Diameter Selective Formation of SWNTs

The diameter distribution of the SWNTs can be controlled by changing the temperature of the furnace. In the electric arc method, the dc arc between the catalyzed carbon anode and the pure carbon cathode produces SWNTs in He gas at 500 Torr. In the arc method, the diameters of the SWNTs are controlled by changing the pressure of the He gas. Increasing the temperature makes larger diameter SWNTs, while the higher ambient gas pressure, up to 760 Torr, makes a larger yield and diameter of SWNTs by the carbon arc method.

The diameter of SWNTs can be controlled, too, by adopting different catalysts and different relative concentrations of the catalyst species, such as NiY (4.2 and 1.0 at. %), NiCo (0.6 and 0.6 at. %), Ni (0.6 at. %) and RhPd (1.2 and

1.2 at.%), which have provided the following diameter distributions by the laser ablation method with a furnace temperature of 1150 to 1200° C, respectively, 1.24-1.58 nm, 1.06-1.45nm, 1.06-1.45nm and 0.68-1.00 nm [3,40]. The diameter distribution in each case was determined from TEM experiments and from measurement of the RBM frequencies using Raman spectroscopy and several different laser excitation energies. It is important to note that the determination of the frequency of the RBM does not provide a measurement of the nanotube chirality, though the diameter dependence is well observed by measurement of the RBM frequency. The diameter distribution is then obtained if the RBM of a (10,10) armchair nanotube is taken to be 165 cm-1 RBM and 70 = 2.75 eV [3]. However, if we adopt the value of 70 = 2.90 eV, the Raman signal is consistent with 177 cm-1 for a (10,10) armchair nanotube. For these larger values of 70 and wrbm (10,10) the diameter distribution for each catalyst given above is shifted upward by 7%. Most of the catalysts, except for the RhPd, show very similar diameter distributions for both the laser vaporization and the electric arc methods at growth conditions giving the highest yield. In the case of the RhPd catalyst, however, no SWNTs are synthesized by the arc discharge method, in contrast to a high yield provided by the laser vaporization method.

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