MSE Seminar: Dr. Ray Phaneuf, UMD
Wednesday, November 1, 2023
2118 Chemical and Nuclear Engineering Building
Hierarchically structured materials via directed self-assembly*
*Work supported by NSF #DMR0705447
Abstract: The next frontier beyond nanotechnology is arguably assembly at the meso-scale: that range of sizes from 10’s to 100’s of nm, with expectations of emergent properties at this scale. It has been argued that a hybrid approach combining top-down, and bottom-up aspects will be needed to accomplish assembly of nanostructures into meso-scale assemblies on a practical time scale. In this talk I present an example of just such an approach – using an artificially fabricated template along with molecular beam epitaxial growth in the presence of a kinetic barrier to direct the assembly on nm-scale growth mounds into mesoscale arrays. We show via both kinetic Monte Carlo (kMC) simulations and molecular beam epitaxial (MBE) growth experiments that patterning in the presence of an “Ehrlich_Schwoebel” diffusion barrier at step edges can be used to direct assembly of a series of “higher-order commensurate” mound-on-template structures. We find that in the initial stage of growth, the pattern directs the spontaneous formation of multilayer islands at 2-fold bridge sites between neighboring nanopits along  crystal orientation. However, as growth continues, the height of mounds at 2-fold bridge “self-limits”: the mounds cease to grow. Beyond this point an initially less favored 4-fold bridge site for mounds dominates and a different pattern of self assembled mounds begins. We find that the transient pattern amplification during growth proposed by Tadayyon-Eslami et al. is correlated with self-limiting behavior of mounds. We also propose that a minimum, ‘critical terrace size’ at the top of each mound is responsible for the observed self-limiting growth (SLG) behavior.