Non-Technical Abstract The development of new materials for advanced technologies usually requires that a large number of different samples be synthesized and tested one by one to find those best suited for the intended application. An alternative to this often-tedious process is to produce a material incorporating a chemical gradient in which the composition gradually varies along a single sample. The speed at which optimum new materials are developed and identified is thus greatly enhanced. These rapid-throughput-screening applications of chemical gradients find possible utility in the development of better catalysts for production of pharmaceuticals, plastics, lubricants and fuels and in the fabrication of surfaces with adhesion properties tailored for the attachment and growth of biological tissues. Chemical gradients also find direct utility in the separation of specific components from complex chemical mixtures and in guiding the delivery of liquids, chemical precursors and cells in miniaturized chemical devices and sensors. For all such applications, the gradients to be employed must have well defined chemical and physical properties. Ideally, they would exhibit gradual, monotonic compositional and properties variations extending from the macroscale down to molecule levels. In reality, such idyllic character seldom exists. Unexpected properties variations may arise from the spontaneous separation of the gradient components during preparation, producing materials that exhibit stepwise rather than gradual properties variations, and limit the participation of often-desirable cooperative interactions. These attributes make gradient materials uniquely more complex and valuable than either single component materials or uniform nongradient films prepared from identical precursors. This activity emphasizes exploration of the chemical and physical complexity of organosilane gradients prepared by novel wet-chemical methods recently developed by the principal investigator's groups. The outcomes will lead to development of gradients having better understood and better controlled properties that can be more effectively implemented in advanced materials. The synergy between the collaborating groups will lead to the enhanced training of a diverse body of undergraduate and graduate students in state-of-the-art materials synthesis and their characterization by advanced chemical imaging methods. These students will be actively mentored by both investigators and will participate in a summer exchange program between the two campuses and weekly web conferences to broaden their educational, scientific and career horizons.
With the support from the Solid State and Materials Chemistry Program in the Division of Material Research, the principal investigators will (1) investigate the extent to which phase separation and synergistic interactions occur along multicomponent organosilane gradients prepared by the sol-gel process and (2) evaluate the sizes and compositions of the resulting domains. Materials to be investigated will include multicomponent polarity, acidity, charge and dopant gradients derived from different organoalkoxysilane precursors. Over the long term, the impacts of nanometer-to-micrometer scale phase separation and cooperative interactions on the macroscopic properties of these gradients will be explored. Gradient composition will be characterized on multiple length scales (micrometer-to-millimeter) by x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman mapping. All three will yield quantitative data on the distance scales over which gradient composition varies and will allow for the chemical origins of any cooperative effects to be fully understood. Single molecule superlocalization microscopy will be used to probe the gradients on nanometer length scales and to elucidate cooperative interactions between the gradient components themselves as well as between a probe molecule and the film components. Quantitative data on materials polarity will be obtained through implementation of unique single molecule level measurements of the local dielectric constant of the films. In all cases, results from the gradients will be compared to those from uniform nongradient samples.
