Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H31: Focus Session: Computational Nanoscience III: Ferroelectrics, Surfaces, and Water |
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Sponsoring Units: DMP DCOMP Chair: Eric Schwegler, Lawrence Livermore National Laboratory Room: Morial Convention Center 223 |
Tuesday, March 11, 2008 8:00AM - 8:36AM |
H31.00001: Properties of Ferroelectric Nanostructures Invited Speaker: Ferroelectric nanostructures (FENs) such as thin films, nanowires and nanodots are receiving a lot of attention due to their potential for technological applications and to the rich variety of underlying physics. Interestingly, properties of FENs can substantially deviate from their bulk counterpart due to their sensitivity to many factors. Examples of such factors are the electrical boundary conditions (associated with the full, partial or non-existent screening of polarization-induced surface charges) and mechanical boundary conditions (arising from the lattice mismatch between the FEN and its substrate). \newline Here, we developed and used computational schemes to predict many properties in various FENs, as well as, to provide atomistic insight to their complex phenomena. In particular, we will show the striking following features and reveal their origins: \begin{itemize} \item The interplay between electrical boundary conditions, mechanical boundary conditions and growth direction results in the appearance of novel dipole patterns and new low-symmetry phases possessing superior dielectric properties in ferroelectric dots, wires and films [1,2]. \item FENs can exhibit dielectric anomalies, such as a {\it negative} dielectric susceptibility [3]. \item Nanobubbles can form in ferroelectric films under an external electric field [4]. \item An homogeneous electric field can be used to control the chirality of vortex structures in asymmetric ferroelectric dots, via the creation of original intermediate states [5]. \end{itemize} [1] I. Ponomareva {\it et al.}, Phys. Rev. B {\textbf 72}, 214118 (2005). \newline [2] I. Ponomareva and L. Bellaiche, Phys. Rev. B {\textbf 74}, 064102 (2006). \newline [3] I. Ponomareva {\it et al.}, to be published in Phys. Rev. Lett. (2007). \newline [4] B.-K. Lai {\it et al.}, Phys. Rev. Lett. \textbf{96}, 137602 (2006). \newline [5] S. Prosandeev {\it et al.}, submitted (2007). \newline These works have been done in collaboration with L. Bellaiche, I. Kornev, B.-K. Lai, I.I. Naumov, R. Resta and S. Prosandeev. Some computations were made possible thanks to the MRI Grants 0421099 and 0722625 from NSF. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H31.00002: Internal electric field effects in ferroelectric nanostructures Byounghak Lee, Zhengji Zhao, Lin-Wang Wang A ground state dipole moment of ferroelectric nanostructures is a long-standing problem. A permanent dipole moment can alter optical and electronic properties of nanostructures, causing state localizations and electron hole separations. A dipole moment in the bulk can have a self-screening effect. In a nanostructure, the different shape of the system can result in an effective screening different from the bulk. In ferroelectric nanostructures, this can lead to a shape dependent total dipole moment, different from the volumetric result derived from its bulk value. Direct ab initio theoretical study on the nanostructure dipole moment is scarce due to the large computational cost. We present a first principles study of this problem using a recently developed Linear Scaling 3-Dimensional Fragment method. We investigated shape and size dependence of the surface and bulk contributions of the electric dipole moment in various ferroelectric nanorods. We compared our results with experiments and provided an insightful physical picture by analyzing the ab initio numerical results with a classical dielectric model. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H31.00003: Nanowettability by saline solutions in electric field Alenka Luzar, C.D. Daub, D. Bratko Molecular simulations of nanosized aqueous droplets and films next to apolar surfaces show a remarkable sensitivity of water contact angles on the applied electric field polarity and direction relative to the liquid/solid interface. We explain the effect by analyzing the influence of the field on interfacial hydrogen bonding which in turn affects the interfacial tensions. When electric field is applied on the aqueous film in the direction perpendicular to the confining hydrophobic surfaces, the competition between field-induced alignment and orientational preference of interfacial water molecules relative to the surfaces results in asymmetric wettability of opposing surfaces (Janus interface). The observed anisotropy in droplet or film wetting is a new nanoscale phenomenon that has so far been elusive as, in current experimental setups, surface molecules represent a very low fraction of the total number of molecules, affected by the field. We discuss amplification of these effects in saline solutions. The work gives basic understanding of field charge effects that can modulate local hydrophilicity of engineered and biological interfaces, as well as surface manipulation in nanofluidic devices. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H31.00004: Electric Field Control of Structure, Dimensionality and Reactivity of Gold Nanoclusters Supported on Thin Films of MgO/Ag(100) Bokwon Yoon, Uzi Landman External electric field control and manipulations of the structural stability, dimensionality, and chemical reactivity of gold nanoclusters deposited on MgO films grown on an Ag(100) substrate, are introduced and illustrated with the use of first-principles electronic structure calculations. Field-controlled interfacial charging and field-induced structural dimensionality crossover are predicted; These structural changes are accompanied by variations of the chemical reactivity of the adsorbed gold nanostructures. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H31.00005: Adsorption of water on anatase TiO$_2$ nanoparticles: the role of a wire of undercoordinated Ti sites. M. Posternak, A. Baldereschi, B. Delley Presence of hydroxyl groups is known to be an important element for the initial integration of titania--coated implants in natural tissues. The existence of OH$^-$ radicals due to water dissociation is indeed responsible for major changes in their surface reactivity, and depends in particular on the preparation of the surface. The stable phase for nanoparticle--sized microstructures is anatase, and crystallites are expected to expose different surface terminations and edges. In this work, we study water adsorption on a model system, consisting of an edge along the $[1\,1\,\overline{1}]$ direction, at the intersection of two major (101) surfaces. Using the DMol$^3$ approach\footnote{B. Delley, J. Chem. Phys. \textbf{113}, 7756 (2000).} within DFT, we find that water dissociative adsorption occurs at the four--fold coordinated Ti atoms present on this edge, in contrast with the case of the unreactive (101) surface. These results provide evidence of an increased chemical activity of such binding sites for adsorption of atoms and molecules. The mechanisms of the hydroxylation process are discussed in terms of the geometric arrangement, coordination numbers, and acidic/basic character of the relevant Ti and O sites. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H31.00006: Characterization of Anatase Nanoparticles Using Tight Bonding DFT Simulation Hong Wang, Ning Ma, Hao Wang, James P. Lewis The structure and electronic properties of anatase nanoparticles with size ranging from 1.5nm to 2.4 nm have been presented in this paper under ab initio Density Functional Theory computational method. Based on the relaxed structures obtained in our calculation, we propose that the portion of surface in the whole structure effects the geometric configuration of anatase nanoparticles. However, as long as the nanoparticles grow bigger, the affection decreases obviously. The analysis of the frontier orbitals of the nanoparticles in our work make us believe that the frontier orbitals (so called HOMOs) are mostly localized on some corner positions of the whole structure. These corner positions are constisted of 4-coordinated Ti atom lack of bridge oxygen atoms. When we adsorb water molecule on different positions on the facets of the smallest particle (TiO$_{2})_{48}$, it turns out the corner position where the frontier orbitals are mostly localized are most energetic favorable adsorption position for water molecule. These special corner positions will likely act as high attractive spots for the external molecules existing in the nanoparticles' environment. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H31.00007: Single oxide overlayers grown on top of another oxide: Their stability, interaction with metal nanoparticles, and contribution to catalytic activity Sergey Rashkeev Oxides are widely used as catalysts as well as supports for catalytically active metal nanoparticles. The catalytic activity of the system depends on many different factors such as anchoring, sintering, decomposition, and diffusion of metal atoms/clusters on the oxide surface. Here we use a combination of first-principles density-functional calculations and molecular dynamics simulations to investigate how all of these factors may change when the surface of the oxide support is modified by an additional single overlayer of another oxide positioned on it. In particular, we found that deposited monolayer oxide films may show instabilities that result in formation of strong anchoring sites for metal atoms/clusters. Also, an atomic-scale roughness introduced in such a way may slow down the surface diffusion processes and inhibit nanocluster growth/sintering. For example, a single layer of SiO2 on a TiO2 substrate may significantly increase the stability of Au nanoparticles and the efficiency of the catalytic CO oxidation. The author thanks Dr. Steven Overbury (Oak Ridge National Laboratory) for attracting his attention to this problem and INL Laboratory Directed Research and Development program and the U. S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07-051D14517 for financial support. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H31.00008: Role of hydrogen on catalytic reduction of nitric oxide on selected transition metal surfaces Faisal Mehmood, Anand U. Nilekar, Manos Mavrikakis Self-consistent periodic DFT-GGA calculations are used to investigate the NO reduction reaction in presence of atomic H on seven close-packed transition metal surfaces namely, Cu(111), Ag(111), Pd(111), Pt(111), Rh(111), Ir(111), and Ru(0001). The chemisorption of atomic (N, O, H) and molecular (NO, NH, OH, HNO, NOH, HNOH, H$_{2}$NO) reaction intermediates has been systematically studied on each metal surface and the preferred sites and binding energies are determined for a 1/4 ML surface coverage. The activation energy barriers for the relevant set of reactions have also been calculated. Based on these results, the potential energy surfaces (PESs) for direct and H-assisted NO reduction reaction on all the metal surfaces have been constructed. These PESs are used for elucidating the trends for various elementary steps involved in the NO reduction reaction, across the periodic table. These PESs indicate that on (111) surfaces of Cu, Ag, Pd and Pt, H assisted NO reduction is quite favorable by either NOH or HNO reaction intermediate. For remaining three close-packed metal surfaces, i.e. Rh, Ir and Ru we do not find significant change due to presence of H. We also find that the presence of extra H may contribute in formation of HNOH or H$_{2}$NO on the surface that cause barrier to reduce even further. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H31.00009: First-principles study of surface stresses induced by target-receptor interactions on a cantilever sensor Varadharajan Srinivasan, Giancarlo Cicero, Jeffrey C. Grossman Nanoscale cantilevers have shown great promise as ultrasensitive, low-power chemical sensors based on the surface stresses induced by interactions between the target species and the receptor coating layer. However, the basic mechanism of these induced stresses is yet to be fully understood, and it is therefore of great fundamental and practical interest to elucidate their electronic and structural origins via the weak interactions that lead to cantilever deflection. An example of such a device is the Au-SiN$_{x}$ cantilever sensor using functionalized long-chain alkanethiols as a coating layer. Even though the target-receptor interactions are often weak, the induced stresses are quite sensitive to the chemistry of the interacting species. Taking water molecules as a model target and $\omega $-hydroxy and $\omega $-carboxy alkanethiols as the receptor layer on a Au(111) surface, we use first-principles surface stress calculations to provide a detailed atomistic-level understanding of the various contributions leading to the deflection of a cantilever. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H31.00010: Atomic motion and electronic structure of alkanethiol monolayer covered gold surfaces Sabri Alkis, Hai-Ping Cheng, Jeffrey Krause Self-assembled alkanethiol monolayers are subjects of great interest because of potential applications in future nano-electronics. In this talk, we report our recent studies of the motions of Au atoms on alkanethiol monolayers using molecular dynamics in conjunction with first-principles calculations. Guided by accurate quantum mechanical calculations, we have calibrated the interactions between Au atoms and monolayers for classical simulations. We then investigate the motions of Au atoms as a function of coverage and temperature. Simulations with improved potential parameters show a good agreement with experimental observations. In addition, we discuss the electronic structure and charge transfer at the interface between the molecular monolayer and gold (111) surfaces. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H31.00011: Comparison between GGA+U and LSDA-GGA on CuN/Cu(100) Pushpa Raghani, Chiung-Yuan Lin, Barbara Jones It has become possible to study atomic magnetism by the Scanning Tunneling Microscope. The spin of a magnetic atom is often screened when it is adsorbed on a metal surface. However, a CuN layer reduces this screening and the atomic spins become detectable by STM. We have applied DFT to calculate the charge transfer, binding energies of the CuN layer on a Cu(100) surface. Within DFT, we use two different techniques for exchange correlation interaction: GGA+U and LSDA-GGA. Then we will calculate the atomic spins for Mn on CuN/Cu(100) using these two techniques. We will compare and discuss the results. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H31.00012: Formation and reactions of hydronium species in silica I.G. Batyrev, L. Tsetseris, S.T. Pantelides Water-related impurities in silica-based glasses are known to affect the properties of the network and corresponding devices in many types of ways. Here, we use first-principles calculations to highlight the special role of one of these species, hydronium (H$_{3}$O). We elucidate the atomic-scale details of formation and reactions of H$_{3}$O molecules in amorphous SiO$_{2}$. We find that the attachment of a migrating H$^{+}$ on a water molecule is an exothermic reaction with an energy gain of 0.4 eV and activation energy of only 0.6 eV. We present results on pertinent features in the vibrational spectra of silica that support the presence of H$_{3}$O, and we describe the role of H$_{3}$O as a passivant of oxide defects, like oxygen vacancies and non-bridging oxygen atoms, and the atomic-scale details of H$_{3}$O-mediated diffusion of H species in SiO$_{2}$. The results bear on the formation and dynamics of defects in electronic devices and the physical properties of hygroscopic silica-based glasses. This work was supported in part by the AFOSR and the US Navy. [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H31.00013: Hydrogen Bonds and the Vibrational Modes of Water at Interfaces: ab-initio Molecular Dynamics meets Neutron Scattering Nitin Kumar, Sanghamitra Neogi, Paul Kent, Andrei Bandura, James Kubicki, David Wesolowski, Jorge Sofo We study the vibrational density of states (VDOS) of a thin water layer on the rutile (110) surface. The VDOS is obtained from the velocity-velocity autocorrelation function calculated from trajectories of large scale ab-initio molecular dynamics simulations. The rutile surface induces a shift to lower frequencies of the stretching modes with respect to pure water. The water vapor surface shows a peak at the vibrational frequency of free hydroxyls. Overall, the average stretching mode vibrational frequency increases with decreasing hydrogen bonding density. This density depends strongly on temperature. The water dissociation percentage at the surface can be correlated with the ratio between the weights of the stretching and the bending modes. Our results are in good agreement with inelastic neutron scattering measurements done on wet titania nanoparticles. [Preview Abstract] |
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