Organic electronics are becoming an increasingly important part of the semiconductor industry, with myriad applications enabled by their low cost, solution processability, and electrical conductivity. Charge transport in electronic applications involving organic semiconductor materials depends strongly on the electronic properties of nanoscale interfaces. Local variations in molecular environments can have a significant impact on the interfacial electronic properties, and subsequently the organic semiconductor electronic structure. Here, we use scanning tunneling microscopy and spectroscopy, supported by theoretical calculations, to investigate the impact of the local adsorption environment on the local density of states of oligothiophenes, carbon nanohoops, and carbon nanotubes. First, we present work showing that, for alkyl-substituted quaterthiophenes, molecular packing and electronic structure at interfaces differ substantially from the bulk, and a significant degree of structural and electronic variation occurs even in this relatively simple system. Then, we report on investigations of longer alkyl-substituted oligothiophenes, were we found a variety of planar molecular conformations that surprising exhibited similar, particle-in-a-box-like progressions of unoccupied molecular orbitals. Next, we share our research that found, for the first time, metal surface electrons confined within single adsorbed molecules. Finally, we study the impact of electrostatic defects in both metal and dielectric substrates on single-walled carbon nanotubes. The research presented in this dissertation increases our understanding of organic semiconductor interfaces and the impact of said interfaces on local molecular electronic structure, thereby aiding future organic semiconductor technological development. / 10000-01-01

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.

Dans les supraconducteurs de type II, le champ magnétique pénètre le matériau sous forme de vortex, tourbillon de courants supraconducteurs circulant autour d'un c¿ur normal. Dans les travaux de cette thèse, nous montrons comment le confinement d'un système à une échelle comparable à la longueur de cohérence ? nanométrique modifie sensiblement ses propriétés supraconductrices.Cette étude est menée dans des nanostructures de plomb déposé in-situ sur un substrat de silicium (111), puis mesurées par spectroscopie à effet tunnel, sous UHV, à 300 mK et sous champ magnétique.En confinement extrême (taille latérale D<<10 ?), le système créé des vortex Géants, objets quantiques prédits théoriquement depuis 45 ans. En confinement plus faible (D~10 ?), les vortex peuvent être piégés, s'organisent ensuite en réseau triangulaire d'Abrikosov, puis s'interpénètrent avec le champ magnétique croissant.Les îlots de Pb cristallins supraconducteurs sont reliés entre eux par une monocouche de mouillage de Pb, ici désordonnée et non supraconductrice. Au voisinage des îlots, cette couche acquiert des caractéristiques spectroscopiques spécifiques qui reflètent la supraconductivité induite par proximité et les interactions entre électrons de type Altshuler-Aronov. L'évolution spatiale des spectres tunnel sont simulés en combinant les équations d'Usadel et la théorie du blocage de Coulomb dynamique.En réduisant la distance entre les îlots, l'effet de proximité autour de chacun se recouvre, ce qui forme une jonction Josephson. Sont étudiés finement le nombre, la position, le spectre et la forme des c¿urs de vortex Josephson sous champ magnétique, à travers une grande panoplie de jonctions. / In type II superconductors, the applied magnetic field penetrates the material in quanta of flux called vortices, vortices of superconducting currents circulating around a normal core. In the work of this thesis, we show how the confinement of a system to a scale comparable to the nanometric coherent length ? substantially modifies its superconducting properties.This study is carried out in nanostructures of lead deposited in-situ on a silicon (111) substrate, then studied by scanning tunneling spectroscopy, under UHV, at 300 mK, and under magnetic field. In extremely confinement (lateral size D<<10 ?), systems create Giant vortices, quantum objects predicted 45 years ago. In the weakly confinement (D~10 ?), vortices may be pinned, then are organized in the triangular Abrikosov lattice, finally interpenetrate in surface superconductivity with the increasing magnetic field. Crystalline superconducting Pb islands are here connected by a disordered non-superconducting wetting layer of Pb. In the vicinity of each superconducting island, the wetting layer acquires specific tunnelling characteristics which reflect the interplay between the proximity induced superconductivity and the inherent electron correlations of this ultimate diffusive two-dimensional metal. Spatial evolution of the tunnel spectra are simulated by combining the Usadel equations and the theory of dynamic Coulomb blockade. With reducing the distance between the islands, the proximity effect around each overlaps and forms a Josephson junction. Thanks to the tunneling spectroscopy, number, position, the spectrum and the form of Josephson vortex cores are studied in detail for a large variety of junctions.

The electronic and magnetic properties of nanographenes strongly depend on their size, shape and topology. While many nanographenes present a closedshell electronic structure, certain molecular topologies may lead to an open-shell structure. Triangular-shaped nanographenes with zigzag edges, which exist as neutral radicals, are of considerable interest both in fundamental science and for future technologies aimed at harnessing their intrinsic high-spin magnetic ground states for spinbased operations and information storage. Their synthesis, however, is extremely challenging owing to the presence of unpaired electrons, which confers them with enhanced reactivity. We report a combined in-solution and onsurface synthesis of π-extended triangulene, a non-Kekulé nanographene with the structural formula C33H15, consisting of ten benzene rings fused in a triangular fashion. The distinctive topology of the molecule entails the presence of three unpaired electrons that couple to form a spin quartet ground state. The structure of individual molecules adsorbed on an inert gold surface is confirmed through ultrahigh-resolution scanning tunneling microscopy. The electronic properties are studied via scanning tunneling spectroscopy, wherein unambiguous spectroscopic signatures of the spin-split singly occupied molecular orbitals are found. Detailed insight into its properties is obtained through tight-binding, density functional and many-body perturbation theory calculations, with the latter providing evidence that π-extended triangulene retains its open-shell quartet ground state on the surface. Our work provides unprecedented access to open-shell nanographenes with high-spin ground states, potentially useful in carbon-based spintronics.

A theoretical framework for calculating electron transport through molecular junctions is presented. It is based on scattering theory using a Green's function formalism. The model can take both elastic and inelastic scattering into account and treats chemical and physical bonds on equal footing. It is shown that it is quite reliable with respect to the choice of functional and basis set. Applications concerning both elastic and inelastic transport are presented, though the emphasis is on the inelastic transport properties. The elastic scattering application part is divided in two part. The first part demonstrates how the current magnitude is strongly related to the junction width, which provides an explanation why experimentalists get two orders of magnitude differences when performing measurements on the same type of system. The second part is devoted to a study of how hydrogenbonding affects the current-voltage (I-V) characteristics. It is shown that for a conjugated molecule with functional groups, the effects can be quite dramatic. This shows the importance of taking possible intermolecular interactions into account when evaluating and comparing experimental data. The inelastic scattering part is devoted to get accurate predictions of inelastic electron tunneling spectroscopy (IETS) experiments. The emphasis has been on elucidating the importance of various bonding conditions for the IETS. It is shown that the IETS is very sensitive to the shape of the electrodes and it can also be used to discriminate between different intramolecular conformations. Temperature dependence is nicely reproduced. The junction width is shown to be of importance and comparisons between experiment as well as other theoretical predictions are made. / QC 20101118

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

High-temperature superconductors belong to the group of strongly correlated materials. In these compounds, complex repulsive electron interactions and a large number of degrees of freedom lead to a rich variety of states of matter. Exotic phases like the pseudogap, charge-, spin- and pair-density waves, but also the remarkable phenomenon of superconductivity emerge, depending on doping level and temperature. However, up to now it is unclear what exactly causes these states, to what extent they are coexisting or competing, and where their borders in the phase diagram lie. A better understanding could help in finding the mechanism behind high-temperature superconductivity, but would also provide a better insight into the puzzling behavior of strongly correlated materials. This thesis tries to resolve some of these questions with focus on the underdoped pseudogap regime. Mesa structures of bismuth-based cuprate superconductors were studied using intrinsic tunneling, which allows spectroscopic characterizations of electronic density of states inside the material. A micro/nano fabrication method was developed to further reduce mesa areas into the sub square-micrometer range, in order to minimize the effect of crystal defects and measurement artifacts caused by heating induced by the measurement current. The comparison of energy scales in Bi-2201 and Bi-2212 cuprates shows that the pseudogap phenomenon is not connected to superconductivity, but possibly represents a competing spin-singlet order that is universal to all cuprates. The analysis of the upper critical field in Bi-2201 reveals a low anisotropy, which gives evidence of paramagnetically limited superconductivity. Furthermore, a new electrical doping method is demonstrated, which enables the reversible tuning the doping level of Bi-2212 and study a broad doping range upon a single sample. Using this method, two distinct critical points were observed under the superconducting dome in the phase diagram: one at the overdoped side, associated with the onset of the pseudogap and a metal to insulator transition, and one at optimal doping, associated with an enhanced "dressed" electron energy. Finally, a novel angular-dependent magnetotunneling technique is introduced, which allows for the separation of the superconducting and non-superconducting contributions to the pseudogap phenomenon. The method reveals that after an abrupt decay of the energy gap for T→Tc, weak superconducting correlations persist up to several tens of degrees above Tc.

Gegenstand dieser Dissertation ist die experimentelle Untersuchung von Schwere-Fermionen-Systemen mittels Rastertunnelmikroskopie und –spektroskopie (RTM/S). In diesen Materialien führen starke elektronische Korrelationen zur Ausbildung einer besonderen Art von \"schweren\" Ladungsträgern, deren Natur bislang nicht abschließend aufgeklärt werden konnte. Einige grundlegende Aspekte der Physik der Schwere-Fermionen-Systeme werden eingangs der Arbeit dargestellt. Im Anschluss daran werden die experimentellen Methoden der RTM und RTS eingeführt sowie die verwendeten Messaufbauten vorgestellt. Dies geschieht mit Hinblick auf die experimentellen Voraussetzungen für die RTS an Schwere-Fermionen-Systemen, insbesondere auf das spektrale Auflösungsvermögen. Die Präparation geeigneter Probenoberflächen von Schwere-Fermionen-Materialien und deren Auswirkung auf RTM-Experimente nehmen eine zentrale Stellung dieser Arbeit ein und werden daher gesondert behandelt. Vorrangig wurde dabei das Spalten einkristalliner Proben untersucht. In RTS-Untersuchungen des Schwere-Fermionen-Supraleiters CeCoIn5 ist es gelungen, die für einen Supraleiter typische Energielücke im Anregungsspektrum zu messen. Die Daten können über einen weiten Temperaturbereich mit theoretischen Voraussagen für die unkonventionelle Supraleitung in diesem Material verglichen werden. Die Resultate sind im Einklang mit früheren experimentellen Befunden, welche auf einen der Supraleitung vorausgehenden sog. „Precursor“-Zustand hindeuten. Allerdings gibt es, wie auch in anderen untersuchten Schwere-Fermionen-Supraleitern, Hinweise auf Inhomogenitäten der Probenoberfläche. Im Fall des nicht-supraleitenden Kondogitter-Systems YbRh2Si2 konnte durch Spalten von Einkristallen bei tiefen Temperaturen großflächig atomar geordnete Oberflächen erzeugt werden. Es zeigen sich starke Indikationen darauf, dass die Spektroskopie-Daten die Volumeneigenschaften des Materials reflektieren. Ein Vergleich mit theoretischen Rechnungen deutet darauf hin, dass der Kondoeffekt der magnetischen Yb3+-Ionen sich in der Tunnelleitfähigkeit widerspiegelt - bis hin zum Einfluss der sich ausbildenden räumlichen Kohärenz des Kondogitters bei tiefen Temperaturen. Diese Ergebnisse gewähren wichtige Einblicke in die thermische Entwicklung der elektronischen Korrelationen in Kondogitter-Systemen, und demonstrieren somit das große Potential der Rastertunnel-Spektroskopie für die weitere Erforschung der Schwere-Fermionen-Systeme. Die im Abschnitt 6.3 'Tunnelspektroskopie-Resultate an YbRh2Si2' dargestellten Ergebnisse sind in ähnlicher Form auch veröffentlicht in Nature Vol. 474 (2011), Seiten 362-366.

Graphene, a honeycomb lattice of sp2-bonded carbon atoms, has received considerable attention in the scientific community due to its unique electronic properties. Distinct symmetries of the graphene wave functions lead to unusual quantum properties, such as a unique half-integer quantum Hall effect. As an added consequence of these symmetries, back-scattering in graphene is strongly prohibited leading to long coherence lengths of carriers. These charge carriers at low energy exhibit linear energy-momentum dispersion, much like neutrinos. Thus, carriers in graphene can be described as massless Dirac fermions. Graphene grown epitaxially on semiconducting substrates offers the possibility of large-scale production and deterministic patterning of graphene for nanoelectronics. In this work, epitaxial graphene is created on SiC(0001) by annealing in vacuum. Sequential scanning tunneling microscopy (STM) and spectroscopy (STS) are performed in ultrahigh vacuum at a temperature of 4.2 K and 300 K. These atomic-scale studies address the growth, interfacial properties, stacking order, and quasiparticle coherence in epitaxial graphene. STM topographic images show the atomic structure of successive graphene layers on the SiC substrate, as well as the character of defects and adatoms within and below the graphene plane. STS differential conductance (dI/dV) maps provide spatially and energy resolved snapshots of the local density of states. Such maps clearly show that scattering from atomic defects in graphene gives rise to energy-dependent standing wave patterns. We derive the carrier energy dispersion of epitaxial graphene from these data sets by quantifying the dominant wave vectors of the standing waves for each tunneling bias.