Four-point conductivity measurement of the new transparent conducting film developed by Cor Koning (left) and Paul van der Schoot (right). Credit: Bart van Overbeeke, TU/e.
According to a news release, a research team at Eindhoven University of Technology (TU/e) believes it might have found a surrogate for indium tin oxide in some applications, based on exploiting the particle size and shape interplay in a colloid combination of carbon nanotubes, electrically conductive latex and polystyrene. They say that the material may provide an alternative to ITO, which might be needed if indium reserves continue to decline.
The trick, apparently, is in manipulating how the spherical latex particles and rod-like nanotubes interact. They say the interplay in the polymer matrix “introduces competing length scales that can strongly influence the formation of the system-spanning networks that are needed to produce electrically conductive composites. Interplay between the different species in the dispersions leads to synergetic or antagonistic [electrical] percolation, depending on the ease of charge transport between the various conductive components.”
The team, whose work is published in Nature Nanotechnology (doi:10.1038/nnano.2011.40), says initial attempts at using CNT-reinforced polymeric composites have led to the creation of a transparent and conductive film that mimics ITO properties. It doesn’t duplicate ITO perfectly: Films made with the current batch of composites have a conductivity much lower than ITO. However, the levels of conductivity they have achieved, for now, are good enough for some antistatic applications and shielding against electromagnetic radiation, even for flexible displays.
Moreover, the group believes it can quickly eliminate the conductivity shortfall by tinkering with the composition and increasing the amount of metallic CNTs.
The group says it started with CNTs disbursed in water. Then they added conductive polymer latex and polystyrene beads, the latter acting as a binder. Application of heat then fused the beads to form a film. Lastly, freeze-drying removes the remaining water. What was left behind was a conducting network of nanotubes and beads from the conducting latex.
Returning to the issue of increasing conductivity in the material, it seems to be mainly a problem of cost. One of the coleaders of the TU/e team, Cor Koning, blames the low conductivity on their use of less expensive “standard” CNTs, which are a mixture of metallic conducting and semiconducting tubes. “But as soon as you start to use 100 percent metallic tubes, the conductivity increases greatly. The production technology for 100 percent metallic tubes has just been developed, and we expect the price to fall rapidly,” says Koning.
Paul van der Schoot is the other coleader of the team. He describes the material as being more environmentally friendly then ITO because it avoids the use of tin.