F-type Defects Research Articles

Accumulation of oxygen interstitial-vacancy pairs under irradiation of corundum single crystals with energetic xenon ions

Single crystals of α-Al2O3 with broad sides oriented perpendicular to the c crystal axis have been irradiated by 231-MeV xenon ions with fluence varying from 5×1011 to 1014 ions/cm2. The spectra of radiation-induced optical absorption (absorption of a pristine crystal is subtracted) have been decomposed into Gaussians serving as a measure of oxygen-related Frenkel defects (vacancy-interstitial pairs). The concentration of all structural defects considered – vacancy-type F and F+ centers as well as oxygen interstitials – continuously increases with ion fluence. Therefore, radiation-induced origin of elementary absorption bands at 5.6 and 6.6 eV tentatively ascribed earlier to charged and neutral oxygen interstitials has been proved for the first time. The concentrations of charged interstitials (in the form of superoxide ions) have been directly determined by the EPR method. The evolution of cathodoluminescence bands typical of self-trapped excitons (VUV band at 7.6 eV) and F-type defects (bands peaked around 3.0 and 3.8 eV) with the rise of Xe-ion-irradiation fluence has been measured and analyzed.

The annealing kinetics of the F-type defects in MgAl2O4 spinel single crystals irradiated by swift heavy ions

The annealing kinetics of the F and F+ color centers induced in MgAl2O4 single crystals under irradiation by 156-MeV 132Xe and 2.25-GeV 197Au ions are precisely measured via relevant optical absorption bands and analyzed in terms of the diffusion-controlled reactions between two pairs of anion Frenkel defects which determine thermal stability of irradiated material. It is demonstrated that two types of mobile oxygen interstitials (neutral and charged ones) are involved into recombination process with very different activation energies, which are practically independent on radiation fluencies.

Fabrication of ceramic scintillators by laser sintering: the case of Lu3Al5O12:Pr

The fabrication of ceramic scintillators by laser sintering is briefly reviewed and current limitations discussed. The experimental work focused on the fabrication and characterization of undoped and Pr-doped Lu3Al5O12 (LuAG). X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the structure of the sintered ceramics, with XRD results suggesting the absence of residual thermal stresses. Collectively, Raman results suggested the incorporation of Pr to affect the structure and its dynamics. Broadening the peaks of the ceramics in relation to those from the single crystal revealed the presence of structural disorder. Scanning electron microscopy revealed intergrain porosity thus explaining the lack of optical transparency. Energy-dispersive X-ray spectroscopy (EDX) measurements showed Pr to be homogeneously distributed. Radioluminescence measurements under X-ray excitation as a function of the temperature were used to investigate intrinsic defects of the host, including anti-sites and F-type defects.

Luminescence of F-Type Defects and Their Thermal Stability in Sapphire Irradiated by Pulsed Ion Beams

We studied the luminescence and the thermal stability of defects formed in α-Al2O3 single crystals after pulsed treatment with a beam of C+/H+ ions with an energy of 300 keV and a pulse duration of ~80 ns. By measuring optical absorption, photoluminescence, and pulsed cathodoluminescence, it is found that this type of exposure leads to intense generation of both single F and F+ centers and more complex defects (F2 aggregate centers or vacancy–impurity complexes) in α-Al2O3. The thermal stability of F-type defects formed in α-Al2O3 upon exposure to a pulsed ion beam is comparable to the stability of radiation-induced defects in neutron-irradiated samples.

Radiation-induced defects in sapphire single crystals irradiated by a pulsed ion beam

The luminescence and thermal stability of defects formed in α-Al2O3 single crystals after powerful (300 keV) pulsed irradiation with C+/H+ ion beam were investigated. It was found by measuring of optical density, photoluminescence, and pulsed cathodoluminescence that ion irradiation induces both single F-, F+-centers and F2-type aggregate centers. An intense thermoluminescence band with a complex shape was observed in the broad temperature range of 350–700 K, its intensity decreases with increasing of the energy density of the ion beam. The thermal stability of the F-type defects produced in α-Al2O3 after irradiation with a pulsed ion beam is comparable to that in neutron-irradiated samples. The appropriate kinetics of annealing of radiation-induced defects has been analyzed in terms of the diffusion-controlled bimolecular reactions between F-type centers and complementary interstitial oxygen ions. Thus, two important kinetic parameters – the migration energy of mobile interstitials and pre-exponential – have been evaluated and discussed.

Люминесценция дефектов F-типа и их термическая стабильность в сапфире, облученном импульсными ионными пучками

Luminescence and thermal stability of defects formed in alpha-Al2O3 single crystals under pulsed ion beam treatment (C+/H+ ions with an energy 300 keV, pulse duration 80 ns) were investigated. This type of irradiation leads to the intensive generation of both single F- and F+-centers and more complex defects (F2-type aggregate centers or vacancy-impurity complexes) in alpha-Al2O3. It was confirmed by the results of optical absorption, photoluminescence, and pulsed cathodoluminescence measurements. The thermal stability of F-type defects formed in alpha-Al2O3 under the pulsed ion beam treatment is comparable to the stability of radiation-induced defects in neutron-irradiated samples.

Accumulation of radiation defects and modification of micromechanical properties under MgO crystal irradiation with swift 132Xe ions

Accumulation of F-type defects under irradiation of MgO crystals by 0.23-GeV 132Xe ions with fluence varying by three orders of magnitude has been investigated via the spectra of optical absorption and low-temperature cathodoluminescence. The number of single centers continuously increases with fluence without any marks of saturation. At the highest fluence, a mean volume concentration of 3.1 × 1019 and 3.35 × 1019 cm−3 is reached for F and F+ centers, respectively. The F+ emission strongly dominates in the cathodoluminescence of irradiated MgO and its enhancement with fluence is detected. However, the creation efficiency of the F2 aggregate centers is very low and fluence dependence has a complicated shape. Radiation-induced changes of micro-mechanical properties of the same samples have been analysed; the depth profiles of hardening correlate with the ion energy loss. A joint contribution of ionization and impact mechanisms in the formation of structural defects under MgO irradiation with Xe ions is considered.

ESR AND RAMAN STUDIES OF CHEMICAL BATH DEPOSITED CdS : Ni FILMS

The CdS : Ni films were fabricated on glass substrates by chemical bath deposition method (CBD), where Ni concentrations are 0%, 10%, 20%, 30% and 40%. X-ray diffractometry (XRD), Raman spectroscopy and electron spin resonance (ESR) were employed to study the film structures. The XRD patterns revealed the presence of cubic CdS and trace of NiS . The Raman spectra were observed at 300 and 600 cm-1, corresponding to the first and second orders of the longitudinal optical phonon modes. Both results confirm that slightly lower order of crystallinity of CdS : Ni was found at the higher concentration of Ni . The ESR spectra showed the presence of F-type defects in CdS : Ni films. The band gaps of the samples were found to increase with the increase of Ni concentration.

Tailoring optical and electrical properties of MgO thin films by 1.5 MeV H+ implantation to fluences

Thin films of magnesia (MgO) with (100) dominant orientations were implanted with 1.5MeV H+ ions at room temperature to various fluences of 1013, 1014 and 1015ions/cm2. X-ray analysis unambiguously showed crystallinity even after a peak damage fluence of 1015ions/cm2. Rutherford backscattering spectrometry combined with ion channeling (RBS/C) was used to analyze radiation damages and defect distributions. Optical absorption band observed at 5.7eV in implanted films was assigned to the anion vacancies and the defect was completely disappeared on annealing at 450°C. Number of F-type defects estimated was 9.42×1015cm−2 for the film implanted with 1015ions/cm2. DC electrical conductivity of 4.02×10−4Scm−1 was observed in the implanted region which was three orders higher than the as-deposited films. In unison, film surface was modified as a result of the formation of aggregates caused by the atomic mixing of native matrix atoms (Mg and O) and precipitated hydrogen.

Oxidative Conversion of Propane in a Microreactor in the Presence of Plasma over MgO-Based Catalysts: An Experimental Study

In this work, oxidative cracking of propane was studied in a microreactor containing a catalyst. A dielectric barrier discharge (DBD) allows one to generate a cold microplasma, which activates propane forming radicals, at room temperature and atmospheric pressure. Homogeneous and well crystalline thin layers of MgO and Li/MgO catalysts, 25 μm thicknesses, were deposited in the microchannel using sol−gel method and by micropipetting. Li/MgO catalyst showed higher propane conversion and olefins selectivity compared to MgO. The latter one suggests that (i) radicals formed by DBD in gas phase are differently terminated depending on the catalyst surface and (ii) the surface of Li/MgO catalyst present more selective sites than MgO, such as [Li+O-] centers and F-type defects that are generated and able to react at room temperature. Surprisingly large amounts of products with higher molecular weight than propane, that is, C4, C4+ were also observed due to only C−C bond formation

Color center formation in α-Al2O3 induced by high energy heavy ions

Single crystals of alpha-alumina were irradiated at room temperature with 1.157GeV 56Fe, 1.755GeV 136Xe and 2.636GeV 238U ions to fluences range from 8.7×109 to 6×1012ions/cm2. Virgin and irradiated samples were investigated by ultraviolet–visible absorption measurements. The investigation reveals the presence of various color centers (F,F+,F22+,F2+andF2centers) appearing in the irradiated samples. It is found that the ratio of peak absorbance of F2 to F centers increases with the increase of the atomic numbers of the incident ions from Fe, Xe to U ions, so do the absorbance ratio of F22+ to F+ centers and of large defect cluster to F centers, indicating that larger defect clusters are preferred to be produced under heavier ion irradiation. Largest color center production cross-section was found for the U ion irradiation. The number density of single anion vacancy scales better with the energy deposition through processes of nuclear stopping, indicating that the nuclear energy loss processes determines the production of F-type defects in heavy ion irradiated alpha-alumina.

Site location and optical properties of Eu implanted sapphire

Synthetic colourless transparent (0001) sapphire crystals were implanted at room temperature with 100keV europium ions to fluences up to 1×1016cm−2. Surface damage is observed at low fluences, as seen by Rutherford backscattering spectrometry under channelling conditions. Optical absorption measurements revealed a variety of structures, most probably related to F-type defects characteristic of implantation damage. Thermal treatments in air or in vacuum up to 1000°C do not produce noticeable changes both in the matrix or the europium profiles. However, the complete recovery of the implantation damage and some redistribution of the europium ions is achieved after annealing at 1300°C in air. Detailed lattice site location studies performed for various axial directions allowed to assess the damage recovery and the incorporation of the Eu ions into well defined crystallographic sites, possibly in an oxide phase also inferred from optical absorption measurements.

Understanding the Synergistic Catalytic Effect between La2O3 and CaO for the CH4 Lean De-NOx Reaction: Kinetic and Mechanistic Studies

Doping of La(2)O(3) crystallites with Ca(2+) ions significantly enhances the intrinsic rate of NO reduction by CH(4) in the presence of 5% O(2) at 550 degrees C compared to pure La(2)O(3) and CaO solids, while the opposite is true after doping of CaO with La(3+) ions. It was found that the 5 wt % La(2)O(3)-95 wt % CaO system has one of the highest intrinsic site reactivities (TOF = 8.5 x 10(-3) s(-1)) reported at 550 degrees C for the NO/CH(4)/O(2) reaction among metal oxide surfaces. The doping process occurred after first dispersing La(2)O(3) and CaO crystallites in deionized water heated to 60 degrees C for 90 min, while the dried material was then ground and heated slowly in air to 800 degrees C and kept at this temperature for 5 h. The doping process had the effect of creating surface oxygen vacant sites (F-type defects) in the oxide lattices the concentration of which is a function of the wt % La(2)O(3) used in the mixed oxide system as revealed by photoluminescence and O(2) chemisorption studies. According to DRIFTS (15)NO transient isotopic experiments (SSITKA), oxygen vacant sites in Ca(2+)-doped La(2)O(3) promote the formation of a more active chemisorbed NO(x) species (NO(2)(-)) that contributes to the enhancement of reaction rate as compared to pure lanthana, calcium oxide, and La(3+)-doped CaO. These results were supported by the kinetic orders of the reaction with respect to NO and O(2) obtained as a function of wt % La(2)O(3) content in the mixed oxide system. Carbon dioxide (a reaction product) competes for the same oxygen vacant sites to form stable adsorbed carbonate-like species, thus lowering the reduction rate of NO. The dependence of the reaction TOF on the wt % La(2)O(3) loading at 550 degrees C was found to follow the trend of the dependence of photoluminescence intensity on the wt % La(2)O(3) content in the La(2)O(3)-CaO oxide system.

SURFACE MORPHOLOGY OF SUBMICRON CRYSTALS IN ALUMINUM NITRIDE FILMS GROWN BY DC MAGNETRON SPUTTERING

Aluminium nitride (AlN) thin films were fabricated on a glass substrate by reactive magnetron sputtering. Raman microscopy was then employed to follow the characteristics of their optical and acoustic phonon modes. At the optimal sputtering time of 30 minutes, the defect-induced first and second order Raman spectra were observed in 400–800 cm-1 band which were mostly related to the coating compositions. However, at the 30-, 60- and 90-minute sputtering, crystals of submicron size order of AlN were achieved. This could be clearly identified by the presence of Raman peak at 658–662 cm-1. Powder X-ray diffraction (PXRD) patterns revealed the development of (002) and (101) planes of hexagonal wurtzite AlN phase. The optimal average grain size measured by atomic force microscopy (AFM) is at 330 nm. It was found that the hardness was strongly dependent on roughness of the film, the maximum of which was achieved at 20.00 GPa. The presence of F-type defects in AlN films was investigated by X-band (~9.44 GHz) ESR spectrometer at 295 K. The ESR experiments were carried out by applying magnetic field perpendicular to AlN film, which showed the ESR six-peak multiplet signal at ~290 mT arising from superhyperfine interactions between nuclear spin I = 5/2 of 27 Al and electron spins trapped in nitrogen vacancies. The ESR signals are simulated and the ESR parameters are calculated. The vacancies are clearly randomly distributed as the ESR signals are independent of rotation angle (φ) about the normal of the film. All these results were analyzed and presented as a function of the deposition parameters and composition, and crystalline phases existed in the films.

Magnetic resonance investigations of LaCl 3 :Ce 3+ scintillators

In undoped and Ce 3+ -doped LaCl 3 EPR has been detected in the X-ray luminescence (XL-EPR) in K-band (25 v GHz) at 1.5 v K. Two excited triplet states with different EPR parameters and spectral shapes could be separated, both triplet states have been attributed to "out-of-plane" self-trapped excitons (STE) in LaCl 3 . No EPR signals of V K centres (self-trapped holes) could be detected in undoped or Ce 3+ -doped LaCl 3 after X-irradiation at low temperatures. X-irradiation of undoped LaCl 3 creates also an EPR spectrum which can be tentatively attributed to F-type defects. The scintillation mechanism is discussed.

Study of radiation defects for AlN ceramics under O+ irradiation

The behavior of radiation defects in AlN ceramics under O+ ion irradiation has been studied with in situ luminescence method. The intensity of luminescence emission had close relationship with absorbed dose. The aggregation of F-type defects in AlN has also been confirmed because of the occurrence of 470 nm emission peak. After O+ irradiation, the behavior of F-type defects was investigated by ESR measurement at 77 K. Influence of radiation defects on the characteristics of emission and ESR was discussed.

Aggregation of radiation defects in AIN ceramics under He+ ion irradiation

The aggregation of F-type defects in AIN has been observed for the first time under He+ ion irradiation by using in-situ luminescence measurement. The concentration of aggregation of F-type defects shows a strong dependence on irradiation temperature (300–773 K) under He+ ion irradiation. The mechanism of growth and annihilation of the aggregation of F-type defects was discussed.

Pretreatment effects on the active site for methane activation in the oxidative coupling of methane over MgO and Li/MgO

The effects of high temperature pretreatments on the activity of MgO and Li/MgO catalysts for the oxidative coupling of methane have been studied. The MgO powder catalyst exhibited a turnover frequency of 3.0×10−3 molecules/sites, at 990K, whereas the Li/MgO catalyst showed a turnover frequency of 7.0×10−2 molecules/sites, under the same reaction conditions. The initial C2 formation rate was observed to increase with pretreatment temperature over the MgO catalyst, supporting our previous proposal that F-type defects are responsible for methane activation.

Investigations of Active Sites for Methane Activation in the Oxidative Coupling Reaction over Pure and Li-Promoted MgO Catalysts

The oxidative coupling of methane to ethane over model MgO catalysts prepared under well-controlled, ultrahigh vacuum (UHV) conditions has been studied using a combination of surface science techniques and elevated-pressure kinetic measurements. The utilization of electron energy loss spectroscopy has allowed us to characterize a variety of defects and to identify the active sites which are responsible for the methane coupling reaction. The results indicate that [Li+O−] centers are not likely directly involved in the methane activation step, but rather promote the production of F-type centers in the near-surface region which are responsible for this key step in the methane coupling reaction. The origin of [Li+O−] and F-type defects and their densities and stabilities are discussed.

Electron-energy-loss-spectroscopy studies of thermally generated defects in pure and lithium-doped MgO(100) films on Mo(100).

Thermally generated defects in ultrathin MgO films on Mo(100) have been investigated using high-resolution electron-energy-loss spectroscopy (HREELS) in the 0--9-eV spectral region. The results indicate that the MgO films are thermally stable and nearly free from defects up to a temperature of 1100 K, above which defects in MgO are generated. The three distinct loss features observed at 1.15, 3.58, and 5.33 eV are due to electronic transitions associated with surface F centers, F aggregates, and F (or ${\mathrm{F}}^{+}$) centers, respectively. The addition of lithium into the MgO films produces [${\mathrm{Li}}^{+}$${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$] centers and promotes the production of F-type defects upon high-temperature treatment. The formation of the [${\mathrm{Li}}^{+}$${\mathrm{O}}^{+}$] centers is likely a consequence of the substitution of ${\mathrm{Li}}^{+}$ for ${\mathrm{Mg}}^{2+}$ in the magnesium oxide lattice. The present studies have demonstrated the capabilities of HREELS for studies of electronic transitions associated with a variety of defects in the near surface region.