CNT Research Points to Higher PV Efficiency, Advanced Electronics
Research results promise direct control over carbon nanotube (CNT) structures during growth. This could lead to CNTs with the potential to transport electricity faster and over greater distances than ever before, with minimal losses in energy.
Alexander E. Braun, Senior Editor -- Semiconductor International, 10/19/2009
Research results promise direct control over carbon nanotube (CNT) structures during growth. This could lead to CNTs with the potential to transport electricity faster and over greater distances than ever before, with minimal losses in energy. It also opens the way to a whole new class of electronic devices, such as more powerful and compact computers, supercapacitor electrodes, fuel cells, and others, as well as the improvement of existing devices, such as photovoltaic cells.
The work was carried out at the University of Louisville (Louisville, Ky.) and Purdue University (West Lafayette, Ind.), and funded by Honda Research Institute USA (Columbus, Ohio). The researchers have grown CNTs on the surface of metal nanoparticles, taking the cylindrical form of rolled honeycomb sheets with carbon atoms in their tips. When these CNTs exhibit metallic conductivity, they possess extraordinary strength compared with steel, show higher electrical properties than copper, their thermal conductivity is as efficient as that of a diamond, and they are extremely light.
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Configuration (n, m) diversity of CNTs. Each (n, m) corresponds to the specific structure and therefore conductivity: red and pink exhibit metallic properties, and blue are semiconducting. Configuration mapping of preferentially grown metallic CNTs by controlling the shape and size of catalyst particles is shown; the height of tunes represents the probability of growing particular (n, m) CNTs in the material. (Source: Honda Research Institute USA) |
According to an article announcing the results of the research, "Preferential Growth of Single-Walled Carbon Nanotubes With Metallic Conductivity," published in the Oct. 2 issue of Science, single-walled CNTs (SWCNTs) are classified as metallic or semiconducting, depending on their conductivity, which is determined by their chirality. Traditional synthesis methods are unable to grow controllably nanotubes with specific conductivities. Past research efforts to control the structural formation of CNTs with metallic conductivity through conventional methodology resulted in a success rate of ~20-50%. However, the researchers determined that by varying the noble gas environment during the catalyst's thermal annealing, and in combination with oxidative and reductive species, it is possible to increase the number of CNTs with metallic conductivity from ~33% to 91% of the total population produced.
The Honda and academic researchers discovered that they could control whether the CNTs become metallic or semiconducting by using either argon or helium as carrier gases during the production process. This is the first time it's been reported that it is possible to control fairly systematically whether CNTs achieve a metallic state, according to Avetik Harutyunyan, principal scientist at the Honda Research Institute. "Further research is in progress with the ultimate goal to take complete control over grown nanotube configurations to support their real-world applications," he said.
Harutyunyan added that the research findings indicate that the CNT configuration that defines its conductivity depends not only on the size of the metal nanocatalyst used to nucleate the tube, as was previously believed, but is also importantly based on its shape and crystallographic structure, which as a result of this research is now controllable. Previously, CNTs have been grown, but it was not possible to control whether they would occur as one of two types - semiconducting or metallic (each with different applications) - because the process was random. The metallic nanotubes are more useful as building blocks that connect other nanostructures or for windows for solar cells or other devices with optical and electrical qualities.
Purdue researchers, led by Professor Eric Stach, used the technique developed at Honda to produce large quantities of CNTs and precisely measuring whether they were metallic or semiconducting. They used a TEM to observe nanotube formation, revealing that changes in the gaseous environment can vary the shape of the metal catalyst nanoparticles from very sharp faceted to completely round. These catalyst rearrangements point to correlations between catalyst morphology and resulting CNT electronic structure, showing that chiral-selective growth may be possible. The Louisville researchers, led by Professor Gamini Sumanasekera, produced the CNTs in thin film, and made careful measurements to determine whether the nanotubes achieved a metallic state.
Sumanasekera expressed the hope that the findings will renew interest in the field. "Carbon nanotubes may replace more commonly used materials for purposes that require electrical conductivity and good light transmission," he said.
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