13 C NMR investigation of CVD diamond: Correlation of NMR and Raman spectral linewidths
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C NMR investigation of CVD diamond: Correlation of NMR and Raman spectral linewidths Lawrence H. Merwin, Curtis E. Johnson, and Wayne A. Weimer Chemistry Division, Research Department, Naval Air Warfare Center Weapons Division, China Lake, California 93555 (Received 19 April 1993; accepted 22 October 1993)
Six CVD diamond thin films were examined by magic angle spinning (MAS) 13C nuclear magnetic resonance (NMR), Raman, and electron spin resonance spectroscopy. The use of film samples cut to the diameter of the magic-angle spinning rotor provided ease of spinning and the opportunity to obtain good signal-to-noise spectra in 4 to 16 h. MAS NMR linewidths exhibit a linear correlation with Raman linewidths and reflect the optical quality of the material. Residual MAS NMR linewidths most likely arise from a combination of crystal defect sites and paramagnetic effects.
I. INTRODUCTION Synthetic diamond thin films produced by chemical vapor deposition (CVD) techniques are of interest due to their unique combination of properties, including hardness, inertness, high thermal conductivity, and optical transparency in the infrared region. While the characterization of CVD diamond has been based largely upon Raman spectroscopy, there has recently been an increased interest in the use of solid-state nuclear magnetic resonance (NMR) spectroscopy. Structural questions have been approached both from the standpoint of ! H NMR spectroscopy of the residual protons in the sample,1"3 and from the perspective of the 13C of the diamond itself.4"8 In a recent study, the quality of diamond films was assessed in terms of sp2/sp3 content as determined by 13C NMR and Raman spectroscopy.8 The majority of the 13 C NMR studies have used either 13C enriched samples or sensitivity enhancement techniques [e.g., cross-polarization (CP) or dynamic nuclear polarization (DNP)] to overcome the generally low sensitivity of 13C NMR and the usual small amount of available sample. These techniques have provided significant insights into the structure of CVD diamond films, but they are limited by the cost of 13C enrichment and by the fact that enhancement techniques tend to selectively sample a portion of the sample near the enhancement source (protons for CP; unpaired electrons for DNP). While these sites are important to understanding synthetic diamond properties and growth mechanisms, it is also of interest to obtain spectra that are representative of the bulk film. We have obtained gram-quantity samples of CVD diamond films in the form of millimeter-thick disks cut to the inner diameter of the magic angle spinning (MAS) rotors. As a result, we have been able to apply 13 C solid-state NMR spectroscopy to investigate bulk CVD diamond films in a reasonable amount of time
and without resorting to sensitivity enhancement techniques. In addition, we have been able to use the NMR technique of magic angle spinning, which removes certain interactions that influence the NMR spectrum, on the as-prepared films without destroying their structural integrity. Previ
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