Types Of Paramagnetic Defects Research Articles

The origin of bright cyan persistent luminescence in Ca2SnO4:La3+

The origin of bright cyan persistent luminescence in La3+-doped Ca2SnO4 is investigated by optical and electron paramagnetic resonance spectroscopy methods. The incorporation of La3+ ions in the material stimulates the formation of Sn2+ emission centers and charge trap centers. After exposure of the material to UV radiation, charge transfer processes to the trap centers result in the creation of two types of paramagnetic defects. Both defects are spin 1/2 systems with rhombic symmetry g-factor values, which correspond to electron and hole centers respectively. The role of the identified defects in the optical processes of the material is evidenced by a correlation of defect decay and persistent luminescence kinetics. Due to appropriate activation energies of the defects, light emission of Sn2+ centers is efficient and long-lasting. In this article, optimal lanthanum ion content is determined, the identity of the defects is discussed and a model of persistent luminescence in Ca2SnO4:La3+ is proposed.

EPR study of radiation-induced defects in carbonate-containing hydroxyapatite annealed at high temperature

Radiation-induced (γ or UV) paramagnetic defects in carbonate-containing hydroxyapatite (HAP) annealed at high (600–950 °C) temperature were studied by EPR. The complex spectra reveal the presence of different paramagnetic species. Their contributions were found to be strongly dependent on the annealing temperature as well as microwave power, thus, by the adjustment of experimental conditions some of the components can be eliminated that allowed to record EPR spectra caused by no more than two types of paramagnetic defects. All experimental spectra were analyzed using computer simulation. The parameters of the paramagnetic defects detected were determined, and the centers models were discussed. It was found that high-temperature annealing influences essentially the formation of radiation-induced defects in HAP. The СО33−, О− centers and oxygen vacancy VO− were shown to be the main stable γ-induced defects in the HAP annealed at high temperatures. New paramagnetic defect with the parameters g|| = 2.002, g⊥ = 2.0135 was detected and tentatively identified as an O-related radical. The γ-induced EPR response from СО33− radicals was found to be more intense than response from CO2− in non-annealed HAP. UV-irradiation was found to create smaller amounts of paramagnetic defects in comparison with γ-rays. Besides, oxygen vacancy VO− was not observed, while two other centers (СО3− and the center of unknown nature) appear in the UV-induced EPR spectra.

Deformation paramagnetic defects in Fz-29Si:P crystals

A new type of paramagnetic defects generated under plastic deformation (at 1223 K) in isotopically enriched floating-zone-grown silicon crystals Fz-Si:P (76% 29Si) are found. The anisotropic spectra of the electron-spin resonance of these defects testify that they are of doping origin and have the spin S = 1. The spectra of nuclear magnetic resonance are Pake doublets split by the spin-spin nuclear interaction and broadened by electron-nuclear dipole-dipole relaxation.

Light controlled magnetoresonant softening of γ-irradiated KCl:Fe crystals

Magnetoresonant softening of γ-irradiated KCl:Fe crystals controlled by visual light illumination was observed. Resonant magnetoplasticity detected by microhardness before illumination of the crystals is provided by two types of paramagnetic defects: (1) Fe2+-vc ion-vacancy pair with 7.0 and 3.5 g-factors, (2) Fe+ ions with g-factors equal to 2.2 and 4.1. Illumination of the γ-KCl:Fe crystals by λ=500–600 nm light causes disappearance of the electron paramagnetic resonance lines with 7.0 and 3.5 g-factors corresponding to Fe2+-vc resonances. The spectrum of the illuminated sample changes due to recombination of F-electrons with Fe2+-vc centers and their transformation to the Fe+ centers. Two lines with 2.2 and 4.1 g-factors corresponding to the Fe+ resonances were observed before and after light illumination.

Exchange coupled pairs of dangling bond spins as a new type of paramagnetic defects in nanodiamonds

Exchange coupled pairs of dangling bond spins as a new type of paramagnetic defects in nanodiamonds

Exchange coupled pairs of dangling bond spins as a new type of paramagnetic defects in nanodiamonds

EPR in detonation nanodiamonds (DND) reveals two different signals associated with intrinsic carbon inherited paramagnetic defects. Main carbon inherited EPR signal is narrow intensive Lorentzian-like singlet with g=2.0028 and spin concentration Ns=(6−7)×1019 spin/g that yields on average 13–15 spins per each DND particle. Additional chemical treatment of DND powder allows practically complete removal of trace amounts of transition metal impurities that reveals a new doublet EPR signal consisting of two relatively narrow lines within the half-field region (g∼4) separated by a distance of 10.4 mT. The intensity of the doublet signal is five orders of magnitude lower than that of the main singlet signal. The former signal has been observed in a wide variety of DND samples disregarding of the impurity level reached and thus may be attributed to some intrinsic defects in DND particles. Such half-field EPR signals correspond to “forbidden” ΔMs=2 transitions within thermally populated triplet (S=1) levels observed in polycrystalline samples containing exchange dimers—antiferromagnetically coupled spin pairs. Estimates suggest that the concentration of such defects is about one dimer per hundreds DND particles.

CW- and pulsed-EPR of carbonaceous matter in primitive meteorites: Solving a lineshape paradox

Insoluble organic matter (IOM) of Orgueil and Tagish Lake meteorites are studied by CW-EPR and pulsed-EPR spectroscopies. The EPR line is due to polycyclic paramagnetic moieties concentrated in defect-rich regions of the IOM, with concentrations of the order of 4 × 10 19 spin/g. CW-EPR reveals two types of paramagnetic defects: centres with S = 1/2, and centres with S = 0 ground state and thermally accessible triple state S = 1. In spite of the Lorentzian shape of the EPR and its narrowing upon increasing the spin concentration, the EPR line is not in the exchange narrowing regime as previously deduced from multi-frequency CW-EPR [L. Binet, D. Gourier, Appl. Magn. Reson. 30 (2006) 207–231]. It is inhomogeneously broadened as demonstrated by the presence of nuclear modulations in the spin-echo decay. The line narrowing, similar to an exchange narrowing effect, is the result of an increasing contribution of the narrow line of the triplet state centres in addition to the broader line of doublet states. Hyperfine sublevel correlation spectroscopy (HYSCORE) of hydrogen and 13C nuclei indicates that IOM centres are small polycyclic moieties that are moderately branched with aliphatic chains, as shown by the presence of aromatic hydrogen atoms. On the contrary the lack of such aromatic hydrogen in triplet states suggests that these radicals are most probably highly branched. Paramagnetic centres are considerably enriched in deuterium, with D/H ≈ 1.5 ± 0.5 × 10 −2 of the order of values existing in interstellar medium.

Positron annihilation study of defects and Si nanoprecipitation in sputter-deposited silicon oxide films

Positron annihilation spectroscopy was conducted to study defects and Si nanoprecipitation in sputter-deposited silicon oxide films. For as-deposited SiO 0.9 and SiO 1.9 films, Doppler broadening spectra are strongly influenced by the type of paramagnetic defects ( P b or E ′). However, the disappearance of these defects after annealing at 1050 °C in a vacuum cannot account for the corresponding change of the Doppler broadening. In annealed film of SiO 1.9, positronium formation is reduced from defect-free SiO 2 because of the presence of Si precipitates; meanwhile, in annealed film of SiO 0.9 with nanocrystalline Si (nc-Si), positron annihilation with high momentum electrons is enhanced as a result of efficient positron trapping at the nc-Si surface with oxygen.

Rapid thermal annealing effects on plasma deposited SiO x:H films

The bonding configuration, hydrogen evolution and defect content of rapid thermally annealed (RTA) SiO x :H films of different initial compositions were studied. Infrared absorption measurements showed that all the hydrogen present in the films was lost at temperatures lower than 700°C without any change in the oxygen to silicon ratio of films. RTA temperatures higher than 700°C promote a change in the Si–O–Si stretching position from the initial unannealed value to the 1070–1080 cm −1 range independent of the initial film composition. Electron Spin Resonance (ESR) measurements show that all the films contained two types of paramagnetic defects: E′ ( Si≡O 3) and D ( Si≡Si 3). Annealing up to 700°C promotes the disappearance of the E′ centre. For films where the D defect is present (all except the film with x≈2), the concentration of these defects initially decreases for annealing temperatures of 400°C, then continuously increases for temperatures up to 700°C, getting a saturation value in the 10 18–10 19 cm −3 range for higher temperatures. ESR characterisation suggests that annealing at higher temperatures promotes the formation of a high-quality SiO 2 matrix in which Si nanocrystals are formed, the D defects being located within these nanocrystals.

Photochemical reactions in GeO2-SiO2 glasses induced by ultraviolet irradiation: Comparison between Hg lamp and excimer laser.

${\mathrm{GeO}}_{2\mathrm{\ensuremath{-}}}$${\mathrm{SiO}}_{2}$ glasses prepared by vapor-phase axial deposition were exposed to ultraviolet (uv) radiation from a Hg discharge lamp (4.9 eV) and excimer lasers (KrF laser: 5.0 eV, XeCl laser: 4.0 eV). Two photochemical reaction channels were ascertained: (1) The exposure of the glasses to the Hg lamp radiation (\ensuremath{\sim}16 mW/${\mathrm{cm}}^{2}$) induced Ge E' centers accompanied by bleaching of the absorption band due to oxygen-deficient defects near 5 eV (5-eV band) and the emergence of an intense band near 6.4 eV. (2) The irradiation with KrF and XeCl lasers (power densities of 10 and 90 mJ/${\mathrm{cm}}^{2}$/pulse, respectively, pulse duration of 20 ns) generated two types of paramagnetic defects, electron trapped centers associated with fourfold coordinated Ge ions (GEC) and a self-trapped hole center (STH: bridging oxygen trapping a hole). The former and the latter were considered to be caused via one-photon and two-photon absorption processes, respectively. These alternative reactions proceeded independently depending on the power densities of uv photons. The formation of GEC's was saturated easily by irradiation with KrF laser pulses, and then the conversion of GEC to Ge E' centers was caused by prolonged irradiation.

Near Interface Oxide Degradation in High Temperature Annealed Si/SiO2/Si Structures

ABSTRACTDegradation of 430 nra thick SiO2 layers in Si/SiO2/Si structures which results from high temperature annealing (1320°C) has been studied using electron spin resonance, infra-red absorption spectroscopy and refractive index measurements. Large numbers of oxygen-vacancies are found in a region ≤ 100 nm from each Si/SiO2 interface. Two types of paramagnetic defects are observed following γ or X-irradiation or hole injection. The 1106 cm−l infra-red absorption associated with O interstitials in the Si substrate is found to increase with annealing time. The infra-red and spin resonance observations can be explained qualitatively and quantitatively in terms of a model in which oxygen atoms are gettered from the oxide into the under or overlying Si, the driving force being the increased O solubility limit associated with the anneal temperature.

On the nature of internal biasing fields in ferroelectrics

Types of paramagnetic defects appearing in the PZT-ceramic as a result of γ-irradiation, of stoichiometry disturbance and modification have been studied. Their contribution to the formation and the stability of the internal biasing field is discussed.

ESR spectra of ClO 2 and BrO 2 exchange coupled to molecular oxygen

Two types of paramagnetic defects in X-irradiated KClO 4 and KBrO 4 crystals, one of which was previously assigned to the species ClO 4 and BrO 4, are shown to have properties characteristic of weakly bound complexes of composition [XO 2,O 2] and [XO 2,2O 2], X = Cl, Br. The bond between XO 2 and O 2 is well represented in terms of an isotropic exchange interaction.