Browsing by Author "Jiang, Hua, Dr., Aalto University, Department of Applied Physics, Finland"

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    Carbon dioxide-assisted synthesis of single-walled carbon nanotubes and their thin film properties
    (Aalto University, 2019) Liao, Yongping; Jiang, Hua, Dr., Aalto University, Department of Applied Physics, Finland; Zhang, Qiang, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Owing to their unique chemical and physical properties, single-walled carbon nanotubes (SWCNTs) have attracted increasing attentation in various fields. The properties of SWCNTs strongly depend on their chirality and geometry. Thus, to realize the applications in desired fields, it is of significant importance to tune the chirality and geometry of SWCNTs. The floating catalyst chemical vapor deposition (FC-CVD) method, as a dry and continuous process, has been widely used in academic and industrial fields. However, tuning the growth of SWCNTs in FC-CVD is a challenge. In this thesis, first of all, certain amounts of carbon dioxide (CO2) is introduced to tune the growth of SWCNTs in a FC-CVD reactor, where carbon monoxide (CO) is used as carbon source and ferrocene as catalyst precursor. We found that the SWCNT thin films display different colors with various CO2 concentration, specifically, a green and brown colors are observed . The optical absorption spectrum of the green film shows a distinct absorption peak in visible range. Further analyzing the chirality by electron diffraction reveals that the green film possesses a very narrow chirality distribution near armchair. Besides the chirality, we also studied the geometry, such as tube diameter, bundle length and diameter, modulated by CO2. The SWCNT diameter and bundle length are found to increase with CO2 concentration. In addition, the yield and bundle diameter are also affected by CO2. Accordingly, SWCNT thin film prepared with certain CO2 concetration displays a remarkably reduced sheet resistance. Therefore, employing CO2 offers new strategy to tune the chirality and geometry of SWCNTs in FC-CVD. Furthermore, we also deposited our aerosol SWCNTs on wafer-scale substrates by large-scale thermophoretic precipitator (TP), such SWCNT thin films display ideal uniformity and conductivity. The scale-up deposition method is applicable in industrial productions of SWCNT transparent conductive films. To futher reduce the sheet resistance of SWCNT thin films, we then deposited SWCNTs on graphene by TP. The SWCNTs/graphene hybrid film exhibits improved conductivity. By measuring the temperature dependent conductance, we found that due to the presence of graphene, the tunnelling barrier between tubes has been reduced, which enhances the carrier tunnelling efficiency and thus, improves the conductivity.
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    Gas-phase synthesis of single-walled carbon nanotubes from liquid carbon source for transparent conducting film applications
    (Aalto University, 2019) Ding, Erxiong; Jiang, Hua, Dr., Aalto University, Department of Applied Physics, Finland; Zhang, Qiang, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; NanoMaterials Group; Perustieteiden korkeakoulu; School of Science; Kauppinen, Esko I., Prof., Aalto University, Department of Applied Physics, Finland
    Owing to the exceptional optoelectronic properties of single-walled carbon nanotubes (SWCNTs), the transparent conducting films (TCFs) incorporating SWCNTs have been applied in areas like solar cells, touch screens, organic light-emitting diodes, and thin-film transistors (TFTs). Particularly, the SWCNT TCFs on a polymer substrate can maintain their properties well under mechanical bending and stretching. Thus, high-yield production of SWCNTs with desired morphological and structural features for the fabrication of highly conductive TCFs is of significance for their scaled-up applications. As for the representative application of SWCNTs in TFTs, semiconducting-enriched nanotubes are preferable.  This dissertation focuses on the high-yield production of SWCNTs for conductive film applications and the synthesis of semiconducting-enriched SWCNTs (s-SWCNTs). A dedicatedly designed aerosol reactor was constructed for SWCNT synthesis using liquid hydrocarbons as the carbon source injected with a syringe pump. Ethanol was first selected as the carbon source to produce SWCNTs. We optimized the growth parameters including thiophene and ferrocene concentrations, the hydrogen flow rate, the temperature as well as the feeding rate of the precursor solution. Limiting the feeding rate reduces the sheet resistance of the SWCNT TCF to ca. 78 Ω/sq at 90% transmittance at 550 nm. The SWCNTs synthesized from ethanol have morphological features like a mean diameter of 2 nm, a mean bundle length of 28.4 μm, a mean bundle diameter of 5.3 nm, and the chiral structures are clustered around the armchair edge. The roles of sulfur were systematically investigated as well using a spark-discharge aerosol reactor for SWCNT synthesis. An optimal amount of sulfur was found to promote the growth of large-diameter and long SWCNTs with high yield and improved quality. Sulfur was proposed to assist the formation of active sites on the catalyst surface to enhance SWCNT growth. To further decrease the sheet resistance and simultaneously keep a high yield, toluene was appointed to be an alternative carbon source. By producing larger-diameter (mean diameter is 2.3 nm) SWCNTs and longer (mean bundle length is 41.4 μm) nanotube bundles, the sheet resistance of the SWCNT TCF was decreased to ca. 57 Ω/sq at 90% transmittance with a much higher yield than that in the ethanol case. The chirality map of the SWCNTs depicted from the electron diffraction results presents a bimodal distribution of the chiral angles. In addition, high-purity s-SWCNTs were also continuously produced with ethanol as the carbon source and methanol as a growth enhancer. The s-SWCNT purity determined from the optical absorption spectrum can be higher than 95% which is beneficial for the high-performance electronics.