PHD, Kent State University, 2018, College of Arts and Sciences / Chemical Physics

Our research uses a dynamic cell to measure azimuthal anchoring energy and disclination line tension in a nematic liquid crystal. The dynamic cell is a cell which has controllable twist angle and cell thickness. By increasing the twist angle to an angle greater than 90 degrees, the cell becomes super twisted. In this state, if the cell thickness is decreased to a critical thickness, the surface anchoring breaks. The azimuthal anchoring energy can be calculated from the twist elastic constant, the twist angle, and the cell thickness. When the anchoring breaks, a disclination line is formed separating regions of opposite handed twist. By balancing the forces due to the disclination line tension and the twist distortion, a stable disclination line can be achieved. We can calculate the line tension of the disclination from the radius of curvature of the disclination line at equilibrium. In order to generate disclination lines more easily, we designed a disclination line nucleation site by photopatterned surface alignment. We found that the line tension increases with the cell thickness. By measuring the line tension with respect to cell thickness and temperature, we investigate the core of the disclination line.

Committee: Hiroshi Yokoyama PhD (Advisor); Peter Palffy-Muhoray PhD (Committee Member); Elizabeth Mann PhD (Committee Member); Antal Jakli PhD (Committee Member); Sam Sprunt PhD (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2021, Photochemical Sciences

Transparent Semiconducting oxides (TSO) belong to a special group of wide bandgap oxide materials that have high optical transmittance and high conductivity at the same time. Wide bandgap semiconductors are extremely important for their use in numerous electronic/ optoelectronic devices including MOSFETs, Photo diodes, solar cell, LED, Laser diode, sensors, etc. Recently, wide bandgap oxide materials, especially Ga2O3 have attracted a great deal of attention from the scientific community. β-Ga2O3 is the most stable polymorphs of Ga2O3 with an ultra-wide bandgap of 4.9 eV, high breakdown voltage, and high Baliga's Figure of Merit (BFM) that make it an ideal candidate for the next generation high power devices. A comprehensive study of material properties of β-Ga2O3 is needed to fabricate high-performance devices. Unfortunately, our understanding of β-Ga2O3 as a semiconductor material is not comprehensive. Point defects (e.g., Cation or anion vacancies, interstitials, etc.) that significantly affect the electrical and optical properties of this material are not yet fully understood. Proper understanding, characterizing and modification of defects can lead to its application in semiconductor-based devices. Moreover, finding suitable donors and acceptors for β-Ga2O3 to tune its electrical conductivity is crucial for its use in electronic devices. In this thesis, different aspects of β-Ga2O3 are addressed as a semiconductor material. We have studied optical, electrical, and structural properties of β-Ga2O3 single crystals and epitaxial thin films grown by several techniques. Major point defects in β-Ga2O3 were investigated using several novel techniques. We have identified and characterized major electronic traps and investigated their effects on the optical, structural, and electrical properties of β-Ga2O3. We discovered an innovative way to dope β-Ga2O3 providing high free carrier density and good mobility while maintaining low defect concentration. A novel spectromete (open full item for complete abstract)

Committee: Farida Selim Ph.D. (Advisor); Amelia Carr Ph.D. (Other); Alexander Tarnovsky Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Chemistry; Materials Science; Physics

PhD, University of Cincinnati, 2020, Engineering and Applied Science: Materials Science

Advances in computing have lead to a sudden burst in simulations involving fully atomistic models. A popular approach for these simulations is the molecular dynamics (MD) technique. Studies employing MD methodology aim to recreate experiments such as nanoindentation, with an aim to extract more information about the model than is possible with experiments. However, while MD does provide a wealth of information such as positions and velocities of all atoms, which can be used to directly study deformation mechanisms as well as provide direct visualization, it has severe drawbacks. The most fundamental restriction of MD is its length-scale limitation, which even on high-performance computing clusters is restricted to the nano-meter range. This makes it difficult to perform one-to-one comparison with experiments. An alternative approach which addresses this length-scale restriction is known as the partitioned-domain method which couples different methods in the same model. One such approach is the quasicontinuum (QC) method which couples atomistic and continuum regions. In the current study we first evaluate the effect of fundamental MD parameters namely: thermostat variables and size of the time-step, on the indentation response of Nickel. We then proceed to show that the length-scales prevalent in MD for nanoindentation simulations have an effect on its response therefore they cannot be used for a direct comparison with larger experimental systems. We then introduce the QC method and improve its computational efficiency by using a hybrid-linear elasticity and Cauchy-Born approach in the continuum region. The QC method is then extended to 3-dimensions and finite temperature for simple and complex lattice systems.

Committee: Woo Kyun Kim Ph.D. (Committee Chair); Donglu Shi Ph.D. (Committee Member); Vijay Vasudevan Ph.D. (Committee Member); Kumar Vemaganti Ph.D. (Committee Member) Subjects: Materials Science

PHD, Kent State University, 2018, College of Arts and Sciences / Department of Physics

In this dissertation research, we propose and develop a new type of photomasks, named plasmonic metamasks (PMMs), for high resolution and high throughput photopatterning of LC molecules into highly complex two-dimensional (2D) or three-dimensional (3D) molecule orientations. Enabled by this new technique, we also explore its applications in engineering topological defects and flat optical elements, and the defect-mediated phase transitions.

Committee: Qi-Huo Wei (Committee Chair); Elizabeth Mann (Committee Co-Chair) Subjects: Optics; Physics

PHD, Kent State University, 2018, College of Arts and Sciences / Department of Physics

Cholesteric liquid crystals experience geometric frustration when they are confined between surfaces with anchoring conditions that are incompatible with the cholesteric twist. Because of this frustration, they develop complex topological defect structures, which may be helicoids or skyrmions. We develop a theory for these structures, which extends previous theoretical research by deriving exact solutions for helicoids with the assumption of constant azimuth, calculating numerical solutions for helicoids and skyrmions with varying azimuth, and interpreting the results in terms of competition between terms in the free energy. We have also performed numerical simulations based on director tensor relaxation and used a finite difference method to demonstrate different defect structures in confined liquid crystals. Using our model, we have studied the formation of Skyrmions and helicoids (stripes) in cholesterics confined in rectangular micron-channels. Depending on the ratio, (cholesteric pitch)/(channel depth), Skyrmions or stripes can form. These results were in agreement with experiments done by QiHuo Wei's group. In another study we demonstrated forming complex defect textures such as disclination loops and arches in Nematics confined between patterned substrates. These results were also compared with experiments done by QiHuo Wei's group. In our final project we have modeled the microstructural evolution in cholesterics under voltage pulses and studied different Cholesteric phases.

Committee: Robin Selinger Professor (Committee Member); Jonathan Selinger Professor (Committee Member); Deng-Ke Yang Professor (Committee Member); John Portman Professor (Committee Member); Elda Hemann Assistant Professor (Committee Member); BARRY DUNIETZ Associate Professor (Committee Member) Subjects: Condensed Matter Physics; Physics

PHD, Kent State University, 2018, College of Arts and Sciences / Chemical Physics

This is an experimental work of liquid crystal physics. Our primary goal is to create arbitrary molecular orientations on the surface imposed with photo-alignment technique, which is considered a cost-effective, non-contact surface alignment technique with higher degrees of freedom and ease of operation. In this thesis, we propose and demonstrate a novel approach for orientationally patterned surfaces with photo-alignment, for disclination generation, as well as optical applications. With conflicted orientationally patterned surfaces, we are able to create half-integer disclinations from one surface, which extends into the bulk and back to the same surface. Pancharatnam-Berry Phase (PBP), also called geometric phase, arises from the continuously spatial variation of polarization directions. By engineering the surface pattern with cyclic orientation variations, we generate half-integer disclination loops floating in the bulk of liquid crystals. When combining two series of disclination loops emanating from the top and bottom surfaces, we obtain real 2D disclination networks in a closed liquid crystal cell. By utilizing a hybrid-aligned liquid crystal cell, with PBP orientations, as a phase shift mask in a projection system, we are able to recursively increase the spatial dynamic range for each projection.

Committee: Hiroshi Yokoyama (Advisor) Subjects: Optics; Physics

Doctor of Philosophy, Case Western Reserve University, 2018, Physics

I present a body of work regarding topological defects (TDs) in nematic liquid crystals. Defects having specific strengths were created in specified locations using atomic force microscope (AFM) lithography and the means by which the defects relieve the diverging strain energy near their cores was characterized as a function of cell depth and by probing with an electric field. I also work towards nanoparticle trapping in the scribed cores by doping a host liquid crystal with fluorescent nano-emitters. The technique of scribing an easy axis by AFM lithography was extended by writing Python scripts that produce densely-packed paths for the AFM tip to follow. I create several arrays of defects using this method in thin cells. I then probe the structure of the nematic director near each scribed core by applying a perpendicular electric field to a positive anisotropy liquid crystal. Of interest is the means by which the TDs relax the diverging energy at the defect cores. I show qualitatively that smaller cell depths promote defect splitting, whereas thicker cells promote defect escape, i.e., the director rotates out of the plane. The voltage profile of the transmitted intensity under crossed polarizers was examined, and shows that the liquid crystal can have a Freedericksz threshold voltage near a split defect, but not near an escaped defect. I then assemble thick cells, such that the disclination lines caused by the scribing run near the master surface and terminate on nearest-neighbors. I apply an in-plane field and show that the disclinations can deflect, interact, and can swap termination partners to effect a change in orientation of 90$^\circ$. The local electric field required to make the disclinations interact was measured. Then I consider a technique whereby the diverging energy of the defect cores can be relaxed by suspending nano-emitters, specifically quantum and carbon dots, into the liquid crystal. We make progress towards trapping these nanoparticles in scr (open full item for complete abstract)

Committee: Charles Rosenblatt (Advisor) Subjects: Physics

PHD, Kent State University, 2015, College of Arts and Sciences / Chemical Physics

Topological defects plays an important role in many physical processes ranging from morphogenesis of phase transitions in condensed matter system to the response to surface confinement and application of external fields. In this dissertation, we investigate the topological defects both in lyotropic and thermotropic nematics in order to characterize the studied materials.

Committee: Oleg Lavrentovich (Advisor); Hiroshi Yokoyama (Committee Member); Liang-Chy Chien (Committee Member); Samuel Sprunt (Committee Member); Elizabeth Mann (Committee Member) Subjects: Engineering; Experiments; Materials Science; Optics; Physical Chemistry; Physics

Doctor of Philosophy, University of Akron, 2009, Polymer Science

One dimensionally (1D) confined crystallization based on semicrystalline diblock copolymers has been widely investigated for twenty years. Highly orientated lamellar samples, after large amplitude oscillation shear, provide the typical 1D confined environment to investigate crystallization behavior, such as crystal orientation and crystallization kinetics. However, inevitable defect generation, such as edge and screw dislocations during mechanical shear lead to “cross-talking” between grain boundaries and significantly affect the ideal 1D confinement crystallization kinetics by releasing the confinement. Meanwhile, the mechanism of the origin of specific crystal orientations (parallel or perpendicular) within the 1D confinement is still under debate. In this research, PS-b-PEO single crystals composed of one PEO nano-layer sandwiched by two PS glassy layers on the nanoscale were chosen as a template to investigate polymer crystallization in a 1D defect-free nanoscale confinement. Since the TgPS is higher than the TmPEO in such a “sandwich” lamellar structure, the PEO single crystal can be melted while keeping the PS layer in the vitrified state. The PEO blocks can be recrystallized between the two confining, glassy PS nano-layers at different re-crystallization temperatures, Trx, and monitored for different recrystallization behavior using electron diffraction (ED). Results indicate that the PEO block does not recrystallize until Trx = -5 °C, the limit of homogeneous nucleation. This observation confirms PEO recrystallization takes place in a defect-free confinement. Next, a puzzling ED pattern taken from PEO microcrystals grown at Trx > -5 °C via self-seeding was analyzed to be the result of the specific interaction between tethered PEO blocks having monoclinic symmetry and the glassy PS substrate. Two different inclined PEO microcrystals having an orthogonal relationship truly coexisting in the lamellar confinement during self-seeding were found. Furthermore, (open full item for complete abstract)

Committee: Stephen Z. D. Cheng Dr. (Advisor) Subjects: Materials Science; Polymers