Characterization of Co-catalyzed Multiwalled Carbon Nanotubes by High-Resolution Transmission Electron Microscopy
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Characterization of Co-catalyzed Multiwalled Carbon Nanotubes by High-Resolution Transmission Electron Microscopy CHIH-CHIN WANG; CHUAN-PU LIU; RUO-MEI LIU; Kuen-Hou Liao Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan 701 ABSTRACT The dependence of characteristics of multiwalled carbon nanotubes (MWCNT) on the microstructure of cobalt catalysts are intensively investigated by high-resolution transmission electron microscopy (HREM). The cobalt catalysts are prepared by either molecule beam epitaxy (MBE) or by DC magnetron sputtering, followed by MWCNT growth by microwave plasma CVD. The MBE-grown Co thin films consist of large epitaxial grains, whereas the Co nanoparticles of FCC or HCP crystal structures can be directly grown on Si(001) substrates by sputtering. Various HREM technologies are applied to completely characterize the microstructures of those Co catalysts and MWCNT. The results reveal that the morphology and microstructures of MWCNT are greatly controlled by the characteristics of Co catalysts. Better quality of carbon nanotube can be grown by FCC cobalt rather than HCP cobalt. The different effects of cobalt catalysts on MWCNT are discussed in this paper. INTRODUCTION Ever since Iijimadiscovered carbon nanotube (CNT) in 1991 [1], CNT has been proposed to have application in field emission display [2] [3], Gene sensor, various probes [4] [5], hydrogen storage, conductors [6] and quantum resistors [7]. However, the mass production of reliable CNT has not been achieved yet due to the difficulty in controlled growth. It is well known that the characteristics of CNT are determined by the size, type and charity of the CNT, which is often grown by catalytic reactions. Therefore, the controlled assembly of catalysts is the significant to the growth of CNT. However, it is less well known that how the microstructure of catalysts influence the CNT formation. Thus, it is our aim to understand the role of the cobalt film with various microstructures in CNT formation. Cobalt has been shown to grow CNT successfully by D. S. Bethune [8] and M. Terrones [9]. Cobalt has hcp and fcc phases and represent a model system to examine the relationship between microstructure and its resulting CNT characteristics. We thus grow CNT by molecular beam epitaxy (MBE) and DC magnetron sputtering followed by CNT growth by microwave plasmon assisted chemical vapor deoposition (MPCVD) in this study. EXPERIMENTAL DETAILS Cobalt thin films are prepared on Si(001) substrates by MBE and DC magnetron sputtering. During MBE growth, the working pressure is 10-8 torr at 60ºC for the thickness of 25 nm. During sputtering deposition, the cobalt thin films is maintained at the thickness of
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300 nm for the applied power of 50 W, substrate bias of -100 W, working pressure of 8m torr while the target-to-substrate distance was varied between 6 and 10 cm for the formation of hcp ε-Co and fcc α-Co phases. Subsequently, the cobalt thin films were etched in CVD chamber using H2 and followed
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