Evidence for re-crystallization process in the irradiated graphite with heavy ions obtained by Raman spectroscopy

Highly oriented pyrolytic graphite (HOPG) was irradiated with 40.5 MeV 112Sn ions at a range of fluence: 1 × 1012-1 × 1014 ions/cm2. A new Raman band was observed at ∼864 cm−1 which grows with increasing the irradiation fluence. This band has been observed in Raman spectra of HOPG and natural graphite crystal edge planes, only when the incident light is polarized perpendicular to the basal planes and it has been assigned to the out-of-plane Raman-inactive B2g mode which becomes Raman active due to a slight rearrangement of lattice structure at the edge. However, we first obtained this band in Raman spectra of basal planes of irradiated HOPG with the polarization of the incident light parallel to the basal planes. Then, a half-wave plate was used to change the polarization direction of the incident light in the HOPG basal planes. The intensity of the band at 864 cm−1 is almost independent of the incident light polarization direction. We suggest that re-crystallized graphite with random lattice orientations are formed during ion irradiation, which gives rise to the B2g band. The conclusion is confirmed by transmission electron microscopy images, showing that there are some randomly orientated nanoscale graphite crystals surrounded by amorphous carbon.