Spatially Selective Removal of Carbon Nanotubes

Satoru Suzuki, and Yoshihiro Kobayashi
Materials Science Laboratory

 Carbon nanotubes are one of the most promising materials for future nanoelectronics, because of the nanometer-scale structure and the unique electronic properties. Actually, a nanotube-based field effect transistor and single electron transistor have been demonstrated to overcome a Si device. For fabricating a nanotube-based integrated circuit, it is necessary to place nanotubes only at specified positions. However, high-density nanotube growth almost always results in the growth of unnecessary nanotubes at unspecified positions, which would cause short circuits, because controlling the growth direction of individual nanotubes is still impossible. Thus, selective removal of carbon nanotubes from a nanotube network is a crucial issue for fabricating an integrated circuit.
 Recently, we found that low-acceleration-voltage electron irradiation induces significant damage in single-walled carbon nanotubes (SWNTs). Moreover, we developed a simple method for removing unnecessary nantoubes selectively utilizing the low-acceleration-voltage electron irradiation damage.
 Raman spectra of SWNTs before and after electron beam irradiation is shown in Fig. 1. The acceleration voltage was 1 kV. The electron irradiation drastically decreased the radial breathing modes (RBM) intensity, which is characteristic in the quasi-one-dimensional structure of SWNTs, indicating that the irradiation caused structural damage in the SWNTs [1]. This damage was found to be extensively caused at around 1 kV. Although SWNTs are chemically highly stable, the electron irradiation significantly reduces the chemical stability. This enables us to simply remove the damaged SWNTs selectively. A typical example of suspended SWNTs after the selective removal procedure is shown in Fig. 2. The SWNTs were grown on Si pillars having diameters of 200 nm. The electron beam was scanned along the dashed line, and then, the sample was annealed in air. In spite of the very simple procedure, the SWNTs initially crossing the line were successfully removed. This technique would make it possible to fabricate various kinds of nanotube circuits.
[1] S. Suzuki et al., Jpn. J. Appl. Phys. 43 (2004) L1118.
[2] S. Suzuki et al., Jpn. J. Appl. Phys. 44 (2005) L133.


Fig. 1. Raman spectra of SWNTs before and after electron irradiation.   Fig. 2. SEM image of the suspended SWNT network after the selective removal procedure. The electron beam was scanned along the dashed line. Scale bar: 2.5 μm.

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