Raman identification of onion-like carbon

A specific Raman spectrum of carbon spherical shells (onions) synthesized by annealing-induced (T = 1800 K) chipping off the outer carbon layers from 5-nm size diamond particles has been observed. It shows a clearly seen band at 1572 cm-1. Its downshift from the position 1582 cm-1 (known for a pure graphite) is interpreted as an influence of shell curvature. The summary contribution of the shells constituting the 5-nm diameter onion is estimated. A narrowness (FWHM = 15 cm-1) of the first order Raman band of onions together with a dominance of the peak at 2712 cm-1 (FWHM = 65 cm-1) in their second order spectrum point to a high degree of structural perfection of a graphitic network in shells. © 1998 Elsevier Science Ltd. All rights reserved.