This dissertation presents results of a project bringing Scanning Tunneling Microscope (STM) into a regime of unlimited operational time at cryogenic conditions. Freedom from liquid helium consumption was achieved and technical characteristics of the instrument are reported, including record low noise for a scanning probe instrument coupled to a close-cycle cryostat, which allows for atomically resolved imaging, and record low thermal drift. Subsequent studies showed that the new STM opened new prospects in nanoscience research by enabling Scanning Tunneling Spectroscopic (STS) spatial mapping to reveal details of the electronic structure in real space for molecules and low-dimensional nanomaterials, for which this depth of investigation was previously prohibitively expensive. Quantum-confined electronic states were studied in single-walled carbon nanotubes (SWCNTs) deposited on the Au(111) surface. Localization on the nanometer-scale was discovered to produce a local vibronic manifold resulting from the localization-enhanced electron-vibrational coupling. STS showed the vibrational overtones, identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. This study experimentally connected the properties of well-defined localized electronic states to the properties of associated vibronic states. Electronic structures of alkyl-substituted oligothiophenes with different backbone lengths were studied and correlated with torsional conformations assumed on the Au(111) surface. The molecules adopted distinct planar conformations with alkyl ligands forming cis- or trans- mutual orientations and at higher coverage self-assembled into ordered structures, binding to each other via interdigitated alkyl ligands. STS maps visualized, in real space, particle-in-a-box-like molecular orbitals. Shorter quaterthiophenes have substantially varying orbital energies because of local variations in surface reactivity. Different conformers of longer oligothiophenes with significant geometrical distortions of the oligothiophene backbones surprisingly exhibited similar electronic structures, indicating insensitivity of interaction with the surface to molecular conformation. Electronic states for annealed ligand-free lead sulfide nanocrystals were investigated, as well as hydrogen-passivated silicon nanocrystals, supported on the Au(111) surface. Delocalized quantum-confined states and localized defect-related states were identified, for the first time, via STS spatial mapping. Physical mechanisms, involving surface reconstruction or single-atom defects, were proposed for surface state formation to explain the observed spatial behavior of the electronic density of states. This dissertation includes previously published co-authored material.

The growth mechanism of chemical vapor deposition (CVD) grown homo-epitaxial diamond (110) and (111) films was studied using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). In addition, the field emission properties of diamond coated molybdenum microtips were studied as a function of exposure to different gases.

Thesis advisor: Ilija Zeljkovic / We used spectroscopic-imaging scanning tunneling microscopy (SI-STM) and spin-polarized STM (SP-STM) to unveil new electronic phenomena in several different quantum systems. We explored: (1) a potential topological superconductor heterostructure Bi₂Te₃/Fe(Te, Se), (2) high-Tc superconductors − Bi₂Sr₂CaCu₂O₈₊ₓ and Fe(Te, Se), and (3) doped spin-orbit Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. In Bi₂Te₃/Fe(Te, Se), we observed superconductivity (SC) on the surface of Bi₂Te₃ thin film, induced by the iron-based superconductor substrate. By annealing the optimally-doped cuprate superconductor Bi₂Sr₂CaCu₂O₈₊ₓ, we drastically lowered the surface hole doping concentration to detect a unidirectional charge stripe order, the first reported charge order on an insulating (defined by the spectral gap with zero conductance spanning the Fermi level) cuprates surface. In the high-Tc SC Fe(Te, Se) single crystal, we found local regions of electronic nematicity, characterized by C₂ quasiparticle interference (QPI) induced by Fermi surface anisotropy and inequivalent spectral weight of dyz and dxz orbitals near Fermi level. Interestingly, the nematic order is locally strongly anti-correlated with superconductivity. Finally, utilizing SP-STM, we observed a short-range antiferromagnetic (AF) order near the insulator-metal transition (IMT) in spin-orbital Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. The AF order inhomogeneity is found not to be strongly correlated with the charge gap. Interestingly, the AF order in the bi-layered Sr₃Ir₂O₇ shows residual memory behavior with temperature cycling. Overall, our work revealed new phenomena in a range of today’s most intriguing materials and set the stage for using SP-STM in other complex oxides. / Thesis (PhD) — Boston College, 2020. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

No description available.

Two dimensional (2D) materials, including graphene, hexagonal boron nitride and layered transition metal dichalcogenides (TMDs), have been a revolution in condensed matter physics and they are at the forefront of recent scientific research. They are being explored for their unusual electronic, optical and magnetic properties with special interest in their potential uses for sensing, information processing and memory. Molybdenum disulfide (MoS2) has been the flagship semiconducting TMD over the past ten years due to its unique electronic, optical and mechanical properties. In this thesis, we grow mono- to few layer MoS2 films using ambient pressure chemical vapor depositions (AP-CVD) to obtain high quality samples. We employ low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) to study the effect of layer number on the electronic density of states (DOS) of MoS¬2. We find a reduction of the magnitude of the quasiparticle band gap from one to two monolayers (MLs) thick. This reduction is found to be due mainly to a shift of the valence band maxima (VBM) where the conduction band minimum (CBM) does not change dramatically. Density functional theory (DFT) modeling of this system shows that the overlap of the interfacial S-pz orbitals is responsible for shifting the valence band edge at the Γ-point toward the Fermi level (EF), reducing the magnitude of the band gap. Additionally, we show that the crystallographic orientation of monolayer MoS2 with respect to the HOPG substrate can also affect the electronic DOS. This is demonstrated with five different monolayer regions having each with a unique relative crystallographic orientation to the underlying substrate. We find that the quasiparticle band gap is closely related to the moiré pattern periodicity, specifically the larger the moiré periodicity the larger the band gap. Using DFT, we find that artificially increasing the interaction between the film and the substrate means that the magnitude of the band gap reduces. This indicates that the moiré pattern period acts like a barometer for interlayer coupling. We investigate the effect of defects, both point and extended defects, on the electronic properties of mono- to few layer MoS¬2 films. Atomic point defects such including Mo interstitials, S vacancies and O substitutions are identified by STM topography. Two adjacent defects were investigated spectroscopically and found to greatly reduce the quasiparticle band gap and arguments were made to suggest that they are Mo-Sx complex vacancies. Similarly, grain boundaries were found to reduce the band gap to approximately ¼ of the gap found on the pristine film. We use Kelvin probe force microscopy (KPFM) to investigate the affect of annealing the films in UHV. The work function measurements show metastable states are created after the annealing that relax over time to equilibrium values of the work function. Scanning transmission electron microscopy (STEM) is used to show that S vacancies can recombine over time offering a feasible mechanism for the work function changes observed in KPFM. Lastly, we report how strain affects the quasiparticle band gap of monolayer MoS2 by bending the substrate using a custom built STM sample holder. We find that the local, atomic-scale strain can be determined by a careful calibration procedure and a modified, real-space Lawler Fujita algorithm. We find that the band gap of MoS2 reduces with strain at a rate of approximately 400 meV/% up to a maximum strain of 3.1%, after which the film can slip with respect to the substrate. We find evidence of this slipping as nanoscale ripples and wrinkling whose local strain fields alter the local electronic DOS. / Physics

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Inventado no início dos anos 1980, o STM evoluiu para uma importante ferramenta na investigação das propriedades de superfícies e interfaces, com aplicações em várias áreas da ciência além da Física, como Ciências de Materiais e Química. O presente trabalho trata da microscopia de tunelamento, que se baseia no fenômeno do tunelamento quântico dos elétrons. No STM, a distância entre ponta e a amostra é reduzida até que as funções de onda dos elétrons na ponta e na superfície da amostra se sobreponham. Nessa situação, observa-se o fenômeno do tunelamento quântico de elétrons através da barreira formada entre os dois eletrodos (ponta e amostra). Como o valor da corrente de tunelamento é fortemente dependente da distância ponta-amostra, um microscópio STM pode ser utilizado para mapear a morfologia da superfície da amostra com alta resolução espacial. Além disso, outra importante capacidade do STM é a possibilidade de atuar no modo espectroscópico (STS). Por vezes, o estudo detalhado das propriedades de um sistema requer o uso de métodos não convencionais de microscopia STM. Um exemplo é o estudo do magnetismo de nanoestruturas por microscopia de tunelamento com resolução em spin (SP-STM). A implementação destes métodos não convencionais normalmente exigem recursos experimentais específicos, nem sempre disponíveis em equipamentos comercias. A versatilidade no controle das características funcionais do equipamento foi a principal razão que nos motivou a construir o microscópio STM. No primeiro capitulo faremos uma introdução geral ao tema da dissertação. Uma breve introdução ao método de STM será dada no capítulo 2, incluindo aspectos fundamentais do tunelamento quântico, bem como sua aplicação técngeralica. O capítulo 3 descreve o estudo das propriedades eletrônicas e magnéticas do grafeno preparado sobre a superfície vicinal Ni(977). O magnetismo observado na camada de grafeno induzido por um substrato ferromagnético é de grande interessante para o desenvolvimento de dispositivos spintrônicos. Relatamos a investigação das propriedades de uma monocamada de grafeno preparada sobre de Ni(977) por CVD, utilizando a microscopia de varredura por tunelamento (STM), dicroísmo circular magnético de raios-x (XMCD) e espectroscopia de fotoelétrons (XPS). No capítulo 4, apresentamos o detalhamento do projeto de construção de um STM para operar em ambiente de ultra alto vácuo (UHV). Características do STM como isolamento de vibração, desenho mecânico, sistema de varredura e processo de preparação de pontas são discutidos. Por fim, analisamos os resultados dos testes de operação no microscópio STM, dificuldades observadas ao longo do projeto e possíveis melhorias. / Invented in the early 1980s, STM has evolved into a standard tool to investigate the properties of surfaces and interfaces, with applications in various research fields, such as physics, material sciences and chemistry. The present work deals with scanning tunnelling microscopy, which refers to the quantum phenomenon of electron tunneling through a potential barrier. The distance between a conductive probe tip and sample is reduced until the electron wave functions of tip and sample surface have significant overlap, and electrons can tunnel through the vacuum barrier. As the so-detected tunneling current is strongly distance-dependant, it can be used to map the morphology of the sample surface with a resolution which goes far beyond the actual meaning of the term microscopy. Besides its unique spatial resolution, one strength of the STM is the possibility to perform local electronic spectroscopy. STM has evolved into a standard tool to investigate the properties of surfaces and interfaces, with applications in various research fields besides physics, such as material sciences, chemistry, or biology. Nevertheless, the detailed study of materials properties requires the use of non-conventional methods of STM microscopy. One example is the study of magnetism in nanostructures by spin polarized scanning tunneling microscope (Sp-STM). The implementation of such unconventional methods typically requires specific experimental features, not always available in commercial equipment. This versatility in controlling the functional characteristics of the equipment was one of the main reasons that motivated us to design and build our own STM microscope. A general introduction to the dissertation theme is presented in chapter 1. In chapter 2, a brief discussion about the method of STM is developed, including theoretical aspects on quantum tunneling. In chapter 3, the experimental study of electronic and magnetic properties of the graphene/Ni(977) is shown. The recent observation of magnetism in graphene layers induced by a ferromagnetic substrate is a very interesting issue and can impact the design of new carbon-based spintronic devices. Here we report on the investigation of the electronic and magnetic properties of the graphene/Ni(977) by using scanning tunneling microscope (STM), x-ray magnetic circular dichroism (XMCD) and x-ray photoelectron spectroscopy (XPS). In chapter 4, it was described the project and construction of a homemade STM operated in ultrahigh vacuum (UHV) condition. We discuss the details of main STM-UHV components: main and sample preparation chambers, STM head, tip scanner, and sample holder design. Characteristics of the mechanical and electronic design, vibration isolating system, tip and sample preparation are discussed. Finally, we report the results of testing experiments as well as discuss the encountered difficulties and some possible solutions.

Since its physical isolation via the "scotch tape method," graphene (a monolayer of graphite) has attracted much attention from both the solid-state and high-energy scientific communities because its elementary excitations mimic relativistic chiral fermions. This has allowed graphene to act as a testbed for exploring exotic forms of symmetry breaking and for verifying certain longstanding theoretical predictions dating back to the very first formulation of relativistic quantum mechanics. In this dissertation I describe scanning tunneling microscopy and spectroscopy experiments that visualize ordered electronic states in graphene that originate from its unique chiral structure. Two detailed investigations of chemical vapor deposition graphene grown on copper are presented. In the first, a heretofore unrealized phase of graphene with broken chiral symmetry called the Kekulé distortion is directly visualized. In this phase, the graphene bond symmetry breaks and manifests as a (√3×√3)R30° charge density wave. I show that its origin lies in the interactions between individual vacancies ("ghost adatoms") in the crystalline copper substrate that are mediated electronically by the graphene. These interactions induce the formation of a hidden order in the positions of the ghost adatoms that coincides with Kekulé bond order in the graphene itself. I then show that the transition temperature for this ordering is 300K, suggesting that Kekulé ordering occurs via enhanced vacancy diffusion at high temperature. In the second, Klein tunneling of electrons is visualized for the first time. Here, quasi-circular regions of the copper substrate underneath graphene act as potential barriers that can scatter and transmit electrons. At certain energies, the relativistic chiral fermions in graphene that Klein scatter from these barriers are shown to fulfill resonance conditions such that the transmitted electrons become trapped and form standing waves. These resonant modes are visualized with detailed spectroscopic images with atomic resolution that agree well with theoretical calculations. The trapping time is shown to depend critically on the angular momenta quantum number of the resonant state and the radius of the trapping potential, with smaller radii displaying the weakest trapping.

Iron-based high temperature superconductors are a large family of materials that exhibit unconventional superconductivity and arise from antiferromagnetically-ordered parent compounds. One of the grand challenges in understanding the behavior of these materials is determining the physical mechanisms responsible for the transition into the superconducting state. This thesis describes two recent investigations to explore the magnetic and superconducting properties of NaFeAs in response to changes in temperature and nickel dopants. The peculiar interplay of magnetism and superconductivity in nickel-doped NaFeAs is elucidated using muon spin rotation. Our experimental findings on this novel system are supported with both computational and theoretical calculations. The second investigation describes an improvement to the analysis framework to the scanning tunneling microscopy technique that leverages recent advances in nonconvex optimization. This novel approach is applied directly to microscopy images of NaFeAs to provide unprecedented phase-sensitive access to the quasiparticle scattering spectrum in the material. These results place constraints on theoretical models that describe the local electronic structure and physics of NaFeAs.