Raman Spectroscopy of Carbon Nanotubes at CVD Growth Temperature and the Assignments of their Chiral Indices


Takashi Uchida and Yoshihiro Kobayashi
Materials Science Laboratory

  Whether we will be successful in applying carbon nanotubes (CNTs) to electronic devices highly depends on our ability to synthesize CNTs with a desired chirality, because the electronic structures of CNTs strongly depends on their chirality. To control the growth of CNTs, it is important to clarify their growth mechanism. We have investigated CVD growth processes of CNTs by in-situ Raman observation. Raman spectra are strongly temperature-dependent because they reflect the vibrational properties of observed materials. In order to discuss the details of the growth process observed by in-situ Raman spectroscopy, temperature effects in the Raman spectra of CNTs should be elucidated.
  Figure 1 shows an atomic force microscope (AFM) image of a CNT-grown sample. Individual CNTs lie on the substrate. Figure 2 shows (a) the experimental Kataura plot and (b) RBM signals observed at 720 ℃ (during CVD) and at room temperature (after CVD). The chiral indices of the RBM signals observed at room temperature can be assigned as shown in Fig. 2(b) on the basis of the experimental Kataura plot. The RBM peak frequencies tend to simply decrease with increasing temperature. This is a commonly observed phenomenon originating from the softening of the force constant with increasing temperature. In addition, the relative intensities of each RBM peak change with temperature. This temperature dependence is explained by the change in the resonance condition of each RBM peak. The optical transition energy of nanotubes decreases with increasing temperature, resulting in a shift of each point in the Kataura plot to the lower left as shown in Fig. 2(a). From this analysis, the chiral indices of the RBM peaks observed at CVD growth temperature of 720 ℃ are successfully assigned as shown in Fig. 2(b) [1]. These results encourage us to further investigation of the chirality-dependent growth behavior of CNTs observed with in-situ Raman spectroscopy and, in future, will lead us to clarifying growth mechanism and thereby achieve the chirality-controlled growth of CNTs.

[1] T. Uchida, et al., Appl. Surf. Sci., in press.

Fig. 1. AFM image of the CNT sample.
Fig. 2. (a) Experimental Kataura plot: RBM frequencies RBM vs the optical transition energies Eii. (b) RBM signals observed during CVD at 720 ℃ and after CVD at 25 ℃.

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