Surface Analysis of Excimer-Laser-Treated Polyethylene-Terephthalate by Surface-Enhanced Raman Scattering and X-Ray Photoelectron Spectroscopy

Abstract

Silver and gold island films have been evaporated onto various polymer surfaces; the samples were characterized by surface-enhanced Raman spectroscopy (SERS), UV-visible spectroscopy, and x-ray photoelectron spectroscopy (XPS). Among the series of investigated polymers, the SERS spectra recorded on stretched polyethylene-terephthalate (PET) foils are unique with respect to resolution and intensity; the signal strengths are comparable to those observed in enhanced resonance Raman scattering. This exceptional behavior is attributed to the semicrystalline nature of the investigated material, and to the preferred interaction of the deposited silver with carbonyl groups of the polymer. The formation of a metal/polymer bond was established by the observation of corresponding bands in the XPS spectrum. Upon melting or amorphization of PET, the SERS intensities are reduced; this result is attributed to a decrease in the metal/polymer bond strength, due to changes in orientation of the phenyl rings and of the carbonyl groups on the polymer surface. The PET surfaces have been modified by excimer laser irradiation at the wavelengths of 308 and 248 nm. The characteristic laser-induced structuring of the surface, as revealed by scanning electron microscopy, is related to the biaxial stretching applied during production of the PET foils, and has been studied for various orientations of the samples and different laser fluences. As a consequence of irradiation, a loss of oxygen is detected by a decrease of the C 1s XPS bands assigned to carbon atoms with singly or doubly bound oxygen, and by an overall decrease of the oxygen-to-carbon ratio. The surface polarity is observed to decrease, as accompanied by a red shift of the absorption maximum for silver island films deposited onto the irradiated surfaces. SERS spectra recorded on these samples are weaker in intensity, as compared to those recorded on untreated PET foils. This decrease is interpreted in terms of partial amorphization of the sample accompanied by polymer reorientation, loss of the preferred anchoring sites for nucleation of silver islands, and changes in the surface structure and morphology.