Technology | Highlight
Study Yields Electric-Field-Induced Layer Formation Method
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ational Institute of Advanced Industrial Science and Technology (AIST) has elucidated the separation mechanism of metallic and semiconducting carbon nanotubes (CNTs) in the electric-field-induced layer formation method (ELF method), which is the core technology for manufacturing high-purity semiconducting CNTs indispensable for the application of CNTs in semiconductors, and details of which were not known. As a result, a guiding principle for producing high-purity CNTs using the ELF method has been obtained, and higher efficiency CNT separation has been achieved. Compared to the conventional method, the separation cost has been reduced more than 90 percent and the separation time has been shortened by half.

Separating Metallic, Semiconducting CNTs
Printed electronics has been attracting attention as an energy-saving and resources-saving manufacturing process. This technology enables the production of flexible and large-area devices at low costs. However, for practical use, high-functional inks, in which semiconductor materials and metal materials are dispersed, are indispensable. CNTs have excellent mechanical strength, chemical stability, and electric characteristics, and hence, they are expected as materials of next-generation high-functional inks.

A schematic of the separation of metallic and semiconducting CNTs using the ELF method
A schematic of the separation of metallic and semiconducting CNTs using the ELF method

This time, AIST has found that metallic and semiconducting CNTs with different electrical properties have different quantities of charges in aqueous dispersions. In addition, by studying the effects of the environment of CNTs’ aqueous dispersions, such as pH and concentration of dispersant, on CNTs’ quantities of charges, the mechanism of the movement of the two types of CNTs, which are both charged negatively, to opposite electrodes in an electric field has been clarified. Semiconducting CNTs with a large quantity of negative charges moved by electrophoresis, while metallic CNTs with a small quantity of negative charges moved by electro-permeation flow. These findings are expected to lead to a design guideline for separation equipment that would enable the purification and mass production and stable supply of separated CNTs.

As a result of this study, conditions for separating CNTs using the ELF method have been optimized, and separation costs (costs of CNTs, dispersants, and solvents used for separation) has been reduced by 90 percent and separation time has been shortened by half compared with the conventional ELF method. Several challenges toward mass production have been overcome. Furthermore, the findings of the study are expected to lead to the advancement of separation technology and the obtainment of design guideline for separation equipment that would enable mass production and stable supply of metallic and semiconducting CNTs. They are expected to contribute to the progress of the application of CNTs to printed electronics.

Based on the technologies developed this time, AIST intends to advance collaboration among industry, government, and academia using semiconducting CNTs obtained by the ELF method to develop new applications through a joint research. At the same time, it will work to develop CNT inks that provide performance for practical use and printing technology for those inks. Leveraging CNT’s characteristics that they are light and flexible, AIST aims to contribute to the creation of an energy-saving and comfortable society using CNT devices.