Carbon Nanotube Network Structuring Using Two‐Dimensional Colloidal Crystal Templates

Abstract

Owing to their extraordinary electrical, physical, and thermal properties as well as a high aspect ratio, carbon nanotubes (CNTs) have been intensively studied for such applications as nanometer-scale electronic devices, chemical and biological sensors, and polymeric nanocomposites. Among the CNT systems considered to date, networks or arrays of CNTs as films on a substrate have attracted a great deal of attention because of their potential applications in the area of transparent conducting materials. Because these CNT films can be mechanically flexible, they are the most reliable candidates for flexible, transparent, conducting materials that complement indium tin oxide (ITO) for certain applications, including electrodes for solar cells, smart windows, and transparent transistors. Therefore, the ability to fabricate uniform and continuous CNT films with good reproducibility is of great importance. Many approaches based on solution processes have been explored to fabricate continuous CNT films, including spin-coating, spray-coating, and filtration. In this Communication, we present a novel method for the production of continuous CNT films with higher optical and electrical properties on a flexible plastic substrate. We also demonstrate that the physical properties of CNT films can be improved by manipulating the network structures. It is no wonder that conductivity and transparency will be major factors in CNT films intended to be used as transparent conducting media. Because these two physical properties depend primarily on the CNT density in the films, we can adjust that density to control these properties. However, it is difficult to fabricate CNT films with both high transparency and high conductivity solely through the adjustment of the CNT surface density, defined as CNT mass per unit surface area. This is due to the fact that conductivity and transparency show an opposite dependence on the CNT surface density. We have aimed to fabricate highly transparent CNT films with CNT densities sufficient for high conductivity. We expected that the transparency of the CNT films could be improved by manipulating the network structure of the CNTs. Specifically, colloidal arrays have been adopted as sacrificial templates in various nanostructuring schemes, termed colloidal lithography. Therefore, we decided to use sacrificial 2D colloidal crystal templates to fabricate special network-structured CNT films for higher transparency. A schematic illustration of our procedure is shown in Figure 1. Using a suspension containing a mixture of colloidal particles and single-walled nanotubes (SWNTs), we first fabricated 2D colloid–CNT complex crystal C O M M U N IC A IO N