Microcrystalline Phosphors Research Articles

Orange Emitting SrS: Eu2+, Dy3+ Afterglow Phosphor: Structural and Luminescence Properties

SrS:Eu is known as an attractive material for imaging, scintillator, or persistent phosphor applications, thanks to its effective photoluminescence (PL) and thermoluminescence (TL) capabilities, but there are still great mysteries of persistent phosphors and attract great interest. Hence, this work investigated the luminescence properties of well-known persistent phosphor based on SrS using various spectroscopic techniques for detailed characterization. Measurements using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to analyze the phase and elements of the phosphor. Belongs to the XRD data the microcrystalline phosphor is cubic, and the diameters ranged from 1 to 100 µm. The morphology of the phosphor was examined by scanning electron microscope (SEM). A time-resolved PL system recorded PL properties, namely the emission and excitation spectra of the phosphor. SrS emits around 626 nm with a broad excitation band peaking around 466 nm. A TLD reader system measured the TL glow curve of the phosphors after UVB radiation. The TL glow curve exhibited a broad peak around 160oC. The dose-response of this peak was obtained up to 600 seconds of exposure to UVB radiation, and it was observed that the dose-response curve exhibits a saturating exponential behavior.

Chemical tuning of photo- and persistent luminescence of Cr3+-activated β-Ga2O3 by alloying with Al2O3 and In2O3

An effect of alloying of the monoclinic β-Ga2O3 with Al2O3 and In2O3 on the photoluminescent, thermoluminescent and persistent luminescent properties of Cr3+ ions has been comprehensively investigated. For this purpose, various series of Cr3+ and Ca2+ co-doped microcrystalline phosphors were synthesized by the solution combustion method, including pseudobinary compounds like (Ga-Al)2O3 with up to 20 % Al and (Ga-In)2O3 with up to 50 % In as well as pseudoternary compounds (Ga-Al-In)2O3 with balanced proportion of Al, Ga and In. The phase composition and crystal structure of the obtained materials were examined by X-ray powder diffraction technique. Detailed luminescence studies were conducted for the (Ga-Al)2O3 and (Ga-In)2O3 compounds which exhibited a single-phase monoclinic structure. Low-temperature and time-resolved photoluminescence investigations of the Cr-doped pseudobinary compounds unveiled several types of Cr3+ centres, attributed to the Al-, Ga- and In-centred octahedra in the studied alloys. The obtained results underscore the benefit of bandgap engineering through alteration in the host lattice chemical composition for efficient tuning of the thermoluminescent and persistent luminescent properties of the near-infrared-emitting β-Ga2O3:Cr based phosphors. Furthermore, it was demonstrated that modification of the chemical composition of the host lattice also adjusts the thermometric performance of the studied phosphors. Indeed, the specific sensitivity of the β-Ga2O3:Cr3+ decay time luminescence thermometer showed nearly twofold enhancement when the host lattice was alloyed with 30 % of In2O3.

Interstitial Zinc Boosted Light Tunability, Afterglow, and Ultrabright White Emission in Zinc Germanate (Zn2GeO4)

To reduce the burden of solid-state lighting and phosphor-converted light-emitting diode technologies on rare earth (RE) and avoid their potential supply risks, we have developed a RE-free Zn2GeO4 (ZGeO) microcrystalline phosphor. It was synthesized by a simple and scalable solid-state reaction of ZnO and GeO2 at 1200 °C. High-temperature synthesized ZGeO demonstrated color tunable emission from bluish-white to green on changing the excitation wavelength from 265 to 335 nm with a photoluminescence quantum yield of 42 and 9%, respectively. They could also convert highly energetic X-ray to bluish-green light emission. First-principles calculations showed that bluish-white and green light emissions are linked to zinc interstitials present, respectively, in zinc (Zni2) and zinc–germanium rings (Zni1). These defects generated in large numbers via high-temperature annealing along with antisites boosted the generation of trapping centers leading to dual persistent luminescence (PerL) of white and green light for more than 60 min as well as exhibited radioluminescence. Luminescent ink based on ZGeO was fabricated and successfully tested for anticounterfeiting applications. An extra effort has been vested on making this solid-state processed ZGeO dispersible in water-soluble deep eutectic solvent which could be a game changer for its marketability. This material has great potential to become an efficient RE and dopant-free phosphor synthesized by a simple, scalable, and cost-efficient route for applications in solid-state lighting, display, and anticounterfeiting, having multifunctionalities such as PerL, tunability, and RL.

Study on the microstructure and properties of a phosphor copper ball during its formation process

Microcrystalline phosphor copper balls with a diameter of 28 mm were prepared via continuous extrusion upsetting. Optical microscopy and electron backscatter diffraction were used to study the microstructural evolution of phosphor copper balls during the formation process. In addition, the hardness distribution and tensile properties were tested. The results show that fine dynamic recrystallisation grains and twins were formed after continuous extrusion and that the grains were further refined after upsetting. After continuous extrusion upsetting, there were typical 〈111〉, 〈100〉, and 〈110〉 fibre textures, and the proportions of these three textures in the individual samples were different. The change in microhardness was affected by the microstructure. The increase in the hardness value from casting and continuous extrusion upsetting was owing to pronounced grain refinement. The grain sizes from the centre to the edge were similar, and the grain refinement was more uniform. Notably, the grain size of the extruded rod was still fairly uniform from the centre to the edge in the radial direction. It can be concluded that the continuous extrusion-upsetting phosphor copper anode is more conducive to the formation of black film, that is, it is more suitable for electroplating anode material.

Thermoluminescence, photoluminescence and optically stimulated luminescence characteristics of CaSO4:Eu phosphor: Experimental and density functional theory (DFT) investigations

The CaSO4:Eu phosphor in nanocrystalline form was obtained by chemical method. The sample was annealed at various temperatures and quenched. The structural, electronic and optical properties are studied using various experimental techniques. As synthesized CaSO4:Eu particles have nanorod shapes with diameter of ~15 nm and length of ~250 nm. After annealing (at around 900 °C) a significant increase in their size (~2–4 μm) with phase transformation from hexagonal to orthorhombic was observed. Thermoluminescence (TL) and optically stimulated luminescence (OSL) intensities were found to increase with temperature up to 900 °C and decrease thereafter for 1 Gy of test dose of β-rays from 90Sr-90Yr source. However, the maximum OSL sensitivity was found to be more than that of CaSO4:Eu microcrystalline phosphor (prepared by acid recrystallization method) contrary to the usually found in the literature but much less than that of commercially available α-Al2O3:C phosphor. The activation energy for thermally assisted OSL process was found to be 0.0572 ± 0.0028 eV. The dose ranges of TL and OSL response was found from 0.04 Gy to 100 Gy and 0.02 Gy–100 Gy, respectively. The experimental results are also correlated with computational calculations based on density functional theory (DFT). The crystal structures and electronic structures of both hexagonal and orthorhombic CaSO4 and CaSO4:Eu materials show that they are direct band gap (5.67–5.86 eV) insulators, with Ca2+ substitution by Eu2+ found to introduce donor states in the band gap near Fermi level and the valence band edge of CaSO4 on doping with Eu2+ impurity ions.

Thermoluminescence studies of CaSO4:Dy nanophosphor for application in high dose measurements

Nanorods of CaSO4:Dy (having diameter ∼20 nm and length ∼200 nm) were prepared by chemical coprecipitation method and their thermoluminescence (TL) characteristics were studied after annealing at different temperatures in the range 400–1000 °C. Microcrystalline material was also prepared through acid recrystallization method. TL glow curves of the nanocrystalline material annealed at 700 °C showed a major peak at around 283 °C. The effect of annealing temperature on the phase structure/morphology of particles was studied. Phase change was observed when CaSO4:Dy was annealed at different temperatures as confirmed by XRD and TG-DTA. It was observed from the TEM images that the nanomaterial originally in the nanorods form broke into nanoparticles due to strain developed by phase change on annealing at higher temperatures. The combined effect resulted in changes in TL glow curve structure. For better clarity the TL glow curves were further deconvoluted by Computerized Glow Curve Deconvolution (CGCD) method and kinetic trapping parameters were determined. It was found that the values of kinetic parameters also changed for the nanocrystalline material annealed at different temperatures. These were also compared with the corresponding values of the microcrystalline material. It was also observed that the TL intensity saturates at about 100 Gy in case of microcrystalline phosphor, while that in case of nanocrystalline phosphor did not do saturate even up to 5 kGy.

Luminescence characterization of Dy and Eu doped Na6 Mg(SO4 )4 phosphors.

A series of single-phase phosphors based on Na6 Mg(SO4 )4 (Zeff =11.70) doped with Dy and Eu was prepared by the wet chemical method. The photoluminescence (PL) and thermoluminescence (TL) properties of Dy3+ - and Eu3+ -activated Na6 Mg(SO4 )4 phosphors were investigated. The characteristic emissions of Dy3+ and Eu3+ were observed in the Na6 Mg(SO4 )4 host. The TL glow curve of the Na6 Mg(SO4 )4 :Dy phosphor consisted of a prominent peak at 234°C and a very small hump at 158°C. The TL sensitivity of the Na6 Mg(SO4 )4 :Dy phosphor was found to be four times less than the commercialized CaSO4 :Dy phosphor. The TL dose-response of the Na6 Mg(SO4 )4 :Dy phosphor was studied from a dose range of 5-10kGy and the linear dose-response was observed up to 1kGy which is good for a microcrystalline phosphor. Trapping parameters for both the samples were calculated using the Initial Rise and Chen's peak shape methods.

La3BaSi5N9O2:Ce3+ – A Yellow Phosphor with an Unprecedented Tetrahedra Network Structure Investigated by Combination of Electron Microscopy and Synchrotron X-ray Diffraction

Due to the relationship between structure and luminescence properties, detailed crystal structure determination for microcrystalline phosphors is necessary for a profound understanding of materials properties. The yellow phosphor La3BaSi5N9O2:Ce3+ (λmax = 578 nm; fwhm ∼4700 cm–1) was characterized by a combination of transmission electron microscopy (TEM) and synchrotron microfocus diffraction as only agglomerates of crystals with a maximum size of a few μm could be obtained yet. La3BaSi5N9O2:Ce3+ was synthesized from LaF3, La(NH2)3, BaH2, Si(NH)2, and CeF3 in a radio frequency furnace. It crystallizes in space group Pmn21 (no. 31) with a = 9.5505(8), b = 19.0778(16), c = 12.1134(9) A, and Z = 8. Its interrupted three-dimensional tetrahedra network contains zehner and dreier rings of vertex-sharing SiN4 and SiN2O2 tetrahedra. The crystal structure was confirmed by high-resolution TEM and Z-contrast scanning TEM. The element distribution was derived by bond-valence sum calculations. The infrared spectrum p...

Elastico-mechanoluminescence and crystal-structure relationships in persistent luminescent materials and II–VI semiconductor phosphors

Elastico-mechanoluminescence (EML) has recently attracted the attention of a large number of researchers because of its potential in different types of mechano-optical devices. For understanding the mechanism of EML the relationships between elastico-mechanoluminescence (EML) and crystal-structure of a large number of persistent luminescent materials and II–VI semiconductor phosphors known to date are investigated. It is found that, although most of the non-centrosymmetric crystals exhibit EML, certain non-centrosymmetric crystals do not show EML. Whereas, many centrosymmetric crystals do not exhibit EML, certain centrosymmetric crystals exhibit EML. Piezoelectric ZnS:Cu,Cl single crystals do not show EML, but piezoelectric ZnS:Cu,Cl microcrystalline phosphors show very intense EML. Piezoelectric single crystals of undoped ZnS do not show EML. It seems that EML is related to local piezoelectrification near the impurities in crystals where piezoelectric constant is high. Suitable piezoelectric field near the local piezoelectric region and stable charge carriers in traps are required for appearance of EML. The EML of persistent luminescent materials and II–VI semiconductor phosphors can be understood on the basis of piezoelectrically-induced trap-depth reduction model of EML. Using suitable dopants both in non-centrosymmetric and centrosymmetric crystals intense elastico-mechanoluminescent materials emitting desired colours can be tailored, which may find applications in several mechano-optical devices.

Monte Carlo simulations of TL and OSL in nanodosimetric materials and feldspars

The study of luminescent materials consisting of nanoclusters is an increasingly active research area. It has been shown that the physical properties of such nanodosimetric materials can be very different from those of similar conventional microcrystalline phosphors. In addition, it has been suggested that traditional energy band models may not be applicable for some of these nanodosimetric materials, because of the existence of strong spatial correlations between traps and recombination centers. The properties of such spatially correlated materials have been previously simulated by using Monte Carlo techniques and by considering the allowed transitions of charge carriers between the conduction band, electron traps and recombination centers. This previous research demonstrated successfully the effect of trap clustering on the kinetics of charge carriers in a solid, and showed that trap clustering can significantly change the observed luminescence properties. This paper presents a simplified method of carrying out Monte Carlo simulations for thermoluminescence (TL) and optically stimulated luminescence (OSL) phenomena, based on a recently published model for feldspar. This model is based on tunneling recombination processes involving localized near-neighbor transitions. The simulations show that the presence of small clusters consisting of a few traps can lead to multiple peaks in both the TL and linearly modulated OSL signals. The effects of donor charge density, initial trap filling and cluster size are simulated for such multi-peak luminescence signals, and insight is obtained into the mechanism producing these peaks.

Synthesis and luminescence properties of Ce3+ doped MWO4 (M=Ca, Sr and Ba) microcrystalline phosphors

The Ce3+ doped and undoped samples of alkali earth metal tungstate MWO4 (M=Ca, Sr, and Ba) phosphors are synthesized by a co-precipitation method in controlled pH environment. The resulting phosphors were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR), photoluminescence (PL) and thermoluminescence (TL). XRD pattern and SEM micrographs reveal the formation of agglomerated microcrystalline phosphor. FTIR spectra show the strong absorption around 821cm−1 due to characteristic vibrations of (WO4)2− complex. PL excitation spectra show broadband in the UV region having peak at 280nm, and the emission spectrum shows broadband in the visible region with peak in the blue region. The PL emission intensity increases with Ce3+ concentration with the most effective concentration at 5mol%. The complex TL glow curve of Ce3+ doped phosphors is deconvoluted by using a TLAnal computer program. The trap parameters obtained by TLAnal were compared with those calculated by Chen׳s method and a possible model for TL is discussed.

Study of TL and optically stimulated luminescence of K2Ca2(SO4)3:Cu nanophosphor for radiation dosimetry

Nanocrytstalline K2Ca2(SO4)3:Cu phosphor was synthesized by the chemical co-precipitation method and annealed at different temperatures (400–900°C) for 2h. The XRD spectrum shows the cubic structure with crystallite size ~20nm. The same was also confirmed from the TEM image which shows the formation of nanorods having diameter ~20nm and length of ~200nm. They are found to be quite uniform in shapes and sizes. These samples were irradiated with gamma radiation for the doses varying from 0.01Gy to 10kGy and their thermoluminescence (TL) characteristics and continuous wave optically stimulated luminescence (CW-OSL) have been studied. The sample annealed at 700°C was found to be most sensitive than others. The glow curves of the nanophosphor show a major peak at around 175°C and other two peaks of low intensity at around 85°C and 305°C. The traps responsible for the three thermoluminescence peaks in K2Ca2(SO4)3:Cu are also found to be sensitive to the OSL. The qualitative correlation between TL peaks and CW-OSL response is established. The TL response of the sample annealed at 700°C for 2h and irradiated with different gamma doses shows a linear behavior from 0.01 up to 300Gy and become sublinear in the range of 300 Gy–1kGy before it saturates with further increase in the dose, while, the OSL response of the same sample shows linearity up to 1kGy. Simple glow curve structure, easy method of synthesis, and linear dose response make the nanocrystalline phosphor a good candidate for radiation dosimetry, especially for the estimation of high doses of gamma rays where the microcrystalline phosphors generally saturate.

Thermoluminescence study of K2Ca2(SO4)3:Cu nanophosphor for gamma ray dosimetry

Nanocrystalline K2Ca2(SO4)3:Cu was synthesized by chemical coprecipitation method and annealed at 700°C. XRD of this sample shows the cubic structure and the crystallite size ∼20nm. The same was also confirmed with TEM and shows the formation of nanorods of quite uniform in shape having diameter ∼20nm and length in the range of 200nm. The FTIR spectrum exhibits the bands corresponding to sulphate anions around 1015 and 600cm−1. These samples were irradiated with gamma radiation for the dose varying from 0.1Gy to 50kGy and their TL characteristics have been studied. The glow curves of the pristine samples consists of two peaks one at around 139°C while another one around 185°C while the glow curve of samples annealed at 700°C show a major peak at around 165°C and other two peaks of low intensity at around 300°C and 390°C. The change in glow curve structures around 700°C is due to the phase transition (from orthorhombic P212121to cubic P213 phase). The energy levels (trapping levels) get reorganized due to phase transition after the phase transition temperature (260°C) and remain the same at higher temperatures (700°C). TL dose response shows a linear behavior up to 1kGy and further saturates with increase in the dose. Simple glow curve structure, easy method of synthesis, linear dose response and low fading make the nanocrystalline phosphor a good candidate for radiation dosimetry and especially, for the estimation of high doses of gamma rays where the microcrystalline phosphors generally saturate.

Thermally-induced ionization of the Ce3+ and Pb2+ excited states in the SrHfO3 microcrystalline phosphor

Although luminescence properties of Pb2+ and Ce3+ in SrHfO3 host (SHO) have been recently studied due to various potential applications [1, 2], the mechanism of the Pb2+ and Ce3+ excited states thermal quenching has not been addressed. Both Pb2+ and Ce3+ doped SHO microcrystalline powders were prepared by multi-step solid state annealing technique. Radioluminescence and photoluminescence (PL) spectra as well as both the prompt and delayed decay kinetics were investigated in the wide temperature range (10 – 500 K) using the time-resolved luminescence spectrocopy techniques. Temperature dependences (TDs) of PL decay time and PL emission intensity of both Pb2+ and Ce3+ centres were fit by the fenomenological model. Corresponding excited state characteristics were determined. The innovative method of measuring the TD of intensity of the delayed radiative recombination was employed to better understand the origin of temperature quenching of the Pb2+ and Ce3+ decay times [3, 4]. The method put in evidence the excited state thermal ionization of both activators in the SHO host. The influence of dopant concentration and suitability of both the Pb and Ce-doped SHO for phosphor and scintillator applications are discussed.

Synthesis and luminescence properties of YVO4:Eu3+ cobblestone - like microcrystalline phosphors obtained from the mixed solvent - thermal method

Abstract The mixed solvent-thermal method has been developed for the synthesis of YVO 4 :Eu 3+ luminescent materials in the N, N-dimethylformamide (DMF)/ de-ionized water (DIW) solution. The samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electronic microscope (TEM), UV/vis absorption and photoluminescence spectroscopies. The results demonstrate that we have obtained the uniform YVO 4 :Eu 3+ cobblestone - like microcrystalline phosphors in the mixed solution of DMF and DIW, which are different to the as-obtained YVO 4 :Eu 3+ nanoparticles in pure DIW. And the as - prepared YVO 4 :Eu 3+ microcrystalline particles are composed of numerous nanoparticles. The assembling phenomenon of the nanoparticles is strongly affected by the pH value of the solution and the volume ratio of DMF/DIW. Under UV excitation, the samples can emit the bright red light. While, the photoluminescence (PL) intensities of YVO 4 :Eu 3+ show some difference for samples obtained under the different reaction conditions. This is because that different microstructures of samples result in different combinative abilities between the surface and the adsorbed species so as to produce the different quenching abilities to the emission from Eu 3+ ions.

Thermally-induced ionization of the Ce 3+ excited state in SrHfO 3 microcrystalline phosphor

We measured temperature dependences of emission spectra, prompt nanosecond decays and millisecond delayed recombination decays for the Ce 3+ center in the SrHfO 3 (SHO) host. We characterized two samples with 0.1% and 5%Ce prepared by the acetate and citrate combustion and solid state reaction, respectively. The decrease of nanosecond decay times above approximately 220 and 260 K, respectively, is due to thermal ionization of 5d 1 excited state of the Ce 3+ center. Energy barrier of 300 meV, governing this process, is calculated for both the 0.1% and 5%Ce:SHO samples. Thermoluminescence glow curve measurement after X and UV (310 nm) irradiation at RT provides another support for the 5d 1 excited-state ionization of Ce 3+ in the SHO host.

Pr3+ luminescence center in Lu2Si2O7 host

AbstractPr3+‐doped Lu2Si2O7 (LPS:Pr) microcrystalline phosphor was prepared by the sol–gel method. We study the LPS:Pr luminescence properties under UV and X‐ray excitation within 80–500 K. The emission spectrum is dominated by fast 5d–4f band peaking at 261 nm having 16 ns decay time. By purely optical contactless methods we determine the energy barrier of 300 meV for thermal ionization of the Pr3+ 5d1 relaxed excited state in LPS host. The barrier is high enough to keep the room temperature quantum efficiency of the Pr3+ luminescence center close to unity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sol–gel synthesis and photoluminescence of RE 3BO 6: Eu 3+/Tb 3+ (RE = Y, Gd) microcrystalline phosphors from hybrid precursors

Novel phosphors of Eu 3+/Tb 3+ doped RE 3BO 6 (RE = Y, Gd) have been prepared using an original modified in situ sol–gel synthesis route. Different optimized organic media were mixed with rare earth coordination polymers, and tri- n-butyl borate was added to assemble inorganic/organic multi-component hybrid precursors. After calcinations of the resulting precursors at 1000 °C, target phosphors were obtained. The microstructure and morphology information of the phosphors were investigated via the technique of X-ray powder diffraction (XRD) and scanning electronic microscopy, and it has been shown that these phosphors present symmetrical distribution and high packing density, whose grain sizes were around 200 nm. Analyzed by luminescent spectra, these phosphor particles show narrow lines of emissions respectively originating from their characteristic transitions, and the dominating emission peak is due to the hypersensitive transition. The RE 3BO 6: Eu 3+/Tb 3+ (RE = Y, Gd) phosphors can be expected to gain more practical applications in commercial phosphors and other luminescent materials used in advanced devices.

In situ co-precipitation synthesis and photoluminescence of YxGd1−xVO4:Tm3+ microcrystalline phosphors by hybrid precursors

YxGd1−xVO4:Tm3+ (5mol%) phosphors were prepared by in situ co-precipitation technology with the different content ratio of Y/Gd (x=0.2,0.3,0.4,0.5,0.6,0.8, respectively). During the process, rare earth coordination polymers with o-hydroxylbenzoate were used as precursors, composing with polyethylene glycol (PEG) as dispersing media. After heat-treatment of the resulting multicomponent hybrid precursors at 900°C, the samples were obtained. SEM indicated the particles present good crystalline state, whose crystalline grain sizes were about 0.2–2μm. Under the excitation of 257nm, all the materials show the characteristic emission of Tm3+ which is the strong blue emission centered at 475nm originating from 1G4→3H6 of Tm3+. Besides this, concentration quenching appears in the system of YVO4:Tm3+ and GdVO4:Tm3+. And when x reaches 0.5, the system of YxGd1−xVO4:Tm3+ shows the strongest blue emission.

Chemical co-precipitation synthesis and photoluminescence of Eu 3+ or Dy 3+ doped Zn 3Nb 2O 8 microcrystalline phosphors from hybrid precursors

Dy 3+, Eu 3+ activated Zn 3Nb 2O 8 phosphors were synthesized via a modified chemical co-precipitation technology with the different concentration. In order to obtain the hybrid precursors, the rare earth coordination polymers with salicylic acid were used as precursors and the polyethylene glycol (PEG) was used as dispersing media. Their microstructure and micromorphology have been analyzed by X-ray powder diffraction (XRD) and scanning electronic microscope (SEM), which indicates that there exist some novel column-like microcrystalline particles with 0.5–5.0 μm sizes in dimension. The characteristic transition of Dy 3+ and Eu 3+ were identified in the emission spectra and the optimum concentration of activators in Zn 3Nb 2O 8 was found.