Nature of confinement of phonons in nanocrystalline CVD diamond

Self-standing nanocrystalline CVD diamond sheets are investigated using Raman spectroscopy and X-ray diffraction. The sheets are composed of diamond phase and an amorphous-carbon-like non-diamond phase whose content increases with increasing gas pressure in the growth chamber. In the samples synthesised at high pressures the 1332 cm(-1) diamond Raman peak exhibits asymmetric broadening. Although the scattering from defects can account for the symmetric part of the line-width, the asymmetry is much more than expected for a confined phonon line-shape calculated according to the Gaussian confinement model for the particle sizes obtained from X-ray diffraction. An alternative model for the confinement of phonons is discussed which involves the standing waves arising from the reflection of the phonons from the elastic-dielectric boundary. The calculated line-shapes arising from the contributions from the allowed q-points in the Brillouin-zone are found to agree with the measured spectra. (C) 2001 Elsevier Science B.V. All rights reserved.