Diameter Control in the Formation of Single-Wall Carbon Nanotubes
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diameter distribution. We were interested to investigate the effect of target position with respect to the hot zone (center) of the furnace on the diameter distribution of the SWNTs. We have also studied the effect of flow rate of the gas on the diameter distribution of the SWNTs. EXPERIMENT SWNTs were grown by the laser ablation of metal-carbon composite targets. The metal composition in the targets were Ni(0.6at.%)/Co(0.6at.%). Second harmonics of Nd:YAG laser (532 rnm, 10Hz) was focused in a 5mm diameter spot on the target. The target was supported by a graphite holder fixed to a rotating molybdenum rod inside a quartz tube. The quartz tube was first evacuated by a rotary pump and then flowing argon gas was introduced into it. The argon gas pressure inside the quartz tube was maintained at 500 Torr and the flow rate was set to a desired value. The quartz tube was heated by an electric furnace and the temperature was maintained at 1200'C. During the laser ablation the flowing argon gas carried the carbon products downstream and the mat-like material deposited on the molybdenum rod was analyzed by transmission electron microscopy (TEM) and Raman spectroscopy. The radial breathing mode of the Raman spectra was used for estimation of the diameter distribution of the SWNTs. The laser excitation line for the Raman spectroscopy was 488 am and the spectral resolution was 4 cm-.
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Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
RESULTS AND DISCUSSION Analysis of the Raman spectra Diameter selective Raman scattering for SWNTs is particularly important for the Raman band at about 180 cm"1 which is associated with the radial breathing mode (RBM) of the carbon nanotube [7]. According to theory the frequency of the radial breathing mode is inversely proportional to the diameter of the SWNT [5,8,9]. Fig. 1 shows a typical RBM band of SWNTs grown using Ni(0.6at.%)/Co(0.6at.%) catalyzed graphite rods at 1200TC. All the RBM bands could be fitted by a sum of Lorentzian peaks. Two constraints have been used for the fitting procedure. Firstly, all the Lorentzian peaks were given the same width of 8.4 cm-1 and secondly, minimum number of frequencies that fitted all the RBM bands were used [4]. By using this technique it was found that five peaks (indicated by arrows in Fig. 1) gave a good fit to observed RBM bands. Each of these peaks corresponds to the RBM frequency for a particular diameter SWNT. The tube diameter is inversely proportional to the RBM frequency. The relative yields were obtained from the ratio of the area of a particular RBM peak to the total area of all the RBM peaks. The RBM frequencies shown in Fig. I indicates that the SWNT diameters range from 1 to 1.5 nm. This was also confirmed by TEM observation.
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Raman shift (cm1 ) FIG. 1. Raman spectra of the radial breathing mode (RBM) of SWNTs grown using Ni(0.6)/Co(0.6) catalyzed graphite rods at 1200 0 C. Ar gas pressure was 500 Torr and the flow velocity was 2.2 mm/sec
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Effect of target position Effect
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