Session H32: Focus Session: Computational Nanoscience III

Tuesday, March 14, 2006

Based on density functional theory with a planewave basis set, the stability, morphology and electronic properties of 2D boron sheets are studied. We suggest that a 2D boron sheet can be stable, and can posses metallic, half-metallic and semiconducting properties, depends on their distinct configurations. In contrast to previous studies, we predict the stability of a new novel 2D reconstructed $\{1221\}$ boron sheet over the idealized $\{1212\}$ triangular plane, together with their corresponding buckled configurations. Despite the high stability in cohesive energy, the unique features in geometry and electronic properties found in both configurations, suggest strong variations in electronic and mechanical properties, which might occur when the plane is rolled into different chiral structures of single-wall boron nanotubes.   

Tuesday, March 14, 2006

We use density functional theory (DFT) to study the relaxation of narrow Cu, Ni, Au, Pt and Ag nanowires originally oriented in the $<$001$>$ direction with a FCC structure. For a small enough diameter (D$<$2 nm) each nanowire, under the compressive influence of its own surface stress, spontaneously relaxes to either a $<$110$>$ orientation (Cu, Ni, Ag) or to a BCT $<$001$>$ (Au, Pt) orientation, both of which are characterized by a compression of the wire axis of at least 30{\%}. To analyze the stability of BCT structures, we calculate the elastic constants for the BCT phases of these metals under bulk, slab, and nanowire conditions. We find that the surface contribution to the elastic constant for shear, C$_{66}$, helps stabilize the BCT phase in Au which would otherwise be unstable under bulk conditions. A large stabilization contribution from the surface also occurs in Ni and Cu, but not enough to overcome the shear instability in the bulk, and these nanowires do not transform to BCT. We discuss the implications of these results on the superelastic shape-memory effect for FCC metals.   

Tuesday, March 14, 2006

Composite structures with high-k dielectrics are important future technologies for high-performance capacitors. We perform generalized-gradient approximation calculations to study interfacial properties of ceramic-polymer composites, focusing in particular on the strength of polymer adhesion to ceramic surfaces. Our results show that several polymers of interest do not bind directly to the ceramic. However, it is possible to functionalize the surface so that polymer attachment occurs. We present various possibilities for attachment to ceramic surfaces, which should lead to the formation of stable composites.   

Tuesday, March 14, 2006

Multiscale modeling is gaining an increasingly important role in guiding the fabrication of artificially structured nanomaterials~with atomic-scale precision and desirable physical properties. In this talk, two~recent examples will be presented to~illustrate~its predictive power in modern materials research. The modeling~approaches range from electronic-scale calculations~based on first principles~to mesoscopic-scale continuum elasticity theory. Specific examples include: (a)~fabrication of~ordered magnetic atom wires on non-magnetic~metal substrates; (b)~optimal dopant control~in dilute magnetic semiconductors via ``Subsurfactant Epitaxy.'' Emphasis will be made on the~substantially improved structure-property relationships achieved through such synergetic efforts between theory and experiment, including in the second example the striking observation of magnetic ordering temperatures well above 300 K.   

Tuesday, March 14, 2006

Metallic structures optimized for surface enhanced Raman scattering (SERS) must combine i) large dipole moments, for stronger coupling with the external field E, and ii) highly localized plasmon modes, which increase Raman Scattering cross sections (proportional to E4) and improve spatial resolution. Since typical molecules are much smaller than the visible wave lengths, the design of nanostructured surfaces for SERS involves an electro-dynamical problem analogous to the one solved with ear shapes in acoustics. In this limit, retardation effects can be ignored, which reduces the problem to the response at the metal surface. We have developed a finite element computational tool that allows the calculation of surface plasmons on arbitrary metallic shapes. We have found that an accurate description of the surface curvature is crucial. Our numerical results agree with exact analytical solutions for the quasi-static model known for spheres and spherical shells. Using this method, we have compared several shapes, where analytical results are not available, and uncover ear-shaped structures that are 12 orders of magnitude more efficient than spheres.   

Tuesday, March 14, 2006

While nanotribological simulations are generally performed for two opposing parallel surfaces, the Atomic Force Microscopy (AFM) experiments to which they are often compared measure the interactions between a curved probe tip and a sample. The parallel plate geometry cannot capture many effects seen in experiments, including load-dependent contact areas and molecular transfer of material from the substrate to the tip. We present the results of true dynamical nanotribological simulations of alkylsilane self-assembled monolayers (SAMs) with realistic tip/substrate geometries. Tips matching experimental dimensions (up to $\sim$~30 nm radius of curvature) were cut out of an amorphous silica substrate (a-SiO$_2$) and either coated with SAMs or annealed for uncoated tips. The adhesion and friction of the tip in contact with a SAM-coated amorphous a-SiO$_2$ substrate were studied with massively parallel molecular dynamics simulations. The effects of load-dependent contact areas are compared to previous simulations with flat plate geometries, and to AFM measurments. Conditions leading to tip fouling, and the effects on nanotribological measurements will also be discussed. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Departme nt of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.   

Tuesday, March 14, 2006

A multiscale approach has been applied to study the growth mechanism of carbon-based nanomaterials such as graphite flakes, fullerenes, nanotubes, and nanohorns. Especially, the role of defects during the growth of different morphologies will be discussed. At high temperatures, interesting structural transitions among given morphologies are observed when defects are introduced. The transitions are driven by entropy which plays a crucial role in determining the structural stability at these temperatures. The stability of large structures is also studied using molecular dynamics simulations and analytical calculations based on ab initio density functional techniques. We will further discuss the role of catalytic particles in determining the structural stabilities of experimentally observed nanomaterials.   

Tuesday, March 14, 2006

We performed ab-initio density functional simulations to study the structural and growth properties of short carbon nanobells. We used a real space approach and the linear combination of atomic orbitals (LCAO) formalism. In the nitrogen-doped carbon nanobells, the nitrogen atoms that are attracted to the open-edge sites of the carbon nanobells play an important role in the growth of the short carbon nanostructures. We also present the calculated electronic structure of the short nanobells. The calculated local density of states of the nanobells revealed field emission characteristics that agree with experimental observations. Acknowledgments: this work was funded in part by NASA (Award No. NCC 2-1344), NSF (Award No. 0508245), and ONR (Grant No: N00014-05-1-0009).   

Tuesday, March 14, 2006

Transition metal nanoparticles are the catalysis of choice for the growth of single-walled carbon nanotubes because of the high miscibility of carbon atoms with their supersaturated liquid droplets. This suggests that, at least for modeling the initial stage of growth, the droplets may be represented by the jellium medium and catalytic action of the droplets may be modeled by the electron redistribution for carbon atoms mediated by the medium. We have tested this scenario by comparing stable configurations of cage-structured C clusters in vacuum with those in jellium, with an average charge transfer from the jellium to C atoms n$_{av }\approx $ 0.2e to model the effect of nickel atoms. The structural optimization was carried out using the semi-empirical self-consistent and environment- dependent Hamiltonian in the framework of linear combination of atomic orbitals developed by our group [1]. Our results showed that the pentagons are more stable with substantial charge transfer occurrence in the defects in the vicinity of pentagons, consistent with previous results [2]. We also found that for a cage-structured cluster of a given size, increasing n$_{av}$ tends to elongate the hexagonal rings. Thus it appears that the catalytic action provided by Ni-droplet may be modeled by the charge fluctuations on C atoms mediated by the jellium. [1] S. Y. Wu et.al. \textit{Handbook of Material Modeling} Vol. I, p.2935 (2005). [2] X. Fan et. al. Phys. Rev. Lett. \textbf{90}, 145501 (2003).   

Tuesday, March 14, 2006

Within the current effort to understand and develop the organic functionalization of silicon surfaces, recent experiments have identified the radical chain reaction of unsaturated organic molecules with H-terminated silicon surfaces as a particularly promising route for controlled formation of such functionalized surfaces. Using periodic Density Functional Theory calculations, we theoretically study and characterize the basic steps of the radical chain reaction mechanism for different (conjugated and unconjugated) aldehyde molecules reacting with the H-Si(111) surface, under the assumption that a Si dangling bond is initially present on the surface. Molecular conjugation is found to play a crucial role in the viability of the reaction, by controlling the delocalization of the spin density at the reaction intermediate. Interesting differences are observed and discussed between our present results for aldehydes and our previous study for the reactions of alkene/alkyne molecules with H-Si(111) [Takeuchi, et al {\it J. Am. Chem. Soc.} {\bf 2004}, {\sl 126}, 15890.]   

Tuesday, March 14, 2006

Storage of hydrogen molecules is a research topic that has deep scientific interests and enormous applications in providing a clean type of energy for the future. Recent experimental breakthrough in encapsulating hydrogen molecules in fullerene C$_{60}$ promises a wider role for the versatile carbon cage in this regard. Probing further, we use density functional theory to study how the enclosed hydrogen molecules interact with the carbon cage. The calculations show that quite a few molecules can be exothermically inserted into the cage and as many as 35 hydrogen molecules can be stored inside without rupturing the cage structure.