Herman Lock

A study of 13C‐enriched chemical vapor deposited diamond film by means of 13C nuclear magnetic resonance, electron paramagnetic resonance, and dynamic nuclear polarization

13C nuclear magnetic resonance (NMR), with and without dynamic nuclear polarization (DNP), and electron paramagnetic resonance (EPR) measurements are presented on two 13C‐enriched chemical vapor deposited (CVD) diamond films. The samples were prepared by the decomposition in a microwave discharge of a gas mixture of CH4 and H2, with and without O2. 13C NMR spectra, obtained with or without magic‐angle spinning (MAS) both at a field of 14 T using direct polarization (DP) and at 3.5 T via cross polarization (CP), are presented. Because the samples contain both nuclear and (unpaired) electron spins, irradiation of the sample with microwaves with a frequency at or near the electron Larmor frequency leads to an enhancement of the nuclear spin polarization via the effect referred to as dynamic nuclear polarization (DNP). This enhancement (at a field of 1.4 T), combined with the effects of isotopic enrichment (14% 13C), makes it possible to measure 13C DNP–CP–MAS spectra within a couple of hours, compared to alm...

Natural-abundance 13C dynamic nuclear polarization experiments on chemical vapor deposited diamond film

13 C cross-polarization (CP) and direct-polarization (DP) spectra of an 83 mg sample of a chemical vapor deposited (CVD) diamond film (combined from 12 separate depositions) have been obtained via dynamic nuclear polarization (DNP) combined with magic-angle spinning (MAS). With DNP, the presence of unpaired electron spins in the sample, measured to be 2 × 10 18 spins/g, provides a way to enhance the 13 C or the “residual” 1 H signal by irradiating the sample with microwaves at or near the electron spin resonance (ESR) Larmor frequency; the interactions between the unpaired electrons and protons or 13 C spins lead to a transfer of polarization from the electron spin system to the 1 H and/or 13 C spin systems. No signal for sp 2 hybridized carbons could be observed. The DNP-CP-MAS spectrum, obtained in an experiment in which the DNP-enhanced proton polarization is in turn transferred via CP to the 13 C spin system, differs significantly from the DNP-DP-MAS spectrum, in which the 13 C spins are directly enhanced.

13C knight shift saturation and 1H dynamic nuclear polarization in a polycrystalline sample of the organic conductor (fluoranthenyl)2PF6

Abstract It is demonstrated that the 013 C Knight shift in the organic conductor (fluoranthenyl) 2 PF 6 can be reduced by saturating the conduction electron ESR line with microwave irradiation. It is also found that microwave irradiation leads to a nuclear polarization enhancement due to the Overhauser effect. The measurements have been used to separate the 13 C Knight shift and the ordinary chemical shift.

29Si dynamic nuclear polarization of dehydrogenated amorphous silicon

Abstract 29Si NMR spectra were obtained on a dehydrogenated amorphous silicon sample by means of dynamic nuclear polarization (DNP). The dependence of the DNP enhancement factor, which peaked at about 40, upon the microwave frequency offset (ω - ωe) from the electron spin Larmor frequency is essentially antisymmetric about ω - ωe = 0, which corresponds to a g value of 2.0059. This observation shows that the paramagnetic centers are fixed, immobile on the time scale of electron spin larmor precession, requiring a reexamination of recently advanced models of the paramagnetic defects. Prospects for the application of DNP in the study of amorphous silicon materials are discussed.

Solid-State 25Mg, 45Sc and 93Nb MAS NMR Studies: Local B-Site Chemical Environments and Ordering in PB(MG1/3NB2/3)O3 (PMN) and Related Relaxor Ferroelectrics

Solid-state 25Mg, 45Sc and 93Nb NMR spectroscopic studies of the local chemical environments and relative degree of B-site ordering in lead-based perovskite relaxor ferroelectrics are reported. 93Nb MAS NMR spectra of PMN and PZN provide conclusive evidence for significant B-site disorder, while 93Nb and 45Sc MAS NMR indicate B-site ordering in PSN. 93Nb MAS NMR spectra of PMN show resonances with different CQ values assigned to different types of Nb(V) B-sites of pseudo-cubic, axial and rhombic symmetry. For PSN, 93Nb and 45Sc MAS NMR spectra support the presence of predominantly a single Nb(V) and Sc(III) site with intermediate CQ values assigned to axially distorted B-sites. 25Mg MAS NMR of PMN also show evidence for at least two different Mg(II) sites of local cubic and axial symmetry due to partial B-site disorder in PMN. 25Mg and 93Nb NMR spectra of La3+-substituted PMN (at 25 mol%) show that A-site substitution by La(III) leads to increased B-site ordering in the Mg-rich and Nb-rich nanodomains of PMN. By contrast, the 25Mg and 93Nb NMR spectra of BMN, a Ba2+ A-site analog of PMN, demonstrate that BMN has a more highly ordered Mg/Nb B-site distribution than PMN. These multinuclear NMR approaches demonstrate the enormous potential of solid-state NMR spectroscopy to provide both qualitative and quantitative information about the chemical environments, cation ordering and motional behavior of the B-site cations in a diverse range of perovskite relaxor ferroelectrics.

NMR of samples containing metal foils

By using spool configurations of a sample containing aluminum foil, in which the axis of the spool is collinear with the RF coil axis, one can obtain high-quality 13C NMR spectra of static samples of organic material attached to the aluminum foil. By combining such a spool configuration (or, alternatively, analogous samples containing equivalent amounts of fine aluminum powder) with the magic-angle hopping (MAH) technique, one can achieve a high degree of isotropic averaging of the 13C spectrum. This opens to NMR techniques the study of a variety of samples containing macroscopic pieces of metal foils, e.g., thin films deposited on metal foils and electrochemical systems with species adsorbed on metal-foil electrodes.