NIRT: Directed Assembly of Nanostructures: Theory, Simulations, and Experiments in Hard and Soft Materials

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This proposal was received in response to Nanoscale Science and Engineering initiative, NSF 03-043, category NIRT. Directed self-assembly and aggregation offers tremendous possibilities for making structures at the nanoscale. The challenge lies in the requirement that atoms, molecules, or particles be assembled into complex and highly organized nanometer-sized structures across centimeters. Potential applications for such structures arise in a wide range of technologies, including nano and molecular electronics, high-density patterned media for data storage, optoelectronics, and nanosensor arrays to name a few. The use of externally applied fields to control and direct micro- and nanostructural evolution is a very promising avenue for achieving such precise control of aggregation. The practical application of external-field interactions with matter to create nanoscale, ordered aggregates will require both fundamental understanding and engineering design methodologies. While many of the initial discoveries of novel phase behavior in microscale and nanoscale systems have arisen from purely experimental investigations, it is increasingly apparent that the ability to model and predict microstructural evolution will be of central importance for achieving effective and practical control at the nanoscale. The proposed research aims to accomplish this goal with a substantial modeling and theoretical effort in conjunction with state-of-the-art experiments in both hard and soft material systems, each of which offers complementary advantages. Hard (or atomic) systems, e.g. nanoprecipitates in crystalline materials, are very difficult to completely characterize either experimentally or theoretically. Colloidal or soft systems, on the other hand, offer greater flexibility both in setting the "microscopic" properties that control particle-particle interactions and in the ability to make direct experimental observations, particularly in the case of time-dependent phenomena. The outcome of this research will be a physically based framework for achieving directed aggregation in both hard and soft systems. In both cases, the unifying theme will be the induction and control of transport by externally applied fields. Examples include stresses in hard materials, entropic fields in soft materials, and chemical potentials in both. The concept of field-assisted directed assembly is not material specific and the proposed research will serve as the general foundation for modeling and experimental design of directed nanoscale aggregation in a very broad range of materials. It may provide entirely new insights into the mechanisms of aggregation and reveal aspects that are common to practically all materials. At the same time, by exploiting common underlying characteristics, a deeper understanding will enable transfer of methodologies among different technological applications. Such unifying concepts are of paramount importance as high-tech materials are becoming more complex and ostensibly unrelated. Finally, the impact of this research on the education of graduate students is expected to be far reaching. It is increasingly apparent that a new generation of scientists and engineers is needed to lead the development of nanotechnology, which requires combined training in chemical engineering, materials science, and mechanics of materials. This closely-knit project will provide an ideal framework for this type of cross-disciplinary training. The research is being funded jointly by the Thermal Transport and Thermal Processing Program of the Chemical and Transport Systems Division, the Interfacial, Transport and Thermodynamics Program of the Chemical and Transport Systems Division, the Mechanics and Structures of Materials Program of the Civil and Mechanical Systems Division, and the Nanomanufacturing Program of the Design, Manufacturing and Industrial Innovation Division.