Nucleation limited crystal shape transformations

Using variable-temperature STM we studied the response of facetted 3D crystallites to temperature changes under volume preserving conditions. Defect-free micron-sized Pb crystallites preserve their initial facet size after a temperature increase from 0.64Tm to 0.8Tm (the melting point). In contrast, a temperature drop from Tm to 0.6Tm results in an increase in facet size, where material is transferred from the facets to the rounded regions of the crystal. A simple model suggests that the corresponding facet expansion stops due to a nucleation barrier, when the facet radius reaches approximately 75% of the equilibrium value. Use of the model to determine the energetic parameters governing the equilibrium crystal shape from non-equilibrium measurements is discussed.