Scientists at the Universities of Queensland and New South Wales in Australia have developed a technique for using plastic to conduct electricity that is tunable by exposure to an ion beam, in the same way as the metals used in electrical wires.
Although the findings run counter to the belief that plastics are not good conductors, the researchers were able to show how ion beams could be used to make cheap, strong, flexible and conductive plastic films for electrical resistance thermometers. They also found the material could be tailored by tuning the initial metal film thickness, as well as the beam dose and energy.
The team used ion beam metal-mixing, placing a thin film of metal onto a plastic sheet and mixing it into the polymer surface using an ion beam, resulting in plastics that can conduct with metallic or even superconducting properties. Although ion beams have long been used in microelectronics to tailor the conductivity of semiconductors such as silicon, their use in plastic films has not been greatly successful until now.
This study has essentially made a temperature sensor from the plastic films. The results of the study, published in the journal ChemPhysChem [Stephenson et al ChemPhysChem (2010) DOI: 10.1002/cphc.201000762], were tested against a platinum resistance thermometer, with comparable accuracy, highlighting a way to tune across the entire conductivity spectrum with this material.
The research, which originated from work by Paul Meredith at the University of Queensland in the early 2000s into organic electronics and conducting plastics, initially examined the potential of tailoring the conductivity of silicon to make conductive plastics through ion-implantation. This proved a better approach than other chemical routes for the manufacture of conducting plastics.
However, the use of argon and krypton ion beams did then help to produce a significant increase in electrical conductivity, although the resulting films were still insulators with low conductivity. They unsuccessfully tried using tin ion beams; however, the mass of the tin ions is quite high, and they carry so much momentum that as the tin starts to accumulate, the incoming tin ions start knocking the deposited tin atoms back out, resulting in just a highly resistive, insulating film.
The key was combining a tin ion-beam with a thin tin film on the plastic, which produced high conductivity from the beginning, while the use of the 'sputtering' effect of the ion-beam to remove metal from the film had the effect of pushing them more towards insulating films. By tuning the process parameters, they were able to access the whole range of conductivity.
Potential applications for the research are now focused on developing plastic films with tailored conductivity. To achieve this, they are currently coating the whole film in metal, and doing a blanket ion-beam exposure; they next hope to pattern this material, at both the metal deposition and ion-beam step.
Laurie Donaldson