Nuclear magnetic resonance study of ultrananocrystalline diamonds

We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique. Analysis of the ¹³C and ¹H NMR spectra, spin-spin and spin-lattice relaxation times in purified UNCD samples is presented. Our measurements show that UNCD particles consist of a diamond core that is partially covered by a sp ²-carbon fullerene-like shell. The uncovered part of outer diamond surface comprises a number of hydrocarbon groups that saturate the dangling bonds. Our findings are discussed along with recent calculations of the UNCD structure. Significant increase in the spin-lattice relaxation rate (in comparison with that of natural diamond), as well as stretched exponential character of the magnetization recovery, are attributed to the interaction of nuclear spins with paramagnetic centers which are likely fabrication-driven dangling bonds with unpaired electrons. We show that these centers are located mainly at the interface between the diamond core and shell.     

F NMR and EPR study of fluorinated buckminsterfullerene C60F58

Solid state ¹⁹F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of fluorinated buckminsterfullerene C₆₀F₅₈ have been carried out. The temperature dependence of the line width and the spin-lattice relaxation time show hindered molecular motion with the activation energy of ΔE_a=1.9 kcal/mol. Neither phase transition nor random rotation of C₆₀F₅₈ have been obtained. The spin-lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C-C bonds yielding localized unpaired electrons. Such broken bonds are caused by C-C bond rupture in a cage-opened structure of hyperfluorinated species.     

A study of the firing temperature of archeological pottery by X-ray diffraction and electron paramagnetic resonance

A fragment of an archeological funerary urn from Campos dos Goytacazes, Brazil, was studied using electron paramagnetic resonance (EPR) and X-ray diffraction (XRD). The thermal stability of all paramagnetic species was studied with isothermal treatment. In the present study, the iron signal (Fe³⁺) cannot be used as a firing temperature reference for archeological pottery. The intensification of this signal with temperature is a consequence of Fe²⁺ oxidation, but this reaction occurs in a short-lived treatment at high temperature or in an extended treatment at lower temperature. However, the iron signal and three other paramagnetic species indicate that the urn was fired for an extended time (up to three days). The thermal stability of the three paramagnetic species indicates a firing temperature of around 500 °C in the inner layer, between 400 and 500 °C in the middle layer, and between 500 and 800 °C in the outer layer. The presence of kaolinite structures only in the middle portion is consistent with the temperature values estimated. A firing method for the funerary archeological urn is suggested.     

A quantum computer based on NV centers in diamond: Optically detected nutations of single electron and nuclear spins

In an effort to realize a two-bit processor for a quantum computer on the basis of single nitrogen-vacancy defect centers (NV centers) in diamond, the optically detected nutations of the electron spin of a single NV center in the ground state and of the nuclear spin of a ¹³C atom located at a diamond lattice site nearest to the NV center are studied. The photodynamics of NV and NV + ¹³C centers under different temperatures and optical excitation conditions is discussed. A seven-level model of a center excited by radiation from an Ar⁺ laser at room temperature is proposed. On the basis of this model, the experimental spectra of optically detected electron paramagnetic and electron-nuclear double resonances of single NV and NV + ¹³C centers in diamond nanocrystals, as well as experimental data on the optically detected nutations of the electron and nuclear spins of these centers caused by the actions of pulsed microwave and radiofrequency fields, respectively, are interpreted.     

Synthesis, crystal structure, Cu doped EPR and voltammetric studies of bis[N-(2-hydroxyethyl)ethylenediamine]zinc(II) squarate monohydrate

Crystal structure of [Zn(hydet-en)₂]·C₄O₄·H₂O (ZHES) (hydet-en is N-(2-hydroxyethyl)ethylenediamine) complex has been synthesized and characterized by analytical, spectroscopic (IR, UV/Vis) and voltammetric techniques. After doping Cu²⁺ ion, its magnetic environment has been identified by electron paramagnetic resonance (EPR) technique. The title complex crystalizes in monoclinic system with space group P2₁/c and with Z=4. Each hydet-en ligand acts as a tridentate ligand through the two N atoms and the hydroxyl O atom, resulting in a six coordinate Zn(II) ion. The EPR spectra were recorded in three perpendicular planes of Cu²⁺ doped ZHES single crystal. The calculated g and A values indicated that the paramagnetic center is rhombic symmetry with the Cu²⁺ ion having distorted octahedral environment. The molecular orbital bond coefficients of the Cu(II) ion in d⁹ state is also calculated by using EPR and optical absorption parameters. The dianion SQ²⁻ is oxidized reversibly in two consecutive steps to the corresponding radical monoanion and neutral form.     

Optical properties of nanodiamond layers

Thin ultradisperse diamond (UDD) layers deposited from a water suspension are studied by optical and x-ray photoelectron spectroscopy (XPS). The effective band gap determined by the 10⁴-cm^?1 criterion for ozone-cleaned UDD is 3.5 eV. The broad structureless photoluminescence band (380–520 nm) is associated with radiative recombination through a system of continuously distributed energy levels in the band gap of diamond nanoclusters. The optical absorption of the material at 250–1000 nm originates from absorption on the disordered nanocluster surface containing threefold-coordinated carbon. The surface of UDD clusters subjected to acid cleaning contains nitrogen-oxygen complexes adsorbed in the form of NO ₃ ^? nitrate ions. Annealing in a hydrogen atmosphere results in desorption of the nitrate ions from the cluster surface. The evolution of the oxygen (O1s) and nitrogen (N1s) lines in the XPS spectra under annealing of a UDD layer is studied comprehensively.     

Electron paramagnetic resonance and thermoluminescence study of Ag ions in Li2B4O7 crystals

Electron paramagnetic resonance (EPR) is used to investigate the effects of ionizing radiation on Ag-doped lithium tetraborate (Li₂B₄O₇) crystals. Two similar, yet distinct, trapped-hole centers (Ag²⁺ ions substituting for Li⁺ ions) are produced by 60 kV x rays. One Ag²⁺ ion, labeled Center A, has no nearby defects and the other Ag²⁺ ion, labeled Center B, has a neighboring impurity which is most likely a Ag⁺ ion substituting for a Li⁺ ion. The production and thermal decay properties of the two Ag²⁺ ions are described and their g matrices and ¹⁰⁷Ag and ¹⁰⁹Ag hyperfine matrices are obtained from the EPR angular dependences. The principal values of the g matrices are similar for the two centers, but the hyperfine principal values differ significantly (Center B has smaller values than Center A). There are also differences in the directions of the principal axes for the two centers. Together, these results imply (1) that the unpaired spin is less localized for Center B and (2) that the ground-state positions of the neighboring oxygen ions are different for Centers A and B. This explains why the peaks of the Ag²⁺ charge-transfer photoluminescence bands associated with Centers A and B occur at different wavelengths (502 and 725 nm, respectively). An isochronal pulsed thermal anneal shows that these radiation-induced Ag²⁺ ions serve as the recombination site for the intense thermoluminescence peak observed near 152 °C.     

EPR investigation of the trivalent chromium complexes in SrTiO3

The trivalent chromium centers were investigated by means of electron paramagnetic resonance (EPR) in SrTiO₃ single crystals grown using the Verneuil technique. It was shown that the charge compensation of the Cr³⁺-V_O dominant centers in octahedral environment is due to the remote oxygen vacancy located on the axial axis of the center. In order to provide insight into spin-phonon relaxation processes the studies of axial distortion of Cr³⁺-V_O centers have been performed as function of temperature. The analysis of the trigonal Cr³⁺ centers found in SrTiO₃ indicates the presence of the nearest-neighbor strontium vacancy. The next-nearest-neighbor exchange-coupled pairs of Cr³⁺ in SrTiO₃ has been analyzed from the angular variation of the total electron spin of S=2 resonance lines.     

Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by H NMR

The temperature dependences of ²H NMR spectra and spin-lattice relaxation time T₁ have been measured for paramagnetic [Mn(H₂O)₆][SiF₆]. The obtained ²H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy E_a and the jumping rate at infinite temperature k₀ for the 180° flip of H₂O were obtained as 35 kJ mol⁻¹ and 4×10¹⁴ s⁻¹, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H₂O)₆]²⁺ around the C₃ axis where the E_a and k₀ values were estimated as 45 kJ mol⁻¹ and 1×10¹³ s⁻¹, respectively. From the almost temperature independent and short T₁ value, the correlation time for electron-spin flip-flops, τ_e, and the exchange coupling constant J were obtained as 3.0×10⁻¹⁰ s and 2.9×10⁻³ cm⁻¹, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H₂O)₆]²⁺ around the C₃ axis.     

Dynamical structure of α-Ag0.99Cu0.01I crystal by Cu NMR chemical shift, spin-lattice relaxation time and molecular dynamics simulation

Solid ⁶³Cu NMR and Molecular Dynamics (MD) methods have been used to investigate the dynamical structure of Ag_0.99Cu_0.01I crystal, through the viewpoint of the chemical shift and the spin-lattice relaxation. In the superionic phase (α-phase), we obtained the temperature variation of the chemical shift as −0.2 ppm/K, and the activation energy of the Cu⁺ ion diffusion as 11 kJ/mol. The temperature dependence of the chemical shift was explained by the calculated chemical shielding surface based on ab initio MO calculation, and by the probability density of Cu⁺ ion estimated by MD simulation. The spin-lattice relaxation was also analyzed by using the MD method in which we assumed two jumping models as the diffusion process of the mobile cations. It is concluded that the temperature dependence of the chemical shift is dominated by the shielding in the vicinity of the 24(h) site, and the observed activation energy is due to the diffusion process of the mobile cations jumping from the 6(b) intrasub-lattice to the nearest-neighbor 6(b) sub-lattice.     

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.     

The role of 14N and 13C hyperfine structure in characterizing point defects in diamond

Hyperfine structure (hfs), of either ¹³C or ¹⁴N, observed in the electron paramagnetic resonance (EPR) spectrum, has been a remarkably powerful indicator of the models of paramagnetic defects in diamond. It has often been valuable or necessary to use the much higher resolution of electron nuclear double resonance (ENDOR). Measurements of hfs have recently allowed considerable progress in understanding the nature of defects which have been uncharacterized for many years. A rich variety of defects involving up to at least 4 N atoms, which readily substitute for C atoms, has been found in diamond. The role of ¹⁴N hfs is reviewed in solving problems where different aspects are relevant. The use of synthetic diamond, enriched by up to 10% in ¹³C, has greatly facilitated the construction of models for centres produced by radiation damage, where the only information is from ¹³C hfs. Both ¹⁴N and ¹³C hfs have confirmed models of Ni related centres. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study of synthetic diamonds by dynamic nuclear polarization-enhanced13C nuclear magnetic resonance spectroscopy

Four I_b-type synthetic diamond crystals were studied by dynamic nuclear polarization (DNP)-enhanced high resolution solid state¹³C nuclear magnetic resonance (NMR) spectroscopy. The home built DNP magic-angle-spinning (MAS) NMR spectrometer operates at a field strength of 1.9 T and the highest DNP enhancement factor of synthetic diamonds came near to 10³. Comparing with I_b-type natural diamonds, the¹³C NMR linewidths of synthetic diamonds in static spectra are broader. The¹³C spin-lattice relaxation time and DNP polarization time of synthetic diamond are shorter than those of I_b-type natural diamond. From the hyperfine structure of the DNP enhancement curve, four kinds of nitrogen-centred free radicals could be identified in synthetic diamond.     

Observation of micropores in hard-carbon using Xe NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using ¹²⁹Xe NMR. For high-pressure (4.0 MPa) ¹²⁹Xe NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.     

Spectral analysis of the structure of ultradispersed diamonds

The structure of ultradispersed diamonds (UDD) is studied by spectral methods. The presence of diamond crystal phase in the UDD is found based on x-ray analysis and Raman spectra. The Raman spectra also show sp²-and sp³-hybridized carbon. Analysis of IR absorption spectra suggests that the composition of functional groups present in the particles changes during the treatment. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 524–528, July–August, 2008.     

EPR investigations of oxidation effects in (V1−xMox)2−δO3

Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V_1−xMo_x)_2−δO₃. The oxidation effects were monitored from the rate of paramagnetic V⁴⁺ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V⁴⁺ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V⁴⁺ content.     

New Data on the Nature of the EPR OK1 and N3 Centers in Diamond

The well-known diamond electron paramagnetic resonance (EPR) OK1 and N3 defects are discussed in the context of incorporation of heavy metal, supposedly Ti, in the double semivacancy and substitutional structures, respectively. X-ray phase analysis of inclusions of the growth medium, eclogite, in the diamonds under study shows an atypically high concentration of titanium compounds. To support proposed models, the hyperfine structure (HFS) from ¹³C in the OK1 center has been analyzed more precisely. The data obtained are consistent with attributing additional satellites around the main EPR lines to HFS from ⁴⁷Ti and ⁴⁹Ti. A double semivacancy model describes the directions of g and A tensor components of the OK1 center. The features of infrared and photoluminescence spectra related to the OK1/S1 and N3/440.3 nm centers are presented.     

EPR and optical absorption studies of Fe ions in sodium borophosphate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectral investigations have been carried out on Fe³⁺ ions doped sodium borophosphate glasses (NaH₂PO₄-B₂O₃-Fe₂O₃). The EPR spectra exhibit resonance signals with effective g values at g=2.02, g=4.2 and g=6.4. The resonance signal at g=4.2 is due to isolated Fe³⁺ ions in site with rhombic symmetry whereas the g=2.02 resonance is due to Fe³⁺ ions coupled by exchange interaction in a distorted octahedral environment. The EPR spectra at different temperatures (123-295 K) have also been studied. The intensity of the resonance signals decreases with increase in temperature whereas linewidth is found to be independent of temperature. The paramagnetic susceptibility (χ) was calculated from the EPR data at various temperatures and the Curie constant (C) and paramagnetic Curie temperature (θ_p) have been evaluated from the 1/χ versus T graph. The optical absorption spectrum exhibits bands characteristic of Fe³⁺ ions in octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have also been evaluated and discussed.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

EPR Spectra of Nitrogen in Ultra-Dispersed Diamonds

The analysis of the electron paramagnetic resonance (EPR) line shape of ultra-dispersed diamond (UDD) obtained by conversion of trotyl and hexogen mixture and purified from other phases and metal compounds is carried out. The observed wide line with g = 2.0028 and a line width of 8.7 G is shown to be formed by superposition of three lines with line widths of 15.3, 8.5 and 3 G and with a ratio of integral intensities of 70:30:1. The procedure of decomposition and subtraction of wide lines has revealed the resolved hyperfine structure (HFS) from donor nitrogen with parameters A = 40.8 G and B = 29.2 G. Experimentally obtained dependence of the line width of the exchange line on the concentration of donor nitrogen in synthetic diamonds assumed that variations in line widths of the EPR spectrum components are caused by different local concentration of donor nitrogen due to distribution of nitrogen impurity during crystallization of diamond nanoparticles. EPR spectra of UDD after annealing in vacuum and at high pressures in the range of diamond phase stability are also discussed. At high-pressure annealing above 973 K, the areas with high concentration of defects are graphitized and a narrow Dyson-shape line from conductivity electrons and a resolved HFS from donor nitrogen can be observed without additional treatment of the EPR spectrum.