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.

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Subject: search.filters.subject.Atomic Force Microscopy

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Now showing 1 - 6 of 6
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    MECHANICAL CHARACTERIZATION OF NORMAL AND CANCEROUS BREAST TISSUE SPECIMENS USING ATOMIC FORCE MICROSCOPY
    (2014) Roy, Rajarshi; Desai, Jaydev P.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Breast cancer is one of the most common malignancies among women worldwide. Conventional breast cancer diagnostic methods involve needle-core biopsy procedures, followed by careful histopathological inspection of the tissue specimen by a pathologist to identify the presence of cancerous lesions. However, such inspections are primarily qualitative and depend on the subjective impressions of observers. The goal of this research is to develop approaches for obtaining quantitative mechanical signatures that can accurately characterize malignancy in pathological breast tissue. The hypothesis of this research is that by using contact-mode Atomic Force Microscopy (AFM), it is possible to obtain differentiable measures of stiffness of normal and cancerous tissue specimens. This dissertation summarizes research carried out in addressing key experimental and computational challenges in performing mechanical characterization on breast tissue. Firstly, breast tissue specimens studied were 600 um in diameter, about six times larger than the range of travel of conventional AFM X-Y stages used for imaging applications. To scan tissue properties across large ranges, a semi automated image-guided positioning system was developed that can be used to perform AFM probe-tissue alignment across distances greater than 100 um at multiple magnifications. Initial tissue characterization results indicate that epithelial tissue in cancer specimens display increased deformability compared to epithelial tissue in normal specimens. Additionally, it was also observed that the tissue response depends on the patient from whom the specimens were acquired. Another key challenge addressed in this dissertation is accurate data analysis of raw AFM data for characterization purposes. Two sources of uncertainty typically influence data analysis of AFM force curves: the AFM probe's spring constant and the contact point of an AFM force curve. An error-in-variable based Bayesian Changepoint algorithm was developed to quantify estimation errors in the tissue's elastic properties due to these two error sources. Next, a parametric finite element modeling based approach was proposed in order to account for spatial heterogeneity in the tissue response. By using an exponential hyperelastic material model, it was shown that it is possible to obtain more accurate material properties of tissue specimens as opposed to existing analytical contact models. The experimental and computational strategies proposed in this dissertation could have a significant impact on high-throughput quantitative studies of biomaterials, which could elucidate various disease mechanisms that are phenotyped by their mechanical signatures.
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    Surface Studies of Graphene and Graphene Substrates
    (2013) Burson, Kristen M.; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Graphene has attracted a great deal of attention for its exceptional electronic and mechanical properties. As graphene, a two-dimensional lattice of carbon atoms, is an `all surface' material, its interactions with the underlying substrate play a crucial role in determining graphene device behavior. In order to tailor graphene device properties, the interaction between graphene and the underlying substrate must be clearly understood. This thesis addresses the question of the relationship between graphene and graphene substrates by considering both the substrate topography and the impact of charged impurities in the substrate. Utilizing scanning tunneling microscopy and high-resolution atomic force microscopy, we measure the topography of silicon dioxide (SiO2) supported graphene and the underlying SiO2(300nm)/Si substrates. We conclude that the graphene adheres conformally to the substrate with 99% fidelity and resolve finer substrate features by atomic force microscopy than previously reported. To quantify the density of charged impurities, simultaneous atomic force microscopy (AFM) and Kelvin probe microscopy are used to measure the potential and topographic landscape of graphene substrates, SiO2 and hexagonal boron nitride (h-BN). We find that the surface potential of SiO2 is well described by a random two-dimensional surface charge distribution with charge densities of ~1011 cm-2, while BN exhibits charge fluctuations that are an order of magnitude lower than this. Charged impurities in the substrate present a scattering source for transport through graphene transistors, and the difference in magnitude in measured substrate charged impurities densities for SiO2 and BN is consistent with the observed improvement in charged carrier mobility in graphene devices on h-BN over graphene devices on SiO2. Finally, this thesis presents a theoretical model elucidating the challenges of imaging corrugated substrates by non-contact AFM and an experimental work using Kelvin probe microscopy to characterize the electrostatic potential steps at interfaces of small-molecule organic heterojunctions.
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    NEAR-GRAZING AND NOISE-INFLUENCED DYNAMICS OF ELASTIC CANTILEVERS WITH NONLINEAR TIP INTERACTION FORCES
    (2012) Chakraborty, Ishita; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Within this dissertation work, numerical, analytical, and experimental studies are conducted with macro-scale and micro-scale elastic structures in the presence of nonlinear force interactions. The specific physical systems explored within this work are an atomic force microscope (AFM) micro-cantilever and a macro-scale cantilever experiencing similar tip interaction forces as the AFM cantilever operated in tapping mode. The tip sample forces in an AFM operation are highly nonlinear, with long-range attractive forces and short-range repulsive forces. In the macro-scale case, magnetic attractive forces and repulsive forces, which arise due to impacts with a compliant surface are used to generate similar nonlinear tip interaction forces. For elastic structures subjected to off-resonance base excitations, bifurcations close to grazing events are studied in detail, and the observed nonlinear phenomena are found to be common across the considered length scales. The dynamics of the considered systems are studied with a reduced-order computational model based on Galerkin projection with a single mode approximation. Along with studies on the bifurcation behavior, the effects of added Gaussian white noise on the system dynamics are also examined. Non-smooth system dynamics is studied by constructing local maps near the discontinuity. Period-doubling events are examined by using Poincaré maps and discontinuity mapping analysis. An important component of this dissertation research is the investigations into the effects of noise on the dynamics of these structures. Experimental and numerical efforts are used to examine the stochastic dynamics of the cantilever structures when a random component is added to the harmonic input. The noise effects are studied when the excitation frequency is close to a system resonance as well as when it is off-resonance. An analytical-numerical method with moment evolution equations is used to study the effects of noise. The effects of noise on contact and adhesion phenomena are explored. Through this dissertation work, the importance of considering noise-influenced dynamics in micro-scale applications such as AFM operations is illustrated. In addition, this work helps shed light on universality of nonlinear phenomenon across different length scales.
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    Kelvin Probe Microscopy Studies of Epitaxial Graphene on SiC(0001)
    (2011) Curtin, Alexandra E.; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Epitaxial graphene on SiC(0001) presents a promising platform for device applications and fundamental investigations. Graphene growth on SiC(0001) can produce consistent monolayer thickness on terraces and good electronic properties. In exfoliated graphene on SiO2, random charged impurities in the SiO2 surface are thought to be the dominant scatterers, explaining the observed transport properties as well as the spatial charge inhomogeneity seen in scanned-probe experiments. In contrast, the scattering mechanisms and charge distribution in epitaxial graphene remain relatively unexplored. Here I use Kelvin probe microscopy (KPM) in ambient and UHV conditions to directly measure the surface potential of epitaxial graphene on SiC(0001). Ambient-environment KPM on graphene/SiC(0001) shows surface potential variations of only 12 meV. Taken together with transport measurements, the data suggest that the graphene samples in ambient are in the low-doped regime, near the minimum conductivity of roughly 4e2/h. I am also able to use UHV KPM of graphene/ SiC(0001) to identify the discrete surface potentials of monolayer and bilayer graphene as well as the insulating interfacial carbon layer and bare SiC, correlated with scanning electron micrographs of the same location. The surface potential differences between monolayer and bilayer graphene and between IFL and monolayer graphene are both suggestive of low doping (≤1012 cm-2). The surface potentials of monolayer and bilayer graphene are relatively smooth, while the IFL and bare SiC, in contrast, showed larger variations in surface potential suggesting the presence of unscreened charged impurities present on the IFL that are later screened by the overgrown graphene. I model the potential variations for unscreened and graphene-screened charged impurities using the self-consistent theory of graphene developed by Adam et al. The results show that although surface potential variations are, as expected, larger in the IFL than in graphene, both surfaces display surface potential variations 10-40 times smaller than predicted by theory. While ambient electronic transport data and surface potential steps suggest our samples are only lightly doped (≤1012 cm-2), in a regime dominated by electron-hole puddles, we do not observe these puddles in UHV. The absence of puddles in UHV leaves the source of doping in these samples an open question.
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    DEVELOPMENT AND APPLICATIONS OF MULTIFREQUENCY IMAGING AND SPECTROSCOPY METHODS IN DYNAMIC ATOMIC FORCE MICROSCOPY
    (2011) Chawla, Gaurav; Solares, Santiago D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Force spectroscopy and surface dissipation mapping are two of the most important applications of dynamic atomic force microscopy (AFM), in addition to topographical imaging. These measurements are commonly performed using the conventional amplitude-modulation and frequency-modulation dynamic imaging modes. However, the acquisition of the tip-sample interaction force curves using these methods can generally be performed only at selected horizontal positions on the sample, which means that a 3-dimensional representation of the tip-sample forces requires fine-grid scanning of a volume above the surface, making the process lengthy and prone to instrument drift. This dissertation contains the development of two novel atomic force spectroscopy methods that could enable acquisition of 3-dimensional tip-sample force representations through a single 2-dimensional scan of the surface. The force curve reconstruction approach in the first method is based on 3-pass scanning of the surface using the recently proposed single-frequency imaging mode called frequency and force modulation AFM. A second, more versatile method based on bimodal AFM operation is introduced, wherein the fundamental eigenmode of the cantilever is excited to perform the topographical scan and a simultaneously excited higher eigenmode is used to perform force spectroscopy. The dissertation further presents the development of a trimodal AFM characterization method for ambient air operation, wherein three eigenmodes of the cantilever are simultaneously excited with the objective of rapidly and quantitatively mapping the variations in conservative and dissipative surface properties. The new methods have been evaluated within numerical simulations using a multiscale simulation methodology, and experimental implementation has been accomplished for two multifrequency variants that can provide 2-dimensional surface property contrast.
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    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.