Luminescence Band Research Articles

Optical properties of non-stoichiometric YAG: Ce luminescent ceramics

The aim of this study was to investigate the mutual influence of two factors - deviation from YAG:Ce stoichiometry and vacuum sintering conditions - on the optical and luminescent properties of ceramics. A series of ceramic powders with the general composition formula Y3-хCe0.02Al5+хO12 was synthesized, where −0.024 ≤ x ≤ 0.018 formula units (f.u.) It was demonstrated that deviation from stoichiometry within the specified limits does not affect the morphology, specific surface area, and dispersion composition of ceramic powders. The surface morphology of YAG:Ce ceramic samples with varying stoichiometry was analyzed. The impact of the stoichiometry shift on pore concentration, average grain size, as well as the nature and concentration of impurity phases in ceramics was determined. The influence of the TEOS sintering additive on the grain size and the nature of the macro-defects in ceramics under conditions of stoichiometry shift has been studied. The study of the optical properties of Y3-хCe0.02Al5+хO12 ceramics clearly demonstrates the possibility of creating highly transparent ceramics with an excess of up to 0.018 f.u. of aluminum, while an excess of yttrium significantly reduces the light transmission of the samples. Studies of the luminescent properties have shown that the shift in stoichiometry does not have a significant effect on the luminescence intensity of ceramics, but leads to a shift in the luminescent band maxima and the width of the luminescence bands.

Optical Properties of Fluorozirconate Glasses Doped with Chromium Ions

Chromium trifluoride-doped fluoride glasses in the ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) system with partial substitution of fluorine for chlorine have been synthesized. The spectral data obtained confirm that chromium ions enter the glass structure and exhibit broadband luminescence caused by the 4T2 → 4A2 transition in the Cr3+ ion. The observed long-wavelength shift of the broadband luminescence band and Cr3+absorption bands in fluoride–chloride glass compared to fluoride glass corresponds to the expected behavior of the Cr3+ luminescence and absorption spectra when fluoride ions are replaced by chloride ions, which should lead to a weakening of the strength of the crystal field acting on Cr3+ ions. At room temperature, the luminescence of Cr3+ ions at 888 and 908 nm is strongly quenched due to the thermally stimulated nonradiative transition from the 4T2 excited state to the 4A2 ground state.

Doping-induced Ti3+ state and oxygen vacancies in TiO2: A single-chip combinatorial investigation

TiO2 is widely recognized as a high performance photocatalyst for photocatalytic applications. Oxygen vacancies (VO) and titanium defects are known to make a significant contribution to the enhanced photocatalytic efficiency of TiO2, accordingly control of these two centers is pivotal to optimizing the surface photochemical activity. In this work, the formation of VO and Ti3+ ions induced by n-type doping of TiO2 with Nb is demonstrated using a combinatorial investigation approach. Chemical Beam Vapour Deposition (CBVD) is used to fabricate Nb concentration-graded (NbxTi1-x)O2 anatase films with 0.009 ≤ x ≤ 0.033 over a 5 mm length on a single chip. Cathodoluminescence (CL) microanalysis shows the luminescence intensity decreases with increasing Nb concentration; however, the presence of a self-trapped exciton (STE) emission, characteristic of anatase TiO2, indicates that the Nb incorporation does not affect the structural properties of TiO2. Notably, the introduction of Nb donors induces the formation of the signature luminescence bands of VO and Ti3+ at 2.05 and 2.60 eV at 80 K, respectively, and their intensities rise by up to 28% with increasing Nb content. The formation of VO and Ti3+ is attributed to the effect of excess electrons locally produced by Nb donor doping. The thermal activation energies for the STE and defect-related emissions are found to be identical at 32 ± 4 meV, suggesting that the recombination kinetics is mediated by Nb donors. Significantly, we demonstrate that the near-surface VO density can be further increased by two different post-growth treatment approaches: (i) remote hydrogen plasma and (ii) low-energy electron beam irradiation (LEEBI), without effecting the valence state of Ti. Our results open opportunities to optimize the photocatalytic properties of TiO2 by defect engineering.

YAG:Ce Nanophosphors Synthesized by the Polymer–Salt Method for White LEDs with Isomorphic Substitution of Yttrium by Gadolinium

Fine-dispersed YGdAG:Ce nanopowders with various degrees of isomorphic substitution of yttrium by gadolinium were synthesized. The structure and luminescent properties were studied by X-ray diffraction, attenuated total reflection Fourier-transform infrared spectroscopy, luminescence spectroscopy and scanning electron microscopy. The possibility of synthesis of YGdAG:Ce nanopowders with a degree of gadolinium substitution up to 60% and nanocrystals with average sizes of 25–30 nm were shown. The red-shift of the cerium luminescence band with an increase in Gd content was studied. The CIE diagram for emission of YGdAG:Ce synthesized by the polymer–salt method shows that the degree 30–40% substitution of Y by Gd is optimal for the fabrication of a white light source based on LED with an emission wavelength of 470 nm.

Role of defects in tailoring optical, electronic, and luminescent properties of Cu-doped ZnO films

We present a detailed characterization study on copper-doped ZnO films to correlate the films' electronic and optical properties with the existing native defects in the lattice. In addition, we describe the variation in the concentration of these defects with Cu dopant and temperature. The results of XRD confirmed the single-phase würtzite-structure of the synthesized films. The SEM images showed a homogeneous and dense grain morphology with a granular form and a signature for a hexagonal-like shape. The EDX, XPS, and UV–Vis spectra showed the proper doping of Cu ions into the lattice. The XPS analysis indicated mixed electronic states of both Cu2+ and Cu1+ and showed a clear increase in the Cu2+ intensity relative to Cu1+, with Cu dopant. The transmittance spectra exhibited an average value above 80% in all doped films in the visible and infrared regions. The overall results indicated a clear link between the films’ optical and electronic responses and the level of the intrinsic defects in the lattice. By increasing the Cu dopant, we find a slight reduction in the energy bandgap (Eg). This is correlated with a clear reduction in the blue emission luminescence band associated with the VZn and in the yellow emission band associated with the Oi. On the other hand, we observed a clear enhancement in the green emission band originating from the VO, and in the emission band related to possible transitions from Zni levels to Oi levels. The slight reduction in the Eg signals a weak sp-d hybridization between the ZnO conduction band electrons and the Cu2+ ions, which is mediated by the intrinsic defects. With reducing the temperature, the photoluminescence temperature profiles indicated a slight increase in the Eg values and a negligible effect on the distribution of the native defects.

Synthesis and characterization of MgZnCo ternary mixed metal oxide by the sonochemical method for solid hydrogen storage

In the present investigation, MgZnCo ternary nanocomposites were developed via a one-step sonochemical technique. The nanocomposite was characterized using structural and optical methods. The formation of different crystalline nanocomposite phases (MgO, ZnO, and Co3O4), and a binary phase (Mg-Zn) has been confirmed by X-ray diffraction. Also, it was indicated that varying of Co molar ratio had an impact on the average sizes of the nanoparticles. SEM showed octahedron-shaped nanoparticle morphology with different average sizes for the nanocomposites prepared. Photoluminescence analysis showed the presence of green and ultraviolet emissions, with green luminescence bands being most often observed. with the intention of discovering samples with suitable structural and optical properties for future solid hydrogen storage applications.

Effective ratiometric optical thermometry based on inverse temperature dependent dual near infrared emission of Cr3+ doped BaAl4Sb2O12 phosphor

The device of reliable non-contact ratiometric optical thermometry technique gradually became a cutting-edge research pot over decades due to its intriguing features of rapid response, high accuracy and simple operation. In which, Boltzmann-based optical thermometry has attracted widespread attention for its exceptional reliability. Here in, we put forward an effective ratiometric optical thermometry based on the highly sensitive and effective Boltzmann-based 2E→4A2/4T2→4A2 emission ratio of BaAl3.9Sb2O12:0.1Cr3+. The phosphor shows dual near-infrared luminescence lines at 698 nm and 710 nm (2E→4A2) and the luminescence band near 750 nm (4T2→4A2). With the temperature elevating from 170 to 470 K, the emission intensity of luminescence lines undergoes continuous enhancement and possess excellent anti-thermal quenching property (3.5-fold at 470 K, compared to 170 K). In contrast, the emission intensity of luminescence band shows a monotonic decline trend. The luminescence intensity ratio shows a good linear relationship in the Arrhenius diagram, and the relative sensitivity reaches 2.08 % K−1 at 170 K, which corroborates the BaAl3.9Sb2O12:0.1Cr3+ has the potential as the optical cryogenic thermometer. In the end, the work provides some insights into the development of novel Cr3+-doped thermometry materials based on the 2E and 4T2 energy level transitions in intermediate crystal field environments.

Energy transfer and color tunable up-conversion luminescence in PrF3 and YbF3 co-doped SrF2 transparent ceramics

Near-infrared pumped up-conversion luminescent PrF3 and YbF3 co-doped SrF2 ceramics were fabricated by the hot-pressing sintering technique, and the pellets were transparent with a primrose yellow appearance. The influence of YbF3 co-doping levels on the optical qualities and microstructures of ceramics was investigated. We identified the mechanisms to realize up-conversion luminescence in the visible range from 3P0,1 levels (Pr3+) under near-infrared wavelength pumping sources other than the near ultraviolet to blue region photons. Yb3+ ions inducing a broad and intense absorption band at the 900–1050 nm region, and the absorption coefficient and cross-section at 980 nm are 7.57 cm−1 and 4.19 × 10−21 cm2 in the ceramic co-doped with 15 wt% YbF3, respectively. The intense up-conversion luminescence bands at 480, 520, 540, and 600 nm under 980 nm excitation were attributed to 3P0→3H4, 3P1→3H5, 3P0→3H5 and 1D2→3H4 transitions of Pr3+ ions, respectively.

Dynamics of laser-induced formation of color centers in a lithium fluoride crystal

The dynamics of the laser-induced formation of aggregate color centers under the action of the first (950 nm) and second (475 nm) harmonics of a femtosecond Ti:sapphire laser has been studied. During the irradiation of the samples with laser radiation, the photoluminescence spectra of the induced centers were repeatedly measured. It was found that intensity of the spectral luminescence band of F3+ centers exceeds intensity of the band of F2 centers in samples with color centers induced by the first harmonic. At the same time, the opposite ratio of the intensities of these bands was observed in the luminescence spectra of color centers produced by second harmonic radiation for the same energy of laser pulses. It has been established that the observed effect is caused by different degree of heating of the crystalline medium in the regions of passage of laser filaments. Heating accelerates the migration of kinetic particles that carry out the aggregation process. The ratio of the concentrations of F2 and F3+ centers depends on the ratio of the duration of irradiation and the lifetime of kinetic particles, which significantly depends on temperature. A multiphoton-tunneling mode of interband ionization of a crystalline substance was implemented in the experiment. It is shown that the identity of the absorption spectra of color centers induced by the first and second harmonics of femtosecond laser radiation is a new distinguishing characteristic of experimental implementation of the tunneling ionization mechanism in experiment.

Effect of synthesis conditions on morphology and spectral properties of SiO2:DAST composites

SiO2:DAST composites based on SiO2 matrices with a porosity of 30–70% saturated with DAST molecules were synthesized both by evaporation from a solution and by lowering the temperature in a crystallizer. X-ray study of the composites showed the absence of DAST crystalline phase. There was only one luminescence band for all the composites with a maximum in the region of 620–640 nm (at their annealing at 100 °C). A hypsochromic shift (590-565 nm) of the luminescence band maxima of the composites was observed with an increase in their annealing temperature to 200 °C. Annealing at 200 °C also led to a change in the structure of the composite – “filling” of the pores of SiO2 matrices. This was probably due to a decrease in the interaction of the adsorbed DAST cations with the surface of SiO2 matrices due to exfoliation of DAST layers from the xerogel surface into the pores.

Sonochemical synthesis and characterization of new nanostructured ternary composites based on Mg, Zn, and Co

A simple and rapid method for synthesizing new ternary nanocomposites based on Mg, Zn, and Co has been developed using ultrasound technology. The nanocomposites were characterized using structural, optical, and thermal methods. XRD diffraction confirmed the formation of different crystalline nanocomposite phases (MgO, ZnO, and Co3O4) and binary phase (Mg–Zn), while Raman spectroscopy and FTIR, identified the vibration modes and provided information on functional groups and chemical bands present in samples. SEM showed the octahedron-shaped nanoparticles morphology with different averages size for the nanocomposites oxide prepared. Photoluminescence analysis showed the presence of green, blue, and ultraviolet emissions, with green luminescence bands being most often observed. Heat capacity and thermal diffusivity measurements were used to study thermal properties.

Efficient upconversion and downshifting luminescence of CaIn2O4: Yb3+/Tm3+/RE3+ (RE=Er/Ho) phosphor: Temperature sensing performance in the visible and near-infrared range

In this paper, the upconversion(UC) and downshifting(DS) luminescence and thermal sensing performances of the CaIn2O4: RE3+/Tm3+/Yb3+(RE=Er, Ho) phosphors are investigated in detail. The bright UC emission bands from 450 to 900 nm and the DS luminescence bands from 1000 to 1700 nm are obtained under 980 nm excitation. The typical near infrared(NIR) emisson band(∼1550 nm) of Er3+ ions and the NIR emission band(∼1200 nm) of Ho3+ ions are gained. The temperature sensing features of CaIn2O4: RE3+/Tm3+/Yb3+ (RE=Er, Ho) phosphors are investigated with the luminescence intensity ratio (LIR) technique. The results indicate that the maximum relative sensitivity (SRmax) of CaIn2O4: Er3+/Tm3+/Yb3+ sample reaches 1.576% K-1 at 313 K, and the SRmax of CaIn2O4: Ho3+/Tm3+/Yb3+ sample reaches 0.797% K-1 at 353 K in UC range. In addition, the higher temperature measuring sensitivities have been obtained in NIR-II region with the DS luminescence of CaIn2O4: RE3+/Tm3+/Yb3+ phosphors. The above results indicate that the CaIn2O4: RE3+/Tm3+/Yb3+ phosphors show bright emission bands in visible and NIR-II, and higher sensitivity for temperature measurement. The CaIn2O4: RE3+/Tm3+/Yb3+ phosphors present great application in optical temperature measurement.

Effect of Nb2O5 on the ∼2.0 μm band luminescence of Ho3+/Tm3+/Ce3+ tri-doped tellurite glass

In this work, the effect of Nb2O5 oxide on the ∼2.0 μm band luminescence of Ho3+/Tm3+/Ce3+ tri-doped tellurite glass with composition of TeO2–ZnO–La2O3 was studied. The doped tellurite glass was synthesized by using the melt-quenching method. Under the pumping of 808 nm laser diode (LD), a broad and relatively flat fluorescence emission ranging from 1600 to 2200 nm with a full width at half maximum (FWHM) of 374 nm was obtained by optimizing the concentration combination of Ho3+, Tm3+ and Ce3+ ions. The observed broad luminescence was attributed to the spectral overlapping between the energy level transitions from 3F4 to 3H6 of Tm3+ at 1.82 μm and that from 5I7 to 5I8 of Ho3+ at 2.0 μm. Further, an increment by about 143% in peak intensity of 1600∼2200 nm broad luminescence was found after the introduction of Nb2O5 oxide, which is due to the enhanced energy transfer from Tm3+ to Ho3+ ions, and the increased multi-phonon relaxation (MPR) rate from higher levels to fluorescence emitting levels. It can be inferred from the results that a potential gain medium suitable for applications in optoelectronic devices operating at ∼2.0 μm band can be produced by incorporating an appropriate amount of Nb2O5 oxide into Ho3+/Tm3+/Ce3+ tri-doped tellurite glass.

Structural, mechanical, dielectric and defect properties of Eu-doped BeO ceramics: A computational and experimental approach

BeO ceramics in the wurtzite (w-BeO) form either pure or doped with lanthanide ions present high Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) sensitivity. However, the mechanism of defect formation governing the charge carrier dynamics associated with the TL and OSL signals is not well understood yet. Thus, this work aims to develop a study on the structural, mechanical, dielectric, and defect properties of Europium ion–doped w-BeO ceramics. We develop a new set of potential parameters to describe the Be–O bond. By using these potential parameters, we describe the physical and chemical properties with good agreement. Defect calculations employing the Mott and Littleton two-region strategy show that the distorted structure of pure w-BeO ceramics favours the Frenkel–type defect formation in the basal plane of the crystal lattice. When doped with Eu3+, we observed a change in the defect arrangement. It replaces preferentially two Be sites, eliminating beryllium interstices and keeping the beryllium vacancy from the initial configuration. We also observed that it is unlikely the Eu3+/Eu2+ coexist due to the solution energy values. Lastly, we have observed a most significant electron–hole interaction which may be associated with the intrinsic luminescence band of the w-BeO ceramics. In addition, TL and OSL measurements are presented to corroborate the defect calculations. The results presented here bring new insight to elucidate the charge carrier dynamic associated with the luminescent properties.

ENHANCING PHOTOMETRIC PERFORMANCE OF YAG:CeCERAMICS: INVESTIGATING THE ROLE OF ANNEALING IN RADIATION-ASSISTED SYNTHESIS

Ceramic samples of cerium-doped yttrium aluminum garnet (YAG:Ce) were successfully synthesized utilizing a high-powered electron flux field with a considerable energy level of 1.4 MeV and a power density of 23 kW/cm2. The ceramics were formed in a remarkable time span of just one second from a specifically prepared mix of yttrium, aluminum, and cerium oxides. The process of radiation-assisted synthesis of ceramics within radiation flux fields fundamentally deviates from the methodologies commonly employed today. Analyzed diffraction patterns closely align with those documented for YAG:Ce crystals, both in peak position and proportion. Furthermore, every sample consistently demonstrated a space group symmetry of Ia-3d. The luminescence and excitation spectra of ceramics synthesized in this study closely resemble those of YAG:Ce ceramics produced by other methods and YAG:Ce -based phosphors. The luminescence bands exhibit high efficiency, and the intensity ratios of the UV bands vary among the studied phosphors. The ceramics' radiation-to-luminescence conversion efficiency was found to be impressive, achieving scores of 0.57 and 0.48 in the industrial phosphors SDL 4000 and YAG-02, respectively. It was also observed that an increase in quantum efficiency of the samples could be achieved via high-temperature annealing. High conversion efficiency underscores the potential of the outlined luminescent ceramics synthesis method.

Characterization of chrysoberyl and its gemmological varieties by Raman spectroscopy

AbstractVarious chrysoberyl varieties (non‐phenomenal chrysoberyl, alexandrite and cymophane) are gemstones of high demand; therefore, they are subjected to numerous substitutions by other materials and synthetic analogues. To address this issue, non‐destructive methods for studying gemstones are highly sought after. Raman spectroscopy is one of the most suitable non‐destructive methods for studying gemstones as it requires no sample preparation and does not leave any price‐reducing signs on the gemstone surface. The research is focused on Raman spectroscopy application on chrysoberyl identification, differentiation between its varieties, inclusion analysis, and detection of synthetic analogues and imitations. The alexandrite variety can be identified by its broad luminescence band that ranges from 640 to 790 nm (15 625–12 658 cm−1). Synthetic alexandrite has sharper and more intensive bands on the luminescence spectrum, especially 690 and 696 nm (14 492–14 367 cm−1). Rutile inclusions can be distinguished by a broad band 580–640 cm−1 in the Raman spectrum. Imitations of chrysoberyl represented by corundum and colour‐changing spessartine was also identified. The orientation of faceted stones can also be determined by specific bands present in the Raman spectrum.

Bandgap narrowing and hole self-trapping reduction in Ga2O3 by Bi2O3 alloying

Ga2O3 is an emerging material with attractive electrical properties for improving the performance of high-voltage power electronics, and it is widely accepted that engineering its band edge energies will expand its applications, especially for bipolar devices. In this work, monoclinic phase (BixGa1−x)2O3 ternary alloys are fabricated on c-plane sapphire via magnetron co-sputtering of Ga2O3 and Bi2O3. The bandgap of Ga2O3 is found to decrease from 4.97 to 4.78 eV by alloying with a Bi x-fraction of 0.04, consistent with the theoretical prediction that alloying with Bi2O3 causes the up-shift of the valence band. Concurrent temperature-resolved cathodoluminescence (CL) measurements of the Ga2O3 and (Bi0.04Ga0.96)2O3 are employed to investigate their intrinsic ultraviolet (UV) luminescence and defect-related visible bands with increasing temperature from 80 K. The CL results reveal a more rapid thermal quenching of the self-trapped hole (STH) emission with an activation energy of only 32 meV, suggesting that alloying with Bi2O3 lowers the self-trapping energy for holes. Temperature-dependent electrical measurements reveal the conductivity of the (Bi0.04Ga0.96)2O3 film is one order of magnitude lower than that of the undoped Ga2O3 counterpart; however, both possess an electrical activation energy of ∼ 26 meV, likely associated with an impurity-related shallow donor. The low electrical conductivity of (Bi0.04Ga0.96)2O3 can be attributed to the compensation of shallow donors by acceptors activated by the Bi2O3-induced upward shift of the valence band edge. The frequency-dependent conduction and ionic polarization mechanisms up to 107 Hz are found to be identical for Ga2O3 and (Bi0.04Ga0.96)2O3, indicating that the conduction in this ternary alloy does not involve the activation of Bi acceptors.

LASER FORMATION OF LUMINESCENT TRACKS IN BISMUTH-DOPED NANOPOROUS GLASS

The formation of microtracks in the volume of porous glass samples by femtosecond laser pulses which is of importance for laser writing of active optical waveguides was studied. It was found that impregnation of nanoporous glass in bismuth nitrate solution leads to the appearance of an orange luminescence band under 488 nm light excitation and this luminescence band is enhanced in the laser-wriiten tracks. It is shown that increasing the laser pulse energy to 200 nJ allows increasing the phase contrast of the tracks laser-written in nanoporous glass doped with bismuth compared to the ones formed in nanoporous glass without additives.

Genesis of granular calcite in lacustrine fine-grained sedimentary rocks and its indication to volcanic-hydrothermal events: A case study of Permian Lucaogou Formation in Jimusar Sag, Junggar Basin, NW China

Granular calcite is an authigenic mineral in fine-grained sedimentary rocks. Core observation, thin section observation, cathodoluminescence analysis, fluid inclusion analysis, scanning electron microscope (SEM), and isotopic composition analysis were combined to clarify the genesis of granular calcite in the lacustrine fine-grained sedimentary rocks of the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin. It is found that the granular calcite is distributed with laminated characteristics in fine-grained sedimentary rocks in tuffite zones (or the transitional zone between tuffite and micritic dolomite). Granular calcite has obvious cathodoluminesence band, and it can be divided into three stages. Stage-I calcite, with non-luminesence, high content of Sr element, inclusions containing COS, and homogenization temperature higher than 170 °C, was directly formed from the volcanic-hydrothermal deposition. Stage-II calcite, with bright yellow luminescence, high contents of Fe, Mn and Mg, enrichment of light rare earth elements (LREEs), and high homogenization temperature, was formed by recrystallization of calcareous edges from exhalative hydrothermal deposition. Stage-III calcite, with dark orange luminescence band, high contents of Mg, P, V and other elements, no obvious fractionation among LREEs, and low homogenization temperature, was originated from diagenetic transformation during burial. The granular calcite appears regularly in the vertical direction and its formation temperature decreases from the center to the margin of particles, providing direct evidences for volcanic-hydrothermal events during the deposition of the Lucaogou Formation. The volcanic-hydrothermal event was conducive to the enrichment of organic matters in fine-grained sedimentary rocks of the Lucaogrou Formation, and positive to the development of high-quality source rocks. The volcanic-hydrothermal sediments might generate intergranular pores/fractures during the evolution, creating conditions for the self-generation and self-storage of shale oil.

Donor-Acceptor Pairs Recombination as the Origin of the Emission Shift In InGaN/GaN Scintillator Heterostructures Doped with Zn

We report luminescence decay characteristics of the InGaN/GaN scintillator heterostructures doped with Zn. Unusually large shifting of luminescence band caused by Zn acceptors incorporated in InGaN is observed both in time-resolved and excitation-dependent spectra. Origins of the shifts are discussed, and model based on donor-acceptor pair recombination is introduced. The results imply a shrinkage of donor Bohr radius compared to the bulk material caused by quantum confinement effect. The slow decay of Zn band points out to the necessity of Zn impurity elimination in applications requiring fast timing characteristics of a scintillator.