Polymer-assisted dispersion for single walled carbon nanotube (SWNT) [electronic resource]
Keywords: Chemical Engineering

Single-walled carbon nanotubes (SWNTs) have a wide range of unique physical properties, from mechanical to chemical to electronic, making them highly desirable for applications such as composites, catalysis, energy storage, IR detectors, transistors and electrodes. SWNTs are essentially perfect one-dimensional quantum wires. Their unique geometric dimensions and unusual method of electrical conduction related to the periodic nature of SWNTs make them ideal candidates as building blocks of nano-scale electronic devices. However, most applications have used SWNTs grown in situ by chemical vapor deposition. This limits throughput and choice of substrate due to the high growth temperatures involved. Solution-based post-synthesis device fabrication, typically involving purification, solubilization, separation of semiconducting or metallic SWNTs, chemical functionalization, cutting, and/or controlled assembly of SWNTs, is more desirable because of low cost, scalability to large areas and compatibility with flexible plastic substrates. Unfortunately, SWNTs are not readily soluble and chemical functionalization strategies for their solubilization usually alter their electronic properties. In the first part of this thesis, I present a high concentration dispersion of SWNT in organic solvent using conjugated polymers and having the nematic phase. The new property of the SWNT dispersion provides another possibility for carbon nanotube (CNT) thin film processing and alignment. I systematically studied the solubilization of SWNTs into various organic solvents using a series of conjugated polymers with different electronic properties and regio-regularities. I found that very high dispersion concentrations (2.75 mg mL-1) in ortho-dichlorobenzene (o-DCB) can be achieved with regio-regular poly (3-hexylthiophene), and SWNT/polymer composite dispersions show a nematic liquid crystalline (LC) phase at this high concentration. While each material separately does not form any LC phase, this nematic LC behavior was induced by the presence of both the conjugated polymers and the SWNTs. A unidirectional domain as large as 6.5 cm2 in a SWNTs/conjugated polymer solid thin film was formed by shear alignment. These types of materials open up new possibilities for CNT processing and are potentially useful as transparent, conductive thin-film electrodes onto which liquid crystals and other molecular materials can be aligned. In the second part, heightening the need for channel materials of high mobility and on/off ratio suitable for application to room temperature thin film processing, an effectively selective dispersion of semiconducting SWNTs will be discussed. I will describe the observation in which poly (3-dodecylthiophene), a commercialized conjugated polymer, highly disperses nearly 100% of pure semiconducting SWNTs due to self-assembling of the polymer at a specific temperature and side-chain length. Optical absorption and resonant Raman spectroscopy (RRS) and photoluminescence excitation (PLE) spectroscopy examine selectively dispersing semiconducting SWNTs. Our molecular modeling suggests that self-organizing poly (3-dodecylthiophene) helically surrounds single walled carbon nanotubes, resulting from crystallization of alkyl-side chains and interaction of the aromatic backbone on the underlying graphene wall. This as-dispersed SWNT/polymer hybrid nanostructure no longer requires viii washing the dispersant unlike other aqueous surfactant-based dispersions in order to achieve high performance of single walled carbon nanotube network transistors with a charge carrier mobility as high as 12 cm2/Vs and an on/off ratio of more than 105. Consequently, this method could offer an effective route to a facile scale-up of the novel dispersion for a range of electronic applications.