Browsing by Subject "Raman spectroscopy"

Now showing 1 - 6 of 6
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    Atomic layer lithography of plasmonic nanogaps for enhanced light-matter interactions: fabrication and applications
    (2016-01) Chen, Xiaoshu
    Enhanced light-matter interactions at the nanometer scale have many potential applications, such as thin film sensing, enhanced Raman scattering, enhanced infrared absorption, particle manipulation, among others. Metal – insulator – metal nanogap structure is one of the most effective plasmonic devices for such applications since they are capable of generating the strongest light field enhancement inside the nanogap. However, current techniques to make such nanogap structures are either very expensive, slow, or lacking of control over nanogap size, pattern shape, and position. In this thesis, two wafer-scale fabrication methods are presented to address the challenges in fabrication. The fabricated devices are then used to demonstrate the above-mentioned applications. Atomic layer deposition is used in both methods to define the width of nanogap with angstrom resolution. The length, position, and shape of the nanogaps are precisely controlled in wafer scale by photolithography and metal deposition. A simple tape peeling and a template stripping process are used to expose the nanogaps. Nanogap devices with different designs are proved to support strong optical resonances in visible, near infrared, mid infrared, and terahertz-frequency regimes. By squeezing electromagnetic waves into nanometer wide gaps, huge field enhancement can be achieved inside the gaps. These novel fabrication methods can easily be duplicated and thus lead to broad studies and applications of the enhanced light-matter interactions.
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    Characterization of Graphene Grown Directly on Crystalline Substrates
    (2015-09) Rothwell, Sara
    Graphene has become one of the most popular materials under research, particularly since the 2010 Nobel Prize in Physics. Many visions posit that graphene electronics will be some of the fastest and smallest circuitry physically feasible, however before this becomes reality the scientific community must gain a firm handle on the creation of semiconducting varieties of graphene. In addition, well understood epitaxial growth of graphene on insulating materials will add to the facility of fabricating all-carbon electronics. This thesis presents experimental work detailing the growth of pristine graphene grown on sapphire (GOS) through the thermal decomposition of acetylene, and the electronic characterization of graphene grown on nitrogen-seeded silicon carbide (NG), a semiconducting variety of graphene grown in collaboration with researchers at Georgia Institute of Technology and Rutgers University. GOS displays turbostratic stacking and characteristics of monolayer graphene as analyzed by Raman spectroscopy and atomic force microscopy. Scanning tunneling microscopy characterization of NG illustrates a topography of pleats from 0.5-2 nm tall, 1-4 nm thick, and 1-20 nm long, as well as atomically flat plateaus and other areas of intermixed features. Scanning tunneling spectroscopy measurements across NG features show peaks interpreted as Landau levels induced by strain. Analysis of these Landau levels in coordination with previous characterization concludes that a model employing a bandgap fits best.
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    Characterizing Freezing and Thawing Responses of Multiple Types of Cells Cryopreserved in Both DMSO and non-DMSO Cryoprotectants
    (2018-09) Yu, Guanglin
    Cryopreservation is the technology used to stabilize cells at subzero temperature for a variety of applications including diagnosis and treatment of disease, and the production of therapeutic proteins. Current theories of cell damage during freezing were developed in the 1960’s, and little has changed since then. However, our understanding of cell biology as well as tools to interrogate cell responses during freezing has improved in the last 50 years. Low temperature Raman spectroscopy has been used to verify, for the first time using chemical spectra, the presence of ice inside the cell during freezing. With this tool, it is possible to internally observe frozen cells, and identify specific chemical and morphological changes that result in cell life or death. In this work, we propose to use this powerful tool to test two hypotheses to enhance our understanding of the mechanism of cell damage during freezing and thawing, and the manner by which the damage can be mitigated to improve cryopreservation outcome. For the first part of this work, we hypothesize that not all intracellular ice formation (IIF) is lethal and the conditions of cell membrane, cytoskeleton and mitochondria play an important role in determining IIF and cryopreservation outcome. Freezing responses of single cells as well as multi-cellular system cryopreserved and thawed in dimethyl sulfoxide (DMSO) solution will be examined to test this hypothesis. DMSO-free cryopreservation has attracted much recent interest due to the toxicity of DMSO. For the second part of this work, we hypothesis that non-DMSO multicomponent osmolyte solutions can be used to preserve cell viability and one component, disaccharide, acts to protect the cell through multiple interactions. Freezing responses of cells cryopreserved in a combination of non-DMSO cryoprotectants such as sugars, sugar alcohols, and amino acids will be examined. Interactions among sucrose (a typical disaccharide), water and cell membrane at low temperature will be also be investigated in order to test the second hypothesis. Enhancing our understanding of freezing damage and strategies to mitigate damage will improve the methods of preserving cell therapy products and therefore enable the treatment to reach the patients who could benefit from them.
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    Electronic transport in mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films.
    (2010-08) Adjallah, Yves Gbemonde
    The opto-electronic properties of amorphous/nanocrystalline hydrogenated silicon (a/nc-Si:H) mixed-phase thin films are investigated. Small crystalline silicon particles (3-5 nm diameter) synthesized in a flow-through reactor are injected into a separate capacitively-coupled plasma (CCP) chamber where mixed-phase hydrogenated amorphous silicon is grown by Plasma Enhanced Chemical Vapor Deposition (PECVD) deposition techniques. This dual-chamber co-deposition system enables the variation of crystallite concentration incorporated into a series of a-Si:H films deposited simultaneously. The structural, optical and electronic properties of these mixed-phase materials are studied as a function of the silicon nanocrystal concentration. That is, we compare a sequence of films deposited in a single run, where the location of the substrate in the CCP chamber determines the density of embedded nanocrystals. Raman spectroscopy is used to determine the volume fraction of nanocrystals in the mixed phase thin films. At a moderate concentration of silicon crystallites, the dark conductivity and photoconductivity are consistently found to be up to several orders of magnitude higher than in mixed phase films with either low or heavy nanocrystalline inclusions. These results are interpreted in terms of a model whereby for low nanocrystal concentrations conduction is influenced by the disorder introduced into the a-Si:H film by the inclusions, while at high nanocrystal densities electronic transport is described by a heterojunction quantum dot model. The thermopower of the undoped a/nc-Si:H has a lower Seebeck coefficient, and similar temperature dependence, to that observed for undoped a-Si:H. In contrast, the addition of nanoparticles in doped a/nc-Si:H thin films leads to a negative Seebeck coefficient (consistent with n-type doping) with a positive temperature dependence, that is, the Seebeck coefficient becomes larger at higher temperatures. The temperature dependence of the thermopower of the doped a/nc-Si:H is similar to that observed in unhydrogenated a-Si grown by sputtering or following high-temperature annealing of a-Si:H, suggesting that charge transport may occur via hopping in these materials.
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    Magnetism from bond-directional anisotropic interactions
    (2023-10) Yang, Yang
    This dissertation presents an overview of my research works on studying magnetism emerging from bond-directional interactions in spin-orbit coupled Mott insulators. The study encompasses two primary activities: the examination of superexchange spin models and the comprehension of magnetic excitations observed in real materials. The first activity includes the study of theoretical Kitaev-Heisenberg model on the kagome lattice, where we identify nontrivial local symmetry of the system. We find the classical and quantum phase diagrams of the model. Our findings reveal striking similarities of the physics between the classical and quantum regimes. The second activity mainly focuses on understanding of Raman responses (inelastic light scattering) from different materials. We first employ the Loudon-Fleury form of the Raman operator to elucidate magnetic excitations in the pyrochlore compound Nd2Ir2O7. Then we extend the Raman operator beyond the conventional Loudon- Fleury formalism, and apply it to understand magnetic excitations observed in the hyperhoneycomb material β-Li2IrO3.
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    Surface-Enhanced Raman Spectroscopy of Analytes in Blood
    (2015-04) Campos, Antonio
    Although Raman scattering has traditionally been considered a weak process, making analysis of low concentration analytes in complex matrices difficult, both methodological and instrumentation advances in the last couple decades have made Raman spectroscopy a viable and useful analytical tool.1,9 This is especially true for analyte species within aqueous environments because the Raman scattering cross-section of water is small; one particular example of a critical aqueous environment is analysis of and in blood. This review will analyze much of the literature related to Raman analysis in blood within the last 20 years, including normal Raman, surface-enhanced Raman, and spatially offset Raman analyses. The first section will focus on direct analysis of blood samples, including determining the age of deposited or donated blood and blood content within body fluid mixtures. The second section will discuss intrinsic Raman-based detection of small molecules and protein analytes within blood as well as extrinsic Raman detection of tumors. The last section will review the recent use of spatially offset Raman and surface-enhanced spatially offset Raman spectroscopy to analyze molecular analytes, tissue, bone, tumors, and calcifications, including in vivo analysis. This focal point closes with perspective on critical gaps and upcoming developments for Raman analysis in blood. Microfluidic sensing platforms facilitate parallel, low sample volume detection using various optical signal transduction mechanisms. Herein, we introduce a simple mixing microfluidic device, enabling serial dilution of introduced analyte solution that terminates in five discrete sensing elements. We demonstrate the utility of this device with on-chip fluorescence and surface-enhanced Raman scattering (SERS) detection of analytes, and we demonstrate device use both when combined with a traditional inflexible SERS substrate and with SERS-active nanoparticles that are directly incorporated into microfluidic channels to create a flexible SERS platform. The results indicate, with varying sensitivities, that either flexible or inflexible devices can be easily used to create a calibration curve and perform a limit of detection study with a single experiment. In current events, ricin has been discussed frequently because of letters sent to high-ranking government officials containing the easily extracted protein native to castor beans. Ricin B chain, commercially available and not dangerous when separated from the A chain, enables development of ricin sensors while minimizing the hazards of working with a bioterror agent that does not have a known antidote. As the risk of ricin exposure, common for soldiers, becomes increasingly common for civilians, there is a need for a rapid, real-time detection of ricin. To this end, aptamers have been used recently as an affinity agent to enable the detection of ricin in food products via surface-enhanced Raman spectroscopy (SERS) on colloidal substrates. One goal of this work is to extend ricin sensing into whole human blood; this goal required application of a commonly used plasmonic surface, the silver film-over-nanosphere (AgFON) substrate, which offers SERS enhancement factors of 106 in whole human blood for up to 10 days. This aptamer-conjugated AgFON platform enabled ricin B chain detection for up to 10 days in whole human blood. Principle component analysis (PCA) of the SERS data clearly identifies the presence or absence of physiologically relevant concentrations of ricin B chain in blood. Spectrophotometry and colorimetry experiments are common in high school and college chemistry courses. Previous work has demonstrated that handheld camera devices can be used to quantify the concentration of a colored analyte in solution in place of traditional spectrophotometric or colorimetric equipment. This paper extends this approach to an investigation of a mesogold mineral supplement. With the addition of free Google applications, the investigation provides a feasible, sophisticated lab experience, especially for teachers with limited budgets.