In situ Investigations of Physical Vapor Deposition of Organic Glass Formers

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With support from Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Professor Mark Ediger of the University of Wisconsin-Madison and Professor Ranko Richert of Arizona State University are studying low-temperature molecular glasses. When liquids are cooled to low temperature, they form solids. Often that solid is a crystal in which molecules are arranged in an ordered structure. However, sometimes the liquid freezes into an amorphous glass and in this case the properties of the solid depend strongly upon how it is made. Working with their students, Professors Ediger and Richert are creating glasses by vapor-depositing molecules onto cold surfaces and using sophisticated experimental methods to understand the unusual glasses that form. Their discoveries could have implications for advancing technologies based on organic electronics, such as organic light emitting diodes (OLEDs). The project contributes to the development of the Nation's scientific workforce by providing research opportunities for graduate students in advanced experimental methods. The team is also working to prepare high school students from under-represented groups for college, through partnership with the University of Wisconsin-Madison’s Pre-college Enrichment Opportunity Program for Learning Excellence (PEOPLE). In contrast to crystallization, fundamental questions about glass formation remain unanswered, including the possible existence of underlying phase transitions and diverging time scales. Professors Ediger and Richert are creating amorphous molecular films through physical vapor deposition (PVD) of low molecular weight organic molecules onto cold surfaces. The resulting films are then characterized by in situ dielectric relaxation measurements, nanocalorimetry, and spectroscopic ellipsometry. Experiments performed at (or near) the glass transition temperature (Tg) aim to determine if a single component system can possess multiple long-lived liquid states. Dielectric relaxation measurements performed during vapor deposition directly characterize surface mobility and test a proposed model for vapor-deposited glasses based upon surface equilibration. Finally, the kinetic stability of co-deposited PVD glasses made from two different organic glass formers are investigated for cases where each component forms a highly stable glass as a pure material.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.