Engineering Physics 2
CNT– pulsed laser deposition – chemical vapour deposition
On completion of this topic you will be able to understand:
1. What is Pulsed laser deposition of CNTs?
2. What is chemical vapour deposition of CNTs?
3. Drawbacks of arc-discharge and pulsed laser deposition methods.
Pulsed laser deposition:
In 1996, a dual-pulsed laser (Nd:YAG) vaporisation technique was used to optimise the laser method to produce single walled nanotube (SWNT) in gram quantities with yields of > 70%. Samples were prepared by laser vaporisation of graphite rods with a 50:50 mixture of Co and Ni powder (particle size ~ 1 µm ) at 1200 ºC in flowing argon, followed by heat treatment in vacuum at 1000 ºC to remove the C60 and other fullerenes. The laser converts a small amount of the composite solid material into a plasma of atoms and molecules which contains mainly C, C2, C3, Ni, Co and their ions. These species leave the target with extremely high initial velocities ~(from 1-5)x106 cm/s. This material violently collides with the background gas, generating a series of shock waves which provides additional heating that dissociates and ionizes the initial ejecta. Basically, however, the initial ejecta are confined by the background gas to remain inside a "bubble" of hot plasma which thermalizes as it expands in a stepwise way. In summary, the starting material for nanotube growth is a hot, expanding plasma of atomic/molecular carbon species and atomic vapor of catalyst.
The initial laser vaporisation pulse was followed by a second pulse to vaporize the target more uniformly. The use of two successive laser pulses minimizes the amount of carbon doposited as soot. The second laser pulse breaks up the larger particles ablated by the first one, and feeds them into the growing nanotube structure.
The material thus produced appears as a mat of 'ropes' 10-20 nm in diameter and up to 100 µm or more in length. Each rope is found to consist primarily of a bundle of SWNTs aligned along a common axis. By varying the growth temperature, the catalyst composition and other process parameters, the average nanotube diameter and distribution can be varied.
Drawbacks of arc-discharge and pulsed laser deposition methods:
1. In both the methods the carbon source is evaporated and it is unclear how to scale-up nanotube production to the industrial level.
2. These methods grow nanotubes in highly tangled forms mixed with unwanted forms of carbon or metal species.
3. It is difficult to purify, manipulate and assemble the nanotubes for building nanotube-device architectures.
Chemical Vapour Deposition(CVD):
There are many routes to prepare CNTs by CVD.
In one of the methods, large amount of CNTs can be formed by catalytic deposition of acetylene over Co and Fe catalysts supported on silica or zeolite (zeolite is a caged structure with large voids into which molecules can enter). The carbon deposition activity seems to relate to the cobalt content of the catalysts, whereas the CNT's selectivity seems to be a function of the pH in catalyst preparation. Fullerenes and bundles of SWNTs were also found among the MWNTs produced on Co/zeolite catalysts.
In another method, CVD of carbon within the pores of a thin alumina template called a mebrane with or without a Ni catalyst has been achieved. Ethylene was used with reaction temperatures of 545 ºC or Ni-catalysed CVD and 900 ºC for an uncatalysed process. The resultant carbon nanostructures have open ends. In this method SWNTs and double-walled nanotubes (DWNTs) can be produced.
Check your understanding
1. Explain pulsed vapour deposition.
2. Mention the drawbacks of pulsed vapour deposition.
3. Explain chemical vapour deposition.
4. State true or false: Apart from the diamond and graphite structure of carbon, it can be there in its amorphous and CNT.
Ans : True
5. State true or false: The CNTs cannot be produced by different chemical routes.
Ans : False
6. State true or false: The CNTs produced by different methods can vary from its forms, purity, production rate, etc.
Ans : True
On completion of this topic you have learned
1. Pulsed vaporisation method of producing CNTs.
2. Chemical vapour deposition method of producing CNTs.
3. The drawbacks of Pulsed vaporisation method of producing CNTs.
1. Find the reason for the different physical (electrical and mechanical) and chemical properties of CNTs.
1. “Nanotechnology-Basic science and Emerging Technologies-M.Wilson, Kalao Kannagara, G.Smith, M.Simmons, B.Raguse, CRC Press
2. “Engineering Physics-II” by P.K.Palanisamy.