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Department Library

2020

Alexandra Christensen (Senior Thesis, April 2020, Advisor: John Colton )

Abstract

Brachytherapy is the use of ionizing radiation at short distances to treat disease, delivering the appropriate dose via an implant through which radionuclides can travel in or on the body. The geometry of implant placement is critical to achieving brachytherapy plans that do not overdose the target region. Implants are typically designed based on the radiation oncologist’s experience, and although some groups have created parameters for quantitative implant comparison, most lack simplicity and objectivity. This project describes an investigation of alternative indices that can be used for brachytherapy implant evaluation. The surface area to volume ratio of isodose surfaces and the number of non-contiguous isodose surfaces were calculated as functions of dose contour value. Several simple geometric target volumes were created to test the ability of these new metrics to differentiate between implant configurations. The surface area to volume ratio demonstrated the highest discernment power. A higher surface area to volume ratio correlates with the configuration expected to represent a more optimal implant. This is particularly useful when comparing implants with similar target coverage and can be used to evaluate implants with geometries for which experience is limited. This approach is sensitive to differences in implant geometry and has the simplicity of being a single number that can be used for comparison or as a parameter in optimization.

James Erikson (Senior Thesis, April 2020, Advisor: David Allred, John Colton )

Abstract

Temperature is an important parameter in many processes being studied with microfluidic devices. As such, improved temperature sensing methods compatible with the size and sensitivity required for microfluidics need to be found. This work investigates the photoluminescence lifetime of Rhodamine B and CdTe quantum dots for potential use in microfluidic devices. Lifetime values were sampled over a range of known temperatures through time-correlated single photon counting. Spectral measurements were also taken at each temperature. In Rhodamine B, lifetime was obtained through numerical deconvolution, however, results obtained in this way were unreliable due to variability within the sample itself over time. Similar methods proved similarly unreliable for CdTe quantum dots, which also show variability over time, though to a lesser extent. Through the application of machine learning algorithms, temperatures in CdTe quantum dots can be accurately determined with uncertainties ranging from 7.7 K at cryogenic temperatures to 0.1 K near room temperature. This success shows that temperature dependent photoluminescence is a valid option for future applications in microfluidic devices.

Charles Lewis (Senior Thesis, June 2020, Advisor: John Colton )

Abstract

Various biological processes require accurate temperature sensing through microfluidic devices,so improved temperature sensors methods are needed. Previously, machine learning techniques have been used for predicting temperatures through thermal images. To develop a new, accurate temperature sensor model, photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra of CdTe quantum dots were measured as functions of temperature for use in training an artificial neural network (ANN). A low-temperature regime from 10-300 K and a high-temperature regime from 298-319 K were measured with additional data provided through interpolation. The optimized neural network is able to determine temperatures with a mean average error of 7.7 K and 0.1 K for the low and high-temperature regimes respectively. The mean absolute error of the low-temperature regime improves to 0.4 K when restricting to temperatures between 100-300 K.

2019

Daniel Boyce (Senior Thesis, August 2019, Advisor: John Colton )

Abstract

Hydrogen (H2) gas is a possible alternative fuel to help meet increasing worldwide energy needs, but a major obstacle in the use of H2 for green, environmentally-friendly fuel is the energetic and chemical requirements to synthesize the gas. I am studying the use of photocatalytic reactions to produce H2, where a light-absorbing substance acts as a catalyst in shuttling electrons from a donor to protons that are reduced into H2. Previous research conducted at BYU showed that platinum nanoparticles bound to ferritin catalyzed the photoreaction of methyl viologen to reduce protons in an organic acid; this catalytic system offered an increase in hydrogen production efficiency by up to 100 times over platinum black (a commonly available platinum-based catalyst). I am reporting on our efforts to optimize the synthesis of the platinum nanoparticles bound to ferritin that are used in this photocatalytic system and how I characterize these nanoparticles, as well as how these characteristics affect H2 production.

Matthew Richards (Senior Thesis, April 2019, Advisor: John Colton )

Abstract

Hydrogen gas has been hailed as the fuel of the future. Unfortunately, significant problems with its production, storage, and transportation prevent its widespread use. One possible solution is to make hydrogen gas using ferritin-bound platinum nanoparticles (FBPNs). I studied the optimum time of UV exposure for making FBPNs, and the ability of FBPNs to synthesize hydrogen gas. FBPN samples were made by reacting chemicals under a UV lamp with stirring. I fractioned the FBPN samples using size-exclusion chromatography and the fraction with the FBPNs was identified using spectrophotometry. I tested the protein concentration using the Lowry protein assay and the platinum concentration using ICP-MS. Using these results, the number of platinum nanoparticles per ferritin was calculated. I then used the FBPNs to catalyze hydrogen gas production. The amount of hydrogen gas was tested using TCD-GC. Preliminary results indicate that the optimum time for production of FBPNs is 30 minutes of UV exposure, resulting in 182.7 platinum nanoparticles per ferritin being formed. I successfully synthesized hydrogen gas as well. While difficulties with the LPA make the results tenuous, the methods, with some modification, would allow the quick analysis of other important parameters in this process and should be pursued.

Micah Shelley (Senior Thesis, April 2019, Advisor: John Colton )

Abstract

Zinc oxide is a semiconductor with a wide band gap (3.37 eV), allowing for applications in optics and optoelectronics. Historically, stable p-type material has been difficult to create. We report deposition of stable p-type ZnO thin films by rf magnetron sputtering. Arsenic acts as the p-type dopant, and is provided through a base layer of Zn3As2 evaporated onto the substrate. Annealing the samples improves crystal structure. P-type material is confirmed, and material properties are quantified by Seebeck effect, Hall effect, photoluminescence, and x-ray diffraction measurements. We identify the effects of substrate temperature, sputter time, rf power, and plasma gas ratio on the electrical and optical properties of the ZnO:As. Future work to improve the quality of the films produced is discussed.

Colter Stewart (Senior Thesis, April 2019, Advisor: John Colton )

Abstract

Zinc oxide (ZnO) is a promising wide band gap semiconductor with applications in ultraviolet optoelectronics. Through a novel sputtering process, our group seeks to create arsenic- doped p-type ZnO. In order to characterize these samples, we must understand the thin evaporated zinc arsenide (Zn3As2) layer in between the sputtered ZnO and our substrates. We characterize these samples by variable-angle spectroscopic ellipsometry (VASE), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Enclosed is a paper our research group has submitted to Optics Express that reports the results of these characterizations; these results show that a five-parameter ellipsometric model is sufficient to find the optical constants for amorphous Zn3As2 samples. Further work needs to be done in order to properly characterize crystalline Zn3As2 samples via ellipsometry.

2018

Scott Crossen (Senior Thesis, April 2018, Advisor: John Colton )

Abstract

Electron spin resonance (ESR) is an important tool in understanding the quantum-mechanical properties of condensed matter. Its applications range from studying lattice defects in solids to studying spin coherence in qubit candidate materials used for quantum computing. When coupled with a photoluminescence measuring component, it is possible to optically record ESR information contained in the resulting induced light. This unique form of ESR is called optically detected magnetic resonance (ODMR). In this thesis we compare experimental ODMR data with ESR predictions generated from a computational modeling system. To investigate the differences between these two methods we will study one spin-system in particular: irradiated 4H silicon carbide. This specimen will serve as the primary means to connect the two very different forms of computational and practical ESR spectroscopy commonly used today. Methods and theory for both methods will be described and resulting spectra will be presented for comparison. Though there will always be some differences, results show that computational ESR predictions match experimental results to the same extent that the underlying Hamiltonian for that particular system is understood.

Heather Hogg (Senior Thesis, April 2018, Advisor: John Colton )

Abstract

Determining the relationship between temperature and photoluminescence lifetime is central to creating temperature probes for microfluidic devices and laser surgery. Rhodamine B, a highly photoluminescent organic dye, is a particularly good candidate for temperature probes. This thesis discusses the use of time-correlated single photon counting to determine photoluminescence lifetimes of rhodamine B at temperatures from 16 K to 296.5 K. The instrument response function is separated from the true photoluminescence lifetime data with deconvolution data analysis techniques. The relationship between temperature and photoluminescence lifetime for rhodamine B is shown to be most accurately represented by a sigmoidal function, with very little variation at low temperature ranges. It is concluded that the behavior of the lifetime follows theoretical quenching regions over different temperature ranges.

J. Ryan Peterson (Senior Thesis, April 2018, Advisor: John Colton )

Abstract

Zinc oxide is a promising wide band gap semiconductor with applications in high-temperature, radiation-hard devices and ultraviolet optoelectronics. The p-type material, however, has historically been difficult to produce. In this work, p-type zinc oxide films are grown by rf magnetron sputtering on c-sapphire substrates. Arsenic doping is provided by a Zn3As2 intermediate layer. Electrical characterization shows that while the conductivity correlates strongly with substrate temperature while sputtering, carrier type is inconsistent for samples grown in similar conditions. Photoluminescence measurements reveal poor optical performance related to deep defects. These defects may explain the n-type conductivity and comparison with previous work suggests future improvements to the growth process.

2017

Kameron Hansen (Senior Thesis, April 2017, Advisor: John Colton )

Abstract

This thesis presents a new synthesis method for PbSe and MoS$_2$ semiconductor nanoparticles inside the protein ferritin. It also presents a simplified synthesis method for PbS, based on the work of Hennequin, et al. To our knowledge, PbSe and MoS$_2$ nanoparticles have not been synthesised inside ferritin before. All three materials could potentially have several applications outside photovoltaics, e.g. as biological markers and cancer therapies; however, this thesis focuses on their application to photovoltaics. PbS-ferritin is used to replace dye molecules in a dye-sensitized solar cell and the device shows an efficiency of $0.28\%$.

2016

Jacob Embley (Senior Thesis, April 2016, Advisor: John Colton )

Abstract

Electron spin states in silicon carbide have shown potential for use as qubits. A qubit requires a quantum state that will remain coherent over a sufficiently long period of time. By measuring spin coherence times for electrons in silicon vacancies, we not only investigate their potential for use as qubits, but we better understand the factors which lead to their eventual decoherence. Using a combination of experimental techniques, including optically detected magnetic resonance and spin echo, we measured electron spin coherence times for two samples of proton-irradiated 4H-silicon carbide. Each sample was studied over a range of temperatures. Results indicate that the longest coherence times for each sample exist at the lowest temperature (8 K). While in general higher temperatures resulted in shorter coherence times, results also show a range of temperature from 60 K to 160 K for which the trend was reversed.

Michael Meehan (Senior Thesis, June 2016, Advisor: John Colton )

Abstract

Titanium-sapphire lasers are useful in condensed matter research because of their ability to be mode-locked, generating ultrafast, regular pulses of coherent radiation. When designing Ti:sapph lasers, their stability in continous wave (CW) operation is often overlooked; however, this feature is often useful and would make a Ti:sapph laser more versatile. We discuss implementing an alternative laser cavity design that provides more stability in CW operation while retaining the ability to be mode-locked.

Cameron Olsen (Senior Thesis, April 2016, Advisor: John Colton )

Abstract

This thesis investigates the reactions of Mn2+ and Co2+ with permanganate as a route for manganese and cobalt oxide nanoparticle synthesis in the protein ferritin. Permanganate serves as the electron acceptor and reacts with Mn2+ and Co2+ in the presence of apoferritin to form manganese and cobalt oxide cores inside the protein shell. Manganese loading into ferritin was studied under acidic, neutral, and basic conditions and the ratios of Mn2+ and permanganate were varied at each pH, while cobalt loading was studied at pH 8.5 only. The manganese and cobalt-containing ferritin samples were characterized by transmission electron microscopy, UV/Vis absorption, and by measuring the band gap energies for each sample. Manganese cores formed in both the acidic and basic conditions, while a mixed cobalt-manganese core formed at the desire pH. New manganese oxide cores formed in the acidic manganese trials and have absorption profiles and band gap energies that are different from the Mn(O)OH cores synthesized by the traditional method of using oxygen. These new manganese cores have indirect band-gap transitions ranging from 1.63 to 1.68 eV, which differ from the band gap energy of 1.53 eV for Mn(O)OH ferritin. In addition, an increased absorption around 370 nm was observed for the new manganese cores, suggestive of MnO2 formation inside ferritin. The mixed cobalt-manganese samples showed band gaps ranging from 1.48 eV up to 1.75 eV, which correlated with the final ratio of cobalt and manganese present in the material.

2015

Stephen Erickson (Senior Thesis, April 2015, Advisor: John Colton )

Abstract

Nanostructured solar cells seek to surpass the present standards of efficiency and affordability in solar energy. The protein ferritin—a 12 nm diameter hollow sphere—serves as a unique template for synthesizing nanoscale solar energy materials. It allows for controlled and uniform nanocrystal synthesis, protection against photo-corrosion, and the ability to be manipulated into ordered arrays. In this thesis, I present a method of tuning the band gap of these encapsulated nanocrystals over a range of 1.60-2.38 eV by controlling their size and chemical composition during synthesis. Band gaps are measured using optical absorption spectroscopy to test the effects of these tunable parameters. Using just these materials, calculations indicate that the maximum solar energy conversion efficiencies under average sunlight could reach a high value of 38.0%. The addition of a material with a band gap similar to silicon (1.12 eV) would raise this maximum efficiency to 51.3%.

Kyle Miller (Senior Thesis, April 2015, Advisor: John Colton )

Abstract

Electrons located in silicon vacancies of 4H silicon carbide (SiC) are potential spintronic devices. In our experiments, electron spin states are polarized with 870 nm laser light, and we manipulate the spins with resonant microwaves at 10.47 GHz and a magnetic field of 350 mT. Spin polarizations are detected by the change in photoluminescence from the silicon vacancy defects, and lifetimes are calculated via measurements of optically detected spin resonance and electron spin echo. We have measured T2 lifetimes in 10^14 cm^−2 proton-irradiated SiC to be about 16 μs between 6 and 295 K, fairly independent of temperature. A sample with decreased defect density, proton-irradiated at 10^13 cm^−2, had a lifetime of about 64 μs. A 10^17 cm^−2 electron-irradiated sample had a lifetime longer than we could measure. These results show that we can increase lifetime by varying defect concentration and type.

2013

Daniel Craft (Senior Thesis, April 2013, Advisor: John Colton )

Abstract

Electron spins in InAs quantum dots have been studied using a pump-probe technique that normally yields the T1 spin lifetime, the time required for initially polarized electrons to relax and randomize. Using a circularly polarized laser tuned to the wavelength response of the quantum dots, the spins are "pumped" into alignment. After alignment, the spins are detected using a second, linearly polarized "probe" laser. The spin response over time is traced out by changing the delay between the two lasers. In contrast with other samples (bulk GaAs and a GaAs quantum well), where the spin response decays exponentially with time, initial data on the quantum dots has shown an unexpected, exponentially decaying sinusoid. This exponentially decaying sinusoid has a decay constant of 190 ns and oscillation frequency of 4.17 MHz, independent of both temperature and magnetic field.

Tyler Drue Park (Masters Thesis, July 2013, Advisor: John Colton )

Abstract

InGaAs quantum dot chains were grown with a low-temperature variation of the StranskiKrastanov method, the conventional epitaxial method. This new method seeks to reduce indium segregation and intermixing in addition to giving greater control in the growth process. We used photoluminescence spectroscopy techniques to characterize the quality and electronic structure of these samples. We have recently used a transmission electron microscope to show how the quantum dots vary with annealing temperature. Some questions relating to the morphology of the samples cannot be answered by photoluminescence spectroscopy alone. Using transmission electron microscopy, we verified flattening of the quantum dots with annealing temperature and resolved the chemical composition with cross-section cuts and plan view cuts.

Jane Tanner (Honors Thesis, April 2013, Advisor: John Colton )

Abstract

As the first step in creating a real-time system that can inexpensively and efficiently measure glass sheet in a production setting, we determined whether current technology, which implements ray tracing from specular reflections from the glass surface, is sufficient to characterize the flatness of the glass sheets. This measurement consists of an optical image of a uniform pattern of vertical stripes as reflected from the sheet of glass, and subsequent ray-trace modeling in order to determine the glass sheet shape that will produce the observed amount of pattern distortion. The system can detect changes as small as 25 um, and can measure amplitudes up to 5 mm accurately. Based on experiments performed, the system shows promise as on online measurement tool for industry.

2012

David Meyer (Senior Thesis, August 2012, Advisor: John Colton )

Abstract

We have measured T1 spin lifetimes of a 14 nm modulation-doped (100) GaAs quantum well using a time-resolved pump-probe Kerr rotation technique. T1 lifetimes in excess of 1 microsecond were measured at 1.5 K and 5.5 T. We observed effects from nuclear polarization, which could be removed by simultaneous nuclear magnetic resonance, along with two distinct lifetimes under some conditions that likely result from probing two distinct subsets of electrons. Finally, we found certain conditions that would produce different cw Kerr rotation responses depending upon the sweep direction of the probe laser wavelength.

2011

Dallas Smith (Senior Thesis, June 2011, Advisor: John Colton )

Abstract

We measured T1 spin lifetimes for electrons in gallium arsenide at various magnetic field strengths. To perform these measurements, we initialized and probed the spin states using optical techniques. By changing the delay between the initializing (pump) and probe laser pulses, we traced out the spin polarization decay curves. From this data we extracted the T1 spin lifetimes. This technique proved to be effective in measuring lifetimes in magnetic fields between 0 T and 7 T and at temperatures of 1.5 K and 5 K. Lifetimes in our sample were measured up to 800 ns.

Scott Thalman (Senior Thesis, August 2011, Advisor: John Colton )

Abstract

We used time-correlated single photon counting to measure the photoluminescence lifetime of solid InGaAs quantum dots (QDs) as well as colloidal CdSe QDs. The InGaAs QDs were grown using a modified Stranski-Krastanov growth method to encourage the formation of QD chains. These QDs had lifetimes between 0.6 ns and 1 ns indicating the possible formation of QD chains. We also investigated the interaction between ruthenium dye molecules and CdSe QDs in solution.

2010

Steven Allen (Honors Thesis, July 2010, Advisor: John Colton )

Abstract

Many diagnostic applications in sodium magnetic resonance imaging (MRI) require accurate flip angle mapping. In search for a mapping technique that performs well in sodium MRI, we evaluated the low signal-to-noise ratio performance of the dual-angle and phase-sensitive techniques. Monte Carlo simulations in MATLAB and measurements of a phantom demonstrate the phase-sensitive technique's superior performance in low SNR environments. The phase sensitive technique has a lower standard deviation of measurement and obtains higher quality flip angle maps than the dual-angle technique. Further, in vivo maps of the human breast demonstrate the phase-sensitive technique's clinical feasibility

Stephen Brown (Capstone, August 2010, Advisor: John Colton )

Abstract

This project describes computer software that helps perform spin dynamics experiments by automating data collection and instrument control in the laboratory. In this report, we review several types of spin dynamics experiments and outline their needs for computer automation. We rewrote an existing data collection application of about 15,000 lines of code using an object-oriented programming paradigm. We developed object class abstractions and interfaces that together form a unified framework for laboratory instrument automation. The new object-oriented implementation is intended to make the application more expandable, robust, and efficient. It enables new types of experiments to be performed that were not possible with the previous version of the software.

Aaron Jones (Senior Thesis, May 2010, Advisor: John Colton )

Abstract

Nanostructures such as quantum dots (QDs) and QD chains have received signifi cant attention because of their applications in quantum information technologies. This paper presents optical investigations of InGaAs QDs and dot chains. The InGaAs samples were grown on a (001) GaAs substrate by Dr. Haeyeon Yang using a novel method similar to the Stranski-Krastonov growth method. Intensity and temperature dependence of the sample photoluminescence (PL) spectra were obtained through optical excitation and detection. The presence of a PL peak at ~1 um suggests the successful creation of QD chains. Also outlined is the method for calculating the PL curve-correcting functions to account for grating effciencies and a changing reciprocal linear dispersion in monochromators.

2009

Benjamin Heaton (Senior Thesis, August 2009, Advisor: John Colton )

Abstract

A brief introduction into quantum computing as well as the background for our experiments will be presented. The work discussed here will show dynamic nuclear polarization which affects the magnetic resonance properties of lightly doped n-GaAs spin states in both bulk and quantum well samples. The nuclear polarization affects spin lifetimes and the ability to use magnetic resonance as a spin manipulation technique. This nuclear polarization can be blocked through double resonance techniques.

2008

Daniel Jenson (Senior Thesis, December 2008, Advisor: John Colton )

Abstract

Quantum computers will use quantum mechanical properties to perform certain tasks much faster than traditional computers. Realization of one quantum computing scheme requires a measurement of the T2 electron spin lifetime in GaAs. Electron and nuclear interactions prevent an accurate measurement of T2 by optically detected spin echo. Attempts to eliminate nuclear effects using NMR have been unsuccessful, probably due to insufficient magnetic field strength at one or more of the three resonant frequencies. Transmission line effects and mismatched impedances may have limited the current delivered to the NMR coil. Several impedance matching methods are considered, using computer models to account for transmission line effects and predict current delivered to the coil. Two methods are selected—both use three coils on the same circuit, each tuned to one of the resonant frequencies. Experiments show that neither method is a viable solution. It is recommended that further tests be performed with three coils on separate circuits, each tuned and driven around one of the resonant frequencies.