(Georgia Institute of Technology, 2019-08)
Weiss, Trent
Nanowire transistor research has intensified in the past two decades due to the need for a novel idea to improve upon shortcomings associated with the modern transistor. A prevailing shortcoming in conventional transistor fabrication is limited throughput due to top-down fabrication steps such as lithography. One solution of interest is the nanowire heterostructure transistor which is fabricated in a bottom-up approach and leverages selective surface chemistry to controllably mask specific segments of the structure. The purpose of this work is to use polymers for selective surface chemical treatment on semiconductors enabling selective deposition or annealing of a dielectric by masking specific substrate chemistries. This occurs in a bottom-up fabrication process. The dielectric will act as the gate oxide for a nanowire transistor. Prior research has indicated that polymers can prevent various molecules from reaching a substrate surface due to their dense network. In this study, atom transfer radical polymerization is utilized to create a soft mask that is selective to silicon. The polymer of interest, poly(methyl methacrylate), is grown on initiating groups anchored to a silicon surface to attain a desired thickness. The intermediate steps are optimized for the best possible performance. The polymer is then subjected to atomic layer deposition with aluminum oxide to determine the stability of the polymer and its practicality as a mask for oxide deposition. These experiments may elucidate a method for gate oxide formation on a nanowire heterostructure. Success could have significant impact on this field and would be the next step to attaining functioning nanowire transistors for use in everyday devices.
(Georgia Institute of Technology, 2019-05)
Mathur, Aarti
Lithium-ion batteries are commonly used in many small electronics around the world. Efforts to
make components of Li-ion batteries more sustainable have ranged from use of a brown algae
extract in Li-ion anodes to efforts to recycle lithium. Linear-low density polyethylene (LLDPE)
has been shown to exhibit the strong conductive properties required of a conductive agent in a
Li-ion anode and can be made from recycled waste plastics such as cling wrap and poly-gloves.
Electrodes were fabricated using polyethylene glycol (PEG) coated magnetite as the active
material, PPBT polymeric binder, and LLDPE Carbon. Electrodes made with 14.3 wt.% LLDPE
did not cycle well and exhibited a poor morphology with a cracked surface and large aggregates.
Simple conductivity testing using a 4-point probe and profilometry measurements showed that
Super-P was orders of magnitude more conductive than LLDPE. Increasing the carbon loading to
33.3 wt.% LLDPE in the electrode drastically improved rate capabilities and capacity retention.
SEM analysis showed that a higher carbon loading of LLDPE had a better morphology overall
and demonstrated less cracking. However, when compared to Super-P, the electrode had larger
aggregates and a higher density of clumping. EDX SEM imaging and elemental (Fe, O, C) image
mapping confirmed the presence of Fe3O4 nanoparticles, carbon additives, and PPBT binder.
XPS analysis after 100 cycles confirmed the presence of an SEI layer in the LLDPE electrode.
XPS on electrode slurries showed the presence of satellite peaks which confirm interactions
between the polymeric binder and active material surface, regardless of carbon used. EIS testing
provided information on the charge transfer resistances of Super-P and LLDPE which was
consistent with the cycling trends. Overall, use of LLDPE in Li-ion batteries has been shown to
work in a half-cell assembly. The performance of LLDPE does not beat the current industry
standard, Super-P, but demonstrates promise for use after further optimization and analysis.
(Georgia Institute of Technology, 2019-05)
Moschella, Emily S.
Porous Matrimid® polymeric membranes were developed as supports for thin film
composites (TFCs) of dense, selective polymers to target organic solvent separations. Selective polymers form dense membranes and must be thinly coated on porous supports to obtain high permeances (pressure-normalized fluxes) of target solvents. The goal of this work was to develop support membranes that demonstrated excellent stability in organic solvents and sufficient porosity to allow toluene permeances of 60-200 Lm-2h-1bar-1 such that a composite has permeance of 0.1-1Lm-2h-1bar-1. A final objective was to quantify the effects of production conditions on membrane morphology and performance. Polymer ‘dopes’ (homogeneous polymer solutions) of Matrimid® containing varying compositions of solvent and non solvent were cast using the phase inversion technique. The resulting flat membranes were then crosslinked to prevent dissolution in harsh solvents. The desired permeance was achieved with a precise control over the dope compositions and production conditions. The impact of several casting parameters such as co-solvent evaporation time, ambient humidity, and post-processing procedures were studied. Generally, a decrease in permeance was observed as the drying time prior to phase inversion was increased. Average permeance at the optimized dope composition was 76.4 Lm-2h-1bar-1, with a standard deviation of 62.5 Lm-2h-1bar-1. The effect of different crosslinkers on membrane swelling and permeability was quantified, with 1,6-hexanediamine emerging as the most suitable crosslinker. Gas porosimetry was used to determine surface pore size, which was found to generally increase with permeance.