Modification of Optical and Electrical Properties of PET Foils by He⁺, Ne⁺ and Ar⁺ Implantation

Thin (3 μm) polyethylene terephthalate (PET) foils were irradiated with 135 keV He⁺, Ne⁺ and Ar⁺ ions with the fluences up to 5×10¹⁵ cm¯². Changes of chemical structure of the polymer were studied with the Fourier transform infrared and Raman spectroscopy - breaking of numerous chemical bonds, polymer chain cross-linking as well as formation of sp² hybridised carbon clusters and cluster networks were demonstrated. The increase of the implanted sample absorbance with the implantation fluence in the UV-VIS spectra as well as the decrease of optical band-gap energy (2.75 and 2.0 eV for He and Ne, respectively, at 5×10¹⁵ cm¯²) are observed. Decrease of bulk resistance of heavily treated samples by ≈5 orders of magnitude is determined. Measurements of the sheet resistance confirm that the sample becomes conducting also on the reverse (unimplanted) side of the foil. Both of these effects depend on the impinging ion mass - they are the strongest for Ar. The increase of both ac conductance and dielectric constant is observed in the frequency range up to 2 MHz and these changes rise with the impinging ion mass.