Published: October 05, 2011 r2011 American Chemical Society 22251 dx.doi.org/10.1021/jp207666w | J. Phys. Chem. C 2011, 115, 22251–22256 ARTICLE pubs.acs.org/JPCC Chemical Strain-Relaxation of Single-Walled Carbon Nanotubes on Plastic Substrates for Enhanced Conductivity Joong Tark Han,* ,† Jun Suk Kim, † Seung Goo Lee, ‡ Hyojin Bong, ‡ Hee Jin Jeong, † Seung Yol Jeong, † Kilwon Cho, ‡ and Geon-Woong Lee* ,† † Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute, Changwon, 641-120, Korea ‡ Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea b S Supporting Information ’ INTRODUCTION Plastic substrates, such as poly(ethylene terephthalate) (PET), polycarbonate, and so on, are used to support deposition of nanomaterials such as carbon nanotubes (CNTs), graphene, silver nanowire, and so on, for flexible transparent conductive electrode film applications. 1À7 In particular, single-walled CNT (SWCNT) network films on plastic substrates are good alter- native electrodes for touch-screen panels and flexible devices because of the excellent electrical and mechanical properties of SWCNTs and their solution processability under ambient conditions. 8À12 For these applications, the optoelectrical (sheet resistance (R s ) vs transmittance) and interfacial adhesive proper- ties of the films should meet several requirements. The con- ductivity of SWCNT network films can be enhanced by chemical doping, which operates via a charge transfer mechanism. 13À15 Top-coating or hybridization with highly hydrophilic materials and acid treatment can increase the density of the network films, which reduces the junction resistance. 16À18 Nanotube or network junction structures at the surface of plastic substrates are par- ticularly important in highly transparent SWCNT films because most nanotubes contact the substrate surface directly. 19,20 Na- notubes bound to the substrate can be strained by the nanoscale roughness of the substrate surface and by the network junction (Scheme 1), and this strain can change the electronic structure of the nanotubes and increase the resistivity. 21À26 Therefore, the optoelectrical properties of SWCNT films may be enhanced by relaxing the strained network structure of the nanotubes on the substrate surface. To this end, chemical swelling of plastic substrates upon introduction of organic solvents may be used to control the interfacial structure of SWCNT network films on plastic substrates, analogous to a chemical welding mechanism. 27 Chemical welding is expected to improve the interfacial adhe- sion and relax the strain induced by substrate interactions or network junctions, as shown in Scheme 1. Previous attempts at improving the interfacial adhesion of SWCNT films on plastic substrates have tested hybridization with binder materials or top-coating with organic or inorganic materials compatible with the substrates. 28,29 In this article, we present a straightforward method for enhancing the conductivity of SWCNT network films as well as the interfacial adhesion to plastic substrates by applying solvents to the films to enable chemical welding. Treatment with aromatic hydrocarbons decreased R s of the nanotube films on polyester by 20% without chemical doping effects. The decrease in R s could be modulated by controlling the welding area with island-structured graphene layer deposited prior to the SWCNT film formation. This observation could be explained in terms of a strain-related R s decrease in the SWCNT bundles, which Received: August 10, 2011 Revised: September 20, 2011 ABSTRACT: We have demonstrated that the conductivity of single-walled nanotube network films on plastic substrates is enhanced by a simple solvent treatment via chemical welding with aromatic hydrocarbons. The welding effect on the con- ductivity was confirmed by welding over a surface, the proper- ties of which were controlled by deposition of reduced graphene oxide sheets on the substrate before deposition of the SWCNTs. The correlation between the sheet resistance and the interfacial structure of nanotubes on polyester substrates is further discussed in conjunction with a solvent-induced strain- relaxation of nanotubes and the geometrical change of nano- tubes confirmed by atomic force microscopy and Raman spectroscopy after introducing solvents. We observe a clear reversible shift to higher energies in the G-mode band after chemical welding, indicating relaxation of the strain induced by the interface structure at the substrate and network junction.