Sonophysically Exfoliated Individual Multi-Walled Carbon Nanotubes in Water Solution and Their Straightforward Route to Flexible Transparent Conductive Films

Abstract

Flexible transparent conducting films (TCFs) with low electric resistance and high optical transmittance have received considerable attention for niche applications such as flexible/foldable displays, touch screens, solar cells, transistors, and transparent electrodes for liquid crystal displays. (Calnan & Tiwari, 2010; Gruner, 2006; Heo et al., 2010; Jo et al., 2010; Takenobu et al., 2006) Deposition of indium tin oxide (ITO) onto plastic substrates to prepare the flexible TCFs has been an attractive strategy owing to its transparency, conductivity, and wide usability. (D. H. Kim et al., 2006; Lin et al., 2008; Na et al., 2008; Sierros et al., 2009; Wang et al., 2008) Nevertheless, the flexible ITO-based TCFs still suffer from several drawbacks. For example, the films are often prepared under vacuum and high temperature conditions, resulting in the limitation of their suitability for many polymer substrates such as polyethylene terephthalate (PET), which are often used in flexible devices and touch screen panels. Also, ITO is expensive owing to the limited supply of indium; it is brittle and will crack under a 2% strain, resulting in loss of conductivity. Furthermore, the film transparency is poor in the near-infrared range. (Feng et al., 2010; Manivannan et al., 2010; Park et al., 2010) Therefore, a substitute for ITO is necessary. To address this issue, carbon nanotubes (CNTs), because of their high flexibility, specific surface area, low density, and excellent electrical, optical, and mechanical properties, have been regarded as the most promising alternative. CNTs have a preeminent status in nanomaterials and have a huge range of potential applications. (Z. R. Li et al., 2007; Paul & Kim, 2009; Simien et al., 2008) Single-walled CNT (SWCNT)–based conductors and semiconductors were used as the active material in the fabrication of flexible CNT-TCFs. Recent efforts towards the fabrication of SWCNTs on several substrates of glass, UV-ozone (UV-O3) treated glass, poly(styrenesulfonate) (PEDOT:PSS) coated glass, flexible PET films, and other substrates to form transparent conducting SWCNT films with sheet resistance and transparency equivalent to ITO have been attempted. These attempts used various approaches such as spin coating, spray coating, solution dipping where the SWCNTs were pre-formulated in combination with surfactants and polymers. (Chen et al.; Dan et al., 2009; Manivannan et al., 2010; Paul & Kim, 2009; Saran et al., 2004; Xiao et al., 2010) However, the