Natural-abundance 13 C dynamic nuclear polarization experiments on chemical vapor deposited diamond film
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Curtis E. Johnson Chemistry Division, Research Department, Naval Air Warfare Center, Weapons Division, China Lake, California 93555-6001 (Received 24 February 1992; accepted 30 June 1992) 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 X 1018 spins/g, provides a way to enhance the 13C or the "residual" *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 13C spins lead to a transfer of polarization from the electron spin system to the *H 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.
I. INTRODUCTION During the past few years, interest in diamond films synthesized with chemical vapor deposition techniques has grown enormously. 12 The reason for this growing interest stems from the many important properties of diamond, such as extreme hardness, chemical inertness, dopability, optical transparency in the infrared region, and the unusual combination of high thermal conductivity and low electrical conductivity. In order to optimize the synthesis of CVD diamond films, several analytical techniques, in particular Raman spectroscopy,3 have been utilized to explore details of the structures, especially defects, of the films produced. Only limited solid-state nuclear magnetic resonance (NMR) data have been reported previously on diamond films4"7; and 13 C results that have been reported have been obtained only on samples that are isotopically enriched with 13 C, mainly because of the small quantities of diamond films normally produced, the inherently low sensitivities of typical NMR techniques, the low natural abundance of the 13C isotope, and the expected long 13C relaxation time8'9 in diamonds. In samples that contain unpaired electrons, the nuclear spin polarization can be enhanced by microwave irradiation at or near the electron-spin Larmor frequency; this is the so-called dynamic nuclear polarization (DNP) effect.10 Duijvestijn et al.8 obtained large 13C enhance-
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)Author to whom correspondence should be addressed. J. Mater. Res., Vol. 7, No. 10, Oct 1992
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ments on natural and synthetic diamonds. Wind et al.n first combined DNP with magic-angle spinning (MAS),12 rapid spinning of the sample about an axis at an angle of 54.7° with respect to the direction of the external magnetic field. Sufficiently fast MAS removes broadening of the signal due to chemical sh
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