Laser vaporization of carbon

Laser vaporization of carbon Long endless carbon nanotube 2.2 [81]... 

SWNT were produced by laser ablation and subsequently purified via acid treatment. Single-wall nanotubes were manufactured by laser vaporization of carbon rods doped with Co, Ni and FeS in an atmosphere of Ar H2. Standard SWNT products made by HiPCO and other methods contain significant amount of sooth, graphite flakes, and remnants of the catalyst, which need to be removed prior to the assembly. The quality of the dispersion directly affects the mechanical performance of the resulting composite. 

Whereas multi-wall carbon nanotubes require no catalyst for their growth, either by the laser vaporization or carbon arc methods, catalyst species are necessary for the growth of the single-wall nanotubes [156], while two different catalyst species seem to be needed to efficiently synthesize arrays of single wall carbon nanotubes by either the laser vaporization or arc methods. The detailed mechanisms responsible for the growth of carbon nanotubes are not yet well understood. Variations in the most probable diameter and the width of the diameter distribution is sensitively controlled by the composition of the catalyst, the growth temperature and other growth conditions. 

In 1984 it was observed that, upon laser vaporization of graphite, large carbon-only clusters C with u = 30-190 can be produced [14]. The mass distribution of these clusters was determined by time-of-flight mass spectrometry. Only ions with... 

Figure 1.3 Time-of-flight mass spectrum of carbon clusters produced by laser vaporization of graphite under the optimum conditions for observation of a dominant Ceo signal [15].

A Co/Ni alloy is the next active catalyst among the binary combinations within the iron-group metals in the arc discharge method (51). Laser vaporization of metal/ carbon composite in argon atmosphere at high temperature (1200°C) can also produce SWNTs (41). Guo et al. (41) reported that the Co/Ni alloy was the most effectual, with a yield of 50-90% in the laser ablation method. 

In 1984. scientists (Rohlling. Cox. and Caldor at Exxon Research and Engineering) created clusters of carbon (soot) by the laser vaporization of a carbon target rod in connection with a supersonic nozzle. By means of mass spectroscopy, the researchers determined the relative abundance of the carbon clusters produced. Small, 20- to 40-atom clusters of carbon were expected inasmuch as these had been produced a number of times by earlier investigators working on the soot problem. In such experiments, an interesting but unexplained question always arose—Why were only even-numbered carbon clusters produced in the complete absence of odd-numbered clusters See Fig. 3. 

Fig. 3. Reasonable facsimiles of mass spectra produced by laser vaporization ot carbon in a supersonic beam, indicating three stages in the process for increasing the extent of clustering. Experiment was carried out by Robbing, Cox, and Kaldor (Exxon Research and Engineering), Original diagrams were featured In Nature (1985)...

Figure 5. Mass spectrum of carbon clusters (C3-C100) generated by laser vaporization of graphite (Rohlfing el al. 1984). (Reprinted with permission of the American Institute of Physics.)...

Single-wall carbon nanotubes (SWNTs) can be prepared by laser-vaporization of a graphite source. A newer process uses carbon monoxide as the source of the carbon and is called the HiPco process. The catalyst is generated in situ from iron carbonyl. The SWNTs from the HiPco process are characterized by a smaller diameter and exhibit greater reactivity with organic reagents. 

Figure 2 Time-of-flight mass spectrum of carbon clusters produced by laser vaporization of graphite (September 4, 1985) imder conditions which first exhibited the dominance of the Cgo cluster and led to the recognition that 60 might be a magic number. (Reprinted with permission from Ref 2a. 1991 American Chemical Society)...

Figure 6.25. Proposed scheme for the formation of the multishell fullerene C6o C24o- Reproduced from Mordkovich, V. Z. Shiratori, Y Hiraoka, H. Takeuchi, Y. Synthesis of Multishell Fullerenes by Laser Vaporization of Composite Carbon Targets, found onhne at http //www.ioffe.rssi.ru/joumals/ ftt/2002/04/p581-584.pdf.

Heath, J.R., Zhang, Q., Obrien, S.C. et al. (1987) The formation of long carbon chain molecules during laser vaporization of graphite. Journal of the American Chemical Society, 109, 359-363. 

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