Bistability in the Shape Transition of Strained Islands

The equilibrium shape of a monatomic strained island on a substrate depends on the step free energies and the difference in surface stress between the island and the substrate. For small island sizes the step free energies dominate, resulting in compact islands. Beyond a critical island size, however, the strain energy becomes dominant and the island maximizes its perimeter, resulting in elongated islands. Here we show that for strained islands with force monopoles pointing in opposing directions at neighboring steps, a regime exists near the critical island size where both compact and elongated shapes can coexist

Vacancy diffusion in the Cu(001) surface II: Random walk theory

We develop a version of the vacancy mediated tracer diffusion model, which follows the properties of the physical system of In atoms diffusing within the top layer of Cu(001) terraces. This model differs from the classical tracer diffusion problem in that (i) the lattice is finite, (ii) the boundary is a trap for the vacancy, and (iii) the diffusion rate of the vacancy is different, in our case strongly enhanced, in the neighborhood of the tracer atom. A simple continuum solution is formulated for this problem, which together with the numerical solution of the discrete model compares well with our experimental results.Comment: 13 pages, 4 figure

Channeling in helium ion microscopy: Mapping of crystal orientation

Background: The unique surface sensitivity and the high resolution that can be achieved with helium ion microscopy make it a\ud competitive technique for modern materials characterization. As in other techniques that make use of a charged particle beam, channeling\ud through the crystal structure of the bulk of the material can occur.\ud Results: Here, we demonstrate how this bulk phenomenon affects secondary electron images that predominantly contain surface\ud information. In addition, we will show how it can be used to obtain crystallographic information. We will discuss the origin of\ud channeling contrast in secondary electron images, illustrate this with experiments, and develop a simple geometric model to predict\ud channeling maxima.\ud Conclusion: Channeling plays an important role in helium ion microscopy and has to be taken into account when trying to achieve\ud maximum image quality in backscattered helium images as well as secondary electron images. Secondary electron images can be\ud used to extract crystallographic information from bulk samples as well as from thin surface layers, in a straightforward manner

The Atomic Slide Puzzle: Self-Diffusion of an Impure Atom

In a series of recent papers van Gastel et al have presented first experimental evidence that impure, Indium atoms, embedded into the first layer of a Cu(001) surface, are not localized within the close-packed surface layers but make concerted, long excursions visualized in a series of STM images. Such excursions occur due to continuous reshuffling of the surface following the position exchanges of both impure and host atoms with the naturally occuring surface vacancies. Van Gastel et al have also formulated an original lattice-gas type model with asymmetric exchange probabilities, whose numerical solution is in a good agreement with the experimental data. In this paper we propose an exact lattice solution of several versions of this model.Comment: Latex, 4 pages, 2 figures, to appear in Phys. Rev. E (RC

Spinodal decomposition driven formation of Pt-nanowires on Ge(001)

Using low energy electron microscopy, we have found that the deposition of Pt on Ge(001) leads to the formation of a surface confined eutectic liquid when the system is heated above 980 K. From the bulk phase diagram we derive the composition of the eutectic phase: Ge$_{0.78}$Pt$_{0.22}$. Upon solidification and further cooling down, two distinct types of terrace emerge, the so-called α and β terraces, which have been assigned previously as relatively Pt-poor and Pt-rich, respectively. Immediately after solidification β terraces fully cover the surface, while further cooling leads to their partial transformation into α terraces. Subsequently, Pt nanowire domains nucleate and grow exclusively on β terraces at about 600 K. The results are discussed using spinodal decomposition concepts and reveal a new pathway for nanowire formation

Imaging ultra thin layers with helium ion microscopy: Utilizing the channeling contrast mechanism

Background: Helium ion microscopy is a new high-performance alternative to classical scanning electron microscopy. It provides superior resolution and high surface sensitivity by using secondary electrons.\ud \ud Results: We report on a new contrast mechanism that extends the high surface sensitivity that is usually achieved in secondary electron images, to backscattered helium images. We demonstrate how thin organic and inorganic layers as well as self-assembled monolayers can be visualized on heavier element substrates by changes in the backscatter yield. Thin layers of light elements on heavy substrates should have a negligible direct influence on backscatter yields. However, using simple geometric calculations of the opaque crystal fraction, the contrast that is observed in the images can be interpreted in terms of changes in the channeling probability.\ud \ud Conclusion: The suppression of ion channeling into crystalline matter by adsorbed thin films provides a new contrast mechanism for HIM. This dechanneling contrast is particularly well suited for the visualization of ultrathin layers of light elements on heavier substrates. Our results also highlight the importance of proper vacuum conditions for channeling-based experimental methods\u

Vacancy-mediated diffusion of Co atoms embedded in Cu(001)

The diffusion of Co atoms in the Cu(001) surface has been studied using Scanning Tunneling Microscopy (STM). Like other impurities in the Cu(001) surface, the diffusion of Co is mediated by single surface vacancies. STM images reveal that diffusion of the embedded atoms takes place through multi-atom jumps separated by long time intervals, which is characteristic for this type of diffusion. The jump length and frequency are measured to establish the nature of the interaction between surface vacancies and the embedded Co atoms and to extract the relevant formation and diffusion energies

Nothing moves a surface: vacancy mediated surface diffusion

We report scanning tunneling microscopy observations, which imply that all atoms in a close-packed copper surface move frequently, even at room temperature. Using a low density of embedded indium `tracer' atoms, we visualize the diffusive motion of surface atoms. Surprisingly, the indium atoms seem to make concerted, long jumps. Responsible for this motion is an ultra-low density of surface vacancies, diffusing rapidly within the surface. This interpretation is supported by a detailed analysis of the displacement distribution of the indium atoms, which reveals a shape characteristic for the vacancy mediated diffusion mechanism that we propose.Comment: 4 pages; for associated movie, see http://www-lion.leidenuniv.nl/sections/cm/groups/interface/projects/therm