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Register a new userInstability driven fragmentation of nanoscale fractal islands
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Authors
C. Brechignac
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Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms.
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We analyze in detail the fluctuations and correlations of the (spatial) Fourier modes of nano-scale single-layer islands on (111) fcc crystal surfaces. We analytically show that the Fourier modes of the fluctuations couple due to the anisotropy of the crystal, changing the power spectrum of the fluctuations, and that the actual eigenmodes of the fluctuations are the appropriate linear combinations of the Fourier modes. Using kinetic Monte Carlo simulations with bond-counting parameters that best match realistic energy barriers for hopping rates, we deduce absolute line tensions as a function of azimuthal orientation from the analyses of the fluctuation of each individual mode. The autocorrelation functions of these modes give the scaling of the correlation times with wavelength, providing us with the rate-limiting kinetics driving the fluctuations, here step-edge diffusion. The results for the energetic parameters are in reasonable agreement with available experimental data for Pb(111) surfaces, and we compare the correlation times of island-edge fluctuations to relaxation times of quenched Pb crystallites.
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Surface catalytic processes produce, under certain conditions, small clusters of adsorbed atoms or groups, called {em islands} which, after they have been formed, move as individual entities. Here we consider the catalytic reduction of NO with hydrogen on platinum. (i) Using video field ion microscopy, we observe the dynamic motion of small hydroxyl islands on the Pt(001) plane; despite changes in their morphology, the islands dimensions are confined to values corresponding to 10 to 30 Pt atoms suggesting cooperative effects to be in operation. (ii) We construct an automaton (or lattice Monte-Carlo) model on the basis of a set of elementary processes governing the microscopic dynamics. The agreement between the simulation results and the experimental observations suggests a possible mechanism for the formation and dynamics of hydroxyl islands.
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In strained heteroepitaxy, two-dimensional (2D) layers can exhibit a critical thickness at which three-dimensional (3D) islands self-assemble, relieving misfit strain at the cost of an increased surface area. Here we show that such a morphological phase transition can be induced on-demand using surfactants. We explore Bi as a surfactant in the growth of InAs on GaAs(110), and find that the presence of surface Bi induces Stranski-Krastanov growth of 3D islands, while growth without Bi always favors 2D layer formation. Exposing a static two monolayer thick InAs layer to Bi rapidly transforms the layer into 3D islands. Density functional theory calculations reveal that Bi reduces the energetic cost of 3D island formation by modifying surface energies. These 3D nanostructures behave as optically active quantum dots. This work illustrates how surfactants can enable quantum dot self-assembly where it otherwise would not occur.
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