The most well known nanotubes are made of carbon , but also other chimical elements have been used to sinthetise nanotubes, like :
The nanotubes can be cathegorised by the number of concentrical cilinders which form the tubes :
Single Wall Nanotubes - SWNT
Multi Wall Nanotubes - MWNT
Carbon Nanotubes
The most widely used are CNT (Carbon Nanotubes). They wer first evidentiated by Ijima in 1991.
The SingleWall Carbon Nanotubes can be imagined as a rolled graphene sheet.
The graphene sheet have an hexagonal structure. The elementary cell it is determinet by the vectors noted in the figure bellow as a1 and a 2.
The Nanotube Type is determined by the Chirality Vector
Ch=na1+ma2.
The origin and the tip of the vector Ch will be superposed when the graphene sheet is rolled to form a carbon nanotube ( it's just a way of seeing the geometry of the problem, the carbon nanotubes are not made by rolling a graphene sheet).
The chiral angle θ is the angle between the Chiral vector and the "zigzag" direction of the hexagonal network of the graphene sheet.
After the chiral angle , the SWCNT can be categhorised as :
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a) «armchair» θ = 30° |
| b) «zigzag» θ = 0° | |
| c) «chiral» 0 < θ< 30° |
The raport between the parameters n and m of the chiral vector determine if the SWCNT is metalic or not. One third of the SWC nanotubes are metallic, the rest are semiconductors.
Fabrication
There are 3 main techniques for building CNTs:
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a) Arc discharghe : the carbon is
vaporised by an electric arc (T>3000C, I ~ 100A), and the tubes ar
formed on the cathode, in a He flux. The tubes are partially aligned, and amorphous carbon is formed also. |
| b) Laser ablation : In a argon gas flux a
high power pulsed laser is applied on a graphite cible, and a Carbon
plasma is created. The obtained nanotubes are ussualy monowall. |
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| c) CVD ( Chemical Vapor Deposition). Very simple technique, with temperatures < 1000°C |