Size dependence of CH stretching features on diamond nanocrystal surfaces: Infrared spectroscopy and density functional theory calculations

Surface CH stretching features on diamond nanocrystals of different origins and sizes have been characterized by Fourier transform infrared spectroscopy and density functional calculations. Independent of the origins of the samples, a single sharp transition, corresponding to the CH stretch on C(111)-1x1, is observed at 2834 cm(-1) for particles of sizes (d) larger than 100 nm. The feature, however, gradually diminishes as the particle size decreases and disappears nearly completely at d=5 nm. We interpreted the observation in terms of a domain size effect, namely, the domains of the {111}-type facets on the 5 nm diamonds are too small to yield the characteristic CH stretching feature [Chen et al., J. Chem. Phys. 116, 1211 (2002)]. The interpretation is supported by the present density functional calculations performed at the B3LYP/6-31G* level for three H-truncated diamond model clusters, C22H28, C38H42, and C58H58. Both "isolated'' and "coupled'' frequencies of the C-H stretching features on the {111}-type facets of these clusters are calculated and the results agree satisfactorily with the values obtained from the mixed isotope measurements conducted on diamond C(111)-1x1 single crystal surfaces [Cheng et al., J. Chem. Phys. 106, 7411 (1997)]. (C) 2003 American Institute of Physics.