Speaker: Xudong Wang, UW-Madison (Materials Science and Engineering)
Abstract: Morphology is one essential element that gives rise to extraordinary physical, chemical, and mechanical properties in nanomaterials. Precise morphology control of nanomaterials is a notorious task, which heavily relies on fundamental understanding of the governing mechanisms and kinetics at the nanoscale. In this talk, I will present our three recent discoveries of the growth kinetics in 1D and 2D nanomaterial evolution. Firstly, the surface-reaction-limited pulsed chemical vapor deposition (SPCVD) technique will be discussed. The SPCVD technique effectively decouples the crystal growth from precursor vapor concentration, thus makes the conform growth of dense NW arrays inside highly-confined submicron-sized spaces possible. The evolution of NRs was found to be a manifestation of the Ostwald-Lussac Law. SPCVD opens a new route toward the design and creation of complex 3D hierarchical nanostructures, which can advantageously impact the devices performance of solar energy harvesting. Secondly, I will present our discovery of the wedding cake growth mechanism in the formation of 1D and 2D ZnO nanostructures. Within a narrow kinetic window, the surfaces of the 1D and 2D structures were covered with a unique concentric terrace feature, different from the screw-dislocation features. An interesting 1D to 2D morphology transition was found during the wedding cake growth, when the adatoms overcome the Ehrlich-Schwoebel (ES) barrier. At last, I will present a new ionic layer epitaxy (ILE) technique that uses surfactant monolayers to serve as soft templates guiding the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. One- to two-nm-thick, single-crystalline free-standing ZnO nanosheets with sizes up to tens of micrometers were synthesized at the water-air interface. ILE of other metals and oxides have also been proved to be successful.