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.

Seminario para optar al grado de Ingeniero Comercial, Mención Administración de Empresas / No autorizada por el autor para ser publicada a texto completo / En el último tiempo se ha manifestado una nueva arista en las investigaciones de las finanzas corporativas, ésta es el entender el rol que tiene la propiedad y el control de las empresas. En línea con esto es que el investigar si las empresas realizan actividades de tunneling es un aporte para comprender este campo de las finanzas. En los distintos campos de investigación han surgido variados estudios en la última década para evidenciar dicho fenómeno en diversos países. No obstante evidencia para Sudamérica y Chile es poco vista básicamente por la falta de un marco teórico que detalle los distintos modos de evidenciarse o estudiarse del tunneling. Dicho esto el presente documento intenta dar un marco teórico de todo lo que éste fenómeno refiere para luego poder realizar alguna investigación que busque responder ante la interrogante de la existencia de actividades de tunneling en empresas no financieras chilenas. El tema a investigar se centra en los problemas de agencia de las empresas en Chile, conflicto que tiene como base la divergencia entre accionistas mayoritarios y minoritarios y trata de investigar qué ocurre cuando hay accionistas administradores (insiders), en cuanto a las decisiones para determinar la estructura de capital en Chile. El trasfondo es determinar si las empresas que tienen accionistas con altos derechos de control realizan tunneling cuando son grupos empresariales piramidales o con controladores concentrados, tunneling que se estima se puede ver reflejado en los altos ratios de apalancamiento y en algunas cuentas contables. Importante es destacar que la identificación del fenómeno como el resguardo de accionistas minoritarios ante este es mínimo ya que en mercados como el chileno existe poca protección a los accionistas minoritarios, los accionistas administradores o controladores pedirán deuda excesiva para realizar el “túnel” entre estas transacciones entre empresas relacionadas o aprovechando el poder gerencial para tener excesivas compensaciones con los ejecutivos. En términos generales se intenta dar una explicación y un marco teórico detallado de lo que el fenómeno tunneling refiere para dar pie a alguna investigación de evidencia a la práctica en Chile o demostrar que el sistema legal existente impone barreras efectivas para la no realización de esta práctica. Del mismo modo se podría ver la relación entre esta actividad y la concentración de las empresas, si la participación de algún grupo económico tiene influencia en la realización de esta práctica, lo mismo con empresas familiares. Por lo tanto este documento tiene una orientación teórica que entregue una base para una posterior investigación práctica. En la sección 2 del documento se dará a conocer un marco teórico para las definiciones que existen en la literatura actual de tunneling, Luego en la sección 3 se establecerán los distintos tipos de prácticas con los que la ya mencionada actividad puede llevarse a cabo. En la sección 4 del documento se da respaldo al ya mencionado marco legal y su eficiencia en cuanto a existencia y aplicación, también se dan líneas generales de cómo son los marcos regulatorios de economías emergentes y cuáles podrían ser sus efectos en la realización de tunneling. En la sección 5 se analiza uno de los principales problemas y probablemente la mayor causa de muchas de las evidencias existentes de la realización de la práctica y por último en la sección 6 se puede ver la bibliografía utilizada para respaldar cada uno de los puntos discutidos.

Plasmonic structures with nanometer scale gaps provide localized field enhancement and allow for engineering of the optical response, which is well described by conventional classical models. For subnanometer scale gaps, quantum effects and nonlocal effects become important and classical electromagnetics fail to describe the plasmonic coupling response. Coupled plasmonic system of gold nanoparticles on top of thin gold film separated with self-assembled monolayers (SAMs) provides a convenient geometry to experimentally explore plasmonic features in subnanometer scale gaps. However, the surface roughness of the thin metal film can significantly influence the plasmonic coupling properties. In this dissertation, I suggest modifying the coupled nanoparticles-film structures by using ultraflat thin metal films. Using these structures, I investigated the far-field optical response for gap size variations by dark field scattering measurements. A red-shift of the plasmon resonance wavelength was observed by reducing the gap width. However, I did not observe the previously reported saturation trend of the resonance shift for subnanometer scale gaps. I attribute the difference to surface roughness effects in past works since as they were not present in my studies with ultraflat films. To study the near-field enhancement in subnanometer scale gaps, I used third harmonic generation as a method that is highly sensitive (as the third power) to the local field intensity. The onset of the quantum tunneling regime was determined for gap thicknesses of 0.51 nm, where there was a sudden drop in the third harmonic when the gap width decreases from 0.69 nm to 0.51 nm. The experimental observations were consistent with analytical calculations that applied the quantum-corrected model for SAM separating two gold regions. In comparison to the gap without SAMs in which the onset of the tunneling regime was reported at 0.31 nm, the onset of tunneling across the gap with SAM occurred for larger gaps. This was an expected outcome because the material in the gap reduced the barrier height to tunneling. Furthermore, I investigated the wavelength dependence of the third harmonic generation for the gold plasmonic system to determine the role of the interband transitions in the nonlinear response of gold. Past works reported a strong wavelength dependence of the nonlinear response of gold for the fundamental wavelength at about 550 nm, attributed to the interband transitions between the 5d to 6s-6p bands. However, the roles of the interband transitions and wavelength-dependent field enhancement in the nonlinear response of gold was not investigated. In this dissertation, results showed the third harmonic generation enhanced by an order of magnitude by the interband transition (as compared to the non-resonant case). In my research I also used an analytic model for the dielectric function of gold in which contributions of the interband transitions were considered. This model was also consistent with the experimental observations. / Graduate / 0752 / 0544 / Ghazal.hajisalem@gmail.com

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.

MSc, Faculty of Science, University of the Witswatersrand, 2011 / The objective of this project is to understand the details of the electronic transport in low dimensional carbon structures at low temperatures as well as high magnetic fields. The emphasis is on the quasi-2 dimensional thin grain boundary regions of nanodiamond films and one dimensional carbon nanotubes. As such nitrogen “doped” and undoped nanodiamond films were synthesized by the hot filament chemical vapor deposition method (HFCVD). The films were micro-structurally and electrically characterized using several techniques such as Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy and magnetoresistance (MR) measurements. The electronic transport properties were compared to the films deposited by microwave plasma enhanced chemical vapour deposition (MWCVD). The conductivity revealed a typical semiconducting and semi-metallic behavior for the HFCVD films depending on the nitrogen percentage in the chamber. The dephasing time of the electronic wave function was found to be weakly temperature dependant i.e. τ T-p with p < 1, a behavior reported in artificial superlattices. These results show potential application of these materials in novel nano-electronic devices. Previously the transport mechanism in nanodiamond films has been attributed to hopping conduction in the grain boundaries which is predominately disordered sp2 phases. Our studies on nanodiamond films have however shown different mechanisms in these films. We observed very little contribution from hopping and pronounced weak localization contributions in nanodiamond films. We thus establish the significance of tunneling transport in nanodiamond films. We also studied the electronic transport in films of metal filled multiwalled carbon nanotubes which show significant contribution from the hopping mechanism and a negative magnetoresistance at low fields that crosses over into positive MR at high magnetic fields.

Wong Yuen-yee. / Thesis submitted in: August 2005. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references (leaves 121-125). / Abstracts in English and Chinese. / THESIS COMMITTEE --- p.ii / ABSTRACT --- p.iii / 摘要 --- p.iv / ACKNOWLEDGEMENTS --- p.v / TABLE OF CONTENT --- p.vi / LIST OF TABLES --- p.viii / LIST OF FIGURES --- p.xi / Chapter Chapter 1 --- Introduction --- p.1 / Chapter Chapter 2 --- Quantum Mechanical Background --- p.9 / Chapter 2.1 --- Molecular energy operator and the Born-Oppenheimer approximation --- p.9 / Chapter 2.2 --- Electronic wavefunctions --- p.16 / Chapter 2.3 --- Vibrational wavefunctions --- p.22 / Chapter 2.4 --- Rotational wave function --- p.28 / Chapter 2.5 --- Tunneling Phenomenon --- p.31 / Chapter Chapter 3 --- Implementation --- p.35 / Chapter 3.1 --- Inversion potential --- p.37 / Chapter 3.2 --- Zero-field vibrational wavefunctions --- p.39 / Chapter 3.3 --- Zero-field rovibrational levels --- p.43 / Chapter 3.4 --- Stark energy and rovibrational levels in DC fields --- p.49 / Chapter 3.5 --- Transition dipole moments and intensities calculations --- p.52 / Chapter Chapter 4 --- Results and Discussions --- p.57 / Chapter 4.1 --- "Inversion potentials, levels, and energies" --- p.57 / Chapter 4.2 --- Rovibrational levels and spectrum at zero-field --- p.70 / Chapter 4.3 --- Rovibrational spectrum in DC fields --- p.83 / Chapter 4.3.1 --- Localization of vibrational wavefunctions --- p.83 / Chapter 4.3.2 --- Rovibrational energies and spectra --- p.89 / Chapter 4.4 --- Discussion --- p.118 / REFERENCE --- p.121

No description available.

A program was written in 'C' to control the functions of an already existing Scanning Tunneling Microscope (STM). A DAS-1601 data acquisition card (from Keithley Data Acquisition) was installed together with its drivers for 'C' on a computer with a 486-DX motherboard. The computer was interfaced to the electronics of the STM. Images taken of HOPG (highly oriented pyrolitic graphite) were of a reasonable quality and showed atomic resolution.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1981. / Vita. / Bibliography: leaves 232-235. / by Mark Hing Chuen Chan. / Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Civil Engineering, 1981.