Novel graphitic spheres: Raman spectroscopy at high pressures

Novel graphitic spheres with a shell structure of curved graphene layers we re discovered as a side product of fullerene synthesis. The Raman spectra o f these carbon spheres with typical sizes of 20-500 mum have been found to be similar to that of micro-crystalline graphite. The silent low frequency B-1g(1) phonon of graphite, however, becomes Raman active in the C-spheres. Raman spectroscopy of the C-spheres under high pressures up to 2.4 GPa sho ws that their vibrational properties are very similar to those of graphite, despite the curvature of the graphene sheets. We have studied the effect o f high pressure on the D* mode near 2700 cm(-1) which is present in all gra phitic materials and shows a peculiar dependence of its frequency on the ex citing laser wavelength.