Anomalous two-phonon absorption in diamond nanocrystals embedded in amorphous carbon

Observation of anomalously strong IR absorption are reported in the region of two-phonon diamond frequencies in amorphous hydrogenated carbon films grown by cosputtering graphite and copper in a plasma. Up to five structural elements of bands having frequencies close to or coinciding with the figures quoted in the literature for two-phonon absorption bands in bulk diamond were observed. This suggests the presence of diamond nanocrystals in the grown layers. Observation of two-phonon absorption in thin films is in itself remarkable because of the smallness of its coefficient in bulk diamond. An estimate of the absorption coefficient in the dominant band at 2140 cm^?1 yields about 200 cm^?1, which exceeds by more than an order of magnitude that for bulk diamond. This can be assigned to an anomalous enhancement of the two-phonon absorption coefficient due to phonon confinement in the small diamond particles nucleated in the amorphous carbon matrix. An estimate of these inclusions from the band width yields about 20 Å. The reasons for the catalytic activity of copper in diamond nucleation are analyzed.