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

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Abstract
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 almost 20 h for the CP–MAS spectra taken at high field without DNP. Data based on DNP–CP–MAS experiments with variable CP‐match time or dipolar‐dephasing time are shown and discussed in terms of structural models. The cross‐polarization signals are rationalized in terms of estimates of the chemical shifts of hydrogenated diamond surfaces; this approach indicates that most of the hydrogens in the polycrystalline sample are at the intergrain boundaries.