Graduate Thesis Defense - 2022 Spring

Speaker: Aswad Alhassan, (PhD Candidacy)

Abstract: 2D moiré materials, by twisting 2D bilayers or multilayers, have enormously attracted the attention of researchers in science and engineering due to their fascinating electronic, mechanical, optical, and thermal properties which are not observed in their untwisted 2D counterparts. Recent new discovery by an experimental group [Nature 556, 44 (2018)] has shown that a transition from the semi-metal to an unconventional superconductivity was observed in twisted bilayer graphene at a magic angle (𝜙 = 1.05!) with a critical temperature 𝑇". Another new discovery is found in the twisted MoS2/WS2 heterostructures [Sci. Adv. 6, 1-9 (2020)]. The delocalized excitons in the bilayer MoS2/WS2 become localized interlayer moiré excitons by twisting it at a magic angle of 3.480. Such moiré excitons could form ordered quantum dot arrays, paving the way for unprecedented optoelectronic and quantum information applications. These discoveries have laid the footprints to explore intriguing materials properties in other 2D materials such as h-BN, transition metal dichalcogenides, and phosphorene. The prudent assembly of these 2D monolayers into multilayer heterostructures with moiré patterns has opened a new area to engineer multilayer systems with fascinating properties.

In this thesis proposal, we plan to explore the fundamental aspect of moiré patterns of twisted phosphorene bilayers from our first-principles study. Phosphorene, the monolayer of black phosphorus, is an emerging semiconductor material with layer dependent bandgap, anisotropic structure, high carrier mobility, and anisotropic thermal and mechanical properties. Since its discovery in 2014, phosphorene has fetched a great amount of attention of researchers worldwide. By twisting phosphorene bilayers, a moiré potential is generated which introduces very interesting effects between these bilayers. It has recently been identified as a promising 2D moiré material to explore despite its limited air-stability. Moreover, twisted bilayer phosphorene is expected to possess more chances to exist in the real world owing to its multiple alternatives. Recent theoretical and experimental work have reported interesting features of moiré patterns of twisted phosphorene bilayers [ACS Appl. Nano Matter 2, 3138-3145 (2019), J. Phys.: Condens. Matter 32, 234001 (2020), J. Mater. Chem. C 8, 6264-6272 (2020)]. Because of the unique anisotropic structural and physical properties of phosphorene, the effects of stacking and twisting bilayer phosphorene including the commensurability and local strain should be considered concurrently. Our preliminary study has shown that the equilibrium interlayer distance, the cohesive energy, the band structures, and the distribution of top valence band and bottom conduction band strongly depend on the stacking arrangement, the twisted commensurate angles, and the local strains.

Based on our preliminary results, we plan to perform a systematic study to seek the whole aspect of the moiré physics of phosphorene bilayers by exploring the intriguing anisotropic properties of phosphorene monolayer coupled with stacking and twisting effects in these bilayer nanostructures. We plan to consider various commensurate structures with different stacking arrangement and local strain. Accordingly, we plan to (1) systematically study the moiré physics of phosphorene bilayers with different stacking orders, (2) systematically study the effect of twist angle on the different stacking orders, (3) systematically study strain effects on the moiré patterns of phosphorene bilayers, and finally, (4) systematically study the electronic and optical properties of these optimized nanostructures. The outcome of these studies is expected to provide a fundamental understanding of the intriguing details of the novel structural, electronic, optical, thermal and mechanical properties of these systems and guide current and future works in designing nanoelectronics devices at the microscopic level.

Speaker:  Diptaparna Biswas

Abstract: This thesis presents a search for a dark leptophilic scalar (phi_L) produced in association with taupair in electron-positron annihilation. This search will put strong constraints on new physics models that can explain the origin of dark matter beyond the current understanding of the particle content of the Standard Model. The dark scalar can decay into an electron-positron pair with invariant mass less than the di-muon mass, or into a pair of muon-antimuon above that. We search for the dark scalar in the mass range 30 MeV to 6.5 GeV. The strength of coupling between this leptophilic dark scalar and standard model leptons is proportional to the mass of the lepton it couples with. This makes the decay channel e+e- -> tau+ tau- phi_L to be the one with the highest sensitivity in the mass range being searched for. The goal of this analysis is to discover such a dark scalar particle or improve upon previous upper limits of the dark coupling constant as a function of the dark scalar mass using the data around Y(4S) resonance from the SVD2 dataset of the Belle experiment.

The Belle II detector, successor of this Belle detector, is built around the world’s highest luminosity electron-positron collider SuperKEKB, located at the KEK accelerator complex in Japan. In this thesis, a general overview of the SuperKEKB accelerator and the Belle II experiment is also presented. On the technical contribution part, my job as the deputy of operations and slowcontrol developer for the K-long and Muon (KLM) detector is discussed in this thesis. Also, my contribution in the upgrade of various parts of the Data Acquisition (DAQ) system of this experiment is described in detail.

Speaker: Ali Ibrahim M, Alzahrani

Abstract: A lithography free technique for the direct fabrication of two-dimensional (2D) material-based tunnel junctions has been developed. Graphene/h-BN/Graphene single barrier tunneling devices were fabricated by direct deposition of graphene, h-BN and graphene sequentially using a plasma enhanced chemical vapor deposition technique on Si/SiO₂ substrates. The I-V data were shown to follow the barrier dependent quantum tunneling behavior. The study was further expanded to realize resonant tunneling devices by (i) asymmetric doping of graphene and (ii) double barriers (DB) consisting of Graphene/h-BN/Graphene/h-BN/Graphene. DB Tunneling junctions with varying well widths were studied while different degrees of doping were used for the other devices. The I-V characteristics of tunneling current of both devices showed resonant tunneling behavior at room temperature with a negative differential conductance. The behavior could be explained with quantum mechanical tunneling models. Finally, Josephson tunneling was demonstrated for MgB₂/h-BN/MgB₂ junctions.

Speaker: Anil Sharma, University of Louisville

Abstract: In the past few years, researchers have been exploring the electrical properties of DNA (and related genetic materials) with a hope to use those highly stable structures in nano-electronic technologies. The charge transport mechanism in DNA, which is yet to be fully understood, has been envisioned as a key component in important applications for a wide variety of fields, for example to design miniaturized 3D electronic circuits and to design genetically-specific biosensors. Although large number of experiments have already been performed, a clear rationale for DNA’s conductivity is still lacking. Results for different research groups show a broad range of outcomes with DNA been reported to act as an insulator, semi-conductor, conductor, and in some exceptional case even as an induced super-conductor. The results of these experiments are highly affected by factors like the type (genetic sequence) of DNA used, the length of DNA sample, and temperature/pressure conditions. It also highly depends on whether the experiment is done in dry or liquid environments. Similarly, the results are seen to be influenced by the different methodologies used to immobilize a DNA molecule onto an electrode. One of the important factors to be considered in the conductivity measurement of DNA is the point of contact between the DNA and the electrode. I am interested in exploring the electronic properties of a chain of nucleic acid attached to a gold surface through well-designed immobilization chemistries and in an aqueous environment that is relevant for bioassay applications. I plan to investigate charge transfer between a gold surface and a DNA molecule by using the analytical tools such as electrochemical modulation of plasmonic surface waves, ac voltammetry, and optical impedance spectroscopy. Such knowledge will create the foundation that I plan to use for developing new biosensing strategies that could eventually detect the genetic code of any living being, as for example for the SARS-CoV2 virus.