Browsing by Subject "Raman"

Now showing 1 - 8 of 8
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    Charge Carrier Transport and Strain in Graphene Grown on Nitrogen-Seeded Silicon Carbide
    (2017-10) Torrey, Ethan R.
    The interest in graphene as a possible basis for new, faster, smaller and more flexible electronics is tempered by its lack of a band-gap. In recent years, several methods by which a gap might be created have been proposed and explored. The work presented here is a part of that exploration. In this case, the specific gap-inducing mechanism under study is a method of engineered strain. Graphene can be grown on silicon carbide. By pre-treating the silicon carbide in a process that leaves small amounts of nitrogen on its surface, the subsequently grown graphene is made to wrinkle. By controlling the wrinkling, i.e. the strain in the graphene layer, it may be possible to induce a band-gap. Indeed, Angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy results provide experimental support for this theory. At the same time, optical absorption measurements appear to contradict it. The primary focus of this dissertation is strain and transport measurements taken on devices fabricated from this type of graphene, with the expectation that these would aid in resolving the apparent contradiction in previous results.In the course of this work, a new tri-layer method of gate oxide deposition, using reactive electron beam deposition and plasma-assisted atomic layer deposition, was developed. Also, a method of enhanced Raman spectroscopy was developed for graphene-on-silicon-carbide devices. These methods were applied to a set of samples of graphene grown on nitrogen-seeded silicon carbide (NG) with the concentration of nitrogen varying between samples. In this dissertation, several transport characteristics are shown to exhibit a monotonic dependence upon the nitrogen concentration. These include changes in strain, broadening of the longitudinal resistivity peak, an offset between that peak and the zero-crossing of Hall conductivity, and a thermally activated n-doping mechanism, all measured with respect to an applied gate voltage. In addition, more complicated changes in temperature dependence and B-field dependence of the longitudinal resistivity are observed. These results, along with the surprising decrease in resistivity with the addition of nitrogen, are explained in the context of weak localization effects, increased transport by charge puddle-mediated tunneling, and edge states. While the presence of a band-gap could not be demonstrated conclusively in this, the first report of charge transport in this material, the results are in keeping with the presence of a band-gap short-circuited by edge states.
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    Detection of Whey Protein in a Hot Dog Using Immunomagnetic Separation Coupled with Surface Enhanced Raman Spectroscopy
    (2017-04) Swanson, Benjamin
    With the passing of recent legislation, most notably the Food Allergen Labeling and Consumer Protection Act in 2006 and the Food Safety Modernization Act of 2011, the focus on allergens in the food supply is a top priority for the food industry. With the consideration of unintentional allergens now being considered an adulteration, companies are trying to find detection methods that can accurately identify an unintentional allergen, but that are also rapid enough to use so as not to interrupt the production line. Immunomagnetic Separation (IMS) coupled with Surface Enhanced Raman Spectroscopy (SERS) was investigated in this research as one possible detection method. We decided to test and compare two types of IMS methods, antibody and aptamer, to see if one or the other would produce better results. The methods were based off of previous work by Dr. Lili He and were adapted to detect whey in a hot dog. During initial testing in a pure solution, both of the IMS methods appeared to show similar results, both being able to detect whey at levels of at least 125μg/mL of solution. But once we switched over testing whey in a hot dog, the antibody based IMS method proved to be the better IMS method. With a detection limit of 600μg of whey protein isolate/g of hot dog, the antibody based IMS method proved to be the more effective method. The aptamer IMS method ran into trouble with non-specific binding to the magnetic beads and was unable to detect any whey protein isolate in the hot dogs during the experiment. It is therefore concluded by the results of this experiment that the antibody based 6 IMS-SERS method is a better method to detect whey protein in a hot dog versus the aptamer method.
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    Developing Stimulated Raman Spectroscopic Techniques For Imaging Below the Optical Diffraction Limit
    (2020-05) Graefe, Christian
    Stimulated Raman spectroscopy (SRS) is a technique that amplifies the normally weak Raman scattering process using an additional laser beam, resulting in increased signal amplitudes. For this reason, it has been developed as a biological imaging platform with the potential to be used as an alternative to fluorescence microscopy due to its chemical specificity. This eliminates the need for fluorescent tags, which can photobleach or disrupt the structure or dynamics of the system of interest. However, due to the optical diffraction limit SRS cannot compete with the spatial resolution that super-resolution fluorescence techniques are capable of. An SRS-based technique capable of breaking the diffraction limit would therefore allow for nanoscale research to occur on systems for which super-resolution fluorescence is not an option. To that end, we developed a method to improve spatial resolution in SRS using a toroidal beam to deplete SRS signal. As a result, signal is only generated in a reduced area at center of the beam. Initial experiments demonstrated up to 97% depletion of the signal and explored the properties of the depletion process. Additionally, we improved spatial resolution by approximately a factor of two using the toroidal beam to deplete signal while scanning the laser beams over the edge of a diamond plate. While the proof-of-concept experiments were successful, they were performed with a laser with high peak power and a relatively low repetition rate of 1 kHz. These high powers were not compatible with soft matter samples, causing significant photodamage. We therefore adapted super-resolution SRS on laser with a 2.04 MHz repetition rate to average faster and increase the peak power flexibility. Experiments on the 2.04 MHz laser corroborated many proof-of-concept results, including resolution improvement by about a factor of two. However, depletion iv was not achieved with the same efficiency and further improvements in resolution were not forthcoming. This is likely due to the inconsistent phase of the laser’s fundamental pulse profile, highlighting the importance of consistent and reproducible pulses when driving sensitive nonlinear optical processes. Additionally, we demonstrate the use of a new Raman tag using carboranes. By scanning a thin film of a carborane-terminated poly(N-isopropylacrylamide) (pNIPAAm), we show that their high density of B-H bonds and their unique vibrational frequency in the cell silent region make carboranes useful Raman imaging tags that expand multiplexing options. Carboranes’ role as reversible addition-fragmentation chain transfer (RAFT) polymerization agents make them especially good endogenous probes for polymers produced in this manner. Finally, we discuss planned experiments to further improve signal-to-noise ratio (SNR) and explore the mechanism of signal depletion. We also discuss applications of Raman imaging in lipid dynamics, using both diffraction-limited and sub-diffraction techniques. We propose possible methods to compare results from Raman and fluorescence microscopy to determine the impact of fluorescent tags on dynamics. In the research described herein, we develop and explore new Raman imaging methods and highlight the potential power of super-resolution SRS as a versatile chemical imaging tool.
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    Development of a surface-enhanced Raman sensor for detection in complex mixtures.
    (2011-12) Bantz, Kyle Christine
    Surface-enhanced Raman scattering (SERS) is a powerful analytical signal transduction mechanism for the detection of analytes in aqueous environments, largely free from interfering water signals and capable of obtaining unique molecular signatures from structurally similar analytes. These characteristics make SERS ideal for the detection of analytes of interest from biological and environmental settings. To achieve the low limits of detection needed for biological and environmental analyte detection, new SERS platforms with the highest possible enhancement factors (EF) need to be developed. Traditionally, SERS has had limited analytical use because the analyte of interest must dwell on or near the Ag, Au or Cu surfaces, regardless of substrate EF. To overcome this limitation this work employs alkanethiol partition layers in combination with novel SERS platforms for the detection of environmental pollutants (eg. polychlorinated biphenyls and polycyclic aromatic hydrocarbons) and bioactive lipids. As the use of partition layers continues to increase and more SERS platforms with higher EF are developed, the use of SERS for analytical applications will increase. Overall, this work demonstrates the power of using novel SERS platforms combined with partition layers and reveals great promise for the future of environmental and biological sensing with SERS. Chapter One reviews the use of surface-enhanced Raman scattering detection in complex mixtures that have emerged in the last 10 years. SERS been employed for small molecule detection all the way to more complex systems, such as detection in living cells, and this chapter reviews the recent advancements and looks toward the future of SERS detection in biological systems. Chapter Two details the fabrication and characterization of new novel substrates for SERS sensing. In this chapter, three substrates are discussed, each with their own fabrication method and SERS sensing application. The majority of our SERS sensing schemes for non-traditional SERS analytes employ a partition layer-covered SERS substrate. In Chapter Three, I investigated what fundamental properties of an alkanethiol partition layer make it an ideal partition layer for particular analytes. I discovered both the thickness of the monolayer and the amount of disorder in the partition layer that allows for analyte partitioning and are critical for facilitating the analyte to dwell within the zone of enhancement for SERS. The last two chapters detail the implementation of partition layer-modified SERS substrates for detection in complex mixtures. Chapter Four demonstrates the use of partition layer-modified SERS substrates for the detection of environmental pollutants: polycyclic aromatic hydrocarbons, polychlorinated biphenyls and polybrominated diphenyl ethers. I was able to show that our substrate made it possible to detect and discriminate between structurally similar analytes in the presence of interfering species at environmentally relevant concentrations. The final chapter of this dissertation describes the steps I have taken towards SERS sensing in complex biological mixtures for the detection of bioactive lipids. The results of this investigation indicate that partition layer-modified AgFON substrates can facilitate the detection of phospholipids and secreted lipids at higher concentrations, but the SERS bands from the partition layer make detection of lipids at physiologically relevant concentrations challenging at this time.
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    MATLAB Code: Raman Scattering Intensity for Quadratic Hamiltonians
    (2015-09-14) Perreault, Brent M; perre035@umn.edu; Perreault, Brent
    These are codes were used to generate the Raman scattering intensity spectra of Kitaev Spin Liquid models using the Loudon-Fleury approach. In its most basic form this code diagonalizes a quadratic fermionic Hamiltonian and computes the spectra by constructing the Raman operator, using the eigenfunctions to compute matrix elements, and the eigenvalues to plot the spectrum as a function of energy. Variants are included that consider 2D and 3D lattices, finite systems, as well as the resonant Raman scattering. Neither interactions nor the bosonic case are considered.
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    Quinine Copolymer Reporters For Enhanced Gene Editing And Raman Imaging
    (2022-01) Van Bruggen, Craig
    After decades of development, gene therapy has finally reached the forefront of medicine and has led to new cures for genetic disorders and the development of life-saving vaccines. The field has been buoyed by the development of more precise and user-friendly targeted nucleases, such as those used for clustered regularly interspersed palindromic repeats (CRISPR)-based editing. These useful gene-editing technologies, however, are still stymied by the challenge of delivering exogenous nucleic acids and proteins into the cells of interest. The emerging gene therapy industry is investing heavily in developing more efficient and safe non-viral vehicles as alternatives to costly and immunogenic viral vectors. Cationic polymers are promising non-viral vectors due to their manufacturing scalability, their chemical stability, and their synthetic tunability. Improvements in delivery efficiency are necessary, however, for widespread adoption of polymeric vehicles for gene therapy. One challenge in improving performance, however, is the difficulty and limited methodology for elucidating the intracellular mechanics of polymeric vehicles. In this thesis, I describe my research focused on the development of a novel quinine-containing polymer, called a Quinine Copolymer Reporter (QCR), that enhanced transient transfections of cultured cells with plasmids and improved gene editing of cultured cells through the simultaneous delivery of the CRISPR-associated protein Cas9 and DNA donor template. In addition, I describe collaborative research performed with colleagues in the research group of Prof. Renee Frontiera that characterized a band in quinine’s Raman spectrum that is diagnostic of its chemical environment. Using this chemical sensitivity in conjunction with Raman microscopic imaging, we help elucidated the intracellular unpackaging mechanisms of the QCR-nucleic acid complexes.
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    Raman spectroscopy data and phonon calculations for ScV6Sn6 in P6/mmm and R-3m structures, 2023
    (2023-06-27) Ritz, Ethan T; Birol, Turan; Gu, Yanhong; Musfeldt, Janice L; eritz@umn.edu; Ritz, Ethan T; Birol Research Group, University of Minnesota; Musfeldt Group, University of Tennessee Knoxville
    We use density functional theory (DFT) to calculate the phonon frequencies and the distortions associated with them in the compound ScV6Sn6 in the P6/mmm and R-3m space groups, then compute the overlap between the Raman-active phonons in each structure. This data includes scripts to generate the DFT submission files, the results of those simulations, as well as MATLAB scripts to plot the results. We also include experimental Raman spectroscopy data at temperatures from 5.5 K to 300 K.
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    Synthesis and characterization of copper zinc tin sulfide nanoparticles and thin films.
    (2012-07) Khare, Ankur
    Copper zinc tin sulfide (Cu2ZnSnS4, or CZTS) is emerging as an alternative material to the present thin film solar cell technologies such as Cu(In,Ga)Se2 and CdTe. All the elements in CZTS are abundant, environmentally benign, and inexpensive. In addition, CZTS has a band gap of ~1.5 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity. CZTS has a high absorption coefficient (>104 cm-1 in the visible region of the electromagnetic spectrum) and only a few micron thick layer of CZTS can absorb all the photons with energies above its band gap. CZT(S,Se) solar cells have already reached power conversion efficiencies >10%. One of the ways to improve upon the CZTS power conversion efficiency is by using CZTS quantum dots as the photoactive material, which can potentially achieve efficiencies greater than the present thin film technologies at a fraction of the cost. However, two requirements for quantum-dot solar cells have yet to be demonstrated. First, no report has shown quantum confinement in CZTS nanocrystals. Second, the syntheses to date have not provided a range of nanocrystal sizes, which is necessary not only for fundamental studies but also for multijunction photovoltaic architectures. We resolved these two issues by demonstrating a simple synthesis of CZTS, Cu2SnS3, and alloyed (Cu2SnS3)x(ZnS)y nanocrystals with diameters ranging from 2 to 7 nm from diethyldithiocarbamate complexes. As-synthesized nanocrystals were characterized using high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and energy dispersive spectroscopy to confirm their phase purity. Nanocrystals of diameter less than 5 nm were found to exhibit a shift in their optical absorption spectra towards higher energy consistent with quantum confinement and previous theoretical predictions. Thin films from CZTS nanocrystals deposited on Mo-coated quartz substrates using drop casting were found to be continuous but highly porous. Annealing CZTS nanocrystal films at temperatures as low as 400°C led to an intense grain growth; however, thin films from CZTS nanocrystals cracked on annealing due to their high porosity. Although quantum confinement in CZTS is only accessible in nanocrystals of diameters less than 5 nm, the high volume of the ligands as compared to the volume of the nanocrystals makes it a challenge to deposit continuous compacted thin films from small nanocrystals. Films deposited from thermal decomposition of a stoichiometric mix of metal dithiocarbamate complexes were found to be predominantly CZTS. These films from complexes were found to be continuous but microporous. The diameter of the spheres making up the microporous structure could be changed by changing the anneal temperature. The structural composition of the final film could be altered by changing the heating rate of the complexes. CZTS exists in three different crystal structures: kesterite, stannite, and pre-mixed Cu-Au (PMCA) structures. Due to the similarity in the crystal structures, it is extremely difficult to distinguish them based on X-ray diffraction. We computed the phonon dispersion curves for the three structures using ab-initio calculations, and found characteristic discontinuities at the Γ-point which can potentially be used to distinguish the three. In addition, the Γ-point phonon frequencies, which correspond to the Raman peak positions, for the three structures were found to be shifted from each other by a few wavenumbers. By deconvoluting the experimental Raman spectra for both CZTS and Cu2ZnSnSe4 (CZTSe) using Gaussian peaks, we observed that the most intense Raman scattering peak in both CZTS and CZTSe is a sum of two different peaks which correspond to scattering from their respective kesterite and stannite phases. The electronic, structural, and vibrational properties of a series of CZTS-CZTSe alloys (CZTSSe) were studied using ab-initio calculations. The S-to-Se ratio and the spatial distribution of the anions in the unit cell were found to determine the energy splitting between the electronic states at the top of the valence band and the hole mobility in CZTSSe alloys and solar cells. X-ray diffraction patterns and phonon distribution curves were found to be sensitive to the local anion ordering. The predicted Raman scattering frequencies and their variation with x agree with experimentally determined values and trends.