Carbon nanotubes and graphene
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|Carbon nanotubes and graphene|
Model of two crossing CNT on a graphene surface
|Other Names||CNT, graphene, SWCNT, MWCNT|
Carbon nanotubes (CNTs) and graphene are allotropes of carbon which have unique electrical, mechanical & other physical properties. Graphene is a two-dimensional material, basically a single layer of graphite, with carbon atoms arranged in a hexagonal, honeycomb lattice. Carbon nanotubes are hollow, cylindrical structures, essentially a sheet of graphene rolled into a cylinder. The angle at which they are rolled (their "chirality"), and their diameter, affect their properties. CNTs can be single-walled (SWCNTs or SWNTs) or multi-walled (MWCNTs or MWNTs).
CNTs and graphene have many remarkable properties, and have been suggested for a wide range of applications. In both graphene and CNTs, the carbon atoms are connected by sp2 bonds, which are even stronger than the sp3 bonds found in diamond, and which give both materials exceptional strength. In addition, both have extremely high thermal conductivity, electron mobility, and chemical reactivity. Both exhibit interesting physics due to their two-dimensional and one-dimensional structures. Graphene is a zero-gap semiconductor and exhibits the anomalous quantum Hall effect. Carbon nanotubes, depending on their structure, can be either semiconducting, with a variable bandgap, or metallic.
CNTs and graphene are of interest for many different applications. Being light and strong, CNTs, or composite materials containing CNTs, have been suggested for many uses that require these properties, from clothing and tennis racquets to tissue engineering, bulletproof gear and space elevators. Other applications have taken advantage of their electrical, chemical and optical properties: CNTs have been studied for use in transistors, as battery electrodes, in solar cells, and sensors. Graphene, being very thin, flexible and yet conductive, is of interest as a transparent conductor for photovoltaics and other flexible electronics. Having a large ratio of surface area to mass makes it promising for applications requiring reactivity or surface adsorption, such as chemical sensing or energy storage.
Both CNTs and graphene can be produced by a variety of methods. Graphene can be produced by exfoliation, epitaxial growth on SiC, and chemical vapor deposition on metal catalysts. After decades of theoretical study, graphene was produced in 2004 by Geim and Novoselov by using Scotch tape to successively remove layers from a flake of graphite until they reached a single layer. This monolayer of carbon, when transferred to an oxidized silicon wafer, can be seen optically. Graphene produced by this type of mechanical exfoliation has shown the highest electron mobility and fewest defects to date, compared with other methods of production. However, exfoliation cannot be used to create graphene over large areas, and is therefore limited technologically. Therefore, catalytic CVD has emerged as the most promising growth method for practical applications.
Graphene has been grown by CVD on Cu, Ni, and other transition metal substrates. Among the various metal catalysts that have been studied, Cu has the lowest solid solubility for carbon. This means that after a single layer of graphene is deposited on the substrate surface, no more Cu is exposed to the gas and available to catalyse the decomposition of the gas and and deposit further carbon. Thus, graphene growth is self-limited, leading to single-layer graphene. In contrast, graphene growth on Ni often results in multiple layers.
After deposition, graphene typically needs to be removed from the metal substrate and transferred to another substrate. This can be done by spin-coating the graphene with polymethyl methacrylate (PMMA), and then chemically etching away the metal substrate. The graphene on PMMA is then placed on the desired substrate, graphene side down, and the PMMA is dissolved away in acetone.
CNTs can be synthesized by arc discharge, laser ablation, catalytic CVD and many other techniques. CVD is the dominant growth technique, and can be used to grow CNTs in bulk or on surfaces. Iron or other transition metals are used as catalysts for CVD growth. To grow forests of vertically aligned CNTs, an extremely thin layer of the metal catalyst is first deposited on a substrate. This is heated to allow the thin catalyst layer to restructure itself into small islands, each of which becomes the nucleation site for a carbon nanotube. During CVD, the hydrocarbon precursor gas dissociates and adsorbs at the surface of the metal catalyst, where the carbon diffuses and then precipitates out to form the nanotube.
In the LNF, the Angstrom Engineering Furnace has source gases available to grow graphene or CNTs.
- List of chemical (or otherwise) processes for this material
- LNF Tech Talk for CNT and Graphene is Coming Soon!
- R. Munoz, C. Gomez-Aleixandre, "Review of CVD Synthesis of Graphene," Chemical Vapor Deposition (2013), 19, 297-322.
- J. Robertson, G. Zhong, S. Esconjauregui, C. Zhang, M. Fouquet, and S. Hofmann, "Chemical Vapor Deposition of Carbon Nanotube Forests," Phys. Stat. Sol. (2012), 249, 2315-2321.
- "Graphene," Wikipedia.
- "Carbon nanotube," Wikipedia.