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Narrower band red and green emission in phosphor‐converted white light‐emitting diodes (wLEDs) can improve the efficacy and color gamut in lighting and display applications. A promising luminescent ion is Mn2+ that can have both narrowband green (tetrahedral coordination) and red (octahedral coordination) emission. Unlike in earlier lighting applications of Mn2+ phosphors, temperature quenching is important in wLEDs. Insight into the thermal quenching behavior of Mn2+ luminescence is lacking. Here systematic research is reported for a variety of Mn2+‐doped phosphors; a huge variation in the luminescence quenching temperature T50, ranging from 50 K for Mn2+ in ZnTe to 1200 K in MgAl2O4, is revealed. The value T50 shows a positive correlation with the bandgap of the host, but no correlation with the full width half maximum (FWHM) of the emission band, indicating that thermally activated photoionization, not thermal crossover, is the operative quenching mechanism. This is confirmed by thermally stimulated luminescence (TSL) measurements that show a rise in TSL signal following photoexcitation at temperatures around T50 providing evidence that quenching is correlated with generation of free charge carriers. Based on these findings, as a design rule is obtained that for temperature‐stable Mn2+ luminescence in (high power) LEDs a wide‐bandgap host material is required. [ABSTRACT FROM AUTHOR]

Direct observation of the growth dynamics of 2D transition metal dichalcogenides (TMDs) is of key importance for understanding and controlling the growth modes and for tailoring these intriguing materials to desired orientations and layer thicknesses. Here, various stages and multiple growth modes in the formation of WS2 layers on different substrates through thermolysis of a single solid‐state (NH4)2WS4 precursor are revealed using in situ transmission electron microscopy. Control over vertical and horizontal growth is achieved by varying the thickness of the drop‐casted precursor from which WS2 is grown during heating. First depositing platinum (Pt) and gold (Au) on the heating chips much enhance the growth process of WS2 resulting in an increased length of vertical layers and in a self‐limited thickness of horizontal layers. Interference patterns are formed by the mutual rotation of two WS2 layers by various angles on metal deposited heating chips. This shows detailed insights into the growth dynamics of 2D WS2 as a function of temperature, thereby establishing control over orientation and size. These findings also unveil the important role of metal substrates in the evolution of WS2 structures, offering general and effective pathways for nano‐engineering of 2D TMDs for a variety of applications. [ABSTRACT FROM AUTHOR]

In a Ce3+-Yb3+ system, two mechanisms are proposed so far namely, the quantum cutting mechanism and the electron transfer mechanism explaining Yb3+ infrared luminescence under Ce3+ excitation. Among them, the quantum cutting mechanism, where one Ce3+ photon (ultraviolet/blue) gives rise to two Yb3+ photons (near infrared) is widely sought for because of its huge potential in enhancing the solar cell efficiency. In present study on Ce3+-Yb3+ codoped borate glasses, Ce3+ sensitized Yb3+ luminescence at ~1?μm have been observed on Ce3+ 5d state excitation. However, the intensity of sensitized Yb3+ luminescence is found to be very weak compared to the strong quenching occurred in Ce3+ luminescence in Yb3+ codoped glasses. Moreover, the absolute luminescence quantum yield also showed a decreasing trend with Yb3+ codoping in the glasses. The overall behavior of the luminescence properties and the quantum yield is strongly contradicting with the quantum cutting phenomenon. The results are attributed to the energetically favorable electron transfer interactions followed by Ce3+-Yb3+ ⇌ Ce4+-Yb2+ inter-valence charge transfer and successfully explained using the absolute electron binding energies of dopant ions in the studied borate glass. Finally, an attempt has been presented to generalize the electron transfer mechanism among opposite oxidation/reduction property dopant ions using the vacuum referred electron binding energy (VRBE) scheme for lanthanide series. [ABSTRACT FROM AUTHOR]

With increasing interest in mixed‐anionic hydrides, a number of interesting properties have been reported. Here, the structural and optical properties of (Eu2+‐doped) MCaH3–MCaF3 (M = Rb, Cs) are investigated. For M = Rb, a complete hydride–fluoride solid solution series is found and for M = Cs, the known solid solution series (0 ≤ x ≤ 1.70) can be extended to x = 3. In case of Cs, a very bright luminescence emission is observed in Eu2+‐doped samples, whereas the luminescence is fairly weak in Rb based compounds. With increasing hydride content, a shift of the emission color from cyan‐green to red can be observed. In contrast to earlier reports for mixed fluoride–hydride host, the redshift is not a gradual shift of a single broad emission band, but the appearance of new narrow emission bands on the low energy side, which are assigned to the occupation of sites with higher hydride content. Consequently, this finding represents the first example in a mixed anionic hydride with a site‐sensitive emission for sites with locally varying hydride content in the first coordination sphere and may serve as a general example for emission color tuning taking advantage of mixed‐anionic compounds. [ABSTRACT FROM AUTHOR]

Co-doped hexagonal Sr1−x/2Al2−xSixO4:Eu2+,Dy3+ (0.1 ≤ x ≤ 0.5) transparent ceramics have been elaborated by full glass crystallization. The compositions with low SiO2 content (x ≤ 0.4) require fast quenching conditions to form glass, i.e. specific elaboration processes such as aerodynamic levitation coupled to laser heating, whereas the x = 0.5 glass composition can be prepared on a large scale by the classic melt-quenching method in commercial furnaces. After a single thermal treatment, the resulting SrAl2O4-based transparent ceramics show varying photoluminescence emission properties when x increases. These variations are also observable in persistent luminescence, resulting in an afterglow colour-tuning ranging from green to light blue. Afterglow excitation spectra highlight the possible activation in the visible range of the obtained persistent luminescence. Indeed, persistent luminescence of hexagonal Sr0.75Al1.5Si0.5O4:Eu2+,Dy3+ large transparent ceramics has been successfully charged using a typical smartphone low power white light source. Moreover, thermoluminescence glow curves of samples containing different Dy3+ doping concentrations are studied to gain insights regarding the traps' origin and depth. Coupling thermoluminescence results together with luminescence thermal quenching and band gap calculations appear useful to understand the charge trapping and detrapping evolution with the material composition. Varying the Si-content in hexagonal Sr1−x/2Al2−xSixO4:Eu2+,Dy3+ compounds appears as a promising strategy to obtain transparent materials with tuneable green to light blue persistent luminescence. [ABSTRACT FROM AUTHOR]

ZnGa2O4:Cr3+, owing to its persistent luminescence properties in the deep red range, is an exceptional material in view of foreseen in vivo imaging applications. In the present work, we report the elaboration process and detailed investigations of the optical properties of nano glass-ceramics composed of spinel ZnGa2O4:Cr3+,Yb3+ nanocrystals embedded in a transparent, silica rich, glass matrix. The as-prepared materials show good incorporation of the dopants in the crystallites leading in both Cr3+ and Yb3+ emissions. These emissions occur while exciting in the Cr3+ bands, indicating an energy transfer process from Cr3+ to Yb3+. Furthermore, excitation in the Yb3+ band in the near-infrared (NIR) range suggests an interesting up-conversion process, which promotes the Cr3+ emission. Persistent luminescence of both Cr3+ and Yb3+ doping ions can be activated by charging the Cr3+ excitation bands, leading to persistent luminescence of zinc gallate nanocrystals in both first and second biological windows. The influence of Yb3+ co-doping on persistent luminescence properties has been investigated by persistent luminescence decay profiles and thermoluminescence studies. Indeed, thermoluminescence glow curves of Yb3+ exhibit similar shape to those of Cr3+ but appear broader and shifted towards higher temperatures. This temperature shift may be explained by the temperature dependence of the involved energy transfer process. [ABSTRACT FROM AUTHOR]

The development of efficient red‐emitting persistent phosphor is still an ongoing challenge. In the search of persistent materials in red range, Pr3+ is a good candidate owing to its transitions between 1D2 and 3H4 state at about 612 nm. In this paper, we investigated the red persistent properties of Pr3+‐doped perovskite oxide ABO3, (A: K, Na and B: Nb), which can be elaborated as large single crystal. KNbO3:Pr3+ appears to have weak photoluminescence and no persistent luminescence. However, the cationic substituted compounds K1−xNaxNbO3:Pr3+ (x = 0, 0.4, 0.5, 0.7 0.9, 1) exhibit intense persistent luminescence, which increases steadily with increase in Na content. We correlated persistence behavior with the position of Metal‐to‐Metal Charge Transfer (MMCT) band, which plays crucial role in tuning of the trap depth. The MMCT band position decreases with the addition of Na contents and the thermoluminescence peak shifts toward higher temperature indicating the formation of deeper traps. This is in good agreement with the enhancement of the persistent luminescence suggesting that a proper tailoring of MMCT is needed to design efficient Pr3+‐persistent phosphors with better performance. A detailed analysis on the trap depth, bandgap energy, and persistent luminescence properties is reported tuning the composition in the K1−xNaxNbO3:Pr powder. Tunable traps depths are obtained by Na/K substitution. Electron and hole traps are evidenced by thermal analysis. This gives new insights in the persistent luminescence mechanism in the Pr3+− doped perovskite compounds. [ABSTRACT FROM AUTHOR]

Wet-chemically synthesized inorganic materials often exhibit luminescence behavior. We have recently shown that the amorphous yttrium-aluminium-borate (a-YAB) powders obtained by sol–gel and modified Pechini methods exhibit organic impurities, responsible for their intense visible photoluminescence and phosphorescence afterglow. However, the heterogeneity of impurity organic compounds and difficulties in their intact extraction from the solid inorganic host matrix limit the extraction-based chemical analysis for luminophore identification. Here, we propose a photo-physical route based on time-gated triplet-state optical spectroscopy (TGTSS) to construct the electronic structures of the trapped unknown luminophores, which successfully illustrates the luminescence properties of a-YAB powders in more detail and also provides important insights intrinsic to the nature of the luminophores. The experimental results accompanied with TD-DFT calculations of the theoretical electronic structures thus help us to propose the probable luminophore compounds trapped in rigid a-YAB matrices. We anticipate that the present approach will open new opportunities for analyzing similar complex luminescent materials, including carbon dots, graphene oxides, etc., which is vital for their improvement. [ABSTRACT FROM AUTHOR]

Highly transparent glass ceramics, synthesized from a glass precursor with a nominal composition of SiO2/ZnO/Ga2O3/Na2O/Cr2O3 (65:17:23:5:0.125), were prepared by glass crystallization. This method enables the synthesis of up to 50 wt.-% of size-controllable Cr3+-doped ZnGa2O4 nanocrystals embedded in an SiO2-rich glass matrix after crystallization at 900-1000 °C for 10 min. Rietveld refinements and transmission electron microscopy imaging showed that the nanoparticle sizes are ca. 16 and 33 nm for the glass ceramics annealed at 900 and 1000 °C, respectively. Photoluminescence measurements of these glass ceramics revealed intense deep red ( λ = 695 nm) persistent emission that is detectable for up to 10 h, similar to that previously observed for ZnGa2O4 powder. As the glass ceramics exhibit good transparency, the persistent glow arises from the entire sample volume, which is of interest for outdoor applications and for more detailed studies of the mechanism for the persistent luminescence. Thermoluminescence measurements showed a wide distribution of trap depths centered at ca. 325 K. The traps can be charged not only under UV light but also under visible-light excitation from λ = 365 to 625 nm. Electron paramagnetic resonance spectroscopy measurements of the Cr3+ ions showed clearly that the ZnGa2O4 nanocrystals in the glass ceramics exhibit the same types of defects and disorder as those in microcrystalline ZnGa2O4 powder, and this suggests that antisite defects are the dominant traps responsible for the persistent luminescence. It is proposed that the ability to charge ZnGa2O4 particles with visible light can be attributed to the combination of a two-photon absorption and a local electric field owing to antisite defects in the vicinities of some of the Cr3+ ions. [ABSTRACT FROM AUTHOR]

Yttrium aluminum borate (YAB) powders prepared by sol-gel process have been investigated to understand their photoluminescence (PL) mechanism. The amorphous YAB powders exhibit bright visible PL from blue emission for powders calcined at 450 °C to broad white PL for higher calcination temperature. Thanks to 13C labelling, NMR and EPR studies show that propionic acid initially used to solubilize the yttrium nitrate is decomposed into aromatic molecules confined within the inorganic matrix. DTA-TG-MS analyses show around 2 wt % of carbogenic species. The PL broadening corresponds to the apparition of a new band at 550 nm, associated with the formation of aromatic species. Furthermore, pulsed ENDOR spectroscopy combined with DFT calculations enables us to ascribe EPR spectra to free radicals derived from small (2 to 3 rings) polycyclic aromatic hydrocarbons (PAH). PAH molecules are thus at the origin of the PL as corroborated by slow afterglow decay and thermoluminescence experiments. [ABSTRACT FROM AUTHOR]

Yttrium aluminum borate (YAB) powders prepared by sol-gel process have been investigated to understand their photoluminescence (PL) mechanism. The amorphous YAB powders exhibit bright visible PL from blue emission for powders calcined at 450 °C to broad white PL for higher calcination temperature. Thanks to 13C labelling, NMR and EPR studies show that propionic acid initially used to solubilize the yttrium nitrate is decomposed into aromatic molecules confined within the inorganic matrix. DTA-TG-MS analyses show around 2 wt % of carbogenic species. The PL broadening corresponds to the apparition of a new band at 550 nm, associated with the formation of aromatic species. Furthermore, pulsed ENDOR spectroscopy combined with DFT calculations enables us to ascribe EPR spectra to free radicals derived from small (2 to 3 rings) polycyclic aromatic hydrocarbons (PAH). PAH molecules are thus at the origin of the PL as corroborated by slow afterglow decay and thermoluminescence experiments. [ABSTRACT FROM AUTHOR]

We developed a persistent phosphor of Y3Al2Ga3O12 doped with Nd3+, Ce3+, Cr3+ ions (YAGG:Nd-Ce-Cr) exhibiting long (>10 h) persistent luminescence at multi-wavelengths of around 880, 1064, and 1335 nm due to f-f transitions of Nd3+ and at 505 nm due to Ce3+:5d1 4f transition. The intense near-infrared (NIR) persistent luminescence bands from Nd3+ match well with the first (650-950 nm) and second (1000-1400 nm) bio-imaging windows. The NIR persistent radiance of the YAGG:Nd-Ce-Cr phosphor (0.33 × 10-1 mW/Sr/m²) at 60 min after ceasing blue light illumination was over 2 times higher than that of the widely used ZnGa2O4:Cr3+ red persistent phosphor (0.15 × 10-1 mW/Sr/m²). [ABSTRACT FROM AUTHOR]

A facile method to describe the electron transfer and energy transfer processes among lanthanide ions is presented based on the temperature dependent donor luminescence decay kinetics. The electron transfer process in Ce3+-Yb3+ exhibits a steady rise with temperature, whereas the Ce3+-Tb3+ energy transfer remains nearly unaffected. This feature has been investigated using the rate equation modeling and a methodology for the quantitative estimation of interaction parameters is presented. Moreover, the overall consequences of electron transfer and energy transfer process on donor-acceptor luminescence behavior, quantum efficiency, and donor luminescence decay kinetics are discussed in borate glass host. The results in this study propose a straight forward approach to distinguish the electron transfer and energy transfer processes between lanthanide ions in dielectric hosts, which is highly advantageous in view of the recent developments on lanthanide doped materials for spectral conversion, persistent luminescence, and related applications. [ABSTRACT FROM AUTHOR]

In this work, a novel and low phonon oxide glass based on the Bi2O3-ZnO-GeO2 system has been formulated and its structural, optical properties together with crystallization kinetics under non-isothermal conditions were investigated in detail. The crystalline phase formed during optimized ceramization process was identified as cubic Bi2O3 by x-ray diffraction. The activation energy (Ec) for the first crystallization event during formation of Bi2O3 was estimated using various well established models and ranges between 258·6-308·5 kJ/mol. The Avrami exponents determined by the Augis- Bennett model and the Matusita model indicate that Bi2O3 phase formation is dominated by two-dimensional surface crystallization. The coupling phonon energy associated with dopant rare earth ions in this host has been established using Eu3+ as a spectral probe and it has been observed from the phonon side band (PSB) that the phonon energy surrounding the dopant rare earth ion is around 465 cm-1. This low phonon energy coupling with Ho3+ ions in the present glass has resulted in mid-infrared emissions at 2·9 μm and 2·0 μm due to the reduced non-radiative relaxations. [ABSTRACT FROM AUTHOR]

Transparent oxyfluoride glass-ceramics containing Eu: BaYF5 nano-crystals in the newly developed SiO2- K2CO3- BaF2- YF3- Sb2O3 glass system are synthesized by melt quenching method followed by optimized ceramization process. The X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy confirmed the precipitation of tetragonal BaYF5 nano-crystals in glass matrix. The coexistence of Eu2+ and Eu3+ ions in both glass and glass-ceramics are ascertained from their emission and excitation spectra. The in situ formation of divalent europium ( Eu2+) along with Eu3+ during high temperature synthesis under ambient atmosphere is explained through optical basicity model. The Eu3+ emission from upper excitation states (5D3−1) and reduced asymmetry ratio ( R = I ED/ I MD) in glass-ceramics have established the dopant ion incorporation into fluoride nano-crystalline environment. The observed luminescence properties of Eu: BaYF5 are compared with that of Eu: BaYF5 nanocrystals containing transparent glass-ceramics and their marked differences are discussed. [ABSTRACT FROM AUTHOR]

Europium doped glass-ceramics containing BaF nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm has been demonstrated from the emission spectra of Eu ions showing the transitions from upper excitation states D (J = 1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu) ions to divalent (Eu) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu to Eu ion by radiative trapping or re-absorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu to Eu under normal air atmospheric condition has been observed in a BaF containing oxyfluoride glass-ceramics system. [ABSTRACT FROM AUTHOR]

This paper reports results on the crystallization kinetics of 35.5CaO–7.25La2O3–57.25B2O3 glass under nonisothermal conditions based on the studies carried out from the differential thermal analysis upon using various well-established models. The crystalline phases formed during the optimized ceramization process have been confirmed from the X-ray diffraction. The activation energies of the first (formation of CaLaB7O13) and second (formation of LaBO3) crystallization events have been estimated using the conventional methods of Kissinger, Augis–Bennett, Ozawa, and Matusita, and the results are found to be in good agreement with each other. The Avrami exponents that are determined by these models for the crystallization of CaLaB7O13 and LaBO3 are found to be in the range of (1.81–2.35) and (4.03–4.65), respectively. This indicates that the formation of CaLaB7O13 is dominated by a surface crystallization, whereas LaBO3 is formed by three-dimensional bulk crystallization with an increased rate of nucleation. This observation is further validated by microstructural investigation, which shows the formation of CaLaB7O13 phase as a surface layer and a bulk crystallization of LaBO3 in optimally ceramized samples. [ABSTRACT FROM AUTHOR]

Transparent glass–ceramics containing Pr3+:BaF2 nanocrystals in the chemical composition of SiO2–BaF2–K2CO3–La2O3–Sb2O3 oxyfluoride glass systems have been prepared from melt quenching and with a subsequent heat-treatment method. The luminescence and structural properties of these materials have been evaluated and the results are reported. Rietveld analysis of X-ray diffraction patterns and investigation of transmission electron microscopy confirmed the presence of BaF2 nanocrystals dispersed in the heat-treated glass matrices. Measured UV-Vis-NIR absorption spectra exhibited nine bands of the transitions 3H4→3P2, (1I6, 3P1), 3P0, 1D2, 1G4, 3F3, 3F2, 3H6, and 3H5 from all the samples with nondegenerated 1I6 and 3P1 levels in the glass–ceramics. The photoluminescence spectra show an enhancement in the intensities upon ceramization, indicating the incorporation of Pr3+ ions into BaF2 nanocrystals that possess a low phonon energy (346 cm−1). This has further been corroborated from the observation of a significant threefold increase in the relative intensity ratio of blue (3P0→3H4) to red (1D2→3H4, 3P0→3H6) emissions from glass–ceramics compared with the glass. This is due to a significant decrease of multiphonon nonradiative relaxation from the 3P0 to the 1D2 level of Pr3+ in glass–ceramics. Time-resolved spectra exhibit 3P0-level decays faster than the 1D2 level. [ABSTRACT FROM AUTHOR]

This paper reports on the preparation and systematic analysis of energy transfer mechanisms in Nd3+-Yb3+-Er3+ co-doped new series of barium-alumino-metaphosphate glasses. The time resolved fluorescence of Nd3+ in triply doped Ba-Al-metaphosphate glasses have revealed that, Yb3+ ions could function as quite efficient bridge for an energy transfer between Nd3+ and Er3+ ions. As a result, a fourfold emission enhancement at 1.54 µm of Er3+ ions has been achieved through an excitation of 4F5/2 level of Nd3+ at 806 nm for the glass having 3 mol% Yb3+ with an energy transfer efficiency reaching up to 94%. Decay of donor (Nd3+) ion fluorescence has been analyzed based on theoretical models such as direct energy transfer model (Inokuti-Hirayama) and migration assisted energy transfer models (Burshtein's hopping and Yokota-Tanimoto's diffusion). The corresponding energy transfer parameters have been evaluated and discussed. Primarily, electrostatic dipole- dipole (s~6) interactions are found to be responsible for the occurrence of energy transfer process in theses glasses. [ABSTRACT FROM AUTHOR]

2D growth mechanisms, catalytic metal substrates, ex situ growth, in situ transmission electron microscopy, thermolysis, twisted bilayers, WS 2 Keywords: 2D growth mechanisms; catalytic metal substrates; ex situ growth; in situ transmission electron microscopy; thermolysis; twisted bilayers; WS 2 EN 2D growth mechanisms catalytic metal substrates ex situ growth in situ transmission electron microscopy thermolysis twisted bilayers WS 2 1 1 1 01/06/22 20220101 NES 220101 B 2D Growth Mechanisms b In article number 2106450, Dnyaneshwar S. Gavhane, Marijn A. van Huis, and Atul D. Sontakke visualize the growth of 2D WS SB 2 sb in real time and at atomic resolution via in situ transmission electron microscopy. Selective Vertical and Horizontal Growth of 2D WS2 Revealed by In Situ Thermolysis using Transmission Electron Microscopy (Adv. Funct. [Extracted from the article]

The present issue on "Transparent Glass Ceramics" has included articles on the absorption library of rare-earth orthophosphates, the crystallization of GeO SB 2 sb -Al SB 2 sb O SB 3 sb -Bi SB 2 sb O SB 3 sb glasses, and the luminescence behavior of GdVO SB 4 sb : Tb nanocrystals in silicate glass ceramics. In the past few decades, glass ceramic (GC) has revolutionized the application of glass [[1]]. In general, transparent glass ceramics are produced through two methods: the melt-quenching of glass and subsequent heat treatment for crystallization, and the sol-gel technique [[5]]. The loss of transparency limits the application of glass ceramics; as such, the development of transparent glass-ceramics is essential for many applications. [Extracted from the article]

An investigation of donor-acceptor energy transfer kinetics in dual rare earths doped precursor oxy-fluoride glass and its glass-ceramics containing NaYF4 nano-crystals is reported here, using three different donor-acceptor ion combinations such as Nd-Yb, Yb-Dy, and Nd-Dy. The precipitation of NaYF4 nano-crystals in host glass matrix under controlled post heat treatment of precursor oxy-fluoride glasses has been confirmed from XRD, FESEM, and transmission electron microscope (TEM) analysis. Further, the incorporation of dopant ions inside fluoride nano-crystals has been established through optical absorption and TEM-EDX analysis. The noticed decreasing trend in donor to acceptor energy transfer efficiency from precursor glass to glass-ceramics in all three combinations have been explained based on the structural rearrangements that occurred during the heat treatment process. The reduced coupling phonon energy for the dopant ions due to fluoride environment and its influence on the overall phonon assisted contribution in energy transfer process has been illustrated. Additionally, realization of a correlated distribution of dopant ions causing clustering inside nano-crystals has also been reported. [ABSTRACT FROM AUTHOR]