Time-resolved cathodoluminescence and photoluminescence of nanoscale oxides

The nanostructured oxide materials such as ZnO, ZrO2, and Y3Al5O12 (YAG) are perspective materials for transparent scintillating and/or laser ceramics. The luminescence properties of single crystals, nanopowders and ceramic were compared. Nominally pure and rare-earth doped nanopowders and ceramics have been studied by means of time-resolved luminescence spectroscopy. The fast blue luminescence band was studied in ZnO ceramics sintering from different raw materials. The luminescence centres of ZrO2:Y were compared in a single crystal, ceramic and nanopowder. It is shown that ceramic sintering parameters have a strong influence on time-resolved luminescence characteristics in cerium-doped YA…

Radiation defects in undoped and Nd‐doped LaGaO 3 crystals

Radiation induced defects have been studied in undoped and Nd-doped (6 mol% and 12 mol%) LaGaO3 crystals. Wide absorption band (2.2–2.8 eV) was observed after crystal irradiation with X-rays at 300 K. Induced defects have been annealed in air at ∼450 K. Similar absorption band was observed in transient absorption spectra after ns-pulsed electron beam excitation. The radiation defect creation efficiency is higher in undoped LaGaO3 crystal. It is shown that small concentration of Nd-doping increases the LaGaO3 crystal radiation hardness. In transient absorption spectra along with 2.7 eV band the absorption bands at 1.5 eV and 2.2 eV were observed. The decay process of transient absorption has…

The Model for Luminescence Center Excitation in PbWO4

Luminescence in SrTiO3and LiNbO3Crystals Under High Density Pulsed Electron Excitation

The time-resolved luminescence as well as stationary excited luminescence in SrTiO3 (undoped, Nb and Pb doped) and LiNbO3 (congruent and stoichiometric) single crystals was studied. The charge-transfer vibronic exciton in the regular oxy-anion site is suggested to be responsible for luminescence bands at 2.8 eV and 2.75 eV for SrTiO3 and LiNbO3 crystals respectively. It is suggested that luminescence band observed in SrTiO3 at ∼2.2-2.3 eV above 80 K possibly is due to some defect common for all SrTiO3 crystals studied.

The electron beam induced short lived absorption in PLZT optical ceramics

Abstract The short lived absorption induced by a pulsed electron beam in the led–lanthanum–zirconate–titanate (PLZT) optical ceramics was studied. The measured absorption spectrum covers 1.1–2.9 eV energy range and consists of several strongly overlapping peaks. The rise of the absorption is delayed with respect to the excitation pulse, due to charge carrier migration before trapping at centers responsible for the absorption. The formation rate of absorption centers and decay rate of absorption depend on the photon energy. The kinetics of short lived absorption varies over spectrum and can be approximated with a stretched exponent. The stretched exponent parameters indicate that the local d…

Intrinsic defect related luminescence in ZrO2

The studies of ZrO 2 and yttrium stabilized ZrO 2 nanocrystals luminescence as well as yttrium stabilized single crystal luminescence and induced absorption showed that the intrinsic defects are responsible for luminescence at room temperature. These defects form a quasi-continuum of states in ZrO 2 band gap and are the origin of the luminescence spectrum dependence on the excitation energy. Luminescence centers are oxygen vacancies related but not the vacancies themselves. At room temperature, in ZrO 2 , deep traps for electrons and holes exist. The oxygen vacancies are proposed to be the traps for electrons.

Swift heavy ion induced modifications of luminescence and mechanical properties of polypropylene/ZnO nanocomposites

Abstract The effect of irradiation with 150 MeV Kr ions at fluencies 10 11 and 10 12  ions/cm 2 on luminescence of PP/ZnO nanocomposites and PP matrix has been studied. In unirradiated composite the luminescence is comparatively weak and is contributed mainly by the excitonic luminescence of ZnO. Irradiation of nanocomposite and PP matrix leads to a strong increase of luminescence intensity in a wide spectral range of 360–600 nm. It follows from the obtained results that the enhancement of luminescence originates mainly from the broken bonds in PP matrix. Ion-induced scission of polymer bonds and fragmentation of polymer chains is deduced also from the nanoindentation tests which show a rem…

The model of recombination process in TlBr

The time-resolved luminescence was used as a tool in the study of recombination process in several undoped TlBr crystals. The spectra and decay kinetics observed under electron beam excitation were investigated. Observation of several luminescence bands with different decay rates shows that more than one recombination center is involved and the recombination process is quite complicated. The band at ∼2.5 eV is dominant under 10 ns excitation pulse (electron beam or nitrogen laser pulses). The results of short-lived absorption and luminescence are used for analysis of possible mechanisms of recombination processes in TlBr.

Time‐resolved absorption and luminescence following electron‐hole pair creation in ZnO

We report transient absorption induced by electron-hole excitation in undoped ZnO. A laser pump/continuum probe method covers 2–300 ps, and an electron pulse with lamp transmission covers 8–300 ns. The broad absorption spectrum increases monotonically with wavelength from 900 to 1600 nm. Following a reasonable hypothesis that the free-carrier-like induced infrared absorption is proportional to the total number of free carriers, excitons, and shallow-trapped carriers in the sample, these data allow setting an upper limit on the quantum efficiency of a specified lifetime component of luminescence. For the undoped commercial ZnO studied in this report, the quantum efficiency of room temperatur…

Luminescence of ferroelectric crystals: LiNbO3and KNbO3

Abstract The thermostimulated luminescence and time-resolved luminescence of LiNbO3 (congruent, stoichiometric, Eu−, Mn−, or Cr−doped) and KNbO3 crystals (undoped, Fe−, or Mn− doped) excited by X-ray, pulsed nitrogen laser or pulsed electron beam were studied. The luminescence decay times of niobium-oxygen groups (regular or perturbed by defect or impurity) were obtained. It is shown that the energy transfer from bulk to the activator in LiNbO3 is not effective during the electron-hole recombination process.

Blue luminescence in ZnO single crystals, nanopowders, ceramic

The luminescence spectra and luminescence decay processes were studied in a ZnO single crystal, nanopowders and ceramic at liquid helium and room temperature under VUV synchrotron radiation as well as under pulsed laser excitation. The exciton-exciton and exciton-multiphonon processes were compared in different ZnO nanopowders (commercial powder, powders obtained by vaporization-condensation technique) and ceramic. The possibility of luminescence decay time modification by Al3+ doping was shown.

Gas sensitive luminescence of ZnO coatings obtained by plazma electrolytic oxidation

Abstract The ZnO coatings on Zn substrate were obtained using plasma electrolytic oxidation method. The XRD and SEM methods were used for structural and morphological characterization of obtained coatings. The luminescence of ZnO coatings were studied and compared with luminescence characteristics of ZnO single crystal. It is shown that luminescence intensity in ZnO defect band depends on oxygen concentration in ambient atmosphere. The effect is of interest for oxygen sensing based on ZnO coating luminescence.

Formation of luminescence centres under excitation with electron beams in PbWO 4 and ZnWO 4 crystals

The leading edge of intrinsic luminescence pulse in PbWO 4 and ZnWO 4 single crystals is studied. The rise of luminescence intensity is observed after excitation with electron beam pulse. The luminescence rise is shown tobe a result of luminescence center formation via electron-hole recombination within spatially correlated pairs. An electron of a geminate electron-hole pair of PbWO 4 crystal self-traps and stimulates self-trapping of a hole at a close distance. In the ZnWO 4 crystal a hole of a geminate electron-hole pair self-traps and stimulates self-trapping of the electron producing a W 5 + center at a short distance. The rise-time of luminescence depends on spatial separation of recom…

Transient absorption spectra and relaxation kinetics in Nb-doped PbWO4 scintillating crystals

Abstract The spectra and relaxation kinetics of absorption induced by a pulsed electron beam were studied for undoped and Nb-doped PbWO 4 single crystals. A significant influence of Nb doping on transient absorption was found. The influence of Nb doping on the relaxation kinetics is proposed to be due to an oxygen vacancy involving electron–hole recombination. A delay was observed (35– 50 ns ) between the growth of F-centers and the irradiation pulse. A large portion of the F-centers decays within ∼16 μs due to recombination with mobile holes. The prolonged recombination processes are responsible for the decrease of the scintillation yield in PbWO 4 : Nb crystals.

Transient and near‐edge absorption in YVO 4 crystals

The process of near-edge absorption annealing in air was studied up to 1500C in YVO4. In this annealing process, two stages with activation energies of 0.16 eV and 0.38 eV were obtained. (Should combine to make this one paragraph)In addition to the annealing study, the transient absorption induced by pulsed electron beam excitation (270 keV, 8 ns) was completed on a set of YVO4 samples with different near-edge absorption levels in the spectral region 3.2-3.5 eV. The spectral range from ∼1.25 eV up to 3.0 eV is covered by strong transient absorption. Transient absorption spectra show at least three broad overlapping bands (∼1.3 eV, ∼2.0 eV and ∼3.0 eV). The 3 eV peak position is close to the…

The study of time-resolved absorption and luminescence in PbWO4 crystals

Abstract The transient absorption and luminescence under pulsed electron beam excitation were studied for undoped and La3+ doped PbWO4 crystals. The absorption band at ∼1.0 eV is suggested to be due to self-trapped electrons and the absorption band at 3.5 eV may be due to self-trapped holes. The formation of luminescence centers via electron–hole recombination is affected by La3+ in doped crystals. The large fraction of electrons and holes undergoes thermostimulated recombination within geminate pairs and the spatial separation of geminate pairs components is important in recombination process. The intrinsic (blue) luminescence arises in this recombination. The mechanism of La3+ influence o…

Luminescence spectra and decay kinetics in ZnWO4 and CdWO4 crystals

The luminescence spectra and luminescence decay kinetics in ZnWO4 (nominal pure and doped with Fe and Mo) under pulsed electron beam and pulsed nitrogen laser excitation have been investigated. It is suggested that the two-stage intrinsic luminescence decay observed under ionizing radiation excitation in ZnWO4 and CdWO4 was due to two different self-trapped exciton (STE) configurations in these crystals. The role of Fe and Mo impurities for scintillation efficiency in ZnWO4 and CdWO4 has been discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Subpicosecond Laser Spectroscopy of Blue-Light-Induced Absorption in KNbO3 and LiNbO3

Pulses of 427-nm light from am amplified Ti:sapphire frequency-doubled laser are absorbed in two-photon band-gap excitations of KNbO3 and LiNbO3. Induced absorption spectra measured in the visible and recently extended in the infrared have been recorded as a function of delay from 0 to 250 ps after two-photon blue excitation. Samples include stoichiometric as well as congruent LiNbO3. Comparison is made to transient absorption in the nanosecond and longer time range induced by electron pulses of~10 ns width and 270 keV energy. Parts of the absorption can be attributed to bound and free polarons, including the intrinsic electron polaron.

Nanocomposites for Novel Sensing Systems

The work performed under the National Research Program of Latvia and presented here is about advanced, chemical stimuli responsive materials. Sensor materials are produced on the basis of metallophthalocyanines or zirconia nanocrystals with the purpose of detecting small size molecular compounds like NH3, O2 and O3. Polymer/nanostructured carbon composite is produced to detect large size molecular compounds like organic vapour. Basic gaseous compound sensing principles are discussed within this paper.

Time-resolved luminescence and absorption in CdWO4

Abstract The luminescence and short-lived absorption are studied in CdWO 4 single crystals under pulsed laser beam as well as pulsed electron beam excitation. The luminescence spectrum and decay kinetics for CdWO 4 powder are studied. The short-lived absorption in single crystals arises from electron transitions from the luminescence center excited state to the conduction band. From luminescence and short-lived absorption spectra the energy depths of luminescence center excited and ground states are estimated.

Optical investigations of TlBr detector crystals

Shift of fundamental absorption edge, the position of main luminescence bands, the luminescence decay and transient absorption spectra in three TlBr crystals were studied. The γ-quanta detector made from TlBr crystals with similar transient absorption and luminescence parameters shows similar detector properties. The iodine impurity in TlBr was detected by optical methods. The role of impurities and crystal defects in γ-quanta detectors manufactured is discussed.

Excitonic luminescence in ZnO nanopowders and ceramics

Abstract Fast photoluminescence spectra in the spectral region of 3.1–3.45 eV in ZnO and ZnO:Al ceramics were studied at 14 and 300 K. Ceramics with grains smaller than 100 nm were sintered from nanopowders by high pressure (8 GPa) and low temperature (350 °C). Ceramics with grain sizes 1–5 μm were sintered at 1400 °C. It is shown that excitonic luminescence spectra depend on the ceramics grain size, post preparing annealing and doping. The excitonic luminescence decay time was faster than 2 ns and the afterglow at 30 ns was ∼0.05%.

Luminescence Properties of ZnO Nanocrystals and Ceramics

The luminescence excitation spectra, luminescence spectra and the nanosecond-scale decay kinetics were studied. The ZnO and ZnO:Al nanopowders were prepared by vaporization-condensation in a solar furnace using different raw powders: commercial, hydrothermal and those obtained by plasma synthesis. Exciton-phonon as well as exciton-exciton interaction processes in nanopowders, a bulk crystal and ZnO ceramics were studied and compared. The fast decay and low afterglow intensity of ZnO nanopowders and ceramics support these materials for scintillators.

The role of Fe and Cu dopants in electron–hole trapping and relaxation process in congruent LiNbO3

Abstract The transient optical absorption and kinetics of absorption decay is studied in undoped, Fe doped, and Cu doped LiNbO 3 crystals irradiated by pulsed electron beam. The 1.6 eV band of electron polaron trapped at antisite niobium was observed in all crystal samples. The nature of centers responsible for transient absorption is discussed. Electron polarons are shown to be less stable in LiNbO 3 :Fe compared with LiNbO 3 :Cu and undoped LiNbO 3 . It is suggested that a major part of electrons and holes created by irradiation are trapped in the vicinity of dopants.

Luminescence of Er/Yb and Tm/Yb doped FAp nanoparticles and ceramics

The nanoparticles of hydroxiapatite and fluorapatite doped with Er/Yb and Tm/Yb were synthesized and characterized by FTIR, XRD, SEM and TEM methods. The results of up-conversion luminescence studies were presented for the samples as prepared, annealed at 500°C and at 900-1000 °C. At annealing above 800°C the ceramic state was formed. It is shown that fluorapatite host is more appropriate than hydroxiapatite host for rare ions luminescence and up-conversion processes. The post preparing annealing of nanarticles significantly enhanced the luminescence intensity. The Tm/Yb doped fluorapatite shows intense up-conversion luminescence in 790-800 nm spectral region and is potentially useful for b…

Transient color centers in complex oxide crystals

The present work is devoted to investigation of induced by a pulsed electron beam (E = 0.25 MeV, τ = 10 ns) transient color centers in pure Gd3Ga5O12, YAlO3 and LiNbO3 single crystals. The relaxation of induced absorption of the crystals was studied within 0-5000 ns range. The origin and nature of transient color centers responsible for induced absorption was discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Induced optical absorption and ITS relaxation in LiNbO3

Abstract Electron pulse induced absorption spectra and their decay kinetics have been investigated in LiNbO3 crystals with various Li/Nb ratios rasnging from congruent to stoichiometric composition. It is shown that the absorption spectra, the optical density and the decay kinetics depend on the Li/Nb ratio. The rising front of the induced absorption is too fast to be resolved. The room temperature decay, characterized by the half-lifetime Δt, is faster in the stoichiometric crystal (∼ 750 ns) than in the congrruent crystal (∼ 3.5 μs).

Pulsed electron beam excited transient absorption in SrTiO3

Abstract Transient absorption (TA) spectra and optical density relaxation kinetics in SrTiO3 (STO), STO–Nb and STO–V were studied. The electron polaron free or bound at intrinsic defect is suggested to be responsible for the TA band at ∼1.4 eV observed in STO. The band peaking below 0.75 eV in STO–Nb is proposed to be from Nb4+. The electron center V3+ or electron polaron bound in the vicinity of V4+ is responsible for the absorption band at 1.0 eV in STO–V. The absorption from hole polarons is suggested to be in the range 1.8–2.3 eV. It is shown that TA in pure STO at room temperature decays due to electron–hole polaron recombination.

Surface processing of TlBr single crystals used for radiation detectors

Abstract The processing method for obtaining the high-quality surfaces of TlBr single crystals, providing removal of a mechanically destroyed surface layer by chemical etching, is developed. The crystals grown from the melt of purified materials by the Bridgman–Stockbarger method were used for the experiments. The Vickers microhardness as a structure-sensitive technique was used in a study of the crystal quality and properties of the plastically deformed surface layer created by cutting. It was shown that even under highly accurate conditions of cutting, the depth of the work-hardened surface layer with a high density of dislocations, vacancies and other structural defects exceeds 20 μm. Th…

Luminescence and Transient Absorption of CdWO4Crystals

Comparison of ZrO2:Y nanocrystals and macroscopic single crystal luminescence

The luminescence spectra of a tetragonally structured ZrO2:Y single crystal and nanocrystals were compared. It was found that the number of luminescence centers contributed to the spectra. The excitation of luminescence within the band gap region led to different luminescence spectra for the single crystal and nanocrystal samples, whereas recombinative luminescence spectra were the same for both samples. The origin of this difference is that in the nanocrystals, even under excitation within the band gap, charge carriers were created. Zirconium- oxygen complexes distorted by intrinsic defects were proposed to be the luminescence centres responsible for the wide luminescence band observed.

The time-resolved luminescence characteristics of Ce and Ce/Pr doped YAG ceramics obtained by high pressure technique

Abstract Transparent Ce and Ce/Pr doped YAG ceramics were prepared under high pressures (up to 8 GPa) and relative low temperature (450 °C). Grain size of the ceramics is less than 50 nm. However unknown defects or disorders strains on grain boundaries caused the additional absorption in these ceramics. The luminescence intensity, spectra and the decay time dependence on pressure applied during ceramic preparation were studied. Concentration of some intrinsic point defect was reduced under the high pressure applied for sintering process. It is shown that formation time of the excited state of Ce luminescence depends on the pressure applied during ceramic sintering.

Luminescence of ZnO nanopowders

Abstract The luminescence of ZnO nanocrystals prepared by different methods was studied under pulsed electron beam excitation. It is shown that the luminescence intensity depends on the nanocrystal sintering conditions and does not depend on the nanocrystal size within the range 10–50 nm. The relative luminescence intensities for the 3.32 eV (free exciton) and ∼3.20 eV (bound exciton) bands showed a dependence on nanocrystal size. The role of the nanocrystal surface in excitonic luminescence is discussed.

The energy transfer to the luminescence centers in PbWO4

Abstract The luminescence spectra, rise and decay kinetics and the yield of luminescence in undoped PbWO 4 crystals were studied after pulsed electron beam irradiation. The luminescence intensity rise observed after irradiation pulse showed two mechanisms—excitonic relaxation and electron–hole recombination—were involved in the formation of the excited states of the blue luminescence centers. It is proposed that the excited states of WO 3 and WO 2− 4 luminescence centers were formed from some intermediate metastable state.

Relaxation of electronic excitations in LiNbO3crystals

Transient absorption both in stoichiometric and Mg doped congruent LiNbO was observed after pulsed electron beam excitation. The luminescence spectra and decay kinetics in these materials show different excitonic relaxation possibilities. The dependence on sample stoichiometry is also discussed.

Optical properties and spectrometric performance of TlBr detector crystals

Abstract It is shown that preliminary TlBr crystals optical parameters testing can be used for detector crystal selection. Absorption in fundamental spectral region was used for the band gap width definition and determination of homological impurities presence. The luminescence was applied for impurity and defects content study. Radionuclide spectra 241Am were measured using the ionizing radiation detectors with planar electrodes made from TlBr crystals. The detector energy resolution for TlBr detectors produced from different row materials was summarized and the results of detector testing parameters and results of optical investigation were compared.

Luminescence and transient absorption in ZnWO4 and ZnWO4–Fe crystals

Abstract The transient absorption spectra and relaxation kinetics under pulse electron beam excitation in ZnWO 4 and ZnWO 4 –Fe, as well as luminescence spectra and decay kinetics, were studied. It is shown that a fraction of transient absorption is due to luminescence center excited state. The energy transfer from intrinsic excitations to the defect or impurity states is not efficient. The role of Fe-impurities in the decay process is discussed.

Time-resolved spectroscopy of ZnWO4

The luminescence spectra of ZnWO 4 excited by a pulsed electron beam, pulsed nitrogen laser and X-ray are studied. The luminescence decay kinetics are compared with transient absorption relaxation kinetics. Broad band transient absorption spectra are observed and it is shown that the slow decay component (∼14 μs at RT) of the transient absorption matches well with the luminescence decay therefore this absorption is due to the excited state absorption of the luminescence center.

The luminescence of ZnO ceramics

Abstract The luminescence properties of ZnO ceramics with grains 100–5000 nm sintered by different techniques from nanopowders were studied. The luminescence decay times were compared with that obtained for ZnO single crystal. The temperature dependence of non-exponential decay of defect luminescence (2.0–2.6 eV) was measured in wide time, intensity and temperature range. The luminescence decay kinetic at T ≤ 20 K shows the decay close to I(t) ∼ t−1 dependence. At temperature region 50–250 K the decay kinetics is more complicate since the TSL was observed in this temperature region. It is shown that the luminescence properties of NP and ceramics strongly depend on defect distribution on gra…

Intrinsic luminescence and energy transfer processes in pure and doped YVO 4 crystals

Luminescence properties of nominally pure and neodymium doped YVO4 single crystals have been studied by means of time-resolved luminescence spectroscopy technique. It was detected that, besides the main well-known blue excitonic emission band near 2.8 eV, an additional UV emission band at ∼3.1 eV appears in the spectra of intrinsic luminescence at low temperatures (∼80 K) for all crystals studied. Decay time of the UV band is much faster in comparison with the blue one and the UV band is quenched at temperatures above 150 K. Origin of the UV emission band is discussed. A significant rise front was observed in the relaxation kinetics of Nd3+ emission under band-to-band excitation. The time c…

Time-resolved optical absorption in YAlO3 crystals

Abstract The present work is devoted to the investigation of transient absorption (TA) induced by a pulsed electron beam (E=250 keV ) in pure and doped YAlO3 (YAP) single crystals. The nature of centers responsible for TA is discussed.