Symposium Organizers
Yue Qi General Motors R&D and Planning
H. Eliot Fang Sandia National Laboratories
Nick Reynolds Accelrys
Zi-Kui Liu The Pennsylvania State University
W1: New Approaches Toward Multiscale Matierals Design
Session Chairs
Monday PM, December 01, 2008
Constitution B (Sheraton)
9:30 AM - **W1.1
Multiscale Modeling in Multilevel Materials Design.
David McDowell 1
1 School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show Abstract10:00 AM - **W1.2
The Prediction of Crystal Structure with Knowledge Methods as a Crucial Ingredient for Computational Materials Design.
Gerbrand Ceder 1 , Geoffroy Hautier 1 , Chris Fischer 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractThe prediction of structure is a key problem in computational materials science that forms the platform on which rational materials design can be performed. Without detailed structure information the prediction of properties rapidly becomes irrelevant. We will present an ab initio approach that rapidly finds the stable crystal structure of materials with > 95% of success. The premise of the approach is that for many materials chemistries standard computational quantum mechanics is highly accurate in selecting the true ground state of a system from a small set of candidate structures, though notable exceptions exist. Finding ground states by traditional optimization methods on quantum mechanical energy models is difficult due to the complexity and high dimensionality of the coordinate space. An unusual, but efficient solution to this problem can be obtained by merging ideas from heuristic approaches and ab initio methods: In the same way that scientist build empirical rules by observation of experimental trends, we have developed machine learning approaches that extract knowledge from a large set of experimental information and a database of over 20,000 first principles computations, and used these to rapidly direct accurate quantum mechanical techniques to the lowest energy crystal structure of a material. Knowledge is captured in a Bayesian probability network that relates the probability to find particular crystal structure at a given composition to structure and energy information at other compositions. We show that this approach is highly efficient in finding the ground states of binary metallic alloys and can be easily generalized to more complex systems. We have already used this approach to identify several hundred new compounds
10:30 AM - W1.3
From Molecular Grand-canonical Density Functional Theory towards the Rational Multiscale Design of Chemical Compounds from First Principles.
Anatole Lilienfeld 1
1 Multiscale Dynamic Material Modeling Department, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThe fundamental challenge of rational compound design, i.e. the reverse engineering of chemical compounds with predefined specific properties, originates in the high-dimensional combinatorial nature of chemical space. Chemical space is the hyper-space of a given set of molecular observables that is spanned by the molecular grand-canonical variables (elementary particle densities of electrons and nuclei) which define chemical composition. A working definition of chemical space has been given within the notion of a molecular grand-canonical ensemble multi-component density functional theory framework [1]. I will discuss this approach, as well as numerical results for controlling molecular properties, described within various levels of theory on the multiscale hierarchy, through variation of chemical composition. Properties include electronic molecular eigenvalues, intermolecular energies, or drug-enzyme affinity [2-5]. Eventually, the effects of erroneous many-body interatomic potentials on cohesive energies, or of details in the pseudopotential approximation on band-gap estimates, shall exemplify the important issue of ensuring sufficient accuracy across the multiple scales in the context of multiscale materials design [6,7].[1] von Lilienfeld and Tuckerman, J Chem Phys, 125, 154104 (2006)[2] von Lilienfeld et al, Phys Rev Lett, 95, 153002 (2005)[3] von Lilienfeld and Tuckerman, J Chem Theory Comput, 3, 1083 (2007)[4] Marcon et al, J Chem Phys, 127, 064305 (2007)[5] von Lilienfeld et al, (in preparation)[6] von Lilienfeld and Schultz, Phys Rev B, 77, 115202 (2008)[7] von Lilienfeld and Tkatchenko, Phys Rev B, accepted (2008)Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
10:45 AM - W1.4
Green's Function Molecular Dynamics: Applications to Tribology and Contact Mechanics.
Ling Ti Kong 1 , Carlos Campana 1 2 , Colin Denniston 1 , Martin Muser 1
1 Applied Mathematics, University of Western Ontario, London, Ontario, Canada, 2 , CANMET-Materials Technology Laboratory, Natural Resources Canada, Ottawa, Ontario, Canada
Show Abstract11:00 AM - W1: Multi1
BREAK
11:30 AM - W1.5
Enhancing Molecular Dynamics to Capture Electronic Effects.
Normand Modine 1 , Reese Jones 1 , David Olmsted 1 , Jeremy Templeton 1 , Gregory Wagner 1 , Ryan Hatcher 2 , Matthew Beck 3
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Lockheed Martin Advanced Technology Laboratories, Cherry Hill, New Jersey, United States, 3 , Vanderbilt University, Nashville, Tennessee, United States
Show AbstractIn modeling non-equilibrium thermal transport in solids, classical molecular dynamics (MD) has the primary strength of explicitly representing phonon modes and the defects that scatter phonons. On the other hand, electrons and their role in energy transport are missing. These effects are vital in applications such as laser processing, thermoelectrics, and current induced thermal failure. In nanoscale and nanostructured systems, the behavior of the system is complicated further by phonon-confinement, ballistic transport, and discrete defect scattering effects. These effects are absent in phenomenological models of heat transport, but naturally captured by MD. Our goal is to couple a MD treatment of the ionic subsystem with a partial differential equation (PDE)-based model of the electronic subsystem in order to accurately capture the aggregate behavior of the coupled electron-ion system. Along these lines, we have enhanced the LAMMPS MD package by coupling the ionic motions to a finite element (FE) based representation of electronic charge and heat transport. The coupling between the subsystems occurs via a local version of the two-temperature model that allows the ionic and electronic subsystems to exchange energy and eventually come into equilibrium. The rate of equilibration between the ionic and electronic temperatures is calculated for a representative system from first principles using a Time Dependent Density Functional Theory (TDDFT) simulation. The ions are initially in thermal motion, while the electrons are initially in their ground state. During the TDDFT simulation, the electrons leave the Born-Oppenheimer surface and gain energy to equilibrate with the ions. Our approach is intrinsically multiscale and multiphysics. Furthermore, the tight coupling between the MD and FE paradigms utilizes the inherent strengths of each. Initial demonstrations of our approach and capabilities have focused on heat transport in representative carbon nanotubes, and these results will be discussed.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE- AC04-94AL85000.
11:45 AM - W1.6
Coupled Quantum Mechanics and Finite-Element Simulations of Mechanical Properties of Defects.
Noam Bernstein 1 , Viacheslav Sorkin 2 , Ellad Tadmor 2 , Gabor Csanyi 3
1 Center for Computational Materials Science, Naval Research Laboratory, Washington, District of Columbia, United States, 2 Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota, United States, 3 Engineering Laboratory, Cambridge University, Cambridge United Kingdom
Show Abstract12:15 PM - W1.8
Phase-field Modeling of Reactive Fluid Flow with Solute Precipitation and Dissolution.
Zhijie Xu 1 , Paul Meakin 1
1 , Idaho National Lab, Idaho Falls, Idaho, United States
Show AbstractA phase-field approach to the dynamics of liquid-solid interfaces that evolve due to solute precipitation and/or dissolution is presented. In contrast to solidification processes controlled by a temperature field that is continuous across the solid/liquid interface (with a discontinuous temperature gradient), precipitation/dissolution is controlled by a solute concentration field that is discontinuous at the solid/liquid interface. The Gibbs-Thomson effect on the interface dynamics was included in the phase-field model, and a sharp-interface asymptotic analysis of the phase-field equations was performed for precipitation/dissolution processes to demonstrate that the phase-field equations converge to the proper sharp-interface limit. The mathematical model was validated by numerically solving the coupled phase-field equations for a one- and two- dimensional precipitation/ dissolution problems and by comparison with the analytical solutions.
12:30 PM - W1.9
Recursive Coarse-Grained Particle Method for Inhomogeneous Materials.
Takahide Nakamura 1 2 , Ryo Kobayashi 1 2 , Shuji Ogata 1 2
1 Department of Scientific and Engineering Simulation, Nagoya Institute of Technology, Nagoya Japan, 2 CREST, Japan Science and Technology Agency, Saitama Japan
Show AbstractA coarse-graining method has been proposed [1] for a crystalline system of atoms to describe the propagation of relatively long-wavelength waves, in which the inter-particle interaction is obtained through coarse-graining of the partition function of the atomic Hamiltonian in the harmonic approximation.Though the method has attractive features such as its natural incorporation of atomistic phonons and its potential suitableness to connection to both atomistic and continuum simulation methods bridging the wide scale-gap, the original formulation limits its application to periodic systems without surfaces.In this paper, we advance the method to be applicable to realistic systems of meso-scales with various shapes under stressed conditions at relatively low computation costs. The points of advancement include: (i) recursive coarse-graining procedure [2] for both homogeneous and inhomogeneous systems, (ii) application to pseudo-2D systems such as the graphene sheet and the carbon nanotube. Accuracy of the present method is analyzed through the phonon spectra in homogenous systems, the propagation and scattering of elastic waves in both homogenous and inhomogenous systems, the elastic properties, and so on. Also we will demonstrate its concurrent hybridization with both the molecular dynamics for atoms and the lattice Boltzmann method for fluids, for multiscale simulation of interesting systems.[1] R.E. Rudd and J.Q. Broughton, Phys. Rev. B 58, R5893 (1998).[2] R. Kobayashi and S. Ogata, Mat. Trans. (2008), in press.
12:45 PM - W1.10
Computational Materials Design with the Configurational Forces Concept.
Otmar Kolednik 1 5 , Jozef Predan 2 , Narendra Simha 3 , Dieter Fischer 4 5
1 , Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria, 5 , Materials Center Leoben Forschung GmbH, Leoben Austria, 2 Faculty of Mechanical Engineering, University of Maribor, Maribor Slovenia, 3 Department of Orthopaedic Surgery, University of Minnesota, Minneapolis, Minnesota, United States, 4 Institute of Mechanics, Montanuniversität Leoben, Leoben Austria
Show AbstractThe concept of configurational forces is a powerful computational tool for the quantitative description of the behavior of defects in materials and structural components. It enables us to(1)evaluate the crack driving force in arbitrary micro- or macroscopically inhomogeneous materials and components, (2)take into account the influences of eigenstrains and residual stresses,(3)estimate the crack growth direction using the criterion of maximum dissipation,(4)assess the shielding and anti-shielding effects of near-tip and remote plasticity. In this presentation, first a short overview shall be given about theory and computational aspects. Then specific applications are shown, e.g. coated steels, ceramic multilayered materials, or biological materials. These examples demonstrate that the concept of configurational forces - applied in combination with sophisticated experimental methods to determine the spatial variations of local material properties and residual strains - opens new prospects for the design of future damage resistant materials and components.
W2: Materials in Energy Applications
Session Chairs
Monday PM, December 01, 2008
Constitution B (Sheraton)
2:30 PM - W2.1
Phase-Field Modeling of Intercalation Processes in Rechargeable Batteries.
Damian Burch 1 , Martin Bazant 2 1
1 Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, Stanford University, Stanford, California, United States
Show Abstract2:45 PM - W2.2
Simulations Lithium Intercalation into a LixFePO4 Battery Electrode During Recharging with Kinetic Monte Carlo Method.
Celine Hin 1 , W. Craig Carter 1 , Yet Chiang 1
1 Materials Science and Engineering , MIT, Cambridge, Massachusetts, United States
Show AbstractThe demand for reliable and high energy density batteries for use in hybrid and plug-in hybrid vehicles requires improvements of currently available lithium batteries systems. One strategy is to use fine-particle electrode materials that provide large surface area for lithium ion intercalation in electrochemical processes. Thus, the role of interfaces in the transport and reaction behaviour becomes more important for informed battery-microstructure design. These roles have been studied experimentally [1-5] and by modelling [6-8]. More particularly, we will show how battery modelling can account for complex and different reactions that takes place at electrode interfaces for different experimental conditions, such as charging rate.A kinetic Monte Carlo algorithm based on rigid lattice was used to study kinetic of lithium intercalation into a LixFePO4 electrode during a galvanostatic discharge. This algorithm takes into account: (i) the structure of both phases in presence (e.g. Li(alpha)FePO4 and Li(1-β)FePO4), (ii) the solubility limit of Li in FePO4, (iii) the role of the Volmer-Butler relation on lithium ion interface transport and (iiii) the configuration dependent activation barriers within the lattice. The results provide insight into the role atomistic behavior in microstructure development and phase transitions during recharging.References:[1] Chiang Y.M., J. Electroceram. , 1997, 1, 205.[2] Chung S.Y., Bloking J.T., Chiang Y.M., Nat. Mater., 2002, 1,123.[3] Chung S.Y., Bloking J.T., Chiang Y.M., Nat. Mater., 2003, 2,702. [4] Meethong N., Shadow Huang H.-Y., Speakman S.A., Carter W.C., Chiang Y.-M., Adv. Funct. Mater. 2007.[5] Meethong N., Shadow Huang H.-Y., Carter W.C., Chiang Y.-M., Electrochem. Solid-State Lett. 2007, 10(5), A134-A138.[6] Doyle M., Fuller T. F., Newmann J., Journal of the Electrochemical Society 1993, 140 (6), 1526.[7] Garcia R.E., Chiang Y.-M., Carter W.C., Limthongkul P., Bishop C. M., Journal of the Electrochemical Society, 2005, 152 (1), A255-263.[8] Garcia R.E., Chiang Y.-M., Journal of the Electrochemical Society 2007, 154 (9), A856-A864.
3:00 PM - **W2.3
Thermoelectric Materials by Design.
Jihui Yang 1
1 Materials and Processes Lab, GM R&D Center, Warren, Michigan, United States
Show Abstract Increasing awareness and concern for energy resources and the environment has prompted the recent worldwide effort on thermoelectric (TE) materials research and technology development. Advances in TE technology can have a significant impact on the automotive industry in terms of fuel economy improvements by generating electricity from waste heat and augmenting air conditioning efficiency. A critical component of developing automotive TE technologies is the discovery and development of highly efficient TE materials. From the fundamental materials research perspective, this is motivated by the more than half-century-long challenge of simultaneously enhancing the electrical conductivity and thermopower, and lowering the thermal conductivity of materials; and of understanding the underlying mechanisms. In this talk I will highlight some of our recent work on void filling limits, effects of misch-metal filling, and dual-frequency resonant phonon scattering in filled skutterudites; and show how we are able to improve the thermoelectric performance of these materials and predicting new materials through experimental effort and first principle-based computational modeling.
3:30 PM - W2.4
A General Strategy For Screening Hydrogen Purification Membranes by Modeling Amorphous Metals.
Shiqiang Hao 1 , David Sholl 2
1 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 School of Chemical &
Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractThe increasing demand for clean and efficient energy has resulted in an increased global willingness to embrace the proposed ``hydrogen economy''. The use of amorphous metal films as membranes to purify hydrogen has potential to overcome at least some of the disadvantages of existing crystalline metal membranes. We introduce a general strategy combining density functional theory and statistical mechanics to quantitatively predict solubility, diffusivity and permeability of interstitial H in amorphous metals. These calculations make it possible for the first time to quantitatively evaluate the performance of amorphous metal films as hydrogen purification membranes. These methods are introduced by examining amorphous Fe3B and a crystalline analogue with the same composition. A membrane made from the amorphous material is predicted to have a hydrogen permeability 1.5-2 orders of magnitude higher than a crystalline membrane. The methods we introduce here will be useful in accelerating the development of amorphous membranes for practical applications.
3:45 PM - W2.5
Metal Decorated Periodic Mesoporous Organosilicas (PMOs) for Hydrogen Storage.
Liping Huang 1 2 3 , Erik Santiso 1 4 , Keith Gubbins 1 , Marco Buongiorno Nardelli 2 5
1 Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Physics, North Carolina State University, Raleigh, North Carolina, United States, 3 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 5 CSMD, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPeriodic mesoporous organosilicas (PMOs) have hexagonal pore arrangement in the mesoscale and crystalline pore wall with alternating hydrophilic silicate layers and hydrophobic organic layers. The silicate layers provide the rigidity to the materials and the organic layers can be functionalized for potential applications including hydrogen separation and storage. By using first-principles density functional theory calculations, we demonstrated that the interaction between H2 and PMOs can be modified by metal atoms. Metal decorated PMOs have been designed for efficient hydrogen storage. Our studies show that the bonding between metal and hydrogen is of a combination of chemical and physical adsorption, which is essential for reversible hydrogen uptake/release. Car-Parrinello molecular dynamics simulations demonstrate that these systems are stable and exhibit associative desorption of H2 upon heating without breaking the bond between PMOs and metal. This fulfills another requirement for reversible hydrogen storage.
4:00 PM - W2: Engy1
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4:30 PM - **W2.6
Multi-scale Simulation Approach for Designing New Polymeric Materials in Fuel Cell Technology.
Seung Soon Jang 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractIn order to develop new polymeric materials for fuel cell applications, we applied multi-scale simulation approach. First, we will summarize our previous studies on Nafion, sulfonated PEEK and Dendrion in which we investigated the effect of various molecular variables such as monomeric sequence and polymer chain architectures on the nanophase-segregation and proton transport. In these studies, we used full atomistic simulation techniques to predict the most probable structures in the presence of water. To evaluate the characteristics of the structures, we analyzed structure factor profiles, solvation of water and sulfonate group, and intermolecular structure in water phase. From these studies, we found that the nanophase-segregation can be controlled as a function of the monomeric sequence and chain architecture determine and that the well-developed nanophase-segregation facilitates the proton transport. And then, we will present 1) the parameterization of the properties of our interest for meso-scale simulations and 2) the results of the meso-scale simulations for the systems above-mentioned. From the results obtained these studies, we believe that the multi-scale modeling protocol is generic for any polymeric materials for fuel cell membrane application.
5:00 PM - W2.7
Energetics and Electronic Properties of TiO2 Nanotubes by DFT Calculations.
Jian-jie Liang 1 , Alexander Goldberg 1 , Mathew Halls 1 , Paul Kung 1
1 , Accelrys Inc, San Diego, California, United States
Show AbstractOrdered heterojunctions templated by semiconducting TiO2 nanostructures had been identified as a key to the future success of highly-efficient polymer-based solar cells. To decipher the structure-energy-property relationships of the TiO2 nanostructures, single-walled TiO2 nanotubes of ca. 3 nm were constructed based on 6-, 5-, and 4-coordinations of the Ti atoms, respectively. Calculations based on Density Functional Theory (DFT) were used to obtain optimized structures, calculate binding energies, and derive electronic band structures of the nanotubes. The binding energies of the nanotubes of all configurations studied were found to be substantially smaller than those of the bulk phases (rutile, anatase, and brookite), and became incrementally smaller as the diameter of the nanotubes grew. The bandgaps of the nanotubes increased by ca. 50% compared to the bulk. The 6-coordinated nanotubes were determined to have direct bandgaps, whereas the 5- and 4-coordinated nanotubes have indirect bandgaps.
5:15 PM - W2.8
Toward the Nanoscale Design of Catalysts for Fuel Cells: A Computational Approach.
Guofeng Wang 1
1 Department of Mechanical Engineering, Indiana University Purdue University Indianapolis, Indianapolis, Indiana, United States
Show AbstractMonday 12/1New Presentation Time/Paper NumberW2.9 @ 4:30 PM to W2.8 @ 4:15Toward the Nanoscale Design of Catalysts for Fuel Cells: A Computational Approach. Guofeng Wang
Symposium Organizers
Yue Qi General Motors R&D and Planning
H. Eliot Fang Sandia National Laboratories
Nick Reynolds Accelrys
Zi-Kui Liu The Pennsylvania State University
W3: Nano Technology & Devices
Session Chairs
Nick Reynolds
Guofeng Wang
Tuesday AM, December 02, 2008
Constitution B (Sheraton)
9:30 AM - **W3.1
First Principles-based Atomistic Modeling for the Synthesis and Structure of Silicon Nanocrystals Embedded in Silica.
Gyeong Hwang 1
1 Chemical Engineering, The University of Texas at Austin, Austin, Texas, United States
Show Abstract10:00 AM - W3.2
Point Defects and Interfacial Phases in Si:HfO2 Heterojunctions: A First principles Study.
Chunguang Tang 1 , Rampi Ramprasad 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractDriven by a need for device miniaturization in the microelectronic industry, Hf-based high-permittivity materials, such as HfO2, have gained interests for their potential application as gate dielectrics. However, between HfO2 and Si substrate undesirable interfacial phases such as Hf silicides, silicates and silica are known to form in nanometer sizes and degrade the performance of devices. It has been postulated that these interfacial phases are related to the segregation of point defects (such as O vacancy or interstitial) to the interface. In this work we have examined the effects of point defects on the evolution of interfacial phases by performing first principles computations on the point defects. Firstly the relationships between the stoichiometry of HfO2 and the processing conditions (oxygen pressure and temperature) have been studied by calculating the formation energies of point defects with various concentrations. Corresponding to different processing conditions, extensive calculations of the formation and migration energies have been performed for O vacancies, O interstitials, and Hf vacancies at various distances from Si:HfO2 interface. The results indicate that there exist strong thermodynamic and kinetic driving forces for these defects to segregate from bulk HfO2 part to the interface, which can result in the formation of interfacial Hf silicides, silica, and Hf silicates, respectively. Then the growth of the interfacial phases has been modeled by an accumulation process of the point defects at the interface. It is found that the thermodynamic driving force for the segregation of defects is well maintained as defect density at the interface increases, which indicates that the growth of interfacial phases is energetically favorable. Finally, by combining thermodynamics with our defect accumulation data, we have predicted interface morphologies as a function of temperatures and oxygen chemical potentials. The results show that as temperature increases, the clean Si:HfO2 interface, which is stable only for a small range of oxygen chemical potentials, tends to decompose into either O rich interface, such as SiOx (upon high chemical potentials), or O deficient interface, such as Hf silicides (upon low chemical potentials). The conclusions agree well with experiments. Our work deepens the atomic-scale understanding of the formation of interfacial phases in Si:HfO2 heterojunctions.
10:30 AM - W3.4
Modeling Facet Nucleation and Growth of Hut Clusters on Ge/Si(001).
John Venables 1 2 , Michael McKay 3 , Jeff Drucker 1
1 Physics, Arizona State University, Tempe, Arizona, United States, 2 LCN, University College, London United Kingdom, 3 , Lawrence Semiconductor Research Laboratory, Inc., Tempe, Arizona, United States
Show AbstractRecent STM observations of homogenous distributions of pyramid and hut clusters on Ge/Si(001) have shown that these clusters grow extremely slowly during annealing at intermediate temperatures, T ~ 450 oC, when there is a super-saturation of mobile ad-particles above and within the wetting layer. Data has been obtained on the absolute length of the clusters as a function of time L(t), and thereby the evolution of the growth rate, over periods of order 100 hours [1]. We model this slow growth as a layer by layer (2D) facet nucleation and growth problem, in the presence of strain-induced energies both on and around the facets. All of these energies can markedly influence the nucleation rate of new facets. First, they justify the observation that facet nucleation occurs from the apex of the hut, as has been observed in several other studies [2, 3]. Second, they indicate a substantial slowing down of the nucleation rate, by many orders of magnitude, relative to the case when such energies are not present. Finally the need to undo the stable reconstruction on the {105} facets as each new layer is formed contributes an extra energy of order 0.5 eV [4] to the energy of the critical nucleus. Inclusion of these effects, with experimental values of the Ge diffusion coefficient, provides a quantitative fit to the L(t) data, and sets bounds on step and facet energies appropriate to hut clusters. 1. M.R. McKay, J.A. Venables and J. Drucker, submitted to Phys. Rev. (2008);2. F. Montalenti et al., Phys. Rev. Lett., 93, 216102 (2004);3. S. Cereda, F. Montalenti and L. Miglio, Surface Sci. 591, 23-31 (2005);4. S. Cereda and F. Montalenti, Phys. Rev B 75, 195321 (2007).
10:45 AM - W3.5
Modeling of Self-organized Pattern Formation on Semiconductor Surfaces under Ion Sputtering.
Kun-Dar Li 1 2 , Qiangmin Wei 1 , Lumin Wang 1 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Nuclear Engineering and Radiological Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractLow-energy ion sputtering on surfaces of various semiconductors to achieve self-organized nanostructures has attracted attention due to their potential applications in optical and electronic devices. Depending on the material properties, temperature, ion energy, angle of incidence, and other experimental parameters, one may observe the development of rough or cellular surfaces, or, most importantly, the more or less regular ripple or nanodot patterns under oblique ion incidence. For example, well-ordered patterns of nanodots formed on Si, GaSb, InP, and InAs surfaces by low-energy ion-beam sputtering at normal incidence. This pattern formation is related to the instability between sputtering that roughens the surface and smoothing by different surface relaxation mechanisms. In contrast, at an off-normal incidence, periodic pattern of highly ordered hexagonal nanodots of Ga were produced on irradiated GaAs surface. A mass of experimental studies on ion sputtered surfaces have motivated extensive theoretical work aiming to reveal the mechanism responsible for such nanoscale pattern formation. In this study, a phase field approach was adopted to model self-organized pattern formation on sputtered surfaces. The microstructure of the system was described by a composition field, and pores were treated as high vacancy concentration regions which represent the pseudo-phase of voids. The effect of tilted ion incidence is included in the phase field model which integrated the production and elimination rates of vacancies with the free energy of mixing and interfacial energy as the driving force for vacancy diffusion. The theoretical model successfully explains the dynamic processes of formation and evolution of partially and highly ordered patterns of nanodots. Our theoretical simulation, supported by the experiments, suggests that one of the main dominating factors for the formation of the hexagonally ordered pattern of nanodots is the preferential migration direction under tilted ion beam. The influence of temperature, material property, vacancy production and recombination rate are all taken into consideration in the model for the formation of surface morphology. The morphology and length scale of our calculation results are consistent well with many experimental observations.
11:00 AM - W3: device
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11:30 AM - **W3.6
Working in Relativity: Material Application Development Through the Use of Molecular Modeling.
Nancy Iwamoto 1
1 , Honeywell Specialty Materials, Ramona, California, United States
Show Abstract The role of molecular modeling often takes on an investigative rather than an exploratory quality regardless of whether it is being used to develop a new material, predict how a material might fail in application, or diagnose why a material has failed at a customer site. This investigative quality is due in large part to the problem of multiple performance requirements that must be simultaneously satisfied when developing a material for a specific application. From this aspect, molecular modeling can be used to determine whether the molecular structure has the inherent characteristics to suspect success in an application, or to suspect that it could be involved during failure. When dealing with profiling inherent character, a relative predictive metric often becomes more useful than an absolute property prediction because of the complications of scale and the fact that a material in use at the customer is usually used in conjunction with other materials that are not necessarily divulged to the material developer. That is, because the molecular model can only sample a small atomistic area and because most performance issues are due to interactions with other materials, the molecular model can be most efficient when predictions are made in a relative manner by examining key interactions while surveying the different possible chemistries in-play. In doing so, it has been found that the use of molecular modeling provides important bridges in understanding between measurement and performance. This is especially true for areas such as electronic materials development where multiple interfaces must be considered in multi-layer constructions or composites and where the interfaces may not be chemically well-defined. In this manner the performance may be traced to issues surrounding the chemical structure and provide possible new chemistries, structural ranges or directions that may be used in the experimental work. This paper will relay different practical examples of the use of relative molecular modeling for material development in various applications such as stationary phases used in micro (MEMS) GCs, thin films used in integrated circuits (IC), cleaners used in lithography, and adhesives used in electronic packaging. For instance, for the micro-GC application relative molecular modeling was used to help select stationary phases by simulating relative separation performance and adsorption leading to a better general understanding of materials needed for separation versus concentration. In an example of the use of molecular modeling for thin films, relative modulus and adhesion trends were developed for interlayer dielectrics in order to understand fundamental performance limitations of the structures under development. And for the electronic packaging adhesives, relative surface energies, adhesion, and stress cycling trends were used to determine chemistries used for new materials designed for increased robustness to moisture.
12:00 PM - W3.7
Multi-scale Simulation of Metal/metal Interfacial Degradation under High Electromagnetic Stress.
Douglas Irving 1 , Clifford Padgett 2 , Donald Brenner 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Chemistry and Physics, Armstrong-Atlantic State University, Savannah, Georgia, United States
Show AbstractNumerous macroscopic-level models of electro-magnetic launchers (EMLs) have been carried out that include coupled electro-magnetics and mechanics of the rail and armature. Although these studies provide a clear picture of engineering principles for EMLs, the problem of rail degradation remains unsolved. Extensive experimental studies continue to search for materials that dramatically extend rail lifetimes. In contrast to macroscopic modeling, there have been no published atomic simulations of rail-armature contact that include the influence of current flow. Given that rail degradation remains a major hurdle to multiple shot EMLs, the insights from this type of simulation can have a major impact on EML technology. In this talk we present results from a novel multi-scale modeling approach we have developed in which a molecular dynamics simulation of metal-metal contact is coupled to a finite difference solution of heat and current flow equations. We will describe the results of a series of simulations that use this methodology to explore the effect of applied voltage, applied load, sliding velocity, and the formation of molten interface layers on the atomic level dynamics of bare metal single asperity Al-Cu contact. Under an applied normal load localized melting of the aluminum is seen. If the voltage is held constant, Al continues to melt while the Cu asperity undergoes a surface disordering and intermixing with the localized molten Al. If the applied voltage changes to maintain a constant current, the initial melt recrystallizes and yields plastically under the applied normal load. In the sliding simulations heat is generated not only from the electric current but also from frictional heat, which results in extensive damage and material transfer. The authors acknowledge support from the U.S. Office of Naval Research through a Multi-University Research Initiative.
12:15 PM - W3.8
Terthiophene on ZnO Surfaces: First-principles Calculations and Experiments.
Guowen Peng 1 , Peerasak Paoprasert 2 , John Uhlrich 1 , Soonjoo Seo 2 , Padma Gopala 2 , Paul Evans 2 , Thomas Kuech 1 , Manos Mavrikakis 1
1 Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractFunctionalizing ZnO with small organic molecules such as terthiophene is of great interest in hybrid organic/inorganic electronic devices. For that purpose, a clear microscopic understanding of the terthiophene/ZnO interfaces is highly desirable. Terthiophene is a small molecule similar to thiophene polymers that can be linked to the surface using siloxane chemistry. Here, we investigate the structural and electronic properties of sub-monolayer coverages of terthiophene on both polar ZnO(000\bar{1}) and non-polar ZnO(10\bar{1}0) surfaces using first-principles methods. We show that an electric dipole is developed at the terthiophene/ZnO heterojunction. The surface dipole is sensitive to the molecular coverage, the molecular orientation relative to the surface, and the surface facet. The dependence of the electronic properties of the terthiophene/ZnO interfaces on the induced surface dipole is discussed and compared with experimental data obtained by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Fourier transform infrared absorption spectroscopy (FTIR).
12:30 PM - W3.9
Effective Work Functions and Phase Equilibria of TaCxN1-x and Pt in Contact with HfO2 Through ab Initio Thermodynamics.
Hong Zhu 1 , Rampi Ramprasad 1
1 Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States
Show AbstractNext generation electronic devices will use a HfO2 based dielectric layer (instead of the currently used silica layer), and a metal gate on top of HfO2 (instead of polycrystalline Si). The choice of the metal on top of HfO2 is determined by the metal work function (i.e., the alignment of the metal Fermi level with the band edges of the Si substrate beneath hafnia). TaCxN1-x is a promising gate metal due to its resistance to O diffusion and due to the variation in the work function that can be accomplished by composition changes, while Pt is a stable, well-studied, model metal gate. There is very strong evidence that shifts in the metal work function occurs depending on the nature of the interface between HfO2 and the metal. Thus, controlling the composition of the gate metal and the metal:HfO2 interface structure are seen as approaches to controlling the “effective” work function of the metal gate. In this work, we will first present the work function of several surfaces of pure Pt and TaCxN1-x (with x spanning from 0 to 1) through first principles calculations. Our computed results for these pure systems are in good agreement with available experimental data. We will then present our results concerning metal work function shifts (resulting in an “effective” work function) when an interface between the metal and HfO2 is created. We find that the effective work function is a strong function of interfacial O coverage. In order to determine the most probable interfacial O coverage for a given temperature and O pressure during deposition, we use density functional theory total energy results within a statistical thermodynamics model. We are thus able to relate experimental processing conditions to interface morphology (for a given composition), and consequently to the effective work function. This approach thus offers a method for guiding the right choice of the metal electrode composition and metal:HfO2 interface morphologies so as to result in a metal gate with the desired effective work function.
12:45 PM - W3.10
Multi-Scale Transport Calculations for Molecular Wires and Carbon Nanotubes.
Kenji Hirose 1 , Hiroyuki Ishii 2 , Nobuhiko Kobayashi 3
1 , NEC, Tsukuba Japan, 2 , AIST, Tsukuba Japan, 3 , Univ. of Tsukuba, Tsukuba Japan
Show AbstractWe present multi-scale calculation methods for the electric transport of molecular wires and carbon nanotubes (CNT) bridged between electrodes. Here we present three kinds of calculation methods. The first is the recursion-transfer-matrix (RTM) method, which is a reliable tool to calculate scattering waves in plane-wave expansions. Combined with the non-equilibrium Green’s function (NEGF) method and density-functional formalism, we perform calculations of transport properties of single molecules and molecular wires attached to metallic electrodes. We show the exponential behaviour of conductance as a function of molecular wires due to tunnelling process determined by the HOMO-LUMO energy gap and detailed data of electronic states of molecular wires at contacts under strong electric fields. We will also discuss the inelastic electron-phonon coupling effect to transport for the local heating problems.The second method is the time-dependent wave-packet approach. Based on the linear-response Kubo formula with tight-binding approximation, we perform O(N) calculations for the transport of huge system up to several μm length. We apply the method for the CNT-FET device and will show the mobility of CNT-FET under electron-phonon scatterings. Also we will present the transitions of transport behaviors of CNT device from ballistic to diffusive regimes.Finally, we present the third method using NEGF with atomically localized basis sets within density-functional formalism. This method enables us to treat fairly large system (up to several 10 nm) without any experimentally determined parameters. We apply this method to the contact problems of CNT to metallic electrodes. We show the effects of electrodes on conductance of metallic and semi-conducting CNT, especially taking various kinds of electrode materials into account.
W4: Hetrogenous Materials
Session Chairs
Seung Soon Jang
Nick Reynolds
Tuesday PM, December 02, 2008
Constitution B (Sheraton)
2:30 PM - **W4.1
Computational Thermodynamics Based Design of Cermets for Long Term Microstructural Stability.
Shiun Ling 1 , Thirumalai Neeraj 1 , ChangMin Chun 1 , Rao Bangaru 1 , Zi-Kui Liu 2
1 Corporate Strategic Research, ExxonMobil Research & Engineering Co., Annandale, New Jersey, United States, 2 Department of Materials Science and Engineering, Pennsylvania State University, State College, Pennsylvania, United States
Show AbstractCeramic-metal composites (Cermets) are good candidate materials for wear and erosion applications that require a combination of high hardness and toughness. Existing commercial cermets, for example, WC-Co are mostly used in ambient temperature applications such as drill bits, cutting tools etc. Applications in the petroleum and chemical industries, however, expose the cermets to high temperature (>400C) for prolonged periods of time. Under these conditions the cermet microstructure can change and degrade over time affecting its performance.In this investigation, we have employed the CALPHAD based computational thermodynamics methodology to understand the long term microstructural stability of two multicomponent cermet systems: chromium carbide in Fe-Ni-Cr alloy binder matrix, and titanium diboride in Fe-Ni-Cr alloy binder matrix. The phase stability calculations performed were validated with microstructural observations of specimens subjected to long-term elevated temperature aging up to 1000 hours in lab experiments. The science insights generated in this study were then used to guide the design of cermet systems with stable microstructures for long term service at high temperature.
3:00 PM - W4.2
Auxetic to Non-auxetic Transition in Porous Carbon Networks.
Vitor Coluci 1 , Pedro Autreto 2 , Douglas Galvao 2 , Ray Baughman 3
1 , Center for Higher Education on Technology, University of Campinas, Limeira, Sao Paulo, Brazil, 2 Applied Physics, State University of Campinas, Campinas, Sao Paulo, Brazil, 3 , Alan MacDiarmid Nanotech Institute, Richardson, Texas, United States
Show AbstractMost materials shrink laterally and become less dense when stretched. Materials that laterally expand (contract) when stretched (compressed) are called auxetic. This unusual property can be quantified by the Poisson’s ratio (PR) values. PR is defined as the ratio of percent lateral contraction to percent applied tensile elongation. While conventional materials present positive PR values, auxetics present negative ones. An important characteristic of many materials exhibiting negative PR is the presence of pores (void spaces) in their structure. The pores allow structural changes that can be responsible for the appearance of the auxetic property. Thus, it would be important to investigate how the filling of these pores would affect the material mechanical properties, in particular under which conditions the auxetic behavior can be suppressed. A few auxetic crystalline phases are known. In 1993 Baughman and Galvao [1] proposed hypothetical auxetic carbon-based crystalline networks with polydiacetylene motifs. In order to investigate the pore filling effect in auxetic materials we selected one of these hypothetical allotropic carbon phases – the hinged polydiacetyilene. This is a very convenient structure to our study since it presents the two characteristics we are interested here, negative PR and pores. We used He atoms to fill the pores in the hinged structure. We then calculated the PR values as a function of the number of He atoms randomly inserted in a supercell (3x3x3 unit cells). For each He concentration the supercell is reoptimized (cell parameters and atomic positions) and the elastic constants calculated using the second derivative procedure. The interactions among He and C atoms and C-C atoms were described using the well-known universal molecular force. Our results show that after a critical limit of number of He atoms (~2% mass fraction) a transition from negative to positive PR was observed, that is, the auxetic behavior is eliminated. These results confirm that the porosity plays a fundamental role defining the auxetic behavior. Although the hinged polydiacetylene is a hypothetical structure we believed that the general trends obtained here should be valid for other porous auxetic structures. In fact a transition of this kind (from negative to positive PR) was recently reported [2] for a porous material composed of mixed single and multiwalled carbon nanotubes (CNTs). [1] R. H. Baughman and D. S. Galvao, Nature 365, 735 (1993).[2] L. H. Hall, V. R. Coluci, D. S. Galvao, M. E. Kozlov, M. Zheng, S. O. Dantas, and R. H. Baughman, Science 320, 504 (2008).
3:15 PM - W4.3
Multiscale Modeling in Polymer Clay Nanocomposites for Simulations Based Design.
Dinesh Katti 1 , Kalpana Katti 1
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
Show Abstract3:30 PM - W4.4
Interfacial Friction of Carbon Nanotubes in Diamond Matrix Nanocomposites.
Lili Li 1 , Zhenhai Xia 1 , William Curtin 2
1 Department of Mechanical Engineering, University of Akron, Akron, Ohio, United States, 2 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractInterfaces play an essential role in toughening in ceramic composite systems. In the failure process of the composites, fiber pullout from the matrix is one of the major sources of energy dissipation. In ceramic nanocomposites the interface roughness approaches atomic length scale due to the extremely small size of the reinforcement and larger interfacial area is available compared with microscale composites. A fundamental understanding of the origins of friction and its role in determining the toughening would be a key to design a new class of tough nanocomposites. In this work, the interfacial friction in carbon nanotube (CNT) reinforced diamond composites were analyzed by using molecular dynamics. The preliminary results demonstrate that multiwall CNTs with interwall sp3 bonding show significant enhancement in interfacial load transfer by more than 2 times over single-wall CNTs. The interfacial frictional stress increases with an increase of normal stress on the CNTs, and shows a relatively high value even when the normal stress is in tension due to van der Waals interaction between nanotube and matrix.
3:45 PM - W4.5
To What Extent Can Current Interatomic Potentials Accurately Describe Grain Boundaries in Strontium Titanate?
Nicole Benedek 1 2 , Alvin Chua 2 , Christian Elsasser 3 , Adrian Sutton 2 , Mike Finnis 1 2
1 Materials, Imperial College London, London United Kingdom, 2 Physics, Imperial College London, London United Kingdom, 3 , Fraunhofer Institute for Mechanics of Materials, Freiburg Germany
Show AbstractMultiscale simulation methods are currently at the forefront of materials modelling research. Interatomic potentials are often at the heart of such approaches, because they provide the bridge between the very detailed electronic level and the coarser-grained mesoscopic level. For example, an interatomic potential model fitted to first-principles data was recently used in large-scale molecular dynamics (MD) simulations of domain wall motion in ferroelectric lead titanate. However, systematic studies on the reliability of these potentials, particularly for ionic oxides, are sorely lacking. Strontium titanate (STO) is a perovskite-structured oxide commonly used to make barrier layer capacitors. The technological importance of STO is directly linked to its interfacial and grain boundary properties, which are at present poorly understood. A complete understanding (including links with experiment) requires information from many length scales, including electronic and atomistic. We have tested the ability of a number of interatomic potentials from the literature to accurately describe the structures and energetics of some simple grain boundaries in STO. Our aim was to identify a reliable potential for use in molecular dynamics simulations of complex, nonstoichiometric STO grain boundaries. The potentials we have tested are of three types: Born model, shell model and partial charge model. We have also performed a detailed first-principles Density Functional Theory (DFT) study of these boundaries and used this data to validate the interatomic potentials. None of our chosen potentials can reproduce the energy ordering of the boundaries predicted by the DFT calculations. Some of the boundary structures produced by the potentials agree reasonably well with the DFT structures. We discuss the implications of our findings for STO grain boundary research and their relevance to other titanate perovskite systems.
W5: Polymer & Biomaterials
Session Chairs
Seung Soon Jang
Nick Reynolds
Tuesday PM, December 02, 2008
Constitution B (Sheraton)
4:30 PM - **W5.1
Investigation of Epoxy Molecular Structural Features for Increased Polymer Deviatoric Deformation.
Stephen Christensen 1
1 Phantom Works, Boeing, Seattle, Washington, United States
Show AbstractRecently developed continuum level structural analysis indicates that the composite polymeric matrix deviatoric deformation is key to maximizing composite ultimate properties. Distortion is a dissipative process controlled chiefly by torsional rearrangements of the polymer structure. Using computational methods coupled with experiment we have established a methodology for new composite matrix formulation that takes advantage of relations we have established between desired bulk properties and the molecular structure. Our hierarchical multi-scale approach required the development of several new techniques. A method to simulate fully dense and equilibrated high glass transition temperature and high cross-link density epoxy resins while maintaining proper stoichiometry and a means for extracting a value for the critical dilatation and distortion that relates to our bulk property measurement. Results of our simulations and experiments will show that improved composite performance is achievable through proper selection of the organic moiety of the amine and epoxy components of the resin system.
5:00 PM - W5.2
Morphologies of Perfloroalkylethylacrylated Co-Oligomers and Water Dynamics at Their Interface.
Gokhan Kacar 1 , Alimet Sema Ozen 2 , Canan Atilgan 3
1 Materials Science and Engineering, Sabanci University, Istanbul Turkey, 2 Materials Science and Engineering, Sabanci University, Istanbul Turkey, 3 Materials Science and Engineering, Sabanci University, Istanbul Turkey
Show AbstractBlock co-oligomers are known as self-organizing synthetic materials. We investigate the equilibrium and dynamical properties of two such systems which are known to form superhydrophobic surfaces, [1,2] (i) styrene-co-perfloroalkylethylacrylate (styrene-co-PFA) and (ii) methylmethacrylate-co-perfloroalkylethylacrylate (MMA-co-PFA). We first study the morphologies of each system in the solvent tetrahydrofuran (THF) using Dissipative Particle Dynamics method.[3] These fluorocarbon oligomer systems form different micellar or lamellar morphologies at various concentrations, which are studied in the range of 5 – 100 mol %. The model systems consist of A10BD7 oligomers, where the coarse grained beads carry the characteristics of the atomic details only through the interaction parameter: A is the styrene or the MMA monomer, B is the linker H2C=(CH)–(C=O)–O–(CH2)2 and D is a CF2 (or CF3) unit. Spherical micelles form at 10 % styrene-co-PFA, and a phase transition to the cylindrical micellar structure is obtained at 80 % concentration. On the other hand, 10 % and 20 % MMA-co-PFA form cylindrical micellar and lamellar structures, respectively. To understand the origins of the interactions between the beads, the global – local hardness and polarizability of the beads are calculated using quantum mechanical methods. Finally, to study the dynamics of water on these surfaces, atomic details of the oligomers are reverse mapped on selected morphologies and detailed molecular dynamics simulations are performed. The relaxation times and ordering of water molecules at the interface of the self-organized units are deduced, and the differences from the bulk water properties are related to the superhydrophobic nature of these materials.References:1. Simsek, E. Wettability of smooth and rough surfaces of perfluoroacrylate copolymers. M.Sc. Thesis, Sabanci University, Tuzla, Istanbul, 2006. 2. Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Angew. Chem. Int. Ed. 2004, 43, 5210 – 5213.3. Ozen, A.; Sen, U.; Atilgan, C. J. Chem. Phys. 2006, 124, 64905 – 64913.
5:15 PM - W5.3
Phase Transition and Morphology of Polydispersed ABA’ triblock copolymers - Determined by Continuous and Discrete Simulations.
Armand Soldera 1 , Yue Qi 2 , Weston Capehart 2
1 Chemistry, Universite de Sherbrooke, Sherbrooke, Quebec, Canada, 2 R&D, General Motors, Warren, Michigan, United States
Show AbstractDifferent paths exist to improve performance of hydrocarbon-based block copolymer proton exchange membrane. A triblock copolymer ABA’ architecture, constituted of hydrophilic (A and A’) and hydrophobic (B) strands, offers special advantages regarding to random copolymers: improved connectivity, preserving mechanical properties when swollen by water. A certain order of the two phases has to be maintained inside the system for high proton conductivity and mechanical stability. However, the polycondensation of the different blocks results in a polydispersity in the weight of the different strands, which greatly affects the order to disorder transition (ODT) of block copolymers. The purpose of the presentation is then to consider polydispersity in triblock copolymer by supplementing calculations using the random phase approximation (RPA) by a discrete approach performed at the mesoscale level to further capture morphology details. In the latter calculations, the Edwards mean field equation is solved in a self-consistent way using the SUSHI code. To carry out this compatible and complementary procedure, a discretization process of polymer length distribution is exposed. Both methods can predict ODT accurately. Effects of individual chain lengths on morphology, phase segregation and mechanical properties of PEM are specifically regarded using the mesoscale modeling.
5:30 PM - W5.4
Forces Between Functionalized Silica Nanoparticles.
Ahmed Ismail 1 , Matthew Lane 2 , Michael Chandross 3 , Kelly Anderson 4 , Chris Lorenz 5 , Gary Grest 2
1 Performance Assessment and Decisions Analysis, Carlsbad Programs Group, Sandia National Laboratories, Carlsbad, New Mexico, United States, 2 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 Computational Materials Science and Engineering, Sandia National Laboratories, Albuquerque, New Mexico, United States, 4 Modeling and Simulation, Proctor and Gamble, Cincinnati, Ohio, United States, 5 Division of Engineering, King's College, London, London United Kingdom
Show AbstractPolymer-coated nanoparticles have a wide variety of applications including drug delivery, adhesives, coatings, and magnetics. Although their complexity precludes atomistic simulations of large numbers of nanoparticles in solution, using large-scale molecular dynamics simulations, it is possible to study the interaction between pairs of nanoparticles in an explicit solvent and express these interactions in terms of empirical forces such as solvation, depletion, and lubrication forces. From these simulations, we can compute the forces exerted on nanoparticles treated with explicit-atom models, which can be used in coarse-grained simulations at larger length and time scales. In particular, we present results between two bare or polymer-grafted silica nanoparticles as a function of chain length, core size, and approach velocity. We show the work required to bring together two bare silica nanoparticles in water, alkylsilane-coated nanoparticles in decane, or poly(ethylene oxide)-coated nanoparticles in water, and compare them to the expected behaviors of nanoparticles in solution. We also examine the “equilibrium” forces on a pair of stationary nanoparticles, as well as the work required to retract the particles following an approach.
5:45 PM - W5.5
Computational Investigations of Assembly and Stabilization of RNA Nanoparticles.
Yaroslava Yingling 1 , Andrey Semichaevsky 1 , Abhishek Singh 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractBiomolecular nanostructures hold a tremendous promise for effective use in such applications as drug delivery, nanoelectromechanical systems, molecular sensors, and molecular lithography. RNA molecules are especially appealing molecules for nanodesign since they can carry genetic information, posses catalytic properties and can naturally fold or can be programmed to self-assemble into complex structures. Moreover, protein-free RNA nanoparticles do not induce a detectable immune response which is crucial for use in medical applications and drug delivery. RNA molecules can make up effective nanoparticles, nanotubes and scaffolds. However, these structures are not determined by base pairing alone and unpaired residues play a critical role in nanodesign and super-assembly. Yet there is a limited understanding of the rules of formation of RNA superstructures. For successful design we need to understand and control the intermolecular associations, base on natural tendency and favorability and various physical components. We use molecular modeling to understand self-assembly processes of natural and synthetic RNA. The most common motifs found in nature and used in bionanotechnology are hairpin loops which consist of a helical part and a loop with unpaired residues. The unpaired residues in these elements can lead to further super-assembly of RNA structures via formation of the loop-loop interactions. These loop-loop interactions regulate biological functions in both prokaryotic and eukaryotic organisms and are also actively used in bionanotechnology for self-assembly of RNA building blocks into novel nanostructures. It has been observed that the super-assembly of RNA directly depends on the presence and specific concentration of ions. In order to understand the role of ions in loop-loop formation and stability, we conducted a series of explicit solvent atomistic molecular dynamics simulations, electrostatics simulations, and normal mode analysis of ten distinct kissing loops elements taken from various organisms. We discovered that in most organisms the loop-loop assembly process depends on the presence of electronegative and hydration channel. The size of this channel and RNA sequence determines the stability, the hydrogen bonding interactions and the angle of the distinct kink between stems. Using these angles we can engineer the assembly of RNA building blocks via kissing loops motifs into nanostructures of a predefined geometry. We will show how we used the specific RNAI/RNAII loop-loop motif to design a novel RNA hexagonal nanoparticle and RNA fabric, the assembly of which was verified by experiment.
W6: Poster Session I: Computational Material Design
Session Chairs
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - W6.1
An efficient Method to Study Ordering in Low-symmetry Materials.
Tim Mueller 1 , Gerbrand Ceder 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMany material properties can be modeled through the use a cluster expansion, in which the property is expressed as a linear combination of localized interactions. Cluster expansions are commonly used to develop effective Hamiltonians of systems with substitutional disorder, and they have been successfully used to identify material ground states, calculate phase diagrams, and predict other material properties. The coefficients of the linear expansion are typically fit to a set of training data generated using ab-initio methods. For highly symmetric systems such as simple bulk crystals, symmetry can be used to greatly reduce the number of significant independent coefficients. For such systems, a small set of training data is required to generate sufficiently accurate cluster expansions. However, for low-symmetry systems such as nanoparticles, a much larger number of unique coefficient values must be determined, requiring a larger set of training data. In addition, the cost of performing ab-initio calculations to generate the training data is greatly increased due to the low symmetry of the system. For these reasons, it has been computationally impractical to use the cluster expansion to study multi-component nano-structured materials with the same level of accuracy as bulk materials. We have addressed this problem by developing a Bayesian framework for determining cluster expansion coefficients, and we present new methods developed using this framework. Our methods explicitly combine physical insight with the training data to significantly reduce the prediction error of a cluster expansion for a given training set size. The new methods also allow for more accurate estimation of the average prediction error for a given cluster expansion, resulting in generally higher quality cluster expansions. We demonstrate how our methods can be used to generate expansions for low-symmetry structures such as nanoparticles at a fraction of the current computational cost. This approach makes it computationally feasible to study atomic ordering and surface segregation in multi-component nanostructures at a nearly ab-initio level of accuracy.
9:00 PM - W6.10
Knowledge-based Approach to Gas Sorption in Glassy Polymers by Combining Experimental and Molecular Simulation Techniques.
Matthias Heuchel 1 , Ole Hoelck 2 3 , Martin Boehning 2 , Martin Siegert 1 , Dieter Hofmann 1
1 Center for Biomaterial Development, Institute of Polymer Research, GKSS Research Center, Teltow Germany, 2 , Federal Institute for Materials Research and Testing (BAM), Berlin Germany, 3 Micro Materials Center, Fraunhofer Institute for Reliability and Microintegration IZM, Berlin Germany
Show Abstract9:00 PM - W6.11
Modeling Equilibrium Concentrations of Bjerrum and Molecular Point Defects and their Complexes in Ice Ih.
Maurice de Koning 1 , Alex Antonelli 1
1 Instituto de Física, Universidade Estadual de Campinas, Campinas Brazil
Show AbstractWe present a density-function theory (DFT) study of Bjerrum-defect trapping centers involving the molecular vacancy and interstitial in ice Ih. As a first step, we compute the intrinsic migration barrier to D-defect motion using the nudged elastic band (NEB) method and find them to be of the same order of magnitude as the energy barriers involving intrinsic L-defect motion. This finding suggests that intrinsic mobility factors cannot explain the experimentally observed inactivity of D defects, supporting the idea that D defects are trapped at other lattice-defect sites. Next we study the defect complexes formed by the combination of isolated D and L defects with a molecular vacancy. The corresponding geometries show that the formation of these aggregates significantly reduces elastic distortions that are present in isolated Bjerrum defects, leading to a significant binding energy. In contrast to the complexes involving the molecular vacancy, however, the results suggest that the molecular interstitial binds preferentially to the D-type Bjerrum defect. Using both theoretical binding and formation free energies as well as available experimental data we find that the preferential binding and the substantial presence of the interstitial as the predominant point defect in ice Ih may lead to conditions in which the number of free D defects becomes considerably smaller than that of free L defects. Such a scenario could possibly be involved in the experimentally observed inactivity of D-type Bjerrum defects in the electrical properties of ice Ih.An analysis of the energetics involved in the formation of both defect complexes reveals a significant binding energy, indicating that the molecular vacancy represents a strong trapping center for Bjerrum defects. On the other hand, the fact that there is no difference between the absolute values of the binding energies for both D and L defects suggests that the vacancy affects both species of Bjerrum defects in a similar fashion, possibly ruling out the vacancy trapping centers as an explanation for the experimentally observed inactivity of D defects.
9:00 PM - W6.12
Modeling of Memorial Effects in Spinodal Decomposition of a Binary Systems.
Nicolas Lecoq 1 , Helena Zapolsky 2 , Peter Galenko 3
1 Groupe de Physique des Materiaux, University of Rouen, Saint Etienne du Rouvray France, 2 Groupe de Physique des Matériaux, University of Rouen, Saint Etienne du Rouvray France, 3 Institue of Materials Physics in Space , German Aerospace Center, Cologne Germany
Show Abstract Considerable recent interest has focused on spinodal decomposition in a wide variety of physical systems. Theoretical efforts thus far have mainly concentrated on understanding the linear stage of this phase separation. This stage of spinodal decomposition has been theoretically described by Cahn-Hilliard (CH) equation [1,2] which is considered as a diffusion-reaction type of equation. This equation is valid if the time of relaxation of system to local equilibrium is shorter than the characteristic time for dynamics of spinodal decomposition. However, if the propagation speed of wave, which characterise a spinodal decomposition, is of the order of the atomic diffusion speed the memory effect in the diffusion field can not be ignored [3]. We consider the modification of the Cahn–Hilliard equation when a time delay process through a memory function is taken into account [4]. As a result, a causality diffusion equation has a similar form to the telegraph equation with the decomposition delay described by the term τd ∂2 c / ∂t2 , where τdis the relaxation time for the atomic diffusion flux with memory effects. Using this model, we study the process of spinodal decomposition in fast phase transitions by computational methods. Finite-time memory effects are seen to affect the dynamics of phase transition at short times and have the effect of delaying, in a significant way, the process of rapid growth of the concentration that follows a quench into the spinodal region. Results of our modeling demonstrate that these effects are important in earlier stage of spinodal decomposition.[1] J.W. Cahn and J.E. Hilliard, J. Chem. Phys. 28, 258 (1958).[2] J.W. Cahn, Acta Metall. 9, 795 (1961).[3] P. Galenko, Phys. Lett. A 287, 190 (2001). [4] P. Galenko and D. Jou, Phys. Rev. E 71, 046125 (2005).
9:00 PM - W6.13
Calculation of Protein Dielectric Response using Molecular Dynamics Simulations.
George Patargias 2 , Sarah Harris 2 , John Harding 1
2 Department of Physics and Astronomy, University of Leeds, Leeds United Kingdom, 1 Engineering Materials , Univ. of Sheffield , Sheffield United Kingdom
Show AbstractProteins contain many ionizable groups spread throughout the structure. It is possible to measure the ionization of the protein experimentally, but difficult to ascribe this to individual groups within the molecule. In principle, it is possible to calculate the ionization state of groups individually but for large proteins, it is not practical to perform the calculations for all the possible combinations of ionized groups coupled with the conformations of the protein. Therefore simple models based on classical dielectric continuum theory have been used. Such models often give acceptable results, but they can fail for no apparent reason. We have performed a series of molecular dynamics calculations on a candidate protein, Hen Egg White Lysozyme, to investigate the ionization of a number of groups and use these results to test the continuum theory. The importance of electrostatic interactions between these ionizable groups in determining protein structure and function is well established. However, the dielectric properties which determine the magnitude of charge-charge interactions within biomolecules have received relatively less attention and remain poorly understood theoretically. The dielectric response of different regions of Hen Egg White Lysozyme has been mapped using molecular dynamics simulations. The dielectric constant for a particular region of the protein is obtained via the following approach. A charged probe is inserted in the region in question and the protein's reorganization energy is calculated through a free energy perturbation method. A value for the dielectric constant is then obtained by relating this reorganization energy to the Born solvation energy of the same charged probe in a continuum dielectric medium. We discuss the effective dielectric constants in the light of the changes in the molecular conformations predicted by the simulations. High values of effective dielectric constants of the protein correlate strongly with large relaxations of the protein. Moreover, our results show that the assumption of a homogeneous dielectric constant for the protein is not valid.
9:00 PM - W6.14
Multiscale Computer Simulation of Tensile Failure in Polymer-Coated Silica Aerogels.
Brian Good 1
1 Materials and Structures Division, NASA GRC, Cleveland, Ohio, United States
Show AbstractThe low thermal conductivities of silica aerogels have made them of interest to the aerospace community for applications such as cryotank insulation. Recent advances in the application of conformal polymer coatings to these gels have made them significantly stronger, and potentially useful as lightweight materials for impact absorption as well. In this work, we perform multiscale atomistic computer simulations to investigate the tensile strength and failure behavior of silica and polymer-coated silica aerogels.The gels' nanostructure is simulated via a Diffusion Limited Cluster Aggregation (DLCA) procedure, modified to introduce loops that increase the connectivity of the gel network and make the model gel more rigid. The procedure produces fractal aggregates that exhibit fractal dimensions similar to those observed in real aerogels. The largest distinct feature of the clusters is the so-called secondary particle, typically tens of nm in diameter, which are composed of primary particles of amorphous silica an order of magnitude smaller. The secondary particles are connected by amorphous silica bridges that are typically smaller in diameter than the particles they connect.We investigate tensile failure via the application of a uniaxial tensile strain to the DLCA clusters. In computing the energetics of tensile strain, the detailed structure of the secondary particles is ignored, and the interaction among secondary particles is described by Morse pair potentials, representing the strain energetics of the silica gel and the polymer coating, parameterized such that the potential ranges are much smaller than the secondary particle size. The Morse parameters are obtained by separate atomistic simulation of models of the interparticle bridges and polymer coatings, with the tensile behavior of these bridges modeled via molecular statics.We consider the energetics of tensile strain and tensile failure, and compare qualitative features of low-and high-density gel failure.
9:00 PM - W6.15
Computational Study on the Morphological Instability of a Nematic/Isotropic Interface in a Temperature Gradient.
Ezequiel Soule 1 2 , Nasser Abukhdeir 1 , Alejandro Rey 1
1 Chemical Engineering, Mcgill University, Montreal, Quebec, Canada, 2 INTEMA, University of Mar del Plata - CONICET, Mar del Plata, Buenos Aires, Argentina
Show AbstractThe morphological stability of the nematic/isotropic interface of 5CB (pentyl-cyanobiphenyl) in a temperature gradient is studied via computational 2-D simulations. A Landau-de Gennes type quadrupolar tensor order parameter model for the first-order isotropic/nematic transition is used. An energy balance, taking anisotropy into account, was derived and incorporated into the time-dependent model.The evolution of a small amplitude sinusoidal interface, representing a planar interface with a small perturbation to the planar shape, was analyzed for different wavelengths of the perturbation and in the presence of different temperature gradients. Dispersion diagrams (growth/shrinkage velocity of the perturbation vs. wavelength), are constructed in the small-amplitude regime and compared with a sharp-interface linear stability theory. It was found that, besides the shape instability, there is a texturing process. When the director is initially homeotropic at the nematic/isotropic interface, a defect forms and then is shed from the interface into the bulk nematic phase, giving rise to a region in the interface where the orientation becomes planar. This can be explained taking into account that the planar orientation is energetically more favorable than the homeotropic orientation. A similar defect formation and shedding mechanism has been observed in simulation of nematic spherulite growth [Wincure, B. M. and Rey, A. D.; Nano Letters, 2007, 7, 1474], due to the presence of strong orientation gradients along the interface. When the director is initially planar and in the same plane that the perturbation, a second nematic region is formed, also with a planar orientation but perpendicular to the plane of the perturbation. This second region is nucleated at the interface and then grows into the bulk. This can be explained considering that the homeotropic orientation parallel to the perturbation implies a distortion in the director as it follows the shape of the interface, while if the orientation is perpendicular to the plane of the perturbation there is no distortion. This type of phenomena has been observed in directional growth experiments [Bechhoefer, J. in “Pattern Formation in Liquid Crystals”, eds. Buka, A. and Kramer L., Springer, N.Y., 1996], although it was related to an anchoring conflict between the nematic isotropic interface and the plates, while in the present case is related with the distortion energy.
9:00 PM - W6.16
Exfoliation of a Layer of Platelets in Polymer Matrix: Effect of Entropic Constraints and Solvent Quality by a Monte Carlo Simulation.
Ras Pandey 1 , Barry Farmer 2
1 Physics and Astronomy, University of Southern Mississippi, Hattiesburg, Mississippi, United States, 2 , Air Force Research Laboratory, Dayton, Ohio, United States
Show AbstractDispersion of layered sheets (a model for clay platelets) in a polymer solvent matrix is studied by an efficient Monte Carlo simulation on a discrete cubic lattice. A layer of four sheets with a small inter-layer distance is placed at the center of the lattice embedded in a random distribution of polymer chains. Sheets and polymer chains are modeled by tethered nodes with fluctuating bonds in planar and linear chain configurations respectively with appropriate molecular weight. Chains and platelets interact with a short range interaction and execute their stochastic movement with the Metropolis algorithm. At low molecular weight of the polymer matrix, the repulsive interaction between polymer and sheets enhances exfoliation while attractive interaction suppresses it. Increasing the molecular weight of the polymer chains leads to entropic trapping in layered structure in an effective cage due to entanglement. Effects of molecular weight of the polymer solvent and its quality on the dispersion will be discussed.
9:00 PM - W6.17
Phase-field Modelling of Partially Molten Grain Systems.
Frank Wendler 1 , Britta Nestler 1
1 Institute of Computational Engineering, Karlsruhe University of Applied Sciences, Karlsruhe Germany
Show AbstractMany material properties of alloys as well as ceramics are to a large extend determined by their specific morphology on the grain scale. As compared to grain growth in completely solid samples, polycrystalline compounds in contact with a liquid phase show a completely different coarsening dynamics, often accompanied by the formation of a liquid pore network. The clarification of the process is important to improve metal forming methods as thixoforming and liquid-phase sintering, and has a long-ranging impact on understanding partial melt rock formation in structural geology [2].To model the microstructure evolution in partially molten 2D and 3D systems, we apply the formerly introduced multi-phase-field model of Allen-Cahn type [1]. Grains of different orientations as well as liquid phase regions are thereby characterized by a set of continuous phase-field parameters. Fist, volume constraints for the melt phase are introduced to establish an equilibrium situation between solid and liquid state, using a method given in [3]. Based on the results of large scale simulations, the key parameters determining the coarsening rate are found to be the solid-solid-liquid dihedral angle (studying values from 10° to 90°), the liquid fraction and distribution of liquid in the grain structure. A comparison with the evolution of an experimental analoque system (norcamphor-alcohol) is presented. As in the experiments, partially wetted grain boundaries form in the course of the simulations, but modify the coarsening dynamics only sligthly. Furthermore, the pore network formation for dihedral angles < 60° and the transition to complete wetting in 3D grain systems is studied. For many polycrystalline minerals and ceramics, the solid-liquid surface energy anisotropy is very large. We present the results of a phase-field study on grain boundary evolution and grain growth when using a hexagonal, facet-forming anisotropy.[1] B. Nestler, H. Garcke and B. Stinner, Phys. Rev. E 71 (2005) 041609-1[2] F. Wendler, J.K. Becker, B. Nestler, P.D. Bons and N.P. Walte, submitted to Computers & Geosciences[3] H. Garcke, B. Nestler, B. Stinner and F. Wendler, Math. Mod. Meth. Appl. Sci. 18 (2008), in press
9:00 PM - W6.20
Ab-initio Molecular Dynamics Simulations of Polyaromatic Hydrocarbons with Palladium (II) Acetylacetonate.
Samir Mushrif 1 , Gilles Peslherbe 2 , Alejandro Rey 1
1 Chemical Engineering, McGill University, Montreal, Quebec, Canada, 2 Centre for Research in Molecular Modeling and Dept. of Chemistry & Biochemistry, Concordia University, Montreal, Quebec, Canada
Show AbstractThe effect of addition of an organometallic salt on the characteristics and applications of activated carbon adsorbents is widely studied in the literature. It is well known that the presence of metal species affects the pore size and surface area generation of the adsorbent and thus controls its end application and performance. However, a limited investigation has been done to gain the fundamental knowledge of the chemistry of organometallic salts with the carbon matrix and their interactions, under different processing conditions. This research focuses on developing an unambiguous understanding of the metal-salt and carbon matrix interactions using ab-initio molecular dynamics simulations. We simulate the carbon matrix using polyaromatic hydrocarbons and palladium (II) acetylacetonate is chosen as the metal salt of interest due to its role in enhancing hydrogen adsorption capacity of active carbon adsorbents. Ab-initio calculations are performed using the planewave-pseudopotential implementation of the density functional theory and the Car-Parrinello scheme is used for molecular dynamics. Simulations are carried out at 500, 800 and 1200 K and the effect of temperature on the organometallic interactions is also studied.
9:00 PM - W6.21
The Effect of Extensional Flow on the Phase Diagram and Orientational Structure of Carbonaceous Mesophase Mixtures.
Mojdeh Golmohammadi 1 , Alejandro Rey 1
1 , McGill University, Montreal, Quebec, Canada
Show AbstractA Maier-Saupe model for binary mixtures of uniaxial discotic nematogens under extensional flow is formulated to describe the phase ordering in carbonacoues mesophases, differing only in molecular weight. The orientational structure and the thermodynamic phase diagram depend on (i) the intrinsic properties of the mixture, i.e. molecular weight difference and the interaction parameter, (ii) the operating properties, i.e. concentration and temperature and (iii) the processing condition, i.e. flow rate. The results obtained from our pervious study [Golmohammadi et al., JCP, 2008, submitted] shows that depending on the intrinsic properties of the system, two types of uniaxial nematic mixtures arise: (i) non-ideal mixtures with a minimum in its Nemtic to Isotropic (NI) transition as a function of the concentration, and (ii) ideal mixtures with a monotonic trend of the NI transition temperature as a function of concentration. In the current work we study the effect of extensional flow on the type of the mixture and the value of the concentration corresponding to the minimum transition temperature. The results can be used to design the processing flow rate to form a desired fiber structure.
9:00 PM - W6.22
Design Active Nanostructures: Simulated Nanorockets Propelled by Catalytic Chemical Reactions.
Yunfeng Shi 1
1 Department of Materials Science and Engineering, North Carolina State Univ., Raleigh, North Carolina, United States
Show AbstractThis talk details a molecular-level design of an autonomous energy transducer that converts chemical energy to mechanical work by means of molecular dynamics simulations. The propulsion comes from asymmetric catalytic chemical reactions. This design is motivated both by the catalytic molecular motors in hydrogen peroxide solutions and by conventional rocket engines. However, due to the differences in size, speed and viscosity of the environment, the nanomotor using the current design works in a different hydrodynamic regime than the other two cases. Thermodynamic efficiency of the nanomotor is measured by imposing various external loads. Moreover, the maximum efficiency is shown to be proportional to a dimensionless number, which can be served as a guideline to design future functional nanostructures for energy conversion. One final goal of this work is to encourage experimental efforts on catalytic molecular motors in reactive gas phases.[1] Y. F. Shi and D. W. Brenner, NanoLetter, under review (2008).*New address: Department of Materials Science and EngineeringRensselaer Polytechnic Institute, Troy, NY 12180
9:00 PM - W6.23
The Design and Calculation of Photonic Bandgaps in Two-Dimensional Photonic Quasi-crystals by FDTD Simulation Method.
Jong-Bin Yeo 1 2 , Sang-Don Yun 1 , Hyun-Yong Lee 2
1 Department of Functional Nano Fine Chemicals, Chonnam National University, Gwangju Korea (the Republic of), 2 Faculty of Applied Chemical Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Show AbstractIn the this present work, we design and calculate photonic bandgaps (PBGs) of 2 dimensional photonic quasicrystal (2D PQCs) structures. After making the rotation symmetrical PQCs model, we establish the FDTD method for model by the operator conversion. We prove that 2D PQCs is good model of the possibility for wide omni-photonic bandgaps. Photonic crystals (PCs) are high index contrast periodic arrays characterized by photonic bandgaps (PBGs) or electromagnetic stop bands. And photonic quasicrystals (PQCs) are aperiodic structure having characteristic of rotation symmetric property. PQCs have omni-PBGs region at low index contrast array. PBGs have a functional similarity to the energy gap that electrons experience in semiconductor crystals. That is PQCs can prohibit the propagation of electro-magnetic waves (or photons) at frequencies which exist within the PBGs. PQCs are expected to be used in many applications in optoelectronics and optical communications. Because It is able to widely perfect omni-PBGs. In order to realize functional PCs devices such as selective transmission filters, optical waveguides, and microcavities, various defect structures have to be inserted into the PCs. So PQCs devices can applied various optical component. We come to the conclusion that 2D PQCs have widely omni-PBGs. So we believe that 2D PQCs are able to applied very many optical component. And multi exposure holo-lithography system is a suitable tool to realize the pattern generation method for 2D PQCs. Futher studies to realize 2D PQCs by phase changing of amorphous chalcogenide films.
9:00 PM - W6.24
Structural and Electronic Properties of a Buckycatcher.
Marcelo Flores 1 , Fernando Sato 1 , Douglas Galvao 1
1 Department of Applied Physics, University of Campinas - UNICAMP , Campinas, Sao Paulo, Brazil
Show AbstractRecently, functional molecular structures have been object of investigation by several groups [1]. As an example of these new nanostructures, Sygula et. al. [2] reported the synthesis and crystal assembly of stable carbon-based molecule, consisting of two corannulene subunits linked by a carbon-based joint forming a chalice-like configuration for its most stable structure. Due to its ability of acting as fullerene receptor, such molecule was named by them as buckycatcher (BCKY). In the present work we present the dynamics, configuration and electronic properties calculated using the universal force field (UFF) and three different density functional theory (DFT) functionals, with both local (PWC) and non-local (BLYP and PBE) corrections for the BCKY with and without incorporating C60. All the simulations were carried out using Cerius2 computer package for classical and quantum mechanical simulations. Our conformational searches for the isolated BCKY (without the presence of C60) showed the existence of three very stable different conformations, with small relative difference energies among them. Molecular dynamics simulations (up to 600 K) showed that the interconversion barrier is high enough to preclude conformational transitions. However when the C60 is ‘captured’ the barriers are smaller and conformational transitions can occur. It has been suggested [2] that the interaction between the BCKY and the C60 is of pi-pi nature. Our DFT calculations produced conflicting results depending on the used functional (significant pi-pi coupling was observed only when PWC functional was employed, while both GGA functionals failed to produce this behavior). The well-known poor description of van der Waals interactions within DFT scheme may account for this. However, all DFT single-point (using as input the UFF optimized geometry calculations, where the van der Waals interactions are better described) calculations produced results compatible with the proposed pi-pi nature of the buckycatcher-C60 interactions. The molecular dynamics simulations of the ‘catching’ processes showed that in vacuum this occurs only when there is a good alignment between the BCKY ‘mouth’ and the C60. Otherwise, the thermal fluctuations and high torsion barriers render the process unlikely to occur. It is expected that in solution these processes will be facilitated and more effective. [1] Chen, C. W. and Whitlock Jr., H. W. J. Am. Chem. Soc. 1978, 100, 4921.[2] Sygula, A. et al. J. Am. Chem Soc. 2007, 129, 3842.
9:00 PM - W6.25
Transmission and Refraction of Electromagnetic Waves in Transmission Lines, Photonic Crystals and Left Handed Materials: Localized States in the Photonic Gap.
John Vassiliou 1 , Kevin Creedon 1
1 Physics, Villanova University, Villanova, Pennsylvania, United States
Show AbstractUsing multiscale modeling, the Maxwell’s equations are solved for different electromagnetic systems including transmission filters, photonic crystals and left-handed materials (metamaterials). The transmitted electromagnetic power is calculated relative to the input power for Transverse Electric (TE) and Transverse Magnetic (TM) modes. The program inputs include the boundary conditions of the system and the index of refraction of the materials involved. The dielectric constant and the permitivity of the materials involved are given as a function of wavelength or as discreet values. The systems studied include transmission filters and photonic crystals configured as pillars of Gas or Si in air or pillars of air (holes) in GaAs or Si. For several of the systems modeled, the transmitted spectrum of photonic crystals shows clearly photonic gaps for various ranges of wavelengths. When disorder is introduced in the photonic crystal localized states appear in the gap which eventually lead to the emergence of additional gaps or the disappearance of the gaps in case of strong disorder. The ratio of width to the middle gap wavelength is studied as a function of the geometric characteristic of the constituents of the photonic crystal. In case of metamaterials, the refraction of the E&M waves from a wedge of an orthogonal prism is clearly demonstrated in agreement with the Snell’s law for negative index of refraction materials. The focusing of the refracted beam from the metamaterial is demonstrated and is compared to the equivalent behavior of a refracted beam by a normal material for the same geometric setting. The above result demonstrates the possibility of construction of a “perfect lens”. For certain combinations of the dielectric constant and permittivity of the left handed material, the electromagnetic wave is totally dissipated rendering the metamaterial impenetrable to the electromagnetic radiation. In particular geometries where a metamaterial is cloaking a region of a normal material a beam of electromagnetic wave is rerouted around the normal material, thus concealing the normal material and rendering it invisible to electromagnetic radiation.
9:00 PM - W6.26
Amorphous SiO2/SiC Interface Defect Structure Generated with First-principles Molecular Dynamics Simulation.
Atsumi Miyashita 1 , Toshiharu Ohnuma 2 , Misako Iwasawa 2 , Hidekazu Tsuchida 2 , Masahito Yoshikawa 1
1 Quantum Beam Science Dirctirate, Japan Atomic Energy Agency, Takasaki, Gunma, Japan, 2 Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, Komae, Tokyo, Japan
Show AbstractSilicon carbide (SiC) semiconductor devices are expected to be used in severe environments such as outer space and/or nuclear power plants. The performance of SiC metal-oxide-semiconductor (MOS) devices to date is low as compared with the ideal performance. This is considered to be attributed to defects generated at the interface between amorphous SiO2 (a-SiO2) layers and 4H-SiC substrates of MOS devices. However, it is not clear at the present what kind of defects affect the degradation of electrical characteristics. In this work, a large-scale slab model was prepared on the basis of an atomic network of beta quartz (β-SiO2) on 4H-SiC crystal and theoretical structures simulating the interface in the actual SiC MOS devices were successfully obtained on a super computer. A basic model was made on the basis of an atomic network of β-SiO2 on 4H-SiC crystal. The atomic networks composed of twelve layers were employed for the models of β-SiO2 and 4H-SiC crystal, respectively. By using 3×3 super-cell composed of the basic structure, a large-scale slab model with the number of 1017 atoms was prepared. The first principles molecular dynamics calculation using Vienna ab initio simulation package (VASP) [1] was performed for the large-scale slab model by heating and quenching method to form theoretical structures which simulate the a-SiO2/4H-SiC interface of actual SiC MOS devices. The heating of slab model was done at 4000 K for 2 ps followed by quenching to room temperature with the cooling speed of -2000 K/s. The obtained SiO2 layers have amorphous like characteristics on the 4H-SiC substrate. It is found that the theoretical structure at the SiO2/SiC interface consists of three different types of defect-free atomic networks, two different types of atomic networks with defect and a dangling bond of Silicon atom involved in a 4H-SiC substrate. The atomic networks with defect contain a Si-Si bond, respectively. Some atomic networks, which were not proposed in the theoretical structure model in the reference [2,3], were found at the interface in our simulation. Reference: [1] G. Kresse, and J. Hafner: Phys.Rev. B 47, 558 (1993); ibid. 49, 14251 (1994). [2] F. Devynck, F. Giustino, P. Broqvist, and A. Pasquarello: Phys.Rev. B 76, 075351 (2007). [3] J.M. Knaup, P. Deák, Th. Frauenheim, A. Gali, Z. Hajnal, and W.J. Choyke: Phys.Rev. B 71, 235321 (2005).
9:00 PM - W6.29
Quasiparticle Properties of Semiconductors from GW-Wannier Method.
Xiaofeng Qian 1 , Nicola Marzari 1 , Paolo Umari 2
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Theory at Elettra Group, CNR-INFM Democritos, Basovizza (Trieste) Italy
Show AbstractDensity functional theory (DFT) has been extensively applied to calculate the ground-state properties of materials successfully. However, the excited-state properties, including band gap, optical absorption spectrum, and electron energy loss spectrum, are less investigated at the ab initio level, which is very important for better theoretical understanding and systematic device design of modern electronics and photonics. Currently there are two major state-of-the-art approaches available for studying excited states. One is many-body perturbation theory such as Hedin's GW approximation [1,2]. The other is time-dependent density functional theory. However, both approaches involve much more intensive computation effort compared to DFT calculations. To study the excited-state properties of materials in larger scale recently we have developed an efficient and accurate ab initio method based on Hedin's quasiparticle GW method and maximally localized Wannier functions on top of plane-wave DFT method [3]. As benchmarks, we have first applied this method to calculate band gaps of various semiconductors and insulators with both norm-conserving and ultrasoft pseudopotentials [4]. It is shown that our GW-Wannier (GWW) method largely increases the DFT band gap by including dynamical electron correlation effect beyond DFT into the self-energy operator. The nice quantitative agreement between the calculated band gaps and experimental data demonstrates the reliability of the GWW method. Meanwhile, the localization property of the maximally localized Wannier functions enables us to largely reduce the total number of polarization function basis, thus greatly improves the computation efficiency of full GW calculations without losing the accuracy.[1] L. Hedin, Phys. Rev. 139, A796 (1965)[2] M. S. Hybertsen and S. G. Louie, Phys. Rev. B 34, 5390 (1986)[3] P. Umari, to be submitted to Phys. Rev. Lett (2008)[4] X. Qian, N. Marzari and P. Umari, to be submitted to Phys. Rev. B (2008)
9:00 PM - W6.30
Surface Stability and Electronic Structure of Hydrogen and Fluorine Terminated Diamond Surfaces: a First Principles Investigation.
Fatih Sen 1 , Yue Qi 2 , Ahmet Alpas 1
1 Department of Mechanical, Automotive and Materials Engineering, University of Windsor, Windsor, Ontario, Canada, 2 Materials and Process Lab, GM Research and Development Center MC 480-106-224, Mound Road, Warren, Michigan, United States
Show AbstractFirst principles calculations were used to investigate the surface properties of fluorine modified DLC and diamond coatings in order to design tool surfaces with improved environmental stability for aluminum machining and forming. The stability and electronic structure of fully H or F terminated and mixed H and F terminated diamond (111) surfaces were studied. It was found that both fluorine and hydrogen caused surface carbons to make sp3 type bonding, which results in a more stable 1x1 construction rather than the p-bonded 2x1 construction. The surface energies corresponding to each H and F concentration were calculated and a surface phase diagram showing the predominance of stable surface composition region was constructed as a function of chemical potentials of H and F sources. The diagram shows that there is no 3F1H surface termination. Considering common gas sources of fluorine and hydrogen, fluorine doping results in a decrease in the surface energy. The F terminated surface is more stable and chemically more inert due to the strong ionic CF bonding and the close packing of the large F atoms. Although the fluorine atoms are larger, they seemed to develop attractive rather than repulsive forces. Consequently, they will hinder the interaction of carbon atoms with the environment, which results in a chemically more inert surface.
9:00 PM - W6.31
A Molecular Dynamics Study of the Rotational Dynamics and Polymerization of C60 in C60-Cubane Crystals.
Vitor Coluci 1 , Fernando Sato 2 , Scheila Braga 2 , Munir Skaf 3 , Douglas Galvao 2
1 , Center for High Education on Technology, University of Campinas , Limeira, SP, Brazil, 2 , Applied Physics Department, Institute of Physics, University of Campinas, Campinas, SP, Brazil, 3 , Institute of Chemistry, University of Campinas , Campinas, SP, Brazil
Show Abstract9:00 PM - W6.32
Adaptive Hybridization of Density-Functional Theory and Molecular Dynamics: Reaction of Pressurized Water Molecule Trapped in Between Nano-Structured Diamond and Silicon.
Shuji Ogata 1 2 , Yuya Abe 1 2 , Ryo Kobayashi 1 2
1 Department of Scientific and Engineering Simulation, Nagoya Institute of Technology, Nagoya Japan, 2 CREST, Japan Science and Technology Agency, Saitama Japan
Show AbstractWe have been developing a hybrid simulation scheme by combining the electronic-density-functional-theory (DFT) and the classical molecular dynamics (MD) method using the empirical inter-atomic potential [1-3]. In the hybridization scheme, multiple DFT regions each of which is composed of a relatively small number of atoms, are embedded in a system of classical MD atoms. For robust coupling of the DFT and the MD regions with reasonable mechanical accuracy, the buffered cluster method has been proposed, which requires no link-atoms and is applicable to a wide range of materials and settings. During a hybrid DFT-MD simulation run, the sizes and number of the DFT regions change adaptively following a simple rule to trace the chemical reaction with reduced computation cost as compared to the full DFT calculation. Recently interesting observation has been reported in the experiments about the chemical reaction in the nano-indentation: when the diamond-tip was pressed repeatedly to the Si surface in moisture environment the resulting indentation mark shrank gradually, while it enlarged if the pressing was done in vacuum as expected [4]. The fact suggests that local oxidation due to the pressing occured in the case of the moisture environment. Motivated by this, in the present paper, we perform the hybrid DFT-MD simulation of pushing and sliding of H-terminated diamond tip on H-terminated Si(100)2x1 surface with a water molecule inserted in between. We thereby find that, depending on the various settings including the sharpness of the diamond-tip, the water molecule can be pushed into a native dimple of the H-terminated Si surface, which then decomposes and oxidizes the Si to form the Si-O-Si structure.[1] S. Ogata, et al., Comp. Phys. Commun. 149, 30 (2002).[2] S. Ogata, Phys. Rev. B 72, 45348 (2005).[3] T. Kouno and S. Ogata, J. Phys. Soc. Jpn 77, 54708 (2008).[4] I. Zarudi, L.C. Zhang, and M.V. Swain, Key Eng. Mat. 233-236, 609 (2003).
9:00 PM - W6.33
Accelerated Kinetic Monte Carlo Simulations of Vacancy-Mediated Arsenic Diffusion and Clustering in Silicon.
Brian Puchala 1 , Michael Falk 3 1 , Krishna Garikipati 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractDuring semiconductor device fabrication, ion implantation of dopants creates large populations of defects, vacancies and interstitials, which mediate dopant diffusion. Experiments have shown large changes in dopant diffusivity in silicon as a function of annealing time and dopant concentration. We perform kinetic Monte Carlo (KMC) simulations of vacancy-mediated arsenic diffusion in silicon to investigate the effect of dopant concentration, distribution and clustering on diffusivity. In order to follow the diffusion and breakup of clusters, on the order of minutes, our KMC simulations are accelerated using absorbing Markov chain analysis on states intelligently chosen on-the-fly to fill trapping basins in the local energy landscape. At lower dopant concentrations, we calculate the diffusivity and breakup rates of different cluster types and a mean field approach can be used to describe the overall cluster population evolution and dopant diffusivity. Above a critical concentration this mean field description fails as dopants become close enough to form a percolating structure throughout the material. At all concentrations, diffusivity decreases significantly over time as larger, less mobile clusters form.
9:00 PM - W6.37
A First Principles Study of Si and Y Doping of HfO2.
Chunguang Tang 1 , Rampi Ramprasad 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show Abstract9:00 PM - W6.38
Heteroepitaxial Modeling on Strained Surfaces using a Mean Field Approach.
James DeGraffenreid 1 , Ramon Grima 3 , John Venables 1 2
1 Physics, Arizona State University, Tempe, Arizona, United States, 3 Systems Biology, University of Edinburgh, Edinburgh United Kingdom, 2 LCN, University College, London United Kingdom
Show Abstract9:00 PM - W6.4
Phase Diagram of Silicon Using a DFT-based Neural Network Potential.
Oliviero Andreussi 1 , Jörg Behler 3 , Michele Parrinello 2
1 DMSE, MIT, Cambridge, Massachusetts, United States, 3 , Ruhr-Universität, Bochum Germany, 2 , ETH , Zürich Switzerland
Show Abstract9:00 PM - W6.6
Renormalization Method for the Multiscale Modeling of Aperiodic Materials.
Vicenta Sanchez 1 , Chumin Wang 2
1 Departamento de Fisica, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, Mexico D.F. Mexico, 2 Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Mexico D.F. Mexico
Show AbstractCrystalline solids are traditionally studied by taking the advantage of the reciprocal space and the Bloch theorem. However, these methods become inadequate when extended defects are present, such as in amorphous or quasicrystalline materials, where cluster calculations with a limited number of atoms are usually employed. In fact, for mesoscopic periodic systems a real space approach would be most desirable. In this work, we report a real-space renormalization method to address multi-scale aperiodic systems. The frequency-dependent electrical conductivity of periodic and quasiperiodic systems is investigated by means of the Kubo-Greenwood formula within the tight-binding formalism. For the periodic case, electronic transport is ballistic, i.e., the electrons can go from one end to the other without being scattered. An analytical expression of its ac conductivity at low frequency region is obtained for periodic chains of N atoms connected to two semi-infinite periodic leads [1]. The dc conductivity of quasiperiodic chains with Fibonacci bond disorder, also connected to two periodic semi-infinite leads, is always smaller than the periodic case, except for the transparent states which have the same conductivity of periodic chains. However, when an oscillating electric field is applied, we observe that the ac conductivity of quasiperiodic systems at certain resonant states can be larger than the periodic one. Analysis of this behavior in systems with mirror symmetry [2] and in nanowires is also presented.[1] V. Sanchez and C. Wang, Phys. Rev. B 70, 144207 (2004). [2] V. Sanchez and C. Wang, J. Phys. A 39, 8173 (2006).
9:00 PM - W6.8
Adsorption in flexible metal organic framework materials : Molecular simulations compared to experiments
Aziz Ghoufi 1 , Nilton Rosenbach 1 , Fabrice Salles 1 , Philip Llewellyn 2 , Sandrine Bourrelly 2 , Christian Serre 3 , Gérard Férey 3 , Guillaume Maurin 1
1 UMR CNRS 5253 UM2, Institut Charles Gerhardt Montpellier, Montpellier France, 2 UMR CNRS 6264, Laboratoire Chimie Provence, Marseille France, 3 UMR CNRS 8637, Institut Lavoisier, Versailles France
Show Abstract9:00 PM - W6.9
Mechanical Testing and Material Modeling of Thermoplastics:Polycarbonate, Polypropylene and Acrylonitrile-Butadiene-Styrene.
Jean-Luc Bouvard 1 , H. Brown 1 , Esteban Marin 1 , P. Wang 1 , Mark Horstemeyer 1
1 Center for Advanced Vehicular Systems, Mississippi State University, Starkville, Mississippi, United States
Show AbstractTuesday, 12/2New Presenter - Poster W6.9Mechanical Testing and Material Modeling of Thermoplastics:Polycarbonate, Polypropylene and Acrylonitrile-Butadiene-Styrene. H.R. Brown