Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B53: Surfaces, Interfaces, and Thin Films: Kinetics and Dynamics |
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Sponsoring Units: DCMP Chair: Brad Conrad, Appalachian State University Room: Mile High Ballroom 2C |
Monday, March 3, 2014 11:15AM - 11:27AM |
B53.00001: Charge transfer and adsorption-desorption kinetics in carbon nanotube and graphene gas sensing Sang-Zi Liang, Gugang Chen, Avetik Harutyunyan, Milton Cole, Jorge Sofo Detection of molecules in the gas phase by carbon nanotube and graphene has great application potentials due to the high sensitivity and surface-to-volume ratio [1, 2]. In chemiresistor, the conductance of the materials has been proposed to change as a result of charge transfer from the adsorbed molecules. Due to self-interaction errors, calculations using LDA or GGA density functionals have an innate disadvantage in dealing with charge transfer situations. A model which takes into consideration the dielectric interaction between the graphene surface and the molecule is employed to estimate the distance where charge transfer becomes favorable. Adsorption-desorption kinetics is studied with a modified Langmuir model, including sites from which the molecules do not desorb within the experimental time. Assuming a constant mobility, the model reproduces existing experimental conductance data [1, 2]. Its parameters provide information about the microscopic process during the detection and varying them allows optimization of aspects of sensor performance, including sensitivity, detection limit and response time. [1] G. Chen et al., Sci. Rep. 2, 343 (2012). [2] G. Chen et al., Appl. Phys. Lett. 101, 053119 (2012). [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B53.00002: Dynamics of H$_{2}$O Molecules Confined in Fullerenes Roxanne Tutchton, Paul Larson, Zhigang Wu Structural and dynamical properties of confined water in nanostructures are expected to be remarkably different than those of bulk water. Water confined in nanotubes and graphene sheets has been extensively investigated theoretically, yet very few computational efforts have been made to study confined water in fullerenes though, experimentally, water molecules have been successfully encapsulated inside fullerenes as small as C$_{60}$. In this work, we carry out classical molecular dynamics simulations to investigate density and H-bond distributions inside fullerenes ranging from C$_{60}$ to C$_{540}$. Our results show that as the size of the fullerene increases, concentric shells of water molecules are formed, and the water density is higher than that of bulk water while the average H-bond per molecule is slightly lower than the bulk value. We also find that these shells of H$_{2}$O are solid-like at room temperature, but they should eventually become liquid-like at high temperatures. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B53.00003: Growth and dynamics of C$_{60}$ molecules on Ag(111) Renee Diehl, Hsin Li, Guilherme Abreu, Heekeun Shin, A.K. Shukla, Julian Ledieu, Vincent Fourn\'ee, Laura Serkovic Loli, Kristin Marino, Stephanie Su, Michael Snyder C$_{60}$ monolayers on Ag(111) form various structures that depend on the surface preparation. For films deposited at 300 K, the monolayer comprises close-packed domains that are incommensurate with the substrate. These domains progressively convert into a commensurate (2$\surd $3x2$\surd $3)R30 structure as the annealing temperature is increased. The annealing process activates the formation of vacancies beneath some of the C$_{60}$ molecules, resulting in a temperature-dependent equilibrium population of about 50{\%} of the molecules sitting on vacancies and 50{\%} sitting on top of Ag atoms in the commensurate phase. In addition to molecules in these two sites, about 0.5{\%} of the molecules image very brightly, and these molecules are in dynamical equilibrium with the other two types. This talk will focus on scanning tunneling microscopy and low-energy diffraction studies of the structure and dynamics of the monolayer phases of C$_{60}$ molecules on Ag(111). [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B53.00004: Modeling the transport of chemical warfare agents and simulants in polymeric substrates for reactive decontamination Thomas Pearl, Brent Mantooth, Mark Varady, Matthew Willis Chemical warfare agent simulants are often used for environmental testing in place of highly toxic agents. This work sets the foundation for modeling decontamination of absorbing polymeric materials with the focus on determining relationships between agents and simulants. The correlations of agents to simulants must consider the three way interactions in the chemical-material-decontaminant system where transport and reaction occur in polymer materials. To this end, diffusion modeling of the subsurface transport of simulants and live chemical warfare agents was conducted for various polymer systems (e.g., paint coatings) with and without reaction pathways with applied decontamination. The models utilized 1D and 2D finite difference diffusion and reaction models to simulate absorption and reaction in the polymers, and subsequent flux of the chemicals out of the polymers. Experimental data including vapor flux measurements and dynamic contact angle measurements were used to determine model input parameters. Through modeling, an understanding of the relationship of simulant to live chemical warfare agent was established, focusing on vapor emission of agents and simulants from materials. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B53.00005: Desorption Kinetics of Water from Poly(methyl methacrylate) Patrick Howard, Brian Familo, Thorin Kane, Ross Netusil, Marie Romano, John St. Leger, Paul Jones, Carolina C. Ilie Understanding the surface behaviors of polymers has many applications in modern medicine and engineering. In this experiment, we are analyzing the thermal desorption of water from poly(methyl methacrylate), polyethylene, and other polymers. Crystalline polymer films are being prepared using a Langmuir-Blodgett method from a water subphase, as well as by a spin coating technique. The organized, crystalline Langmuir-Blodgett films are different in structure from the more disorganized spin-coated films. The thermal desorption spectra from both techniques will be compared to show the effects of the way polymer films are obtained. Arrhenius analysis on the desorption spectra yields desorption activation energies for these polymers. The purpose of this analysis is to provide insight into the dipole interactions between the water and polymer molecules. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B53.00006: Reactive decontamination of absorbing thin film polymer coatings: model development and parameter determination Mark Varady, Brent Mantooth, Thomas Pearl, Matthew Willis A continuum model of reactive decontamination in absorbing polymeric thin film substrates exposed to the chemical warfare agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (known as VX) was developed to assess the performance of various decontaminants. Experiments were performed in conjunction with an inverse analysis method to obtain the necessary model parameters. The experiments involved contaminating a substrate with a fixed VX exposure, applying a decontaminant, followed by a time-resolved, liquid phase extraction of the absorbing substrate to measure the residual contaminant by chromatography. Decontamination model parameters were uniquely determined using the Levenberg-Marquardt nonlinear least squares fitting technique to best fit the experimental time evolution of extracted mass. The model was implemented numerically in both a 2D axisymmetric finite element program and a 1D finite difference code, and it was found that the more computationally efficient 1D implementation was sufficiently accurate. The resulting decontamination model provides an accurate quantification of contaminant concentration profile in the material, which is necessary to assess exposure hazards. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B53.00007: Estimating Atomic Diffusivity in Metallic Multilayered Systems Manav Vohra, Omar Knio Nanostructured multilayered systems can support self-propagating reactions due to exothermic intermixing and small atomic diffusion distances. In the Zr-Al system under stoichiometric and adiabatic conditions, the temperature increases by about 1500 K over ambient as a result of the formation reaction. This usually results in melting in the individual Al layers, and consequently to enhanced rates of intermixing. In order to characterize this phenomenon, and accordingly quantify the associated heat release rates, we rely on transient temperature measurements of homogeneous ignition, as well as measurements of the velocity of self-propagating fronts. The former enables us to infer averaged intermixing rates in a temperature range falling below the melting point of Al, whereas the latter yield estimates at high temperatures. Implementation of the formalism leads to correlations of the atomic diffusivity that exhibit two Arrhenius branches, with a jump across the melting temperature of Al. The resulting composite Arrhenius relation can be readily incorporated into reduced reaction models,\footnote{M. Salloum and O. M. Knio. \textit{Combustion and Flame} 157.2 (2010): 288-295.} and thus exploited to predict transient, multidimensional reaction phenomena. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B53.00008: Boron diffusion mechanism and effect of interface Ge atoms in Si/SiO$_{2}$ and SiGe/SiO$_{2}$ interfaces Geun-Myeong Kim, Young Jun Oh, Chang Hwi Lee, K.J. Chang In metal-oxide-semiconductor field effect transistors (MOSFETs) it is known that implanted B dopants easily segregate to the oxide during thermal annealing after ion implantation, causing threshold voltage shift and sheet resistance increase. On the other hand, SiGe alloys have been considered as a promising material for $p$-type MOSFETs due to reduced B diffusion and high hole mobility. However, there is a lack of studies for B diffusion in Si/SiO$_{2}$ and SiGe/SiO$_{2}$ interfaces. In this work, we perform first-principles density functional calculations to study the mechanism for the B diffusion in Si/SiO$_{2}$ and SiGe/SiO$_{2}$ interfaces. We investigate the diffusion pathways and migration barriers by using the climbing nudged elastic band and dimer methods. For Si/SiO$_{2}$ interface, B in Si turns into an interstitial B and tends to intervene between the Si and bridge O atoms at the interface. The overall migration barrier is calculated to be about 2 eV, comparable to that in bulk SiO$_{2}$. In SiGe/SiO$_{2}$, interface Ge atoms enhance the stability of B-related defects in the interface region, resulting in the higher migration barrier of about 3.7 eV. Our results indicate that Si/SiO$_{2}$ interface does not hinder the B diffusion, however, the B diffusion is suppressed in the presence of interface Ge atoms. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B53.00009: X-ray Scattering from Ag (001) and Ag (111) Surfaces in Electrochemical Environments: Dynamics and Structure Robert Karl, Jr., Michael Pierce, Vladimir Komanicky, Hoydoo You, Andi Barbour, Chenhui Zhu, Alec Sandy We have investigated the Ag (001) and Ag (111) single crystal surfaces in weak electrolyte using a combination of x-ray Crystal Truncation Rod (CTR) experiments to determine structural information and X-ray Photon Correlation Spectroscopy (XPCS) to examine the nano-scale dynamics. Our structural measurements confirm earlier potential dependent measurements of H$_2$O over the Ag (111) surface [1], and also show that the Ag (001) H$_2$O interface behaves in an analogous fashion. The XPCS dynamics data reveal how the surface evolves, in real-time, relative to the point of zero charge. Both the CTR and XPCS data were collected at different applied potentials. By comparing the CTR data with the XPCS data, along with ex-situ Atomic Force Microscopy (AFM), we will investigate relationships between the dynamics of the Ag surface and the distribution of H$_2$O molecules above the crystal surface. [1] M. F. Toney et al. Nature 368, 444 (1994). [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B53.00010: Evolution of $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface of Al$_{2}$O$_{3}$(0001) generated in air Hawoong Hong, Aaron Gray, T.-C. Chiang As reported by S. Baik et al\footnote{S. Baik, D. E. Fowler, J. M. Blakely, and R. Raj, J. Am. Ceram. Soc. 68(5), 281 (1985).} $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface of Al$_{2}$O$_{3}$(0001) can be generated by annealing at a high temperature in air. We reproduced this $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface and investigated surface structures with x-ray diffraction using synchrotron radiation at Advanced Photon Source and RHEED techniques. We also annealed this $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface in a UHV chamber until the superstructure disappeared and a new $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface was generated. We will compare the results to the previous x-ray diffraction experiments\footnote{G. Renaud, B. Villette, I. Vilfan, and A. Bourret, Phys. Rev. Lett. 73, 1825 (1994).} and recent AFM/DFT investigation.\footnote{J. V. Lauritsen, M. C. R. Jensen, K. Venkataramani, B. Hinnemann, S. Helveg, B. S. Clausen, and F. Besenbacher, Phys. Rev. Lett 103, 076103 (2009). } The UHV generated $\sqrt {31} \times \sqrt {31} R9^{\circ}$ surface also appeared to preserve the $\sqrt {31} \times \sqrt {31} R9^{\circ}$ symmetries as Pd films were deposited. However, the intensity ratios between superlattice peaks went through large changes. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B53.00011: Capillary Condensation Pathways of CO2 under Templated Mesoporous Silica Confinement Bo Wang, Paul Sokol Adsorption of CO2 in porous medium has been of great current interest due to its potential for mitigating the global warming caused by greenhouse gases. In particular, the behavior of confined CO2 in mesoporous media near room temperature is particularly relevant to sequestration efforts. Realistic mesoporous systems, such as shales and coals, represent a complex fractal pore structure that complicates the interpretation of adsorption studies. We present the results of a study focused on the adsorption of CO2 in model mesoporous media with well-defined pore structures. Templated porous glasses, such as MCM-41 which has a regular network of 1D pores, provide an ideal system for fundamental studies of the adsorption process. In this study, we focus on the structure of adsorbed CO2 films which evolves in a mixture of phases and the development of nucleation occurs during the formation of high density liquid CO2 inside the confining matrix. We have used Small Angle Neutron Scattering to study the spatial distribution of material radially and transversely within the pores. The 30m SANS NG7 at NIST was used to map out the details of CO2 condensation pathway under mesoporous silica confinement. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B53.00012: Local Investigation in Dynamic Behavior of Excited Water Nanoclusters on Cu(111) Surface Yang Guo, Huiqi Gong, Li Dong, Lailai Li, Jinchuan Wang, Xinyan Shan, Xinghua Lu Dynamic behavior of water molecules on surfaces is important for surface-mediated water dissociations and reactions. Here we present investigations in dynamic behavior of excited water nanoclusters on Cu (111) surface by using a low temperature scanning tunneling microscope (STM). It is found that excess electrons in a single water nanocluster can be injected from a metallic STM tip under a positive voltage. Such injection of electrons results in both the diffusion of single H$_{\mathrm{2}}$O molecules within the nanocluster and directional diffusion of water nanoclusters on surface. The range of lateral diffusion is limited to several nanometers from the tip because of the electrical screening effect from Cu substrate for the excess electrons in the nanocluster. In addition, femto-second laser pulses are employed to excite the water nanoclusters during STM imaging with tip in the tunneling condition. Significant changes in topographic profile of H$_{\mathrm{2}}$O nanoclusters are observed under the photoexciation, as compared with that of the nanoclusters in the ground state. The results obtained in this study provide a microscopic understanding of the diffusion mechanism of excited water nanoclusters on surface. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B53.00013: Self-consistent elastic and inelastic scattering of helium atoms by the monolayer solid Xe/Pt(111) Ludwig Bruch, Flemming Hansen The inelastic scattering of a helium beam by an incommensurate monolayer solid of Xe/Pt(111) for incident energies in the range 2.5 -- 50 meV is evaluated in the one-phonon approximation. The calculations treat specific one-phonon excitations and use an extension of our wave packet propagation formulation of helium scattering to treat the elastic and inelastic scattering self-consistently. There is an inelastic (diffuse) background scattering for phonon modes with wavevectors that uniformly sample the Brillouin zone. The fraction of the beam that is scattered elastically into diffraction channels varies from almost 90\% to about 15\% over this energy range. A low energy case with trapping for more than 200 ps has been simulated. Results of the self-consistent calculations for the inelastic scattering are compared to those of previous calculations that did not include the feedback (coherent phonon annihilation) to the elastic channels. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B53.00014: Simultaneous observation of ultrafast structural dynamics and transient electrical field by picosecond electron pulses Run-Ze Li, Pengfei Zhu, Long Chen, Jie Chen, Jianming Cao, Zheng-Ming Sheng, Jie Zhang Ultrafast electron diffraction and microscopy are very promising methods to study transient structural dynamics with atomic spatial-temporal resolution. However, in these laser-pump electron-probe studies of structural dynamics, a transient electric field induced by laser excitation of the sample could deflect probing electrons, which may eventually leads to a misinterpretation of the diffraction data. Here, picosecond structural dynamics and transient surface electric field evolution, excited by femtosecond laser interaction with a metallic thin film, have been observed simultaneously in real time by ultrashort electron pulses in a transmission configuration. By tracing time dependent changes of electron diffraction and deflection angles, these two processes are found to be significantly different and distinguishable in their temporal behavior. This observation provides an effective approach to extract the otherwise obscured ultrafast structural dynamics and may help to improve the spatiotemporal resolution in ultrafast electron diffraction and microscopy studies. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B53.00015: Two-stage melting of the ice Ih (0001) surface by molecular dynamics: A molecular dynamics study Mizuki Kuga, Hiroshi Iyetomi We report a molecular dynamics simulation study of the structure and energetics of Ice Ih with a free basal (0001) surface. Especially we cast a new light on the formation of a quasi-liquid layer (QLL) at the surface prior to the melting of bulk ice. Detailed analysis of functional behavior of the total and potential energies with respect to temperature clearly separates the surface melting process into two stages. As temperature is increased, the outermost bilayer of the ideal surface first starts disordering in a progressive way with detachment of water molecules from the surface. During this first stage of surface melting, the energies show no discontinuous change as a function of temperature. Subsequently to fully developed disordering of the top layer, the surface shows stepwise melting in a narrow temperature range of around $10\,{\rm K}$. This second stage of surface melting has first-order phase-transition-like characteristics as demonstrated by structural relaxation from an initial metastable (super-heating) state to the stable state during which the surface is liquefied layer by layer. Accordingly, it turns out that each of the stable QLL states designated by the number of liquefied bilayers has its own branch of the potential energy. [Preview Abstract] |
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