In-situ observation of single-walled carbon nanotube (SWNT) growth is an essential approach for investigating the growth mechanism of SWNTs. However, previous works have not been able to specify chiralities of growing SWNTs using SEM, TEM and Raman spectroscopy [1-3]. Here, we succeeded in detecting chirality-sensitive signals of radial breathing modes (RBM) during CVD growth of SWNTs by in-situ Raman spectroscopy and examined the distribution of SWNT diameters at the initial growth stage to clarify the key factor determining SWNT chirality.
SWNTs were synthesized by ethanol-CVD using Co film (0.5,2 Å) as the catalyst. Typical growth temperature and ethanol gas pressure were 650 °C and 0.1-0.6 Torr, respectively. in situ Raman spectra during CVD growth were measured using a tiny CVD system specially designed for in-situ observation under the typical condition of 633-nm excitation wavelength, a 2-μm laser spot, and integration time of 30 sec.
After the growth had proceeded for an adequate time, evident RBM signals were detected, in addition to intense G-bands. By analyzing the time evolution of the spectra, it is clearly recognized that the RBM signals around 130-150 cm-1 from relatively thick SWNTs delayed in appearing, compared with the signal at 190 cm-1. Occasionally at a latter growth stage, new signals around 200-240 cm-1 from relatively thinner SWNTs were observed to appear. This indicates that the "incubation time" of SWNT growth observed for G-bands in in-situ Raman spectra [2] should depend on the chirality. Moreover, it was found that the growth conditions, such as catalyst film thickness and ambient pressure during growth, also significantly affect initial growth behaviors of SWNTs even if they have the same chirality. These chirality-sensitive growth behavior‚" must be related to " the detailed mechanism of SWNT nucleation on catalyst particles and should be elucidated by a comprehensive analysis of in-situ Raman spectra in conjunction with analysis of catalyst particle diameters and chemical structures.
References:
[1] Y. Homma et al., Appl. Phys. Lett. 88 (2006) 023115.
[2] M. Lin et al., Nano Lett. 6(2006)449.
[3] S. Chiashi et al., Chem. Phys. Lett. 386(2004)89. |