Low-Energy Irradiation Damage in Single-Walled Carbon Nanotubes
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0994-F04-02
Low-Energy Irradiation Damage in Single-Walled Carbon Nanotubes Satoru Suzuki, and Yoshihiro Kobayashi NTT Basic Research Laboratories, NTT Corp., 3-1, Morinosato Wakamiya, Atsugi, 243-0198, Japan
ABSTRACT We show that low-energy (20 eV-20 keV) electron or photon irradiation extinguishes the characteristic physical and chemical properties of single-walled carbon nanotubes, indicating that the irradiation damages the nanotubes. The irradiation-induced defects convert the electric properties of metallic SWNTs to semiconducting, and the nominal bandgap can be tuned simply by the irradiation dose. The defects also have the following interesting properties. The damage and recovery are reversible, indicating that the number of carbon atoms is preserved. The damage and recovery strongly depend on the diameter, suggesting that the damage is prominent in a rolled up graphene sheet, but not in a planar one. The activation energy of the defect healing is so small, depending on the diameter, that the defects can be healed even at room temperature or below.
INTRODUCTION Carbon nanotubes have a nanometer-scale cylindrical structure, consisting of the sp2bonding carbon network (graphene). Their quasi-one-dimensional structure and the excellent electric and mechanical properties make them attractive for a wide variety of various future applications. They also have very high chemical stability, owing to the robust graphene sheet with no dangling bonds. Because of the chemical stability, low-energy (In this paper, low-energy means that the energy is much lower than the threshold energy of knock-on damage.) irradiation has been generally assumed not to cause damage in nanotubes. Thus, low-energy electrons and photons are often used as analytical and lithographic tools. For example, a scanning electron microscope (SEM) is commonly used for quick observations of nanotubes. Photoemission spectroscopy is also often used for characterizing nanotube samples and for studying their electronic structure. The occurrence of low-energy irradiation damage has not been reported for multi-walled nanotubes (MWNTs), in spite of the very common use of SEM in nanotube research. In this paper, however, we show that low-energy irradiation damages single-walled carbon nanotubes (SWNTs). We focus on the irradiation damage solely caused by the ìlowenergyî. The knock-on, thermally induced, and gas-radical-assisted damages are beyond the scope of this paper. Physical, chemical, and electric property changes caused by the damage are studied. The low-energy irradiation-induced defects also have some unique properties, which are significantly different from those of well-known or well-studied defects in nanotubes. Based on the results, energetics of the defect formation and healing are discussed.
EXPERIMENT SWNTs used in this study were grown by the thermal chemical vapor deposition (CVD) method, using either ethanol or methane as the carbon source and Co thin film (nominal thickness: ∼0.2 nm) as the catalyst. However, we have confirmed that the
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