Charged organic species are ubiquitous throughout organic chemistry. They are ideal ascomponents in organic electronics, and are common as transition states or intermediates in many organic transformations. This dissertation details the investigation of the effect of external electric fields on reactions with charged transition states, as well as the incorporation of triarylmethylium and triangulenium cations as components in single-molecule and organic electronics. Chapter 1 provides an introduction to the scanning tunneling microscope break junction (STM-BJ) technique, which is used as a tool to assess the effect of external electric fields on reaction rate and the performance of single-molecule devices. In this chapter, the different design criteria for molecules used in both single-molecule and optoelectronic devices are discussed. Chapter 2 evaluates the effect of electric fields on two classes of reactions with charged transition states – the Menshutkin reaction and the Chapman rearrangement. Chapter 3 describes triarylmethylium and triangulenium dyes as single-molecule devices, and how their conductance can be tuned through different pendant substituents via Fano resonances. Chapter 4 details an ongoing project that incorporates triangulenium cores to yield a modular donor-acceptor system that shows tunable quenching of emission. Finally, Chapter 5 chronicles the progress of our TikTok account called @IvyLeagueScience and outlines the success criteria for using short-form videos as a way of conducting scientific outreach.

Electric fields influence reactions by stabilization of charge-separated transition states. While this has been a longstanding hypothesis supported computationally, recent experimental confirmations highlight the potential for leveraging electric field effects to drive small molecule reactions far from equilibrium. Herein we report electric-field catalysis of an alkane solvent-derived acylation reaction in the scanning tunneling microscope-break junction (STM-BJ), providing additional support for this hypothesis. Additionally, the design and reactivity of an internally charged zwitterionic ligand are disclosed. Synthetic access of metal ligands bearing opposing charged functional groups permitted the examination of stochiometric metalation and catalytic behavior of electric field-bearing ligands.While traditionally computation has been used to rationalize why a particular catalyst is successful descriptively, it has been rarely used to screen candidates and prescriptively provide optimal catalyst structure. We report a neural network-enabled catalyst screening platform that dramatically reduce the resource intensity for examining a large chemical space. We leverage this platform to examine azolium N-heterocyclic carbene (NHC) precursors to address the lack of compatibility for electron-rich aryl aldehydes in the NHC-catalyzed Stetter reaction. This led to the discovery of a new class of azolium NHC precursor: oxadiazoliums that proved competent in achieving the target reaction addressing current limitations in Stetter catalysis.

Graphene is one of the most important low dimensional materials. Ever since the inception of graphene, attempts to scale up production of graphene have never stopped. Chemical vapor deposition synthesis of graphene on copper is one of the most promising pathways. However, the poor quality of CVD-derived graphene has hindered synthetic graphene for large scale basic science and commercial applications. The lack of reproducibility of CVD graphene research, and the inferior quality of CVD graphene points to potential hidden variables and misunderstanding of the graphene CVD process. In this thesis, it is identified that trace oxygen is a key factor in determining the quality and growth trajectory for graphene grown by low-pressure CVD. By innovative design in the CVD system layout and meticulous control over the substrate quality, it is demonstrated that by eliminating oxygen below µTorr level in the growth chamber, high quality graphene comparable to exfoliated graphene can be obtained. The ultrahigh graphene quality is showcased by a combination of Raman spectroscopy, electrical transport measurement and various scanning probe techniques including scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Moreover, a graphite-gated device encapsulated by hexagonal boron nitride (h-BN) shows well developed fractional quantum hall effect. Using the oxygen free CVD (OF-CVD) system as a platform, the effect of oxygen is revealed to be inducing etching effect at the µTorr limit. Addition of hydrogen delays the etching effect with reduced graphene growth rate. At high hydrogen concentrations, µTorr-level of oxygen is found to be inducing amorphous carbon contaminations to the graphene surface, at the same time, the quality of the resulting graphene deteriorates with increasing level of oxygen, as characterized by AFM, XPS, Raman spectroscopy and electrical transport measurements. OF-CVD enables unprecedented tunability of the graphene growth behavior in terms of growth rate and nucleation density. Controlled experiments reveal the individual effect of all experimental parameters such as temperature, and partial pressure of methane and hydrogen are studied. Their effect on the initial growth rate of graphene can be modeled by a compact model based on competitive adsorption of methane and hydrogen onto the copper surface. The mechanism of overall coverage-time evolution is further revealed by phase-field modeling. Collectively, the theoretical insights of the CVD process of graphene pave way for graphene synthesis by design. The underlying mechanism and principles provide insights for understanding and optimizing other 2D materials growth mediated by catalytic substrates.

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This thesis presents investigations on hydrogenated graphene by scanning tunneling microscopy and spectroscopy (STM/STS) as well as the implementation of spin-polarized STM. Preparation processes for a magnetic standard sample and spin-sensitive chromium tips are developed. The measurements on graphene reveal specific hydrogen adsorption sites in low coverage and the formation of a pattern at higher coverage. Both is found to be in agreement with previous predictions and calculations. Upon hydrogenation, an impurity midgap state emerges in the density of states which is measured directly for the first time. Complementing angle resolved photoemission experiments confirm that this state is dispersionless over the whole Brillouin zone. A routine is developed to prepare the standard sample system of ultra-thin iron films on tungsten (Fe/W(110)). Investigations on this system confirm the magnetic properties known from literature, including the presence of a spin spiral, and prove that it is well suited for the characterization of spin-polarized tips. Different approaches for the preparation of tips from the antiferromagnetic material chromium are tested. Among these, a promising new method is presented: The coating of crystalline chromium tips with fresh chromium material suggests reproducibility of the tip characteristics. The performance of the produced tips in STM measurements is excellent in regard to a fixed spin-polarization, high resolution and stability. Especially, a recovery of the tip magnetization direction proposed in this thesis makes this new preparation method superior to all processes yielding antiferromagnetic tips reported so far. / Inhalt der vorliegenden Arbeit sind Untersuchungen von hydogeniertem Graphen mittels Rastertunnelmikroskopie und -spektroskopie (RTM/RTS) sowie die Einführung spin-polarisierter RTM. Im Rahmen dessen wurden Präparationsprozesse für magnetische Standardproben und spin-sensitive Chrom-Spitzen entwickelt. Die Messungen an Graphen zeigen spezifische Wasserstoff-Adsorptionsstellen bei geringer Bedeckung und die Ausbildung eines Musters bei höherer Bedeckung, jeweils in Übereinstimmung mit Vorhersagen und Berechnungen. Der durch Hydrogenierung entstehende Störstellenzustand in der Bandlücke der Zustandsdichte wurde zum ersten Mal direkt gemessen. Ergänzende winkelaufgelöste Photoelektronenspektroskopieexperimente bestätigen, dass dieser Zustand in der gesamten Brillouinzone dispersionsfrei ist. Ein Verfahren zur Herstellung magnetischer Standardproben aus ultradünnen Eisenfilmen auf Wolfram (Fe/W(110)) wurde entwickelt. RTM-Untersuchungen an diesem System bestätigen die bereits aus der Literatur bekannten magnetischen Eigenschaften, insbesondere das Vorhandensein einer Spinspirale. Damit ist Fe/W(110) hervorragend geeignet für die Charakterisierung spin-polarisierter Spitzen. Verschiedene Ansätze, die zur Herstellung von Spitzen aus dem antiferromagnetischen Material Chrom verfolgt wurden, werden präsentiert, darunter auch eine vielversprechende neue Methode: Das Aufwachsen eines frischen Chromfilms auf kristalline Spitzen desselben Materials verspricht eine Reproduzierbarkeit von Spitzeneigenschaften. Der Einsatz von so hergestellten Spitzen in RTMMessungen ist geprägt von einer festgelegten Spin-Polarisation, hohem Auflösungsvermögen und Stabilität. Insbesondere die mögliche Reproduzierbarkeit der Magnetisierungsrichtung, die in dieser Arbeit diskutiert wird, macht diese Methode allen bisher berichteten Herstellungprozessen antiferromagnetischer Spitzen überlegen.

We investigated the electronic structure of the two nematic materials Sr₃Ru₂O₇ and Ca(Fe₀.₉₇Co₀.₀₃As)₂ using spectroscopic – imaging scanning tunneling microscopy (SI-STM) and angle resolved photoemission spectroscopy (ARPES). – – – Sr₃Ru₂O₇ is an itinerant metamagnet that shows a putative quantum critical endpoint at 8 Tesla, submersed by the formation of a nematic electronic phase. Using ARPES, we identified at least 5 Fermi pockets in agreement with quantum oscillation measurements. Surprisingly, we found Fermi velocities up to an order of magnitude lower than in single layer Sr₂RuO₄ and up to 35 times lower than predicted by ab initio calculations. Many bands are confined in an energy range of only ∼10 meV below the Fermi level. This, as well as distinct peak-dip-hump shapes of the spectra with a characteristic energy of around ∼5 meV indicate strong correlations and a possible nontrivial mechanism that is absent in single layer Sr₂RuO₄ and connected to the nematicity. The quasiparticle interference of one of the bands was detected by SI-STM, which was also used to measure subatomic features with the symmetries of the relevant Ru d orbitals. – – – In the second mate- rial, the iron-based high-temperature superconductor Ca(Fe[subscript(1-x)]Co[subscript(x)]As)₂, we discovered electronic nematic nano-pattern in its under-doped ‘parent’ state. We spectroscopically imaged this state in real space over large areas and across domain boundaries that change the directionality of the nano-pattern by 90°. We propose that oriented, dimer-shaped electronic nematogens are responsible for this pattern, in striking contrast to what has been expected and observed in electronic nematic materials. The dimers consist of two Gaussian conductance peaks separated by about 8 a[subscript(FeFe)]. Unidirectionality also shows in the quasiparticle interference pattern of the delocalized electrons. The dispersion is in agreement with scattering from the α₂ band discovered by ARPES but has distinct C₂ symmetry, not inconsistent with a C₄-symmetric band scattered by the proposed dimers.

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De nos jours, l’élaboration d’objets de dimensions nanométriques constitue un champ de recherches particulièrement prometteur pour la conception de systèmes de petite taille. La possibilité d’exploiter ces systèmes dans des applications telles que l’électronique moléculaire ou la modification des propriétés de surface a suscité l’engouement auprès de la communauté scientifique. Cependant, afin de construire des dispositifs électroniques complexes à partir de molécules organiques, l’assemblage covalent de briques moléculaires sur une surface est primordial. Les recherches menées dans le cadre de cette thèse portent sur l’élaboration de réseaux bidimensionnels à partir de briques moléculaires déposées sur surface. La réalisation de tels réseaux consiste d’une part, en la synthèse des différents précurseurs, et par la suite, au dépôt de ces briques moléculaires sur des surfaces métalliques ou de graphite. La croissance de ces réseaux est contrôlée en variant les conditions de dépôts qui s’opèrent dans un milieu sous-vide ou liquide. Le réseau peut être construit à partir d’un même précurseur, qui réagit dès lors sur lui-même pour former le réseau. Ainsi, des réactions telles que l’auto-condensation, la polymérisation oxydative ou encore la cyclotrimérisation sont abordées. De plus, les réactions entre deux précurseurs de natures différentes sont également décrites. Après la synthèse des briques moléculaires, leur étude sur surface est détaillée dont la caractérisation de réseaux est suivie par microscopie à effet tunnel. / Nowadays, the engineering of nanometer-sized systems is a promising field for the development of little-sized systems. The possibility of extending these systems to applications such as molecular electronics or surface property tuning has attracted much attention to the scientific community. However, in order to construct complex electronic devices from organic molecules, covalent assembly of building blocks on surface is primordial. The researches carried out in this work thesis rest on the construction of two-dimensional frameworks from molecular building blocks deposited on surface. The achievement of such networks consists, first of all, in the synthesis of different precursors and afterwards, in the deposition of these molecular buildingblocks on metallic or graphite surfaces. The growth of such networks is controlled by changing deposition conditions which occurs in ultra-high vacuum or in liquid media. The framework can be built from the precursor itself, which reacts with each other to give rise to the network. Reactions such as self-condensation, oxydative polymerization or either cyclotrimerization are broached. Moreover, reactions between two different precursors are also described. After molecular building block synthesis, on-surface study is detailed which framework characterization is followed by scanning tunneling microscopy.