Evaluation of Carrier Density in Doped Single-Wall Carbon Nanotubes


Satoru Suzuki and Hiroki Hibino
Materials Science Laboratory

 The thermal chemical vapor deposition (CVD) method has been widely used to grow single-wall carbon nanotubes (SWNTs). However, direct growth of doped SWNTs and tuning of the impurity concentration in them are still a big challenge. Another very important issue for doped SWNTs is their characterization. In particular, it is very difficult to evaluate carrier concentrations in individual SWNTs. Thus, the carrier concentration has not been clarified in previous studies of doped SWNTs. In this study, we grew boron (B)- and nitrogen (N)-doped SWNTs from B- and N-containing feedstocks by the thermal CVD method. Raman spectral shifts induced by carrier doping were clearly observed and the carrier densities were evaluated from the shifts [1].
 We used triisopropylborate (C9H21BO3) and benzylamine (C7H9N) as B- and N-containing feedstocks. These chemicals also acted as a carbon source, and we did not use any other carbon sources. B- or N-doped SWNTs were grown from either one or the other feedstock, with a Co thin film deposited on a SiO2/Si substrate used as a catalyst. We were also able to synthesize BN-doped SWNTs by supplying both triisopropylborate and benzylamine simultaneously, as shown in Fig. 1. Transmission electron microscopy and Raman (radial breathing mode) measurements indicated that the diameter of the doped SWNTs is mostly 1-2 nm, like that of undoped SWNTs grown using a similar catalyst. Figure 2 shows Raman (G band) spectra of B-, N-, and BN-doped SWNTs, and undoped SWNTs. As can be seen in the figure, the G band position in the doped SWNTs is shifted to the high-wavenumber side by 3-6 cm-1, regardless of the choice of feedstock. The hardening of the G band regardless of doping type (electron or hole doping) can be understood as renormalization of phonon energy through electron-phonon coupling induced by a Fermi level shift in a semiconducting SWNT. Originally, the energy of the G band phonon is softened by the electron-phonon interaction (Kohn anomaly). However, a Fermi level shift reduces the electron-phonon interaction and thus, reduces the effect of the Kohn anomaly. Consequently, the Fermi level shift induced by electron or hole doping causes a hardening of the G band. The G band hardening is expected to become prominent when the Fermi level reaches the valence or conduction band. Therefore, the results shown in Fig. 2 indicate that the Fermi level in the doped SWNTs is located inside the valence or conduction band. We can also estimate the carrier density. Assuming that the SWNT diameter is 1.5 nm, the estimated carrier concentration reaches 0.4-0.8%, which is a considerably large value.

[1] S. Suzuki and H. Hibino, Carbon 49 (2011) 2264.

Fig. 1. SEM image of BN-doped SWNTs.
Scale bar: 5 µm.
Fig. 2. G band spectra of B-, N-, and BN-doped SWNTs.
The peak positions are denoted.
The excitation wave length was 785 nm.

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