Delivered by Ingenta to: Main CID is 80004805 (JPP) IP : 95.24.31.138 Tue, 26 Jun 2012 16:10:36 Copyright © 2006 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 6, 1939–1944, 2006 CarbonNanotubeBasedTransparent ConductiveThinFilms X. Yu, R. Rajamani ∗ , K. A. Stelson, and T. Cui Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA Carbon nanotube (CNT) based optically transparent and electrically conductive thin films are fabri- cated on plastic substrates in this study. Single-walled carbon nanotubes (SWNTs) are chemically treated with a mixture of concentrated sulfuric acid and nitric acid before being dispersed in aque- ous surfactant-contained solutions. SWNT thin films are prepared from the stable SWNT solutions using wet coating techniques. The 100 nm thick SWNT thin film exhibits a surface resistivity of 6k/ with an average transmittance of 88% on the visible light range, which is three times better than the films prepared from the high purity as-received SWNTs. Keywords: Carbon Nanotube, Transparent, Conductive, Thin Film, Acid Treatment. 1. INTRODUCTION Transparent conductive thin film electrodes are widely used for liquid crystal displays (LCDs), touch screens, solar cells, and flexible displays. Due to high electrical conductivity and high optical transparency, indium tin oxide (ITO) thin films are often used in these applications. Typically, ITO thin films needs to be deposited or post- annealed at high temperatures (>300  C) to achieve an optimal combination of electrical and optical properties. 1 2 However, high processing temperatures are not suitable for many plastic substrates, such as polyethylene terephthalate (PET) films, which are often used for touch panels and flexible electro-optical devices. Another shortcoming of ITO films prepared by conventional methods is their brit- tleness, a 2% strain will make the films crack and thus lose conductivity. 3 Recently investigated wet coating tech- niques can provide ATO/polymer films with flexibility 4 (ATO, antimony tin oxide, is a material similar to ITO), but the conductivity is greatly reduced to about 10 −2 S/cm, which limits its applications. An alternative material to ITO is poly(3,4-ethylenedio- xythiophene)-poly(4-stryrenesulfonate) (PEDOT:PSS) which is a conductive polymer material. With PEDOT: PSS, flexible films can be made that can continue to con- duct beyond 60% strain. 5 However, the low transparency and conductivity (∼1 S/cm) (Ref. [6]) of PEDOT:PSS restricts its suitability for many applications. ∗ Author to whom correspondence should be addressed. With high conductivity (in the order of 10 3 ∼ 10 4 S/cm (Ref. [7])) and high aspect ratio (>100), single-walled car- bon nanotubes (SWNTs) have been investigated for fab- ricating transparent conductive thin films/coatings. One approach is to add carbon nanotubes into a polymer matrix, such as polyimide 8 or ASTAA (alkoxysilane ter- minated amide acid) polymer. 9 These polymeric compos- ites have good mechanical properties but low conductivity. Therefore, they are just suitable for those applications that do not require high electrical conductivity such as electro- static charge mitigation. To improve the conductivity, films fabricated directly from SWNT solutions have also been investigated. In previously published work, SWNTs were first dispersed in aqueous solutions, then thin films were made by wet coating techniques, 10 or filtering through membrane and transferring to substrates thereafter. 11 Thin films made by these methods show much better conduc- tivity than the CNT-based composites. Since SWNTs are insoluble in most organic solvents and water, a surfactant (such as Triton X-100 or SDS) has to be used to make a stable SWNT solution. However, the surfactant adsorbed on the surface of SWNTs will greatly affect electrical properties. Therefore, removing surfactant in the coatings is one important issue for making transparent conductive SWNT thin films. The cited filtration method in Ref. [11] is effective for washing away the surfactant, but the fabri- cation process in Ref. [11] is not easy to be scaled up and the bonding between the SWNT film and the substrate is quite weak. This motivates the need to improve the fab- rication processes so as to achieve CNT thin films with improved electrical and optical transmission properties. J. Nanosci. Nanotechnol. 2006, Vol. 6, No. 7 1533-4880/2006/6/1939/006 doi:10.1166/jnn.2006.332 1939