[show abstract][hide abstract] ABSTRACT: The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO4 as well as the Znx
WO4 solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO6]6− anions, becomes modulated by the optical absorption of Ni2+ ions in the Znx
WO4 solid solutions; and (ii) no significant change in the excitation spectra of Zn0.9Ni0.1WO4 is observed compared to pure ZnWO4. At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO4.
Central European Journal of Physics 08/2013; 9(2). · 0.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: a b s t r a c t MgO nano-powder with an average crystallite size of nanoparticles ranging 10–15 nm was synthesized using the extractive-pyrolytic method and was studied by room temperature VUV spectroscopy under synchrotron radiation excitation. Comparative analysis of their luminescent properties with that of mac-rocrystalline powder analogues and an MgO single crystal, grown by the arc-fusion method, has been per-formed under excitation by pulsed VUV synchrotron radiation. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources. A considerable blue shift of about 0.3 eV in the excitation spectra of 2.95 eV emission band, was revealed in nanocrystalline MgO samples. Ó 2013 Elsevier B.V. All rights reserved.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2013; 310:23-26. · 1.27 Impact Factor
[show abstract][hide abstract] ABSTRACT: h i g h l i g h t s < Luminescence properties of SrI 2 and SrI 2 :Eu single crystals have been investigated. < Excitation spectroscopy technique using synchrotron radiation was applied. < Excitonic properties in nominally pure and europium doped crystals have been studied. a b s t r a c t We report measurements of luminescence and its ultraviolet excitation spectra in SrI 2 and SrI 2 :Eu 2þ at temperatures of 10 and 300 K. Attention is focused on determining the exciton energy and its temper-ature shift from features of the excitation spectra and limits placed by absorption spectroscopy on a 120 mm thin crystal, on observation of a broadened Eu emission band attributed to trace Eu associated with oxygen in nominally undoped crystals, and on adding observations concerning the 3.4 eV band at low temperature attributed by Pustovarov et al. to the self-trapped exciton.
[show abstract][hide abstract] ABSTRACT: Comparative analysis of the luminescent properties of nanocrystalline LaPO4:Ce,Tb and YVO4:Eu luminescent materials with macrocrystalline analogues, commercially produced by Philips, has been performed under excitation by pulsed vacuum ultraviolet (VUV) synchrotron radiation, ranging from 3.7-40 eV. Special attention was paid to VUV spectral range, which is not reachable with commonly used lamp and laser sources. Our results clearly show distinct difference in the excitation spectra for nano- and macrocrystalline samples, especially at energies, when the spatial separation of electron-hole pairs is comparable with sizes of nanoparticles. Differences in the region of multiplication of the electronic excitations are also demonstrated and discussed.
Journal of Applied Physics 09/2011; 110(5):053522-053522-7. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Photoluminescence excitation and transmission spectra of Si nanocrystals of different diameters embedded in a SiO2 matrix have been investigated in the broad visible-vacuum ultraviolet spectral range using synchrotron radiation. The dependence of the photoluminescence excitation spectra on the nanocrystals size was experimentally established. It is shown that the photoluminescence excitation and absorption spectra are significantly blueshifted with decreasing Si nanocrystal size. A detailed comparison of photoluminescence excitation and absorption spectra with data from theoretical modeling has been done. It is demonstrated that the experimentally determined blueshift of the photoluminescence excitation and absorption spectra is larger than the theoretical predictions. The influence of point defects in the SiO2 matrix on the optical and luminescence properties of the embedded Si nanocrystals is discussed. Moreover, it is demonstrated that no energy transfer takes place between the SiO2 and Si nanocrystals when the excitation energy is higher than the band-to-band transition energy in SiO2.
[show abstract][hide abstract] ABSTRACT: The photoluminescence spectra and luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO(4) as well as the Zn (x) Ni(1-x) WO(4) solid solutions were studied using vacuum ultraviolet (VUV) synchrotron radiation. The samples were also characterized by x-ray powder diffraction. We found that: (i) the shape of the photoluminescence band at 2.5 eV, being due to radiative electron transitions within the [WO(6)](6-) anions, becomes modulated by the optical absorption of Ni(2+) ions in the Zn (x) Ni(1-x) WO(4) solid solutions; and (ii) no significant change in the excitation spectra of Zn(0.9)Ni(0.1)WO(4) is observed compared to pure ZnWO(4). At the same time, a shift of the excitonic bands to smaller energies and a set of peaks, attributed to the one-electron transitions from the top of the valence band to quasi-localized states, were observed in the excitation spectrum of nano-sized ZnWO(4).
Central European Journal of Physics 01/2011; 9:432-437. · 0.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: a b s t r a c t Comparing the luminescence properties of nanosized and macroscopic LaPO 4 :Ce,Tb powders are per-formed in wide spectral range using synchrotron radiation. In the present study, LaPO 4 :Ce,Tb nanopow-der was produced by means of a microwave-induced synthesis in ionic liquids, whereas the bulk sample represents a commercial lamp phosphor. Emission and excitation of both, Ce 3+ and Tb 3+ luminescence, is observed to be different when comparing bulk and nanosized LaPO 4 :Ce,Tb. In particular, it was shown that the fine structure of the Ce 3+ as well as the Tb 3+ related emission is poorly resolved for the nanoma-terial. It is suggested that the nanoparticles surface plays a key role regarding the perturbation of rare-earth ions and changes their luminescence properties. Furthermore, it is demonstrated that allowed f–d transitions on Tb 3+ at high energy are significantly suppressed for nanosized LaPO 4 :Ce,Tb. Energy transfer is required to initiate Tb 3+ emission even in the vacuum ultraviolet spectral range.
[show abstract][hide abstract] ABSTRACT: The incorporation site of Cu in the scintillator and thermoluminescent dosimeter material lithium tetraborate Li2B4O7:Cu has been shown to depend on the charge state of the dopant. As confirmed by a refined analysis, the EPR spectra of an unpaired hole can be ascribed to Cu2+ situated near the Li lattice site with C1 symmetry, in contrast to the higher (C2) overall symmetry reported earlier for Cu+ in its relaxed excited state, the latter suggesting interstitial incorporation for the monovalent dopant. Consequences for charge transfer processes involving copper ions are discussed.
Journal of Physics Conference Series 11/2010; 249(1):012008.
[show abstract][hide abstract] ABSTRACT: The relaxation of electronic excitations in CdWO4 and CaWO4 crystals was studied using the method of time-resolved interferometry with 100-fs temporal resolution at temperatures 15–295 K. The electronic system was excited in the one-photon and two-photon regime within the excitonic band in CaWO4 and in the electron-hole continuum in CdWO4. Immediate trapping of charge carriers was detected under pumping in the excitonic band of CaWO4. This result is in agreement with decay kinetics measurements with nanosecond time resolution under direct creation of excitons by 100-fs laser pulses. Fast relaxation of charge carriers followed by formation of excitons was observed in CdWO4. The comparison with previous work allows suggesting the formation of bulk excitons and surface-perturbed excitons in the multi-photon and one-photon regime. The corresponding models of self-trapped exciton creation in tungstate crystals are discussed.
[show abstract][hide abstract] ABSTRACT: Time-resolved interferometry with a 100-fs temporal resolution was applied for the first time to studying the relaxation of
electronic excitations in complex oxides, namely, tungstates CDWO4 with a crystal lattice of the wolframite-type and CaWO4 with a scheelite-type lattice. Two stages of charge carrier relaxation, namely, very fast carrier trapping in 200 fs resulting
in self-trapped exciton formation and a relatively slow picosecond relaxation process probably due to configurational relaxation
within the oxyanion molecule and modification of the surrounding lattice, are revealed in tungstate crystals. Corresponding
models of self-trapped exciton creation in tungstate crystals are discussed.
Physics of the Solid State 08/2008; 50(9):1784-1788. · 0.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: A low-temperature study of the thermoluminescent dosimeter material, lithium tetraborate (Li2B4O7) doped by Cu, has been carried out by the methods of electron paramagnetic resonance (EPR) and time-resolved polarization spectroscopy using 4–20 eV synchrotron radiation and 1 µs Xe flash lamp pulses in the region 3–6 eV. The observed EPR spectra of an unpaired hole with strong d-character and characteristic hyperfine splittings can be ascribed to Cu2+ substituted at a Li lattice site and displaced due to relaxation. The results on the Cu+-related luminescence strongly support the conclusion about a low-symmetry position of copper impurity ions in the lithium tetraborate lattice. The temperature dependence of the decay kinetics of the Cu+-related 3.35 eV emission indicates a triplet nature for the relaxed excited state of the Cu+ centres. An off-centre position of the Cu+ ion in the relaxed excited state is suggested.
[show abstract][hide abstract] ABSTRACT: Scheelite CdMoO4 and wolframite CdWO4:Mo (0.04–0.4 wt%) single crystals were studied using the time-resolved spectroscopy under UV excitation over a temperature range of 1.85–300 K. The threshold energies for the creation of free charge carriers were measured using the method of photostimulated luminescence. The decay kinetics of the main emission in CdMoO4 and the molybdenum-related emission in CdWO4:Mo was studied and the parameters of the triplet excited states of molybdenum-related oxyanions were calculated.
[show abstract][hide abstract] ABSTRACT: The emission spectra of lithium orthosilicates (Li4SiO4) ceramics have been measured in the range of 1.8–5.8eV under irradiation by 6–30eV photons or 1–30keV electrons at 6–300K. The tunnel recombination phosphorescence, as well as luminescence, stimulated by 1.5–2.5eV photons has been detected in the sample preliminarily irradiated at 6 or 80K. The main peaks of thermally stimulated luminescence (TSL) in the irradiated ceramics have been observed at 72, 118 and 265K. The creation spectra of the 118K TSL peak, as well as the excitation spectrum of photostimulated luminescence (PSL) span the region of the intrinsic absorption of a lithium orthosilicate (9–30eV). The intensity of PSL and the TSL peaks in Li4SiO4 ceramics prepared in hydrogen/argon atmosphere is several times lower than that in the mainly investigated Li4SiO4 ceramics prepared in the atmosphere of dry argon. The optical characteristics of Li4SiO4 are compared with the ones known for Li2O and SiO2. Low-temperature luminescent methods are promising for the investigation of electron–hole processes and radiation defects serving as the traps for tritium released in D–T fusion reactor blanket systems.
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms - NUCL INSTRUM METH PHYS RES B. 01/2006; 250(1):159-163.
[show abstract][hide abstract] ABSTRACT: The decay kinetics of the 2.88eV emission of pure and Bi-doped CaWO4 crystals has been studied in the temperature region of 4.2–300K. The decay components connected with the radiative transitions from the triplet excited states of regular WO42− oxyanionic complexes and WO42− complexes perturbed by Bi3+ ions (WO42−(Bi3+) centres) have been identified. It is shown that WO4(Bi3+) centres can be excited directly above the edge of the fundamental absorption of CaWO4, or due to the energy transfer from the metastable state of a Bi3+ ion at the excitation in the Bi3+ absorption band at 4.4eV. At temperatures T>170K, the excited state of the WO42−(Bi3+) centre is thermally ionised, giving rise to phosphorescence in the blue spectral region. These processes are compared with those known for excitons in PbWO4 crystals.
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment - NUCL INSTRUM METH PHYS RES A. 01/2005; 537(1):61-65.
[show abstract][hide abstract] ABSTRACT: For the first time the characteristics of the main emission of Al2(WO4)3 are reported. In order to distinguish the excitonic and electron-hole processes in Al2(WO4)3 we measured the creation spectrum of photostimulated luminescence under irradiation by synchrotron light and compared it to the excitation spectrum of the main emission. It is shown that the onset of the excitation spectrum of the main 2.7 eV emission of Al2(WO4)3 is at 5 eV, while the threshold of the creation of electron-hole pairs is measured to be 8.7 to 9.5 eV at 8 K. The latter value is in good agreement with the band-gap value of alpha-Al2O3 (9.4 eV). This supports the suggestion that free electrons and holes are created in tungstates due to electronic transitions from oxygen to cation states situated several eV above the bottom of the conduction band.
[show abstract][hide abstract] ABSTRACT: Thermostimulated and photostimulated luminescence of ZnWO4,ZnWO4:Fe and ZnWO4:Mo crystals irradiated at low temperatures by X-rays or UV photons was studied in the temperature range 4.2–300K in order to clarify the creation and recombination processes of the elementary colour centres. The connection of the luminescence phenomena with the self-trapped holes has been revealed.
[show abstract][hide abstract] ABSTRACT: ZnWO4, ZnWO4:Fe and ZnWO4:Mo crystals were investigated by the methods of time-resolved spectroscopy in the temperature range of 4.2–300K. It is shown that the Mo and Fe impurities significantly reduce the light yield of ZnWO4. The main 2.5eV emission of ZnWO4 and the 1.77eV emission band of ZnWO4:Mo are shown to originate from the triplet excited state of the WO6 and MoO6 complex, respectively. In ZnWO4:Fe,Mo the MoO6 emission band is shifted to lower energies due to the perturbing influence of the iron impurity. No perturbing effect of Fe or Mo ions was observed for the main emission of ZnWO4:Fe and ZnWO4:Mo. The creation spectrum of self-trapped holes was measured for ZnWO4, ZnWO4:Fe and ZnWO4:Mo crystals in the energy region of 4–30eV.