Electronic and optical properties of selected polymers studied by reflection electron energy loss spectroscopy

Abstract

We have determined the electronic and optical properties of six polymers: Polymethyl-methacrylate (PMMA), polyethylene (PE), polyvinyl chloride (PVC), polyester (PET), polypyrrole (PPY), and polyamide (PA6) for energy losses from 0 to 70 eV by analysis of reflection electron energy-loss spectroscopy (REELS) spectra. We found that the surface was easily damaged by the incident electron beam, in particular for energies above 500 eV. The damage results in new peaks in the bandgap region and the polymers become metallic. Great care was exerted to determine experimental conditions under which these effects are minimized. The REELS spectra were corrected for multiple inelastically scattered electrons with the QUASES-XS-REELS software to determine the effective inelastic-scattering cross sections. From these cross sections, we found that the band gaps for PMMA, PE, PVC, PET, PPY, and PA6 are 5.0 eV, 7.5 eV, 7.0 eV, 3.0 eV, 3.5 eV, and 5.1 eV, respectively. Quantitative analysis of the experimental cross sections was carried out by using the QUEELS-ɛ(k,ω)-REELS software to determine the dielectric function and optical properties. This is done by comparing the experimental REELS inelastic electron-scattering cross-section with a simulated cross section in which the only input is Im(−1/ɛ). The dielectric function is expressed as a sum of oscillators and the oscillator parameters are determined. Good agreement between the experimental and theoretical cross section is achieved for all polymers. From Im(−1/ɛ), the real and imaginary parts of ɛ (ω), the refractive index, and the extinction coefficient were determined for all polymers in the energy range ћω = 0 to 70 eV. An oscillator is clearly observed for PPY, PET, and PA6 at ∼ 6.7 eV, which corresponds to the π plasmon. This oscillator is not found for PMMA, PE, and PVC. A set of oscillators in the 20–30 eV energy range corresponding to the σ+π plasmon is found for all polymers.