Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P12: Focus Session: Characterization and Modeling of Complex Surfaces and Interfaces |
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Sponsoring Units: DMP DCMP Chair: Yue Qi, General Motors Room: 308 |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P12.00001: Atomistic Structure and Energy of Solid-Liquid Interfaces Invited Speaker: As microstructural length-scales are reduced, the role of interfaces in determining the properties of materials becomes more dominant. The importance of the correlation between interface structure and chemistry with interface (and bulk) properties is evident in a range of material systems, and is a topic of intense experimental and theoretical work for solid-solid interfaces. While detailed thermodynamic analysis of solid-liquid interfaces is routinely conducted, knowledge of the local structure at solid-liquid interfaces is still incomplete. To be more specific, the correlation between the structure of the solid, and the structure in the liquid near the interface, has not been fully addressed. In this presentation, in-situ ($\sim $750\r{ }C) high resolution transmission electron microscopy (HRTEM) of liquid Al in contact with sapphire ($\alpha $-Al$_{2}$O$_{3})$ will be presented. Contrast perturbations in the liquid Al adjacent to the crystalline substrate were determined to be due to ordering of the liquid, via detailed multi-slice dynamic electron scattering simulations. Details on the type of ordering were interpreted by molecular dynamics simulations of liquid Al in contact with crystalline substrates, and compared to sessile drop studies of liquid Al on sapphire. These results are compared with recent HRTEM investigations of equilibrium amorphous films at metal-Al$_{2}$O$_{3}$ interfaces, where partial ordering of the film plays an important entropic role in reaching stable nanometer-thick films. This will then be extended to equilibrium segregation, and the concept of extremely small volumes of liquids confined by crystals. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P12.00002: Calculation of excess interfacial entropy, stress and energy for solid-liquid interfaces Brian B. Laird, Ruslan L. Davidchack, Mark Asta, Yang Yang The solid-liquid interfacial free energy, $\gamma_{\rm sl}$, governs a number of important phenomena, e.g., crystal nucleation and growth, and wetting. For an equilibrium crystal-melt interface, $\gamma_{\rm sl}$ can be calculated via simulation using thermodynamic integration or capillary fluctuations [Phys. Chem. B {\bf 109}, 17802 (2005)]. The calculation of $\gamma_{\rm sl}$ away from coexistence requires the temperature and strain dependence of $\gamma_{\rm sl}$, which can be determined from the excess interfacial entropy, $\eta_{\rm sl}$, and stress tensor, $\mbox{\boldmath$\tau$}_{\rm sl}$. We determine $\eta_{\rm sl}$ and $\mbox{\boldmath$\tau$}_{\rm sl}$ for a system of Lennard-Jones particles and for particles with an inverse-power interaction [$\phi(r) = \epsilon (\sigma/r)^{n}$] for $n = $ 6, 8 (fcc and bcc) and 12, 20 (fcc). We determine $\eta_{\rm sl}$ and $\mbox{\boldmath$\tau$}_{\rm sl}$ for the (100), (110) and (111) orientations. We calculate $\eta_{\rm sl}$ using two methods, both using the Gibbs dividing surface defined so that the excess interfacial particle number is zero. In the first, we calculate $\eta_{\rm sl}$ from the temperature dependence of $\gamma_{\rm sl}$, $\mbox{\boldmath$\tau$}_{\rm sl}$ and the number density, $\rho$, along the coexistence curve. In the second, we calculate the excess interfacial energy, $e_{\rm sl}$, and use the equation $\gamma_{\rm sl} = e_{\rm sl} - T \eta_{\rm sl}$. The results agree within estimated errors. One surprising observation is that $\eta_{\rm sl}$, $e_{\rm sl}$ and $\mbox{\boldmath$\tau$}_{\rm sl}$ are significantly more anisotropic than $\gamma_{\rm sl}$. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P12.00003: Determination of Spin Order in Magnetic Organic Semiconductor V[TCNE]$\sim $2 Hailemariam Ambaye, Valeria Lauter, Stephen Nagler, Christina Hoffmann, Hal Lee, Andrew Payzant, Arthur Epstein, Chen Chi-Yi, Richard Goyette These Organic-based magnets are new area of materials research. The discovery of V[TCNE]$\sim $2 with its high Tc $\sim $ 400 K and semiconducting behavior similar to silicon, as well as its photonic response unique for magnetic materials, opens up many issues of fundamental physics and chemistry as well as the potential opportunities for use of these and related materials in technologies ranging from spintronics to sensing. To understand the magnetic state and the evolution of the magnetic at and near interfaces with other magnetic and non magnetic materials we have performed a polarized neutron measurement at the SNS magnetism reflectometer instrument. The measurements show the presence of magnetic response at 5K temperature. The room temperature measurements show no magnetic responses. The systems considered are V[TCNE]$\sim $2(1500A) and 6000A on Si substrate. Work supported by DOE. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P12.00004: Stabilization of a ferromagnetic insulating phase with colossal magnetoresistance at the interface of manganite bilayers Jonathan Laverdi\`ere, Serge Jandl, Patrick Fournier The charge ordered phase observed in colossal magnetoresistive manganites motivated many theoretical and experimental efforts. Charge order is an insulating electronic phase that becomes metallic in sufficiently high magnetic field. This high ``melting'' field hinders any applications for magnetic storage devices. Here, we present the study of the proximity effect in Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (NSMO) / Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ (NCMO) bilayers. NSMO is dominated by ferromagnetic double exchange, producing a ferromagnetic metallic phase, while NCMO is strongly influenced by the Jahn-Teller lattice distortion, localizing charges on the Mn$^{3+}$ sites. Our study addresses the following question: Which one will dominate at the NSMO/NCMO interface? We will present Raman scattering and magnetotransport measurements on NCMO/NSMO bilayers grown on SrTiO$_{3}$ substrate. A ferromagnetic insulating phase has been observed for very thin NSMO films. This phase becomes metallic and gives rise to colossal magnetoresistance at a low field compared to the usual melting field. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P12.00005: Effects of surfactant to the physical properties of single-walled carbon nanotube buckypaper. Jin Gyu Park, Charlie Lin, Jesse Smithyman, Adam Cooke, Shu Li, Richard Liang, Chuck Zhang, Ben Wang, Ade Kismarahardja, James Brooks Single-walled carbon nanotubes (SWCNTs) were dispersed in aqueous medium using surfactant and filtered to get an entangled network, called buckypaper (BP). Thermogravimetric analysis shows that the remaining surfactant has significant weight percentage and has effects on the physical properties. Raman spectrum of BP, especially the radial breathing mode is related to the entanglement degree and residual surfactant. The G-band peak shift shows different temperature dependence with the reduction of residual surfactant in the BP and oxidation of nanotube. The electrical conductivity was improved after removing surfactant and temperature dependence of electrical resistivity followed variable range hopping type conduction. Mechanical properties are also affected from their integration degree, alignment, and residual surfactant. Therefore, tensile modulus and strength were improved after washing surfactant. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P12.00006: Low-Temperature Absorption Studies in Bismuth Nanowires Jason Reppert, Malcolm Skove, Marcie Black, Mildred Dresselhaus, Apparao Rao Bulk bismuth has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Previously, we have shown evidence for strong quantum confinement in Bi nanorods with diameters $\sim $10 nm which undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect band gap. These quantum confinement effects can be potentially useful in optical and electro-optical devices. Here, we report the low temperature (77 K) optical absorption properties of $\sim $10 nm diameter Bi nanorods using Fourier Transform Infrared spectroscopy. The Bi nanorods exhibit a strong absorption peak ($\sim $1000 -- 1400 cm$^{-1}$, depending on the diameter) in the mid-IR that is not present in bulk bismuth. The full width at half maximum intensity of the IR absorption peaks decrease from 26 cm$^{-1}$ at 300 K to 15 cm$^{-1}$ at 77 K. No significant blue-shift in energy was observed, and these changes will be discussed in terms of the temperature dependence of the L-point and T-point electron energies. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P12.00007: Nanopatterned Biomimetic Surfaces to Probe the Role of cytoskeletal Proteins in cell Adhesion Justin Abramson, Matteo Palma, Mark Schvartzman, Shalom Wind, Michael Sheetz, James Hone Nanometer level spatial organization has been shown to play a crucial role in cell mechanics, in particular in cell adhesion to the extracellular matrix. Combining nanolithography and biomolecular self-assembly strategies, we report on the fabrication of nanopatterned biomimetic surfaces to probe the importance of both the spatial ordering of transmembrane proteins as well as the role played by peptide sequences as cell binding domains in the formation of cell focal adhesions. We have fabricated arrays of Au/Pd nano-dots using electron-beam and nanoimprint lithography. Different chemical strategies have been pursued to biofunctionalize such nanostructures, both through the formation of mixed Self Assembled Monolayers as well as via chemical reactions at surfaces. Fluorescence microscopy allowed us to monitor single-molecule chemisorption of cell-adhesion proteins in vitro, as well as to follow cell spreading on the nanopatterned bio-arrays, in order to investigate cytoskeletal protein binding interactions in vivo. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P12.00008: Molecular Dynamics Simulations of Interfaces in Complex Materials Invited Speaker: Molecular details of structure and chemistry play and important role in the properties and engineering performance of composites and complex materials. Experimental investigation of the interfaces most often presents itself as a major challenge. This, in turn, becomes an opportunity in disguise for the molecular level simulation approaches. However, in order to convincingly address to the problem of elucidating the structure and chemistry at the interfaces, one must employ reliable and accurate and transferrable interaction potentials for dissimilar materials - this is the case for composites and complex materials. In this, talk we will present examples of molecular dynamics studies on the structure and properties of silicon nano-crystals in a silica matrix, piezoelectric CNT-polyimide nano-composites, and on the role of super lattice structures for enhancing mechano-electric coupling in ferro-electric ceramic alloys. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P12.00009: Evaluation of dispersion interactions in general geometries Anthony Maggs, Pasquali Samuela Dispersion interactions are very often approximated by pairwise van der Waals interactions between molecules. In dense media, however, there are important corrections due to the many-body nature of fluctuations. These many-body forms can be calculated in closed form in the simplest of geometries using the methods of Casimir and Lifshitz. Here we study dispersion interactions between bodies in general geometries. We map the calculation of the partition function onto a determinant which we discretize and evaluate with the help of Cholesky factorization. We study the efficiency of the factorization in two and in three dimensions and conclude that accuracies of the order of one per cent are readily achieved in the total interaction energy. We compare the approximations of pairwise additivity and proximity force with our numerical methods. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P12.00010: Phase-field Simulation of Phase Coarsening at Ultra-high Volume Fractions Ke-Gang Wang, Xueru Ding The study of phase coarsening kinetics during microstructure evolution is critical to a variety of industrial applications involving two-phase systems in which the dispersed phase controls the properties of the material. Liquid-phase sintering, casting and spray deposition are just a few examples of processes in which the coarsening process has important technological implications. In this talk, the dynamics of phase coarsening at ultra-high volume fractions (V$_{V}>$0.9) will be presented based on 2-D phase-field simulations. Kinetics of phase coarsening and spatial correlations in microstructures will be revealed. Pair distribution functions in microstructures will be shown. The scaled particle-size distribution as functions of the dispersoid volume fraction will be demonstrated. Finally, computational results are compared with experimental observations. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P12.00011: The impact of self-healing on the life-time of materials John Gaddy, Wouter Montfrooij, Alexander Schmets Structural materials that are attributed with the (new) property of ``self-healing'' will obviously lead to safer, longer lasting and more reliable structures. The property of ``self-healing'' can be defined as the ability of a material to mitigate autonomously early stages of damage such as micro cracks, and many examples of materials with this properties have been reported in recent years [1]. In this contribution we investigate the effect of healing on the expected service life time of a model material. We apply a statistical mechanics' inspired computational approach to model the process of damage accumulation and on-site healing of a material under well defined loading conditions. We define a material as being at the end of its service life when a percolative path of damaged cells has passed a prescribed length. The variation of service life for various scenarios, such as healing times and distribution of healing centers is investigated. Finally we show how this type of models may be useful for the design of optimized self healing materials. \\[3pt] [1] S. van der Zwaag (editor), (2007). \textit{Self Healing Materials: An Alternative Approach to 20 Centuries of Materials Science}. Springer Netherlands. [Preview Abstract] |
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