EQUILIBRIUM AND NON-EQUILIBRIUM PROPERTIES OF FINITE-VOLUME CRYSTALLITES

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2006-03-15

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Abstract

Finite volume effects on equilibrium and non-equilibrium properties of nano-crystallites are studied theoretically and compared to both experiment and simulation.

When a system is isolated or its size is small compared to the correlation length, all equilibrium and close-to-equilibrium properties will depend on the system boundary condition. Specifically for solid nano-crystallites, their finite size introduces global curvature to the system, which alters its equilibrium properties compared to the thermodynamic limit. Also such global curvature leads to capillary-induced morphology changes of the surface. Interesting dynamics can arise when the crystallite is supported on a substrate, with crossovers of the dominant driving force from the capillary force and crystallite-substrate interactions.

To address these questions, we introduce thermodynamic functions for the boundary conditions, which can be derived from microscopic models. For nano-crystallites, the boundary is the surface (including interfaces), the thermodynamic description is based on the steps that define the shape of the surface, and the underlying microscopic model includes kinks.

The global curvature of the surface introduces metastable states with different shapes governed by a constant of integration of the extra boundary condition, which we call the shape parameter c. The discrete height of the steps introduces transition states in between the metastable states, and the lowest energy accessible structure (energy barrier less 10kBT) as a function of the volume has been determined. The dynamics of nano-crystallites as they relax from a non-equilibrium structure is described quantitatively in terms of the motion of steps in both capillary-induced and interface-boundary-induced regimes. The step-edge fluctuations of the top facet are also influenced by global curvature and volume conservation and the effect yields different dynamic scaling exponents from a pure 1D system.

Theoretical results are compared with experimental results for Pb crystallites supported on a Ru substrate. The thermodynamic and kinetic parameters are obtained from the observations of equilibrium structures and rates of structure evolution. The predicted dynamic scaling exponents of facet-edge fluctuations are also obtained and compared to an isolated step. Facet-edge fluctuations are also tested with Monte Carlo Simulations (MCS) using the Terrace-Step-Kink (TSK) model with thermally exited kink formations and conserved microscopic processes.

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