2016: May 11 - July 1
The Graduate Student Symposium will be held on -
March 21, 2014
at 6:00 p.m.
Wilson Commons (3rd floor)
6:00 pm - Pizza • 6:30 pm - Talks begin
The symposium features a series of scientific presentations by graduate students and Post-docs whose research is based at CHESS and a brief overview of CHESS.
Speakers and Talk Titles:
Darren Pagan - Materials Science & Engineering, CHESS, Cornell University
"Studying Slip System Activity in Deforming Single Crystals Using High-Energy X-Ray Diffraction"
Abstract: High-energy x-ray diffraction can be used as an effective tool to observe the change of lattice state in crystalline materials as they plastically deform. Evolving diffracted intensity distributions are directly related to internal structure within a crystal, and the “smearing” of diffraction peaks that is observed during plastic deformation is consistent with the evolving lattice state. A forward modeling diffraction framework is introduced and employed to identify slip system activity in High Energy Diffraction Microscopy (HEDM) experiments. In the frame- work, diffraction simulations are conducted on virtual mosaic crystals with orientation gradients consistent with Nye's model of heterogeneous single slip. Simulated diffraction peaks are then compared against experimental measurements to identify slip system activity. Simulation results compared against diffraction data measured in-situ from a silicon single crystal specimen plastically deformed under single slip conditions indicate that slip system activity can be identified during HEDM experiments. Next, high angular resolution pole figure measurements, obtained in-situ from a plastically deforming silicon single crystal, will be shown. These pole figure measurements are then used to determine misorientation distributions, which provide a direct measure of the current lattice state. Preliminary results suggest possible connections between the observed lattice state evolution and polyslip in the crystal.
Anna M. Hiszpanski - Chemical & Biological Engineering, Princeton University
"Traversing the crystalline phase space of contorted hexabenzocoronene to maximize charge transport"
Abstract: Alternative crystal structures of molecular semiconductors may exhibit increased intermolecular charge transport, but methods to controllably access non-thermodynamically-favored crystal structures are lacking. Starting with an amorphous film of contorted hexabenozocoronene (HBC) and applying thermal and solvent-vapor annealing to induce crystallization, we have accessed three distinct HBC polymorphs, two of which have previously not been observed. HBC films crystallize as polymorph I upon thermal annealing and as polymorph II upon solvent-vapor annealing with tetrahydrofuran. Subsequent solvent-vapor annealing of polymorph I converts it to polymorph II; thermal annealing polymorph II transforms HBC to yet a different crystal structure, denoted polymorph II’. Though the crystal structure can be tuned through sequential processing, the preferred out-of-plane molecular orientation adopted by HBC is determined primarily by the first processing step. By imposing different processing sequences, we can access films having different polymorphs but the same molecular orientation, and also films having the same polymorph but different molecular orientations, thereby allowing us to decouple the relative contributions of polymorphism and preferential orientation to charge transport. In the case of HBC, polymorphism and molecular orientation are equally important; with the optimal polymorph and molecular orientation each improving the field-effect mobility of thin-film transistors by an order of magnitude.