Non-classical nucleation pathways revealed by scanning tunneling microscopy of epitaxy of covalent materials

摘要： Identifying atomic/molecular mechanism is crucial for understanding crystallization and epitaxial growth and to enable controlled synthesis of high-quality devices and desired crystalline structures. Using scanning tunneling microscopy, we studied early stages of Si epitaxy on a Si(111) surface mediated by a Pb monolayer. A type of highly mobile magic clusters was observed only when they were trapped at boundary or defect sites. Magic clusters also formed cluster aggregates temporarily. Adding more Si transformed the aggregates into immobile metastable or stable structures. Here we propose a scenario involving concerted chemical reactions of multiple, co-localized magic clusters with the activation energies decreasing with increasing number of clusters. This scenario may be modified to understand nucleation and growth of many covalent materials as well as crystallization in numerous synthetic and natural systems.