13 C NMR Spectroscopy of Amorphous Hydrogenated Carbon and Amorphous Hydrogenated Boron Carbide
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C NMR SPECTROSCOPY OF AMORPHOUS HYDROGENATED CARBON AND AMORPHOUS HYDROGENATED BORON CARBIDE
JANET BRADDOCK-WILKING, SHIU-HAN LIN, AND BERNARD J. FELDMAN Departments of Chemistry and Physics and the Center for Molecular Electronics University of Missouri-St. Louis, St. Louis, MO 63121 ABSTRACT We report the 13C NMR spectra of amorphous hydrogenated carbon and boron carbide. The amorphous hydrogenated carbon spectra consist primarily of a sp 2 carbon peak at 40 ppm and a sp 3 carbon peak at 140 ppm and are in reasonable agreement with recent theoretical calculations of Mauri, Pfrommer, and Louie, but there are some noteable discrepancies. The amorphous hydrogenated boron carbide spectra are very different from that of amorphous hydrogenated carbon, showing two sharp lines at 135 and 170 ppm at low boron concentrations and an intense, broader line at 15 ppm at high boron concentrations. We suggest the two sharp lines at 135 and 170 ppm could be due to carbon atoms in boron-containing aromatic rings, and the broader line at 15 ppm as due to carbon atoms in boron carbide icosahedra. These lines provide evidence of nanocrystalline structure imbedded in an amorphous hydrogenated boron carbide matrix. INTRODUCTION Very recently, Mauri, Pfrommer, and Louie [1 ] reported ab initio calculations of the C NMR spectra of amorphous hydrogenated carbon (a-C:H) that were in good agreement with the experimental results of Jager, Gottwald, Spiess, and Newport [2]. This theoretical calculation motivated us to repeat the experiment using a higher magic angle spinning (MAS) frequency, giving more highly resolved spectra and a more challenging test of the theory. The second motivaton for this work was to resolve a controversy about the carbon bonding in amorphous hydrogenated boron carbide (aB:C:H). The goal here was to use 13C NMR spectroscopy to distinguish between two 3 different carbon binding configurations -- icosahedral [3] vs impurity [4]. The 1 C NMR measurements on a-C:H and a-B:C:H taken together provide a demonstration of the usefulness and sensitivity of this experimental technique to unravel the atomic structure of these amorphous alloys. Kaplan, Jansen, and Machonkin first reported the 13 C NMR spectra of a-C:H, which consisted of a sp 3 carbon peak at 40 ppm and a sp carbon peak at 140 ppm [5]. They also used the integrated intensities of these two peaks to measure the ratio of sp 3 bound carbon to sp 2 bound carbon for a-C:H thin films grown by different techniques. They were the first to report that the hardness of a-C:H thin films is inversely proportional both to this sp 3 /sp 2 ratio and the films' hydrogen concentration, a result that is supported by 13 this work and our previous work [3]. These results have also been confirmed by C NMR measurements on a-C:H thin films by other workers 16-81. The controversy involving the carbon bonding in a-B:C:H is also related to the issue of hardness. We reported that the addition of boron to a-C:H dramatically increased the 13
401 Mat. Res. Soc. Symp. Proc. Vol. 555 ©1999 Ma
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