Identification of paramagnetic nitrogen centers (P1) in diamond crystallites synthesized via the sintering of detonation nanodiamonds at high pressure and temperature

Diamond single crystals synthesized from powder detonation nanodiamonds (DNDs) by means of treatment at high pressures (P ~ 7 GPa) and temperatures (T > 1300°C) have been studied by electron paramagnetic resonance (EPR). A key feature of treatment (high-pressure high–temperature (HPHT) sintering) is the use of low molecular weight alcohols in the process. The appearance of a hyperfine EPR signal structure due to “paramagnetic nitrogen” (P1 centers) is explained by the growth of submicron and micron diamond single crystals from DND nanocrystals by the oriented attachment and coalescence mechanism. Such growth and coarsening of crystals appreciably decreases the concentration of paramagnetic centers, the presence of which hinders the detection of a hyperfine structure in the EPR signal from P1 centers, in the near-surface areas of coalesced and grown together DND particles. It has been shown that the concentration of paramagnetic defects of all types decreases to ~3.1 × 1018 g^–1 (~60 ppm) during HPHT treatment at T = 1650°C. This causes the successful identification of P1 centers, whose fraction is no less than ~40% of the total amount of paramagnetic centers in microcrystals synthesized by HPHT sintering.     

Probing spin-orbit quenching in Cl (2P) + H2 via crossed molecular beam scattering

In our previous work we investigated electronically non-adiabatic effects in using crossed molecular beam scattering coupled with velocity mapped ion imaging. The prior experiments placed limits on the cross-section for electronically non-adiabatic spin-orbit excitation and electronically non-adiabatic spin-orbit quenching . In the present work, we investigate electronically non-adiabatic spin-orbit quenching for which is the required first step for the reaction of Cl^* to produce ground state HCl+H products. In these experiments we collide Cl (²P) with H₂ at a series of fixed collision energies using a crossed molecular beam machine with velocity mapped ion imaging detection. Through an analysis of our ion images, we determine the fraction of electronically adiabatic scattering in Cl^* +H₂, which allows us to place limits on the cross-section for electronically non-adiabatic scattering or quenching. We determine the following quenching cross-sections σ _quench(2.1 kcal/mol) = 26 ± 21 ?², σ _quench(4.0 kcal/mol) = 21 ± 49 ?², and σ _quench(5.6 kcal/mol) = 14 ± 41 ?².     

Transitions of microstructure and photoluminescence properties of the Ge/ZnO multilayer films in certain annealing temperature region

The Ge/ZnO multilayer films have been prepared by rf magnetron sputtering. The effects of annealing on the microstructure and photoluminescence properties of the multilayers have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectrometry and photoluminescence (PL) spectrometry. The investigation of structural properties indicates that Zn₂GeO₄ has been formed with (2 2 0) texture and Zn deficiency from Ge/ZnO multilayer films in the process of annealing. However, lower Zn/Ge ratio can be improved by annealing. The annealed multilayers show three main emission bands at 532, 700, and 761 nm, which originate from the transition between oxygen vacancy () and Zn vacancies (V_Zn), the radiative recombination of quantum-confined excitons (QCE) in Ge nanocrystals, and the optical transition in the GeO color centers, respectively. Finally, the fabrication of thin film Zn₂GeO₄ from Ge/ZnO multilayer films by annealing at low temperature provides another approach to prepare the green-emitting oxide phosphor film:Zn₂GeO₄:Mn.     

Dissymmetrization in X-irradiated RbTiOPO4 crystal

Electron paramagnetic resonance has been used to study the hole and electron paramagnetic centers formed in X-irradiated RbTiOPO₄, the crystals of the KTP family. X-irradiation of RbTiOPO₄ crystals at 77 K produced an oxygen hole center and four different trivalent titanium electron centers I₁, II, III and IV. Theg-tensors, their principal values and axes for the defects were calculated and compared with those for KTiOPO₄ centers. X-irradiation at 300 K produced another two oxygen hole centers and three electron centers I₁, I₂ and II. EPR spectra of the center II revealed dissymmetrization, i.e., irregular distribution of growth defects, between the physically equivalent sites lowering the point group symmetry of the local environment of paramagnetic centers Ti³⁺.     

Effect of point defect concentration in NaCl and LiF crystals on the saturation field of the magnetoplastic effect

The effect is studied of the calcium impurity concentration in NaCl crystals and of preliminary x-ray irradiation of NaCl and LiF crystals on the magnetic saturation field B₀ characterizing the transition from the conventional proportionality of the dislocation mean path length l to the magnetic induction B squared(l∝B²) to saturation (l=const). B₀ is shown to increase with the calcium concentration in NaCl crystals and with the dose of x-ray irradiation of NaCl and LiF. This finding indicates that the dislocation breakaway from local defects in weak magnetic fields is controlled by the mechanism of longitudinal spin relaxation in a system of radical pairs that form due to interaction between dislocation cores and paramagnetic centers.     

Luminescence of ZnO Having a Superstoichiometric Content of Oxygen

Using a unique method of radical-beam heteroepitaxy (RBHE) based on annealing of the crystals of the II–VI (ZnSe) compounds in a flow of radicals of a metalloid component — oxygen O — ZnO layers with a superstoichiometric content of oxygen were obtained. The conductivity of the layers = 10² ·cm, the mobility of the holes = 23 cm²/V·sec, and the concentration N _A = 10¹⁵ cm^–3. The luminescence spectra of pure ZnO single crystals and those doped with Li and Na, both treated by the RBHE method, are studied at helium temperatures. The luminescence centers associated with the intrinsic defects (V _Zn) are identified.     

Effect of halide ion concentration and irradiation dose dependent photostimulated luminescence investigation in the image screen phosphor CsBr1-xClx:Euy 2+

Photoluminescence(PL) characterization is carried out on CsBr_1-xCl_x:Eu_y²⁺ (x = 0.05, 0.1, 0.2, 0.3 y = 100 ppm, 200 ppm) crystals grown in vacuum with the Bridgman technique. PL studies show an increase in luminescence intensity with a decrease in bromide ion content. F(Br^–) and F(Cl^–) centers are formed due to -ray irradiation at room temperature. Photostimulated luminescence (PSL) emission is found to increase with an increase in irradiation dose from 7.5 Gy to 30 Gy at room temperature. From the results it is demonstrated that out of the different compositions studied, CsBr_0.9Cl_0.1:Eu²⁺ (200 ppm) phosphor has a linear PSL response with respect to irradiation dose.     

13C-selective infrared multiple-photon decomposition study of CBrClF2

The infrared multiple-photon single-frequency decomposition (IRMPD) of CBrClF₂ was examined as functions of laser wavenumber, laser fluence, and partial pressure of CBrClF₂. The initial step was the scission of a C-Br bond. In the presence of O₂ the carbon-containing product was CF₂O and its subsequent hydrolysis gave CO₂. The initial dissociation was highly ¹³C selective at wavenumbers below 1014 cm^–1. CBrClF₂ decomposed at relatively low fluences as compared to CHClF₂. However, the decomposition yield rapidly decreased with increasing pressure. In the large-scale irradiation experiment using about 8 J pulse at 1 Hz, we obtained a carbon yield of 0.41 mol per pulse at a ¹³C-atom fraction of 17% for a mixture of 10 Torr CBrClF₂ and 10 Torr O₂, and a carbon yield of 0.17 mol per pulse at a fraction of 29% for a mixture of 20 Torr CBrClF₂ and 20 Torr O₂. The IRMPD of CHClF₂ gave a carbon yield of 0.18 mol per pulse at 48% for 10 Torr neat CHClF₂ and yield of 0.25 mol at 52% for 20 Torr CHClF₂. The large-scale irradiation experiment was also carried out for mixtures of CBr₂F₂ and O₂. CHClF₂ is the most productive of ¹³C.     

Radiation-induced defects in annealed carbonate-containing hydroxyapatite

The effect of preliminary (before irradiation) annealing of synthetic carbonate-containing hydroxyapatite powders on the formation of paramagnetic centers under γ-ray and ultraviolet irradiation has been investigated. Annealing of the samples has been performed in the temperature range from 100 to 700°C. It has been found that electron paramagnetic resonance spectra of radiation-induced defects depend substantially on the annealing temperature. The paramagnetic centers CO ₂ ^? dominate in the samples annealed to 250°C (γ-ray irradiation) and 500°C (ultraviolet irradiation). In the samples annealed above 400°C, other defects, in particular, the O^? and CO ₃ ^3? centers, play a significant role. Annealing at some temperatures leads to an increase in the radiation sensitivity of the material. The observed effects can be associated with the escape of molecular water from the annealed hydroxyapatite samples and with the corresponding transformation of the defect subsystem of the material.     

Electrical properties and defect model of tin-doped indium oxide layers

Tin-doped In₂O₃ layers were prepared by the spray technique with doping concentrationsc _Sn between 1 and 20 at. % and annealed at 500 °C in gas atmospheres of varying oxygen partial pressures. The room-temperature electrical properties were measured. Maximum carrier concentrationsN=1.5×10²¹cm^–3 and minimum resistivities =1.3×10^–4 cm are obtained if the layers are doped withc _Sn9 at. % and annealed in an atmosphere of oxygen partial pressurep _O2 10^–20 bar. At fixed doping concentration, the carrier mobility increases with decreasing oxygen pressure. The maximum obtainable mobility can be described in terms of electron scattering by ionized impurities. From an analysis of the carrier concentration and additional precision measurements of the lattice constants and film thicknesses, a defect model for In₂O₃:Sn is developed. This comprises two kinds of interstitial oxygen, one of which is loosely bound to tin, the other forming a strongly bound Sn₂O₄ complex. At low doping concentrationc _Sn4 at. % the carrier concentration is governed by the loosely bound tin-oxygen defects which decompose if the oxygen partial pressure is low. The carrier concentration follows from a relationN=K ₁ ·p _O2 ^–1/8 ·(3 ×10¹⁰ × c_Sn –N)^1/4 with an equilibrium constantK ₁=1.4×10¹⁵ cm^–9/4bar^1/8, determined from our measurements.     

EPR investigation of the accumulation of F+ centers in irradiated MgO:Mn crystals

According to electron paramagnetic resonance spectra of MgO:Mn crystals irradiated in a reactor, the dependences of the concentration of F⁺ centers on the irradiation dose and on the content of impurities have been obtained. The strong influence of paramagnetic Mn²⁺ ions on the efficiency of the accumulation of F⁺ centers is determined.     

Oxygen agglomeration at111In in silver in the presence of radiation defects

The internal oxidation process of¹¹¹In in Ag has been studied by means of the perturbed angular correlation technique. A homogeneous¹¹¹In concentration of typically 0.05 at ppm has been produced by irradiating the Ag samples with 35 MeV -particles. In particular, the effects of solving oxygen before the irradiation and of radiation induced defects have been explored. It is found that oxygen stabilizes the radiation damage by several hundred degrees. Four molecule-type complexes have been identified, consisting each of one¹¹¹In atom, vacancies and oxygen atoms, among them one with cubic environment _Q =0. The formation and desintegration kinetics of these complexes have been studied as function of the oxygen concentration and the irradiation and measuring temperature. A model is suggested which explains the oxidation process as due to diffusing oxygen and oxygen-vacancy pairs trapped by¹¹¹In.     

Low temperature thermal properties and intrinsic defects in vitreous silica

Measurements of the low temperature specific heatC _p (0.04KT2.4K) and thermal conductivityk (0.5KT20K) of high purity vitreous silica (Suprasil W) in the as-received and electron-irradiated states are reported. In the temperature range where the localized excitations inherent in glasses dominateC _p (T0.5K) no change is observed upon electron irradiation of up to 10¹⁹e^– (total dose). An anomalyC around 1.8K is observed inC _p which is reduced by 40% upon irradiation. However, ask is not affected by electron irradiation, localized excitations as origin forC can be ruled out. The density of paramagnetic centers in the most heavily irradiated sample is 510¹⁷cm^–3 as obtained from the ESR signal. It has been suggested that these centers have diamagnetic precursors in the unirradiated glass. Our measurements indicate that those defects are not associated with the localized excitations.     

A comparison of the magnetic properties of proton- and iron-implanted graphite

In this work we have investigated the changes of the magnetic properties of highly oriented pyrolytic graphite samples after irradiation either with ~3×10¹⁴ protons or 3.5×10¹³ ... 3.5×10¹⁴ iron ions with energies in the MeV range. Our results show that iron and proton irradiations can produce similar paramagnetic contributions depending on the implantation temperature. However, only protons induce a ferromagnetic effect.     

Defects and impurities in nanodiamonds: EPR, NMR and TEM study

EPR, ¹³C NMR and TEM study of ultradisperse diamond (UDD) samples is reported. The compounds show a high concentration of paramagnetic centers (up to 10²⁰ spin/g), which are due to structural defects (dangling C-C bonds) on the diamond cluster surface. The anomalous reduction in the spin-lattice relaxation time of ¹³C (from several hours in natural diamond to ∼150 ms in UDD clusters) is attributed to the interaction between the unpaired electrons of the paramagnetic centers and nuclear spins. ¹³C NMR line-width reflects the fact that the structure of the UDD surface is distorted in comparison to the ‘bulk’ diamond structure.     

Positron annihilation in diamond,silicon and silicon carbide

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×10¹⁶/cm³. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes an increase in the amount of monovacancies retained.In porous silicon very long-lived positronium lifetimes in the range 40–90 ns are found. Polycrystalline diamond films contain various types of vacancy agglomerates but these are found to be inhomogeneously distributed from crystallite to crystallite. Electron irradiation of silicon carbide results in two vacancy-related lifetimes which are interpreted as resulting from carbon and silicon vacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Characterization of vacancy-related defects introduced into silicon during heat treatment by deep-level transient spectroscopy and gamma-ray diffraction techniques

Vacancy-related defects introduced into n-Si during annealing or aluminium diffusion at high temperature (1000–1250°C) have been studied. Different ambients (argon, nitrogen, vacuum and chlorine-containing atmosphere) were used to create a vacancy supersaturation during heat treatments. Three deep-level centers whose formation is governed by the presence of vacancies have been identified. They were characterized by the following temperature dependences of the thermal emission rate:e3 = 7.92 × 10⁷ T ² × exp(– 0.455/kT),e ₅ = 2.64 × 10⁶ T ² × exp( – 0.266/kT),e ₇ = 7.26 × 10⁶ T ² × exp (– 0.192/kT). The influence of different factors, such as heat-treatment conditions, concentration of oxygen and doping level in initial crystals, on center formation was studied. An asymmetric diffuse-ray scattering was observed near the surface of a crystal irradiated by thermal neutrons and annealed in a chlorine-containing atmosphere. This scattering is related to the formation of structural defects of the vacancy type. In the same region of the crystal, the concentration of the E7 center was one order of magnitude higher than that of other deep-level centers. Comparison of the-ray diffraction and deeplevel transient spectroscopy (DLTS) data suggests that the formation of the center occurs under the conditions of Si supersaturation with vacancies.     

Nitrogen-containing species in the structure of the synthesized nano-hydroxyapatite

Synthesized by the wet chemical precipitation technique, hydroxyapatite (HAp) powders with the sizes of the crystallites of 20–50 nm and 1 μm were analyzed by different analytical methods. By means of electron paramagnetic resonance (EPR) it is shown that during the synthesis process nitrate anions from the reagents (byproducts) could incorporate into the HAp structure. The relaxation times and EPR parameters of the stable axially symmetric NO ₃ ^2? paramagnetic centers detected after X-ray irradiation are measured with high accuracy. Analyses of high-frequency (95 GHz) electron-nuclear double resonance spectra from ¹H and ³¹P nuclei and ab initio density functional theory calculations allow suggesting that the paramagnetic centers and nitrate anions as the precursors of NO ₃ ^2? radicals preferably occupy PO ₄ ^3? site in the HAp structure.     

Concentration of Cr<Superscript>4+</Superscript> impurity ions and color centers as an indicator of saturation of forsterite crystals Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> with oxygen

The influence of the oxygen partial pressure P _{O 2} in the growth atmosphere on the coefficient of chromium distribution between the crystal and the melt of forsterite, the Cr³⁺ and Cr⁴⁺ ion contents in crystals, and the concentration of color centers induced by irradiation has been investigated. It has been established that the crystals grown at low oxygen partial pressures P _{O 2} (0.01–0.05 kPa) are characterized by low concentrations of Cr⁴⁺ ions and color centers. A change in the oxygen partial pressure to P _{O 2} ∼ 0.85 kPa leads to an increase in the Cr⁴⁺ center concentration by a factor of ∼10 and in the color center concentration by a factor of ∼5. A further increase in the oxygen partial pressure to P _{O 2} to 12 kPa remains the concentration of these centers almost unchanged. A model has been proposed according to which the intrinsic defects formed under conditions of a relative excess of oxygen leads to both the self-oxidation of chromium and the formation of color centers in the forsterite crystals under irradiation.     

Low-field anomalous muonium depolarization in isotopically enriched germanium

The depolarization rate of anomalous muonium, Mu^*, in germanium isotopically enriched in⁷⁴Ge (I=0) was measured as a function of field. The concentration of⁷³Ge (I=9/2) was about 9 times less than natural abundance. The depolarization rate at 10 K in this isotopically enriched crystal for both lines of those Mu^* centers whose symmetry axes make an angle of 90° to the field is less than 1sec^–1 at all fields down to the lowest one measured, 14.5 gauss. This is in sharp contrast to the wide lines reported at low field in germanium having natural isotopic abundance. The spectrum of Mu^* in the isotopically enriched Ge crystal was also seen at zero field. These results confirm that the increased depolarization rate for Mu^* at low fields arises from unresolved nuclear hyperfine structure. The depolarization rates observed were consistent with an average hyperfine interaction with a single⁷³Ge nucleus of 2.5 MHz, a value requiring nearly 1% of the spin density to be on a typical atom.