Impurity Luminescence Research Articles

Intrinsic and impurity luminescence of rare earth ions doped KYF4 nanophosphors

The KYF4 nanopowders, non-doped and doped with Ce3+ or Tb3+, having well-crystallized, unaggregated, monodisperse (±15%) nanoparticles with the cubic (the size in the range from ∼15 to ∼30 nm) or hexagonal (from ∼30 to ∼50 nm) crystal structure have been successfully synthesized by microwave-hydrothermal treatment of as-precipitated gels. In KYF4 hexagonal nanopowders an intense STE-type luminescence at ∼4.4 eV was observed which is not quenched at room temperature. In contrast to single crystals or cubic nanopowders, in KYF4 hexagonal nanopowders doped with Ce3+ or Tb3+, a rather efficient energy transfer is observed from the host to Ce3+ or Tb3+ ions, respectively, because of overlapping the emission spectrum of STE-type luminescence and the spectrum of efficient absorption on 4f-5d transitions in Ce3+ or Tb3+.

Electron levels and luminescence of dislocation networks formed by the hydrophilic bonding of silicon wafers

Local energy levels produced by dislocations at the interface between bonded n- and p-Si wafers are studied by deep level transient spectroscopy and by a new technique for the detection of impurity luminescence, induced by the occupation of electron states upon the application of electric pulses (the pulsed trap-refilling-enhanced luminescence technique). It is established that only the shallow levels of the dislocation network, with activation energies of about 0.1 eV, are responsible for the D1 dislocation-related luminescence band in both n- and p-type samples. The occupation of deep levels has no effect on the D1-band intensity. A model of coupled neutral trapping centers for charge carriers is proposed. In this model, the difference between the energy position of the D1 band (0.8 eV) and the corresponding interlevel energy spacing (0.97 eV) is attributed to the Coulomb interaction between charge carriers trapped at the levels.

Study of the impurity photoconductivity and luminescence in ZnSe:Ni crystals in the visible spectral region

The photoconductivity and photoluminescence spectra of ZnSe:Ni crystals in the visible spectral region are studied. It is established that the high-temperature impurity photoconductivity of ZnSe:Ni crystals is controlled by the optical transitions of electrons from the ground state 3T1(F) to high-energy excited states, with subsequent thermally activated transitions of electrons to the conduction band. A photoconductivity band associated with the photoionization of Ni impurities is revealed. The intracenter luminescence of ZnSe:Ni crystals is efficiently excited with light corresponding to the intrinsic absorption region of Ni2+ ions.

Temperature dependence of the luminescence of Li6Gd x Y1 − x (BO3)3:Eu crystals

The luminescence and recombination processes in crystals of lithium borates Li6Gd x Y1 − x (BO3)3:Eu (LGYBO:Eu) have been studied. The photoluminescence (PL) spectra upon selective photoexcitation to lower excited states of Gd3+ and Eu3+ ions and the temperature dependences of the photoluminescence intensity at different energies of excitation photons have been measured in a wide temperature range: 10–500 K. We observed a photoluminescence band at 3.97 eV, which is due to the 6 P J → 8 S 7/2 transitions in Gd3+ ions, and a characteristic line spectrum in the range of 1.6–2.2 eV, which is related to the radiative f-f transitions in impurity Eu3+ ions, occurring mainly from the lower excited 5 D 0 level to the 7 F J states (J = 0, 1, …, 6). The influence of the O-Eu charge transfer states and the vibrational relaxation between the 6 I J and 6 P J levels of the Gd3+ ion on the temperature dependences of the intrinsic and impurity luminescence intensities is discussed.

Photoluminescence of Al2O3 nanopowders of different phases

Photoluminescence was studied in six samples of Al2O3 nanopowders produced from the same initial material by calcination in the 800–1400°C temperature range. At temperature around 1200°C phase transition in aluminum oxide lattice occurs; the samples produced at temperatures up to 1200°C contain mainly δ phase, while those obtained at 1400°C contain pure α phase. In all studied samples of nominally pure aluminum oxide nanopowders photoluminescence is determined by trace level concentrations of uncontrolled impurities. It was found that phase transition is accompanied with modification of the emission spectrum: a broad band centered around 750nm presumably ascribed to emission of Fe3+ ions is characteristic for photoluminescence of the samples of δ phase, while narrow band emission of Mn4+ is observed in the samples of α phase. Aside from that emission of Cr3+ ion is observed in all studied samples with the difference that intensity, position and shape of emission bands are characteristic either to transient forms or to α phase of aluminum oxide. Switching of the active luminescence centers in the samples of the same composition with phase transition is tentatively explained by change of the crystal field symmetry affecting probability of electron transitions in impurity centers. An assumption is done about the decisive role of surface hydroxyl groups in energy transfer to impurity luminescence centers.

Pulsed cathodoluminescence of nanoscale aluminum oxide with different phase compositions

The methods of pulsed cathodoluminescence have been used to study compacted powders and ceramics containing different phases of aluminum oxide. An intensive luminescence of the samples under study in the visible, NIR, and UV regions of the spectrum has been found. The luminescence bands are very broad and include a few components. The number of the bands depends on the phase composition of the samples. The oxygen vacancies, which capture one or two electrons, produce luminescence centers in the near UV region. The most probable in the visible region is the luminescence of aggregate defects, impurities, and surface centers.

Time-resolved luminescence spectroscopy of LiF(U) crystals

We studied the luminescence spectra and decay kinetics of LiF(U) crystals excited by pulsed laser radiation (λ = 337 nm) in temperature range of 15–300 K. Two groups of lines with different exponential decay times are revealed in spectra. Based on comparative analysis of fast and slowly decaying luminescence spectra, temperature dependences of decay time constants, and line intensities, we conclude that electronic transitions from radiation levels of the impurity luminescence center can occur, not only in a vibrationally relaxed state, but also in a state with an excited high-frequency mode in O-U-O bonds.

Pre-evaluation method for the spectroscopic properties of YAG bulk materials by sol-gel synthetic powder

The sol-gel synthetic powder evaluation method is the most effective and the easiest technique for pre-evaluating the spectroscopic properties of luminescent impurity doped YAG. The luminescent elements doped sol-gel powder agrees well with that of a single crystal or ceramic YAG regarding the luminescent spectra, the intensity, and the lifetime, even though the powder sintered at a low temperature of 1100°C in comparison with the YAG crystal melting point of 1970°C, because a good crystallinity of the sol-gel powder is led by the hard agglomerates of very small primary particles. It is considered that the significance of this method is to apply for not only of YAG but also of other oxides, for instance Y2O3, ZrO2, TiO2, and MgAl2O4

Photoluminescence and photostimulated luminescence of oxygen impurities in CsBr

Oxygen impurities have been detected in undoped CsBr by photoluminescence (PL) spectroscopy and their contribution to photostimulated luminescence (PSL) properties of powdered CsBr is discussed. When excited at 200 nm, PL is observed from CsBr which consists of three separate emission peaks. The intrinsic luminescence of CsBr at 379 nm is accompanied by two emission peaks at 395 and 460 nm which arise from oxygen impurities, the latter of which is also PSL active following x-irradiation. Sintering of CsBr with the reducing agent NH4Br removes the oxygen impurities so that the 395 and 460 nm emissions are no longer detectable, and subsequently the PSL emission is significantly reduced. PSL storage time measurements of these materials show that oxygen impurities favorably increase both the PSL sensitivity and radiation induced charge-center stability in CsBr:Eu2+. The oxygen impurities and their associated luminescence properties can be reintroduced to the CsBr matrix with a controlled concentration by doping with CsOH and then subsequently sintering the resultant CsBr:OH− with NH4Br, which in this system reduces the OH− centers to O2−.

On temperature dependence of impurity absorption and luminescence bands in dielectric crystals

In terms of the adiabatic approach to determining the states of an impurity dielectric crystal, the temperature dependence of the first moment (maximum) of the optical band that arises upon vibronic transitions in a local center is studied. It is shown that the frequency of the band maximum and absolute temperature are linked with each other by a relation that is analogous to the Wien’s displacement law for equilibrium radiation. This result is consistent with experimental data obtained, in particular for F absorption and luminescence bands in alkali halide crystals.

Theoretical analysis of the static quenching of optical excitations in luminescent nanoparticles

A compact analytical expression has been obtained for the kinetics of the donor-acceptor transfer of the energy and the quenching of optical excitations in spherical nanoparticles with luminescent impurities. A series of numerical experiments simulating the process have been performed using the Monte Carlo method for nanoparticles of various sizes and concentrations of acceptors. The analytical expression well describes the numerical simulation results.

10.1007/s11449-008-3009-2

Luminescence properties of a pure CdWO4 crystal and a CdWO4:Mo crystal doped with molybdenum in different concentrations have been investigated. The effect of molybdenum impurity on the intrinsic luminescence of CdWO4 has been found, and the role of the impurity in the formation of new luminescence centers has been investigated. The features of the formation of the intrinsic and impurity luminescence excitation spectra of CdWO4 and CdWO4:Mo crystals in the fundamental-absorption region have been considered. The reflection spectra of the CdWO4 crystal have been investigated taking into account the crystal structure anisotropy.

Blinking Behavior of Surface-Defect and Impurity Luminescence in Nondoped and Mn2+-Doped CdS Nanocrystals

We report on the photoluminescence (PL) blinking behavior of nondoped and Mn-doped CdS/ZnS nanocrystals studied using single nanocrystal spectroscopy at room temperature. The blinking phenomenon is clearly observed in the surface-defect luminescence of nondoped single CdS/ZnS nanocrystals. In Mn-doped CdS/ZnS nanocrystals, however, PL blinking is markedly suppressed, and the Mn 2+ -related PL has a long on -time. Our findings indicate that Mn 2+ ions are excited by fast energy transfer from photoexcited excitons in both neutral and ionized CdS/ZnS nanocrystals. We discuss energy transfer processes from the host nanocrystals to the Mn 2+ ions.

Pre-evaluation method for the spectroscopic properties of YAG bulk materials by sol–gel synthetic YAG powder

We propose a convenient method for pre-evaluating the spectroscopic properties of luminescent impurity doped YAG by sol–gel synthetic powder. Three synthetic methods, the sol–gel, normal-strike, and reverse-strike methods, are examined for their ability as a predictor and the sol–gel method proved the most effective of the three in experiments on crystallinity, particle size, and spectroscopic properties. The sol–gel Nd:YAG powder agrees well with that of a single crystal or ceramic YAG regarding the luminescent spectrum, the intensity, and the lifetime, even though the powder sintered at a low temperature of 1100 °C in comparison with the YAG crystal melting point of 1970 °C. These facts suggest that the sol–gel synthetic powder is the most effective and the easiest technique for evaluating spectroscopic properties of bulk YAG materials before fabricating the final materials.

Dynamic fluorescence quenching and the highest excited states of molecules

The dynamic fluorescence quenching in organic molecules, or quenching of the second kind according to Vavilov’s classification, is an efficient method of investigating excited states in solutions and is widely used in various fields. The effect of quenching on the intensity of the fluorescence from the first and higher singlet states of organic molecules is studied. The results may serve as a basis for determining the nature of the short-wavelength luminescence and can be used to distinguish the Sn fluorescence from the comparably intense luminescence of impurities, which is a very important problem when investigating such emissions. A method for obtaining dynamic quenching by specially chosen quenchers is proposed. The method is based on an experimentally found strong increase in the constants of bimolecular collisions of luminophore and quencher molecules when the luminophore is excited through the highest singlet states.

Optical and luminescence properties of CdWO4 and CdWO4:Mo single crystals

Luminescence properties of a pure CdWO4 crystal and a CdWO4:Mo crystal doped with molybdenum in different concentrations have been investigated. The effect of molybdenum impurity on the intrinsic luminescence of CdWO4 has been found, and the role of the impurity in the formation of new luminescence centers has been investigated. The features of the formation of the intrinsic and impurity luminescence excitation spectra of CdWO4 and CdWO4:Mo crystals in the fundamental-absorption region have been considered. The reflection spectra of the CdWO4 crystal have been investigated taking into account the crystal structure anisotropy.

Synthesis and characterization of ZnGeN 2 grown from elemental Zn and Ge sources

Polycrystalline ZnGeN 2 was synthesized by a new method, via vapor growth from Zn and Ge with thermal cracking of NH 3. Measured lattice parameters indicate a monoclinic structure with a= b=0.3157±0.0024 nm, c=0.5137±0.0021 nm, and γ=119°30′±22′, consistent with previous reports. Planar stacking faults lying on the (0 0 1) plane were identified by transmission electron microscopy. Analysis indicates that these stacking faults are bound by a prismatic dislocation loop having in-plane and out-of-plane Burgers vector components of ( a/3[1 1¯ 0]) and ( c/2[0 0 1]), respectively. Photoluminescence spectroscopy at low temperature (∼4 K) showed broad “yellow-band” impurity luminescence centered around 2.6 eV and a narrower near-band-edge luminescence peak at 3.40±0.01 eV. The near-band-edge luminescence peak is higher in energy by approximately 50 meV, and higher in efficiency by at least two orders of magnitude than any previously reported. These observations indicate that this material has superior optical quality than previously obtained by other growth methods. The temperature dependence of the near-band-edge luminescence efficiency is consistent with excitons bound to a shallow donor band.

Formation mechanism and photoluminescence of AlN nanowhiskers

Aluminium nitride (AlN) nanowhiskers were synthesized on Si substrate by an oxide-assisted vapour transport and condensation method at 900 °C. Electron scanning microscope observations reveal that the whisker-like nanostructured architectures are composed of nanotips growing out from the surfaces of central sphere-like cores. The stem diameter of the nanotip is ∼100 nm and decreases with increasing distance from the core. The formation mechanism of the nanowhiskers is discussed on the basis of structural characterization. It is inferred that Fe2O3 plays an important role in the relatively low temperature growth of the AlN nanowhiskers. The photoluminescence of the nanowhiskers shows a broad blue luminescence band which can be Gaussian divided into three subbands. Spectral examinations and analysis reveal that the three subbands are connected with nitrogen vacancy, oxygen impurity and surface luminescence, respectively. The successful growth of AlN nanowhiskers would provide significant applications in modern optoelectronic and luminescence nanodevices.

Some aspects of radiation resistance of wide-gap metal oxides

Wide-gap oxides drastically differ in radiation resistance against nonimpact mechanisms of defect creation depending on the ratio between the values of the energy gap E g and the formation energy of a pair of Frenkel defects (FD) E FD . Materials with E g > E FD are radiation-sensitive even at a low excitation density, while the efficiency of FD creation in the materials with E g < E FD is noticeable only under a high excitation density or in the presence of impurity centers serving as the promoters of radiation damage due to the nonimpact mechanisms. Novel experimental results on the FD creation in the bulk of MgO single crystals ( E g < E FD ) irradiated by swift uranium ions at 300 K and 5 keV electrons at 6 K are presented. The prospects of luminescent protection against radiation damage as well as of the decrease of the luminescence efficiency due to the suppression of nonradiative recombination of electrons and holes (both relaxed and nonrelaxed) by doping the material with a sufficient amount of luminescent impurity ions are considered on the example of spectral transformers for plasma display panels.

Synthesis and impurity doping of GaN powders by the two‐stage vapor‐phase method for phosphor applications

AbstractThe analysis of particle formation and the doping of luminescent impurities during the two‐stage vapor‐phase synthesis of GaN powder were carried. GaN particles were grown very fast during the second stage of this method, and the increment in particle size was larger for higher reaction temperature in the region between 800 and 1000 °C. The analysis on the behaviour of particle growth based on the reaction kinetics suggested that the growth almost finishes in a few seconds with an extremely high rate at the early stage at 1000 °C, whereas the growth lasts with relatively low rates for a time longer than the actual growth duration for the case of lower temperature synthesis. GaN powders doped with various impurity atoms were synthesized by supplying impurity sources with GaCl during the second stage. The samples doped with Zn, Mg and Tb showed emissions characteristic for each doped impurity. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)