Gas-phase synthesis of single-walled carbon nanotubes from liquid carbon source for transparent conducting film applications

Abstract

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.