Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references (leaves 70-77). Available also in an electronic version.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.S.)--University of Texas at Dallas, 2006. / Includes vita. Includes bibliographical references (leaves 50-58)

Thesis (MTech(Electrical Engineering))--Cape Technikon, Cape Town,1999 / This thesis describes the design and implementation of a new feedback controller for a scanning tunneling microscope or STM. The previous controller had several shortcomings when it came to the data throughput rate of the data acquisition system, the scan rate, and the way the data was stored and displayed. The initial investigation was done to determine the most cost effective way to implement the data acquisition system. Various approaches such as DSP systems, analogue systems and microcontroller systems were looked at. The investigation also looked at the best way to get the data from the Z directional control loop to the PC for displaying the data. The final choice was to use an ultra fast microcontroller for the control loop implementation and to change the DOS based software for Windows based software. The embedded system was divided into two parts. The first was the controller for the X and Y scan directions, and the second was for the Z scan direction. A digital PI control loop was implemented on the Z controller to control the height of the scan tip above the specimen surface. The microcontroller that was chosen for this was the Microchip PIC17c43. The data transfer to the PC was done with a PC-14 programmable digital input/output card. Two options for the implementation of the PC-14 software were considered. The first option was the software that was bundled with the card. This software proved to be very slow, so special device-driver-based software was developed to control the PC-14 card and the data transfer to and from the Pc. The PC software was implemented using Visual C++. Both the XY and the Z controllers proved to be working satisfactorily in the existing STM arrangement. It was discovered that the XY controller was overloaded with the many tasks that it has to perform, and a suitable alternative system to replace the XY controller is proposed. The selection of the PC that will be used for the data acquisition system is also discussed. It was found that this choice had a very big influence on the design of the final system because of the difference in PC bus design. Several proposals to increase the functionality of the PC software are also made.

A study of the nucleation, growth and final microstructure of vacuum deposited nickel films has been performed using scanning tunneling microscopy (STM) as the primary research instrument. Typical nucleation conditions are reported for nickel films grown on partially shadowed highly-oriented pyrolytic graphite (HOPG), and techniques are developed for using the STM to catalog film islands instead of more conventional electron microscopes. Values for the activation energy of surface diffusion, critical nucleus size, changes in the saturation nucleation density with temperature, and spatial variations in the nucleation rate are included. Roughening and microstructure changes observed with STM are reported as functions of substrate temperature and deposition angle for nickel films grown on highly-oriented pyrolytic graphite and fused silica. Conventional film RMS roughness values are compared to microRMS values derived from STM data and STM images of film microstructure are compared with SEM and optical microscope photographs.

Semiconductors are characterized by atomic resolution imaging and density of states measurements (DOS) obtained through the use of a scanning tunneling microscope (STM). The DOS of the conduction and valence bands can be measured separately with a STM as opposed to an optical measurement which measures only the joint DOS. Layered-structure semiconductors are characterized both in the bulk form and in the isolated cluster form. Images of three bulk layered-structure semiconductors, MoS₂, WSe₂, and SnS₂, were obtained with both positive and negative sample-to-tip bias voltages. Curves of tunneling current as a function of bias voltage were measured, from which the DOS of the valence and conduction bands can be inferred. We obtained an atomically resolved image of an isolated fragment of a semi-conductor cluster which was deposited on a graphite surface from a colloidal suspension of BiI₃. Also imaged were clusters of MoS₂ layered-structure semiconductors.

Many deductions made about STM images are based upon the model of Tersoff and Hamann, in which images are given in principal by a combination of surface atomic positions and local charge density. There is a now a need for a fuller understanding of this technique in order to explain experimental evidence which indicates that the tip and sample can interact strongly during normal imaging. In order to investigate the fundamental STM imaging process, a method for deducing the tunnel barrier height has been developed which is based on corrugation height measurements of constant current topographs. From experiments on clean Cu(100), values of the tunnel barrier height have been shown to be somewhat below the workfunction (~ 1-2.5eV) but are in good agreement with other reports of atomically resolved barrier height data. At large values of the tunnel conductance (~ 1μS), a fall-off (based upon extrapolation of large separation data) in the corrugation heights is observed with increasing conductance. This effect is quantitatively explained using a Molecular Dynamics simulation of the tip approaching the sample. The simulation gives a good estimate of both the absolute tip-sample separation and site-dependent tip-surface forces. Distributions of corrugation heights indicate that variations in both tip geometry and chemistry are likely to occur in practice and strongly influence the phenomena described above. Similarly, it is found that increased local tunnel barrier heights are measured when the Cu(100) surface is modified with small numbers of single halogen atoms. This data has been used to estimate the contributions to the increase in local barrier height of both adsorbate induced dipoles and geometric topography. Values for the charge transfer between the surface and adsorbate have been established. The process of tip-induced adsorbate manipulation has also been demonstrated at room temperature.

Thesis advisor: Vidya Madhavan / Spin-orbit coupling (SOC) is the interaction of an electron's intrinsic angular momentum (spin) with its orbital momentum. The strength of this interaction is proportional to Z<super>4</super> where Z is the atomic number, so generally it is stronger in atoms with higher atomic number, such as bismuth (Z=83) and iridium (Z=77). In materials composed of such heavy elements, the prominent SOC can be sufficient to modify the band structure of the system and lead to distinct phase of matter. In recent years, SOC has been demonstrated to play a critical role in determining the unusual properties of a variety of compounds. SOC associated materials with exotic electronic states have also provided a fertile platform for studying emergent phenomena as well as new physics. As a consequence, the research on these interesting materials with any insight into understanding the microscopic origin of their unique properties and complex phases is of great importance. In this context, we implement scanning tunneling microscopy (STM) and spectroscopy (STS) to explore the surface states (SS) of the two major categories of SOC involved materials, Bi-based topological insulators (TI) and Ir-based transition metal oxides (TMO). As a powerful tool in surface science which has achieved great success in wide variety of material fields, STM/STS is ideal to study the local density of states of the subject material with nanometer length scales and is able to offer detailed information about the surface electronic structure. In the first part of this thesis, we report on the electronic band structures of three-dimensional TIs Bi₂Te₃ and Bi₂Se₃. Topological insulators are distinct quantum states of matter that have been intensely studied nowadays. Although they behave like ordinary insulators in showing fully gapped bulk bands, they host a topologically protected surface state consisting of two-dimensional massless Dirac fermions which exhibits metallic behavior. Indeed, this unique gapless surface state is a manifestation of the non-trivial topology of the bulk bands, which is recognized to own its existence to the strong SOC. In chapter 3, we utilize quasiparticle interference (QPI) approach to track the Dirac surface states on Bi₂Te₃ up to ~800 meV above the Dirac point. We discover a novel interference pattern at high energies, which probably originates from the impurity-induced spin-orbit scattering in this system that has not been experimentally detected to date. In chapter 4, we discuss the topological SS evolution in (Bi_1-xIn_x)₂Se₃ series, by applying Landau quantization approach to extract the band dispersions on the surface for samples with different indium content. We propose that a topological phase transition may occur in this system when x reaches around 5%, with the experimental signature indicating a possible formation of gapped Dirac cone for the surface state at this doping. In the second part of this thesis, we focus on investigating the electronic structure of the bilayer strontium iridate Sr₃Ir₂O₇. The correlated iridate compounds belong to another domain of SOC materials, where the electronic interaction is involved as well. Specifically, the unexpected Mott insulating state in 5<italic>d</italic>-TMO Sr₂IrO₄ and Sr₃Ir₂O₇ has been suggested originate from the cooperative interplay between the electronic correlations with the comparable SOC, and the latter is even considered as the driving force for the extraordinary ground state in these materials. In chapter 6, we carried out a comprehensive examination of the electronic phase transition from insulating to metallic in Sr₃Ir₂O₇ induced by chemical doping. We observe the subatomic feature close to the insulator-to-metal transition in response with doping different carriers, and provide detailed studies about the local effect of dopants at particular sites on the electronic properties of the system. Additionally, the basic experimental techniques are briefly described in chapter 1, and some background information of the subject materials are reviewed in chapter 2 and chapter 5, respectively. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.