Chemical States of Metal Catalysts in CVD Ambient for Single-walled Carbon Nanotube Growth
Materials Science Laboratory
Single-walled carbon nanotubes (SWCNTs) are promising for the generation beyond silicon-based microfabrication technology. However, the mixture of various characteristics, such as metals and semiconductors with various band gaps, are obtained for CNTs. The control of their characteristics is essential in order to achieve large-scale integration for CNT-based devices. Hence, we have been investigating the growth mechanism of CNTs with the aim of controlling their growth and obtaining the desired CNTs. Since catalytic nanoparticles play an important role in growing CNTs, their chemical states are of great interest for this investigation. Recently, we have succeeded in growing SWCNTs by CVD and performing successive in--situ x-ray photoelectron spectroscopy measurements. As a result, we have been able to elucidate the chemical state of catalytic nanoparticles after the growth [1, 2].
Figure 1 shows the core-level photoelectron spectra of Co, which is a catalyst for the CNT growth. These spectra were captured before the growth process, at the heating stage, and after CNT growth using ethanol. In the case of ex-situ deposition, which is the conventional growth procedure, Co was oxidized by exposure to air, resulting in the formation of Co3O4. This oxide turned to CoO during the heating process under ultra-high vacuum and, finally, metallic Co was obtained after CNT growth. In the case of in-situ deposition, the new process without air-exposure after metal deposition, Co remained metallic throughout the growth process. These two results indicate that the metallic state is stable under the growth ambient. This is inconsistent with existing CNT growth models, which need the carburization of catalytic nanoparticles. Meanwhile, the CNT yield for the in-situ deposition was higher than that for the ex-situ deposition. From the analysis of core-level photoelectron intensity, we found that the amount of decomposed carbon on the surface for the in-situ deposition is larger than that for the ex-situ deposition. The situation after CNT growth in the former case is schematically illustrated in Fig. 2. The formation of thick graphitic films indicates that the uniform metallic nanoparticles have a high ability for the decomposition and that a large amount of migrating carbon remains without being included in the nanoparticles during CVD. We will further investigate the reactions of CNT growth and clarify the growth mechanism.
 F. Maeda, et al., Jpn. J. Appl. Phy. 46 (2007) L148.
 F. Maeda, et al., Mater. Res. Soc. Symp. Proc. 96 (2007) 30963-Q05-04.
Fig. 1. Co 2p photoelectron spectra.
Fig. 2. Schematic representations after CNT growth.
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