Location of paramagnetic defects in detonation nanodiamond from proton spin-lattice relaxation data.

We developed an approach for determining location of intrinsic paramagnetic defects in nanodiamonds from the data of proton spin-lattice relaxation of the surface hydrogen atoms. The approach was applied to the detonation nanodiamond (DND) of the diameter of 5 nm. We found that dangling bonds with unpaired electron spins are located within the near-surface belt at the distance of 0.3-0.9 nm from the DND surface. The NMR data are compared with the results of EPR measurements.

Towards Determination of Distances Between Nanoparticles and Grafted Paramagnetic Ions by NMR Relaxation.

We developed an approach for determining distances between carbon nanoparticles and grafted paramagnetic ions and molecules by means of nuclear spin-lattice relaxation data. The approach was applied to copper-, cobalt- and gadolinium-grafted nanodiamonds, iron-grafted graphenes, manganese-grafted graphene oxide and activated carbon fibers that adsorb paramagnetic oxygen molecules. Our findings show that the aforementioned distances vary in the range of 2.7-5.4 Å and that the fixation of paramagnetic ions to nanoparticles is most likely implemented by means of the surface functional groups. The nuclear magnetic resonance data data are compared with the results of electron paramagnetic resonance measurements and density functional theory calculations.

23Na and 27Al NMR Study of Structure and Dynamics in Mordenite.

Temperature dependencies of 27Al and 23Na nuclear magnetic resonance spectra and spin-lattice relaxations in mordenite have been studied in static and magic angle spinning regimes. Our data show that the spin-lattice relaxations of the 23Na and 27Al nuclei are mainly governed by interaction of nuclear quadrupole moments with electric field gradients of the crystal, modulated by translational motion of water molecules in the mordenite channels. At temperatures below 200 K, the dipolar interaction of nuclear spins with paramagnetic impurities becomes an important relaxation mechanism of the 23Na and 27Al nuclei.

(1)H NMR study of water molecules confined in nanochannels of mordenite.

Behavior of water molecules entrapped in nanochannels of zeolite mordenite has been investigated by (1)H NMR technique. The (1)H spectra and spin-lattice relaxation times in the laboratory and rotating frames, T1 and T1ρ, respectively, as well as the dipolar relaxation time T1D have been measured in the temperature range from 96 to 351K. Diffusion of water molecules along the channels was observed above ~200K. While in bulk liquid the dipolar ordered state of nuclear spins is not formed owing to complete motional average of dipolar interactions, we show that such a state is observed for mobile molecules confined in a restricted geometry. At temperatures below ~140K the relaxation was found to be mainly caused by interaction of (1)H nuclear spins with paramagnetic impurities. Complete lost of the fine structure of (1)H spectra above ~320 K is attributed to isotropic molecular reorientation or/and proton exchange. We show that the dipolar relaxation in mordenite is responsive to slow 180° reorientations of water molecules. The correlation times of nuclear and electron spin fluctuations were determined.

Effect of magic angle spinning on (13)C spin-lattice and spin-spin relaxation in nanodiamonds.

We report on (13)C spin-lattice (R 1) and spin-spin (R 2) relaxation rate dependence on magic-angle-spinning (MAS) rate in highly purified synthetic nanodiamonds. Noticeable slowdown of both relaxation processes and reduction of nuclear spin diffusion coefficient D with increasing MAS rate was obtained. This effect is attributed to suppression of nuclear spin diffusion by MAS. We developed a theoretical approach that describes the MAS rate dependence of R 1, R 2 and D, allows quantitative analysis of the data and shows good compliance with the experiment.

Size dependence of 13C nuclear spin-lattice relaxation in micro- and nanodiamonds.

Size dependence of physical properties of nanodiamond particles is of crucial importance for various applications in which defect density and location as well as relaxation processes play a significant role. In this work, the impact of defects induced by milling of micron-sized synthetic diamonds was studied by magnetic resonance techniques as a function of the particle size. EPR and (13)C NMR studies of highly purified commercial synthetic micro- and nanodiamonds were done for various fractions separated by sizes. Noticeable acceleration of (13)C nuclear spin-lattice relaxation with decreasing particle size was found. We showed that this effect is caused by the contribution to relaxation coming from the surface paramagnetic centers induced by sample milling. The developed theory of the spin-lattice relaxation for such a case shows good compliance with the experiment.

(13)C spin-lattice relaxation in nanodiamonds in static and magic angle spinning regimes.

We report on (13)C nuclear spin-lattice relaxation time (T1) dependence on the magic-angle-spinning (MAS) rate in powder nanodiamond samples. We confirm that the relaxation is caused by interaction of nuclear spins with fluctuating electron spins of localized paramagnetic defects. It was found that T1 is practically not affected by MAS for small particles, while for larger particles with lower defect density T1 is different in static and MAS regimes and reveals elongation with increasing MAS rate. This effect is attributed to suppression of nuclear spin diffusion by MAS. We propose an approach that describes T1 dependence on the MAS rate and allows quantitative analysis of this effect.

Proton spin diffusion in a nanodiamond.

We report on a proton magnetic resonance study of a powder nanodiamond sample. We show that (1)H spin-lattice relaxation in this compound is mainly driven by the interaction of nuclear spins with unpaired electron spins of paramagnetic defects. We measured the spin-lattice relaxation time T1 by means of a saturation comb pulse sequence followed by dipolar dephasing, and plotted T1 as a function of the dephasing time [Formula: see text] in different external magnetic fields. The received T1([Formula: see text]) dependence provides a striking manifestation of the spin diffusion-assisted relaxation regime. The obtained experimental data allow us to estimate the spin diffusion coefficient and spin diffusion barrier radius.

Nanodiamond graphitization: a magnetic resonance study.

We report on the first nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) study of the high-temperature nanodiamond-to-onion transformation. (1)H, (13)C NMR and EPR spectra of the initial nanodiamond samples and those annealed at 600, 700, 800 and 1800 ° C were measured. For the samples annealed at 600 to 800 ° C, our NMR data reveal the early stages of the surface modification, as well as a progressive increase in sp(2) carbon content with increased annealing temperature. Such quantitative experimental data were recorded for the first time. These findings correlate with EPR data on the sensitivity of the dangling bond EPR line width to air content, progressing with rising annealing temperature, that evidences consequent graphitization of the external layers of the diamond core. The sample annealed at 1800 ° C shows complete conversion of nanodiamond particles into carbon onions.

Hahn-echo decay for exchange-coupled nuclear spins in solids.

In this paper we present a simple model to calculate the Hahn-echo decay of the exchange-coupled nuclear spins in solids. Satisfactory agreement between the calculated and experimentally observed echo decay of the exchange-coupled spins of T203l and T205l isotopes in thallium chloride TlCl and thallium tantalum sulfide TlTaS(3) is obtained.

Phase transitions and incommensurability in the layered semiconductor TlInS2--an NMR study.

We report on the first NMR study of powder and single crystal samples of thallium indium sulfide, TlInS(2). The crystal under study is a pure single-layer TlInS(2) polytype. Our findings show that transformation from the high temperature paraelectric phase to the low temperature ferroelectric phase occurs via an incommensurate phase that exists in the temperature range from T(c) = 192 K to T(i) = 205 K. On approaching the phase transition at T(i) from above, the crystal exhibits a soft mode behavior. A discrepancy in the literature data on the phase transitions in TlInS(2) is discussed and ascribed to polytypism of the TlInS(2) crystals.

Spin diffusion in multiple pulse spin-locking in solids containing paramagnetic impurities.

Spin diffusion and spin-lattice relaxation in solids containing paramagnetic impurities under influence of a multiple-pulse spin-locking radio-frequency sequence are studied theoretically and experimentally. The diffusion equation obtained provides a clue for determination of the time dependent magnetization. The spin-lattice relaxation time is calculated as a function of the correlation time and multiple-pulse field parameters. From the experimental data the spin diffusion coefficient, the radius of the spin diffusion barrier, and the correlation time for very slow molecular motion in polycrystalline (C(2)F)(n) system are estimated and found to be D∼7.1×10(-12)cm(2)/s, r(c)∼4.8×10(-10)m, and τ(c)∼10.2µs, respectively.

A magnetic resonance study of MoS(2) fullerene-like nanoparticles.

We report on the first nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) investigation of inorganic fullerene-like MoS(2) nanoparticles. Spectra of bulk 2H-MoS(2) samples have also been measured for comparison. The similarity between the measured quadrupole coupling constants and chemical shielding anisotropy parameters for bulk and fullerene-like MoS(2) reflects the nearly identical local crystalline environments of the Mo atoms in these two materials. EPR measurements show that fullerene-like MoS(2) exhibits a larger density of dangling bonds carrying unpaired electrons, indicative of them having a more defective structure than the bulk sample. The latter observation explains the increase in the spin-lattice relaxation rate observed in the NMR measurements for this sample in comparison with the bulk 2H- MoS(2) ones.

Nuclear magnetic resonance study of fullerene-like WS2.

Inorganic fullerene-like nanoparticles of WS2 (IF-WS2), are synthesized by a reaction of tungsten oxide with molecular hydrogen and hydrogen sulfide. The synthesized nanoparticles appear as large agglomerates (>40 microns), each one counting thousands of IF nanoparticles. 1H nuclear magnetic resonance study of these nanoparticles is reported. The measurements show that the prepared product contains water (and possibly some hydrogen) molecules that occupy the voids in the central part of the fullerene-like nanoparticles and the nanopores between the adhering IF-WS2 particles. Defects in the IF-WS2 structure, arising due to the strain release during the folding of the layers, may result in additional sites for the absorbed water. Vacuum annealing of the powder leads to substantial reduction in the amount of absorbed water molecules.

Simple two-pulse time-reversal sequence for dipolar and quadrupolar-coupled spin systems.

We demonstrate, both theoretically and experimentally, that two-pulse sequence (2n+1) X 90 degrees(Y) - 90 degrees(X) -Acq(t) without delay between pulses, yields the reverse of time evolution of spin system with dipolar and quadrupole interactions. This process results in refocusing of the spin magnetization into magic echo at te = t1/2 after the second pulse, where t1 is the length of the first pulse.

Spin diffusion and nuclear spin-lattice relaxation in irradiated solids: a multiple-pulse NQR study.

We present a detailed theoretical and experimental NQR multiple-pulse spin-locking study of spin-lattice relaxation and spin diffusion processes in the presence of paramagnetic impurities in solids. The relaxation function of the nuclear spin system at the beginning of the relaxation process is given by exp (-t/T1rho)alpha, where T1rho is spin-lattice relaxation time in rotating frame and alpha = d/6, d is the sample dimensionality. Then the relaxation proceeds asymptotically to an exponential function of time, which was attributed to the spin-diffusion regime. Using the experimental data obtained from the analysis of those two relaxation regimes in gamma-irradiated powdered NaClO3, spin diffusion coefficient has been determined and the radius of the diffusion barrier has been estimated.

Spin-locking in one pulse NMR experiment.

The response of a spin system to a long (in comparison to spin-spin relaxation time T2) radiofrequency pulse has been studied. We observed that the magnetization after the long pulse does not fall to zero at time t >> T2 for both on-resonance and off-resonance conditions. The dependencies of the magnetization on frequency offset, linewidth and radiofrequency power are investigated, both theoretically and experimentally. The question of the effective field direction is also discussed.