Collaborative Research: Highly Ordered Nanoscale Patterns Produced by Ion Bombardment of Solid Surfaces: Theory and Experiment

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When a flat solid surface is bombarded with a broad ion beam, patterns can emerge spontaneously, including surface ripples with a wavelength as short as one ten-thousandth of the width of a human hair. If the patterns formed were not almost always disordered, this fascinating type of self-organization could become a widely employed method of fabricating structures with feature sizes too small to be produced by conventional techniques. A central goal of this collaborative theoretical and experimental research is therefore to develop and refine methods of producing highly ordered surface ripples by ion bombardment. Surfaces patterns of this kind could be used in a variety of applications, including ones in which extreme ultraviolet light must be manipulated or analyzed. This project involves graduate, undergraduate and high school students who are working in a field that bridges basic research and applications. In addition to experiencing, and contributing to, an important topic in current materials research, the students are learning to use a wide range of theoretical, simulation and materials characterization tools that will translate well to future careers in industry or academia. Beyond working with the materials research communities at their home institutions, the project is providing the students with a broader perspective through participation in the exciting, multi-disciplinary environments at national x-ray scattering facilities.This collaboration brings together two principal investigators who have individually made seminal theoretical and experimental contributions to the field of ion beam nanopatterning, producing a team that is uniquely positioned to advance the field. Two key strategies are being employed in working toward the goal of producing surface ripples with an exceptional degree of order. These strategies are also leading to important steps forward in our understanding of the fundamental processes that take place during ion bombardment. The first strategy is to study the onset of pattern formation that occurs as the angle of ion incidence or the sample temperature is varied through a threshold value. Near-threshold behavior is being examined in three different regimes: 1) the higher energy regime in which sputtering is important; 2) the low-energy regime in which sputtering is negligible, a largely unexplored frontier of the field; and 3) the high-temperature regime in which the surface remains crystalline, resulting in better quality material for electronic/photonic applications. The equation of motion becomes simpler near these transitions and can be rigorously derived. In addition, it has a high degree of universality, i.e., it is to a large extent independent of the choice of target material and ion species. Studying near-threshold behavior was the key to making fundamental advances in the theory of phase transitions, and the same is proving to be true for ion-induced pattern formation. Finally, and most importantly, almost perfectly ordered ripple patterns are in certain circumstances predicted to form close to threshold. The second central strategy of the project is to exploit the power of real-time x-ray scattering. The extreme sensitivity of grazing-incidence small-angle x-ray scattering to surface disturbances makes it an ideal method to study the near-threshold behavior of the nanopatterns and to rigorously test the theory. Moreover, the research team is taking advantage of the increasing coherent x-ray flux available at new and upgraded synchrotron sources to develop coherent x-ray scattering as a powerful new approach to investigating surface dynamics at the nanoscale. Coherent scattering gives entirely new insight, going beyond study of the average surface evolution to measure the time-dependent fluctuations of the microstructure around the average kinetics, enabling novel tests of the detailed theoretical predictions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.