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.

For the investigation of single molecules on surfaces, STM under UHV and at low temperatures is the experimental technique of choice. Inelastic STM is, furthermore, able to manipulate the target structures and to induce chemical reactions or to control single molecule mechanics precisely. This thesis presents inelastic STM experiments on three different molecules as a tool for the electronic manipulation at a single molecule level. The first part of this work concerns the on-surface synthesis of dodecacene, the longest acene molecule obtained so far. Acenes as smallest zigzag edge graphene nanoribbons and model 1D electronic system play an important role in both experimental and theoretical science. Due to the high reactivity and low solubility of long acenes, precursor molecules were deoxygenated step wise in an on-surface reaction triggered by inelastic tunneling and through annealing at increasing temperatures. The molecular structure was proven by high resolution STM employing a CO functionalized tip. Additionally, the electronic states of the molecule were observed in the energy spectrum by STS and their spatial distribution was measured in dI/dV maps. The increase in the band gap compared to shorter acenes was explained by increasing contributions of multiradical states to the electronic states and higher orbitals participating in virtual tunneling states. In the second part of this work, the inelastic tunneling effect was used to investigate the conversion of electrical into mechanical energy in azulene derivatives carrying a large dipole moment. Metal organic complexes consisting of gold adatoms and pristine as well as cleaved molecules were formed upon evaporation of BCA. These structures were identified with the aid of theoretical calculations. Voltage pulse experiments at different tunneling resistance revealed that the electric field in combination with the charge distribution of the structures is the origin of the motion. Metal organic complexes of cleaved molecules could be moved on the surface in a controlled way and driven along an arbitrarily chosen parcours. The third part of this work concerns the investigation of DMBI-P molecules as rotors for molecular machines. Demethylation during evaporation was used to create an open radical bond stably anchoring the molecule on the surface. This was utilized for a step wise rotation where the direction is controlled by the voltage sign and chirality of the molecule on the surface. A C-H stretch mode was identified as its origin, serving as energy entry channel excited by inelastic tunneling electrons. Temperature dependent measurements and theoretical calculations yielded the potential barrier for the rotation. / Rastertunnelmikroskopie (RTM) unter UHV Bedingungen und bei tiefen Temperaturen ist die experimentelle Methode der Wahl zur Untersuchung von Einzelmolekülen auf Oberflächen. Darüber hinaus ist inelastische RTM in der Lage, die Zielstrukturen zu manipulieren und chemische Reaktionen auszulösen oder die Mechanik der einzelnen Moleküle präzise zu kontrollieren. Diese Dissertation behandelt inelastische RTM-Experimente an drei verschiedenen Molekülen als Werkzeug zur elektronischen Manipulation einzelner Moleküle. Der erste Teil der Arbeit behandelt die Oberflächensynthese von Dodecacen, des längsten bisher erzeugten Acens. Als kleinste Graphen-Nanobänder mit Zickzack-Rand und Modell für eindimensionale elektronische Systeme spielen Acene sowohl in Theorie als auch Experimentalphysik eine wichtige Rolle. Aufgrund der hohen Reaktivität und geringen Löslichkeit langer Acene wurden Vorläufermoleküle sowohl durch inelastisches Tunneln als auch durch Heizen des Substrates schrittweise deoxygeniert. Die Molekülstruktur wurde durch hochaufgelöste RTM mittels einer CO-funktionalisierten Spitze nachgewiesen. Zusätzlich konnten die elektronischen Zustände des Moleküls im Energiespektrum identifiziert und ihre räumliche Verteilung in dI/dV-Karten festgehalten werden. Die Vergrößerung der Bandlücke im Vergleich zu kürzeren Acenen konnte hierbei durch zunehmenden Einfluss multiradikaler Zustände auf den Grundzustand des Moleküls und den Beitrag höherer Molekülorbitale zu den virtuellen Tunnelzuständen erklärt werden. Im zweiten Teil dieser Arbeit wird die inelastische RTM dazu genutzt, um die Umwandlung von elektrischer Energie in mechanische mittels Azulen-Derivaten mit großem Dipolmoment zu untersuchen. Bei der Verdampfung bilden diese metallorganische Komplexe aus Goldatomen und sowohl intakten als auch gespaltenen Molekülen. Deren Strukturen wurden mit Hilfe von Berechnungen identifiziert. Experimente mit Spannungspulsen bei unterschiedlichen Tunnelwiderständen enthüllten das elektrische Feld in Kombination mit der Ladungsverteilung der Strukturen als Ursprung der Bewegung. Die metallorganischen Komplexe aus gespaltenen Molekülen konnten zielgerichtet auf der Oberfläche durch einen zufällig gewählten Parcours bewegt werden. Der dritte Teil dieser Arbeit behandelt die Untersuchung von DMBI-P Molekülen zur Verwendung als Rotoren für molekulare Maschinen. Eine Demethylierung während der Verdampfung erzeugt eine offene Bindung, die das Molekül stabil auf der Oberfläche verankert. Dies wurde für eine schrittweise Rotation genutzt, deren Richtung durch das Vorzeichen der Spannung und die Chiralität auf der Oberfläche kontrolliert werden konnte. Eine C-H Streckschwingung dient hierbei als Eintrittskanal der durch inelastische Elektronen bereitgestellten Energie. Temperaturabhängige Messungen und theoretische Berechnungen lieferten die Potentialbarriere für die Rotation.

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The work in this thesis is basically divided into two related but separate investigations. The first part treats simple chemical reactions of adsorbate molecules on metallic surfaces, induced by means of a scanning tunneling probe (STM). The investigation serves as a parameter free extension to existing theories. The theoretical framework is based on a combination of density functional theory (DFT) and non-equilibrium Green's functions (NEGF). Tunneling electrons that pass the adsorbate molecule are assumed to heat up the molecule, and excite vibrations that directly correspond to the reaction coordinate. The theory is demonstrated for an OD molecule adsorbed on a bridge site on a Cu(110) surface, and critically compared to the corresponding experimental results. Both reaction rates and pathways are deduced, opening up the understanding of energy transfer between different configurational geometries, and suggests a deeper insight, and ultimately a higher control of the behaviour of adsorbate molecules on surfaces. The second part describes a method to calculate STM images in the low bias regime in order to overcome the limitations of localized orbital DFT in the weak coupling limit, i.e., for large vacuum gaps between a tip and the adsorbate molecule. The theory is based on Bardeen's approach to tunneling, where the orbitals computed by DFT are used together with the single-particle Green's function formalism, to accurately describe the orbitals far away from the surface/tip. In particular, the theory successfully reproduces the experimentally well-observed characteristic dip in the tunneling current for a carbon monoxide (CO) molecule adsorbed on a Cu(111) surface. Constant height/current STM images provide direct comparisons to experiments, and from the developed method further insights into elastic tunneling are gained.

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Molecules adsorbed on two-dimensional (2D) materials can show interesting physical and chemical properties. This thesis presents scanning probe microscopy (SPM) investigation of emerging 2D materials, molecular nanostructures on 2D substrates at the nanometer scale, and biophysical processes on the biological membrane. Two main techniques of nano-probing are used: scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The study particularly emphasizes on self-assembled molecules on flat 2D materials and quasi-1D wrinkles. First, we report the preparation of novel 1D C60 nanostructures on rippled graphene. Through careful control of the subtle balance between the linear periodic potential of rippled graphene and the C60 surface mobility, we demonstrate that C60 molecules can be arranged into a 1D C60 chain structure of two to three molecules in width. At a higher annealing temperature, the 1D chain structure transitions to a more closely packed, quasi-1D hexagonal stripe structure. The experimental realization of 1D C60 structures on graphene is, to our knowledge, the first in the field. It could pave the way for fabricating new C60/graphene hybrid structures for future applications in electronics, spintronic and quantum information. Second, we report a study on nano-morphology of potential operative donors (e.g., C60) and acceptors (e.g., perylenetetracarboxylic dianhydride, aka. PTCDA) on wrinkled graphene supported by copper foils. We realize sub-monolayer C60 and PTCDA on quasi-1D and quasi-2D real periodic wrinkled graphene, by carefully controlling the deposition parameters of both molecules. Our successful realization of acceptor-donor binary nanostructures on wrinkled graphene could have important implications in future development of organic solar cells. Third, we report an STM and spectroscopy study on atomically thin transition-metal dichalcogenides (TMDCs) material. TMDCs are emerging 2D materials recently due to their intriguing physical properties and potential applications. In particular, our study focuses on molybdenum disulfide (MoS2) mono- to few-layers and pyramid nanostructures synthesized through chemical vapor deposition. On the few-layered MoS2 nanoplatelets grown on gallium nitride (GaN) and pyramid nanostructures on highly oriented pyrolytic graphite (HOPG), we observe an intriguing curved region near the edge terminals. The measured band gap in these curved regions is consistent with the direct band gap in MoS2 monolayers. The curved features near the edge terminals and the associated electronic properties may contribute to understanding catalytic behaviors of MoS2 nanostructures and have potential applications in future electronic devices and catalysts based on MoS2 nanostructures. Finally, we report a liquid-cell AFM study on the endosomal protein sorting process on the biological lipid membrane. The sorting mechanism relies on complex forming between Tom1 and the cargo sorting protein, Toll interacting protein (Tollip). The induced conformational change in Tollip triggers its dissociation from the lipid membrane and commitment to cargo trafficking. This collaborative study aims at characterizing the dynamic interaction between Tollip and the lipid membrane. To study this process we develop the liquid mode of AFM. We successfully demonstrate that Tollip is localized to the lipid membrane via association with PtdIns3P (PI(3)P), a major phospholipid in the cell membrane involved in protein trafficking. / Ph. D. / Two-dimensional (2D) materials are layered materials with thickness of single atom or few atoms. The ultimate thickness leads to novel properties that are useful for a wide range of applications in photovoltaics, electronics and quantum information. In order to explore these properties at the nanometer scale, we used scanning probe techniques, i.e., scanning tunneling microscopy (STM) and atomic force microscopy (AFM), to perform comprehensive investigations on these emerging materials. 2D materials, such as graphene and atomically thin transition-metal dichalcogenides (TMDCs), are promising candidates for building economic, safe and mechanically flexible solar cells with desirable optical and electronic properties, e.g. tunable sunlight absorption. The first part of the thesis focuses on graphene, a single-atom-thick carbon sheet. We deposited key components in organic solar cells, such as perylenetetracarboxylic dianhydride (PTCDA) and C₆₀ molecules, on graphene. On these materials we observed various novel nanostructures, like quasi-1D C₆₀ nanochains. The second part of the thesis focuses on mono- to few-layered MoS₂, which can be used as an active layer in high-efficiency solar cells. Our study has important implications in improving efficiency of organic solar cells in the future. In the final part of the thesis, we extended our subject to the biological lipid membrane, a 2D material critical in biology, and biophysical processes occurring on the membrane. Using a liquid-cell AFM, we investigated the endosomal protein sorting process on the biological membranes. Our study contributes to understanding of the interactions between the adaptor proteins and cell membranes in the protein sorting process that guides proteins to their proper destinations.

The scanning tunneling microscope (STM) is one of the most advanced surface science tools capable of atomic resolution imaging and atomic manipulation. Unfortunately, STM has many time-consuming bottlenecks, like probe conditioning, tip instability, and noise artificing, which causes the technique to have low experimental throughput. This dissertation describes my efforts to address these challenges through automation and machine learning. It consists of two main sections each describing four projects for a total of eight studies. The first section details two studies on nanoscale sample fabrication and two studies on STM tip preparation. The first two studies describe the fabrication of graphene-based Josephson Junction devices and the factorial optimization of patterned carbon nanotube forest synthesis. The second two studies focus on the factorial optimization of electrochemical STM tip etching and automated STM tip functionalization via in-situ silicon nanocolumn growth. The second section details four studies on the use of neural networks for STM image and spectroscopy analysis. The third two studies are on the effectiveness of convolutional neural networks for identifying images of conditioned STM tips on the Au(111) surface and on the detection and metrology of atomic scale defects in single crystal tungsten diselenide, a transition metal dichalcogenide. The fourth two studies are on the use of variational autoencoders to autonomously classify scanning tunneling spectra of various materials, molecules, and surface structures and to identify bismuth and nickel atoms from cross sectional STM images of doped gallium arsenide.