Preparation of Transparent Conductive Films Using Carbon Nanotubes
Authors: Matsumiya Yusuke , Tsushima Yusuke , Konishi Toshifumi 
Published: February 2024

Carbon nanotubes (CNTs), since the discovery by Iijima, have attracted tremendous interests in scientific research mainly due to their exceptional structure and physical properties. The high aspect ratio, light weight, extraordinary stiffness, and strength make CNTs a potentially very functional material to be used in polymer nanocomposites application. Carbon nanotubes are single or multilayered coaxial tubes of six-membered ring networks composed of carbon. It is an allotrope of carbon and is sometimes classified as a type of fullerene. Carbon nanotubes are expected to be applied to materials that normally do not conduct heat or electricity, such as resin, rubber, ink, and paint. In addition, it is expected to be applied to the electronics field because of its conductivity and thermal conductivity even in small quantities and high strength when made into long lengths. One of the problems of CNTs is the tendency of CNTs to aggregate with each other due to intermolecular interactions. In the aggregated state, CNTs cannot exhibit their inherently useful characteristics, and therefore, a technology to disperse CNTs at the nano-level is required. Currently, two major methods are being considered for dispersing CNTs: The first is dispersion by chemical modification. By introducing carboxylic acid into strong acid treated CNTs and introducing hydrophilic or hydrophobic substituents here, solubilization in water or organic solvents is possible. However, there is a problem that this destroys the structure of CNTs and thus greatly impairs their original properties.

The other method is to physically disperse CNTs. This is a very simple method that uses ultrasonic irradiation to loosen bundled CNTs, but the dispersion is only temporary, and re-agglomeration occurs quickly. Problems have also been reported, such as excessive ultrasonic irradiation destroying the structure of the CNTs. We have succeeded in producing CNTs nano dispersion gels by adding an aromatic compound to agglomerated CNTs and applying agitation and ultrasonic irradiation. The following describes the process of preparing CNTs dispersion gels. After agglomerated CNTs are temporarily dispersed by ultrasonic irradiation, an aromatic compound (dispersant) is added and mixed and agitated. The dispersed CNTs and the aromatic compound are combined by π-π interaction, and the aromatic compound is adsorbed on the CNTs surface. When CNTs molecules adsorbed with aromatic compounds approach each other, the aromatic compounds on the surface form π-π interactions. At this point, the CNTs are in a gel-like state.

Since this dispersion gel does not chemically modify the CNTs, it can be prepared without destroying the structure of the CNTs. The CNTs gel proved to be free from aggregation even after the dispersant component was removed, and the network structure was maintained. In addition, this dispersion gel shows high electrical conductivity because the CNTs are dispersed three-dimensionally and the CNTs form three-dimensional conductive paths in the gel.Next, we introduce the preparation of CNT composite resin. CNT composite resin is prepared by adding binder resin to the carbon nanotube dispersion gel prepared earlier, mixing, and stirring, and ultrasonic irradiation. A transparent conductive film is created by forming the prepared CNT composite resin on a glass slide.

In recent years, transparent conductive films have been in high demand due to the development of electronics products, and the use of CNTs as the main material is expected to significantly reduce the cost of development, as they are less expensive than Metallic materials such as indium tin oxide and can be stably supplied. The challenge of this research is that the amount of black CNTs added, which imparts conductivity, significantly affects the transparency of the transparent conductive film. Therefore, it is necessary to select aromatic compounds and resin materials that achieve both high conductivity and transparency with low amounts of added CNTs from a molecular chemistry perspective.

In our previous research, we focused on the chemical bonding between resin materials and CNTs for the first time and attempted to improve electrical conductivity by using hydrogen bonding for CNTs. In addition, by combining polycarbonate, which exhibits high transparency and impact resistance, with CNT dispersion gel, we attempted to fabricate a transparent conductive film with high durability. As a result, we succeeded in developing a transparent conductive film material with performance applicable to touch panels.

In this study, CNTs dispersion gels prepared with aromatic compounds were evaluated using absorbance measurements and Raman spectroscopy. The Raman spectra measurements allowed us to quantitatively evaluate the interaction between SWCNTs and aromatic compounds. The possibility of charge-transfer complex formation was also suggested by the results of absorbance measurements.

Figure 1