text:
You searched for +publisher:"University of Michigan" +contributor:("Clarke, Roy"). Showing records 1 – 30 of 85 total matches.
◁ [1] [2] [3] ▶
2017 – 2021 (13)
1. Jackson, Nefertiti Patrick. Characterization and evaluation of surface modified superparamagnetic iron oxide nanoparticles for uptake into human prostate carcinoma cells.
Degree: PhD, Applied Physics, 2011, University of Michigan
URL: http://hdl.handle.net/2027.42/84573
► Magnetic nanoparticles (MNPs) were synthesized from Fe2+ and Fe3+ by the co-precipitation method. Two nanoparticle surface coatings (gum arabic and sodium citrate) provided additional functionalization… (more)
▼ Magnetic nanoparticles (MNPs) were synthesized from Fe2+ and Fe3+ by the co-precipitation method. Two nanoparticle surface coatings (gum arabic and sodium citrate) provided additional functionalization and cell selectivity. The incorporation of surface modifiers to the synthesized nanoparticles generated three nanoparticle systems from which the research was based. The physical and chemical properties were determined with a variety of standard characterization techniques. Upon characterization of the nanoparticles, in vitro cell culture experiments were conducted. Nanoparticles were allowed to co-exist with prostate cells for 48 hours under sterile conditions. A range of iron oxide nanoparticle concentrations (0.5 x 1016 – 50 x 1016 particles/mL) were studied and compared for their effects on cell viability, intercellular uptake and quantitative analysis of prostate cell selectivity. Nanoparticles have a natural tendency to agglomerate; therefore, the addition of gum arabic as a stabilizing agent provided increased electrostatic repulsion which freely dispersed the particles and greatly contributed to more stabilized particles in both water and cell nutrient media. Sodium citrate was an excellent capping agent for nanoparticles in water but not in serum rich nutrient media. The contents of the media contributed to the destabilization of the nanoparticle solution; hence, particle size measurements decreased over time for sodium citrate coated particles in media as well as non-coated nanoparticles in media, which formed larger particles (350 – 375 nm), then gradually fell out of solution. Initial measurements for sodium citrate MNPs were more than four times greater in media as compared to stabilized sodium citrate particles in water (160 nm). Non-coated nanoparticles demonstrated agglomeration in water and size measurements increased to 700nm over the 48 hour period. Despite the degree of particle stability, prostate cells intracellularly received nanoparticles from each of the three nanoparticle systems. Cellular encapsulation of iron nanoparticles by prostate cells was demonstrated with TEM. Iron filing and cell mobility in response to a magnet was captured with video. The gum arabic nanoparticle system exhibited the highest differential uptake (ratio 6.8) at a lower nanoparticle concentration (2.5 x 1016). There is possibility of greater specificity when the size of prostate cancer and normal cells are compared. Advisors/Committee Members: Orr, Bradford G. (committee member), Clarke, Roy (committee member), Harris, Gary L. (committee member), Kurdak, Cagliyan (committee member).
Subjects/Keywords: Superparamagnetic; Prostate Cancer; MNP; Cellular Encapsulation; Quantification of Nanoparticle Uptake; Iron Oxide Nanoparticles; Physics; Science
Jackson, N. P. (2011). Characterization and evaluation of surface modified superparamagnetic iron oxide nanoparticles for uptake into human prostate carcinoma cells. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/84573
Jackson, Nefertiti Patrick. "Characterization and evaluation of surface modified superparamagnetic iron oxide nanoparticles for uptake into human prostate carcinoma cells." 2011. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/84573.
Jackson, Nefertiti Patrick. "Characterization and evaluation of surface modified superparamagnetic iron oxide nanoparticles for uptake into human prostate carcinoma cells." 2011. Web. 20 Jan 2021.
Jackson NP. Characterization and evaluation of surface modified superparamagnetic iron oxide nanoparticles for uptake into human prostate carcinoma cells. [Internet] [Doctoral dissertation]. University of Michigan; 2011. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/84573.
Jackson NP. Characterization and evaluation of surface modified superparamagnetic iron oxide nanoparticles for uptake into human prostate carcinoma cells. [Doctoral Dissertation]. University of Michigan; 2011. Available from: http://hdl.handle.net/2027.42/84573
2. Walrath, Jenna Cherie. Probing the Band Structure and Local Electronic Properties of Low-dimensional Semiconductor Structures.
Degree: PhD, Physics, 2016, University of Michigan
URL: http://hdl.handle.net/2027.42/120828
► Low-dimensional semiconductor structures are important for a wide variety of applications, and recent advances in nanoscale fabrication are paving the way for increasingly precise nano-engineering… (more)
▼ Low-dimensional semiconductor structures are important for a wide variety of applications, and recent advances in nanoscale fabrication are paving the way for increasingly precise nano-engineering of a wide range of materials. It is therefore essential that the physics of materials at the nanoscale are thoroughly understood to unleash the full potential of nanotechnology, requiring the development of increasingly sophisticated instrumentation and modeling. Of particular interest is the relationship between the local density of states (LDOS) of low-dimensional structures and the band structure and local electronic properties. This dissertation presents the investigation of the band structure, LDOS, and local electronic properties of nanostructures ranging from zero-dimensional (0D) quantum dots (QDs) to two-dimensional (2D) thin films, synthesizing computational and experimental approaches including Poisson-Schrodinger band structure calculations, scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and scanning thermoelectric microscopy (SThEM). A method is presented for quantifying the local Seebeck coefficient (S) with SThEM, using a quasi-3D conversion matrix approach to directly convert temperature gradient-induced voltages S. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces. The local carrier concentration, n, is profiled across epitaxial InAs/GaAs QDs, where SThEM is used to profile the temperature gradient-induced voltage, which is converted to a profile of the local S and finally to an n profile. The S profile is converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. The combined computational-experimental approach suggests a reduced n in the QD center in comparison to that of the 2D alloy layer. The surface composition and band structure of ordered horizontal Sb2Te3 nanowires induced by femtosecond laser irradiation of a thin film are investigated, revealing a band gap modulation between buried Sb2Te3 nanowires and the surrounding insulating material. Finally, STM and STS are used to investigate the band structure of BiSbTe alloys at room temperature, revealing both the Fermi level and Dirac point located inside the bulk bandgap, indicating bulk-like insulating behavior with accessible surface states. Advisors/Committee Members: Goldman, Rachel S (committee member), Pipe, Kevin Patrick (committee member), Clarke, Roy (committee member), Uher, Ctirad (committee member), Qi, Liang (committee member).
Subjects/Keywords: Condensed matter physics; Nanoscience; III-V semiconductors; Topological Insulators; Thermoelectric materials; Quantum dots; Physics; Science
Walrath, J. C. (2016). Probing the Band Structure and Local Electronic Properties of Low-dimensional Semiconductor Structures. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/120828
Walrath, Jenna Cherie. "Probing the Band Structure and Local Electronic Properties of Low-dimensional Semiconductor Structures." 2016. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/120828.
Walrath, Jenna Cherie. "Probing the Band Structure and Local Electronic Properties of Low-dimensional Semiconductor Structures." 2016. Web. 20 Jan 2021.
Walrath JC. Probing the Band Structure and Local Electronic Properties of Low-dimensional Semiconductor Structures. [Internet] [Doctoral dissertation]. University of Michigan; 2016. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/120828.
Walrath JC. Probing the Band Structure and Local Electronic Properties of Low-dimensional Semiconductor Structures. [Doctoral Dissertation]. University of Michigan; 2016. Available from: http://hdl.handle.net/2027.42/120828
3. Jones, Christina. Compound Semiconductor Quantum Wells and Dots for Light-Emitting Diode Applications: Insights on Structural and Optoelectronic Properties.
► Compound semiconductor nanostructures have revolutionized solid-state devices through their unique electronic, optical, and strain properties. To engineer nanostructures and optimize devices, we must understand their… (more)
▼ Compound semiconductor nanostructures have revolutionized solid-state devices through their unique electronic, optical, and strain properties. To engineer nanostructures and optimize devices, we must understand their structural and optoelectronic properties. In this dissertation project, we have characterized the structural coherence of homoepitaxial quantum dot growth on bulk GaN substrates, and we have calculated the effects of alloy composition fluctuations on recombination rates in InGaN-based light-emitting diodes. Additionally, we have explored the effects of composition, width, and strain of GaAsSb/GaAs quantum wells on band alignment type. This work allows us to better develop nanostructure-based devices for efficient, solid-state lighting and optical communications. Advisors/Committee Members: Clarke, Roy (committee member), Kioupakis, Emmanouil (committee member), Goldman, Rachel S (committee member), Kurdak, Cagliyan (committee member).
Subjects/Keywords: nanostructures; light-emitting diodes; X-ray diffraction; k · p theory; Physics; Science
Jones, C. (2017). Compound Semiconductor Quantum Wells and Dots for Light-Emitting Diode Applications: Insights on Structural and Optoelectronic Properties . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/138680
Jones, Christina. "Compound Semiconductor Quantum Wells and Dots for Light-Emitting Diode Applications: Insights on Structural and Optoelectronic Properties." 2017. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/138680.
Jones, Christina. "Compound Semiconductor Quantum Wells and Dots for Light-Emitting Diode Applications: Insights on Structural and Optoelectronic Properties." 2017. Web. 20 Jan 2021.
Jones C. Compound Semiconductor Quantum Wells and Dots for Light-Emitting Diode Applications: Insights on Structural and Optoelectronic Properties. [Internet] [Doctoral dissertation]. University of Michigan; 2017. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/138680.
Jones C. Compound Semiconductor Quantum Wells and Dots for Light-Emitting Diode Applications: Insights on Structural and Optoelectronic Properties. [Doctoral Dissertation]. University of Michigan; 2017. Available from: http://hdl.handle.net/2027.42/138680
4. Del Gaudio, Davide. Morphological Design of Semiconductors During Molecular-Beam Epitaxy and Pulsed-Laser Deposition.
Degree: PhD, Materials Science and Engineering, 2020, University of Michigan
► Low-dimensional semiconductors exhibiting quantum confinement are promising for enhanced efficiency in energy-conversion devices, including photovoltaics, thermoelectrics, and light-emission devices. Ideal arrays of semiconductor nanostructures enable… (more)
▼ Low-dimensional semiconductors exhibiting quantum confinement are promising for enhanced efficiency in energy-conversion devices, including photovoltaics, thermoelectrics, and light-emission devices. Ideal arrays of semiconductor nanostructures enable the formation of well-defined minibands that enhance light absorption. Nanostructure arrays are often achieved using extrinsic patterning methods which introduce lattice defects. Processes for the spontaneous formation and ordering of both QDs and NWs have emerged, but these are insufficiently mature for full integration into device manufacturing. Indeed, for molecular beam epitaxy (MBE) of InAs QD arrays, the influence of local surface curvature on semiconductor QD positioning after the nucleation stage is not well understood. On the other hand, for pulsed-laser deposition (PLD) of In2O3:Sn, the relative roles of vapor-solid (VS) and vapor-liquid-solid (VLS) growth modes during the morphological transitions resulting in semiconductor NW formation remain controversial. In the first part of this thesis, we quantify the aligned positioning of MBE-grown InAs/(Al)GaAs QDs and the effect of local surface curvature on the evolution of the InAs layer. In the second part of this thesis, we investigate the role of plasma expansion on the VS-VLS growth mode transition during PLD of In2O3:Sn. The influence of surface curvature on the placement of InAs quantum dots (QDs) on (Al)GaAs surfaces is examined using an experimental-computational approach. We simulate QD deposition with a 2D phase field model, which describes the time evolution of the InAs layer driven by a chemical potential gradient. Both atomically flat and mounded surfaces, which contain elongated corrugations generated via a surface instability, are employed as templates for the subsequent deposition of InAs QDs. For flat surfaces, simulations result in QD densities comparable to experimental observations following MBE deposition. For mounded surfaces, the simulations reveal QDs preferentially positioned in regions of positive curvature (valleys), e.g., at along the side of surface mounds, consistent with the anisotropic QD placement observed experimentally. This substrate curvature-driven direction of QD placement offers a method for immediate in-plane QD crystals and is extendable to a wide range of groups III-V compounds systems. Complex oxides such as ITO are widely utilized as transparent conductors in a variety of functional devices. Using PLD, both high transparency and high conductivity In2O3:Sn has been achieved without annealing, using instead selected gas species and pressures. However, the relative roles of VS and VLS growth modes during morphological transitions remains controversial. Here, we report on PLD of In2O3:Sn in an inert-gas environment, identifying the role of plasma plume expansion in the selection of VLS vs. VS growth. For the lowest N2 pressure, indium-tin droplet formation, followed by self-catalyzed VLS growth, is observed. With increasing N2 pressure, a transition from VLS to VS growth… Advisors/Committee Members: Goldman, Rachel S (committee member), Heron, John Thomas (committee member), Clarke, Roy (committee member), Kioupakis, Emmanouil (committee member), Ziff, Robert M (committee member).
Subjects/Keywords: thin films deposition; molecular beam epitaxy; pulsed laser deposition; quantum dots; nanowires; energy conversion devices; Materials Science and Engineering; Engineering
Del Gaudio, D. (2020). Morphological Design of Semiconductors During Molecular-Beam Epitaxy and Pulsed-Laser Deposition . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/155153
Del Gaudio, Davide. "Morphological Design of Semiconductors During Molecular-Beam Epitaxy and Pulsed-Laser Deposition." 2020. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/155153.
Del Gaudio, Davide. "Morphological Design of Semiconductors During Molecular-Beam Epitaxy and Pulsed-Laser Deposition." 2020. Web. 20 Jan 2021.
Del Gaudio D. Morphological Design of Semiconductors During Molecular-Beam Epitaxy and Pulsed-Laser Deposition. [Internet] [Doctoral dissertation]. University of Michigan; 2020. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/155153.
Del Gaudio D. Morphological Design of Semiconductors During Molecular-Beam Epitaxy and Pulsed-Laser Deposition. [Doctoral Dissertation]. University of Michigan; 2020. Available from: http://hdl.handle.net/2027.42/155153
5. Oraiqat, Ibrahim Malek. Ultrafast Pulsed-Laser Applications for Semiconductor Thin Film Deposition and Graphite Photoexfoliation.
► This thesis focuses on the application of ultrafast lasers in nanomaterial synthesis. Two techniques are investigated: Ultrafast Pulsed Laser Deposition (UFPLD) of semiconductor nanoparticle thin… (more)
▼ This thesis focuses on the application of ultrafast lasers in nanomaterial synthesis. Two techniques are investigated: Ultrafast Pulsed Laser Deposition (UFPLD) of semiconductor nanoparticle thin films and ultrafast laser scanning for the photoexfoliation of graphite to synthesize graphene. The importance of the work is its demonstration that the process of making nanoparticles with ultrafast lasers is extremely versatile and can be applied to practically any material and substrate. Moreover, the process is scalable to large areas: by scanning the laser with appropriate optics it is possible to coat square meters of materials (e.g., battery electrodes) quickly and inexpensively with nanoparticles. With UFPLD we have shown there is a nanoparticle size dependence on the laser fluence and the optical emission spectrum of the plume can be used to determine a fluence that favors smaller nanoparticles, in the range of 10-20 nm diameter and 3-5 nm in height. We have also demonstrated there are two structural types of particles: amorphous and crystalline, as verified with XRD and Raman spectroscopy. When deposited as a coating, the nanoparticles can behave as a quasi-continuous thin film with very promising carrier mobilities, 5-52 cm2/Vs, substantially higher than for other spray-coated thin film technologies and orders of magnitude larger than those of colloidal quantum dot (QD) films. Scanning an ultrafast laser over the surface of graphite was shown to produce both filamentary structures and sheets which are semi-transparent to the secondary-electron beam in SEM. These sheets resemble layers of graphene produced by exfoliation. An ultrafast laser "printing" configuration was also identified by coating a thin, transparent substrate with graphite particles and irradiating the back of the film for a forward transfer of material onto a receiving substrate. A promising application of laser-irradiated graphene coatings was investigated, namely to improve the charge acceptance of lead-acid battery electrodes. We demonstrated improvements of 63 % in the cycle lifetime and 23 % in the electrode charging conductance. Advisors/Committee Members: Clarke, Roy (committee member), Kurdak, Cagliyan (committee member), Sih, Vanessa (committee member), Thornton, Katsuyo S (committee member).
Subjects/Keywords: Quantum Dot Thin Films; Ultrafast Laser Materials Processing; Graphene-based Energy Storage; Physics; Science
Oraiqat, I. M. (2016). Ultrafast Pulsed-Laser Applications for Semiconductor Thin Film Deposition and Graphite Photoexfoliation. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/120790
Oraiqat, Ibrahim Malek. "Ultrafast Pulsed-Laser Applications for Semiconductor Thin Film Deposition and Graphite Photoexfoliation." 2016. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/120790.
Oraiqat, Ibrahim Malek. "Ultrafast Pulsed-Laser Applications for Semiconductor Thin Film Deposition and Graphite Photoexfoliation." 2016. Web. 20 Jan 2021.
Oraiqat IM. Ultrafast Pulsed-Laser Applications for Semiconductor Thin Film Deposition and Graphite Photoexfoliation. [Internet] [Doctoral dissertation]. University of Michigan; 2016. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/120790.
Oraiqat IM. Ultrafast Pulsed-Laser Applications for Semiconductor Thin Film Deposition and Graphite Photoexfoliation. [Doctoral Dissertation]. University of Michigan; 2016. Available from: http://hdl.handle.net/2027.42/120790
6. Schumaker, William Alexander. Pump-Probe Experiments & Radiation Generation using Laser Wakefield Accelerators.
Degree: PhD, Nuclear Engineering & Radiological Sciences, 2014, University of Michigan
► This thesis describes pump-probe and radiation generation experiments using Laser Wakefield Acceleration (LWFA) as the driving source. LWFA systems generate highly relativistic electron beams in… (more)
▼ This thesis describes pump-probe and radiation generation experiments using Laser Wakefield Acceleration (LWFA) as the driving source. LWFA systems generate highly relativistic electron beams in a compact geometry by driving a nonlinear plasma wave with an ultraintense laser pulse. These electrons beams, or the secondary radiation that they create, can be used to pump or probe interactions on fs-timescales due to inherent synchronization with the laser driver. In this thesis, the first sub-ps measurements of magnetic field dynamics in ultra-intense laser-solid interactions are presented. This experiment employed the LWFA electron beam to probe the laser-irradiated target at different time delays, and by measuring the subsequent beam deflections, the evolution of the magnetic field could be inferred. The effect of laser temporal contrast on the laser-target interaction was observed to play a crucial role in the magnetic field dynamics. High-contrast laser conditions were observed to rapidly evolve over the course of ps-timescale as electrons propagated radially along both the front and rear of the target, establishing an azimuthal field that was stronger on the front surface. On the other hand, low-contrast laser conditions allowed ablated plasma on the front surface of the target to limit magnetic field growth to only the rear of the target. Using high-energy LWFA electron beams, bremsstrahlung radiation was created by interaction with various solid targets. Secondary processes generate high-energy electrons, positrons, neutrons, and pions, which can be measured using magnetic spectrometers, nuclear activation, bubble detectors, and Compton scattering. Presented in this thesis are proof-of-principle results from a high-resolution, high-energy gamma-ray spectrometer capable of single-shot operation as well as high repetition rate activation diagnostics. The first measurements of laser-generated neutral electron-positron plasma beams are also presented. One promising application of LWFA is the compact implementation of nonlinear Thomson scattering (NLTS). Using high-energy LWFA electrons as a pump, a second, counter-propagating, ultraintense laser pulse can Thomson backscatter off these electrons and get upshifted to sim MeV energies, yielding a high-brightness source of high-energy photons. Two experimental attempts of NLTS are presented here. Advisors/Committee Members: Krushelnick, Karl M. (committee member), Thomas, Alexander George Roy (committee member), Clarke, Roy (committee member), Maksimchuk, Anatoly M. (committee member).
Subjects/Keywords: Laser Wakefield Acceleration; Bremsstrahlung; Radiation Generation; Thomson Scattering; Pump-probe; Electron Radiography; Electrical Engineering; Nuclear Engineering and Radiological Sciences; Engineering
Schumaker, W. A. (2014). Pump-Probe Experiments & Radiation Generation using Laser Wakefield Accelerators. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/107288
Schumaker, William Alexander. "Pump-Probe Experiments & Radiation Generation using Laser Wakefield Accelerators." 2014. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/107288.
Schumaker, William Alexander. "Pump-Probe Experiments & Radiation Generation using Laser Wakefield Accelerators." 2014. Web. 20 Jan 2021.
Schumaker WA. Pump-Probe Experiments & Radiation Generation using Laser Wakefield Accelerators. [Internet] [Doctoral dissertation]. University of Michigan; 2014. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/107288.
Schumaker WA. Pump-Probe Experiments & Radiation Generation using Laser Wakefield Accelerators. [Doctoral Dissertation]. University of Michigan; 2014. Available from: http://hdl.handle.net/2027.42/107288
7. Li, Yuwei. Time-Resolved Studies and Nanostructure Formation in Sb2Te3 Films Using Femtosecond Lasers.
► Antimony telluride (Sb2Te3) is an important material with a wide range of applications in thermoelectrics, data storage devices and topological insulator research. Our work on… (more)
▼ Antimony telluride (Sb2Te3) is an important material with a wide range of applications in thermoelectrics, data storage devices and topological insulator research. Our work on femtosecond laser studies of Sb2Te3 films has significance for insights into femtosecond laser interaction with Sb2Te3 above the damage threshold, as well as providing a new pathway for novel fabrication of highly-ordered nanostructured Sb2Te3. These new developments are made possible by careful control of the laser scanning conditions, opening the way to future nanoscale studies and materials applications. The pump-probe scheme for the time-resolved studies employed a novel asynchronous optical sampling (ASOPS) technique, which has distinctive advantages over the traditional mechanical-delay scheme including superior stability of beam alignment during scans, faster data acquisition rates, and the ability to monitor a much wider range of dynamics up to ten nanoseconds. With ASOPS, it is shown that a sequence of connected processes can be studied in Sb2Te3 films, from coherent optical phonons and acoustic echoes at picosecond timescale, through thermal transport at nanosecond timescale. In particular, the coherent phonons were used, for the first time, to monitor the element segregation in Sb2Te3 films under high-fluence pump laser irradiation conditions. These results are important for the ultrafast spectroscopy research community: they highlight the need for careful interpretation of coherent phonon spectra in tellurides, which are susceptible to fs laser damage. Femtosecond laser irradiation of Sb2Te3 above ~6 mJ/cm2 was also found to produce highly-ordered nanotracks with a periodicity an order of magnitude below the laser wavelength. A variety of characterization techniques identified these nanotracks as single crystalline Sb2Te3 nanowires separated by polycrystalline phases including a large amount of the insulating Sb2O3. Laser irradiation in different gas environments revealed the sensitivity of the Sb2Te3 surface morphology to the surrounding gas species, especially O2, highlighting the critical role of the ambient environment interactions with Sb2Te3 for nanostructure formation in the thin films. These results provide valuable experimental input for the future analysis of the generation mechanism of these nanostructures. Additionally, the results open up new opportunities for fabrication of in-plane Sb2Te3 nanowires for planar applications. Advisors/Committee Members: Clarke, Roy (committee member), Pipe, Kevin Patrick (committee member), Kurdak, Cagliyan (committee member), Goldman, Rachel S. (committee member), Uher, Ctirad (committee member).
Subjects/Keywords: Femtosecond Lasers; Antimony Telluride; Nanostructures; Pump-probe; Physics; Science
10 more images…
Li, Y. (2014). Time-Resolved Studies and Nanostructure Formation in Sb2Te3 Films Using Femtosecond Lasers. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/108790
Li, Yuwei. "Time-Resolved Studies and Nanostructure Formation in Sb2Te3 Films Using Femtosecond Lasers." 2014. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/108790.
Li, Yuwei. "Time-Resolved Studies and Nanostructure Formation in Sb2Te3 Films Using Femtosecond Lasers." 2014. Web. 20 Jan 2021.
Li Y. Time-Resolved Studies and Nanostructure Formation in Sb2Te3 Films Using Femtosecond Lasers. [Internet] [Doctoral dissertation]. University of Michigan; 2014. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/108790.
Li Y. Time-Resolved Studies and Nanostructure Formation in Sb2Te3 Films Using Femtosecond Lasers. [Doctoral Dissertation]. University of Michigan; 2014. Available from: http://hdl.handle.net/2027.42/108790
8. Kinnunen, Paivo. Asynchronous Magnetic Bead Rotation (AMBR) for Biosensors.
► As antibiotic resistance of pathogenic bacteria is now a declared global threat, dubbed by the US Centers for Disease Control (CDC) as one of the… (more)
▼ As antibiotic resistance of pathogenic bacteria is now a declared global threat, dubbed by the US Centers for Disease Control (CDC) as one of the most pressing public health problems worldwide, faster, growth-based methods are needed to be able to treat infections more effectively. Here we present the latest developments in the Asynchronous Magnetic Bead Rotation (AMBR) biosensor toward this goal. Asynchronous rotation of a magnetic bead in a fluid occurs when a rotating magnetic field exceeds a critical driving frequency. The frequency of the asynchronously rotating magnetic bead is a linear function of its volume, as well as of the fluid's viscosity, and can therefore be used as a volumetric sensor. These sensors, called here Asynchronous Magnetic Bead Rotation (AMBR) sensors, were first used for bio-applications in 2007. This dissertation shows the development of various aspects of the AMBR biosensor: (1) the effect of the sensor's frequency on its sensitivity of detection is investigated, (2) an AMBR sensor is optimized for measuring the growth of individual bacterial (E. coli) cells, achieving an 80 nm sensitivity to the cell length, (3) an off-the-microscope method for the observation of AMBR sensor signals is demonstrated, and (4) self-assembled AMBR sensors are developed for potentially rapid and scalable antibiotic susceptibility testing of bacteria. AMBR offers a simple and robust method for translating nanoscale volumetric changes into easily measurable frequency changes. It is a platform technology applicable to a multitude of high resolution volumetric and viscosity measurements, one of the primary applications being in healthcare: bacterial growth can be measured even on the single cell level. Due to AMBR biosensors high sensitivity, the bacterial resistance to antimicrobials can be rapidly determined. The resistance was determined within one hour for a clinical E. coli isolate, potentially leading to faster information on what would be the appropriate therapy for the specific case/patient. Advisors/Committee Members: Kopelman, Raoul (committee member), Clarke, Roy (committee member), Doering, Charles R. (committee member), Hunt, Alan J. (committee member), Orr, Bradford G. (committee member).
Subjects/Keywords: Magnetic Bead; Biosensor; Asynchronous Rotation; Asynchronous Magnetic Bead Rotation; Biomedical Engineering; Physics; Engineering; Science
Kinnunen, P. (2011). Asynchronous Magnetic Bead Rotation (AMBR) for Biosensors. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/86488
Kinnunen, Paivo. "Asynchronous Magnetic Bead Rotation (AMBR) for Biosensors." 2011. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/86488.
Kinnunen, Paivo. "Asynchronous Magnetic Bead Rotation (AMBR) for Biosensors." 2011. Web. 20 Jan 2021.
Kinnunen P. Asynchronous Magnetic Bead Rotation (AMBR) for Biosensors. [Internet] [Doctoral dissertation]. University of Michigan; 2011. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/86488.
Kinnunen P. Asynchronous Magnetic Bead Rotation (AMBR) for Biosensors. [Doctoral Dissertation]. University of Michigan; 2011. Available from: http://hdl.handle.net/2027.42/86488
9. Harton, Renee Michelle. Fabrication and Control of Magnetic Pd,Fe Thin Film Heterostructures.
► The goal of this dissertation research is to investigate the use of multiferroic (ferroelectric- ferromagnetic) thin-film interfaces to control the magnetization in materials of interest… (more)
▼ The goal of this dissertation research is to investigate the use of multiferroic (ferroelectric- ferromagnetic) thin-film interfaces to control the magnetization in materials of interest for perpendicular recording. We explore the relationship between magnetization and structure in thin films of Fe/Pd compounds deposited onto ferroelectric BaTiO3 substrates. Utilizing magnetostrictive coupling, the magnetism of a magnetic film was controlled by the epitaxial strain at the film/substrate interface. BaTiO3 is particularly favorable as it develops a significant elongation along the tetragonal c-axis as a result of symmetry breaking at the cubic to tetragonal phase transition. A novel aspect of the work is to tune the Curie point of the ferromagnetic transition to match the ferroelectric Curie point of the substrate, so that the magnetostrictive effect is maximized. This is achieved by alloying Fe with Pd to produce a Pd3Fe compound. We report for the first time, the elastic control of the perpendicular magnetic anisotropy of Palladium-Iron (Pd,Fe) films deposited onto a barium-titanate (BaTiO3) (100) substrate. Using Magneto-optic Kerr Effect magnetometry, we observed the behavior of the magnetization through the tetragonal-to-cubic phase transition of the BaTiO3 substrate. We found that such films exhibited in-plane magnetization reversal below the T-C transition temperature, and out-of-plane magnetization reversal above the transition. This change in behavior demonstrates the elastic control of the perpendicular magnetic anisotropy of the deposited Pd,Fe film. In addition, we grew an ordered FePd3 film on SrTiO3 using the inter-diffusion of an Fe/Pd multilayer heterostructure. Each layer was deposited using Ultra-High Vacuum deposition. Utilizing in-situ RHEED (Reflection High Energy Electron Diffraction), we observed that each deposited layer was both ordered and exhibited the crystalline structure of the bulk material. Once deposited, the multi-layer heterostructure was heated above the FePd3 formation temperature. Upon heating, the RHEED pattern began to exhibit the crystalline structure of FePd3. Using X-ray Diffraction analysis and MOKE magnetometry, we found that the epitaxial film consisted primarily of FePd3. This example of atomic layer epitaxy of Fe, Pd alloys represents a successful approach to forming high quality magnetic heterostructures on perovskites with excellent control over their composition and structural ordering. Advisors/Committee Members: Clarke, Roy (committee member), Kioupakis, Emmanouil (committee member), Neal, Homer A (committee member), Winful, Herbert Graves (committee member), Goldman, Rachel S (committee member).
Subjects/Keywords: elastic control; magnetism; multiferroic; Physics; Science
Harton, R. M. (2016). Fabrication and Control of Magnetic Pd,Fe Thin Film Heterostructures. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/120869
Harton, Renee Michelle. "Fabrication and Control of Magnetic Pd,Fe Thin Film Heterostructures." 2016. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/120869.
Harton, Renee Michelle. "Fabrication and Control of Magnetic Pd,Fe Thin Film Heterostructures." 2016. Web. 20 Jan 2021.
Harton RM. Fabrication and Control of Magnetic Pd,Fe Thin Film Heterostructures. [Internet] [Doctoral dissertation]. University of Michigan; 2016. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/120869.
Harton RM. Fabrication and Control of Magnetic Pd,Fe Thin Film Heterostructures. [Doctoral Dissertation]. University of Michigan; 2016. Available from: http://hdl.handle.net/2027.42/120869
10. Mathis, James. Optical and Electrical Characterization of ZnSnN2 Thin-Films.
► This dissertation focuses on a new thin-film semiconductor material for optoelectronic applications such as solar energy harvesting and solid state lighting. The work represents a… (more)
▼ This dissertation focuses on a new thin-film semiconductor material for optoelectronic applications such as solar energy harvesting and solid state lighting. The work represents a detailed exploration of the optical and electrical properties of epitaxial thin films of ZnSnN2 (ZSN), a member of the II-IV nitride family of compounds in which cation ordering was predicted as a novel route to tuning the relevant optoelectronic characteristics. It also represents the first attempt at quantum confinement in a ZSN isocompositional heterostructure using cation order/disorder as a control parameter. The increasing demand for alternatives to fossil fuels and the scarcity of certain metals commonly used in LED lighting, together highlight the need for earth-abundant optoelectronic materials. This has led to the development of the II-IV-V family of semiconductors – a wide variety of materials that enable access to the VIS/IR portions of the electromagnetic spectrum. As a member of this family, ZSN is composed entirely of earth-abundant elements and early theoretical investigations of it predict compatibility with solar cell applications, with band gaps ranging between 1.0 eV and 2.5 eV, depending on the degree of cation ordering. The methodology for synthesizing ZSN with specific properties was developed in collaboration with a group at Western Michigan University, resulting in the first fully ordered, orthorhombic ZSN films. We present original results on the optical and electrical characterization of ZSN thin-films grown on (111) yttria-stabilized zirconia (YSZ) via plasma-assisted molecular beam epitaxy (MBE). A key finding is that the band-gap of the material is correlated with cation ordering across a large portion of the infrared and visible spectrum. The degree of ordering was quantified by an order parameter, defined as S = ralpha + rbeta - 1, where ralpha and rbeta represent the fractions of Zn and Sn cation sites occupied by Zn and Sn atoms, respectively. Values of S from 0.05 to as high as 0.92 were achieved in this work, as measured using in-situ Reflection High-Energy Electron Diffraction (RHEED) and ex-situ X-Ray Diffraction (XRD). Optical characterization included Photoluminescence (PL), Cathodoluminescence (CL) and Diffuse Reflectance Spectroscopy (DRS). The CL results are the first evidence of light emission from single-crystal ZSN thin-films. Another innovative aspect of our work was to modify conventional DRS to enhance optical absorption contrast from our thin-film samples, namely ``waveguided diffuse reflectance spectroscopy'' (WDRS), providing vital data on how the band-gap depends on the degree of cation ordering. Electrical measurements were carried out to explore fundamental properties relevant to device fabrication – specifically mobility and carrier concentration. Hall measurements reveal high carrier concentrations ranging from 3.37x1020 cm-3 to 2.08x1021 cm-3, while measured mobilities ranged from 1.27 cm2/Vs to 32.5 cm2/Vs. These results suggest that the density of carrier-generating… Advisors/Committee Members: Clarke, Roy (committee member), Durbin, Steven M (committee member), Kurdak, Cagliyan (committee member), Li, Lu (committee member), Sih, Vanessa (committee member).
Subjects/Keywords: Thin-film studies; Optical characterization; Photovoltaics; Physics; Science
Mathis, J. (2020). Optical and Electrical Characterization of ZnSnN2 Thin-Films . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/155209
Mathis, James. "Optical and Electrical Characterization of ZnSnN2 Thin-Films." 2020. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/155209.
Mathis, James. "Optical and Electrical Characterization of ZnSnN2 Thin-Films." 2020. Web. 20 Jan 2021.
Mathis J. Optical and Electrical Characterization of ZnSnN2 Thin-Films. [Internet] [Doctoral dissertation]. University of Michigan; 2020. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/155209.
Mathis J. Optical and Electrical Characterization of ZnSnN2 Thin-Films. [Doctoral Dissertation]. University of Michigan; 2020. Available from: http://hdl.handle.net/2027.42/155209
11. Senabulya, Nancy. Structural Characterization Studies on Semiconducting ZnSnN2 Films using Synchrotron X-ray Diffraction.
► This work is motivated by the need for new visible frequency direct bandgap semiconductor materials that are earth abundant and low-cost to meet the increasing… (more)
▼ This work is motivated by the need for new visible frequency direct bandgap semiconductor materials that are earth abundant and low-cost to meet the increasing demand for optoelectronic device applications such as solid state lighting and photovoltaics. Zinc-Tin-Nitride (ZnSnN2), a member of the II-IV nitride semiconductor family has been proposed as an alternative to the more common III-nitride semiconductors for use in optoelectronic devices. This material has been synthesized under optimized conditions using plasma assisted molecular beam epitaxy. Though a lot of research has recently been done computationally to predict the electronic and structural properties of ZnSnN2, experimental verification of these theories in single crystal thin films is lacking and warrants investigation because the accurate determination of the crystal structure of ZnSnN2 is a fundamental prerequisite for controlling and optimizing optoelectronic properties. In this synchrotron x-ray diffraction study, we present experimental validation, through unit cell refinement and 3d reciprocal space maps, of the crystal structure of single domain ZnSnN2 films deposited on (111) Yttria stabilized zirconia (YSZ) and (001) Lithium gallate (LGO) substrates. We find that ZnSnN2 films grown on (111) YSZ can attain both the theoretically predicted disordered wurtzite and ordered orthorhombic Pna21 structures under carefully controlled MBE growth conditions, while films grown on (001) LGO have the ordered Pn21a orthorhombic crystal structure. Through a systematic annealing study, a temperature induced first order structural phase transition from the wurtzite to orthorhombic phase is realized, characterized by the appearance of superstructure reflections in Advisors/Committee Members: Clarke, Roy (committee member), Durbin, Steven M. (committee member), Kioupakis, Emmanouil (committee member), Kurdak, Cagliyan (committee member).
Subjects/Keywords: ZnSnN2; Synchrotron x-ray diffraction; Single crystal; Disorder-Order phase transition; Crystal structure; Reciprocal space maps; Materials Science and Engineering; Physics; Engineering; Science
Senabulya, N. (2017). Structural Characterization Studies on Semiconducting ZnSnN2 Films using Synchrotron X-ray Diffraction . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/138487
Senabulya, Nancy. "Structural Characterization Studies on Semiconducting ZnSnN2 Films using Synchrotron X-ray Diffraction." 2017. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/138487.
Senabulya, Nancy. "Structural Characterization Studies on Semiconducting ZnSnN2 Films using Synchrotron X-ray Diffraction." 2017. Web. 20 Jan 2021.
Senabulya N. Structural Characterization Studies on Semiconducting ZnSnN2 Films using Synchrotron X-ray Diffraction. [Internet] [Doctoral dissertation]. University of Michigan; 2017. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/138487.
Senabulya N. Structural Characterization Studies on Semiconducting ZnSnN2 Films using Synchrotron X-ray Diffraction. [Doctoral Dissertation]. University of Michigan; 2017. Available from: http://hdl.handle.net/2027.42/138487
12. Bayerl, Dylan J. First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications.
► Modern semiconductor technology and nanoengineering techniques enable rapid development of new materials for energy applications such as photovoltaics, solid- state lighting, and thermoelectric devices. Yet… (more)
▼ Modern semiconductor technology and nanoengineering techniques enable rapid development of new materials for energy applications such as photovoltaics, solid- state lighting, and thermoelectric devices. Yet as materials engineering capabilities become increasingly refined, the space of controllable properties becomes increasingly large and complex. Selecting the most promising materials and parameters to focus on represents a significant challenge. We approach this challenge by applying state-of-the-art predictive first-principles calculation methods to guide research and development of materials for energy applications. This work describes our first-principles investigations of nanostructured group-III-nitrides for solid-state lighting applications and bulk titanium dioxides for thermoelectric applications. We demonstrate several remarkable properties of nanostructured group-III-nitrides. In InN nanowires with diameters on the order of 1 nm, we predict that quantum confinement shifts optical emission into the visible range at 2.3 to 2.5 eV (green to cyan) and results in a large exciton binding energy of 1.4 eV. These findings offer a new approach to addressing the "green-gap" problem of low efficiency in solid-state lighting devices emitting in this part of the spectrum. In ultra-thin GaN-AlN quantum wells, we show how to adjust the well and barrier thicknesses for tuning the optical gap in the deep ultraviolet range between 3.85 and 5.23 eV. Furthermore, we predict that quantum confinement in ultra-thin GaN wells results in large exciton binding energies between 80 and 210 meV and enhances radiative recombination by reducing the exciton lifetime to as short as approximately 1 ns at room temperature. These findings highlight the capability of quantum-confined group-III-nitrides to improve the efficiency and utility of visible and ultraviolet solid-state light emitters. Additionally, we calculate the n-type thermoelectric transport properties of the naturally occurring rutile, anatase, and brookite polymorphs of TiO2 and predict optimal temperatures and free-carrier concentrations for thermoelectric energy conversion. We also predict a theoretical limit on the figure of merit ZT of 0.93 in the rutile polymorph, demonstrating that TiO2 can potentially achieve thermoelectric energy conversion efficiency comparable to that of commercialized thermoelectrics. Advisors/Committee Members: Kioupakis, Emmanouil (committee member), Clarke, Roy (committee member), Goldman, Rachel S (committee member), Phillips, Jamie Dean (committee member).
Subjects/Keywords: optoelectronics; thermoelectrics; first-principles; III-nitride; titanium oxide; Materials Science and Engineering; Engineering
Bayerl, D. J. (2015). First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/116701
Bayerl, Dylan J. "First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications." 2015. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/116701.
Bayerl, Dylan J. "First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications." 2015. Web. 20 Jan 2021.
Bayerl DJ. First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications. [Internet] [Doctoral dissertation]. University of Michigan; 2015. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/116701.
Bayerl DJ. First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications. [Doctoral Dissertation]. University of Michigan; 2015. Available from: http://hdl.handle.net/2027.42/116701
13. Katcher, Adam. Optical Studies of Indium Gallium Nitride Nanostructures.
► Indium gallium nitride (InGaN) is a semiconductor material that is in widespread use in blue light emitting diodes (LEDs) and blue laser diodes and is… (more)
▼ Indium gallium nitride (InGaN) is a semiconductor material that is in widespread use in blue light emitting diodes (LEDs) and blue laser diodes and is being used in solid-state lighting, displays, and scientific applications. The scientific understanding of the physical mechanisms responsible for the performance of these devices is still developing; this includes the description of localization of carriers in this material – a fundamental issue which is believed to be responsible for the origin of luminesce in blue LEDs – as well as that of performance limitations of existing devices, including the so-called "green gap" and "efficiency droop," which are related in part to the exciton-phonon interaction. This thesis studies top-down etched InGaN quantum disks (QDs) embedded in GaN nanopillars, focusing on the effect of localization and of the exciton-phonon interaction. The exciton-phonon interaction is studied with two experiments which examine the effect of quantum dot size and shape on the strength of the optical phonon replica in the PL. First, we study the effect of asymmetrical strain on the exciton-phonon coupling by examining the optical phonon replica in the PL of nineteen individual elliptical QDs with dimensions of 22nm x 36nm. We show that the effect of strain on the phonon coupling strength should be observable by a reduction in the degree of polarization (DOP) of the optical phonon replica. Measurements confirm that there is a reduction in the DOP of the optical phonon replica, with reasonable agreement with theory for the high DOP dots. Lower DOP dots, which arise to due irregularities in the shape and size of the fabricated nanopillars, also show a reduction in DOP of the phonon replica but are more sensitive to the effect of asymmetrical phonon coupling and warrant further study. Second, we examine the effect of nanopillar diameter on exciton-phonon coupling strength in InGaN quantum disks. We observe an enhancement of the phonon replica as the nanopillar diameter is reduced from 1000nm to 60nm. This effect is explained by a reduction in the lateral Bohr radius of the exciton which accompanies the decrease in vertical electron-hole separation in smaller nanopillar diameters. To quantify this effect, a simple model is used to infer that, based on the measured phonon coupling strengths, the Bohr radius reduces from approximately 2.5nm to 2nm as the diameter is reduced over the observed range. In order to study the effect of localization, we measure the Stokes shift, which is the energy difference between emission and absorption. By measuring this quantity as a function of nanopillar diameter, we demonstrate the ability to separately determine the contributions of the strain-induced quantum confined Stark effect and of localization to the observed Stokes shift. In our case, we find that the two effects have approximately equal contributions for the range of nanopillar diameters studied here. Furthermore, the site control of InGaN/GaN quantum disks using this top-down fabrication method assists… Advisors/Committee Members: Deng, Hui (committee member), Ku, Pei-Cheng (committee member), Kioupakis, Emmanouil (committee member), Clarke, Roy (committee member), Sih, Vanessa (committee member).
Subjects/Keywords: InGaN; nanostructures; exciton-phonon interaction; LED; Physics; Science
Katcher, A. (2018). Optical Studies of Indium Gallium Nitride Nanostructures . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/147624
Katcher, Adam. "Optical Studies of Indium Gallium Nitride Nanostructures." 2018. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/147624.
Katcher, Adam. "Optical Studies of Indium Gallium Nitride Nanostructures." 2018. Web. 20 Jan 2021.
Katcher A. Optical Studies of Indium Gallium Nitride Nanostructures. [Internet] [Doctoral dissertation]. University of Michigan; 2018. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/147624.
Katcher A. Optical Studies of Indium Gallium Nitride Nanostructures. [Doctoral Dissertation]. University of Michigan; 2018. Available from: http://hdl.handle.net/2027.42/147624
14. Ledbetter, Abram. Investigations of Excited States and Radical Ions in ?-Conjugated Polymers of Interest for Photovoltaic Applications.
► Photovoltaics using polymer semiconductors as active materials require donor/acceptor (D/A) interfaces to convert excitons into freed charges. In the absence of chain defects, excited states… (more)
▼ Photovoltaics using polymer semiconductors as active materials require donor/acceptor (D/A) interfaces to convert excitons into freed charges. In the absence of chain defects, excited states and charges migrate most efficiently via intramolecular pathway. This thesis is concerned with understanding dynamics of intramolecular transport, charge transfer at intramolecular D/A interfaces, and the nature of charges and excited states which give rise to the observed phenomena. Three conjugated polymers are investigated: MEH-PPV (typical bandgap), pDPP2FT (low bandgap), and pCVPPV (high bandgap). Compounds investigated as electron acceptors relative to these polymers were oligomers fSF-PPV and SF-PPV for MEH-PPV, perylene diimide (PDI) for pDPP2FT, and MEH-PPV for pCVPPV. As a prototypical conjugated polymer, MEH-PPV is investigated most extensively – exploring the physical and electronic properties of its charged and excited states, energetics of those states, and the rates and nature of charge and exciton capture via radiolysis. Comparisons and contrasts are made with non-conventional pCVPPV and pDPP2FT. Radiolysis is employed as a powerful technique for obtaining optical signatures and reaction kinetics of radical ions and triplets for oligomers and polymers by ensuring a small fraction of chains in solution are injected with a single charge or excited state. This technique facilitates unambiguous assignment of specific species in contrast with other techniques for which multiple species are formed but difficult to resolve. Mistaken assignment of species have resulted from such ambiguities as well as conflicting theoretical models predicting the number of optical transitions expected for these species. Rates of formation for charged states are compared with theoretical diffusion controlled rates, and optical transitions of the optical signatures are compared with those predicted by the commonly referenced Fesser-Bishop-Campbell (FBC) model predicting two transitions for a radical ion (polaron) and one for a radical diion (bipolaron). Departures from this model included fSF-PPV and SF-PPV radical ions which exhibited 3 transitions (one quasi-forbidden) and pDPP2FT radical ions which exhibited 1 transition while pDPP2FT diions exhibited two. Using optical detection provided the known optical signatures, electron and exciton dynamics in acceptor-capped MEH-PPV and acceptor-capped pDPP2FT are investigated. Acceptor radical anion formation following electron or exciton capture by the chain is interpreted as rate limited by either electron transport along the chain or by electron transfer at the D/A interface. In PDI-capped pDPP2FT, a lower limit for electron mobility under zero-field conditions is estimated as 3.37 x 10-3 cm2/Vs, more than an order of magnitude larger than observed in polythiophene. This mobility study for a D-A copolymer under zero field conditions is the first of its kind. In a series of Donor-bridge-Acceptor (DBA) diblocks consisting of MEH-PPV and (f)SF-PPV with varying lengths of aliphatic… Advisors/Committee Members: Clarke, Roy (committee member), Miller, John R. (committee member), Guo, L Jay (committee member), Kurdak, Cagliyan (committee member), Maldonado, Stephen (committee member).
Subjects/Keywords: conjugated polymers; radiolysis; diblock copolymers; Chemistry; Physics; Science (General); Science
Ledbetter, A. (2017). Investigations of Excited States and Radical Ions in ?-Conjugated Polymers of Interest for Photovoltaic Applications . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/140906
Ledbetter, Abram. "Investigations of Excited States and Radical Ions in ?-Conjugated Polymers of Interest for Photovoltaic Applications." 2017. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/140906.
Ledbetter, Abram. "Investigations of Excited States and Radical Ions in ?-Conjugated Polymers of Interest for Photovoltaic Applications." 2017. Web. 20 Jan 2021.
Ledbetter A. Investigations of Excited States and Radical Ions in ?-Conjugated Polymers of Interest for Photovoltaic Applications. [Internet] [Doctoral dissertation]. University of Michigan; 2017. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/140906.
Ledbetter A. Investigations of Excited States and Radical Ions in ?-Conjugated Polymers of Interest for Photovoltaic Applications. [Doctoral Dissertation]. University of Michigan; 2017. Available from: http://hdl.handle.net/2027.42/140906
15. Cui, Sunan. Incorporating Deep Learning Techniques into Outcome Modeling in Non-Small Cell Lung Cancer Patients after Radiation Therapy.
► Radiation therapy (radiotherapy) together with surgery, chemotherapy, and immunotherapy are common modalities in cancer treatment. In radiotherapy, patients are given high doses of ionizing radiation… (more)
▼ Radiation therapy (radiotherapy) together with surgery, chemotherapy, and immunotherapy are common modalities in cancer treatment. In radiotherapy, patients are given high doses of ionizing radiation which is aimed at killing cancer cells and shrinking tumors. Conventional radiotherapy usually gives a standard prescription to all the patients, however, as patients are likely to have heterogeneous responses to the treatment due to multiple prognostic factors, personalization of radiotherapy treatment is desirable. Outcome models can serve as clinical decision-making support tools in the personalized treatment, helping evaluate patients' treatment options before the treatment or during fractionated treatment. It can further provide insights into designing of new clinical protocols. In the outcome modeling, two indices including tumor control probability (TCP) and normal tissue complication probability (NTCP) are usually investigated. Current outcome models, e.g., analytical models and data-driven models, either fail to take into account complex interactions between physical and biological variables or require complicated feature selection procedures. Therefore, in our studies, deep learning (DL) techniques are incorporated into outcome modeling for prediction of local control (LC), which is TCP in our case, and radiation pneumonitis (RP), which is NTCP in our case, in non-small-cell lung cancer (NSCLC) patients after radiotherapy. These techniques can improve the prediction performance of outcomes and simplify model development procedures. Additionally, longitudinal data association, actuarial prediction, and multi-endpoints prediction are considered in our models. These were carried out in 3 consecutive studies. In the first study, a composite architecture consisting of variational auto-encoder (VAE) and multi-layer perceptron (MLP) was investigated and applied to RP prediction. The architecture enabled the simultaneous dimensionality reduction and prediction. The novel VAE-MLP joint architecture with area under receiver operative characteristics (ROC) curve (AUC) [95% CIs] 0.781 [0.737-0.808] outperformed a strategy which involves separate VAEs and classifiers (AUC 0.624 [ 0.577-0.658]). In the second study, composite architectures consisted of 1D convolutional layer/ locally-connected layer and MLP that took into account longitudinal associations were applied to predict LC. Composite architectures convolutional neural network (CNN)-MLP that can model both longitudinal and non-longitudinal data yielded an AUC 0.832 [ 0.807-0.841]. While plain MLP only yielded an AUC 0.785 [CI: 0.752-0.792] in LC control prediction. In the third study, rather than binary classification, time-to-event information was also incorporated for actuarial prediction. DL architectures ADNN-DVH which consider dosimetric information, ADNN-com which further combined biological and imaging data, and ADNN-com-joint which realized multi-endpoints prediction were investigated. Analytical models were also conducted for comparison purposes. Among… Advisors/Committee Members: El Naqa, Issam (committee member), Clarke, Roy (committee member), Ten Haken, Randall K (committee member), Tewari, Ambuj (committee member).
Subjects/Keywords: Outcome modeling; Deep neural networks; Radiation therapy; Multi-omics; Machine learning; Personalization of treatment; Oncology and Hematology; Health Sciences
Cui, S. (2020). Incorporating Deep Learning Techniques into Outcome Modeling in Non-Small Cell Lung Cancer Patients after Radiation Therapy . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/162923
Cui, Sunan. "Incorporating Deep Learning Techniques into Outcome Modeling in Non-Small Cell Lung Cancer Patients after Radiation Therapy." 2020. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/162923.
Cui, Sunan. "Incorporating Deep Learning Techniques into Outcome Modeling in Non-Small Cell Lung Cancer Patients after Radiation Therapy." 2020. Web. 20 Jan 2021.
Cui S. Incorporating Deep Learning Techniques into Outcome Modeling in Non-Small Cell Lung Cancer Patients after Radiation Therapy. [Internet] [Doctoral dissertation]. University of Michigan; 2020. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/162923.
Cui S. Incorporating Deep Learning Techniques into Outcome Modeling in Non-Small Cell Lung Cancer Patients after Radiation Therapy. [Doctoral Dissertation]. University of Michigan; 2020. Available from: http://hdl.handle.net/2027.42/162923
16. McPeak, Brian. Holography, Supergravity, and the Weak Gravity Conjecture.
► This dissertation represents my work on three different subjects relating to quantum gravity and the AdS/CFT correspondence. First, we review a holographic computation of the… (more)
▼ This dissertation represents my work on three different subjects relating to quantum gravity and the AdS/CFT correspondence. First, we review a holographic computation of the one-loop corrections to the Weyl anomaly on Ricci flat backgrounds in six dimensions. This allows us to determine the correction to one linear combination of the anomaly coefficients. Then, we will show that these corrections may be obtained from the six-dimensional superconformal index. The second section will cover consistent truncations on the Lunin-Maldacena (LM) background cite{Lunin:2005jy}. We show how to restore minimal supersymmetry to the model of cite{Lunin:2005jy} by determining the reduction ansatz which includes the graviton and a gauge field, which comprise the graviton multiplet of mathcal{N} = 2 supergravity in five dimensions. Then we discuss our attempt to construct a truncation which includes a scalar field corresponding to the β-deformation parameter of the dual field theory. We show that if such a solution exists, it must differ somewhat drastically from the LM background. Finally, we discuss higher-derivative corrections to black hole solutions and the weak gravity conjecture in a few settings. We consider black holes which are charged under an arbitrary number of U(1) gauge fields in four dimensional flat space. In this setting, we compute the effect of higher-derivative corrections on the extremality bound, and we discuss the constraints placed on the effective field theory coefficients by the requirements that near-extremal black holes are unstable to decay to smaller black holes. Next we consider the shifts to thermodynamic quantities due to higher-derivative corrections to charged black holes in Anti-de Sitter space. We confirm and clarify a previously noted relationship between the shift to the extremality bound and the shift to the Wald entropy. We also show that if the shift in the Wald entropy is assumed to be positive, then the coefficient of the Rmu nu rho sigma Rmu nu rho sigma term in the effective Lagrangian must be positive as well. Advisors/Committee Members: Liu, James T (committee member), Gultekin, Kayhan (committee member), Clarke, Roy (committee member), Elvang, Henriette (committee member), Pando Zayas, Leopoldo A (committee member).
Subjects/Keywords: AdS/CFT; String Theory; Holography; Physics; Science
McPeak, B. (2020). Holography, Supergravity, and the Weak Gravity Conjecture . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/163204
McPeak, Brian. "Holography, Supergravity, and the Weak Gravity Conjecture." 2020. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/163204.
McPeak, Brian. "Holography, Supergravity, and the Weak Gravity Conjecture." 2020. Web. 20 Jan 2021.
McPeak B. Holography, Supergravity, and the Weak Gravity Conjecture. [Internet] [Doctoral dissertation]. University of Michigan; 2020. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/163204.
McPeak B. Holography, Supergravity, and the Weak Gravity Conjecture. [Doctoral Dissertation]. University of Michigan; 2020. Available from: http://hdl.handle.net/2027.42/163204
17. Elbez, Remy A. Nanoparticle Induced Cell Magneto-Rotation for the Multiplexed Monitoring of Morphology, Stress and Drug Sensitivity of Suspended Single Cancer Cells.
► The metastatic process of a cancer relies on the transformation of some of the primary tumor cells into cells capable of migrating through the Extra-Cellular… (more)
▼ The metastatic process of a cancer relies on the transformation of some of the primary tumor cells into cells capable of migrating through the Extra-Cellular Matrix (ECM), surrounding the tumor, into the bloodstream and the lymph nodes, and then settle in distant tissue, growing new secondary tumors. By identifying, characterizing and quantifying these cells, the progression of cancer in a patient during therapy can be more accurately assessed. Here we describe the development of a new method for quantitative real time monitoring of cell size and morphology, on single live suspended cancer cells, unconfined in three dimensions. The enabling cell magnetorotation (CM) method is made possible by nanoparticle induced cell magnetization. Using a rotating magnetic field, the magnetically labeled cells are actively rotated, then imaged, using a high definition CCD camera. Under proper conditions, the rotation period of a magnetic object is proportional to its shape factor. We demonstrate first that the rotational period, when measured in real-time, can serve to track cellular response to drugs, cytotoxic agents and other chemical stimuli. In addition, while cells are rotated, they exhibit very specific morphological activities, even without a chemical stimulus. Described also is how to multiplex the CM method, to image several dozens to several thousands of cells simultaneously, and using morphology to classify cells into different phenotypic categories, with each phenotype being correlated with malignancy level. The intrinsic tumor heterogeneity, at the cellular level, can be visualized with relationship graphs. Shown is the ability to monitor cell morphological changes over long periods of time, in real time, in order to detect the metastatic potential for heterogeneous populations of cancer cells, using tools from statistical analysis methods. The method relies on unsupervised Machine Learning algorithms which do not require human inputs. Overall it is demonstrated that the CM method can be used as a diagnostic tool to evaluate the phenotypical heterogeneity in a cell population in general, and in a cancer cell population in particular. This fast and high throughput method promises to efficiently assess the efficacy of personalized therapeutic strategies. Advisors/Committee Members: Kopelman, Raoul (committee member), Hunt, Alan J. (committee member), Takayama, Shuichi (committee member), Ziff, Robert M. (committee member), Doering, Charles R. (committee member), Clarke, Roy (committee member).
Subjects/Keywords: Cancer diagnostic; Metastasis; Clustering and classification using machine learning algorithms; Magneto-rotation; Magnetic nanoparticles; Single cell trapping; Physics; Science
Elbez, R. A. (2015). Nanoparticle Induced Cell Magneto-Rotation for the Multiplexed Monitoring of Morphology, Stress and Drug Sensitivity of Suspended Single Cancer Cells. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/111434
Elbez, Remy A. "Nanoparticle Induced Cell Magneto-Rotation for the Multiplexed Monitoring of Morphology, Stress and Drug Sensitivity of Suspended Single Cancer Cells." 2015. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/111434.
Elbez, Remy A. "Nanoparticle Induced Cell Magneto-Rotation for the Multiplexed Monitoring of Morphology, Stress and Drug Sensitivity of Suspended Single Cancer Cells." 2015. Web. 20 Jan 2021.
Elbez RA. Nanoparticle Induced Cell Magneto-Rotation for the Multiplexed Monitoring of Morphology, Stress and Drug Sensitivity of Suspended Single Cancer Cells. [Internet] [Doctoral dissertation]. University of Michigan; 2015. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/111434.
Elbez RA. Nanoparticle Induced Cell Magneto-Rotation for the Multiplexed Monitoring of Morphology, Stress and Drug Sensitivity of Suspended Single Cancer Cells. [Doctoral Dissertation]. University of Michigan; 2015. Available from: http://hdl.handle.net/2027.42/111434
18. Kidner, Shirley Lynn Matson. Ion-assisted sputter deposition and structural characterization of cubic boron nitride.
► We have grown cubic boron nitride (c-BN) films on silicon (100) substrates by rf magnetron sputter deposition in the presence of an electron cyclotron resonance… (more)
▼ We have grown cubic boron nitride (c-BN) films on silicon (100) substrates by rf magnetron sputter deposition in the presence of an electron cyclotron resonance source (ECR) nitrogen plasma. By applying a negative dc potential to the substrate, we are able to accelerate the nitrogen ions from the ECR providing more energy to the growth surface. We have investigated the growth conditions, particularly the effect of substrate bias and ion energies on growth. The structure of the films has been analyzed by reflective high energy electron diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and high resolution electron microscopy. We find evidence that the formation of c-BN requires irradiation of the growth surface with energetic species on the order of 100 eV, much lower than previously reported. For our deposition system, this minimum kinetic energy manifests itself as a threshold in the substrate bias potential. The structure of the films changed from entirely hexagonal (h-BN) to predominantly cubic over a very small range in the substrate bias voltage (±5 V). Moreover, the crystal structure is optimized in a small range beyond the threshold with bias voltages just a few volts higher causing a significant decrease in crystal quality. Our finding is, to the best of our knowledge, the first evidence of a sharp, low energy threshold for c-BN formation. High resolution electron microscopy studies show that the films have a layered structure with a thin (40 A) amorphous layer next to the silicon, followed by a thin (300 A) layer of textured h-BN aligned with the c-axis parallel to the substrate surface. Single-phase, polycrystalline c-BN comprises the rest of the film. The h-BN is confined to the interfacial region and once c-BN forms it is the preferred phase in the deposition. We discuss the important role of energetic ions in the formation of c-BN. In particular, recent molecular dynamics simulations provide quantitative evidence for bonding kinetics at impulse energies similar to those we have observed experimentally. Advisors/Committee Members: Clarke, Roy (advisor).
Subjects/Keywords: Physics, Condensed Matter; Engineering, Materials Science
Kidner, S. L. M. (1994). Ion-assisted sputter deposition and structural characterization of cubic boron nitride. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/104152
Kidner, Shirley Lynn Matson. "Ion-assisted sputter deposition and structural characterization of cubic boron nitride." 1994. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/104152.
Kidner, Shirley Lynn Matson. "Ion-assisted sputter deposition and structural characterization of cubic boron nitride." 1994. Web. 20 Jan 2021.
Kidner SLM. Ion-assisted sputter deposition and structural characterization of cubic boron nitride. [Internet] [Doctoral dissertation]. University of Michigan; 1994. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/104152.
Kidner SLM. Ion-assisted sputter deposition and structural characterization of cubic boron nitride. [Doctoral Dissertation]. University of Michigan; 1994. Available from: http://hdl.handle.net/2027.42/104152
19. Lamelas, Francisco Javier. Growth and x-ray scattering studies of epitaxial cobalt superlattices.
► In this thesis we describe the MBE growth and x-ray scattering analysis of magnetic Co-Au and Co-Cu superlattices. The superlattices are epitaxially ordered in that… (more)
▼ In this thesis we describe the MBE growth and x-ray scattering analysis of magnetic Co-Au and Co-Cu superlattices. The superlattices are epitaxially ordered in that the in-plane crystallographic orientation of individual layers is preserved throughout the growth process, as shown by in-situ reflection high energy electron diffraction (RHEED) studies. This epitaxial ordering is confirmed by x-ray scattering measurements performed in transmission, with the scattering vector parallel to the plane of the superlattice. In addition, these x-ray measurements determine the in-plane epitaxial strains in the superlattice layers. These strains are useful in estimating magnetoelastic contributions to the magnetic anisotropy. The composition modulation in the growth direction has been analyzed via x-ray scans performed with the scattering vector normal to the film plane. The measured intensities have been compared to computer simulations which incorporate interfacial diffusion and layer thickness fluctuations. We find that the interfaces in these superlattices are limited in thickness to a 2 monolayer region. Finally, x-ray scans along 101ℓ are used to distinguish hcp and fcc stacking sequences in Co layers. We find that, in the case of Co-Au superlattices, the Co planes are stacked according to the hcp sequence (which is stable in the bulk below 400\sp∘C). However, in Co-Cu superlattices, the Co planes are stacked according to the fcc sequence, providing an example of the epitaxial stabilization of a structure which does not occur in the bulk under ambient conditions. Advisors/Committee Members: Clarke, Roy (advisor).
Lamelas, F. J. (1990). Growth and x-ray scattering studies of epitaxial cobalt superlattices. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/103783
Lamelas, Francisco Javier. "Growth and x-ray scattering studies of epitaxial cobalt superlattices." 1990. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/103783.
Lamelas, Francisco Javier. "Growth and x-ray scattering studies of epitaxial cobalt superlattices." 1990. Web. 20 Jan 2021.
Lamelas FJ. Growth and x-ray scattering studies of epitaxial cobalt superlattices. [Internet] [Doctoral dissertation]. University of Michigan; 1990. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/103783.
Lamelas FJ. Growth and x-ray scattering studies of epitaxial cobalt superlattices. [Doctoral Dissertation]. University of Michigan; 1990. Available from: http://hdl.handle.net/2027.42/103783
20. Smith, Elizabeth Ruth. Probing the magnetic anisotropy of Co/Cu thin films.
Degree: PhD, Pure Sciences, 1998, University of Michigan
► The magnetic anisotropy of ultrathin magnetic Co/Cu (111) and Co/Cu (001) films was investigated. A comparison of two experimental techniques, Ferromagnetic Resonance (FMR) and Magneto-Optic… (more)
▼ The magnetic anisotropy of ultrathin magnetic Co/Cu (111) and Co/Cu (001) films was investigated. A comparison of two experimental techniques, Ferromagnetic Resonance (FMR) and Magneto-Optic Kerr Effect (MOKE), revealed surprising differences in the observed in-plane magnetic anisotropy. For the Co/Cu (111), FMR observed two resonance signatures: one exhibiting 6-fold symmetry and another having uniaxial symmetry in the plane of the film. However, MOKE measurements displayed only the uniaxial symmetry plus strong singularities when the external magnetic field was aligned along the magnetic hard axis. Similar results were seen for the Co/Cu (001) films, in which FMR observed the anticipated 4-fold symmetry, while MOKE observed the 4-fold symmetry plus singularities along the magnetic hard axes. These differences highlight the role of domain dynamics and indicate that the singularities are an inherent signature of the magnetization reorientation. The dynamics of the magnetization reorientation were investigated through the use of a simple theoretical model which predicted the shape of the hysteresis curve. The model is based on a free-energy minimization approach in which spin reorientation kinetics are included via coherent rotation with the additional provision that the magnetization can jump between local minima. The field at which the magnetization is allowed to jump is determined by the competition between the energy gained by spin reorientation verses the energy cost of nucleating and propagating magnetic domains. This introduces the possibility of a latched state when the magnetization is saturated along a magnetic hard axis, leading to marked singularities in the coercive field and remanent moment. These singularities emerge as a general consequence of the kinetics imposed by the constrained geometry in ultrathin films. Advisors/Committee Members: Clarke, Roy (advisor).
Subjects/Keywords: Anisotropy; Co; Cobalt/copper; Cu; Films; Magnetic; Probing; Thin
Smith, E. R. (1998). Probing the magnetic anisotropy of Co/Cu thin films. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/131097
Smith, Elizabeth Ruth. "Probing the magnetic anisotropy of Co/Cu thin films." 1998. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/131097.
Smith, Elizabeth Ruth. "Probing the magnetic anisotropy of Co/Cu thin films." 1998. Web. 20 Jan 2021.
Smith ER. Probing the magnetic anisotropy of Co/Cu thin films. [Internet] [Doctoral dissertation]. University of Michigan; 1998. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/131097.
Smith ER. Probing the magnetic anisotropy of Co/Cu thin films. [Doctoral Dissertation]. University of Michigan; 1998. Available from: http://hdl.handle.net/2027.42/131097
21. He, Hui David. Correlations between the structural and magnetic properties of MBE-grown cobalt-based superlattices.
► We describe the MBE (Molecular Beam Expitaxy) technique for growth of the first transition metal magnetic superlattices. The structural and magnetic properties of Co-Au and… (more)
▼ We describe the MBE (Molecular Beam Expitaxy) technique for growth of the first transition metal magnetic superlattices. The structural and magnetic properties of Co-Au and Co-Cu superlattices are characterized with various methods. The focus of this thesis is an attempt to reveal some of the relationship between them. The magnetic Co superlattices are grown epitaxially on GaAs substrates, as shown by in-situ RHEED (reflection high energy electron diffraction) studies. The in-plane crystallographic orientation of individual layers, which is preserved throughout the growth process, revealed with RHEED, TEM (transmission electron microscopy) and x-ray scattering measurements. The structural properties of Co superlattices are studied with cross-sectional and plan-view TEM imaging and diffraction techniques. The superlattice layers are found to be uniform with sharp interfaces across the entire film, with no evidence of interlayer mixing or disruption of the layers. Diffraction studies indicate that Co layers are grown in an hcp phase in Co-Au superlattices, while they are primarily stacked in the metastable fcc ordering in Co-Cu superlattices. Elastic strains exist in both systems, as determined by the x-ray scattering, and these are shown to be intimately related to the magnetic anisotropy in these epitaxial systems. SQUID and FMR (ferromagnetic resonance) measurements have been used to characterize the magnetic properties. The easy magnetization axis is found to change from in-plane to out-of-plane when the Co layer thickness reduces. Effective magnetic anisotropies are derived from both measurements. The cross-over thicknesses are 18A and 9A for Co-Au and Co-Cu superlattices, respectively. The low temperature perpendicular coercive field appears to oscillate as a function of the Au interlayer thickness in the Co-Au superlattices with constant Co layers. This may be the result of interlayer coupling interactions between individual Co layers. We suggest a simple calculation that includes the demagnetization energy, the magnetocrystalline, and magnetoelastic anisotropy energies, to account for the observed magnetic anisotropies in Co superlattices. The magnetic layer thickness dependence of the anisotropy is presented through the strains that have been measured by x-ray scattering. The calculation agrees quite well with experiments in the case of Co-Au superlattices; in particular, it can account for the saturation behavior of the effective anisotropy at smaller Co thickness. In the Co-Cu superlatice, the magnetocrystalline anisotropy constants have to be reduced by nearly 80% to obtain a satisfactory fit. This result is consistent with the growth of Co in a metastable fcc phase. Advisors/Committee Members: Clarke, Roy (advisor).
He, H. D. (1990). Correlations between the structural and magnetic properties of MBE-grown cobalt-based superlattices. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/103594
He, Hui David. "Correlations between the structural and magnetic properties of MBE-grown cobalt-based superlattices." 1990. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/103594.
He, Hui David. "Correlations between the structural and magnetic properties of MBE-grown cobalt-based superlattices." 1990. Web. 20 Jan 2021.
He HD. Correlations between the structural and magnetic properties of MBE-grown cobalt-based superlattices. [Internet] [Doctoral dissertation]. University of Michigan; 1990. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/103594.
He HD. Correlations between the structural and magnetic properties of MBE-grown cobalt-based superlattices. [Doctoral Dissertation]. University of Michigan; 1990. Available from: http://hdl.handle.net/2027.42/103594
22. Cionca, Codrin N. Imaging interfaces in epitaxial heterostructures.
► We present high resolution electron density maps of two types of systems investigated. The first type consists of two InAs films grown on GaSb (001)… (more)
▼ We present high resolution electron density maps of two types of systems investigated. The first type consists of two InAs films grown on GaSb (001) substrates (one grown using As4, the other one grown using As 2) and two GaSb films grown on InAs (001). The semiconductor samples were 9 monolayers thick and were grown using MBE. The three dimensional electron densities were obtained measuring the intensities along the Bragg rods and analyzing them with the newly developed Coherent Bragg Rod Analysis (COBRA) phase retrieval method. The study represents the first attempt to use a phase retrieval method in the characterization of a buried semiconductor heterostructure. The semiconductor was treated as a quaternary of form Ga mIn1-mAs nSb1-n. The m(z) and n( z) compositional fractions and the vertical lattice spacing profiles were extracted. The results tend to indicate that the transition regions have a quaternary composition and are relatively narrow (1.5 unit cells). In the case of the GaSb film, significant presence of As is observed. The lattice constant in the film is not constant and has a minimum ∼1.5 unit cells below the surface. The second type of system investigated consists of a thin film of PbTiO 3 on SrTiO3 (001), with and without a Pt electrode on top. The 50 A thick PbTiO3 film was grown by sputtering. The ED maps were investigated for peak position shifts. The O1 lattice seems to be shifted toward the film substrate interface in the case of the bare film. In the case of the Pt electrode region, its behavior is more complex. The O2 sublattice exhibits a shift toward the surface of the film, with a maximum of ∼0.25 A at 12 unit cells away from the substrate. The effect relaxes toward the surface. In the case of the bare film, the Ti peak width is slightly increased in the vicinity of both film interfaces. The presence of the Pt electrode seems to inhibit this behavior, fact that can be correlated with the inhibition of the presence of surface charge. Advisors/Committee Members: Clarke, Roy (advisor).
Subjects/Keywords: Epitaxial; Gallium Antimonide; Heterostructures; Imaging Interfaces; Indium Arsenide; Lead Titanate; Strontium Titanate; Thin Films
Cionca, C. N. (2005). Imaging interfaces in epitaxial heterostructures. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/124762
Cionca, Codrin N. "Imaging interfaces in epitaxial heterostructures." 2005. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/124762.
Cionca, Codrin N. "Imaging interfaces in epitaxial heterostructures." 2005. Web. 20 Jan 2021.
Cionca CN. Imaging interfaces in epitaxial heterostructures. [Internet] [Doctoral dissertation]. University of Michigan; 2005. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/124762.
Cionca CN. Imaging interfaces in epitaxial heterostructures. [Doctoral Dissertation]. University of Michigan; 2005. Available from: http://hdl.handle.net/2027.42/124762
23. Taylor, Charles Albert, II. Growth and optical studies of boron nitride films on silicon substrates.
► This dissertation details significant advances in the thin film growth of the wide bandgap semiconductor cubic boron nitride. We have developed a novel high temperature… (more)
▼ This dissertation details significant advances in the thin film growth of the wide bandgap semiconductor cubic boron nitride. We have developed a novel high temperature growth process which combines RF-magnetron sputtering, a high density nitrogen plasma produced by an electron cyclotron resonance (ECR) ion source, and negative substrate bias potentials to precisely control the ion energy at the substrate surface. We find that under high ion flux conditions, the growth of cubic BN is possible only within narrow, ∼30 eV, ranges of ion energy. Through a better understanding of the growth kinetics, we have significantly reduced the ion energy needed to form the metastable cubic phase, with values now substantially less than 100 eV. The reduction in ion energy has led to improvements in film crystallinity and orientation, with an associated reduction of the high film stress which has severely limited film thickness in the past. The BN films exhibit a layered morphology: a thin amorphous region at the silicon substrate interface, followed by an oriented hexagonal layer upon which the cubic phase nucleates. Scanning force microscopy images show that cubic BN nucleates as well aligned triangular crystalites, indicative of (111) cubic growth. More specifically, the cubic BN appears to nucleate on the edges of perpendicularly oriented hexagonal BN planes, such that the cubic BN (111) is normal to the hexagonal BN (0002) (c-axis). The crystallites coalesce to a smooth film (RMS roughness ∼20 A) with an average grain size of ∼700 A. Selected area electron diffraction and infrared spectroscopy confirm that the cubic layer is single phase cubic BN. The results suggest a path to "compliant" oriented growth on a variety of substrates in which hexagonal BN planes serve to both absorb strain and provide a nucleation mechanism for cubic BN. New Raman and cathodoluminescence (CL) spectroscopy results are presented for both hexagonal and cubic BN films. Raman scattering from cubic BN samples, in which the cubic LO and TO modes are clearly resolved, are the first reported well resolved Raman features from cubic BN films. Significant results from CL spectroscopy include the first reported observation of near bandgap emission from hexagonal BN films in the range 5.5 eV to 4.9 eV. The emission features are attributed to recombination processes involving shallow donors and deep acceptors. Deep level defect-related luminescence is observed from cubic BN films centered near 3.75 eV. The deep level luminescence is strongly correlated with growth conditions, decreasing in intensity and shifting to higher energy as parameters are optimized. Advisors/Committee Members: Clarke, Roy (advisor).
Taylor, Charles Albert, I. (1996). Growth and optical studies of boron nitride films on silicon substrates. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/105158
Taylor, Charles Albert, II. "Growth and optical studies of boron nitride films on silicon substrates." 1996. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/105158.
Taylor, Charles Albert, II. "Growth and optical studies of boron nitride films on silicon substrates." 1996. Web. 20 Jan 2021.
Taylor, Charles Albert I. Growth and optical studies of boron nitride films on silicon substrates. [Internet] [Doctoral dissertation]. University of Michigan; 1996. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/105158.
Taylor, Charles Albert I. Growth and optical studies of boron nitride films on silicon substrates. [Doctoral Dissertation]. University of Michigan; 1996. Available from: http://hdl.handle.net/2027.42/105158
24. Litvinov, Dmitri. Growth of semiconducting cubic boron nitride thin films.
► This thesis details the growth mechanisms and deposition processes for semiconducting cubic boron nitride. Through experimental research a better understanding of the nucleation and growth… (more)
▼ This thesis details the growth mechanisms and deposition processes for semiconducting cubic boron nitride. Through experimental research a better understanding of the nucleation and growth mechanisms of the metastable cubic phase of boron nitride is achieved. A deposition process based on reduced-bias ion-assisted sputtering is developed. We found that relatively high nitrogen ion energy (∼100eV) necessary for initial nucleation and coalescence of c-BN islands can be reduced substantially (to ∼60eV) leading to a dramatic improvement in the film quality in terms of fewer defects, larger grain size (∼2000A), lower residual stress, and well defined texture. It was found that the films grown at temperatures above 1050°C are stable at ambient conditions and consist of pure-phase cubic boron nitride. An important aspect of the studies was the use of a novel technique for in-situ real-time stress monitoring which allowed us to produce films of cubic boron nitride with record thicknesses of more than 2mum. Cubic BN films prepared in this way are typically polycrystalline exhibiting partial crystallographic alignment relative to the substrate (i.e. textured morphology). A kinematic theory of reflection high energy electron diffraction (RHEED) is developed for textured polycrystalline thin films. Analytical expressions for RHEED patterns are derived for arbitrary texture situations and for any general crystallographic orientation that may be encountered in thin-film growth. We showed that the RHEED can be used as a fast and convenient tool for in-situ texture characterization. The approach also permits quantitative extraction of texture angular dispersion parameters which are useful for optimizing thin-film growth conditions. Analysis of RHEED patterns showed that c-BN films are textured with the texture axis along the (100) crystallographic direction perpendicular to the film surface with the texture spread of less than 20 degrees, a dispersion that can be reduced substantially by optimizing the growth conditions. The deposition technique devised in this research enables p-type semiconducting cubic boron nitride thin films with low a carrier activation energy of 60meV, high room temperature carrier mobility of ∼500cm2/V-s and high room-temperature carrier concentrations of ∼5 · 10 18cm-3. Advisors/Committee Members: Clarke, Roy (advisor).
Subjects/Keywords: Cubic Boron Nitride; Deposition; Growth; Ion-assisted; Rheed; Semiconducting; Stress Monitoring; Thin Films
Litvinov, D. (1999). Growth of semiconducting cubic boron nitride thin films. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/131711
Litvinov, Dmitri. "Growth of semiconducting cubic boron nitride thin films." 1999. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/131711.
Litvinov, Dmitri. "Growth of semiconducting cubic boron nitride thin films." 1999. Web. 20 Jan 2021.
Litvinov D. Growth of semiconducting cubic boron nitride thin films. [Internet] [Doctoral dissertation]. University of Michigan; 1999. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/131711.
Litvinov D. Growth of semiconducting cubic boron nitride thin films. [Doctoral Dissertation]. University of Michigan; 1999. Available from: http://hdl.handle.net/2027.42/131711
25. Wellman, Joanne Marie. Growth and annealing study of MBE-growth Rh(111) films.
► We present a comprehensive study of the formation and evolution of homoepitaxial Rh(111) thin film surface morphology. This set of scanning tunneling microscopy (STM) and… (more)
▼ We present a comprehensive study of the formation and evolution of homoepitaxial Rh(111) thin film surface morphology. This set of scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED) experiments is unique in terms of the combination of imaging and diffraction techniques and combining both growth and annealing in a single study. Rh(111) in many ways represents a model system for such studies in that it can be used to access a variety of growth regimes over a wide temperature range. A series of 10.1 monolayer (ML) Rh(111) films were grown via molecular beam epitaxy (MBE) at temperatures ranging from 375K to 875K. These surfaces show three distinct growth regimes, low temperature 3D growth, a mid-range (500K-700K) extended 2D layer-by-layer growth regime, and high temperature 3D growth. Both the high and low temperature 3D regimes exhibit patterned surfaces with well-defined feature separation (R), illustrating that they cannot be classified as self-affine. As growth proceeds, R increases following a power-law dependence on film thickness with a morphology-independent exponent of 0.33 ± 0.03. We observe a morphological transition in nucleation and growth at 600K. STM images of Rh(111) surfaces show fingered step edges for 10.1 ML films grown below 600K and compact features for higher temperature growth. Submononlayer deposition studies indicate that dimers become unstable at temperatures above 600K, with the binding energy of dimers set at 0.6 ± 0.4 eV. These results highlight the effect of one-bond detachment on the overall growth kinetics on the Rh(111) surface. Finally, we utilize the power of in-situ RHEED analysis to study the equilibrium-bound smoothing kinetics of 2D and 3D Rh(111) surface features during annealing. The 2D islands coarsen rapidly throughout the anneal. The 3D features are more stable initially, and then flatten rapidly. The transition activation energy is ∼0.6 eV, which corresponds to the one-bond detachment energy discussed above. These findings suggest that the smoothing kinetics of 3D features is limited by adatom detachment at low temperatures, with faster processes activated at higher temperatures to facilitate rapid smoothing. The results provide a roadmap for achieving smooth layer-by-layer growth in close-packed metals. Advisors/Committee Members: Clarke, Roy (advisor).
Subjects/Keywords: Annealing; Growth; Mbe; Molecular Beam Epitaxy; Rh; Rhodium; Study; Thin Films
Wellman, J. M. (1998). Growth and annealing study of MBE-growth Rh(111) films. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/131128
Wellman, Joanne Marie. "Growth and annealing study of MBE-growth Rh(111) films." 1998. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/131128.
Wellman, Joanne Marie. "Growth and annealing study of MBE-growth Rh(111) films." 1998. Web. 20 Jan 2021.
Wellman JM. Growth and annealing study of MBE-growth Rh(111) films. [Internet] [Doctoral dissertation]. University of Michigan; 1998. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/131128.
Wellman JM. Growth and annealing study of MBE-growth Rh(111) films. [Doctoral Dissertation]. University of Michigan; 1998. Available from: http://hdl.handle.net/2027.42/131128
26. Medjahed, Djamel. A structural and lattice vibrational study of graphite-alkali metal ternary compounds.
► We study the structure and lattice vibrations of one member of the Alkali-metal/Graphite ternary compound family, namely C8K1-xRbx. The relative concentration (x) is varied in… (more)
▼ We study the structure and lattice vibrations of one member of the Alkali-metal/Graphite ternary compound family, namely C8K1-xRbx. The relative concentration (x) is varied in a series of samples so as to encompass the entire range available (0 to 1). The in-plane structure is monitored by conducting a series of hoko0 x-ray scans and appears to remain unchanged. The lattice parameter in the out-of-plane direction, and certain vibration modes, exhibit a non-linear behavior in the concentration range x = 0.5 to 0.7 for samples based on disordered artificial graphite matrices, but is otherwise linear for structurally "clean" samples. We find that the anomalous softening of phonon modes previously reported in highly oriented pyrolitic graphite (HOPG) samples does not occur in naturally occurring single-crystal (SC) samples. X-ray and Raman scattering experiments show that the anomaly is an artifact due to the inhomogeneity of synthetic polycrystalline graphite. Advisors/Committee Members: Clarke, Roy (advisor).
Subjects/Keywords: Physics; Condensed Matter
Medjahed, D. (1992). A structural and lattice vibrational study of graphite-alkali metal ternary compounds. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/102957
Medjahed, Djamel. "A structural and lattice vibrational study of graphite-alkali metal ternary compounds." 1992. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/102957.
Medjahed, Djamel. "A structural and lattice vibrational study of graphite-alkali metal ternary compounds." 1992. Web. 20 Jan 2021.
Medjahed D. A structural and lattice vibrational study of graphite-alkali metal ternary compounds. [Internet] [Doctoral dissertation]. University of Michigan; 1992. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/102957.
Medjahed D. A structural and lattice vibrational study of graphite-alkali metal ternary compounds. [Doctoral Dissertation]. University of Michigan; 1992. Available from: http://hdl.handle.net/2027.42/102957
27. Daniel, Abishai. Electrical studies on cubic boron nitride MIS structures.
► A systematic investigation of the electrical properties of Metal-cBN-p-Silicon MIS structures through Capacitance-Voltage and Leakage Current measurements was undertaken. Electrical leakage measurements were performed using… (more)
▼ A systematic investigation of the electrical properties of Metal-cBN-p-Silicon MIS structures through Capacitance-Voltage and Leakage Current measurements was undertaken. Electrical leakage measurements were performed using an HP4145B Semiconductor Parameter Analyzer and Capacitance-Voltage results were obtained using an LCZ Meter. Poole-Frenkel conduction was observed to be dominant in cBN MIS structures at low electric fields. Trap height values obtained for the films were ∼0.8 eV. Schottky conduction was observed to be the dominant conduction in predominantly cBN MIS structures. The gold-cBN barrier height was obtained to be ∼0.6 eV. For a fraction of MIS structures incorporating indium gate metal, Poole-Frenkel and Schottky conduction was observed. For other contacts evidence was observed of the alloying of indium with cBN resulting in Ohmic-like behavior analogous to the behavior of indium with GaAs. Evidence of the pinning of the Fermi level at the Metal-Insulator interface was obtained from the similarity of the gold-cBN and indium-cBN barrier heights. MIS structures incorporating tBN as the insulating layer exhibited similar conduction behavior to the cBN MIS structures. The gold-tBN barrier height obtained was ∼0.68 eV and the trap height obtained was ∼0.86 eV. A correlation between leakage current and RMS roughness of the surface was observed providing evidence of roughness mediated field enhancement at the metal insulator interface resulting in higher leakage currents. Leakage current of the cBN MIS structures at high electric fields exhibit a similar functional relationship to post Soft Breakdown I-V curves of ultrathin oxides at high electric fields. Hard Breakdown was not observed for our films at fields up to 2 MV/cm. This sets a lower bound for the breakdown field for our BN films. Resistivity values in the range 109 O-cm to 1012 O-cm were obtained for cBN and tBN films. To our knowledge, we obtained the first Capacitance-Voltage results on BN MIS structures with a cBN phase close to 100% and with silicon as the semiconductor material. No hysteresis was observed in the C-V curve when the voltage was swept from accumulation to inversion and back to accumulation. A small shift in VFB was observed and negative fixed nitride charge density of 5x1011 cm-2 was determined. High Dit values were obtained for the cBN films compared to hBN films reported in the literature indicating that the cBN nucleation process is accompanied with significant trap creation similar to our leakage results on cBN MIS structures. Advisors/Committee Members: Clarke, Roy (advisor).
Subjects/Keywords: Cubic Boron Nitride; Electrical; Metal-insulator-semiconductor; Mis; Structures; Studies; Wide-bandgap
Daniel, A. (2005). Electrical studies on cubic boron nitride MIS structures. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/125336
Daniel, Abishai. "Electrical studies on cubic boron nitride MIS structures." 2005. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/125336.
Daniel, Abishai. "Electrical studies on cubic boron nitride MIS structures." 2005. Web. 20 Jan 2021.
Daniel A. Electrical studies on cubic boron nitride MIS structures. [Internet] [Doctoral dissertation]. University of Michigan; 2005. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/125336.
Daniel A. Electrical studies on cubic boron nitride MIS structures. [Doctoral Dissertation]. University of Michigan; 2005. Available from: http://hdl.handle.net/2027.42/125336
28. Elagoz, Sezai. Structural characterization of epitaxial cobalt-chromium superlattices.
► A series of epitaxial Co-Cr superlattices has been grown by molecular beam epitaxy and characterized with x-ray diffraction and in-situ RHEED. Using some x-ray techniques… (more)
▼ A series of epitaxial Co-Cr superlattices has been grown by molecular beam epitaxy and characterized with x-ray diffraction and in-situ RHEED. Using some x-ray techniques newly developed in my laboratory I have been able to provide complete descriptions of both the growth process and an analysis of epitaxial phase transitions. The superlattices are epitaxially ordered such that the in-plane crystallographic orientations of the individual layers are preserved during the growth process as indicated by the RHEED studies. This epitaxial ordering is confirmed (for Cr layers up to 6A) by in-plane x-ray scattering measurements. We have accomplished this work with only standard laboratory x-ray equipment. While synchrotron radiation offers several advantages, e.g., for higher resolution measurements, the techniques developed here are convenient for a laboratory setting which combines growth and structural characterization. X-ray scans along 101l are used to distinguish hcp and fcc stacking sequences in both Co and Cr layers. We find that the Co layers and the first 2-3 monolayers of Cr are stacked according to an hcp sequence. The data clearly show that we were able to synthesize a new close-packed metastable phase of Cr up to a layer thickness of 6A, providing an example of epitaxial stabilization of a structure which does not occur in the bulk. In addition, we showed that beyond this thickness there is an abrupt structural transition to bcc (110) Cr in the classical Nishiyama-Wasserman and Kurdjumov-Sachs orientations. Advisors/Committee Members: Clarke, Roy (advisor).
Elagoz, S. (1993). Structural characterization of epitaxial cobalt-chromium superlattices. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/103573
Elagoz, Sezai. "Structural characterization of epitaxial cobalt-chromium superlattices." 1993. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/103573.
Elagoz, Sezai. "Structural characterization of epitaxial cobalt-chromium superlattices." 1993. Web. 20 Jan 2021.
Elagoz S. Structural characterization of epitaxial cobalt-chromium superlattices. [Internet] [Doctoral dissertation]. University of Michigan; 1993. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/103573.
Elagoz S. Structural characterization of epitaxial cobalt-chromium superlattices. [Doctoral Dissertation]. University of Michigan; 1993. Available from: http://hdl.handle.net/2027.42/103573
29. Huang, Simon. Formation, Structure, and Properties of InAs/GaAs Quantum Dots.
► Semiconductor quantum dots (QDs) have shown significant promise for a wide range of optoelectronic devices, including solar cells, photodetectors, and lasers. Typically, QDs are fabricated… (more)
▼ Semiconductor quantum dots (QDs) have shown significant promise for a wide range of optoelectronic devices, including solar cells, photodetectors, and lasers. Typically, QDs are fabricated by the misfit-driven Stranski-Krastanov (SK) mode, which results in elliptical-shaped QDs with sizes and densities limited by the lattice misfit. Recently, the nucleation of metal droplets and their conversion to QDs, often termed droplet epitaxy (DE), has attracted much attention because it allows QD fabrication without lattice misfit. However, the mechanisms for the conversion of In islands to InAs QDs and the origins of misfit dislocation (MD) displacement are still unclear. Thus, further examination on the formation mechanism and microstructures of DE QDs is essential. Here, we report on the structure and properties of InAs/GaAs QDs formed by DE and SK approaches. These results suggest that DE is promising for tuning QD sizes and densities, as well as tailoring carrier confinement in the vicinity of QDs. Using a finite-element Schrodinger-Poisson model that considers experimentally measured QD and wetting layer (WL) shapes, sizes, and spacings, we examined the influence of InAs/GaAs SK QDs on the solar cell external quantum efficiency (EQE). A comparison between the computed and measured EQEs reveals a broadening of sub-bandgap EQE induced by QD size distribution, and a weak EQE contribution from the WLs. To further enhance the control of QD size, density, and microstructure, we investigate alternative QD fabrication approaches via annealing In islands under an As flux. We revealed the influence of As surface coverage on the QD formation mechanisms. On c(4x4) GaAs surfaces, QD formation follows DE. For the As capped surfaces, QDs nucleate by solid phase epitaxy during annealing of an amorphous film. Furthermore, we revealed the origin of interlayer formation and MD displacement in the vicinity of InAs/GaAs QDs. For SK QDs, MDs nucleate at the QD/buffer interface. For DE QDs with low In exposure, an InGaAs interlayer at the QD/buffer interface results in MD vertical displacement. For DE QDs with high In exposure, the formation of an InAs interlayer at the island/buffer interface leads to MDs at the QD/buffer interface. Advisors/Committee Members: Goldman, Rachel S. (committee member), Phillips, Jamie Dean (committee member), Kioupakis, Emmanouil (committee member), Clarke, Roy (committee member).
Subjects/Keywords: quantum dot; compound semiconductor; molecular beam epitaxy; Materials Science and Engineering; Engineering
…Microbeam Analysis Laboratory (EMAL) at the University of Michigan. We used etched… …at the University of Michigan. The TEM measurements were performed by Mr. Sung Joo Kim in… …Prof. X.Q. Pan's group at University of Michigan. Cross-sectional TEM specimens were prepared…
Huang, S. (2015). Formation, Structure, and Properties of InAs/GaAs Quantum Dots. (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/111371
Huang, Simon. "Formation, Structure, and Properties of InAs/GaAs Quantum Dots." 2015. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/111371.
Huang, Simon. "Formation, Structure, and Properties of InAs/GaAs Quantum Dots." 2015. Web. 20 Jan 2021.
Huang S. Formation, Structure, and Properties of InAs/GaAs Quantum Dots. [Internet] [Doctoral dissertation]. University of Michigan; 2015. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/111371.
Huang S. Formation, Structure, and Properties of InAs/GaAs Quantum Dots. [Doctoral Dissertation]. University of Michigan; 2015. Available from: http://hdl.handle.net/2027.42/111371
30. Che, Xiaozhou. High Efficiency Single and Multijunction Organic Photovoltaics.
► Organic photovoltaics (OPVs) offer a lightweight and potentially cost-effective approach for solar energy harvesting. The first OPV heterojunction device was reported in 1985 with 1%… (more)
▼ Organic photovoltaics (OPVs) offer a lightweight and potentially cost-effective approach for solar energy harvesting. The first OPV heterojunction device was reported in 1985 with 1% efficiency. There has been a rapid development of small molecule and polymer materials, as well as different growth techniques such as vapor-deposition and solution-processing over the past 30 years. With recent emergence of the non-fullerene acceptors (NFAs), the efficiency has been improved to 15%; a benchmark for OPV commercialization. The flexible and semitransparent form factors of OPVs also lead to unique applications such as flexible electronics and building integrated photovoltaics (BIPV), showing considerable market potential. This dissertation will focus on high efficiency single and multijunction OPVs. Following the background introduction of organic materials and solar cells, the simulation methods based on transfer matrix is discussed. The rest of the work can be divided into two parts. The first part studies a group of dipolar donor molecules with donor-acceptor-acceptor' structures, either with the propeller or coplanar donor unit. The molecular conjugation length and side chain configuration are adjusted to better understand the structure-property-performance relationships. In the second part, two multijunction structures will be discussed, both of which employ subcells with the d-a-a' donors. The first structure focuses on the fully vacuum-deposited tandem and triple junction cells with efficiencies of 10-11%, delivering several important multijunction design principles. It is followed by discussion of a tandem cell combining the vacuum-deposited and solution-processed NFA-based subcells, achieving a record 15% efficiency with close to 100% fabrication yield. A variety of characterization methods including crystallography, photoluminescence, external quantum efficiency and current density-voltage measurements, optical simulation etc. will be presented along with these results. With improved donor and NFA materials along with their inherent structural design flexibility, further improvement of OPV performance is expected to be achieved in the near future with plenty of market potential. Advisors/Committee Members: Forrest, Stephen R (committee member), Clarke, Roy (committee member), Kurdak, Cagliyan (committee member), Phillips, Jamie Dean (committee member).
Subjects/Keywords: Organic; Solar cell; Efficiency; Small molecule; Electrical Engineering; Engineering
Che, X. (2018). High Efficiency Single and Multijunction Organic Photovoltaics . (Doctoral Dissertation). University of Michigan. Retrieved from http://hdl.handle.net/2027.42/146071
Che, Xiaozhou. "High Efficiency Single and Multijunction Organic Photovoltaics." 2018. Doctoral Dissertation, University of Michigan. Accessed January 20, 2021. http://hdl.handle.net/2027.42/146071.
Che, Xiaozhou. "High Efficiency Single and Multijunction Organic Photovoltaics." 2018. Web. 20 Jan 2021.
Che X. High Efficiency Single and Multijunction Organic Photovoltaics. [Internet] [Doctoral dissertation]. University of Michigan; 2018. [cited 2021 Jan 20]. Available from: http://hdl.handle.net/2027.42/146071.
Che X. High Efficiency Single and Multijunction Organic Photovoltaics. [Doctoral Dissertation]. University of Michigan; 2018. Available from: http://hdl.handle.net/2027.42/146071