Highly Ordered Organic Layers and Wires

This thesis deals with the synthesis of highly ordered organic thin films and the characterization of the molecule-substrate interaction through spectroscopy and diffraction. Organic devices, such as organic light emitting diodes (OLEDs) and organic field effect transistors (OFETs) have become ubiquitous in modern times. The transistor and high frequency performance of such organic devices crucially depends on charge carrier mobility. In inorganic semiconductors, which are bonded covalently, the band masses are typically lower and their crystallinity higher in comparison to their organic counterparts. In a simple Drude model, the charge carrier mobility is inversely proportional to the effective charge carrier mass. The low effective mass and high crystallinity of inorganic semiconductors result in large carrier mobilities of up to 1E7 cm^2/Vs for GaAs at low temperatures. The large effective mass in van der Waals bonded organic materials and their poorer molecular order decrease the carrier mobility. This thesis addresses the limitations of the inherently low mobility and disorder in organic thin films in a twofold way. The first is the introduction of a novel synthesis method for graphene nanoribbons, which are covalently bonded long stripes of graphene. This new method, developed in this thesis, is based on laser induced photothermal heating. It allows for the synthesis of atomically precise graphene nanoribbons with a higher degree of control over the reaction than conventional methods and is shown to work in a multitude of different nanoribbon species. The growth takes place in an area that is solely governed by the spotsize of the incoming laser light (4 µm). This method has an advantage over present methods through the exact control of the growth kinetics with regards to chemical uniformity and local area distribution. Additionally, the physical properties and growth kinetics of photothermally grown graphene nanoribbons are investigated by means of Raman spectroscopy. In a second way, the growth of organic moire structures on a topological insulator is studied. We show the growth of C60 thin films on the topological insulator Bi4Te3 through electron diffraction and observe a moire pattern. This indicates very long range order in the form of a (4x4) on (9x9) superstructure that is observable on the entire 1x1 cm^2 sample surface. The growth of the structure is performed using molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) in ultra-high vacuum (UHV) conditions and the properties of the interface are studied using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that a C60 induced surface reconstruction and the softness of the underlying, layered topological insulator are responsible for the high order. The theoretical calculations find that the structure bonds mostly through physisorption and both the theory and band structure measurements show no perturbation of the electronic states of the topological insulator by the overlayer. Finally, we extend the concept of well ordered growth of organic thin films on topological insulators to superconducting alkali metal doped C60. These organic films are metallic at room temperature but turn into s-wave superconductors at a critical temperature of 28 K. The combination of this relatively high transition temperature in combination with the well defined growth opens up a new playground for both experimental and theoretical studies. The van der Waals bond nature of the interface protects the interface from alloying, which can be a problem for inorganic topological insulator--superconductor interfaces. We show a novel synthesis route for the growth of well ordered superconducting alkali metal doped fullerenes on the topological insulator Bi4Te3. The growth process is studied using LEED and ultra violet photoemission spectroscopy (UPS) and makes the phase pure synthesis of thin film Rb3C60 possible, which is crucial to avoid contamination through an insulating Rb6C60 phase. ARPES spectra confirm the intactness of the interface by measurements of both the Fermi surface of the topological insulator as well as the newly formed Rb3C60 metallic film

Silicon Cluster Arrays on the Monolayer of Hexagonal Boron Nitride on Ir(111)

Periodic structures of silicon are of interest in quantum-dot-based applications because of their unique optical and electronic properties. We report on the fabrication of stable quasi-ordered Si nanocluster arrays on the moiré of a hexagonal boron nitride (h-BN) monolayer on Ir(111). The h-BN monolayer promotes the growth of regular Si nanoclusters at 130 K and electronically decouples the clusters from the underlying metallic substrate. Using scanning tunneling microscopy and spectroscopy, we have investigated the cluster binding sites, their electronic structure, and their thermal stability. We find that the clusters display a size-dependent bandgap and that they are stable up to 577 K, after which cluster coalescence degrades the arrays

Morphological and crystallographic orientation of hematite spindles in an applied magnetic field

The magnetic response of spindle-shaped hematite (-Fe2O3) nanoparticles was investigated by simultaneous small-angle and wide-angle X-ray scattering (SAXS/WAXS) experiments. The field-dependent magnetic and nematic order parameters of the magnetic single-domain nanospindles in a static magnetic field are fully described by SAXS simulations of an oriented ellipsoid with the implemented Langevin function. The experimental scattering intensities of the spindle-like particles can be modeled simply by using the geometrical (length, radius, size distribution) and magnetic parameters (strength of magnetic field, magnetic moment) obtained from isotropic SAXS and macroscopic magnetization measurements, respectively. Whereas SAXS gives information on the morphological particle orientation in the applied field, WAXS texture analysis elucidates the atomic scale orientation of the magnetic easy direction in the hematite crystal structure. Our results strongly suggest the tendency for uniaxial anisotropy but indicate significant thermal fluctuations of the particle moments within the hematite basal plane

Morphological and crystallographic orientation of hematite spindles in applied magnetic field

The magnetic response of spindle-shaped hematite (-Fe2O3) nanoparticles was investigated by simultaneous small-angle and wide-angle X-ray scattering (SAXS/WAXS) experiments. The field-dependent magnetic and nematic order parameters of the magnetic single-domain nanospindles in a static magnetic field are fully described by SAXS simulations of an oriented ellipsoid with the implemented Langevin function. The experimental scattering intensities of the spindle-like particles can be modeled simply by using the geometrical (length, radius, size distribution) and magnetic parameters (strength of magnetic field, magnetic moment) obtained from isotropic SAXS and macroscopic magnetization measurements, respectively. Whereas SAXS gives information on the morphological particle orientation in the applied field, WAXS texture analysis elucidates the atomic scale orientation of the magnetic easy direction in the hematite crystal structure. Our results strongly suggest the tendency for uniaxial anisotropy but indicate significant thermal fluctuations of the particle moments within the hematite basal plane

Controlling the rotation modes of hematite nanospindles by dynamic magnetic fields

The magnetic field-induced actuation of colloidal nanoparticles has enabled tremendous recent progress towards microrobots, suitable for a variety of applications including targeted drug delivery, environmental remediation or minimally invasive surgery. Further size reduction to the nanoscale requires enhanced control of orientation and locomotion to overcome dominating viscous properties. Here we demonstrate how the coherent precession of nanoscale hematite spindles can be controlled via dynamic magnetic fields. Using time-resolved Small-Angle Scattering and optical transmission measurements, we reveal a clear frequency-dependent variation of orientation and rotation of an entire ensemble of hematite nanospindles. Our findings are in line with the different motion mechanisms observed for much larger, micron sized elongated particles near surfaces. The different dynamic rotation modes promise hematite nanospindles as a suitable model system towards field-induced locomotion in nanoscale magnetic robots

Controlling the rotation modes of hematite nanospindles using dynamic magnetic fields

The magnetic field-induced actuation of colloidal nanoparticles has enabled tremendous recent progress towards microrobots, suitable for a variety of applications including targeted drug delivery, environmental remediation, or minimally invasive surgery. Further size reduction to the nanoscale requires enhanced control of orientation and locomotion to overcome dominating viscous properties. Here, control of the coherent precession of hematite spindles via a dynamic magnetic field is demonstrated using nanoscale particles. Time-resolved small-angle scattering and optical transmission measurements reveal a clear frequency-dependent variation of orientation and rotation of an entire ensemble of non-interacting hematite nanospindles. The different motion mechanisms by nanoscale spindles in bulk dispersion resemble modes that have been observed for much larger, micron-sized elongated particles near surfaces. The dynamic rotation modes promise hematite nanospindles as a suitable model system for field-induced locomotion in nanoscale magnetic robots

Photothermal Bottom-up Graphene Nanoribbon Growth Kinetics

We present laser-induced photothermal synthesis of atomically precise graphene nanoribbons (GNRs). The kinetics of photothermal bottom-up GNR growth are unravelled by in situ Raman spectroscopy carried out in ultrahigh vacuum. We photothermally drive the reaction steps by short periods of laser irradiation and subsequently analyze the Raman spectra of the reactants in the irradiated area. Growth kinetics of chevron GNRs (CGNRs) and seven atoms wide armchair GNRs (7-AGNRs) is investigated. The reaction rate constants for polymerization, cyclodehydrogenation, and interribbon fusion are experimentally determined. We find that the limiting rate constants for CGNR growth are several hundred times smaller than for 7-AGNR growth and that interribbon fusion is an important elementary reaction occurring during 7AGNR growth. Our work highlights that photothermal synthesis and in situ Raman spectroscopy are a powerful tandem for the investigation of on-surface reactions

Origin of the Flat Band in Heavily Cs-Doped Graphene

A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we show that the flat band in graphene can be achieved by sandwiching a graphene monolayer by two cesium (Cs) layers. We investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculations. Our work highlights that charge transfer, zone folding of graphene bands, and the covalent bonding between C and Cs atoms are the origin of the flat energy band formation. Analysis of the Stoner criterion for the flat band suggests the presence of a ferromagnetic instability. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas

Massive and massless charge carriers in an epitaxially strained alkali metal quantum well on graphene

We show that Cs intercalated bilayer graphene acts as a substrate for the growth of a strained Cs film hosting quantum well states with high electronic quality. The Cs film grows in an fcc phase with a substantially reduced lattice constant of 4.9 angstrom corresponding to a compressive strain of 11% compared to bulk Cs. We investigate its electronic structure using angle-resolved photoemission spectroscopy and show the coexistence of massless Dirac and massive Schrodinger charge carriers in two dimensions. Analysis of the electronic self-energy of the massive charge carriers reveals the crystallographic direction in which a two-dimensional Fermi gas is realized. Our work introduces the growth of strained metal quantum wells on intercalated Dirac matter

Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates. Here, the authors characterize the spectroscopic and transport properties of heterojunctions composed of quasi-metallic and semiconducting graphene nanoribbons (GNRs) with different widths, showing a predominant quantum tunnelling mechanism. The GNR heterojunctions can also be used to realize adsorbate sensors with high sensitivity