Grain boundaries in graphene grown by chemical vapor deposition

The scientific literature on grain boundaries (GBs) in graphene was reviewed. The review focuses mainly on the experimental findings on graphene grown by chemical vapor deposition (CVD) under a very wide range of experimental conditions (temperature, pressure hydrogen/hydrocarbon ratio, gas flow velocity and substrates). Differences were found in the GBs depending on the origin of graphene: in micro-mechanically cleaved graphene (produced using graphite originating from high-temperature, high-pressure synthesis), rows of non-hexagonal rings separating two perfect graphene crystallites are found more frequently, while in graphene produced by CVD—despite the very wide range of growth conditions used in different laboratories—GBs with more pronounced disorder are more frequent. In connection with the observed disorder, the stability of two-dimensional amorphous carbon is discussed and the growth conditions that may impact on the structure of the GBs are reviewed. The most frequently used methods for the atomic scale characterization of the GB structures, their possibilities and limitations and the alterations of the GBs in CVD graphene during the investigation (e.g. under e-beam irradiation) are discussed. The effects of GB disorder on electric and thermal transport are reviewed and the relatively scarce data available on the chemical properties of the GBs are summarized. GBs are complex enough nanoobjects so that it may be unlikely that two experimentally produced GBs of several microns in length could be completely identical in all of their atomic scale details. Despite this, certain generalized conclusions may be formulated, which may be helpful for experimentalists in interpreting the results and in planning new experiments, leading to a more systematic picture of GBs in CVD graphene

Colloquium: Graphene spectroscopy

Spectroscopic studies of electronic phenomena in graphene are reviewed. A variety of methods and techniques are surveyed, from quasiparticle spectroscopies (tunneling, photoemission) to methods probing density and current response (infrared optics, Raman) to scanning probe nanoscopy and ultrafast pump-probe experiments. Vast complimentary information derived from these investigations is shown to highlight unusual properties of Dirac quasiparticles and many-body interaction effects in the physics of graphene.Comment: 36 pages, 16 figure

Transient displacement analysis using double-pulsed ESPI and fringe processing methods

This thesis deals with techniques for the displacement measurement of fast transient phenomena using ESPI. Four main contributions are presented. First, a computer model for speckle noise and ESPI fringe generation is proposed. An assessment methodology for speckle noise reduction algorithms is then derived using the computer model. Then the noise in the ESPI fringe patterns is analysed using computer generated speckle and several solutions for its reduction are proposed and assessed. Finally, a fast electro-optical system is presented as a solution to the unambiguous phase extraction problem from a single interferogram. With this novel system, whole field transient displacements occurring in time intervals as short as 20ns can be successfully registered and retrieved. [Continues.

Out-of-Plane Magnetic Anisotropy in Ordered Ensembles of Fey_yN Nanocrystals Embedded in GaN

Phase-separated semiconductors containing magnetic nanostructures are relevant systems for the realization of high-density recording media. Here, the controlled strain engineering of Ga$\delta$FeN layers with Fe$_y$N embedded nanocrystals (NCs) \textit{via} Al$_x$Ga$_{1-x}$N buffers with different Al concentration $0<x_\mathrm{Al}<41$\% is presented. Through the addition of Al to the buffer, the formation of predominantly prolate-shaped $\varepsilon$-Fe$_3$N NCs takes place. Already at an Al concentration $x_\mathrm{Al}$\,$\approx$\,5\% the structural properties---phase, shape, orientation---as well as the spatial distribution of the embedded NCs are modified in comparison to those grown on a GaN buffer. Although the magnetic easy axis of the cubic $\gamma$'-Ga$_y$Fe$_{4-y}$N nanocrystals in the layer on the $x_\mathrm{Al} = 0\%$ buffer lies in-plane, the easy axis of the $\varepsilon$-Fe$_3$N NCs in all samples with Al$_x$Ga$_{1-x}$N buffers coincides with the $[0001]$ growth direction, leading to a sizeable out-of-plane magnetic anisotropy and opening wide perspectives for perpendicular recording based on nitride-based magnetic nanocrystals.Comment: 29 pages, 10 figures, submitte

Molecular organisation of water and alcohols at solid-liquid interfaces

The organisation and self-assembly of molecules at solid-liquid interfaces is central to numerous natural processes and can be used to create supramolecular architectures with functional applications. Historically, studies into surface-based self-assembly in liquids in ambient conditions are limited to molecules with significant surface interactions predisposed to durably reside at the surface. Outside of extreme conditions, such as low temperatures or under confinement, the self-assembly of small molecules (< 20 atoms) without significant surface interactions remains relatively unexplored. Here, a joint approach involving atomic force microscopy and molecular dynamics simulations is used to explore the self-assembly of small alcohols and water into supramolecular structures on hydrophobic surfaces in ambient conditions. This self-assembly can occur because of the formation of extended hydrogen bonded networks between the assembling molecules at the interface, enabling the molecules to adsorb as a group. Investigations into this system has led to three, major, novel observations. The first is that graphite catalyses a reaction involving water and volatile organics to produce small quantities of methanol. This reaction is enhanced by applied electric fields and the methanol produced can subsequently self-assembly with the water, thus changing the behaviour of the interfacial liquid. The second is that at hydrophobic interfaces, the structure of small alcohol-water mixtures displays a strong concentration dependence; with alcohol molecules at the surface switching between states of hydrogen bonding with the bulk liquid and with other molecules in the same plane. The final result is that due to the weak molecular surface interactions, the hydrogen bond networks of these group-effect stabilised assemblies can be influenced through multiple approaches to create a wide variety of supramolecular structures. The generality and importance of group-effect self-assembly is demonstrated to be applicable to multiple hydrophobic interfaces. Overall these results form the foundation for further investigations into small molecule self-assembly, along with having wider implications for other fields including the development of novel carbon-based catalytic materials, studies into transfer properties at electrodes as well as understanding friction and lubrication in many systems

Microstructure of Al2O3 scales formed on NiCrAl alloys

The structure of transient scales formed on pure and Y or Zr-doped Ni-15Cr-13Al alloys oxidized for 0.1 hr at 1100 C was studied by the use of transmission electron microscopy. Crystallographically oriented scales were found on all three alloys, but especially for the Zr-doped NiCrAl. The oriented scales consisted of alpha-(Al,Cr)2O3, Ni(Al,Cr)2O4 and gamma-Al2O3. They were often found in intimate contact with each other such that the close-packed planes and directions of one oxide phase were aligned with those of another. The prominent structural features of the oriented scales were approximately equal to micrometer subgrains; voids, antiphase domain boundaries and aligned precipitates were also prevalent. Randomly oriented alpha-Al2O3 was also found and was the only oxide ever observed at the immediate oxide metal interface. These approximately 0.15 micrometer grains were populated by intragranular voids which decreased in size and number towards the oxide metal interface. A sequence of oxidation was proposed in which the composition of the growing scale changed from oriented oxides rich in Ni and Cr to oriented oxides rich in Al. At the same time the structure changed from cubic spinels to hexagonal corundums with apparent precipitates of one phase in the matrix of the other. Eventually randomly oriented pure alpha-Al2O3 formed as the stable oxide with an abrupt transition: there was no gradual loss of orientation, no gradual compositional change or no gradual decrease in precipitate density