Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P26: Computational Nanoscience IV |
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Sponsoring Units: DCOMP DMP Chair: R. Klemm, Kansas State University Room: LACC 501B |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P26.00001: Water at a hydrophilic solid surface probed by ab-initio molecular dynamics: inhomogeneous thin layers of dense fluid. Giancarlo Cicero, Jeffrey Grossman, Alessandra Catellani, Giulia Galli We present a microscopic model of the interface between liquid water and a hydrophilic, solid surface, as obtained from \textit{ab-initio} molecular dynamics simulations. In particular, we focused on the (100) surface of cubic SiC, a leading candidate semiconductor for bio-compatible devices. Our results show that, in the liquid in contact with the clean substrate, molecular dissociation occurs in a manner unexpectedly similar to that observed in the gas phase. After full hydroxylation takes place, the formation of a thin ($\sim $ 3 {\AA}) interfacial layer is observed, which has higher density than bulk water and forms stable hydrogen bonds with the substrate. The liquid does not uniformly `wet' the surface, rather molecules preferably bind along directions parallel to the Si dimer rows. Our calculations also predict that at $\sim $ 1 nm, the structural and electronic properties of liquid water are weakly affected by one-dimensional confinement between hydrophilic, solid substrates. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P26.00002: One dimensional growth of styrene on H-Si(001)-(3×1): a Density Functional Theory study. Noboru Takeuchi, Annabella Selloni Recent experimental work on the addition of styrene on hydrogenated Si (001) and (111) surfaces has provided evidence for a surface chain reaction mechanism initiated at isolated H vacancies. In contrast with the island-type growth on the H-terminated Si(111) surface, styrene is found to form one dimensional lines on the hydrogenated Si(001) surfaces. Using periodic Density Functional Theory (DFT) calculations, together with a recently developed method to find reaction pathways, we have studied the initial steps of the radical chain mechanism on the H-Si(001)-(3 $\times $1) surface. Our results suggest a preference for a one dimensional growth in the direction perpendicular to the Si dimer rows, in agreement with experiment. This preference is partly due to a smaller activation energy for hydrogen abstraction from the nearest dihydride site compared with hydrogen abstraction from a neighboring dimer, as well as to a strong repulsion between hydrogen atoms of the styrene molecules and monomer sites, when the growth is parallel to the dimer rows. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P26.00003: Surface Reaction of Alkynes and Alkenes with H-Si(111) : A DFT study Yosuke Kanai, Noboru Takeuchi, Annabella Selloni There is currently a strong interest in the organic functionalization of semiconductor surfaces. One of the most promising approaches for preparing such functionalized surfaces is via a radical-initiated surface chain reaction of terminally unsaturated molecules with hydrogen-terminated surfaces. The adsorbing organic molecule reacts with the Si dangling bond, and forms an intermediate metastable state in which a carbon centered radical is present. Abstraction of a hydrogen atom from a neighboring H-Si surface unit results in a stable adsorbed species and a new hydrogen vacancy. We have studied the initial stages of this chain reaction on the H-Si(111) surface using the first principles string molecular dynamics approach [1] which couples the Car-Parrinello scheme with an efficient method to determine reaction pathways. We find that the relative values of the energy barrier for hydrogen abstraction and for the desorption of the adsorbed carbon centered radical at the metastable state is the crucial factor in determining the viability of the surface chain reaction. The roles of the molecule and surface electronic structures in the reaction are discussed. Results for the adsorption of C$_2$H$_2$, C$_2$H$_4$, Phenylacetylene, and Styrene on the H-Si(111) surface are presented. \newline \newline [1] Y. Kanai, A. Tilocca, A. Selloni, and R. Car, J. Chem. Phys. 121, 3359 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P26.00004: Structure, Bonding, and Dynamics of Alkanethylhiolates on Copper and Gold Clusters and Surfaces Martin Konopka, Roger Rousseau, Ivan Stich, Dominik Marx The interaction of alkanethiolates with small coinage metal clusters and (111) surfaces of copper and gold was studied based on density functional theory with a focus on the metal-thiolate junction. Calculation of fragmentation energies indicate that for Cu cluster-thiolate (n=1,3,5,7, and 9) there is a progressive lowering in energy for the fragmentation of the S-C bond in the thiolate from a value of 2.9 eV for n=1 to 1.4 eV for n=9. The detailed electronic origins of this specific weakening are attributed to a polarization of electron density in the S-C bond as induced by bonding with the Cu cluster. For the gold analogues this effect is not observed and fragmentation at the S-C bond experiences only a slight 10\% destabilization as n increases from 3 to 9 On the Cu(111) surface the metal to thiolate charge transfer which leads to a non-direction partially ionic bonding with a concurrent flat adsorption energy landscape, As a result, occupation of fcc-hollow, hcp-hollow and fcc-bridge sites is observed during the coarse of a short finite temperature ab-initio molecular dynamics simulation as opposed to a static model where only the hollow sites are stable minima. Comparison of our results with the available experimental evidence and consequences of the electrostatic profile of the metal-molecule interface are presented. The difference between Cu and Au are discussed in the context of relativistic effects. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P26.00005: Strain in Layered Nanocrystals Youir Bae, Russel Caflisch Layered nanocrystals consist of a core of one material surrounded by a shell of a second material. We present computation of the atomistic strain energy density in a layered nanocrystal, using an idealized model with a simple cubic lattice and harmonic interatomic potentials. These computations show that there is a critical size r$_{\ast }$ for the shell thickness r$_{s}$ at which the energy density has a maximum. This critical size is roughly independent of the geometry and material parameters of the system. Moreover it agrees with the shell thickness at which the quantum yield has a maximum, as observed in several systems. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P26.00006: Molecular Dynamics Simulation of Nanostructured Thin Film Growth X.W. Zhou, D.A. Murdick, J.J. Quan, B. Gillespie, H.N.G. Wadley, R. Drautz, D.G. Pettifor Properties of nanostructured films are sensitive to atomic defects. Molecular dynamics (MD) simulations of growth can reveal defect formation mechanisms that are difficult to explore using other approaches. There are mainly two challenges. First, nanostructures often utilize different materials with metallic, ionic and covalent bondings. The MD must hence use an interatomic potential transferable to different local bonding environments encountered during growth. Secondly, growth is simulated by randomly adding atoms on the surface. The interatomic potential must hence accurately predict surface properties under various surface configurations. The newest MD approaches have begun to enable the growth simulation for a wide range of materials. The embedded atom method (EAM) potential was successfully used to simulate the growth of giant magnetoresistance metal multilayers. Our integrated EAM and charge transfer ionic potential is transferable between metallic and ionic materials and has been successfully used to simulate the growth of spin tunnel junction multilayers. Stillinger-Weber, Tersoff, and our analytical bond order potentials are compared for simulating covalent semiconductor growth. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P26.00007: Self-Teaching Kinetic Monte-Carlo Scheme For Small Cluster Diffusion on Cu(111) Petr Vikulov, Oleg Trushin, Victor Naumov, Altaf Karim, Abdelkader Kara, Talat S. Rahman We have developed a new version of Self-Teaching Kinetic Monte Carlo technique (ST-KMC) for simulations of processes relevant to growth on fcc(111). In this method, adsorbed atoms may occupy fcc or hcp sites for 2D cases. Additionally, adatoms may occupy ``top'' sites in the case of 3D simulations. The environment of any diffusing adatom is mapped using a 211-site template mimicking the layer stacking on fcc surfaces. We applied this new approach to the diffusion of small copper adatom clusters with size from 1 to 7 atoms on a Cu(111) surface. Activation energies for all mechanisms were calculated using the drag method, for saddle points search, and Embedded Atom Method (EAM) for interaction potentials. With this new approach, it was possible to incorporate multi-atom concerted motion involving the occupancy of fcc and hcp sites, which has been suggested by experimental findings and MD simulations of small clusters diffusion on fcc(111). Diffusion coefficients and their scaling with size and temperature will be presented and contrasted with MD and KMC results. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P26.00008: Preferential Growth of Pt Particles on Rutile TiO$_2$ Vladimir Skavysh, Hakim Iddir, Serdar Ogut, Nigel Browning Pt/TiO$_2$ is the prototype system exhibiting strong-metal- support-interaction phenomenon. The characterization of a real Pt/TiO$_2$ catalyst system through a combination of atomic resolution Z-contrast images and electron energy loss spectroscopy in the scanning transmission electron microscope has revelaed an unexpected result: Pt particles have a strong tendency to nucleate on the rutile phase of TiO$_2$ rather than anatase. In order to address the selective growth of Pt on rutile, Pt atom binding energies on stoichiometric and reduced TiO$_2$ surfaces and surface oxygen vacancy formation energies have been calculated using first principles density functional theory calculations for both rutile and anatase phases. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P26.00009: Mechanical properties of diamond/a-C nanocomposite films Ioannis Remediakis, Maria Fyta, Pantelis Kelires Nanostructured amorphous carbon (na-C) is a hybrid form of carbon in which nanocrystallites are embedded in an a-C matrix. It has attracted considerable attention, because it offers the possibility to tailor the mechanical and electronic properties of a-C. We present here our studies of a particular form of na-C, containing diamond nanocrystals, using tight-binding molecular dynamics and empirical-potential Monte Carlo simulations. The calculations allow us to shed light into several properties of this material. We examine its structure, stability as a function of temperature and size of nanocrystals, stress state, and hardness. We find that the nanocrystals are stable only in dense, highly coordinated a-C matrices. The nanocomposite films possess negligible intrinsic stresses. The elastic moduli and yield stresses under tensile or shear load are consistently and considerably higher than those of pure a-C, making the nanocomposite diamond/a-C films suitable for ultra-hard mechanical coatings. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P26.00010: Theoretical Investigation of the Vibrational and Electronic Properties of Titanium Carbide Nanocrystals Qin Zhang, Steven Lewis Stable titanium carbide nanoclusters with \verb+~+1:1 stoichiometry were first discovered in molecular-beam experiments in the early 1990's. These clusters are all indexed to perfect or nearly perfect $N_1 \times N_2 \times N_3$ fragments of bulk TiC in the rocksalt structure and are thus termed ``nanocrystals''. The most abundant member of this family is the $3\times 3 \times 3$ nanocrystal Ti$_{14}$C$_{13}$, indicating special stability for this species. Using Density Functional Theory, we have carried out a detailed theoretical analysis of the structural, electronic, and vibrational states of Ti$_{14}$C$_{13}$ and its $3 \times 3 \times 3$ sibling, Ti$_{13}$C$_{14}$, which is not observed in the experiments. In this talk, we will present our theoretical results and show how our analysis sheds light on several previously unresolved experimental findings. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P26.00011: Evolution of Electronic and Vibrational Polarity of NaF Nanocrystals Philip B. Allen, Christian Schmidt, Tunna Baruah, Mark R. Pederson Density functional theory is used to study vibrations, electrical dipole moments, and polarizabilities of NaF clusters. We use the NRLMOL code with GGA exchange and correlation, and a large basis set of Gaussian orbitals. Because of prior experimental and theoretical studies, this is a good model system for tracking the evolution of the properties from diatomic molecule to bulk crystal. The predicted ratio of vibrational to electronic contributions to the polarizability increases dramatically with size $N$ in the closed shell clusters Na$_N$F$_N$. The open shell system Na$_{14}$F$_{13}$ has a greatly enhanced electronic polarizability. Contrary to previous studies on this system which treated only the outer electron by quantum mechanics, we find the O$_h$ cubic structure to be stable relative to the polar distorted structures such as C$_{3v}$. The size of the permanent dipole is predicted to be 2.01 and 5.12 in units $ea_B$ for the C$_4v$ systems Na$_9$F$_9$ and Na$_{18}$F$_{18}$ respectively. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P26.00012: Simulations of amorphous nanoparticles: the effect of shape on surface structure and subsequent interactions with the surroundings Susan Fullerton, Chunxia Chen, Janna Maranas Molecular dynamics simulation is employed to study the effect of varying nanoparticle shape on the structure of boron oxide nanoparticles, and their subsequent influence on surrounding polyethylene oxide. While previous studies have focused on crystalline nanoparticles, this study is unique because the nanoparticles are amorphous. Shape has been shown to affect the electrical and optical properties of nanoparticles, in addition to the structure and dynamics of polymers surrounding nanoparticles. In this study, two nano-shapes of boron oxide are compared: a 16 angstrom diameter sphere, and a 16 x 16 x 16 angstrom cube. The networks are described by a short-range structure consisting of BO3 units, while the intermediate-range structure is described by six-membered planar boroxol rings. Both the fraction of boroxol rings and their locations differ between the two nano-shapes. All planar boroxol rings within the spherical simulation are located on the interior, while planar rings within the cubic simulation aggregate to the cube walls. Structural differences also appear at longer ranges, including the formation of ``layers'' aligned parallel to the walls of the cube. We also investigate how varying the nanoparticle shape influences the structure and dynamics of the surrounding polymer. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P26.00013: Cage structures based on Polyhedral Oligomeric Silsesquioxanes (POSS) with atomic and ionic impurities Frank Hagelberg, Sung Soo Park, Chuanyun Xiao, Delwar Hossain, Charles Pittman, Svein Saebo Endohedral and exohedral polyhedral cage molecules of the form (HAO$_{3/2}$)$_{8}$ (A = C, Si, Ge) with double four-membered ring D4R units complexed with the atomic or ionic species: Li$^ {+}$, Na$^{+}$, K$^{+}$, F$^{-}$, Cl$^{-}$, Br$^{-}$, He, Ne, Ar have been investigated using Density Functional Theory (DFT). Geometric, electronic and energetic properties were obtained. The symmetry of the endohedral complexes when X is a cation turned out to depend critically on the relative cation and cage sizes. The binding energies of the endohedral and exohedral complexes document a clear preference for the latter, except for halides, where the endohedral complexes are more stable. The formation of the endohedral complexes is discussed in terms of transition states that connect the exohedral and endohedral minima, as well as the activation barriers for insertion of the guest into the cage. Our studies predict that a fluoride anion can penetrate into the (HAO$_ {3/2}$)$_{8}$ cage without destroying it. For X = Cl$^{-}$, in contrast, the cage ruptures upon insertion of the impurity. [Preview Abstract] |
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P26.00014: Structure of nanocrystals embedded in amorphous carbon Ioannis Remediakis, Maria Fyta, Pantelis Kelires, Georgios Kopidakis Amorphous carbon (a-C) has been often found to contain crystalline regions with diameters in the nanometer scale. The so-called nanostructured amorphous carbon has attracted considerable attention, because of possible applications in MEMS/NEMS and optoelectronic devices. In this work, we study embedded nanocrystals in various a-C matrices using tight-binding molecular dynamics and empirical-potential Monte Carlo simulations. We are especially interested in faceted nanocrystallites, that deviate significantly from a spherical shape. We start by calculating the interface energy between various faces of the crystal and a-C. We then use the so obtained interface energies to predict the shape of the nanocrystal by means of a Wulff construction. Finally, we construct the atomistic configuration of the embedded nanocrystallite and examine its stability as a function of temperature and the nanocrystallite size. [Preview Abstract] |
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