Dipolar Molecular Rotors in Surface and Bulk Inclusion Compounds.

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Non-Technical AbstractThis project aims to construct a new class of materials, "roto-electrics", in which crystalline materials, normally thought of as composed of rigidly bound atoms, also contain molecular components that are free to rotate. Of particular interest are cases where the rotating components carry a permanent electric dipole moment. These materials have long been predicted but appear not to exist in nature and provide an opportunity to learn how to build new and potentially useful materials by design. Roto-electric phases are special examples of ferroelectric phases and ferroelectrics find application in such areas as memory sticks, high frequency filters in cell phones, and for controlling light polarization in displays. Roto-electrics offer the opportunity to miniaturize such components and to make them operate faster. The project supports the education of a graduate student, who receives Ph.D.-level training in the design of dipolar materials, in a wide variety of experimental spectroscopy techniques, and in the underlying materials theory. The project looks to involve under-represented minorities in nano-engineering research. It further leverages the training of future engineers in both nano-scale modeling and experiment, by the integrating the research results into materials science and engineering curricula. Project results and materials design insights are further distributed through physics, engineering, and educational websites.Technical AbstractThe research team builds crystalline arrays of nearly-freely rotating dipolar molecules with a host / guest molecular-engineering approach. The present host systems are crystals, thin-films, and crystalline powders of the well-documented tris (o-phenylenedioxy) cyclotriphosphazene (TPP) material, which forms a hexagonal structure with open channels directed along the hexagonal c-axis. The channels are known to accept a large variety of guest molecules. The team investigates the dielectric phases that result from placing specially synthesized dipolar molecules, specifically designed to allow for nearly free rotation of the dipolar components, in the bulk of the TPP channels (3-dimensional materials) or at TPP surfaces (2-dimensional materials). These compounds are studied using a combination of radio-frequency dielectric spectroscopy, x-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, bulk and micro-Raman spectroscopy, and, for surface-localized and thin-film materials, with atomic force microscopy. Other probes, especially neutron diffraction and tip-enhanced Raman are investigated via collaborations. The new roto-electric materials are predicted to have ferroelectric waves that propagate at speeds far below those of traditional ferroelectrics, allowing e.g., for miniaturized surface acoustic wave devices. This research makes contributions to the assembly of molecular systems of nearly-freely rotating molecular components, and in demonstrating ordered dielectric phases in such materials. The project works to provide new insights into the construction of dipolar phases of matter, and has the potential to provide new useful materials.