Water Adsorption and Ion Induced Defect Formation: A Comparative Study of Graphene and Noble Metal Surfaces

The thesis at hand is subdivided into parts. In the first part we study the thermodynamics and structure of water adsorption on graphene and noble metal surfaces. The second part of this thesis is dedicated to surface damage caused by grazing incidence ion bombardment of graphene on Ir(111). Both parts are completed by a comparison of the various sample systems. We study the structure and stability of the first water layer on Pt(111) by variable temperature scanning tunneling microscopy. Two structures observed previously by diffraction techniques are imaged for coverages at or close to completion of the wetting layer. At 140K only a (√37x√37)R25.3° superstructure can be established, while at 130K also a (√39x√39)R16.1° superstructure with slightly higher molecular density is formed. In the temperature range under concern the superstructures reversibly transform into each other by slight changes in coverage through adsorption or desorption. The superstructures exhibit a complex pattern of molecules in different geometries. We find that a high Pt step edge density considerably increases the long range order of the equilibrium superstructures, presumably due to the capability of step edges to trap residual adsorbates from the surface. Passivating the step edges with CO or preparing a flat metal surface leads to the formation of disordered structures, which still show the same structural elements as the ordered ones. Coadsorption of Xe and CO proves that the water layer covers the metal surface completely. Moreover, we determine the two-dimensional crystal structure of Xe on top of the chemisorbed water layer which exhibits a Xe-Xe distance close to the one in bulk Xe and a rotation angle of 90° between the close-packed directions of Xe and the close-packed directions of the underlying water layer. CO is shown to replace H2O on the Pt(111) surface as has been deduced previously. Finally, a so far not understood restructuring of the adlayer by an increased tunneling current has been observed. We present experimental data regarding the wetting behaviour and structure formation of water on Ir(111). Studying the thermodynamics of water adsorption and pointing out the possibility to form a wetting layer, we perform a coverage dependent analysis of the sub-monolayer regime. We show that water molecules form planar, polar clusters upon adsorption. A continuous water layer forms out of the cluster phase for Θ>0.5ML. The molecular arrangement is nearly identical to the wetting layer observed on Pt(111). The water growth mode changes from wetting to non-wetting for Θ>1ML as the closed water monolayer is hydrophobic. For both Pt(111) and Ir(111), we demonstrate that tunneling of electrons into the antibonding state or from the bonding state of H2O leads to dissociation of the molecules and a corresponding reordering of the adlayer into a (√3x√3)R30°-structure. An analogous study is carried out for water adsorption on a graphene covered Ir(111) surface. We show that water dewets graphene at 20K and forms three-dimensional, electronically insulating clusters aligned in the Gr/Ir(111) moiré for Θ=3ML. Higher coverage results in coalescence and the formation of an amorphous adlayer. The occurence of this structure can be promoted by an increase of the adsorption temperature above 100K. By combining ion beam experiments and atomistic simulations we study the production of defects in graphene on Ir(111) under grazing incidence of low energy noble gas ions. Low fluences are chosen to make the damage patterns of individual ions visible. We demonstrate that the ions are channeled in between graphene and the substrate, giving rise to chains of vacancy clusters, and control the defect structure via tuning of the bombardment parameters. Combining our experiments on thermal stability of defects and density functional theory calculations, we discuss the atomic structure of defects in detail and show that their edges are bending down towards the substrate to saturate dangling bonds by the metal surface. With the onset of vacancy cluster mobility around 800K, the vacancies sense formation energy differences within the graphene moiré supercell, which paves the way towards the formation of a graphene nanomesh. In order to realize this struture a transition from isolated ion impacts towards a defect density in the magnitude of one per moiré cell has to be accomplished. We discuss the sample morphology and thermal evolution of the damage patterns following prolonged ion exposure. We show that the temperature during bombardment is of great importance to drive the carbon vacancies to these sites, where the formation energy is lowest, and thus allow for an immediate reconstruction of the graphene layer. Elaborating appropriate parameters for the formation of a high quality nanomesh regarding ion energy and fluence, we give an experimental realization of this structure

Інтегральне числення функції однієї змінної. Матеріали методичного забезпечення поглибленого вивчення розділу студентами технічних спеціальностей

Подано основні теоретичні відомості до розділу «Інтегральне числення функції однієї змінної», наведено розв’язання задач підвищеної складності та приклади для самостійного розгляду

Tuning the van der Waals Interaction of Graphene with Molecules via Doping

We use scanning tunneling microscopy to visualize and thermal desorption spectroscopy to quantitatively measure that the binding of naphthalene molecules to graphene (Gr), a case of pure van der Waals (vdW) interaction, strengthens with $n$- and weakens with $p$-doping of Gr. Density functional theory calculations that include the vdW interaction in a seamless, ab initio way accurately reproduce the observed trend in binding energies. Based on a model calculation, we propose that the vdW interaction is modified by changing the spatial extent of Gr's $\pi$ orbitals via doping

Trails of Kilovolt Ions Created by Subsurface Channeling

Using scanning tunneling microscopy, we observe the damage trails produced by keV noble-gas ions incident at glancing angles onto Pt(111). Surface vacancies and adatoms aligned along the ion trajectory constitute the ion trails. Atomistic simulations reveal that these straight trails are produced by nuclear (elastic) collisions with surface layer atoms during subsurface channeling of the projectiles. In a small energy window around 5 keV, Xe(+) ions create vacancy grooves that mark the ion trajectory with atomic precision. The asymmetry of the adatom production on the two sides of the projectile path is traced back to the asymmetry of the ion's subsurface channel

H2O on Graphene/Ir(111): A Periodic Array of Frozen Droplets

Scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS) show that deposition of water molecules onto epitaxial graphene on Ir(111) leads to the formation of an extended and well ordered array of amorphous water clusters. We trace the evolution of this cluster phase as dependent on water exposure and deposition temperature. The formation of separated clusters is due to binding energy differences within the moire superstructure

Pronunciation and Intonation [in Greek]

The molecular structure of the wetting layer of ice on Pt(111) is resolved using scanning tunneling microscopy. Two structures observed previously by diffraction techniques are imaged for coverages at or close to completion of the wetting layer. At 140 K only a root 37 x root 37R25.3 degrees superstructure can be established while at 130 K also a root 39 x root 39R16.1 degrees superstructure with slightly higher molecular density is formed. In the temperature range under concern the superstructures reversibly transform into each other by slight changes in coverage through adsorption or desorption. The superstructures exhibit a complex pattern of molecules in different geometries

H 2

Scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS) show that deposition of water molecules onto epitaxial graphene on Ir(111) leads to the formation of an extended and well ordered array of amorphous water clusters. We trace the evolution of this cluster phase as dependent on water exposure and deposition temperature. The formation of separated clusters is due to binding energy differences within the moire superstructure

Ion Impacts on Graphene/Ir(111): Interface Channeling, Vacancy Funnels, and a Nanomesh

By combining ion beam experiments and atomistic simulations we study the production of defects in graphene on Ir(111) under grazing incidence of low energy Xe ions. We demonstrate that the ions are channeled in between graphene and the substrate, giving rise to chains of vacancy clusters with their edges bending down toward the substrate. These clusters self-organize to a graphene nanomesh via thermally activated diffusion as their formation energy varies within the graphene moiré supercell