Session B31: Focus Session: van der Waals Bonding in Advanced Materials: Applications to Systems and Behaviors

Physisorbed molecules: How their frictional and diffusive properties impact lubricity

Friction and its consequences are of great concern from both a national security and quality-of-life point of view, and the economic impact of energy efficiency, wear, and manufacturing cannot be underestimated [1]. Lubrication schemes for many macroscopic applications have been solved, but an era of science and engineering is emerging where control of mechanical and electrical systems at the atomic level will be required [2]. A fundamental understanding of the dissipative and frictional properties of weakly adsorbed films, which are ubiquitous in these systems, is key to a vast range of emerging applications. This talk will begin with a discussion of how diffusive and frictional properties of adsorbed atoms and molecules governed by van der Waals interactions can be measured experimentally [3]. Selected example of how atomic scale mobility in physisorbed materials, even at very low coverage, can directly impact friction, tribological performance and/or device viability will then be presented, for systems spanning Micro- and Nano- ElectroMechanical Systems to avalanches in granular materials [4,5]. \\[4pt] [1] ``Surface science and the atomic-scale origins of friction: what once was old is new again'', J. Krim, Surface Science \textbf{500} (1-3): 741-758, (2002)\\[0pt] [2] ``QCM tribology studies of thin adsorbed films'', J. Krim, Nano Today 2 (5): 38-43, (2007)\\[0pt] [3] ``Sliding friction measurements of molecularly thin ethanol and pentanol films: How friction and spreading impact lubricity'', B.P. Miller and J. Krim, J. Low. Temp. Phys., \textbf{157,} Special issue on Wetting, Spreading, and Filling, p 252 (2009)\\[0pt] [4] ``Friction, Force Chains and Falling Fruit '', J. Krim and R.P. Behringer, Physics Today, \textbf{62, }pp. 66-67 (Sept. 2009)\\[0pt] [5] ``Atomic-scale lubrication at ultra-low vapor coverages'', D.A. Hook, B.P. Miller, B.M. Vlastakis, M.T. Dugger and J. Krim, submitted   

Edge-dependent Static Friction of Adsorbed van der Waals Islands

Rare gas islands adsorbed through van der Waals forces on metal surfaces do not slide freely, but exhibit static friction in QCM experiments. Static friction appears, unexpectedly, even for incommensurate and defect-free crystal surfaces, where sliding should be frictionless. Via atomistic simulations of Kr islands on Au(111), we show that the island edges may be the ultimate culprits. Adsorbate sliding requires the flow of solitons - tiny density and corrugation modulations with the beat periodicity between the two periodicities. For an island, we find an edge-originated energy barrier that blocks the soliton flow, keeping the island pinned. As the static friction force is reached, the barrier vanishes at one point on the edge, and new solitons enter the island, which becomes depinned. Unsurprisingly, we find that low surface corrugation and high temperature facilitate this edge depinning. However, the island's thermal expansion is large and leads to changeable commensurability upon heating, which gives rise to the possibility of re-entrant static friction.   

Why viscosities of Ne and Kr monolayers are so different on the Pb(111) surface

Adsorption and segregation of Ne and Kr monolayers on the Pb(111) surface are examined through density functional calculations to understand the puzzling experimental observations of different tribological properties of these two rare gases. Theoretical results reveal weak but non-negligible interaction between rare gas and Pb(111), manifested as charge polarization and orbital intermixing. Because of its large atomic size, orbital polarizability and wave function extension, Kr binds with Pb(111) more strongly than does Ne. The activation energy of Kr segregation from the ground state hcp site to the metastable fcc site is 3.8 meV, substantially larger than that of Ne, 2.1 meV. This explains the drastic difference between the viscosities of Ne and Kr over Pb(111), observed at low temperatures using a quartz-crystal microbalance technique.   

Tribological Characterization of Nanoclustered Lead Films

For thin films of Pb on Ti, a system which does not wet, it is known that when studying coverages below the percolation transition measurement of surface friction via a sliding gas monolayer is an effective probe of electronic structure for the isolated lead nanoclusters.\footnote{Highland, M.\ et al.\, in preparation.} This technique is capable of studying superconductors as they pass through the transition temperature. Motivated by on-going reports of quantum size effects in thin lead films grown on Si(111)\footnote{Ozer, M.\ et al.\ {\it J.\ Low Temp Phys.} {\bf 2009}, 157: 221-251.} and Cu(001),\footnote{Li, W.\ et al.\ {\it Phys.Rev.B} {\bf 1993}, 48, 11: 8336-8344.} we examine the issue of nitrogen adsorption\footnote{Krim, J.and Widom, A.\ {\it Phys.\ Rev.\ B} {\bf 1988}, 38: 12184-12189.} onto such nanostructured films. Funding provided by NSF DMR.   

van der Waals bonded materials: vanadium pentoxide bulk structure

In this work we present a computational study of the layered oxide structure of vanadium pentoxide (V$_2$O$_5$) using the vdW-DF functional (M. Dion et al., Phys.Rev.Lett. \textbf{92}, 246401 (2004); T. Thonhauser et al., Phys. Rev. B \textbf{76}, 125112 (2007); K. Lee et al., Phys. Rev. B \textbf{82}, 081101 (2010)) which has proven to be able to capture the essential van der Waals interactions across matter separated by charge voids. We show that these forces play a substantial role for the description of the lattice constants and cohesion of this compound. In addition we document and handle a sensitivity to numerical noise in the evaluation of some exchange versions used with nonlocal correlation.   

Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride

The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to anchor the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which band gap modulations of ~0.6 eV are obtained. We propose a simple geometric model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes. Reference: Marom et al., Phys. Rev. Lett. 105, 046801 (2010).   

van der Waals binding and band structure effects in graphene overlayers and graphane multilayers

We study graphene formation (by selective Si evaporation) and adhesion on SiC surfaces as well as stacking and binding of graphane multilayers [1] using a number of versions of the van der Waals Density Functional (vdW-DF) method [2] and plane-wave density functional theory calculations. For the graphene/SiC systems and for the graphane multilayers we document that the bonding is entirely dominated by van der Waals (vdW) forces. At the same time we find that dispersive forces acting on the layers produce significant modifications in the graphene and graphane band structure. We interpret the changes and discuss a competition between wave function hybridization and interaction with the charge enhancement (between the layers) that results from density overlap. \\[4pt] [1] J. Rohrer and P. Hyldgaard, http://arxiv.org/abs/1010.2925\\[0pt] [2] Dion et al, PRL 92, 246401 (2004); V.R.Cooper, PRB 81, 161104(R) (2010), K. Lee et al PRB 82, 081101(R) (2010).   

Noble gas adsorption on carbon nanotubes: insight from a van der Waals density functional study

Adsorption of nobel gases (Ar, Kr, Xe) on metallic and semiconducting carbon nanotubes (CNTs) is investigated using the van der Waals density functional (vdW-DF) developed by the Lundqvist and Langreth groups. Standard local and semi-local density functional methods do not describe nonlocal dispersive forces and fail in these systems. We found that the noble gases are underbound or even unbound with the generalized gradient approximation, while the bonding distance is underestimated at the local density approximation level of theory. In contrast, the vdW-DF approach gives considerable improvement in the description of the adsorption energies. We found no difference in the adsorption between the metallic and semiconducting nanotubes, indicating that the adsorption energies for rare gases on carbon nanotube are not strongly influenced by differences in the electronic structure of the nanotubes. The adsorption energies predicted from classical potentials are smaller than those from vdW-DF calculations by about 10-35\%.   

Water droplet distributions in pure and functionalized single walled carbon nanotubes

In this study we investigated water droplet distribution in (1-2 nm diameter and 30 $\mu $s long) single walled carbon nanotubes (SWCNT) using time domain nuclear magnetic resonance (NMR). Annealing SWCNTs at 400\r{ }C resulted in a carbon nanotube with closed ends. We attached various amounts of water on the annealed SWCNT samples and measured the NMR spin-spin relaxation (T$_{2}$ ) distribution profile. The T$_{2}$ distributions were analyzed using the inverse Laplace transform to estimate the amount of water attached to the SWCNT. We performed NMR measurements on water distributions in pure CNT and functionalized CNT with OH and COOH radicals. The T$_{2}$ distribution curves for pure and functionalized SWCNTs show significant difference in water attachment. We also studied water distribution profile with the SWCNTs annealed at 800\r{ }C. Annealing at 800\r{ }C opens the ends of the SWCNTs. T$_{2}$ distribution curves at 400\r{ }C and 800\r{ }C will be compared to obtain the amount of free water attached on the outer and inner surface of pure and functionalized SWCNTs.   

How do hybrid functionals, dispersion interactions and quantum nuclei affect the structure of liquid water?

We report {\it ab-initio} molecular dynamics simulations of liquid water at STP and at the volume corresponding to experimental equilibrium density. These simulations are based on the hybrid functional PBE0 for the electrons and include approximate dispersion interactions according to Ref \footnote{A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. {\bf 102}, 073005 (2009).}. Nuclear quantum corrections were included as estimated by Ref \footnote{J. Morrone and R. Car, Phys. Rev. Lett. {\bf 101 }, 017801 (2008).}. We find that all of these components are important to significantly improve the agreement of the simulated structure with recent experimental analyses based on neutron and X-ray diffraction\footnote{ A. Soper and C. Benmore, Phys. Rev. Lett. {\bf 101 }, 065502(2008).} and on NMR experiments. \footnote{K. Modig, B. Pfrommer, B Halle, Phys. Rev. Lett. {\bf 90 }, 075502 (2003).}   

Density, structure and dynamics of water: the effect of Van der Waals interactions

We present a DFT AIMD study of liquid water using several GGA functionals as well as the van der Waals density functional (vdW-DF) of Dion et al. [PRL 92, 246401(2004)]. As expected, we find that the density of water is grossly underestimated by GGA functionals. When a vdW-DF is used, the density improves drastically and the experimental diffusivity is reproduced without the need of thermal corrections. We analyze the origin of the density differences between all the functionals. We show that the vdW-DF increases the population of non-H-bonded interstitial sites, at distances between the first and second coordination shells. However, it excessively weakens the H-bond network, collapsing the second coordination shell. This structural problem is partially associated to the choice of GGA exchange in the vdW-DF. We show that a different choice for the exchange functional is enough to achieve an overall improvement both in structure and diffusivity. Jue Wang et al. J. Chem. Phys, 133, (2010).   

Van der Waals Density Functional Simulations of Liquid Water

We compare two versions of van der Waals density functionals (DRSLL [1], LMKLL [2]) in electronic structure computations of weakly bonded systems. The functionals are implemented in the Qbox code [3] and are verified by reproducing published binding energies and equilibrium separations of several weakly bonded dimers. Vibrational frequencies of the water monomer and dimer computed using the above van der Waals functionals are not improved compared to PBE results. We present results of molecular dynamics simulations of liquid water using the DRSLL and LMKLL functionals and compare radial distribution functions with corresponding results obtained with GGA functionals.\\[4pt] [1] M. Dion et al. Phys. Rev. Lett. 92, 246401 (2004).\\[0pt] [2] K. Lee et al. Phys. Rev. B 82, 081101 (2010).\\[0pt] [3] http://eslab.ucdavis.edu/software/qbox   

Ice under pressure: the role of van der Waals forces in hydrogen bonding

We will discuss the evolution of the role of van der Waals interactions in hydrogen bonding in high pressure phases of ice. Here, we compare first principles results of the structural and electronic properties of ice using several different approaches to the calculation of exchange and correlation energies. These include the non-local vdW density functional of Dion \textit{et al}\footnote{M. Dion, H. Rydberg, E. Schr\"oder and D. Langreth, Phys. Rev. Lett. \textbf{92}, 246401 (2004)}, the revised vdW density functional of Lee \textit{et al}\footnote{K. Lee, \'E. D. Murray, L. Kong, B. I. Lundqvist and D. C. Langreth, Phys. Rev. B \textbf{82}, 081101 (2010)} and the EXX/RPA approach based on an eigenvalue representation of the dielectric matrix\footnote{D. Lu, Y. Li, D. Rocca and G. Galli, Phys. Rev. Lett. \textbf{102}, 206411 (2009)} along with the semilocal functional PBE and hybrid functional PBE0.