Defect Structures and Properties of Liquid Crystalline Polymer Semiconductors, DMR-0802655, Prof. David C. Martin, The University of Michigan, Department of Materials Science and Engineering.
Polymer and organic molecular semiconductors are of considerable interest for creating inexpensive electronic devices. Previous studies of polycrystalline organic molecular films have shown evidence that grain boundary defects play an important role in limiting the performance of these materials. However the detailed relationship between the microstructure and macroscopic properties of these materials remains obscure and controversial. Liquid crystalline polymer and organic molecular semiconductors are of particular current interest, because it is expected that the more modest distortions near grain boundaries and other defects in the solid-state may not lead to the large reductions in properties seen in polycrystalline films. The microstructure of liquid crystalline polymer and organic molecular semiconductors will be investigated using an array of instruments and techniques including optical microscopy, wide angle X-ray diffraction, small angle X-ray diffraction, scanned probe microscopy, scanning electron microscopy, transmission electron microscopy, low voltage electron microscopy, and high resolution electron microscopy. These microstructural details will be correlated with information about sample performance on thin-film transistor devices, and with impedance spectroscopy of the carrier transport. The fundamental scientific challenge is to determine if and how the increased order provided by liquid crystalline order makes it possible to create films that have better performance than either the amorphous or polycrystalline structures seen in other organic molecular solids.
NON-TECHNICAL SUMMARY
There is considerable interest and future potential in developing new materials for all-organic "plastic" electronics. Examples of such devices include soft, flexible computers and displays printed on flexible substrates, and biomedical components that can be directly implanted into the body. However there are many important questions about these materials that are not yet known, such as the detailed relationship between their local structure and the properties that they exhibit when used in devices. This research project will make it possible to develop improved materials that can be easily and inexpensively processed and manufactured into components. This project will provide for financial support for graduate students from Materials Science, Macromolecular Science, and Biomedical Engineering. Research opportunities for undergraduate students will be provided through the University Research Opportunity Program, the Sarah Marian Parker Women in Engineering and the Minority Engineering Programs. We also have summer students from high schools in the Ann Arbor area. Established collaborations and interactions will be continued with colleagues at the University of Michigan, the University of Kentucky, Georgia Tech, the Fraunhofer Institute in Bremen, Germany, the National Institute of Science and Technology, Chulalongkorn University in Bangkok, Thailand, the University of Wollongong, Australia, and Suwan University in Seoul, South Korea. Industrial interactions will be extended with companies including Ford Motor Company (Dearborn, MI), Cochlear (Australia), BioControl (Israel), Plexon (Dallas, TX), Biotectix (Quincy, MA) and NeuroNexus (Ann Arbor, MI).
This project examined the structure and properties of defects in polymer semiconductors that have liquid crystalline order. Traditional semiconductor materials used in computers and other types of devices are inorganic, hard, and highly regular single crystals like silicon or gallium arsenide. There is currently interest in creating similar devices from inexpensive, flexible, "plastic" materials that are all organic. Such devices might include transistors (for computers and logic), light emitting diodes (for energy-efficient lights), or fully integrated biomedical devices for next generation bionic prosthetics. In order to fully utilize these materials, however, we need to know more about the fundamental relationship between their structure and molecular organization and their electrical and mechanical properties. This study focused on a particularly interesting class of these compounds that have "liquid crystalline" symmetry that is intermediate between that of liquids (like water) and crystalline solids (like ice). Our research revealed new information about the changes in structure of these liquid crystalline organic semiconductors, and compared this information with a relatively new technique for investigating the dynamics of charge transport (impedance spectroscopy). The results from our research have been presented in more than 25 peer-reviewed publications, and have provided support for post-doctoral research scientists, graduate students, and undergraduate students in Materials Science and Engineering and Macromolecular Science and Engineering at the University of Michigan as well as the University of Delaware (where our lab moved in the summer of 2009). Students who were involved in this research are now working in a number of locations including the Oak Ridge National Laboratory, Dow Chemical Company, Penn State University. One former student (Dr. Laura Povlich) is currently a Congressional Fellow for the Materials Research Society and Optical Society of America in Washington, DC, and is now working in Congressman Sandy Levin's (12th Michigan) office on Health Care and Biomedical Research Policy. Prof. David Martin has taken a leadership role in the development of curricula in the area of polymers and other soft materials. He served as the Director of the the University of Michigan Macromolecular Science and Engineering Center, an intercollegiate, interdisciplinary degree-granting program between the College of Engineering; the College Literature, Science, and the Arts; Medical School; and Dental School (www.engin.umich.edu/prog/macro). Under his direction, the Macro program initiated new degree concentrations in Biomaterials, Biomedical Engineering, and Organic Electronics and Photonics. He lead an effort to create Sequential Graduate/Undergraduate Studies (SGUS) degrees that started in fall 2004. These SGUS degrees now provide opportunities for highly qualified undergraduates in Biology, Biomedical Engineering, Chemistry, Chemical Engineering, Materials Science and Engineering, Mechanical Engineering, and Physics obtain a Macro M.S. degree in an accelerated fashion by coordinating requirements between the units. Since arriving at Delaware in July 2009, he has created 4+1 Degree programs for Delaware undergraduates in Chemical Engineering, Mechanical Engineering, Electrical and Computer Engineering, Civil and Environmental Engineering , as well as Chemistry, Physics and Astronomy, and Biological Sciences. Prof. Martin has also been aggressive in employing undergraduate students in his laboratory, including students from the University Research Opportunities Program (UROP), as well as the Minority Engineering Program Office and the Sarah Marian Parker Women in Engineering summer research programs administered by the College of Engineering. He has also provided educational opportunities for high school students from Greenhills in Ann Arbor, through a program organized by Prof. Rachel Goldman in the Materials Science and Engineering department. Since starting at the University of Michigan in 1990, Prof. Martin has directly mentored 80 undergraduate students on projects in his laboratory, of which 33 were women.
