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.
Browse
11 results
Search Results
Item PHYSICAL CHARACTERIZATION OF DNA CONDENSED WITH CATIONIC AGENTS(2016) Salgado, Eddy; Briber, Robert M; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gene therapy using non viral vectors remains a challenging problem of maximizing efficiency while minimizing risks due to the multiple biological hurdles for a carrier agent to deliver its genetic cargo. The precise connection between the physical properties of the vectors and their transfection behaviors remains to be fully realized. We have used atomic force microscopy as well as dynamic light scattering and zeta potential measurements in order to image and characterize DNA complexes with polyethylenimine (PEI), histidine-lysine (HK) peptide, and triethylenetetramine (TETA)-functionalized gold nanoparticles. The resulting complex structures are analyzed as a function of amine to phosphate (N/P) ratios and as a function of sample preparation protocols. This work aims to not only characterize these specific complexes, but to aid in the general understanding of complex formation and how it relates to transfection observations to promote a more rational design of future gene delivery agents.Item Leveraging Porous Silicon Carbide to Create Simultaneously Low Stiffness and High Frequency AFM Microcantilevers(2014) Barkley, Sarice; Solares, Santiago; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many operative modes of the atomic force microscope (AFM) are optimized by using cantilever probes that have both a low force constant and a high resonance frequency. Due to fabrication limitations, however, this ideal cannot be achieved without resorting to sizes incompatible with standard AFM instrumentation. This project proposes that cantilevers made from electrochemically etched porous silicon carbide (SiC) enjoy reduced force constants without significantly sacrificing frequency or size. The study includes prototype fabrication, as well as parametric experiments on the etching recipe and suggestions to improve the process. Analysis of the mechanical properties of the prototypes proves that introducing porosity to the structure greatly reduces the force constant (porous k = 0.27 bulk k) while only slightly reducing the resonance frequency (porous f0 = 0.86 bulk f0).Item Novel Interactions of Liquid Crystals with Coated Nanoparticles(2013) Taylor, Jefferson; Martinez-Miranda, Luz J; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Functionalized nanoparticles have a wide range of applications in liquid crystal systems, including displays, photovoltaics, and drug delivery. We need to understand the interactions between the nanoparticles and the liquid crystal molecules in order to utilize them fully and safely. We investigate the short-range interaction of coated nanoparticles with a liquid crystal membrane or bulk sample through the use of atomic force microscopy (AFM) and X-ray scattering techniques. We identify the role the functionalization plays in the phase behavior of the liquid crystal both as a thin film and in bulk. Our research produced three results. We identify differing behavior in thin film samples of liquid crystal and coated nanoparticles dependent upon particle functionalization using AFM. Using X-ray scattering we measure the alignment and smectic layer formation in the presence of coated nanoparticles, even above the smectic-A to nematic transition temperature. We find evidence of a "halo" that forms around coated nanoparticles, particularly with longer coating molecules.Item Nanomechanical Properties and Buckling Instability of Plasma Induced Damaged Layer on Polystyrene(2012) Lin, Tsung-Cheng; Phaneuf, Raymond J; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we report on an investigation of an elastic buckling instability as a driving force for the roughening of polystyrene, a model resist, during Ar+ plasma etching. Polystyrene films etched by pure Ar+ plasma with different ion energies were characterized using both atomic force microscopy topography and force curve measurements. By using height-height correlation function in analyzing the AFM measured topography images, we find that surface corrugation of etched polystyrene film surfaces all display a dominant wrinkle wavelength (ë), which is a function of ion energy. Next, we characterized the mechanical properties of these samples using AFM force curve measurements in an controlled ambient environment. We analyzed the measured force curves using a systematic algorithm based on statistical fitting procedures, and taking into account the adhesive interaction, in order to determine the effective elastic modulus of the films. We find that the effective elastic modulus (EBL) of the etched samples increases monotonically with increasing ion energy, but the changes are rather subtle as compared to the elastic modulus (EPS) of the unetched one. In order to test the validity of a buckling instability as the mechanism for surface roughening in our polystyrene-Ar plasma system, the elastic modulus of individual layer (i.e. ion-damaged layer plus unmodified foundation) needs to be determined. We present a determination of the damaged layer elastic modulus (EDL) from the effective elastic modulus of the damaged layer/polystyrene bilayer structure (EBL), based upon a finite element method simulation taking into account the thickness and elastic modulus of the damaged layers. We extract the damaged layer elastic modulus versus etching ion energy initially within the approximation of a spherical tip in contact with a flat sample surface. We next extend our model, by considering a periodic corrugated film surface, with its amplitude and wavelength determined by AFM, to take into account the effect of roughness induced by plasma exposure. The damaged layer elastic modulus extracted from these two approximations gives of quantitative agreement, and thus evidence for the correlation between buckling instability and plasma-induced roughening.Item SURFACE CHARACTERIZATION OF VISCOELASTIC MATERIALS THROUGH SPECTRAL INTERMITTENT CONTACT ATOMIC FORCE MICROSCOPY(2012) Williams, Jeffrey Charles; Solares, Santiago D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to recover material properties at the atomic scale has been the ongoing objective of the Atomic Force Microscope (AFM). More specifically, the most popular operation of the probe with this microscope (Intermittent Contact AFM) has not yet been able to resolve material properties of viscoelastic samples. By using the force and position time signals of the AFM and the constitutive equations for linear viscoelasticity, a method is developed by which such material properties are extracted in real-time scanning. A parametric study is then performed by simulating surface and AFM system conditions to understand the limits under which the method can accurately be performed in experiment. Suggestions are made to help experimentalists optimize the method to cater to the range of viscoelastic materials being measured and the results are related to measured material properties in literature. The method is found to be accurate for a wide range of viscoelastic materials.Item USING SINGLE MOLECULE TECHNIQUES TO DETERMINE THE MECHANISM OF DNA TOPOLOGY SIMPLIFICATION BY TYPE IIA TOPOISOMERASES(2011) Hardin, Ashley Harris; Thirumalai, Devarajan; Neuman, Keir C; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Type IIA topoisomerases are essential, universally conserved proteins that modify DNA topology by passing one segment of duplex DNA (the transfer, or T-segment) through a transient double strand break in a second segment of DNA (the gate, or G-segment) in an ATP-dependent reaction. Type IIA topoisomerases decatenate, unknot, and relax supercoiling in DNA to levels below equilibrium, resulting in global topology simplification. The mechanism underlying non-equilibrium topology simplification remains speculative, though several plausible models have been proposed. This thesis tests two of these, the bend angle and kinetic proofreading models, using single-molecule techniques. The bend angle model postulates that non-equilibrium topology simplification scales with the bend angle imposed on the G-segment DNA by a type IIA topoisomerase. To test this model, we used atomic force microscopy and single molecule Förster resonance energy transfer to measure the extent of bending imposed on DNA by three type IIA topoisomerases that span the range of topology simplification activity. We found that all proteins bent DNA, but the imposed bends are similar and cannot account for the differences among the enzymes. These data do not support the bend angle model and suggest that DNA bending is not the sole determinant of non-equilibrium topology simplification. Based on the assumption that the rates of collision between DNA segments is higher in knotted, linked, and supercoiled DNA than in topologically free or relaxed DNA, the kinetic proofreading model proposes that two successive binding events between a G-segment bound topoisomerase and a putative T-segment are required to initiate strand passage. As a result of the two step process, the overall rate of strand passage should scale with the square of the collision probability of two DNA segments. To test this model, we used magnetic tweezers to manipulate a paramagnetic bead tethered to the surface by two DNA molecules. By rotating the bead, we varied the proximity, and thus collision rate, of the two molecules to determine the relationship between collision probability and rate of strand passage. Our data indicate that the strand passage rate scales linearly with the collision probability, which is inconsistent with the kinetic proofreading model.Item Quantitative Prediction of Tip-Sample Repulsive Forces and Sample Deformation in Tapping-Mode Frequency and Force Modulation Atomic Force Microscopy(2008-08-27) Crone, Joshua C; Solares, Santiago D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to predict sample deformation and the resultant interaction forces is a vital component to preventing sample damage and acquiring accurate height traces in atomic force microscopy (AFM). By using the recently developed frequency and force modulation (FFM) control scheme, a prediction method is developed by coupling previously developed analytical work with numerical integration of the equation of motion for the AFM tip. By selecting a zero resonance frequency shift, the sample deformation is found to depend only on those parameters defining the tip-sample interaction forces. The results are represented graphically and through a multiple regression model so that the user can predict the tip penetration and maximum repulsive force with knowledge of the maximum attractive force and steepness of the repulsive regime in the tip-sample interaction force curve. The prediction model is shown to be accurate for a wide range of imaging conditions.Item Ferrocene-based molecular electronics and nanomanufacturing of Pd nanowires.(2007-11-27) Wang, Lixin; Sita, Lawrence; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two test structures were tried out for molecular junction formation and subsequent I-V characteristics measurements. One is formed by insertion of certain dithiol molecules into an alkanethiol self-assembled monolayer (SAM), followed by tethering the free thiol end with gold nanoparticles. The test structure can then be measured with CP-AFM. The matrix SAM, mixed monolayer with inserted dithiol molecules, and final test structure with gold nanoparticles were prepared and characterized by ellipsometry, AFM and STM. However, the CP-AFM measurements were very irreproducible, even on an alkanethiol SAM. This problem was analyzed and believed to come from two possible causes, namely thermal drift and deformation of the metalized tips. The other test structure was from insertion of molecules into nanogaps made by electromigration technique. Two molecules were tested and drastically different properties were observed from junctions with each molecule. For Fc-OPE molecules, near perfect conductance peaks (>0.6G0) were observed in some junctions and analysis indicates that such molecular junction contains only one or two molecules inside the nanogap. The formation of conductance peaks was analyzed with Landauer formula and a simple metal-molecule-metal model. Computational calculation also predicted high conductance through such junctions and the existence of resonant peaks. The junctions with OPE molecules, however, showed poor conductance. Possible causes such as molecular structure and easiness of molecular junction formation were discussed. In the second part of this dissertation, a new method was developed to fabricate Pd nanowires on HOPG surface using a sacrificial Cu film. The morphology and composition of the nanowires were characterized by AFM, SEM and XPS. The formation of such Pd nanowires was explained with a galvanic displacement mechanism and some test experiments were carried out to prove such growth mechanism. It was also found that the size of the Pd nanowires can be directly controlled by the thickness of the Cu film that was initially deposited. However, attempts to make Au, Pt and Ag nanowires with the same method failed, and possible causes were discussed.Item Characterization of Electrodeposited Chitosan Films by Atomic Force Microscopy and Raman Spectroscopy(2006-05-08) Dreyer, Erin C; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chitosan has served as a robust and reproducible scaffold for biological reactions by electrodeposition at specific sites in microfluidic channels. However, its growth and properties are not well understood as a function of deposition parameters. To better understand the materials and process science, in-vitro characterization techniques and post-deposition measurements of air-dried films were performed. AFM images of dried films depicted variable, rough morphology not directly correlated to deposition conditions while hydration increased surface homogeneity. Dry roughness increased logarithmically with thickness supporting growth by nucleation. In-vitro fluorescence images showed fairly smooth distribution of chitosan, whereas dried films were much rougher, indicating non-uniform collapse of structure during drying. Raman spectroscopy revealed the presence of primary amine groups active in biofunctionalization and served as a technique for evaluating the spatial selectivity of chitosan by electrodeposition. Further study of hydrated films is needed to fully understand chitosan as a platform for biotechnology applications.Item CRYOGENIC ATOMIC FORCE MICROSCOPE FOR CHARACTERIZATION OF NANOSTRUCTURES(2005-07-28) Li, Changyi; Yang, Chia-Hung; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we present the design and applications of a cryogenic atomic force microscope (AFM) for characterization of nanostructures. The cryogenic AFM with a conductive tip can measure DC current through nanostructures. We use quartz tuning fork (QTF) as the force sensor. Unique coarse z motor design provides reliable autoapporach in the Z direction. AFM imaging with 10nm horizontal and ~2 angstrom vertical resolution has been achieved. We have used this AFM in the current-voltage characterization of diodes, and, with a modified sensing mechanism, electrical force microscopy (EFM) and magnetic force microscopy (MFM) have been demonstrated.