Crystallite Relaxation by Confined Screw Dislocation Motion


Nanoscale lead crystallites supported on Ruthenium substrate were prepared in unstable shapes by rapidly cooling the system from its high temperature state. It was observed that about 30% of these crystallites had a noncentered screw dislocation on the circular top facet. These crystallites relax to their equilibrium state by turning of the spiral step emerging from the screw dislocation. The turning of these spiral steps was observed using Scanning Tunneling Microscopy. These experiments showed that when the radius was 385 nm a single revolution took 23 minutes. As the spiral turns the shape and rotation rate changes significantly as can be seen in this movie of successive STM images.

Step Model

We construct a model of the top surface consisting of a step that starts the core of the dislocation and smoothly joins a circle corresponding to the boundary of the top facet. The local motion of the step is governed by the attachment and detachment of atoms from the step edge but the global shapes are strongly influenced by the confinement due to the finite size of the facet. The time dependent step shapes predicted using a curvature driven law of motion are very similar to the experimental shapes and the kinetic parameters are in agreement with other experiments on the same system.

Uniformly Rotating Shapes

We also look at approximate solutions called Uniformly Rotating Shapes (URS). These shapes have, at any given time a constant angular velocity along the step from the core to the point where they join the circle smoothly with a continuous first derivative. But unlike the case of a centered spiral, the angular velocity and the total arc length from the core to the joining point will change for solutions at different time instants. These shapes are very close to the time dependent solution when they join the crystallite in the upper half plane, but there are no URS that join in the other half plane. Experiment (a), the time dependent shapes (b), and the URS (c), are compared in this Figure.

This work has been supported by NSF-MRSEC at University of MD, DMR #0080008.