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

¹³C 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 × 10¹⁸ spins/g, provides a way to enhance the ¹³C or the “residual” ¹H 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 ¹³C spins lead to a transfer of polarization from the electron spin system to the ¹H and/or ¹³C spin systems. No signal for sp² 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 ¹³C spin system, differs significantly from the DNP-DP-MAS spectrum, in which the ¹³C spins are directly enhanced.