Browsing by Subject "AFM"
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Item Advancing a nuclear magnetic resonance force microscopy (NMRFM) probe and simulating NMRFM in thin films(2023-12) Paster, Jeremy W.; Markert, John T.; Lai, Keji; Lewis-Peacock, Jarrod A; Tsoi, Maxim; Marder, MichaelThis research endeavor centers around the development of a nuclear magnetic resonance force microscopy (NMRFM) probe for investigating thin-film samples. Of particular interest is the conducting region that forms when lanthanum aluminate (LAO) is grown epitaxially on strontium titanate (STO). These materials are insulating in their bulk form. We propose NMRFM as a tool to detect whether there is diffusion of atoms across the interface, which could explain the emergence of the conducting region. While conventional scanning probe techniques are constrained to the surface of a sample, NMRFM features the non-invasive and subsurface detection capabilities of conventional nuclear magnetic resonance (NMR) spectroscopy. Unlike conventional NMR, for which a cubic millimeter-sized sample is required to produce a measurable signal, we can readily scale down NMRFM detection sensitivity, extending its application to smaller samples. In combination, these features suggest that NMRFM is well-suited to study the LAO/STO interface. Force detection of nuclear spins is made possible by coupling NMR spin flip sequences to a mechanical oscillator (cantilever). A small magnetic tip deposited on the cantilever establishes a large field gradient and an interaction force with the magnetic moments of the sample nuclei. This tip traces out constant-field slices perpendicular to the magnetic field. Within a particular slice, nuclear spins resonate with the perturbing oscillating field conventionally employed in NMR spectroscopy. We anticipate that evidence of atomic diffusion across the LAO/STO interface is limited to a 10-nanometer region. Before the reconfiguration outlined herein, this NMRFM probe previously resolved a sample whose smallest dimension was 60 microns. To develop this probe for thin films, we adopted the Interrupted OScillating Cyclic Adiabatic Reversal (iOSCAR) protocol. This method distinguishes the minuscule force interaction between the cantilever and the sample by implementing an NMR-induced modulation of the cantilever frequency. iOSCAR generates a distinguishable signal at an established frequency that is far from spurious artifact signals that limited the signal-to-noise ratio of the previous NMRFM protocol. This dissertation involves contextualizing NMR and NMRFM and assesses the need for further experimental investigation of LAO/STO. Furthermore, it details the evolution of an NMRFM probe to enable the exploration of thin-film samples using iOSCAR. While this research project largely involved the creation of experimental components, it concluded by modeling the expected experimental results. We created simulations of thin-film NMRFM, calculating the z component of the sample magnetization near an oscillating cantilever with a magnetic tip. These simulations explore the dynamic interactions between a thin-film sample and a cantilever as sample nuclei undergo magnetic resonance.Item Exploration of voltage controlled manganite phase transitions as probed with magnetic force microscopy(2010-05) Ruzicka, Frank Joseph; de Lozanne, Alejandro L.; Tsoi, Maxim; Shih, Chih-Kang; Markert, John T.; Shi, LiLow-temperature magnetic force microscopy was used to study the phase diagram of a La1/3Pr1/3Ca1/3MnO3 thin film grown on a (110) NdGaO3 (NGO) substrate by pulsed laser deposition. Traditionally, one can observe the phase change at the nanoscale level as the sample is cooled from room temperature through the transition temperature to liquid nitrogen temperatures, but in this case a fixed voltage ranging from 0 V to 31 V was applied before each cooling cycle. From in and ex situ transport measurements, it is observed that the temperature of the peak of the transition increases with applied field; however, the MFM images show that the magnetic transition begins at a lower temperature with the same increase in field. Thus, this dissertation shows that a new voltage control exists for the phase transition in certain manganites.Item Nanoscale electronic and thermal transport properties in III-V/RE-V nanostructures(2013-12) Park, Keun Woo; Yu, Edward T.The incorporation of rare earth-V (RE-V) semimetallic nanoparticles embedded in III-V compound semiconductors is of great interest for applications in solid-state devices including multijunction tandem solar cells, thermoelectric devices, and fast photoconductors for terahertz radiation sources and receivers. With regard to those nanoparticle roles in device applications and material itself, electrical and thermal properties of embedded RE-V nanoparticles, including nanoscale morphology, electronic structure, and electrical and thermal conductivity of such nanoparticles are essential to be understood to engineer their properties to optimize their influence on device performance. To understand embedded RE-V semimetallic nanostructures in III-V compound semiconductors, nanoscale characterization tools are essential for analysis their properties incorporated in compound semiconductors. In this dissertation, we used atomic force microscopy (AFM) with other secondary detection tools to investigate nanoscale material properties of semimetallic RE-V and GaAs heterostructures, grown by molecular beam epitaxy. We used scanning capacitance microscopy and conductive AFM techniques to understand electronic and electrical properties of ErAs/GaAs heterostructures. For the electrical properties, this thesis investigates details of statistical analysis of scanning capacitance and local conductivity images contrast to provide insights into (i) nanoparticle structure at length scales smaller than the nominal spatial resolution of the scanned probe measurement, and (ii) both lateral and vertical nanoparticle morphology at nanometer to atomic length scales, and their influence on electrical conductivity. To understand thermal properties of ErAs nanoparticles, in-plane and cross-sectional plane of ErAs/GaAs superlattice structure were investigated with a scanning probe microscopy technique implemented with 3[omega] method for thermal measurement. By performing detailed numerical modeling of thermal transport between thermal probe tip and employed samples, and estimation of additional phonon scattering induced by ErAs nanoparticles, we could understand influences of ErAs nanoparticles on the host GaAs thermal conductivity. Investigation of ErAs semimetallic nanostructure embedded in GaAs matrix with scanned probe microscopy provided detailed understanding of their electronic, electrical and thermal properties. In addition, this dissertation also demonstrates that an atomic force microscope with secondary detection techniques is promising apparatus to understand and investigate intrinsic properties of nanostructure materials, nanoscale charge transports, when the system is combined with detailed modeling and simulations.Item Optical characterization of asphalt binder microstructure(2019-04-30) Ramm, Adam Steven; Downer, Michael Coffin; Bhasin, Amit; Marder, Michael; Sitz, GregThis dissertation presents results of optical characterization of asphalt binder microstructure. We use noncontact optical microscopy methods to observe microstructure at the surface as well as in the bulk of asphalt binder samples. Normal incidence optical microscopy is used to image elongated, striped, surface microstructures (known as ’bees’ since they resemble bumble-bees) with comparable resolution to atomic force microscopy (AFM). Narrow band dark field optical microscopy is used to measure surface microstructure areal density and bulk microstructure volume density. Short wavelength illumination images surface microstructures, while longer wavelength near infrared (NIR) illumination penetrates into the bulk and images sub-surface microstructures. We subject the binder to temperature cycles and observe the resulting microstructure kinetics. We benchmark these optical measurements with rheometry measurements obtained from identical thermal cycles. We also use optical methods to observe surface ’bee’ microstructure variations due to interfacial tension modifications and compare these results to thin film mechanics theoryItem Physical characterization of bacterial biofilm polymer networks to determine the role of mechanics in infection and treatment(2018-11-29) Kovach, Kristin N.; Gordon, Vernita Diane; Florin, Ernst-Ludwig; Marder, Michael P; Smyth, Hugh D; Lynd, NathanielBiofilms are communities of microorganisms that produce a matrix of extracellular polymers to surround and protect themselves from external forces in their environment. This communal lifestyle is incredibly beneficial for microorganism survival. Characterization of the mechanical properties of biofilms is a vital and understudied component of fully understanding these biological systems. In this dissertation, we break down the mechanical response of the Pseudomonas aeruginosa biofilm by its constituent polymers. These bacteria produce unique polymers to resist a variety of stresses. In the first part of this dissertation, using oscillatory bulk rheology, we characterize the viscoelasticity of biofilm polymer networks. Using genetically manipulated lab strains of P. aeruginosa, we isolate the mechanical response of each polymer by analyzing biofilms comprised primarily of one type of polymer. We find that the polymers have unique mechanical properties: some increase the yield strain and others increase elastic modulus. In strains of P. aeruginosa isolated from chronic infections, we find that the bacteria evolve to increase production of polymers that maximize the energy required to yield the matrix. In the second part of this dissertation, we work to mechanically compromise each of the polymers in the matrix. By attacking different matrix components, we learn more about the structural properties that give rise to mechanical properties as well as identify the most promising therapeutic treatments to break down biofilm infections. We find that specific enzymes are useful for decreasing yield strain of biofilms and increasing the diffusivity of the matrix. Decrease in yield strain means that biofilms will take less deformation before losing mechanical integrity, and the increase in matrix diffusivity means that current treatments such as antibiotics are more effective as the antibiotics can more easily reach the bacteria in the matrix to effectively kill them. This dissertation treats biofilms as polymer networks, divorcing the analysis from biological responses, in an attempt to well-characterize the understudied mechanical properties of biofilms. By approaching these systems from a physical standpoint, we are able to learn more about biofilms by breaking the mechanical response into constituent components, as well as learn about how enzymatic treatments alter biofilm properties.Item Preparation of biomimetic surfaces that facilitate the native cellular process of peptide self-assembly(2015-11-24) Dugger, Jason Wade; Webb, Lauren J.; Vanden Bout, David; Roberts, Sean; Maynard, Jennifer; Elber, RonThe ability to maintain or reproduce biomolecular structures on inorganic substrates has the potential to impact diverse fields such as sensing and molecular electronics, as well as the study of biological self-assembly and structure-function relationships. Because the structure and self-assembly of biomolecules are exquisitely sensitive to their local chemical and electrostatic environment, the goal of reproducing or mimicking biological function in an abiological environment, including at a surface, is challenging. However, simple and well-characterized chemical modifications of prepared surfaces can be used to tune surface chemistry, structure, electrostatics, and reactivity of inorganic materials to facilitate biofunctionalization and function. Here, we describe the covalent attachment of 13-residue β-stranded peptides containing alkyne groups to a flat gold surface functionalized with an azide-terminated self-assembled monolayer (SAM) through a Huisgen cycloaddition, or “click,” reaction. The chemical composition and structural morphology of these surfaces were characterized using X-ray photoelectron spectroscopy (XPS), grazing incidence angle reflection-absorption infrared spectroscopy (GRAS-IR), surface circular dichroism (CD), atomic force microscopy v (AFM), and neutron reflectometry (NR). The surface-bound β-strands self-assemble into antiparallel β-sheets to form fibrillar structures 24.9 ± 1.6 nm in diameter and 2.83 ± 0.74 nm in height on the reactive surface. The results herein provide a platform for studying and controlling the self-assembly process of biomolecules into larger supermolecular structures while allowing tunable control through chemical functionalization of the surface. Interest in the mechanisms of formation of fibrillar structures have most commonly been associated with neurodegenerative diseases such as Alzheimer’s and Parkinson’s, but fibrils may actually represent the thermodynamic low-energy conformation of a much larger class of peptides and proteins. The protocol developed here is an important step towards uncovering not only the factors that dictate self- assembly, but also the mechanisms by which this fibrillar class of superstructures form.Item Surface enhanced Raman spectroscopy of olivine type battery cathode LiFePO4(2010-08) Delone, Nicholas Ryan; Stevenson, Keith J.; Vanden Bout, David A.This thesis explores the use of Raman Spectroscopy to study the battery cathode material LiFePO4. Surface Enhanced Raman Spectroscopy (SERS) was incorporated into the study due to fluorescence that traditionally plagues Raman. By imaging LiFePO4 nanoparticles, an understanding can be gained of the complex chemistry taking place when the material is lithiated and delithiated at the nanoscale level and the phase changes of the material that occur during this process. The use of bimetallic (Au/Ag) SERS substrates allowed for more stable substrates with longer shelf life compared single metal Ag substrates. Further tuning of these substrates can be applied to the ever evolving science of energy storage material technology as a way to track phase changes in the material.Item Tailoring nanoscale metallic heterostructures with novel quantum properties(2013-05) Sanders, Charlotte E.; Shih, Chih-Kang; Raizen, Mark G.Silver (Ag) is an ideal low-loss platform for plasmonic applications, but from a materials standpoint it presents challenges. Development of plasmonic devices based on Ag thin film has been hindered both by the dificulty of fabricating such film and by its fragility out of vacuum. Silver is non-wetting on semiconducting and insulating substrates, but on certain semiconductors and insulators can adopt a metastable atomically at epitaxial film morphology if it is deposited using the "two-step" growth method. This method consists of deposition at low temperature and annealing to room temperature. However, epitaxial Ag is metastable, and dewets out of vacuum. The mechanisms of dewetting in this system remain little understood. The fragility of Ag film presents a particular problem for the engineering of plasmonic devices, which are predicted to have important industrial applications if robust low-loss platforms can be developed. This dissertation presents two sets of experiments. In the first set, scanning probe techniques and low energy electron microscopy have been used to characterize Ag(111) growth and dewetting on two orientations of silicon (Si), Si(111) and Si(100). These studies reveal that multiple mechanisms contribute to Ag film dewetting. Film stability is observed to increase with thickness, and thickness to play a decisive role in determining dewetting processes. A method has been developed to cap Ag film with germanium (Ge) to stabilize it against dewetting. The second set of experiments consists of optical studies that focus on the plasmonic properties of epitaxial Ag film. Because of the problems posed until now by epitaxial Ag growth and stabilization, research and development in the area of plasmonics has been limited to devices based on rough, thermally evaporated Ag film, which is robust and simple to produce. However, plasmonic damping in such film is higher than in epitaxial film. The optical studies presented here establish that Ag film can now be stabilized sufficiently to allow optical probing and device applications out of vacuum. Furthermore, they demonstrate the superiority of epitaxial Ag film relative to thermally evaporated film as a low-loss platform for plasmonic devices spanning the visible and infrared regimes.