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New Carbon Nanotube Composite

18 March 2008

Fujitsu Laboratories has formed a new nano-scale carbon composite featuring a self-organizing structure, by combining carbon nanotubes and graphene.

The newly-discovered composite structure is synthesized at a temperature of 510 C, cooler than for conventional graphene formed at temperatures too high for electronic device applications, thereby paving the way for the feasible use of graphene as a material suitable for future practical use in electronic devices which are vulnerable to heat. Carbon nanotubes have properties including high thermal conductivity and high current-density tolerance, while graphene is known for its high electron mobility. Carbon nanostructures combining these two materials hold the promise of creating new potential for material research and applications.

Details of this technology will be presented at the 34th Fullerene Nanotubes General Symposium to be held from March 3 to March 5 in Nagoya, Japan.

Carbon nanotubes and graphene are both nano-scale structures consisting of carbon atoms. Graphene is a sheet-like hexagonal lattice of carbon atoms, while nanotubes can be described as graphene wrapped into a cylinder with a nano-scale diameter.

Despite the fact that both are made from the same carbon atoms, each has very distinct characteristics. Of any material found in nature, carbon nanotubes feature the highest thermal conductivity and mechanical strength as well as the ability to withstand the highest current density, making them an attractive material for wiring, heat dissipation, field electron emitters, and other potential applications. Research and development is underway to find technologies to synthesize carbon nanotubes at temperatures as low as approximately 400 C, a temperature that would enable its use in electronic devices vulnerable to heat. Since the discovery of its high electron mobility in 2004, graphene has become attractive as a channel material for future transistors. However, conventional methods for synthesizing graphene only work at temperatures over 700 C - considered too high for use in electronic devices - or involve a time-consuming and unreliable process of stripping away graphite crystals.

Fujitsu Laboratories is researching ways to develop electronic devices that take advantage of the superior properties of carbon nanostructures.

In order to better understand the growth mechanism of carbon nanotubes, Fujitsu Laboratories conducted experiments using chemical vapour deposition, a technique in which a feedstock gas is heat-cracked in a vacuum chamber to synthesize film or structures on a substrate. This resulted in the formation and discovery of aligned growth multi-walled carbon nanotubes featuring layers of graphene (from a few layers to a few dozen) on top formed in a self-organizing way, thereby forming a complex composite.

Carbon-based materials come in a variety of different forms that depend on how their atoms link together, such as zero-dimensional fullerenes, one-dimensional nanotubes, two-dimensional (2-D) graphene, and three-dimensional (3-D) diamonds. Complex structures consisting of zero-dimensional and one-dimensional elements, known as ""peapod[8]"" structures, have already been created. The new complex composite developed by Fujitsu Laboratories is the world's first composite featuring one-dimensional and two-dimensional elements based on graphene layers and nanotubes, which are perpendicularly connected. The composite was synthesized at the relatively low temperature of 510 C.

Due to the fact that carbon nanotubes are linear, one-dimensional structures, in the two-dimensional directions perpendicular to the tube axis they have nearly no thermal or electrical conductivity between tubes. Graphene, on the other hand, possesses electrical and thermal conductivity across two dimensions. The newly-discovered carbon nanostructure is expected to have electrical conduction and thermal dissipation in all directions. Conventionally aligned-growth carbon nanotubes have had relatively poor uniformity in length, thus being inconsistent when joined in the upper areas and resulting in increased thermal and electrical resistance. As the new carbon nanostructures from Fujitsu Labs feature carbon nanotubes that nearly all connect to the graphene with good uniformity at their endpoints, and since the graphene surface is planar, it is anticipated that the new carbon nanostructures will enable excellent electrical and thermal conductivity. This technology brings the application of graphene for electronic devices one step closer to practical use.