UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Creating Localized Amyloid Nucleation of Silk-Elastin-Like Peptide Polymer Using Atomic Force Microscopy(2015) Stock, Brian; Seog, Joonil; Biophysics (BIPH); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Research into amyloids was initially motivated by pathogenic amyloids involved in disease states such as Alzheimer's; however, new research implicates small oliogmeric species and not the mature fibers. This lack of toxicity has allowed for the development of amyloid-based biomaterials for use as nanowires, biosensors, and tissue regeneration. The directed self-assembly of peptides into amyloid-like fibers for use as biomaterials requires the ability to control both the nucleation location and growth direction of the fiber. We have used Atomic Force Microscopy to repeatedly stretch Silk-Elastin-Like Peptide Polymer (SELP) in the normal direction using continuous pulling in a force acquisition mode which has the ability to create nanodots of SELP at a specified location which are capable of nucleating SELP nanofibers. This work, if generalized to other amyloidogenic systems, may aid in the mechanistic understanding of the assembly process of both pathogenic and functional amyloids.Item On Mapping Electron Clouds with Force Microscopy(2012) Wright, Charles Alan; Solares, Santiago D.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)At its core, this is a story about electrons. Electrons drive the interactions of matter at the nanoscale, so an understanding of electron behavior offers significant insight into the behavior of nanoscale materials. Atomic force microscopy (AFM) has demonstrated great success as a tool for probing matter at the nanoscale, and recent reports suggest that it may even be capable of mapping electron clouds on atomic surfaces. The most recent of these claims came in 2004, when Hembacher et al. [Science 305] observed subatomic features while imaging a graphite surface with a tungsten tip using higher-harmonics frequency modulation AFM (FM-AFM). The authors' interpretation of these features as the footprint of the electron density at the tungsten tip's apex atom has been met with much skepticism. But despite the potential significance of the results, a detailed theoretical study has not been performed. In this work, a computational method based in density functional theory (DFT) is developed in order to simulate the imaging process and draw fundamental conclusions regarding the feasibility of subatomic imaging with higher harmonics FM-AFM. The application of this method to the tungsten/graphite system reveals that the bonding lobes of increased charge density are in fact present at the tungsten tip's apex atom and that the corresponding higher harmonics images can exhibit subatomic features similar to those observed experimentally. We further show that the filtering process used to experimentally measure the harmonics does not introduce imaging artifacts but that harmonics averaging is not an appropriate method for enhancing contrast. We then suggest an alternate approach: the individual mapping of the first two harmonics, which are expected to dominate the contrast under the experimental conditions studied. Finally, we demonstrate the important role played by the surface atom used to probe the AFM tip. We find that a small, non-reactive atom is necessary for resolving subatomic features. Most importantly, we show that the observed features are not a direct reflection of the electron density at the AFM tip's front atom. Instead, they represent a measure of the bonding stiffness between the tip's front atom and the atoms in the layer above.Item ACCELERATED SELF-ASSEMBLY OF PEPTIDE-BASED NANOFIBERS USING NANOMECHANICAL STIMULUS(2010) Chang, Jonathan Paul; Seog, Joonil; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One-dimensional nanostructures are ideal building blocks for functional nanoscale assembly. Peptide-based nanofibers have great potential for building smart hierarchical structures due to their tunable structures at a single residue level and their ability to reconfigure themselves in response to environmental stimuli. In this study, it was observed that a pre-adsorbed silk-elastin-based protein polymer self-assembled into nanofibers through a conformational change on the mica substrate. Furthermore, using atomic force microscopy, it was shown that the rate of the self-assembling process was significantly enhanced by applying a nanomechanical stimulus. The orientation of the newly grown nanofiber was mostly perpendicular to the scanning direction, implying that the new nanofiber assembly was locally activated with a directional control. The method developed as a part of this study provides a novel way to prepare a nanofiber patterned substrate using a bottom-up approach.Item Electronic transport in low dimensions: carbon nanotubes and mesoscopic silver wires(2008-12-08) Ghanem, Tarek Khairy; Fuhrer, Michael S.; Williams, Ellen D.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis explores the physics of low-dimensional electronic conductors using two materials systems, carbon nanotubes (CNTs) and lithographically-defined silver nanowires. In order to understand the intrinsic electronic properties of CNTs, it is important to eliminate the contact effects from the measurements. Here, this is accomplished by using a conductive-tip atomic force microscope cantilever as a local electrode in order to obtain length dependent transport properties. The CNT-movable electrode contact is fully characterized, and is largely independent of voltage bias conditions, and independent of the contact force beyond a certain threshold. The contact is affected by the fine positioning of the cantilever relative to the CNT due to parasitic lateral motion of the cantilever during the loading cycle, which, if not controlled, can lead to non-monotonic behavior of contact resistance vs. force. Length dependent transport measurements are reported for several metallic and semiconducting CNTs. The resistance versus length R(L) of semiconducting CNTs is linear in the on state. For the depleted state R(L) is linear for long channel lengths, but non-linear for short channel lengths due to the long depletion lengths in one-dimensional semiconductors. Transport remains diffusive under all depletion conditions, due to both low disorder and high temperature. The study of quantum corrections to classical conductivity in mesoscopic conductors is an essential tool for understanding phase coherence in these systems. A long standing discrepancy between theory and experiment regards the phase coherence time, which is expected theoretically to grow as a power law at low temperatures, but is experimentally found to saturate. The origins of this saturation have been debated for the last decade, with the main contenders being intrinsic decoherence by zero-point fluctuations of the electrons, and decoherence by dilute magnetic impurities. Here, the phase coherence time in quasi-one-dimensional silver wires is measured. The phase coherence times obtained from the weak localization correction to the conductivity at low magnetic field show saturation, while those obtained from universal conductance fluctuations at high field do not. This indicates that, for these samples, the origin of phase coherence time saturation obtained from weak localization is extrinsic, due to the presence of dilute magnetic impurities.