Cryogenic ion implantation of polyethylene terephthalate (PET) thin films: structural and electronic properties

Abstract

Conventional inorganic semiconductors such as silicon, germanium and gallium arsenide have been the backbone of the semiconductor industry over the past few decades. In recent years, organic electronics have attracted attention in the semiconductor industry for many different applications which include photovoltaic cells, light emitting diodes, transistors and so on. The development of organic electronics stems from the improvement of the properties of polymers through various fabrication methods, one such method is ion implantation. Ion implantation offers an effective solution in modifying surface properties of polymers such as hardness and wear resistance, physical and chemical properties are also altered. These include: cross linking which enhances the conductivity of the implanted material, chain scission and carbonization. Ion implantation of polymers have some advantages over other fabrication methods, these methods include: chemical synthesis, vacuum deposition, plasma polymerization, and etc. Low filling factor and a large dispersion in sizes and shapes of nanoparticles are a few of the problems these methods pose. Ion implantation enables a high metal filling factor to be reached in a solid matrix. Polyethylene terephthalate (PET) is a very common and readily available polymer in the plastics industry. Since PET is a naturally insulating material, the possibility of modifying its conductance opens up a range of possible applications for this polymer in the semiconductor industry. In this work samples of PET films were implanted at liquid nitrogen to various fluences using 100 keV argon and titanium ions with the objective to characterise its properties using different material characterization techniques. The samples were cleaned through a process of inserting them in beakers of small amounts of acetone, ethanol and distilled water respectively at two minute intervals, they were dried by using nitrogen gas. The chemical structure of the films was investigated using x-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy; the semi-crystalline structure exhibit properties that indicate that they have undergone degradation with breakage and destruction of some bonds. The morphology, via scanning electron microscopy (SEM), show formation of granular materials. Depth profiles acquired by using heavy-ion elastic recoil detection analysis (HI-ERDA) show an accumulation in carbonization in the films with increase in fluence; a decline in the carbon content is observed in depths where there is accumulation of the implanted species. The ultraviolet-visible (UV-Vis) spectroscopy results show a redshift in the spectra with a decrease in the optical energy band gap from 3.55 eV for the pristine to 2.01 and 1.84 eV for argon and titanium respectively for samples implanted at the highest fluence. This is accompanied by increases in the activation energy and number of carbon cluster. The electrical structure of the films has been modified, with current-voltage (I-V) results showing a decrease in the surface resistivity and increase in the current. The characteristic measurements show that there is an overall increase in carbonization and formation of carbon clusters thereby enhancing the electrical properties of PET.