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Radio frequency (RF) cavities are the foundational infrastructure which facilitates much of the fundamental research conducted in high energy particle physics. These RF cavities utilize their unique shape to produce resonant electromagnetic fields used to accelerate charged particles. Beside their core role in fundamental physics research, RF cavities have found application in other disciplines including material science, chemistry and biology which take advantage of their unique light sources. Industry has been keen on taking advantage of accelerator technology for a multitude of applications. Particle accelerators like the one found at Jefferson Lab’s Continuous Electron Beam Accelerator Facility must produce stable beams of high energy particles which is an incredibly costly endeavor to pursue. With the gargantuan size of these facilities, the cost of high-quality beam production is a matter of great importance. The quest to find highly efficient RF cavities has resulted in the widespread use of superconducting radio frequency (SRF) cavities which are the most efficient resonators that exploit a superconductor’s incredibly low AC surface resistance. While metals like Cu are up to the demanding job of RF cavity particle acceleration, their efficiency at transferring RF power to the particle beam is low when they are compared with SRF Nb cavities. Nb is the standard material for all SRF cavity technology particularly for its reproducibly low surface resistance, comparatively high transition temperature and thermodynamic critical field. Using superconducting Nb is not without its drawbacks. Keeping hundreds of Nb cavities in their superconducting state under extreme RF conditions is quite a daunting task. It requires the normal state not nucleate during operation. This is achieved by producing high-quality cavities with as few defects and impurities as possible while also keeping the cavities at low temperature, usually 2K. Again, due to the sheer scale of the facilities, hundred million-dollar cryogenic plants are required to handle the heat loads during SRF cavity operation. This means even small increases in maximum accelerating gradients or decrease in cavity surface resistance results in a sizably reduced operation cost. Considerable effort has been put forth to increase the efficiency of Nb cavities toward and even beyond the theoretical maximum accelerating gradients and quality factor for a clean superconductor. Recently, a new method to produce high quality factor cavities has emerged that involves nitrogen doping the cavity. The mechanism by which N doping causes the improvement is still not well understood, but the experimental research described in this dissertation shines some light into the mechanisms behind such a drastic improvement. These insights are universal for all superconductors and may prove useful for SRF cavities beyond Nb. With Nb approaching its fundamental limits, new materials are being proposed to increase the performance of future SRF cavities which MgB2 finds itself among. MgB2 is a two-band superconductor that possesses many properties that are very attractive for the next generation of SRF cavities. One of the most important properties is MgB2’s comparatively large critical temperature which in part predicts it will have a lower surface resistance than Nb at higher operating temperatures. Such behavior of MgB2 may unlock the possibility of using cryocoolers instead of costly liquid helium plants for large scale industrial use. This dissertation starts with an introduction to superconductivity, its theory, and application to SRF cavities as well as the open questions that can be addressed in Nb and the next generation of SRF materials. A description of the experimental techniques of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy is presented. Our experimental investigation into Nb SRF cavity cutouts starts with a discussion of the material’s limitations for SRF applications with an emphasis on the proximity effect which arises at the surface of this material due to its myriad of naturally forming oxides. The results of our scanning tunneling microscopy measurements for typically prepared Nb and nitrogen doped Nb follows and comparisons are made which show that the surface oxides are fundamentally different between these samples likely resulting in the profound enhancement of the cavity’s quality factor. Experimental investigation into the native oxide of hot spot nitrogen doped Nb shows a degraded oxide and superconducting properties as compared with the cold spot. The dissertation continues with a brief introduction to MgB2, followed by our scanning tunneling and electron tunneling insights into MgB2. The dissertation is concluded with a summary of our investigations and broader impact of our research on the SRF community. / Physics

<p> This thesis consists of two projects: alignment of copper phthalocyanine compounds on Au(111) and sidewall functionalization of single-walled carbon nanotubes on graphite. Both of these projects are performed with scanning tunneling microscopy (STM), which is used to study the structure of modified surfaces that are of interest in molecular electronics.</p> <p> In the first project, copper phthalocyanine compounds are made into a thin film with different methods, such as solution deposition, self-assembly and Langmuir-Blodgett film deposition. Those films are important materials in photoelectric devices such as organic light emitting diodes (OLED's). Molecules in these films are aligned on the solid surface with face-on orientation or edge-on orientation. However, the films of molecules with face-on orientation are preferentially used in LED's. In this project, we focus on finding a method to force molecules with face-on orientation in the film. The structure of copper octakisalkylthiophthalocyanine films on Au(111) was investigated with STM under ambient conditions. Columns of molecules are commonly observed due to the π-π interaction between molecules. The presence and length of alkyl chains in the molecules affects the alignment of molecules on the gold surface. The weak interaction between molecules and substrate caused the structure to be easily modified by an STM tip.</p> <p> In addition, chemical sidewall functionalization of SWCNTs was also explored with STM under ambient conditions. It was found that the spatial distribution of functional groups on nanotube sidewall is not random. Understanding the rules behind the distribution of functional groups will allow scientists to better control carbon nanotube functionalization and improve the properties of nanotubes. High resolution STM images provide direct evidence of the distribution and the effects of functional groups on nanotubes. Possible mechanisms are proposed to elucidate the process of SWCNT functionalization by free radicals and via the Bingel reaction.</p> / Thesis / Master of Science (MSc)

In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta-to-beta' phase transformation was investigated by temperature dependent transmission electron microscopy and Raman spectroscopy, combined with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. STS measurements show that the indirect bandgap of monolayer beta' In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications like non-volatile memory devices. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in two-dimensional (2D) ferroelectric beta' In2Se3 is visualized with scanning tunneling microscopy and spectroscopy (STM/S) combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials. / Doctor of Philosophy / Two-dimensional (2D) materials are materials consisting of a single layer or a few layers of atoms. They exhibit unique and interesting properties distinct from their bulk counterparts. Over the past decade, much effort has been devoted to a large family of 2D materials — 2D metal chalcogenides that exhibit fascinating structural and electronic properties. These 2D metal chalcogenides can also be stacked together to form various heterostructures. The scanning tunneling microscope (STM) is a powerful tool to study these materials with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. It can also be used to manipulate nanometer-scale structures on the material surface. In this dissertation, we use scanning tunneling microscopy and spectroscopy (STM/S) to investigate 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers transform from beta phase to a more stable beta' phase when the sample is cooled down from room temperature to 77 K. This thermally driven beta-to-beta' phase transformation was found to be reversible by temperature dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in 2D ferroelectric beta' In2Se3 is visualized by using STM/S combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

Graphene, one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has grabbed appreciable attention due to its exceptional electronic, mechanical and optical properties. Chemical functionalization schemes are needed to integrate graphene with the different materials required for potential applications. Molecular self-assembly behavior on graphene is a key method to investigate the mechanism of interaction between molecules and graphene and the promising applications related to molecular devices. In this thesis, we report the molecular self-assembly behavior of phenyl-C61-butyric acid methyl ester (PCBM), C60, perylenetetracarboxylic dianhydride (PTCDA) and Gd3N@C80 on flat and rippled graphene 2D material by the experimental methods of scanning tunneling microscope (STM) and atomic force microscope (AFM) and by the theoretical method of density functional theory (DFT). We found that molecules form ordered structures on flat graphene, while they form disordered structure on rippled graphene. For example, PCBM forms bilayer and monolayer structures, C60 and Gd3N@C80 form hexagonal close packed (hcp) structure on flat graphene and PTCDA forms herringbone structure on flat graphene surface. Although C60 and Gd3N@C80 both form hcp structure, C60 forms a highly ordered hcp structure over large areas with little defects and Gd3N@C80 forms hcp structure only over small areas with many defects. These differences of structure that forms on flat graphene is mainly due to the molecule-molecule interactions and the shape of the molecules. We find that the spherical C60 molecules form a quasi-hexagonal close packed (hcp) structure, while the planar PTCDA molecules form a disordered herringbone structure. From DFT calculations, we found that molecules are more effected by the morphology of rippled graphene than the molecule-molecule interaction, while the molecule-molecule interaction plays a main role during the formation process on flat graphene. The results of this study clearly illustrate significant differences in C60 and PTCDA molecular packing on rippled graphene surfaces. / Doctor of Philosophy / As the first physical isolated two-dimensional (2D) material, graphene has attracted exceptional scientific attention. Due to its impressive properties including high carrier density, flexibility and transparency, graphene has numerous potential applications, such as solar cell, sensors and electronics. 2D molecular self-assembly is an area that focuses on organization and interaction between self-assembly behaviors of molecules on surface. Graphene is an excellent substrate for the study of molecular self-assembly behavior, and study of molecular study is very important for graphene due to potential applications of molecules on graphene. In this thesis, we present investigations of the molecular self-assembly of PCBM, C60, PTCDA and Gd3N@C80 on graphene substrate. First, we report the two types of bilayer PCBM configuration on HOPG with a step height of 1.68 nm and 1.23 nm, as well as two types of monolayer PCBM configuration with a step height of 0.7 nm and 0.88 nm, respectively. On graphene, PCBM forms one type of PCBM bilayer with a step height of 1.37 nm and one type of PCBM monolayer with a step height of 0.87 nm. By building and analyzing the models of PCBM bilayers and monolayers, we believe the main differences between two configurations of PCBM bilayer and monolayer is the tilt angle between PCBM and HOPG, which makes type I configuration the higher molecule density and binding energy. Secondly, we report the investigation of self-assembly behaviors of C60 and PTCDA on flat graphene and rippled graphene by experimental scanning tunneling microscope (STM) and theoretical density functional theory (DFT). On flat graphene, C60 forms hexagon close pack (hcp) structure, while PTCDA forms herringbone structure. On rippled graphene, C60 forms quasi-hcp structure while PTCDA forms disordered herringbone structure. By DFT calculation, we study the effect of graphene curvature on spherical C60 and planar PTCDA. Finally, we report a STM study of a monolayer of Gd3N@C80 on graphene substrate. Gd3N@C80 forms hcp structure in a small domain with a step height of 0.88 nm and lattice constant of 1.15 nm. According to our DFT calculation, for the optimal organization of Gd3N@C80 and graphene, the gap between Gd3N@C80 and graphene is 3.3 Å and the binding energy is 0.95 eV. Besides, the distance between Gd3N@C80 and Gd3N@C80 is 3.5 Å and the binding energy is 0.32 eV.