Alternative conductive coatings based on multi-walled carbon nanotubes

Abstract

The aim of this work was to develop coatings as possible replacements for tin-doped indium oxide (ITO) systems. The alternative material of choice was carbon nanotubes, due to their flexibility, the abundance of carbon element in nature, their high aspect ratio and high electrical conductivity. Focusing on cost benefits, very thin multi-walled carbon nanotubes (MWNTs) were investigated rather than single-walled nanotubes (SWNTs) normally presented in the literature. Moreover, spin coating sol-gel technique was performed as a less expensive alternative to sputtering techniques used in the production of ITO films. MWNTs were studied both as pure networks as well as embedded in conductive and insulating matrixes. As networks, MWNTs presented sheet resistances as low as 20 kΩ/sq with transparency in the visible range of 87%, values comparable to some of SWNT networks presented in the literature. MWNTs were investigated as additional conductive elements in antimony-doped tin oxide (ATO) matrix. The results showed that the addition of concentrations as low as 0.1 wt.-% MWNTs is sufficient to decrease the resistivity of conducting ATO films by a factor of 16, with preserved transparency (90%) in the visible range. Less ATO nanoparticles and lower temperatures of sintering are required in order to obtain films with comparable resistivities and even higher transparency than that presented by ATO films reported in the literature. MWNTs have also provided resistivities in the order of 10 Ω.cm-1 to an initially insulating TiO2 matrix. The transparency of these films was, however, affected by the concentration of MWNTs necessary in order to reach the percolation threshold. Although the studied coatings did not meet the requirements necessary for a proper substitution of ITO in opto-electronic devices, their optical and electrical response as well as their low cost and simplicity of preparation allow them to be used in other applications where the high conductivity of ITO is not a requirement. The structural, optical, mechanical and electrical properties of all coatings were studied using different techniques and are demonstrated in this work.