Browsing by Author "Kauppinen, Esko I., Aalto University, Department of Applied Physics, Finland"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Thermophoretic and diffusive gas-phase transport of single-walled carbon nanotubes and their applications in thin film electronics
    (Aalto University, 2018) Laiho, Patrik; Kauppinen, Esko I., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Aalto University, Department of Applied Physics, Finland
    Thin films of single-walled carbon nanotubes (SWNTs) show great promise as a building block for transparent conducting films (TCFs), used in display applications, touch sensors and photovoltaic devices, and thin film transistors (TFTs). In contrast to currently used materials, SWNT thin films also maintain their properties under flexing and stretching. Previously, both TFTs and TCFs with performances comparable or superior to current industrial components have been realized using the floating catalyst CVD (FCCVD) method. In the FCCVD method, SWNT growth occurs on catalyst nanoparticles in the gas phase, producing an SWNT aerosol that can be deposited into thin films without dispersion. However, a clearer understanding of the effects of the synthesis conditions on the thin film properties and the transport of SWNT aerosols are still needed to achieve the full potential of the method and the thin films produced using it. This dissertation focuses in particular on the sampling of SWNTs by thermophoresis, or transport of aerosols by a temperature gradient, and the mechanisms of gas-phase agglomeration, or bundling, of SWNTs during and after FCCVD growth. Thermophoretic deposition avoids previously observed shortcomings related to electrostatic deposition of SWNTs, and can provide unbiased size distributions of the SWNTs, aiding significantly in process development. The bundling of SWNTs in the gas phase and its effects on the TCF performance were studied in unprecedented detail, resulting in a semi-empirical model for TCF performance as a function of the SWNT dimensions and bundle size. Similarly, limiting the catalyst precursor feeding rate was found to decrease bundling in the synthesized SWNTs and result in a clear improvement of the TCF performance. By monitoring the FCCVD process using aerosol measurements, SWNT films with predetermined densities were collected for the fabrication of TFTs with high uniformity, and a stable process concentration was achieved by adjusting catalyst feeding based on the monitoring. As part of this work two thermophoretic precipitators, one used for mechanistic studies and the fabrication of centimetre-scale arrays of TFTs, and a scaled-up unit capable of deposition on ca. 50 mm substrates, were built and tested. Thermophoretic transport of SWNTs was studied experimentally for the first time, and the thermophoretic terminal velocity was found to differ from the free molecular regime model typically used. SWNT TCFs and TFTs fabricated by thermophoretic deposition were showed equivalent or superior performance compared to previously reported devices fabricated by other sampling methods. In addition to thin film applications, the processes and methods developed can be applied in e.g. the basic research of interfaces between SWNTs and other low-dimensional materials and in spectroscopic characterization using methods previously difficult to use with FCCVD SWNTs.