Browsing by Author "Kauppinen, Esko I., Prof."

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    Aerosol synthesis of carbon nanotubes and nanobuds
    (Aalto-yliopiston teknillinen korkeakoulu, 2010) Anisimov, Anton; Nasibulin, Albert G., Dr.; Teknillisen fysiikan laitos; Department of Applied Physics; Aalto-yliopiston teknillinen korkeakoulu; Kauppinen, Esko I., Prof.
    This thesis presents the results of experimental investigations of single-walled carbon nanotube (SWCNT) and nanobud synthesis. These carbon nanomaterials were synthesized by two different methods based on CO disproportionation on the surface of iron particles produced by hot-wire generator and ferrocene decomposition methods. Studies of CO disproportionation in the presence of etching molecules (H2O and CO2) led to the discovery of a novel hybrid carbon material — SWCNTs covered by covalently bonded fullerenes — carbon nanobuds. The reagent concentrations required for nanobud synthesis were found to be between 45 and 245 ppm for H2O and between 2000 and 6000 ppm for CO2. The growth mechanism of the nanobuds and their properties were examined. On the basis of in situ sampling investigations, the kinetics of the SWCNT growth was studied. For temperatures of 804, 836, 851, and 915 °C, the average growth rates were found to be 0.67, 1.11, 1.01, and 2.70 µm/s, respectively. It was found that the growth rate constant complies with the Arrhenius dependence with an activation energy of Ea = 1.39 eV, which can be attributed to the diffusion of carbon atoms in the solid iron catalyst. A new method for separating bundles and individual SWCNTs is proposed. This method is based on the fact that bundled SWCNTs coming from the reactor are charged, while individual SWCNTs remain electrically neutral. Studies of the charging phenomenon revealed that SWCNT bundles were charged (up to 99%) and could carry up to 5 elementary charges. It is proposed that SWCNT bundles were positively charged due to electron emissions and negatively charged due to the emission of impurities from the surface. As a potential SWCNT application, a simple and direct thermo-compression method for integrating SWCNT films with adjustable thicknesses, transparency, and conductivity into polymer films is demonstrated. The produced SWCNT/polyethylene composite films exhibited good optical transparency and conductivity as well as high mechanical flexibility. SWCNT/polyethylene thin films demonstrated excellent cold electron field emission properties.
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    Non-catalytic growth of metal oxide nanowires : properties and growth mechanism investigations
    (Aalto University, 2011) Rackauskas, Simas; Nasibulin, Albert G., Dr.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; Kauppinen, Esko I., Prof.
    This thesis is devoted to the non-catalytic syntheses of metal oxide nanowires (NWs), and investigations of their properties and growth mechanisms. Two different approaches were applied for the syntheses - metal resistive heating and vapor growth methods. The products were thoroughly characterized by electron microscopy, optical and X-ray characterization techniques. The synthesized NWs were examined for field emission (FE) and ultraviolet (UV) sensing applications. The resistive heating of various metals was demonstrated to be an efficient, simple and rapid method for the synthesis of CuO, Fe2O3, V2O5 and ZnO NWs under ambient air conditions. Fe2O3 NW formation was detected after just 2 s of heating; other metal oxide NWs were grown after 10 s. The NW growth mechanism during metal oxidation was explained based on observations of ZnO and Fe2O3 NW growth. The mechanism is based on the diffusion of metal ions to the surface through grain boundaries and to the tip of the growing NW through defect diffusion and by surface diffusion. FE from NWs grown by the resistive heating method exibited promising results for applications in vacuum electronic devices. Cold electron FE measurements showed that CuO NWs have a very low threshold electric field of 4 V/µm at a current density of 0.01 mA/cm². For the vapor growth of ZnO tetrapods (ZnO-Ts) a vertical flow reactor was designed and constructed. It was shown that the morphology of ZnO-Ts could be adjusted via the Zn vapor pressure in the reactor. The highest aspect ratio of ZnO-T legs was obtained at 700 °C, at a Zn partial pressure of 0.08 atm. ZnO-Ts demonstrated application possibilities for transparent and flexible UV sensors. Sensors based on ZnO-Ts showed a 45-fold current increase under UV irradiation with an intensity of 30 µW/cm² at a wavelength of 365 nm, and a response time of 0.9 s. The high performance of the device was explained by the multiple contact barriers.