Chemistry

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This is the collection for the University of Waterloo's Department of Chemistry.

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Browsing Chemistry by Author "Baugh, Jonathan"

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    Development and Electronic Characterization of Graphene-Based Hall Effect Devices
    (University of Waterloo, 2024-09-24) Lacroix, Camille; Baugh, Jonathan
    Graphene is a two-dimensional carbon material with a unique honeycomb lattice structure and exceptional electronic properties. Its band structure confines carriers to a single plane, allowing them to act like relativistic massless particles at low carrier densities. This has made graphene a focal point in condensed matter physics, particularly following the groundbreaking discovery of the first topological state using a graphene lattice. Research into graphene's potential as a platform for quantum topological computing has surged. In addition to its distinct band interactions, graphene is also being studied as a potential standard for electrical resistance. However, progress in its isolation since its initial synthesis in 2004 has been limited. This thesis focuses on the synthesis of single-layer graphene (SLG) through low-pressure chemical vapor deposition (LPCVD) on copper films at temperatures above 1000 °C. The graphene films are transferred using a wet transfer technique and characterized with atomic force microscopy (AFM) and Raman spectroscopy. Hall devices for electrical transport studies are patterned using maskless alignment photolithography, with palladium as ohmic contacts. Electronic transport measurements are conducted at cryogenic temperatures up to a magnetic field of 5T using 4-terminal measurement techniques. Moreover, this work explores electronic transport in twisted bilayer graphene (TBG) - tungsten diselenide WSe2 Hall devices. This structure facilitates the study of strongly correlated electronic states enhanced by spin-orbit coupling induced by WSe2. Preliminary experiments to detect unconventional Hall states in similar devices are carried out at millikelvin temperatures and in magnetic fields up to 18 Teslas.
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    Molecular Nanomagnets for Novel Spintronics Devices
    (University of Waterloo, 2016-01-08) Walker, Sean; Baugh, Jonathan
    Molecular nanomagnets possess interesting quantum properties that make them potential candidates for qubits in quantum information processing. Heterometallic antiferromagnetic wheels specifically have been shown to have a coherence time long enough to permit quantum computing operations. The field of molecular spintronics deals with the integration of molecular nanomagnets into nanoelectronic devices for the purpose of probing and manipulating these quantum properties. In order for a nanomagnet to be incorporated into such a device it needs to be both magnetically and structurally stable when in contact with nanoelectronic components, and its coupling to the environment needs to be controlled. The first part of this thesis deals with the synthesis and characterization of a derivative of a member of the Cr7Ni family of heterometallic antiferromagnetic wheels. The synthetic process involved introducing long alkyl chains into the organic shell of the nanomagnet in order that it may interact with carbon-based nanoelectronic devices in a non-destructive capacity. The molecule was characterized in order to confirm the results of the synthesis, to gain a greater understanding of how its magnetic properties can be modelled, and to fingerprint the system in order to acquire data that will help determine whether its properties remain intact when attached to a graphene surface. The second part of this thesis concerns a series of experiments conducted toward developing a process for determining the viability of the synthesized nanomagnet as the molecular component in spintronics devices. To begin to determine the molecule's magnetic and structural stability when deposited on a graphene surface the first step is to realize clean graphene substrates that are suitably pristine such that a nanometre sized particle can be detected. Graphene flakes were fabricated using the mechanical exfoliation technique and a procedure was developed for cleaning the resulting flakes of residues. The next step of this research will consist of conducting systematic studies to quantify the binding affinity of the nanomagnet species to both graphene and to pristine carbon nanotubes, and to determine whether the system retains its structural and magnetic properties when attached to graphitic surfaces. The work described here lays the foundation for the novel use of Cr7Ni-eth and other functionalized magnetic molecules in spin-based nanoelectronic devices.
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    Proximity Superconductivity in Indium Arsenide Two-Dimensional Electron Gas Devices
    (University of Waterloo, 2024-09-23) Thompson, Fiona; Baugh, Jonathan
    Of the many theoretical proposals for quantum computers, topological quantum computing is unique in its resistance to decoherence and the reliability of its gate operations. One proposed method for achieving these topological qubits is to harness the unusual non-Abelian exchange statistics of quasiparticle excitations known as Majorana bound states. Historically, research devoted to realizing these states has primarily been in nanowires, but purely one-dimensional devices are limited in their applications. Two-dimensional electron gas devices are an alternative with the benefit of future scalability and increased options for device geometries. To this end, we developed InAs/AlGaSb surface quantum well devices compatible with the proximity-induced superconductivity required to realize a Majorana device. Magnetotransport measurements investigating mobility-density relationships, I-V characteristics, the Shubnikov de Haas effect, and the quantum Hall effect confirm the very high quality of our dielectric deposition method and growths. Even with quantum wells so near the surface of the device, we achieve high mobilities and stable, reproducible gating characteristics. These devices have high spin-orbit coupling coefficients, confirming that we can simultaneously benefit from the inherent bulk properties of InAs and properties imparted by the rest of the growth and lithography steps. Analytical comparisons of devices with different quantum well widths, interface characters, and dielectric deposition methods reinforce the need for the rigorous optimization of numerous factors. From this analysis, we conclude that devices with smooth surface morphologies, SiO2 dielectric deposited by atomic layer deposition, and InSb-like interfaces provide the ingredients necessary to achieve near-record mobilities and consistent gating properties. On these same excellent wafers, we fabricated superconductor-normal-superconductor (SNS)-type devices of three different normal region dimensions with ex-situ deposited niobium as the proximitizing superconductor. The universally high quality of these devices challenges the long-held norm that epitaxial aluminum is the best choice for the superconductor in these types of devices. Specifically, we achieved figures of merit much higher than those previously reported in Nb-InAs-Nb devices and on par with those using epitaxial aluminum. Using two separate mathematical models, we found that our devices have very high transparencies, indicating high-quality interfaces. Detailed plans for future devices are also discussed in this thesis, including gated SNS devices, quantum point contacts, and an attempt at observing Majorana signatures.
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    Scanning Tunneling Microscopy of Electrically Driven Phase Transitions in a Charge Density Wave Material
    (University of Waterloo, 2022-03-25) Walker, Sean; Baugh, Jonathan
    The 1T polytype of the van der Waals material TaS2, has been studied extensively as a strongly correlated system. Exhibiting several different charge density wave (CDW) states, the phase diagram of 1T-TaS2 has been explored for close to fifty years, as the forces driving the formation of the charge modulation and determining the characteristics of each of the phases, have been elucidated. In recent years, interest in this system has concerned expanding the phase diagram, with the bulk equilibrium states being further probed and manipulated. While much of the research has focused on the low-temperature phase, where 1T-TaS2 has emerged as a test bed for Mott physics, as the material is thinned towards the 2D limit, its phase diagram shows significant deviations from that of the bulk system, even in the higher temperature range. Optoelectronic maps of ultrathin (< 10 nm thick) 1T-TaS2 have indicated the presence of non-equilibrium CDW phases within the hysteresis region of the nearly commensurate (NC) to commensurate (C) transition. The work in this thesis investigates the nature of these non-equilibrium phases, and in doing so, further elucidates the phase diagram of 1T-TaS2. We perform scanning tunneling microscopy (STM) on exfoliated ultrathin flakes of 1T-TaS2 within the NC-C hysteresis window. Imaging exfoliated flakes using STM poses certain difficulties. Specifically, STM is very sensitive to surface contamination and is ill-suited for locating a specific region of interest in a sample with micron scale dimensions. We address the challenges associated with performing STM on exfoliated materials, utilizing a simple device design that allows for the in situ measurement of both the electrical properties of an exfoliated flake and the topography of the flake. With such a device design, it is possible to correlate changes in electronic structure with changes in the bulk properties of the material. When imaging ultrathin 1T-TaS2 within the NC-C transition region, we find that rather than possessing distinct electronic order, the topography of the flake indicates the presence of intertwined, irregularly shaped NC-like and C-like domains. After applying lateral electrical signals to the sample, we image changes in the geometric arrangement of the different regions. The ability to measure the change in electronic structure with the application of a driving signal provides an invaluable perspective on the evolution of the CDW phases in the material at the nanoscale. Inhomogeneity similar to what we measure in ultrathin 1T-TaS2, has been seen in related strongly correlated systems, such as perovskite manganites and doped Mott insulators. Starting with a phase separation model to simulate the observed inhomogeneity, we incorporate ideas derived from percolation theory to explore the relationship between the electronic structure present in ultrathin 1T-TaS2 and its bulk resistivity. With this model, we are able to qualitatively reproduce many of the features observed when driving the inhomogeneous CDW state. These results highlight the importance of understanding the role of phase competition in determining the properties of strongly correlated systems.