Dominant contributions to the apparent activation energy in two-dimensional submonolayer growth: comparison between Cu/Ni(111) and Ni/Cu(111).

For surface-mediated processes in general, such as epitaxial growth and heterogeneous catalysis, a constant slope in the Arrhenius diagram of the rate of interest, R, against inverse temperature, log R vs 1/T, is traditionally interpreted as the existence of a bottleneck elementary reaction (or rate-determining step), whereby the constant slope (or apparent activation energy, [Formula: see text]) reflects the value of the energy barrier for that elementary reaction. In this study, we express [Formula: see text] as a weighted average, where every term contains the traditional energy barrier for the corresponding elementary reaction plus an additional configurational term, while identifying each weight as the probability of executing the corresponding elementary reaction. Accordingly, the change in the leading (most probable) elementary reaction with the experimental conditions (e.g. temperature) is automatically captured and it is shown that a constant value of [Formula: see text] is possible even if control shifts from one elementary reaction to another. To aid the presentation, we consider kinetic Monte Carlo simulations of submonolayer growth of Cu on Ni(111) and Ni on Cu(111) at constant deposition flux, including a large variety of single-atom, multi-atom and complete-island diffusion events. In addition to analysing the dominant contributions to the diffusion constant of the complete adparticle system (or tracer diffusivity) and its apparent activation energy as a function of both coverage and temperature for the two heteroepitaxial systems, their surface morphologies and island densities are also compared.

Multiscale Analysis of Phase Transformations in Self-Assembled Layers of 4,4'-Biphenyl Dicarboxylic Acid on the Ag(001) Surface.

Understanding the nucleation and growth kinetics of thin films is a prerequisite for their large-scale utilization in devices. For self-assembled molecular phases near thermodynamic equilibrium the nucleation-growth kinetic models are still not developed. Here, we employ real-time low-energy electron microscopy (LEEM) to visualize a phase transformation induced by the carboxylation of 4,4'-biphenyl dicarboxylic acid on Ag(001) under ultra-high-vacuum conditions. The initial (α) and transformed (ß) molecular phases are characterized in detail by X-ray photoemission spectroscopy, single-domain low-energy electron diffraction, room-temperature scanning tunneling microscopy, noncontact atomic force microscopy, and density functional theory calculations. The phase transformation is shown to exhibit a rich variety of phenomena, including Ostwald ripening of the α domains, burst nucleation of the ß domains outside the α phase, remote dissolution of the α domains by nearby ß domains, and a structural change from disorder to order. We show that all phenomena are well described by a general growth-conversion-growth (GCG) model. Here, the two-dimensional gas of admolecules has a dual role: it mediates mass transport between the molecular islands and hosts a slow deprotonation reaction. Further, we conclude that burst nucleation is consistent with a combination of rather weak intermolecular bonding and the onset of an additional weak many-body attractive interaction when a molecule is surrounded by its nearest neighbors. In addition, we conclude that Ostwald ripening and remote dissolution are essentially the same phenomenon, where a more stable structure grows at the expense of a kinetically formed, less stable entity via transport through the 2D gas. The proposed GCG model is validated through kinetic Monte Carlo (kMC) simulations.

Complex chiral colloids and surfaces via high-index off-cut silicon.

Silicon wafers are commonly etched in potassium hydroxide solutions to form highly symmetric surface structures. These arise when slow-etching {111} atomic planes are exposed on standard low-index surfaces. However, the ability of nonstandard high-index wafers to provide more complex structures by tilting the {111} planes has not been fully appreciated. We demonstrate the power of this approach by creating chiral surface structures and nanoparticles of a specific handedness from gold. When the nanoparticles are dispersed in liquids, gold colloids exhibiting record molar circular dichroism (>5 × 10(9) M(-1) cm(-1)) at red wavelengths are obtained. The nanoparticles also present chiral pockets for binding.

VisualTAPAS: an example of density functional theory assisted understanding and simulation of anisotropic etching (abstract only).

VisualTAPAS is a self-contained, user-friendly, graphical user interface based simulator of wet etching and deep reactive ion etching with multi-masking capabilities, built upon an octree representation of the silicon substrate (www.fyslab.hut.fi/∼mag/VisualTAPAS/Home.html). The program allows the use of a wide range of kinetic Monte Carlo (KMC) and cellular automata time-evolution algorithms, including a fast octree search algorithm for the KMC simulations. 'VisualTAPAS' stands for 'visual three-dimensional anisotropic processing at all scales'. The use of the term 'visual' stresses the interactive visual capabilities of the program. Here, a brief history of the evolution of VisualTAPAS as a research tool will be presented: from the initial efforts, explaining the anisotropy of wet etching as a result of steric hindrance using a combination of density functional theory (DFT) and KMC simulations, to the most recent implementation, focusing on the propagation of the etch front for engineering applications by making use of the analytical solution of the continuous cellular automaton (CCA) method; and in between, a recent example of DFT assisted understanding of the effects of metal impurities on the surface morphology of the etched surfaces will be presented. We try to bridge together three complementary simulation tools, namely, DFT, KMC and CCA methods. Our experience with the use of the three methods for the simulation of anisotropic etching shows that DFT is very useful and, many times, an unavoidable approach. Similarly, the KMC approach is the method of choice for understanding the large variety of etched surface morphologies while the CCA is an outstanding tool for the simulation of the process at an engineering level.