Heating and acid doping thin film carbon nanotube assemblies for high transparency and low sheet resistance

Transparent electrodes made from metal oxides suffer from poor flexibility and durability. Highly transparent and electrically conductive thin films were assembled as a potential indium tin oxide (ITO) replacement using layer-by-layer (LbL) assembly of single-walled carbon nanotubes (SWNTs), stabilized with negatively charged deoxycholate (DOC), and positively charged poly(diallyldimethylammonium chloride) [PDDA]. Ellipsometry, quartz crystal microbalance, and UV-vis were used to measure the linear growth of these films as a function of the number of bilayers deposited, while TEM and SEM were used to visualize the morphology of these films. The PDDA/(SWNT + DOC) system produced transparent (>82% visible light transmittance) and electrically conductive (~160 S cm^-1) 20-bilayer films with a 38.4 nm thickness. Moreover, a series of posttreatments, involving heating and nitric acid doping, were used to increase conductivity to 1430 S cm^-1 (Rs ≈ 300 Ω sq^-1), with no change in transparency, owing to the removal of PDDA and the charge transfer doping. This study demonstrates high visible light transmittance and electrical conductivity of SWNT-based thin films, which are potentially useful as flexible transparent electrodes for a variety of optoelectronic applications.