title: Global Alignment of Single-Wall Carbon Nanotubes via Dead-End Filtration
Creators: name: Rust, Christian
Publisher: name: UNSPECIFIED
Year: 2023

description: Carbon nanotubes have been envisioned to open up new pathways for many applications since their discovery by Iijima in 1991. Their band gap being defined by their diameter allows to use them as semiconductors with sharp absorption bands in the infrared and UV-VIS making them often interesting for optics and photonics. Additionally, they exhibit one of the largest intrinsic charge and phonon mobilities known for any material, which in turn sparks the interest of using them as metal interconnects in electronics or heat-management systems for high-performance electronics. As the combined advances in synthesis and sorting processes are making a lot of carbon nanotubes species accessible in larger quantities, a challenge still remains to control their location and orientation in device architectures. Thus, a lot of research nowadays focuses on the alignment of carbon nanotubes in thin films and this is where the efforts made in this work are focused at. Just recently in 2016, a new method of alignment single wall carbon nanotubes has gathered a lot of attention, as it allows for large scale films, spanning over several cm², while maintaining very high order-parameters. The so-called filtration method, which comprises of a very slow dead-end filtration process, is seemingly just needing basic lab equipment and moderate practical skills, but anecdotal reports of many groups being unable to reproduce the astonishing results of He et al. come to the conclusion, that many factors making this alignment possible may be unknown. In order to investigate the mechanisms behind that method and improve the area, quality of alignment, usable carbon nanotubes species and reproducibility, this thesis aims at working out some of those factors by trying to isolate them one by one and applying the findings to a custom-made microfluidic filtration setup. At first the common used polycarbonate track-etched membranes are evaluated by means of filtration resistance and zeta-potential and experiments with varying ionic strength and surfactant concentration are conducted in order to establish a basic understanding of the membrane charges. Similar to the membranes, the zeta-potentials of the carbon nanotubes dispersions are measured for different surfactants and their concentrations as well, defining their stability. In the next step, the precise volume-rate and pressure readings of the custom-made setup are used to optimize the filtration conditions, comprising of an initial slow- and a final fast-filtration step. This data is gathered by only using a single type of single wall carbon nanotube, that has been deemed to align better than others as stated by researchers in the field. The data gained from the setup is then compared to the carbon nanotube thin films being still on the membrane and after transfer to several substrates, allowing for detailed analysis by cross-polarization microscopy and scanning electron microscopy. The domains of carbon nanotube crystals formed at various volume-rates are also measured by a shape-identifying machine learning algorithm, providing large data-sets, that are then used to quantify the impact of the slow-filtration conditions on the size and morphology. However, the fast filtration step has been found not to impact the actual alignment, but result in drying rings formed, which are measured by photographs. Eventually the dependence on deposited mass, as well as appropriate concentrations are investigated under the given filtration parameters, which then finally lead to globally aligned films. Having realized the alignment on pristine membranes, hot-embossing in conjunction with custom made shims is utilized to imprint uni-axial and radial patterns into the membranes. Thereby the bar-spacing of the former is varied to study the needed width and imprinting force. Additionally, radial symmetric patterns are designed and the resulting films were exposed to radial symmetric light patterns, to evaluate their possible use as optical elements, while terminal two-probe measurements in conjunction with custom-made flexible printed circuit boards are conducted to quantify the effect of nanotube alignment on electrical resistance. After investigating the effects on alignment from the perspective of the membranes, length-sorted single wall carbon nanotubes with different average diameters are used to reproduce the global alignment obtained before, using similar conditions. Comprehensive characterization comprising of various methods, covering the macroscopic and nanoscopic alignment under the influence of changing deposited mass are used again to determine trends of improving alignment. In order to warrant comparability to other research groups investigating this method, all champion films throughout this work are additionally mapped with Raman spectroscopy and quantified by calculating the two-dimensional order parameter after determining the wavelength dependent dichroic-ratio obtained from absorption spectroscopy. With the zeta-potentials, geometric data measured of both, the carbon nantoubes and membranes and the experimental data at hand, Derjaguin, Landau, Verwey, Overbeek analysis tailored for an individual single wall carbon nanotube is used to propose models for the alignment mechanisms, which were differentiated by the surface modification of the membrane and the sign of charge of the carbon nanotube. Eventually, this analysis also allows to establish a connection between carbon nanotube diameter and required length, and thus the conclusion given in the end, explores several new use cases and improvements, that might come from this work.