Explosive Nucleation and Mesoscale Collective Diffusion
A very unusual type of nucleation was seen in Pb/Si(111) where fully crystalline Pb islands with ~105 atoms emerge out of the wetting layer when its density becomes 22% above the density of metallic Pb. In classical nucleation the process is more gradual because a critical size cluster should form first and the islands should grow from the attachment of randomly diffusing adatoms. The two successive 1.5 x 1.5 μm2 images show that neighboring islands grow in the same direction and collect material from farther away than their neighborhood. This directional growth persists over hours indicating a very novel collective type diffusion which is responsible for the explosive nucleation.
Phys.org article about this research.
M. T. Hershberger, M. Hupalo, P. A. Thiel, C. Z. Wang, K. M. Ho, M. C. Tringides. Phys. Rev. Lett. 113, 236101 (2014). Nonclassical explosive Nucleation in Pb/Si(111) at Low Temperatures