Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B25: Focus Session: Computational Nanoscience II |
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Sponsoring Units: DMP DCOMP DAMOP Chair: Talat S. Rahman, Kansas State University Room: LACC 501A |
Monday, March 21, 2005 11:15AM - 11:51AM |
B25.00001: Magic structures of binary metallic clusters Invited Speaker: The structure of binary metallic clusters is investigated by a variety of computational tools, ranging from genetic and basin-hopping global optimization algorithms, to molecular dynamics, and to density-functional calculations. Three different binary systems are investigated: Ag-Cu, Ag-Ni, and Ag-Pd. A new family of magic cluster structures is found. These clusters have the common feature of presenting a perfect core-shell chemical arrangement (with an outer Ag shell of monoatomic thickness) and of being polyicosahedra, that is being made of interpenetrating icosahedra of 13 atoms. Core-shell polyicosahedra are of special stability, which originates from the interplay of different factors. First of all, polyicosahedra are very compact structures, so that they maximize the number of nearest-neighbor bonds for a given size. However, in single-element clusters, these bonds are not optimal, since inner bonds are strongly compressed and surface bonds are expanded. This is the contrary of what is required from the bond order -bond length correlation in metals, which favors contracted surface bonds. In binary clusters, the situation is different. Substituting the inner atoms of a single-element polyicosahedron with different atoms of smaller size, the bonds can relax close to their optimal distance. This leads naturally to the appearance of core-shell polyicosahedra. In Ag-Cu, Ag-Ni and Ag-Pd the formation of these structures is reinforced by the tendency of Ag atoms to surface segregation. A similar mechanism of structural relaxation, originating from the interplay of cluster geometry and bond order - bond length correlation, is also the cause of the destabilization of icosahedral structures in pure Pt and Au clusters . In these clusters, the compressed inner atoms of the icosahedra can relax because of the formation of \textit{rosette }structures at vertices in the outer layer. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B25.00002: Density functional study of structural properties of bimetallic Pt-Ni nanoparticles Luis A. Perez, Karo Michaelian, Ignacio L. Garzon A study has been made of the structures and energy ordering of the lowest-lying isomers of the bimetallic nanoparticles (PtNi$_{3})_{n}$, (Pt$_{3}$Ni)$_{n}$, and (PtNi)$_{n}$, with n=1-20. The lowest energy structures of binary Pt-Ni nanoclusters, modeled by the many-body Gupta potential, were obtained by using a genetic-symbiotic algorithm. These structures were further reoptimized within the DFT-GGA framework. The Gupta potential parameters used for the Pt-Ni interaction are the geometric average of the Pt-Pt and Ni-Ni ones. In agreement with the experimental evidence, segregation is observed in the Pt-Ni clusters, with the Ni atoms in the core and the Pt atoms on the cluster surface, even for the (PtNi$_{3})_{n}$ case. A discussion of segregation versus mixing effects will be presented. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B25.00003: Origin of electric dipole moments in free niobium clusters Kristopher Andersen, Vijay Kumar, Yoshiyuki Kawazoe, Warren Pickett Niobium clusters Nb$_n$ ($5 \leq n \leq 200$) with surprisingly large, spontaneous electric dipole moments were recently observed in low-temperature ($T < 100$ K) molecular beam experiments by Moro {\it et al.} (Science {\bf 300}, 1265). We address the origin of the electric dipole using first-principles electronic structure calculations for Nb$_n$ ($n \leq 15$). The calculated moments generally agree with the experimental data, in which negligible moments are found for $n$ = 4, 10, and 15; moderate moments for $n$ = 5--9; and large moments for $n$ = 11--14. A strong correlation is found between the geometrical asymmetry of the cluster and electric dipole: its magnitude is proportional to the spread in the principal moments of inertia and its direction tends to align along the axis of the largest principal moment. Charge deformation densities reveal directional, partially covalent bonds that stabilize the structural asymmetry. Classical simulations of the deflection of a cluster in a molecular beam provide evidence that the electronic dipole may persist at higher temperatures, but is masked by the rotational dynamics of the cluster. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B25.00004: Saturation effects in silver-particle aggregates due to nonliearity A. Buin, P. DeChatel, H. Nakotte, V. Drachev2, V. Shalaev We present results for the extinction and absorption spectra of fractal aggregates consisting of silver particles, calculated in the quasistatic dipole approximation. We have solved the coupled-dipole equations iteratively, taking into account the nonlinear susceptibility kai3(Kerr), which is introduced in terms of an intensity-dependent dielectric function. To achieve self consistency, we used a modified version of Broyden's mixing method. Nonlinearity is seen to supress the giant enchancement effects found for linear polarizability. At intermediate intensitites, we observe a tendency toward saturation and some hysteretic behaviour which is reminiscent of ferro-electrics. For fractal aggregates of particle radii in the 3-5 nm range, the behaviour is determined by saturation and nohysteretic effects are observed. \newline \newline [1] G. Amoretti et al., Phys. Rev. B 15 (1989) 1856 [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B25.00005: Size Dependence of the Static Polarizabilities and Absorption Spectra of Ag clusters Serdar Ogut, Juan C. Idrobo, Julius Jellinek We present results for the structures, static polarizabilities, and absorption spectra for ground state and low-energy isomers of Ag$_n$ ($n\le 10$) clusters. The calculations for ground state properties are performed using local density approximation and pseudopotentials. For excited state properties, we use a real-space higher-order finite difference pseudopotential method within the time dependent local density approximation (TDLDA). The structures and spectra are in very good agreement with previous calculations and available experimental data. The absorption spectra show very sharp features (plasmon peaks) around 3 to 5 eV for clusters with even number of atoms as well as for Ag$_7$. In most cases, there are significant differences between the absorption spectra of the ground state structures and higher lying isomers, which could be used to identify the relevant structure in the experiments. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B25.00006: Metastability of a gold nano ring Qian Wang, Qiang Sun, Puru Jena Due to the unique electronic, optical and catalytic properties, it is believed that gold nano-structures can have important applications in catalysis, molecular electronics, sensors, bioconjugate probes, gene mapping, and in the treatment of cancer and tumours. First principles calculations based on gradient corrected density functional theory show that a cluster of as few as 90 gold atoms can be stabilized in a ring structure having FCC(111) motif with the binding energy/atom and inter-atomic distance approaching 91{\%} and 96{\%} of the bulk values respectively. Although the ring structure lies 0.139eV/atom higher in energy than a poly-icosahedral structure, the calculated frequencies are real. Thus under appropriate experimental conditions it may still be possible to synthesize a metastable form of gold nano ring, as found in the recent experiment. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B25.00007: Single-walled (5,3) gold nanotube-a theoretical study Chih-Kai Yang Total-energy calculations based on first principles reveal that a (5,3) gold nanotube has a helical pitch of 10.05 nm or a natural length of 20.1 Å per unit cell at the minimum total energy. The figure is about 9.5 {\%} shorter than the experimental pitch of 11.0 nm under stress from the experimental setup. Energy bands for both pitches are similar, giving five units of quantum conductance. The calculations indicate that the single-walled gold (5,3) nanotube is a robust and stable conducting wire suitable for nanoelectronic applications. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B25.00008: First-principles study of structure and energetics of supported Ru/Pt clusters on C(0001) and BN(0001) surfaces Lin-Lin Wang, Duane D. Johnson Using density functional methods, we have studied the properties of Ru and Ru/Pt clusters on C(0001) and BN(0001), for comparison to on-going experiments on supported clusters made from metal-organic complexes. We find that physical and chemical properties of the clusters are strongly affected by the cluster-substrate interaction, and, as cluster size increases, the preferred binding sites for the metal atoms in the cluster change. Detail comparisons are made for geometric, magnetic, and electronic properties of clusters on C(0001) and BN(0001) will be presented. The results shows that the Ru clusters interact stronger with C(0001) than BN(0001). We also compare the supported Ru clusters with a Ru monolayer of different coverages. Support from the Department of Energy Catalysis grant (DEFG02-03ER15476) and the NSF at the Materials Computation Center (DMR-0312448). [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B25.00009: Computational Study of Atomistic Strain Relaxation Mechanisms in Biaxially Strained Ultra-Thin Metallic Films M. Rauf Gungor, Dimitrios Maroudas Understanding the mechanical response under certain loading conditions of nanometer-scale-thick metallic films is fundamentally and technologically important. In this presentation, we report a comprehensive computational analysis on the atomistic mechanisms of strain relaxation under a wide range of applied biaxial tensile strain in free-standing ultra-thin Cu films based on multi-million-atom molecular-dynamics simulations. Our analysis reveals that after an elastic response at low strain ($<$ 2{\%}), plastic deformation occurs accompanied by dislocation emission from the void surface, void surface morphological transition, dislocation jogging, vacancy generation and migration along dislocation cores, as well as formation and propagation of threading dislocation loops from the film's surfaces. At higher ($>$ 6{\%}) strain levels, a transition to a new plastic deformation regime gives rise to a practically uniform distribution of dislocations in the metallic thin film. Under such conditions, dislocations are emitted from the thin film's surfaces and inhibit void growth due to their interactions with the dislocations emitted from the void surface and the resulting pinning of the latter defects. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B25.00010: Comparative study of CO adsorption on nanostructured Cu and Au surfaces Faisal Mehmood, Abdelkader Kara, Talat S. Rahman, Claude R. Henry The chemisorption of CO on Cu surfaces is regarded as a prototype system to understand molecular adsorption on transition metals surfaces. To understand the observed trends$^{1}$ in the site specific (flat, stepped, and kinked) adsorption energies of CO on Cu surfaces, we have performed a density functional theory based first principle electronic structure calculations on several low and high Miller index surfaces of Cu with CO adsorbed on-top site. Our calculated values show that adsorption energies increase with decrease in the local coordination of CO with the substrate, as seen in the experiment, although this dependence is not trivial. For in depth understanding of this, we have made a detailed study of electronic structure of these systems. Our calculated vibrational frequencies of CO-molecule on these surfaces are very similar, as expected. However, the frequency of the CO-metal stretch mode shows a small increase for the lower coordinated surfaces as compared to that on the low Miller index surfaces. Similar trends are also found for workfunctions, charge densities and the local density of states. Results will also be presented for CO adsorption on Au(111), Au(211), Au(532) and the behavior compared to that on the Cu surfaces. This work is supported in part by NSF Grant No. CHE0205064. $^{1}$S. Vollmer, G. Witte, C. W\"{o}ll, Cat. Lett., \textbf{77}, 97 (2001). [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B25.00011: Interaction of O$_2$ with Silver Cluster Cations Seung Bum Suh, Bokwon Yoon, Uzi Landman The interaction of oxygen molecules with silver cluster cations, Ag$_n^+$ for n=8-11, is investigated using the first- principles calculations based on the density functional theory with generalized gradient correction. The binding energy of single O$_2$, either molecularly or dissociatively adsorbed on Ag$_n^+$, exhibits an even-odd alternation as a function of n, where O$_2$ binds more strongly for even n. When two oxygen molecules are adsorbed, the dissociative adsorption of the first molecule favors the molecular adsorption of the second one. Analyses of the local density of states (LDOS) and the spatial distribution of the highest occupied molecular orbital (HOMO) are presented to explain our observations. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B25.00012: A DFT study of the effects of preparation conditions on the reactivity of Pt/$\alpha$-alumina catalysts Valentino R. Cooper, Andrew M. Rappe In this paper, we present a density functional theory study on how the presence of water products on the surface of an $\alpha$-alumina substrate affects the adsorption of Pt adatoms and dimers on the stoichiometric (Al$_T$) and oxygen terminated(O$_T$) surfaces. Studies have shown that the catalytic activity of oxide-supported clusters are sensitive to preparation conditions, with catalysts prepared through the impregnation of alumina with a metal ion solution exhibiting increased rates of reactivity. Here, we demonstrate that changes in the charge distribution at the metal-oxide interface and the partial pressures of water and Pt atoms affect the stability and adsorption sites of Pt adatoms and dimers to the alumina surface. Finally, we show that these differences have a significant effect on the electronic properties of the Pt adsorbates, thereby affecting the interaction of CO and NO at the metal surface. [Preview Abstract] |
Monday, March 21, 2005 2:03PM - 2:15PM |
B25.00013: First-principles study of ammonia synthesis on a supported iron nanocluster Zeljko Sljivancanin, Alfredo Pasquarello Possibility of ammonia synthesis on a MgO(100) supported iron nanocluster has been studied using density functional theory. The N$_2$ molecules were sequentially adsorbed on the Fe$_7$/MgO(100) and the most favorable adsorption geometries and dissociation pathways investigated. The first two N$_2$ molecules were found to dissociate easily upon adsorption, producing nitrogen atoms which bind strongly to the cluster. The third N$_2$ molecule adsorbs in a favorable molecular state without dissociating. Adding H atoms one by one to the adsorbed N$_2$ molecule we found that two H atoms are required to break the molecule and then, after adding another two H atoms, the first ammonia is formed. The process is accompanied with a small energy barrier and the microscopic mechanism is similar to the one proposed for the biological nitrogen fixation. [Preview Abstract] |
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B25.00014: Structure, Bonding, and Dynamics of Thiolates on Copper and Gold Clusters and Surfaces Martin Konopka, Roger Rousseau, Ivan Stich, Dominik Marx The interaction of alkanethiolates with small Cu and Au clusters and (111) surfaces was studied based DFT modeling with a focus on the metal--thiolate junction. Calculation of fragmentation energies indicate that for Cu$_n$--thiolate ($n=$ 1,3,...,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 $.The electronic origins of this weakening are attributed to a polarization of electron density in the S--C bond as induced by bonding with the Cu$_n$ cluster. For the gold analogues this effect is not observed. On the Cu(111) surface the metal to thiolate charge transfer leads to a non-direction partially ionic bonding with a concurrent flat adsorption energy landscape. As a result, occupation of {\sc fcc}-hollow,{\sc hcp}-hollow and {\sc fcc}-bridge sites is observed during 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] |
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