Complex Matter and Biophysics Seminar: Fall 2009
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Thursdays 4:00 to 5:00pm, 314 Riddick unless otherwise noted.
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Contents |
Tues Aug 18: Timothy J. Ulrich
Los Alamos National Laboratory
Time Reversal, Elasticity and Nonlinearity: the New TREND in Acoustics
Time Reversal (TR) is a technique used in wave propagation to redirect wave energy "backwards" in time to focus, both in time and space, upon the source of the wave field. Performing TR in elastic media, i.e., 3-dimensional solids, complicates the physics of the process due to the presence of a variety of wave motion possibilities (e.g., compressional, shear, and surface waves). Further adding to the complexity is the phenomenon of nonlinear elasticity often, though not exclusively, attributed to fractures in the solid. This talk will present the fundamentals of TR, elasticity and nonlinearity. Current and future applications of performing TR using linear and nonlinear elastic wave propagation will also be discussed with the support of recent experimental and modeling studies performed at the Los Alamos National Laboratory.
Host: Karen Daniels
Aug 27: John J. Sakon
Weninger Lab, NCSU Physics
Studying Conformation and Dynamics of Individual Proteins in Live Cells using Single Molecule FRET
This talk reports progress towards the use of single molecule fluorescence resonance energy transfer (smFRET) in the cytoplasmic environment of live cells to study individual proteins. Recombinantly expressed, externally dye-labeled SNARE proteins were microinjected into cultured cells, tracked and imaged to observe real-time conformational dynamics. We discuss the many obstacles that lower signal:noise in vivo (cellular and coverslip autofuorescence, dye photobleaching) and our methods for overcoming them. Initial findings and the applications for this technique will also be discussed.
Sep 17: Michael Dickey
Chemical Engineering, NCSU
A Micromoldable Fluid Metal for Ultra-Flexible Electronics
Conventional electronics are typically fabricated from rigid materials (e.g., silicon for transistors, copper for antennas). New materials are being explored as candidates for flexible electronics because of the novel applications that emerge from their mechanical properties. While the flexibility of current devices stems from thin-film geometries, rigid materials cannot be stretched or deformed significantly without inducing irreversible damage. Liquids flow in response to stress and are therefore highly deformable; most liquids, however, have low electrical conductivities, tend to evaporate, and cannot be micromolded into desirable shapes because they typically resolve to hemispherical shapes to minimize surface energy. This talk focuses on a gallium-based metal alloy that is a liquid at room-temperature with low-viscosity (water-like) and high conductivity and has mechanical properties that are governed by a thin, passivating oxide skin with highly-tunable characteristics. The presence of the skin allows the low-viscosity fluid to be micromolded into desirable structures and to form mechanically stable components in microfluidic channels at room temperature. Since the metal flows in response to stress, it can be elongated by orders of magnitude more than conventional materials. The ability to shape the liquid metal holds great promise for forming soft and highly deformable electronics, such as conformal electrodes, stretchable antennas, and self-healing wires.
Host: Karen Daniels
Sep 24: David Carlson
Lord Corporation
Magnetically Controlled Jamming – A Fourth Mode of MR fluid Operation
Magnetorheological (MR) fluids are suspensions of micron-sized, magnetically responsive particles in a liquid carrier. The essential feature of MR fluids is their ability to change from a freely flowing liquid into a semi-solid exhibiting a controllable yield strength when exposed to an applied magnetic field. MR fluids are presently being used in a variety of applications, providing viable solutions to many engineering challenges. Controllability in MR fluid devices is achieved through one of three commonly recognized operating modes: valve mode, direct-shear mode, and squeeze mode. Recently, the possibility of fourth control mode for MR fluids has appeared. This so-called pinch or jamming mode is a non-linear effect in which an applied magnetic field gradient can cause can be used to cause spontaneous, reversible jamming of MR fluid particles flowing through an appropriate orifice. The present talk will review the current status of MR fluid technology, evidence for this fourth, pinch mode of MR fluid operation and some of the possibilities that may be enabled.
Host: Karen Daniels
Oct 1: Alex Pronschinske
Dougherty Lab, NCSU Physics
Scanning Tunneling Spectroscopy of Benzoate Coordination Networks on Copper [110]
Metal-ligand interactions have recently emerged as an important motif driving molecular self-assembly on surfaces [1]. These interactions create surface-bound analogs of coordination compounds (or coordination polymers) and can form both spontaneously [2,3] and by deliberate growth strategies [1]. While the structural diversity that can be achieved by exploiting metal ligand interactions is immense, their functional physical properties have yet to be revealed. This talk will present the first investigations of the local electronic structure of coordination networks for benzoate grown on the [110] surface of copper.
[1] J.V. Barth, Annu Rev Phys Chem 58, (2007)
[2] D. Dougherty et al., Surf. Sci. 600, (2006)
[3] P. Maksymovych et al., Phys. Rev. Lett. 97, 046804 (2006)
Oct 29: Hongping Yan
Ade Research Group, NCSU Physics
Organic/Organic Interfacial Width Study Using Resonant Soft X-ray Reflectivity and Relation to Performance of Organic Photovoltaic Devices
The performance of polymer light emitting diode (PLED) and Organic Solar Cell is strongly affected by the structure of the interface between the active materials and needs to characterized and controlled. Resonant Soft X-ray Reflectivity (RSoXR) is a novel characterization tool that allows the interfacial width measurement without having to resort to deuteration and neutron reflectivity. we report high-precision measurements of organic/organic interfacial widths in conjugated polyelectrolyte (CPE)-containing model bilayers that are nearly isostructural to those successfully used for improving electron injection into PLEDs. Also, by studying the evolution of interface roughening in polymer/polymer bilayers with resonant soft x-ray reflectivity and its influence on device operation, we show that non-equilibrium, sharp interfaces that cannot be achieved by thermodynamic processes are best for interfacial charge separation. Even small changes of the interfaces have strong effects on the electronic characteristics. This observation places limits on the efficiency of charge separation that can be achieved in solution-processed blends and highlights the need for greater control and understanding of interface structure.
Nov 5: Brian A. Collins
Ade Research Group, NCSU Physics
Formerly Tsui Group, UNC Physics and Astronomy
Synchrotron x-ray studies on structural and chemical ordering in group IV-based magnetic epitaxial films
Novel experiments on spin transport and manipulation have recently stoked enthusiasm for the rapidly growing field of spin-based electronics. However, promising materials candidates such as doped magnetic semiconductors and Heusler half metals have shown deficiencies linked to poor ordering at the atomic scale, while complex synthesis and limited experimental techniques to probe the defects make progress on this front slow. In this talk, complementary synchrotron x-ray techniques have been used to study compositional and epitaxial effects on structural and chemical ordering of Ge-based magnetic semiconductor films and Heusler alloys Co_2 MnGe and Co_2 MnSi on the atomic scale. In magnetic semiconductors, dopant states are crucial to ferromagnetic ordering, and it was found that codoping with two magnetic elements could increase the success of dopant incorporation into the host lattice by altering energetics and kinetics of epitaxial growth. Epitaxial constraints and composition also play an important role in the atomic ordering of Co_x Mn_y Ge_z films containing the Heusler alloy as revealed by standard and anomalous diffraction techniques. Full spectrum anomalous diffraction analysis (FSADA) has been developed in this work to quantitatively probe atomic structural and chemical disorders that can contribute to the suppression of the half metallic state in these Heusler films. The findings from these studies demonstrate that compositional and epitaxial effects can be used as viable means to control structure and magnetism on the atomic scale.
Brian A. Collins, PhD dissertation, UNC-Chapel Hill, 2009.
Nov 12: TBD
Nov 19: Brad R. Conrad
Semiconductor Electronics Division, National Institute of Standards and Technology
Electrical Characterization of Soluble Anthradithiophene Derivatives
Organic semiconductors remain an active subject for device physics and material science because of their varied electrical properties and potential for low-cost, high-throughput roll-to-roll processing. Several high-mobility oligomers, such as pentacene and rubrene, have been identified yet most are incompatible with current manufacturing methods because of issues such as insolubility or poor thin film structure. One benefit of working with organic semiconductors is the ability to modify chemical composition to tune both the electrical and physical properties, specifically crystallinity and solubility. Such recently developed organic semiconducting materials are among the forefront of recent advances in the field of organic electronics [1,2]. Electrical characterization of several newly developed partially fluorinated anthradithiophene (F-ADT) derivatives will be presented. Insights gained from single crystal and spin cast transistor devices on a variety of surfaces will be presented.
[1] V. Podzorov, E. Menard, A. Borissov, V. Kiryukhin, J Rogers, and M. Gershenson, PRL, 93, 8, 086602, (2004) [2] O. D. Jurchescu, S. Subramanian, R. J. Kline, S. D. Hudson, J. E. Anthony, T. N. Jackson and D. J. Gundlach, Chemistry of Materials, 20, 21, 6733-6737, (2008)
Host: Dan Dougherty