2017 March 15 - April 24
2017 May 17 - June 29
2017 BTR deadline: 04/17/17
2017 October 11 - December 21
2017 Proposal deadline: 08/01/17
2017 BTR deadline: 09/10/17
The Graduate Student Symposium will be held on -
May 16, 2016
at 5:15 p.m.
Wilson Commons (3rd floor)
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.
5:15 pm - Welcome
5:20 pm - Al Kovaleski (Cornell University)
5:40 pm - Mark Obstalecki (Cornell University)
6:00 pm - Pizza
6:30 pm - Doug Nevers (Cornell University)
6:50 pm - Matt Krogstad (Northern Illinois University)
Speakers and Talk Titles
Al Kovaleski - Cornell University
"X-ray phase contrast imaging of grapevine buds"
Abstract: Low temperatures are the greatest limiting factor governing plant distribution on Earth. Plants located in higher latitudes experience below freezing temperatures and may be subject to freezing of their tissues, but overwintering in these areas likely led to the development of freezing tolerance mechanisms. Water freezes in different tissue compartments at separate times in some plants, grapevines (Vitis spp.) included. However, direct observation of the freezing is difficult considering the opacity of the structures. Previous studies have used x-ray phase-contrast imaging to visualize ice formation in insects. Initial data on plants were acquired at the Cornell High Energy Synchrotron Source using a highly parallel 15 KeV monochromatic beam. However, the structures in grapevine buds were found to be much more intricate and difficult to image, compared to those of insect larvae, making it difficult to visualize ice in a time-lapse 2D fashion. Therefore, using 3-dimensional reconstruction of the bud is proposed. The objective is to visualize in 3D the structures within grapevine buds. Buds of different Vitis species were imaged to evaluate structural differences between species. The images obtained are parallel images of a rotating sample. The setup used is a 15 KeV, 7 × 7 mm monochromatic beam. The sample is placed on a small goniometer mounted on a Huber 4-circle diffractometer. As the beam passes through the sample, the wavefront is slightly distorted by sample density variations. Wavefront angular deviations interfere with the unperturbed beam and become intensity variations recorded on 2D images with spatial resolution better than 5 μm. With the use of appropriate software (e.g. Octopus Reconstruction), images were reconstructed from parallel beam data. 3D renderings were built using OsiriX for better visualization of the intricate structure of the bud. An initial data collection demonstrated that the 3D reconstruction can be used to identify structures within a bud.
Mark Obstalecki - Cornell University
"Understanding Microplasticity Processes Related to Fatigue Damage Using High Energy X-rays and Crystal-based Modeling"
Abstract: High energy x-ray diffraction presents a unique opportunity to study material evolution associated with the onset of microcrack nucleation within ductile polycrystals during low cycle fatigue conditions. Similar to microcrack initiation that accompanies persistent slip bands within deforming single crystals aligned for single slip, we search for regions of heterogeneous cyclic slip within polycrystalline aggregates. The plastic strain amplitude initially being carried by the entire aggregate is eventually focused into regions within individual grains. We have created a combined experiment/ simulation methodology for tracking heterogeneous cyclic plasticity at the size scale of individual grains. The high energy x-ray diffraction experiments supply us with the average stress state and misorientation heterogeneity of each grain; however, they lack spatial information other than the center of mass position of each grain. Details of the subgrain response such as the stress, lattice orientation, and plastic strain rate distributions are extracted from the simulation.
Doug Nevers - Cornell University
"Probing the Structural Origins of Quantum Dots: in-situ Characterization and Isolation"
Abstract: Despite many years of impressive advances in the synthesis of colloidal quantum dots (QDs) with precisely programmable size, shape and composition, it may be surprising that several fundamental questions about the basic nucleation and growth of QDs persist. For example, the detailed structure and composition of the ‘critical nucleus’ that defines the transition from molecular precursors to colloidal crystals is poorly understood. Moreover, the active growth species (i.e. monomer) have not been investigated.
This knowledge gap derives in part from the difficulty in isolating and characterizing these small short-lived reaction intermediates. Nevertheless, controlling these early-stage species is crucial to minimizing QDs size dispersion, tailoring QD size, and reliability producing large-batches of identical QDs. Total X-ray scattering methods (PDF) provide a unique, powerful, and in-situ analytical tool to characterize these crucial, but fleeting initial structures and their subsequent growth.
We have investigated two model systems, PbSe and CdS magic-sized clusters. The former presents an advantageous experimental platform, as an air-stable and slow growing reaction, for in-situ characterization of QD intermediates. Notably, MSC QDs grow through discrete transitions rather than continuous shifts in size, thus reducing the number of possible structural products. Yet, traditional MSC syntheses still yield a mixture of MSC sizes, complicating the unambiguous identification of intermediate structures.
The second model system (CdS MSC) emerged from parallel research efforts to isolate high-purity and high-selectivity MSC families (or discrete states for CdS QDs). Access to high purity samples has enabled the precise structural characterization of individual MSC building blocks. Further, we show that reversible transformation between two related MSC families in response to changes in the solvent environment. We hypothesize that the change between MSC ‘states’ relates to changes in the ligand binding mode and MSC surface reconstruction. We show that 1) PDF analysis captures small structural reorganizations in MSC caused by changes in solution environment, 2) that MSC structures deviate substantially from bulk crystal phases, yet still have a reproducible and distinct atomic structure, and 3) structural identification via PDF requires accounting for all potential sources of scatter. Ultimately, in-situ structural analysis, high-purity MSC standards, and computational techniques enable characterization of crucial QD building blocks as they structurally evolve along the synthetic path from raw reagents to pristine MSC and QDs.
Matt Krogstad - Northern Illinois University
"Diffuse Scattering from Relaxor PMN-xPT"
Abstract: Relaxor ferroelectrics possess intriguing electromechanical and dielectric properties, the microscopic physics of which is widely regarded to be related to local, correlated atomic displacements from long-range symmetry. However, despite numerous studies over the last few decades, the details of how short range correlations and disorder drive the relaxor behavior remain unresolved. Single crystal diffuse scattering provides a powerful probe of such deviations from an average structure correlated over varying length scales, and over the last few years, techniques and instruments for measuring diffuse scattering with both x-rays and neutrons have seen a dramatic improvement, allowing for large volumes of reciprocal space to be measured in little time. We present our recent complementary neutron and x-ray measurements on solid solutions of PMN-xPT which revealed new structure to the diffuse scattering of relaxors close to the morphotropic phase boundary.