manganese Halide Microrods for Dual-Mode Low-Loss Optical Waveguides.

Zero-dimensional (0D) structured lead-free metal halides have recently attracted widespread attention due to their high photoluminescence quantum yield (PLQY) and negligible self-absorption, showing enormous potential as optical waveguides towards miniaturized photonic devices. However, due to the great difficulty in growth of rod-like nano/micro-sized morphologies, such applications have been less explored. Herein, a new-type emissive organic-inorganic manganese (II) halide crystal (TPS2MnCl4, TPS = C18H15S, triphenylsulfonium) in the form of microrods is synthesized via a facile chloride ion (Cl-) induced oriented growth method. Due to a combination of attractive features such as a high PLQY of 86%, negligible self-absorption and smooth crystal surface, TPS2MnCl4 microrods are well suited for use in optical waveguide with an ultra-low optical loss coefficient of 1.20·10-4 dB µm-1, superior to that of most organic-inorganic metal halide hybrids, organic materials, polymers and metal nanoclusters to the best of our knowledge. Importantly, TPS2MnCl4 microrods can further work as dual-mode optical waveguides, combining active and passive light transmission functionalities in one single crystal. In addition, TPS2MnCl4 microrods also display remarkable performance in lighting and anti-counterfeiting due to their distinct optical properties and commendable stability.

Spinel-Type Structured Phosphor Near-Infrared-II Emission: Intervalence Charge Transfer and Hetero-Valent Chromium Pairs.

Near-infrared (NIR) emitting phosphors draw much attention because they show great applicability and development prospects in many fields. Herein, a series of inverse spinel-type structured LiGa5O8 phosphors with a high concentration of Cr3+ activators is reported with a dual emission band covering NIR-I and II regions. Except for strong ionic exchange interactions such as Cr3+-Cr3+ and Cr3+ clusters, an intervalence charge transfer (IVCT) process between aggregated Cr ion pairs is proposed as the mechanism for the ~1210 nm NIR-II emission. Comprehensive structural and luminescence characterization points to IVCT between two Cr3+ being induced by structural distortion and further enhanced by irradiation. Construction of the configurational energy level diagram enabled elucidation of this transition within the IVCT process. Therefore, this work provides insight into the emission mechanism within the high Cr3+ concentration system, revealing a new design strategy for NIR-II emitting phosphors to promote its response.

Visualizing Triplet Energy Transfer in Organic Near-Infrared Phosphorescent Host-Guest Materials.

Limited by the energy gap law, purely organic materials with efficient near-infrared room temperature phosphorescence are rare and difficult to achieve. Additionally, the exciton transition process among different emitting species in host-guest phosphorescent materials remains elusive, presenting a significant academic challenge. Herein, using a modular nonbonding orbital-π bridge-nonbonding orbital (n-π-n) molecular design strategy, we develop a series of heavy atom-free phosphors. Systematic modification of the π-conjugated cores enables the construction of a library with tunable near-infrared phosphorescence from 655 to 710 nm. These phosphors exhibit excellent performance under ambient conditions when dispersed into a 4-bromobenzophenone host matrix, achieving an extended lifetime of 11.25 ms and a maximum phosphorescence efficiency of 4.2%. Notably, by eliminating the interference from host phosphorescence, the exciton transition process can be visualized in hybrid materials under various excitation conditions. Spectroscopic analysis reveals that the improved phosphorescent performance of the guest originates from the triplet-triplet energy transfer of abundant triplet excitons generated independently by the host, rather than from enhanced intersystem crossing efficiency between the guest singlet state and the host triplet state. The findings provide in-depth insights into constructing novel near-infrared phosphors and exploring emission mechanisms of host-guest materials.

Phosphors Ba2 LaTaO6 :Mn4+ and its alkali metal charge compensation for plant growth illumination.

A series of Mn4+ -doped and Mn4+ ,K+ -co-doped Ba2 LaTaO6 (BLT) double-perovskite phosphors was synthesized using a high-temperature solid-state reaction. The phase purity and luminescence properties were also studied. The optimum doping concentration of Mn4+ and K+ was obtained by investigating the photoluminescence excitation spectra and photoluminescence emission spectra. The comparison of BLT:Mn4+ phosphors with and without K+ ions shows that the photoluminescence intensity of K+ -doped phosphors was greatly enhanced. This is because there was a charge difference when Mn4+ ions were doped with Ta5+ ions in BLT. Mn4+ -K+ ion pairs were formed after doping K+ ions, which hinders the nonradiative energy transfer between Mn4+ ions. Therefore, the luminescence intensity, quantum yield, and thermal stability of phosphors were enhanced. The electroluminescence spectra of BLT:Mn4+ and BLT:Mn4+ ,K+ were measured. The spectra showed that the light emitted from the phosphors corresponded well with chlorophyll a and phytochrome PR . The results show that the BLT:Mn4+ ,K+ phosphors had good luminescence properties and application prospects and are ideal materials for plant-illuminated red phosphors.

On the Quantum Yield Determination of UV Emitting Up-Converters.

This work deals with the determination of the external quantum yield of some selected inorganic up-conversion materials, which are able to convert blue light, as typically emitted using blue (In,Ga)N LEDs, into UV radiation. Recently, these materials have drawn tremendous attention due to their potential application in antimicrobial coatings of surfaces. To judge the viability of this approach to reduce the density of germs onto arbitrary surfaces upon indoor or outdoor illumination, the quantum efficiency for the conversion of blue light into UV is of large interest. We found that the quantum efficiency is between about 0.1% and 1%, which might be good enough if the illumination of the respective surface is performed for several hours. Then, a relevant reduction of the number of active microorganisms per area can be achieved.

A novel Mn4+ -activated far-red Sr2 MgWO6 phosphor: synthesis, luminescence enhancement, and application prospect.

A novel far-red emitting phosphor Sr2 MgWO6 : Mn4+ was fabricated using high-temperature solid-state reaction. X-ray diffraction patterns, scanning electron microscopy images, and photoluminescence excitation and photoluminescence spectra for this phosphor were analyzed in detail. The analysis revealed that its emission ranged from 600 to 800 nm and peaked at 699 nm, which was attributed to the 2 Eg →4 A2g transition of Mn4+ under 314 nm excitation. Moreover, we introduced rare-earth Yb3+ ions into the Sr2 MgWO6 :Mn4+ to improve its far-red emitting intensity. The photoluminescence (PL) intensity of the Yb3+ co-doped phosphor was three times higher than that of the single-doped phosphor. Therefore charge compensation is an efficient approach to improving PL intensity. The phosphor emitted a far-red light that resembled the pigments essential for plant growth in terms of the absorption spectrum. Therefore, the obtained phosphor, Sr2 MgWO6 :0.006Mn4+ ,0.2Yb3+ , had the potential to be a new type of far-red luminescent powder for indoor plant growth LEDs.

Impact of ligand environment on optical, luminescence and thermometric behavior of A3 (PO4 )2 :Sm3+ (A = Ca, Sr) phosphors.

The present study investigates the impact of the ligand environment on the luminescence and thermometric behavior of Sm3+ doped A3 (PO4 )2 (A = Sr, Ca) phosphors prepared by combustion synthesis. The structural and luminescent properties of Sm3+ ions in the phosphate lattices were investigated using powder X-ray diffraction (PXRD) and photoluminescence (PL) techniques. PXRD results of the synthesized phosphors exhibit the expected phases that are in agreement with their respective standards. Fourier-transform infrared (FTIR) spectroscopy confirms the presence of PO4 vibrational bands. Upon excitation with near ultraviolet light, the PL studies indicated that Sr3 (PO4 )2 :Sm3+ phosphors exhibit a yellow light emission, whereas Ca3 (PO4 )2 :Sm3+ phosphors exhibit an emission of orange light. The PL emission results are in accordance with the CIE coordinates, with the Sr3 (PO4 )2 :Sm3+ phosphors showing coordinates of (0.56, 0.44), and the Ca3 (PO4 )2 :Sm3+ phosphors displaying coordinates of (0.60, 0.40). Thermal analysis shows improved stability of Ca3 (PO4 )2 :Sm3+ based on lower weight reduction in thermogravimetric analysis. The effect of temperature on the luminescence properties of the phosphor has been examined upon a 405 nm excitation. By using the fluorescence intensity ratio (FIR) method, the temperature responses of the emission ratios from the Sm3+ : the 4 F3/2 → 6 H5/2 transition to the 4 G5/2 → 6 H7/2 and 4 F3/2 → 6 H5/2 transition to the 4 G5/2 → 6 H9/2 emissions are characterized. The Ca3 (PO4 )2 :Sm3+ phosphors are more sensitive as compared with the Sr3 (PO4 )2 :Sm3+ phosphors. The earlier research findings strongly indicate that these phosphors hold great promise as ideal candidates for applications in non-invasive optical thermometry and solid-state lighting devices.

Novel bandpass filter for far UV-C emitting radiation sources.

Disinfection with far UV-C radiation (<230 nm) is an effective method to inactivate harmful microorganisms like the SARS-CoV2 virus. Due to the stronger absorption than regular UV-C radiation (254 nm) and hence limited penetration into human tissues, it has the promise of enabling disinfection in occupied spaces. The best far-UV sources so far are discharge lamps based on the KrCl* excimer discharge peaking at 222 nm, however they produce longer wavelength radiation as a by-product. In current KrCl* excimer lamps usually a dichroic filter is used to suppress these undesired longer wavelengths. A phosphor-based filter is an alternative which is cheaper and easier to apply. This paper describes the results of our exploration of this opportunity. Various compounds were synthesized and characterized to find a replacement for the dichroic filter. It was found that Bi3+-doped ortho-borates with the pseudo-vaterite crystal structure exhibit the best absorption spectrum i.e. high transmission around 222 nm and strong absorption in the 235-280 nm range. Y0.24Lu0.75Bi0.01BO3 showed the best absorption spectrum in the UV-C. To suppress the unwanted Bi3+ emission (UV-B), the excitation energy can be transferred to a co-dopant. Ho3+ turned out to be the best co-dopant, and Ho0.24Lu0.75Bi0.01BO3 appeared to be the best overall candidate for the phosphor filter material. A suitable formulation for a coating suspension containing this material was found, and quite homogeneous coatings were achieved. The efficiency of these filter layers was investigated and the results in terms of exposure limit increase i.e. gain factor vs. no filter were compared with the dichroic filter. We achieved a gain factor for the Ho3+ containing sample of up to 2.33, i.e. not as good as that of the dichroic filter (∼4.6), but a very relevant improvement, making Ho0.24Lu0.75Bi0.01BO3 an interesting material for a cost-effective filter for KrCl* far UV-C lamps.

Improvement of Luminescence Properties of Eulytite Single-Phase White Emitting Ca3Bi (PO4)3: Ce3+/Dy3+ Phosphor.

To reduce the issue of tri-primary color reabsorption, a new approach for single-phase phosphors as light-emitting diodes (LEDs) has been recommended. The structures, morphology, photoluminescence, thermal stability, and luminescence mechanism of a variety of Ca3Bi (PO4)3 (CBPO): Ce3+/Dy3+ phosphors were investigated. XRD characterization showed that all CBPO samples were eulytite structures. Furthermore, the energy transfer process from Ce3+ to Dy3+ in CBPO is systematically investigated in this work, and the color of light can be adjusted by changing the ratio of doped ions. Under UV light, energy is transferred from Ce3+-Dy3+ mainly through quadrupole-quadrupole interactions in the CBPO host, and doping with different Dy3+ concentrations tunes the emission color from blue to white. The thermal stability of the CBPO: 0.04Ce3+, 0.08Dy3+ samples is outstanding, and the CIE coordinates of the samples after emission have little effect with temperature, while their emission intensity at 423 K is as strong as that at room temperature, reaching 90%. The above results indicate that this CBPO material has great potential as a white light phosphor under near-UV excitation at the optimized concentration of Ce3+ and Dy3+.

Co-Doping Effect of Mn2+ and Eu3+ on Luminescence in Strontiowhitlockite Phosphors.

A new series of Sr-based phosphates, Sr9-xMnxEu(PO4)7, were synthesized using the high-temperature solid-state method in air. It was found that these compounds have the same structure as strontiowhitlockite, which is a ß-Ca3(PO4)2 (or ß-TCP) structure. The concentration of Mn2+ ions required to form a pure strontiowhitlockite phase was determined. An unusual partial reduction of Eu3+ to Eu2+ in air was observed and confirmed by photoluminescence (PL) and electron spin resonance (ESR) spectra measurements. The PL spectra recorded under 370 nm excitation showed transitions of both 4f5d-4f Eu2+ and 4f-4f Eu3+. The total integral intensity of the PL spectra, monitored at 395 nm, decreased with increasing Mn2+ concentration due to quenching effect of Eu3+ by the Mn2+ levels. The temperature dependence of Eu2+ photoluminescence in a Sr9-xMnxEu(PO4)7 host was investigated. The conditions for the reduction of Eu3+ to Eu2+ in air were discussed.

Organic Excitonic State Management by Surface Metallic Coupling of Inorganic Lanthanide Nanocrystals.

The ability to harness charges and spins for control of organic excitonic states is critical in developing high-performance organic luminophores and optoelectronic devices. Here we report a facile strategy to efficiently manipulate the electronic energy states of various organic phosphors by coupling them with inorganic lanthanide nanocrystals. We show that the metallic atoms exposed on the nanocrystal surface can introduce strong coupling effects to 9-(4-ethoxy-6-phenyl-1,3,5-triazin-2-yl)-9H-carbazole (OCzT) and some organic chromophores with carbazole functional groups when the organics are approaching the nanocrystals. This unconventional organic-inorganic hybridization enables a nearly 100 % conversion of the singlet excitation to fast charge transfer luminescence that does not exist in pristine organics, which broadens the utility of organic phosphors in hybrid systems.