Surface enhanced Raman spectroscopy as a probe for local modification of carbon films

The possibility of achieving nanometer-scale sensitivity in a surface-enhanced Raman (SERS) experiment while using larger-sized probes (0.1-1 mum) is investigated. The application targeted is carbon film transformation under high-energy beam irradiation, and, primarily, the transformation of amorphous carbon into nanocrystalline graphite. The carbon film covers nanometer-size Ag particles which enhance the signal from zones of material adjacent to them. This geometry gives access to the film/substrate interface, and in this way it complements scanning near-field techniques which have similar spatial sensitivity but are mainly surface sensitive. The SERS effect has been studied as a function of the Ag nanoparticle size, carbon film thickness, and excitation wavelength. A selective enhancement of the Raman cross section of the D band of amorphous carbons was observed. The dielectric properties of the carbon film, when used as an overlayer, strongly affect the SERS enhancement, so that changes in the dielectric function upon irradiation can be used to produce local enhancement contrast, and to establish an identification procedure for material transformation.