| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 97 === To properly utilize the anisotropic properties of carbon nanotubes (CNTs) in device applications, the control of the CNT orientations on patterned positions is a necessity since this would largely determine the device funtionality and performance. The growth, orientation control, and related assembly of CNTs are explored in this study. The main purposes are to develop the direct-growth and post-growth assembly techniques to achieve orientation control and patterning. The growth mechanism of CNTs is qualitatively examined by simultaneously supplying Ti species for CNT growth. By employing the physical characteristics of electric fields in plasma and the fabrication of catalyst dots, the directional and patterned growth of CNTs are investigated. In addition, the self-assemblies of the CNT orientations and arrangemnts by external magnetic fields and capillary forces respectively are also described.
Under a TiCl4/CH4/H2/N2 plasma ambient, a continuous TiCN film is formed in the absence of catalysts; however, only CNTs are grown through the catalytic vapor growth, with the amorphous Ti-containing layers coated on their surfaces. The obtained coaxial structure and the electron diffraction analysis of catalysts support that Ti atoms hardly dissolve into the catalyst and pass through it. Selective dissolution and diffusion between the Ti and C atoms in Ni nanoparticles suggest that catalysts during the growth of CNTs at 580oC are in solid state.
The aligned growth of isolated CNTs and carbon nanofibers (CNFs) via a simple sheath-dependent technique for orientation control is demonstrated. The electric field within the plasma sheath contributes to the aligned growth with an absolute orientation, and can be used to direct the assembly of CNTs/ CNFs in a predetermined manner relative to the substrate by tilting the sample in a sheath region. However, the alignment of high-density CNT bundles grown on inclined substrates is always perpendicular to the surfaces. The alignment mechanism of CNT bundles is dominated by the van der Waals forces even under the electric fields.
CNFs with specific and symmetrically aligned configurations are grown on a flat surface during hollow cathode discharge (HCD). The alignment results from a unique electric field distribution imposed on the substrate surface in the HCD environment. Due to the field reversal characteristic, two alignment trends simultaneously occur. For the transverse direction of the channel, the grown CNFs on the substrate appear in a convergent manner, while those along the longitudinal direction exhibit a radiative arrangement.
Magnetic post-assembly of aligned CNTs only having catalyst nanoparticles capped at one end and the related assembly mechanisms are described. The reason why the CNT orientation aligns parallel to the field direction is mainly attributed to the magnetic shape anisotropy rather than the crystalline anisotropy, of the encapsulated ferromagnetic nanoparticle, based on the identification of the hysteresis loops for the as-grown CNT films as well as the analysis of the relative crystal orientations of catalysts with respect to the alignment axis.
Ordered arrays of aligned CNTs are grown from periodic arrays of Ni dots defined by nanoimprint lithography. Due to the low surface energy of the TiN interlayers employed, the patterned Ni dots tend to break into smaller nanoparticles during growth, thus resulting in the growth of smaller diameter nanotubes and the decrease in ordering. However, by adjusting the growth conditions with consequent modification of the surface tension of catalysts, isolated ordered CNT arrays can be prepared without the occurrence of catalyst breaking.
Finally, a wetting-induced self-assembly strategy is proposed to enhance the field emission properties of CNT films.�n The turn-on field reduces from 5.75 V/μm (for the as-grown CNT films) to 2.5 V/μm (for the re-arranged CNTs), where the improvement arises from the reduction of screening effect since the CNT films are assembled into patterned tower-like structures.
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