Applications of Transition Finding Techniques
to Island Diffusion on Surfaces
J. C. Hamilton
Sandia National Laboratories, Livermore, CA
Determining activation barriers for complex processes typically requires precise information regarding initial and final states and often some additional information or intuition regarding the transition state. Often it is essential to obtain at least some of this information from laboratory or computer experiments. For laboratory experiments the Hamiltonian is exact, however atoms are not generally tagged and transition states generally cannot be observed. For computer experiments the atoms are tagged, however the Hamiltonian is approximate and time scales for molecular dynamics are drastically limited.
For island diffusion on surfaces, laboratory and/or computer experiments have revealed a variety of complex cooperative diffusion mechanisms including exchange diffusion of adatoms and dimers on (100) surfaces1, dimer shear of islands on (100) surfaces2, and dislocation nucleation and glide on close-packed surfaces.3
In this talk new theoretical calculations of island diffusion are considered, specifically Ag on Ni(100) which is an example of a highly strained overlayer on a (100) surface, and Ir on Ir(111).
Ag forms a 2x8 reconstruction on Ni(100). This relaxes the large (16%) difference in lattice constants allowing Ag to form a hexagonal overlayer on the Ni substrate. I show that islands of Ag on Ni(100) can readily glide along the long period axis of the reconstruction, but not in the perpendicular direction. This theoretical prediction is substantiated by experimental observations of Ag island growth on Ag(100) and on Ni(100). On Ag(100) small square islands are grown at room temperature. On Ni(100) the islands are large and ramified, consistent with my calculations.
Ir on Ir(111) is one of the most exhaustively studied island diffusion systems.4 We have endeavored to understand the complex and unusual diffusion of compact clusters containing 7 or 19 Ir atoms on Ir(111) using both first principles calculations and the newly developed technique of extrapolated-temperature dynamics.5