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　　The results demonstrate that it is possible to control the structure of carbon nanotubes (CNTs) directly during their growth. This increases the application potential of CNTs, which can transmit electrical signals at higher speeds and over longer distances with minimal energy loss. This also provides the possibility of obtaining new electronic devices, such as high-performance streamlined computers, electrodes with supercapacitors, fuel cells and other applications, while also improving the performance of existing devices such as photovoltaic cells.

　　The research was conducted at the University of Louisville (Louisville, Ky.) and Purdue University (West Lafayette, Ind.) and was funded by Honda Research Institute USA (Columbus, Ohio). The researchers grew CNTs on the surface of metal nanoparticles to form cylindrical structures on a thin layer of honeycomb rings, with carbon atoms at the top of these structures. While these CNTs exhibit metallic conductive characteristics, they are mechanically stronger than steel, more electrically conductive than copper, have thermal conductivity comparable to diamond, and they are very light.

　　Various combinations of CNT (n, m) forms. Each combination (n,m) corresponds to a specific structure and corresponding conductivity: red and pink exhibit metallic properties, while blue is semiconducting. By controlling the shape and size of the catalytic particles, preferentially grown metallic CNTs can be obtained. The combination is as shown in the figure. The height of the cylinder represents the relative probability of growing specific (n, m) CNTs on the material. (Source: Honda Research America)

　　The research results have been published in the "Science" journal published on October 2, titled "preferenTIalGrowthofSingle-WalledCarbonNanotubesWithMetallicConducTIvity". According to research reports, single-walled CNTs (SWCNTs) can be divided into metallic types and semiconductor types based on their conductivity, which in turn depends on the chirality of the carbon tube. Traditional synthetic methods cannot grow nanotubes with specific conductive capabilities. In the past, metal conductive CNT structures were controlled through common methods, but the success rate was only 20-50%. However, the researchers believe that by changing the gas environment during thermal annealing of the catalyst and adding some oxidizing and reducing components, the formation probability of metallic conductive CNTs can be increased from about 33% to 91%.

　　Researchers from Honda and academia have found that by using argon or helium as a carrier gas during the fabrication process, growing CNTs can be controlled to become either metallic or semiconductor. According to AveTIk Harutyunyan, chief scientist at Honda Research Institute, this is the first report on systematically controlling the growth of metallic CNTs. "Further research is ongoing, with the ultimate goal of fully controlling the nanotube growth mix to support real-world applications."

　　Harutyunyan also said that the past view was that metal nanocatalysts were mainly used for CNT nucleation, and the size of the catalyst determined the conductivity of CNTs, but current research shows that the shape and crystal structure of the catalyst also have a great impact, and It is through the control of these factors that the control of CNT conductive properties is achieved. In the past, the entire process of growing CNTs was random, so it was impossible to determine whether the resulting carbon tube was a semiconductor type or a metal type (different types correspond to different applications). Metallic nanotubes are more useful in many areas, including as conductive materials to connect other nanostructures and as window materials in solar cells and other optical and electronic devices.

　　A research team at Purdue University, led by Eric Stach, used technology developed by Honda to create large quantities of CNTs and precisely measure their conductive properties. They used TEM to observe nanotube formation and showed that changes in the gas environment can change the shape of the metal catalyst nanoparticles, from a very sharp surface to a completely spherical surface. These catalyst structural rearrangements indicate that there is a relationship between the catalyst morphology and the obtained CNT electrical structure, which in turn indicates the chirality of the selective growth. Louisville researchers, led by Gamini Sumanasekera, made thin films of CNTs and made careful measurements to determine whether the nanotubes formed a metallic state.

　　Sumanasekera hopes the discovery will spark renewed interest in the field. "Carbon nanotubes may replace some currently commonly used materials and can meet higher demands for electrical conductivity and light transmission capabilities," he believes.

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