Interfacial directed assembly and attachment of interconnected nanoparticle networks

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Nanoparticles are particles with diameters roughly one thousandth the width of a human hair. This award supports research to investigate how nanoparticles assemble and attach at the surface of a fluid. The process is not unlike how some fine powders can form a layer that floats on top of the surface of quiescent water. Nanoparticle building blocks with programmable size, shape, and composition have become available thanks to recent advances in chemistry. Connecting these building blocks to each other to form "sheets" can give rise to new classes of materials and devices with emergent properties that have intrigued scientists and engineers alike. Unfortunately, assembly instructions are not yet available. This project will close this knowledge gap with a combination of experiments and computer models. This synergistic approach will unveil key details of the mechanism by which the building blocks self-assemble and attach. Hence, the results will lead to strategies to design new building blocks that assemble into desirable patterns with minimal defects. This project will produce new knowledge of scientific and societal importance and may lead to the development of new nanostructured materials. Graduate students will receive extensive training in advanced experimental and modeling approaches, and research opportunities will be provided for undergraduates. Interactive learning modules for local K-12 programs will also be developed.The combination of self-assembly and directed attachment of colloidal nanoparticles at fluid interfaces presents scientifically interesting and technologically important research challenges. Remarkable strides have been made in the synthesis of polyhedral nanoparticle building blocks with precisely defined shapes and their self-assembly into highly ordered superstructures. Recent advances have revealed intriguing synergies between interfacial self-assembly and directed epitaxial attachment into ordered and connected superstructures. Access to superstructures with programmable symmetry opens new opportunities to create materials with properties by design. The main goal of this work is to attain a better understanding of the basic kinetic and thermodynamic factors governing the interplay of self-assembly and directed-attachment. The investigators hypothesize that the key to predicting and creating coupled assemblies with programmable structures lies in understanding and controlling the nanoparticle orientation at the fluid interface as well as the interactions among particles. The nanoparticle orientation at the liquid-liquid interface and subsequent directed attachment is a complex function of the interfacial energies, the nature of multi-particle interactions and the coupled dynamics of interfacial nanoparticle diffusion, ligand displacement from the nanoparticle surface and epitaxial fusion of adjacent nanoparticles through mutually exposed facets. The mechanism describing how the nanoparticle assembly transforms into an epitaxially connected superstructure presents an interesting unresolved scientific question, and competing hypotheses will be tested via a combination of experiments and simulations. In fact, this provides both a challenge and an opportunity to closely integrate in-situ X-ray structure analysis with multi-scale modeling. The proposal presents a hypothesis-driven collaborative approach with two objectives that aim to understand: (1) how specific nanoparticle superlattice polymorphs assemble at fluid interfaces and (2) how directed attachment can transform the assembled superlattice into an epitaxially connected quasi-2D solid.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.