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

Abstract

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.