Vedran Vonk

Formation of Wurtzite InP Nanowires Explained by Liquid-Ordering

We report an in situ surface X-ray diffraction study of liquid AuIn metal alloys in contact with zinc-blende InP (111)(B) substrates at elevated temperatures. We observe strong layering of the liquid metal alloy in the first three atomic layers in contact with the substrate. The first atomic layer of the alloy has a higher indium concentration than in bulk. In addition, in this first layer we find evidence for in-plane ordering at hollow sites, which could sterically hinder nucleation of zinc-blende InP. This can explain the typical formation of the wurtzite crystal structure in InP nanowires grown from AuIn metal particles.

Surface structure refinement including anomalous crystal truncation rods

A surface structure refinement procedure is introduced, which uses both anomalous and non-anomalous crystal truncation rod data simultaneously. It is shown how a single structural model can be refined against data sets measured at different wavelengths and how this can greatly reduce correlations between fit parameters. The structure factors are computed taking into account the anomalous dispersion corrections and are scaled to the data, whereby each data set is assigned its own scale factor. The procedure is implemented in the widely used surface diffraction program ROD. The structure of a one unit cell thin LaAlO3 film on an SrTiO3(001) substrate is refined by making use of a non-anomalous data set and one taken at the La L1-edge.

Observation of Ultrathin Precursor Film Formation during Ge-Si Liquid-Phase Epitaxy from an Undersaturated Solution

Our in situ X-ray study shows that a silicon substrate in contact with an undersaturated In(Ge) solution is wetted by an approximately 1 nm thin germanium film, which does not grow any thicker. The results can be understood by the use of thickness-dependent correlated interfacial energies. This near-equilibrium heterogeneous interface structure marks the initial stage of crystal growth before the formation of bulk material, which can only form under conditions of supersaturation. This finding uncovers a fundamental aspect of the thermodynamics at solid-liquid interfaces relevant for understanding the transition from equilibrium to supersaturation and is of importance for nanoscale solution growth methods.

Single orientation graphene synthesized on iridium thin films grown by molecular beam epitaxy

Heteroepitaxial iridium thin films were deposited on (0001) sapphire substrates by means of molecular beam epitaxy, and subsequently, one monolayer of graphene was synthesized by chemical vapor deposition. The influence of the growth parameters on the quality of the Ir films, as well as of graphene, was investigated systematically by means of low energy electron diffraction, x-ray reflectivity, x-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. Our study reveals (111) oriented iridium films with high crystalline quality and extremely low surface roughness, on which the formation of large-area epitaxial graphene is achieved. The presence of defects, like dislocations, twins, and 30° rotated domains in the iridium films is also discussed. The coverage of graphene was found to be influenced by the presence of 30° rotated domains in the Ir films. Low iridium deposition rates suppress these rotated domains and an almost complete coverage of graphene was o...

In situ x-ray study of the oxidation of a vicinal NiAl(6,7,1) surface

We present an in situ surface x-ray diffraction study of the clean, oxidized and subsequently annealed surfaces of regularly stepped NiAl(6,7,1). Our results show that the UHV stable, clean surface is not faceted and consists of a regular array of (1,1,0) terraces and (0,1,1) steps. The topmost Al and Ni atoms on the terraces exhibit a rippled relaxation while the step atoms are relaxed towards the bulk. Preferential Al oxidation at 540 K and 6×10−6 mbar O2 leads to the formation of a 5 A thin, disordered alumina layer and induces Al vacancies and Ni anti-sites in the Al-depleted interfacial region. The terrace-step structure of the clean surface is maintained, but strong inward relaxations of the interfacial atoms change the strain field around the steps. Massive (1,1,0) faceting with facets up to 50 times larger than the original terraces occurs after high-temperature annealing, during which the surface oxide develops a complex long-range ordering. These results can be understood by the change of interfacial strain, which removes the energy barrier for mass transport. In addition, unlike in the case of low-index (1,1,0) surfaces, we find the step-induced suppression of twin domain formation in the alumina film grown on NiAl(6,7,1). Our results show that the interplay between oxidation and strain can have dramatic effects on the morphology of vicinal surfaces.

Nanosecond laser pulse heating of a platinum surface studied by pump-probe X-ray diffraction

We report on the quantitative determination of the transient surface temperature of Pt(110) upon nanosecond laser pulse heating. We find excellent agreement between heat transport theory and the experimentally determined transient surface temperature as obtained from time-resolved X-ray diffraction on timescales from hundred nanoseconds to milliseconds. Exact knowledge of the surface temperatures temporal evolution after laser excitation is crucial for future pump-probe experiments at synchrotron storage rings and X-ray free electron lasers.

In-situ X-ray Diffraction at Synchrotrons and Free-Electron Laser Sources

X-ray Diffraction (XRD) is an outstanding tool for structural analyses at the atomic scale, and both the experimental techniques and the theoretical interpretations are well established. X-rays also have the advantage of being highly penetrating, as compared to electrons for instance, allowing for the study of bulk materials, or to study samples in complicated environments. The high photon fluxes available at third generation synchrotron sources make it possible to collect full diffraction patterns in relatively short times, and thus to follow time varying processes in-situ. In the first part of this chapter we briefly discuss the advantages and disadvantages of X-rays as compared to other probes like electrons or neutrons. In the second part as an example in-situ surface X-ray diffraction studies of growing films using pulsed laser deposition (PLD) will be presented. The hetero-epitaxial growth process, especially of the first mono-layers can only be understood by in-situ diffraction studies in the PLD chamber under deposition conditions. Also high energy diffraction of buried interfaces will be discussed briefly. The final part of this chapter will present the possibilities for in-situ diffraction studies at the upcoming Free-Electron Laser sources, with fully coherent beams and sufficient intensities to collect full diffraction patterns with single 100 femto-second pulses. The characteristics of the Free-Electron-Lasers and various planned experiments will be presented.