Crystal Growth at the Nanoscale: New Phenomena Revealed by In situ X-ray Scattering

University Of Missouri-Columbia, Columbia MO

Investigators

Abstract

Non-Technical Abstract With new device technologies moving towards significantly smaller length-scales, the need for fundamental science to explore how atoms move and arrange themselves on surfaces is becoming imperative for understanding how materials grow. Although there has been significant recent progress towards understanding some of the mechanisms that dictate atomic assemblies on surfaces, the role of defects (meaning atoms that do not occupy expected positions) has been overlooked. Because defects can be initially incorporated at the surface and then become buried below it, conventional experimental tools which are sensitive only to the surface cannot see the defects. In addition, very small crystals (nanocrystals) are subject to new forces (quantum-mechanical forces) that have not been previously considered in theories of crystal growth. This individual investigator award will utilize a beam of x-rays from the Advanced Photon Source, which is the brightest source of x-rays in the United States, to probe, both, the surface and the subsurface structure as atoms are deposited onto a surface. It is expected that these experimental studies will lead to essential new insight to growth mechanisms and, thus, provide a comprehensive framework for understanding how materials grow at surfaces. The project will train and mentor graduate students and postdoctoral researchers as well as undergraduate students. The research activities at the Advanced Photon Source will enhance their educational experience while they will become part of a national resource of future scientists who can utilize these new state-of-the-art facilities. Technical Abstract The main thrust of the project investigates new atomic-scale mechanisms that strongly affect the growth and evolution of nanocrystals and epitaxial films. Particular emphasis is given to elusive subsurface phenomena, which are uniquely addressed by in situ x-ray scattering. Experiments will be performed on simple "model" systems that lead to clear interpretations and the establishment of fundamental concepts. The subsurface sensitivity of in situ x-ray scattering enables new insight and solutions to problems that have eluded other surface sensitive techniques. It will be learned, at the atomic-scale, how defects become incorporated into a film, how these affect the subsequent growth once incorporated, and how defects relate to the evolving surface morphology. The new results will provide the foundation and a "stepping stone" for investigating more complex systems. These studies will also reveal how interfaces influence the growth of epitaxial nanocrystals and how the small crystal size is manifested in its growth kinetics. Exploring and exposing the novel growth kinetics of quantum size effect nanocrystals is expected to lead to a new conceptual framework for understanding these systems. The project will utilize the in situ surface scattering facility developed by the PI at the Advanced Photon Source (APS), where the high brightness of the x-ray source and the instrument design provide new capabilities that enable these studies. Because the project addresses fundamental issues that help understand how to manipulate nanomaterials, these studies will broadly impact emerging technologies that have important societal benefits such as computer, communication and medical technologies. Education and research are extensively integrated into the project, as it involves graduate students, postdocs and undergraduates. State-of-the-art infrastructure at the APS offers excellent training opportunities for students and postdocs.