The effect of laser-induced heating upon the vibrational raman spectra of graphites and carbon fibres

Abstract

Laser-induced sample heating has been shown to significantly shift the E 2 g vibrational Raman mode (1,580 cm −1) of graphite to lower frequency, with typical bandshifts of ~10 cm 1−20 cm −1, depending upon laser power. These shifts were only observed when small samples (particles or fibres) were examined using a Raman microprobe. The temperature sensitivity of the band position was determined to be ~ −0.03 cm −1 K −1 for a Lonza KS-5-75 graphite. A variety of carbons were studied, from amorphous carbons through graphites to carbon fibres, and all showed the same heating effect with similar bandshifts. Frequency shifts were also noted for the 1,355 cm −1 A 1 g mode, but the magnitude of these shifts was generally smaller than those of the E 2 g mode. These results have important ramifications for measurements of strain in carbon fibres, in that a Hercules HMS4 fibre showed an E 2 g mode frequency shift of −0.7 cm −1 per mW of laser beam power incident on the fibre surface with a 5 μm spot size. Laser power fluctuations can therefore cause bandshifts of magnitude comparable to those induced by fibre strain if they are not carefully controlled.