Das Ziel der vorliegenden Arbeit ist es ein Molekül zu finden und mittels hochauflösender Techniken zu untersuchen, das auf passivierten Halbleiteroberflächen als Schalter in atomaren Schaltkreisen wirken kann. Für diesen Zweck stehen Moleküle zur Verfügung, die aus mindestens einem aromatischen Ring und einer Ankergruppe bestehen, die kovalent auf Silizium bindet. Um einzelne Moleküle auf leitenden Substraten zu untersuchen, hat sich die Nutzung eines Tieftemperatur-Rastertunnelmikroskops (low-temperature scanning tunneling microscope, LT-STM) als geeignetes Werkzeug erwiesen. Zum Einen ist damit die topographische und spektroskopische Charakterisierung von leitenden Proben auf atomarer Ebene möglich, zum Anderen können einzelne Moleküle und Nanostrukturen hochpräzise bewegt oder elektrisch angesprochen werden. Atomare Schaltkreise können besonders präzise auf passivierten Halbleiteroberflächen hergestellt werden. So ist es zum Beispiel möglich, eine Reihe Wasserstoffatome gezielt mit Hilfe einer STM-Spitze von der Oberfläche zu desorbieren. Durch die Überlappung der dann freien Orbitale entstehen, je nach Richtung auf der Oberfläche, atomare Drähte mit unterschiedlichen elektrischen Eigenschaften. Da die Drähte empfindlich hinsichtlich ihrer chemischen Umgebung sind, können diese auch als logische Schaltelemente verwendet werden. Dafür werden die Drähte mit einzelnen Molekülen angesteuert. Geeignete Schaltmoleküle wurden zunächst auf der Au(111)-Oberfläche getestet. Dabei konnten grundlegende und interessante Eigenschaften von selbst-assemblierten Strukturen untersucht werden. Am Modellsystem von nicht-kovalent gebundenen 4-Acetylbiphenyl-Nanostrukturen auf Gold (111) wurde eine neue Methode entwickelt diese Molekülgruppen behutsam zu bewegen. Durch Anlegen eines Spannungspulses auf den Nanostrukturen konnten diese auf der Oberfläche über weite Strecken gezielt und ohne Beeinflussung der internen Struktur positioniert werden. Um Moleküle für zukünftige elektronische Anwendungen zu untersuchen wurde zunächst das Verfahren zur Präparation von sauberen Siliziumoberflächen in die hier verwendeten Anlage implementiert. Es konnten reproduzierbar saubere, (2×1) rekonstruierte Si(100)- Oberflächen präpariert und charakterisiert werden. Nach der erfolgreichen Präparation von Silizium-Oberflächen und der Entwicklung geeigneter Präparationsrezepte für das Schalter-Molekül 4-Acetylbiphenyl (ABP) wurden beide Systeme vereint. Das Molekül konnte erfolgreich auf die Silizium(100)-Oberfläche aufgebracht und die native Adsorptionskonfiguration durch das Anlegen von Spannungspulsen geändert werden. Das Schalten zwischen zwei Konfigurationen ist reproduzierbar und umkehrbar. ABP ist somit der erste umkehrbare molekulare Schalter, der jemals auf Silizium realisiert werden konnte. Bei der Untersuchung technomimetischer Moleküle in Radachsen-Form konnte bisher die Rollbewegung nur anhand der Analyse der Manipulationskurven nachvollzogen und belegt werden. In dieser Arbeit wurde das Rollen eines Nano-Radmoleküls bewiesen. Dazu wurde bei der Synthese in einem Teil der Subphthalocyanin-Räder eine Markierung in Form eines Stickstoffatoms gesetzt. Bei der lateralen Manipulation der Räder auf Gold(111) konnte dann durch Vergleich der STM-Bilder die Markierung verfolgt und darauf geschlossen werden, ob das Rad gerollt oder verschoben wurde. / The aim of this thesis is the investigation of switching properties of single organic molecules, which can be used as molecular latches on a passivated silicon surface. Suitable molecules should be composed of an anchor group that can bind covalently to the silicon surface as well as an aromatic ring for the latching effect. For the imaging as well as the manipulation of single molecules on conductive substrates, a low-temperature scanning tunneling microscope, LT-STM, is a versatile and powerful tool. On the one hand, STM provides topographical and spectroscopic characterization of single molecules on conductive surfaces at the atomic level. On the other hand, under the tip of a STM single molecules and nanostructures can be moved with atomic precision or can be addressed by voltage pulses. Moreover, by STM it is possible to build atomic-scale circuits on passivated semiconducting surfaces as silicon (100). The STM tip is used to extract single hydrogen atoms from the surface to built atomic wires. As the orbitals of the depassivated dangling bonds of the silicon surface overlap differently depending on the direction of the wire in reference to the surface reconstruction, the electrical properties of the wires differ. Moreover, the properties of the wires vary depending on the chemical environment. Taking advantage of these characteristics, the atomic wires can be used as atomic-scale logic elements. However, to bring the input signal to a single logic element, latches are required to controllably passivate and depassivate single dangling-bond pairs. During preliminary studies on possible molecular latches, interesting experiments could be performed on 4-acetylbiphenyl (ABP) on Au(111). The molecules self assemble in non-covalently bond groups of three or four molecules. These groups can be moved controllably by applying voltage pulses on top of the supramolecular structure. The manipulation is possible over long ranges and without losing the internal structure of the assemblies. For the investigation of promising candidates for future molecular electronics on silicon, a preparation procedure tailored to the used UHV machine was developed. During this process, clean (2×1) reconstructed Si(100) surfaces could be prepared reproducibly and were characterized by means of STM imaging and spectroscopy. Switches are essential for electronic circuitry, on macroscopic as well as microscopic level. For the implementation of molecular devices on silicon, ABP is a promising candidate for a latch. In this thesis, ABP was successfully deposited on Si(100) and was switched by applying voltage pulses on top of the molecule. Two stable conformations were found and switching was realized reproducibly and reversibly. In the last part of this work, the rolling of a double-wheel technomimetic molecule was demonstrated. This thesis shows the rolling of a nanowheel on Au(111) as opposed to pushing, pulling or sliding. For this, the subphthalocyanine wheels were tagged by nitrogen during their synthesis. As this tag has different electronic properties than the rest of the wheel, it can be monitored in the STM images. By comparing the images before and after the manipulation the position of the tag proves the actual rolling.

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The two-dimensional phase of carbon known as graphene is actively being pursued as a primary material in future electronic devices. The goals of this thesis are to investigate the growth and electronic properties of epitaxial graphene on SiC, with a particular focus on nanostructured graphene. The first part of this thesis examines the kinetics of graphene growth on SiC(0001) and SiC(0001 ̅) by high-temperature sublimation of the substrate using a custom-built, ultra-high vacuum induction furnace. A first-principles kinetic theory of silicon sublimation and mass-transfer is developed to describe the functional dependence of the graphene growth rate on the furnace temperature and pressure. This theory can be used to calibrate other graphene growth furnaces which employ confinement controlled sublimation. The final chapter in this thesis involves a careful study of self-organized epitaxial graphene nanoribbons (GNRs) on SiC(0001). Scanning tunneling microscopy of the sidewall GNRs confirms that these self-organized nanostructures are susceptible to overgrowth onto nearby SiC terraces. Atomic-scale imaging of the overgrown sidewall GNRs detected local strained regions in the nanoribbon crystal lattice, with strain coefficients as high as 15%. Scanning tunneling spectroscopy (STS) of these strained regions demonstrate that the graphene electronic local density of states is strongly affected by distortions in the crystal lattice. Room temperature STS in regions with a large strain gradient found local energy gaps as high as 400 meV. Controllable, strain-induced quantum states in epitaxial graphene on SiC could be utilized in new electronic devices. / Per request of the author and the advisor, and with the approval of the graduate office, the Acknowledgements page was replaced with an errata.

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Obtenir une interface transparente entre le graphène et un supraconducteur s'est révélé être difficile et pourtant essentiel pour induire des corrélations supraconductrices dans le graphène par effet de proximité. Cette thèse présente une étude par spectroscopie tunnel (STS) à très basse température (50 mK) d'un système nouveau qui réalise ce bon couplage électronique en faisant croitre du graphène par épitaxie sur du rhénium supraconducteur. La fabrication et sélection des films minces de rhénium de haute qualité cristalline sont brièvement expliquées, suivies par le procédé de croissance CVD du graphène sur divers métaux et en particulier sur du rhénium. Les images topographiques obtenues par STM révèlent un moiré qui résulte de la différence de paramètre de maille entre le graphène et le rhénium. Nous identifions ce système à une monocouche de graphène en forte interaction avec le substrat, résultat corroboré par des calculs DFT. Des analyses STS dans une gamme d'énergie de plusieurs centaines de meV montrent une modulation spatiale de la densité d'états (DOS) à l'échelle du moiré, indiquant différentes forces de couplage entre les ‘collines' et les ‘vallées' du moiré. Les propriétés supraconductrices de l'échantillon en volume sont sondées par des mesures de transport, desquelles nous extrayons la température de transition Tc~2K et la longueur de cohérence supraconductrice ξ=18nm. Le gap supraconducteur est extrait de la DOS mesurée par STS à 50 mK (Δ=330µeV) et trouvé homogène à l'échelle du moiré. L'état mixte supraconducteur est étudié sous champ magnétique et un réseau de vortex d'Abrikosov est mis à jour. Enfin, une étude sur diverses morphologies de surface présente un effet de proximité supraconducteur latéral anormal, en contradiction avec les modèles existants. / Obtaining a transparent interface between graphene and a superconductor has proved to be very challenging and yet essential to induce superconducting correlations in graphene via the so-called proximity effect. This thesis presents a scanning tunneling spectroscopy (STS) study at very low temperature (50 mK) of a novel system achieving such a good electronic contact by the growth of epitaxial graphene on superconducting rhenium. The fabrication and selection of high-crystallographic quality rhenium thin films are briefly explained, followed by the CVD growth process of graphene on various metal substrates and in particular rhenium. STM topographic images reveal a moiré pattern due to the lattice mismatch between graphene and rhenium. We identify this system to a graphene monolayer in strong interaction with the underlying substrate, as corroborated by DFT calculations. STS analyses in the hundreds-meV energy range show a spatial modulation of the density of states (DOS) at the moiré scale, indicating different coupling strengths between ‘hills' and ‘valleys' regions. The bulk superconducting properties are probed by transport measurements, from which we extract the transition temperature Tc~2K and a superconducting coherence length ξ=18nm. The superconducting gap is extracted from the DOS at 50 mK (Δ=330µeV) and found homogeneous at the moiré scale. The superconducting mixed state is studied under magnetic field and an Abrikosov vortex-lattice is uncovered. Finally, a study on various surface morphologies exhibits an anomalous lateral superconducting proximity effect in contradiction with the existing models.

The growth of atomically flat silver films on Si(111) usually requires a two-step growth, including deposition at low temperature (≈100 K) followed by slowly annealing to room temperature. In addition, flat silver films are usually only obtained on Si(111) for film thicknesses larger than the critical thickness of 6 monolayer. In this work, Ag thin film formation at ambient temperature on Sn/Si(111)-√3×√3 has been investigated experimentally using a combination of scanning tunneling microscopy, scanning tunneling spectroscopy and low-energy electron diffraction. The first buffer layer, probably consisting of both Ag and Sn, formed a partially ordered structure consisting of atomic rows which mainly followed the high-symmetry directions of the underlying Si(111) lattice. From 3 ML coverage, an atomically flat Ag film was formed. Low-energy electron diffraction confirmed that the films grew in the [111]-direction. This shows that atomically flat Ag films as thin as 3 ML can be grown on Sn/Si(111)-√3×√3 by conventional deposition at room temperature. The electronic structures of the films were studied for a range of different coverages by scanning tunneling spectroscopy. The normalized tunneling conductance showed quantum well states in the occupied electronic states, which moved towards the Fermi energy with increasing film thicknesses.

Orientador: Douglas Soares Galvão / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica "Gleb Wataghin" / Made available in DSpace on 2018-08-14T05:39:30Z (GMT). No. of bitstreams: 1 Brunetto_Gustavo_M.pdf: 28278571 bytes, checksum: 7cba216c6b133f4a224e2155791937f4 (MD5) Previous issue date: 2009 / Resumo: Recentemente, a primeira nanoroda molecular foi caracterizada a partir de experimentos com o microscópio de tunelamento eletrônico (STM). Foi demonstrado que a molécula de hidrocarboneto (C44H24) especificamente desenhada poderia rolar sobre a superfície de cobre ao longo da direção [110] da superfície. A molécula consiste em duas rodas baseadas no grupo triptycene as quais são conectadas por um eixo. Nós reportamos um estudo teórico da simulação desse processo. Usamos métodos ab initio (DMol 3) e de dinâmica molecular clássica (UFF). Consideramos diferentes orientações cristalográficas ([111], [110], e [100]) para a superfície de cobre, a fim de determinar como estas diferentes orientações afetam o processo de rolamento molecular. Nossos resultados estão em boa acordância com os dados experimentais disponíveis. As simulações mostraram que o mecanismo de rolamento só é possível para a direção [110]. Para as outras direções ([111] e [100]) a superfície é muito suave e não pode prover o torque necessário para o processo de rolamento. Para estes casos a molécula somente desliza (movimento de translação), sem rolar quando interage com a ponta do microscópio. Para a direção [110] a separação espacial entre as colunas de cobre é suficiente para travar a molécula e criar um torque. Além da superfície correta, a posição relativa da molécula sobre a superfície é muito importante. A molécula deve estar com seu eixo principal paralelo à direção [110]. Este efeito de comensurabilidade, entre a molécula e a superfície, é similar a difusão seletiva na superfície recentemente observada para outras classes de moléculas orgânicas. Os perfis experimentais observados para o empuramento, puxamento e rolamento também podem ser explicados em termos destas características geométricas entre a molécula e as diferentes direções cristalográficas do cobre / Abstract: Recently, the first molecular nanowheel was characterized with scanning tunneling micro-scope experiments. It was demonstrated that a specifically designed hydrocarbon molecule (C44H24) could roll over a copper substrate along the [110] direction of a surface. The molecule consists in two wheels based on two triptycene groups which are connected by an axle. We report a theoretical study of the simulations of this process. We used ab initio (DMol 3) and classical molecular dynamics methods (UFF). We have considered different crystallographic orientations ([111], [110], and [100]) for the copper surface, in order to determine how these different orientations affect the molecular rolling processes. Our results are in good agreement with the available experimentally data. The simulations showed that the rolling mechanism is only possible for the [110] direction. For the others directions ([111] and [100]) the surfaces are too smooth and cannot provide the necessary torque to the rolling process. For these cases the molecule just slides (translational movement), without rolling when interact with the STM tip. For the [110] direction the spatial separation among rows of copper atoms is enough to trap the molecule and to create a torque. Besides the correct surface the relative position of the molecule on the surface is very important. The molecule should be with its main axis in the parallel direction to [110]. This commensurability effect, between the molecule and the surface, is similar to the surface selective diffusion recently observed for other classes of organic molecules. The experimental observed pushing, pulling, and rolling profiles can also be explained in terms of these geometrical features between the molecule and the different Cu crystallographic directions / Mestrado / Mestre em Física