Novel Graphitic Spheres: Raman Spectroscopy at High Pressures

Novel graphitic spheres with a shell structure of curved graphene layers were discovered as a side product of fullerene synthesis. The Raman spectra of these carbon spheres with typical sizes of 20–500 μm have been found to be similar to that of micro-crystalline graphite. The silent low-frequency B1g(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 shows that their vibrational properties are very similar to those of graphite, despite the curvature of the graphene sheets. We have studied the effect of high pressure on the D* mode near 2700 cm—1 which is present in all graphitic materials and shows a peculiar dependence of its frequency on the exciting laser wavelength.