@phdthesis{Li2015,
  author    = {Li, Han},
  title     = {Fabrication of Carbon Nanotube Thin Films by Evaporation-Induced Self-Assembly},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123407},
  school      = {Universit{\"a}t W{\"u}rzburg},
  pages     = {159},
  year      = {2015},
  abstract  = {In summary, we have prepared single-wall carbon nanotube (SWNT) thin films by the method of evaporation-induced self-assembly (EISA). Using the scalable two-plate or lens setups, sorts of different film types or patterns of SWNTs has been successfully fabricated directly from the evaporation of solvents and could be precisely controlled by the concentrations of SWNT in ambient conditions. The special geometry of meniscus as the capillary bridge has not only given rise to a much higher efficiency of fabrication than what previously reported but also allowed us to monitor the pinning and depinning process carefully and further investigate the mechanism underlying the formation of different film morphologies. In contrast with the conventional "stick-slip" model, we have provided the new dynamical pinning and zipping model for the contact line (CL) behavior. By analyzing the motion of CL and varying deposited patterns, the traditionally so-called "stick" state should be treated as a dynamical pinning process due to the interfacial tension contrast between SWNT-covered and bare silicon surface. Besides, the plausible one-step "slip" motion could be dominated by the zipping-like kink propagation. In addition, the experiments with heated substrates at higher temperatures between 30°C and 50 °C have shown that the striped pattern could be fabricated by both much lower SWNT and SDS concentrations than that in room temperature, which is consistent with our model of interfacial tension contrast. In this situation, the deposition rate was increased but the quality of SWNT alignment was undermined because the corresponding moving velocity of SWNT was also too fast for SWNTs to rotate when the evaporative rate was high. The similar results were identified by the SWNT/polymer conjugates dispersed in chloroform under the similar setups and other identical conditions. The typical breathing motion of dynamical pinning and zipping-like propagation for depinning were confirmed by the new suspensions despite that some morphological parameters changed dramatically compared with that from the aqueous solution. For example, the spacing between stripes reached 100 µm ~ 200 µm because the large contact angle contrast between HDMS- and SWNT-covered surface accompanies with the high evaporation rate of chloroform in the pinning and depinning process. Likewise the average CL velocity for fabrication reached around 20 µm/s due to the much higher evaporation rate of chloroform than water. Using alike suspensions, the modified EISA method called dose-controlled floating evaporative self-assembly (DFES) was employed to implement the self-assembly of SWNTs on the water/air interface and then deposit them on solid substrate by directed floating. Although the stripes were fabricated successfully by drops with certain doses and SWNT concentrations, there inevitably existed randomly oriented SWNTs from the water surface that built networks between the stripes containing well-aligned tubes. In order to slow down the evaporation rate and monitor the process detailedly, we used chlorobenzene as the solvent instead of chloroform and find the typical pinning/depinning movement of the CL. A preliminary analysis of the results in terms of chlorobenzene implied that the CL possibly followed the similar pinning/depinning process in consistence with our model with capillary bridge. In the last part of the thesis, the primary research on the optical properties of these stripes of ultrahigh purity semiconducting nanotubes was conducted by fluorescence microscopy and photoluminescence excitation (PLE) spectroscopy. The energy transfer of the photogenerated excitons was confirmed between different tube species with controlled band gaps. In short, the experiments performed in this thesis allowed to gain new insights about the fabrication of large-area SWNT thin films by the cost-effective solution-processed method and most importantly to uncover its intrinsic mechanism as well. Combined with the separation and selection technique like density gradient centrifugation or polyfluorene derivatives assisted method, highly monodisperse semiconducting nanotubes could be deposited into organized, controllable and functional arrays. Beyond the ambient conditions, precise control for the evaporation under preset temperature and vapor pressure could possibly extend the technique to the industry level. Assisted by some other mature techniques such as roll-to-roll printing, the cost-effective method could be widely used in the manufacture of various thin film devices. More complex 2D or even 3D structures could be designed and accomplished by the method for the functional or stretchable requirements. Further research on the fundamental exciton transition and diffusion in different networks or structures of SWNTs will be the significant precondition for the real applications. Looking ahead, from the individual carbon nanotube to its thin film, this promising material with outstanding properties had many challenges to overcome before the real-world applications. Thanks to the availability of pure and well-defined materials, the scalable solution-processed approaches for fabrication of thin films should be able to unlock the potential of carbon nanotubes and exploit them in (opto-)electronic devices in the foreseeing future.},
  subject      = {Kohlenstoff-Nanor{\"o}hre},
  language  = {en}
}