4-point conductivity measurement of the new transparent conducting film developed by prof. Cor Koning (left) and prof. Paul van der Schoot (right). The black pot contains a dispersion of carbon nanotubes in water, and the white pot contains the conducting latex. Photo: Bart van Overbeeke.
A replacement material for indium tin oxide has been developed by researchers at Eindhoven University of Technology. It is a transparent, conducting film that is produced in water using carbon nanotubes and plastic nanoparticles. While the conductivity of the film is still a factor 100 lower than that of ITO, the researchers say it is already good enough to be used as an antistatic layer for displays, or for EMI shielding to protect against electromagnetic radiation. The researchers also expect the gap in electrical conductivity between their film and ITO to be quickly closed.
Indium tin oxide (ITO), an important material used in displays for all kinds of everyday products such as TVs, telephones and laptops, as well as in solar cells. Unfortunately indium is a rare metal, and the available supplies are expected to be virtually exhausted within as little as ten years.
“We used standard carbon nanotubes, a mixture of metallic conducting and semiconducting tubes”, says Cor Koning. “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.”
Another advantage the new film has over ITO is that it is environmentally friendly. All the materials used to produce it are water based and, unlike ITO, no heavy metals are used. The film is also more suited to flexible displays than ITO layers, which are fragile and lack flexibility.
Carbon nanotube reinforced polymeric composites can have favourable electrical properties, which make them useful for applications such as flat-panel displays and photovoltaic devices. However, using aqueous dispersions to fabricate composites with specific physical properties requires that the processing of the nanotube dispersion be understood and controlled while in the liquid phase. Here, using a combination of experiment and theory, we study the electrical percolation of carbon nanotubes introduced into a polymer matrix, and show that the percolation threshold can be substantially lowered by adding small quantities of a conductive polymer latex. Mixing colloidal particles of different sizes and shapes (in this case, spherical latex particles and rod-like nanotubes) 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 percolation, depending on the ease of charge transport between the various conductive components.