[en] We have investigated experimentally and theoretically the role that ripening processes play in the evolution of supported transition metal nanoclusters on solid surfaces. We discuss some avenues of analysis that can be more informative of the dominant mechanisms than mere temporal measures of island size. Our model system is formed by the growth of Pd nanoclusters at room temperature on the cross-linked (1 x 2) reconstructed TiO₂(110) surface. The reconstructed surface contains a rectangular array of 'defects' that nucleate and pin the clusters. We have followed the surface evolution by variable temperature scanning tunnelling microscopy as the temperature was raised stepwise to 973 deg. K. We find that the ripening is dominated by an Ostwald-type mechanism with particles remaining immobile during growth (decay). Monte Carlo simulations of the ripening of arrays of three-dimensional islands have been undertaken for comparison with the experiments. These reproduce the spatial properties of the experimental arrays and the scaling nature of the height distribution. In addition the classic mean field theory for the size distribution is modified to include island size-separation correlations and is found to recover the size distribution found in both the simulation and the experiment. We conclude that detailed atomistic understanding of ripening is not always necessary to understand important features of nanostructure evolution

[en] Natural asphaltenes are defined as polyaromatic compounds whose chemical composition and structure are dependent on their geological origin and production history, hence are regarded as complex molecules with aromatic cores and aliphatic tails that occur in the heaviest fraction of crude oil. The aggregation of asphaltenes presents a range of technical challenges to the production and processing of oil. In this work we study the behaviour of the model asphaltene-like molecule hexa- tert -butylhexa- peri -hexabenzocoronene (HTBHBC) using molecular dynamics simulation. It was found that the regular arrangement of the tert -butyl side chains prevents the formation of strongly-bound dimers by severely restricting the configurational space of the aggregation pathway. In contrast, a modified molecule with only 3 side chains is readily able to form dimers. This work therefore confirms the influence of the molecular structure of polyaromatic compounds on their aggregation mechanism, and reveals the unexpected design rules required for model systems that can mimic the behavior of asphaltenes. (paper)

[en] The distribution of capture zones formed during the nucleation and growth of point islands on a one-dimensional substrate during monomer deposition is considered for general critical island size i. A fragmentation theory approach yields the small and (for i = 0) large-size asymptotics for the capture-zone distribution (CZD) under the assumption of no neighbour–neighbour gap-size correlation. These CZD asymptotic forms are different to those of the generalized Wigner surmise which has recently been proposed for island nucleation and growth models, and we discuss the reasons for the discrepancies. (paper)