Session A6: Focus Session: van der Waals Bonding in Advanced Materials - Physisorption and Self Assembly on Metals

Application of vdW-DF Methods to Hydrogen Adsorptions and an Organic Ferroelectric

In recent years, several schemes have been proposed to include van der Waals (vdW) interactions into the framework of density functional theory (DFT). These offer the potential to extend the scope and usefulness of DFT, allowing applications to an entire new class of sparse materials. For finite systems, we validated our non-empirical van der Waals density functionals (vdW-DFs) with respect to very accurate quantum chemical calculations of the potential energy curves (PECs) for small molecular duplexes [1]. For extended systems, however, typical tests focus on comparisons with a few accessible observations, such as binding energy and bond length [1]. In this talk, we present a third approach in which full PECs from accurate experiments are used for the assessment of vdW methods for extended systems [2]. We calculate the PECs of the H$_2$ molecule and light atoms on the (100), (110), and (111) surfaces of Cu. The gas-surface-scattering experiments provide the rich data bank that covers results for the whole shape of the physisorption potentials. We also present an application of vdW-DF2 to an organic ferroelectric, phenazine-chloranilic acid [3]. In spite of extensive experimental efforts to characterize this rare organic material, the origin of the long-range order is unclear and even the ground state structure is not completely determined yet. We study its structure, energetics, ferroelectric properties, structural instability, and the proton-transfer process in it in comparison with PBE results and experiments wherever possible. \\[4pt]% [1] K.~Lee, E.~Murray, L.~Kong, B.~I.~Lundqvist, and D.~C.~Langreth, \textit{Phys. Rev. B} \textbf{82}, 081101 (2010). \\[1pt]% [2] K.~Lee, A.~K.~Kelkkanen, K.~Berland, S.~Andersson, D.~C.~Langreth, E.~Schr\"oder, B.~I.~Lundqvist, and P.~Hyldgaard, \textit{Phys. Rev. B} \textbf{84}, 193408 (2011). \\[1pt]% [3] K.~Lee, B.~Kolb, T.~Thonhauser, D.~C.~Langreth, and D.~Vanderbilt, \textit{in preparation}.   

Structure and Energetics of Benzene Adsorbed on Transition-Metal Surfaces: Density-Functional Theory with Screened van der Waals Interactions

The adsorption of benzene on metal surfaces is an important benchmark system for more complex hybrid inorganic/organic interfaces. Here, the recently developed DFT+vdW$\rm^{surf}$ method (density-functional theory including screened van der Waals (vdW) interactions) [1] is used to study the structure and energetics of benzene on transition-metal surfaces (Cu, Ag, Au, Pd, Pt, Rh, and Ir). Benzene adsorbs in a planar configuration at coinage metal surfaces, with almost zero distortion and a flat potential-energy surface. In contrast, benzene is strongly bound to the (111) surface of Pd, Pt, Rh, and Ir, and located at the bridge-30$^\circ$ site. The vdW interactions significantly enhance the binding energy by more than 0.75 eV for all metals. The screening of the vdW energy plays a critical role in coinage metals, shortening the equilibrium distance by 0.2 {\AA}, and lowering the binding energy by 0.25 eV. The validity of our results is confirmed by comparison with calculations using the random-phase approximation including renormalized single excitations (EX+cRPA+rSE scheme [2]), and the experimental data from temperature-programmed desorption and calorimetry measurements. [1] V. G. Ruiz-L\'{o}pez et al., submitted. [2] X. Ren et al., Phys. Rev. Lett. 106, 153003 (2011).   

Physisorption of three amine terminated molecules (TMBDA, BDA, TFBDA) on the Au(111) Surface: The Role of van der Waals Interaction

Recently, the electronic properties and alignment of tetramethyl-1,4-benzenediamine (TMBDA), 1,4-benzenediamine (BDA) and tetrafluro-1,4-benzenediamine (TFBDA) molecules were studied experimentally. Discrepancies were found for both the binding energy and the molecule tilt angle with respect to the surface, when results were compared with density functional theory calculations [1]. We have included the effect of vdW interactions both between the molecules and the Au(111) surface and find binding energies which are in very good agreement with experiments. We also find that at low coverages each of these molecules would adsorb almost parallel to the surface. N-Au bond lengths and charge redistribution on adsorption of the molecules are also analyzed. Our calculations are based on DFT using vdW-DF exchange correlation functionals. For BDA (since we are aware of experimental data), we show that for higher coverage, inclusion of intermolecular van der Waals interaction leads to tilting of the molecules with respect to the surface and formation of line structures. Our results demonstrate the central role played by intermolecular interaction in pattern formation on this surface.\\[4pt] [1] M. Dell'Angela et al, Nano Lett. 2010, 10, 2470; M. Kamenetska et al, J. Phys. Chem. C, 2011, 115, 12625   

Effect of physisorbed molecules and an external external fields on the metallic Shockley surface state of Cu(111): A density functional theory study

To manipulate the Cu(111) partially-filled Shockley surface state, we study its response to an external field\footnote{KB, TLE, PH; arXiv 1109:6706} $E$ and physisorbed PAHs and quinone molecules. We use density-functional theory calculations with periodic-boundary conditions. The van der Waals density functional version vdW-DF2 accounts for the molecular adsorption. The issue that the Kohn-Sham wave functions couple to both sides of the Cu slab is handled with a decoupling scheme based on a rotation in Hilbert space. A convergence study reveals that to obtain a proper Shockley surface state, 6 Cu layers is sufficient, while 15 is optimal. We use 6 layers for the response to the molecules and 15 to external field. We find that the surface state displays isotropic dispersion (up to order $k^6$), free-electron like until the Fermi wave vector but with a significant quartic component beyond. The shift in band minimum and effective mass depend linearly on $E$, with a smaller fractional change in the latter. Charge transfer occurs beyond the outermost copper atoms, and most of the screening is due to bulk electrons. We find that the molecular physisorption increases the band minimum, with the effect the of a quinone being much stronger than the corresponding PAH.   

Computational Study of Supramolecular Self-Assembly Using CH/$\pi$ Bonds

Self assembly is an important research area in supramolecular engineering. We show that CH/$\pi$ bonds can be exploited as a vehicle to assemble clusters of well-defined sizes on metal surfaces. Specifically, we theoretically explain the observations of largely uniform distribution of phenylacetylene magic clusters, each consisting of six molecules, on Au(111) surfaces. Using density functional theory with a van der Waals functional, we discuss the reasons for the preference of the hexamer structure, the key effect of CH/$\pi$ bonding on the self-assembly, and the critical role of the metal surface. Our calculated STM images and electronic properties are in good agreement with experiment. The cooperative, multi-center CH/$\pi$ interactions offer an attractive tunability via chemical functionalization, and thus may provide a new avenue towards rationally designing a desired supramolecular shape and size.   

Noble Gases on Metal Surfaces: New Insights on Adsorption Site Preference

Experiments have previously found that noble gases (Kr, Xe) adsorb on low-coordination atop sites on several different metal surfaces, rather than on high-coordination hollow sites. This unexpected preference for low-coordination sites has been previously ascribed to reduced Pauli repulsion mostly due to exchange energy at the atop site, based on density functional theory calculations within the local density approximation (LDA). In contrast, our calculations using non-local van der Waals (vdW-DF) density functional show that site preference is due to a delicate balance between the electrostatics which favor the hollow site and kinetic energy which favors the atop site; exchange-correlation energies has a very little role. Moreover, we find, using LDA, GGA, and vdW-DF functionals, that the hollow site is a saddle point of index 1 or 2 on the 2-dimensional potential energy surface, while the atop site is the only true minimum. Therefore, the reason that hollow site occupation is not observed is that it is a transition state and so has a very short life-time. Our results show that the inclusion of non-local vdW interactions is crucial for obtaining results in quantitative agreement with experiments for adsorption energies, equilibrium distances, and vibrational energies.   

Adsorption of polyvinylpyrrolidone on Ag surfaces: Insight into the workings of a structure-directing agent

We use density-functional theory to resolve the role of polyvinylpyrrolidone (PVP) as a structure-directing agent in the shape-selective synthesis of Ag nanostructures. We identify several different binding states for PVP segments on Ag(100) and Ag(111) and find an energetic preference for Ag(100), which arises from a surface-sensitive balance between direct binding and van der Waals attraction. At the chain level, correlated segment binding leads to a strong preference for PVP bind to Ag(100). Our study underscores differences between small-molecule and polymeric structure-directing agents.   

Adsorption of halogenated molecule on stepped metallic surfaces: the role of van der Waals interactions

The deposition of halogenated aromatic molecules on metallic surfaces is the first step for the bottom-up fabrication of atomically precise graphene nanoribbons [1]. Interest in the binding properties in this type of systems and in general in organic/inorganic interfaces has stimulated the inclusion of van der Waals (vdw) interactions within density functional theory (DFT). Using a fully non-local Van der Waals density functional (vdW-DF) [2,3] we have studied the adsorption of dichlorobenzene on stepped Au(332) and Pt(332) surfaces and on flat (111) surfaces. For several different adsorption sites, the energies, and equilibrium geometries have been computed, and electron density analysis has been performed using both conventional generalized gradient approximation (GGA) and vdW-DF. The two approaches yield qualitatively different results, highlighting the importance of non-local dispersions in this class of systems. The non-trivial role of steps edges on adsorption energies and geometries is also elucidated. [1] J. Cai \textit{et al}., Nature 466, 470 (2010). [2] M. Dion \textit{et al}., Phys. Rev. Lett. 92, 246401 (2004). [3] A. Gulans\textit{ et al}. Phys. Rev. B 79, 201105 (2009).   

The Nature of Binding in the Phenalenyl Dimer and its Derivatives

The biradical phenalenyl \(\pi\)-dimer and its derivatives have attracted interest recently because of their potentially useful electrical, optical, and magnetic properties. These properties can be tuned by adjusting the binding characteristics between the monomers within the dimer. Typically, this is done by substituting electron withdrawing or donating groups onto the \(\alpha\) or \(\beta\)-site carbons. An understanding of this binding lies at the heart of useful application of these materials. In this work, the binding characteristics of phenalenyl dimers were investigated using density functional theory. In particular, the vdW-DF functional was used to explore the role of van der Waals interactions in the binding within this system. A comparison of the binding curves with those of the closed shell derivatives wherein the central carbons have been replaced by either nitrogen or boron sheds light into the nature of the interactions between the monomers.   

Binding nature of adenine and C60 on graphene: a van der Waals density functional analysis

Based on van der Waals density functional theory (using vdW-DF1 and vdW-DF2), we study and analyze the adsorption of adenine\footnote{K. Berland et al; J. Phys.: Condens. Matter 23 135001 (2001)} and C60\footnote{A. Bergvall et al; Phys. Rev. B 84, 155451 (2011)}on graphene. Understanding molecular binding on graphene helps development of functionals because the infinite graphene shifts the balance between short-range and long-range contributions to binding as compared dimers or molecular crystals. The potential of graphene as contacts in single-molecule electronics also motivates the study of the interaction between aromatic molecules and graphene; the binding separation affects the magnitude of hopping parameters. We present results on binding energy and separation, vibrational states, overlayers, and charge transfer. We find that the hexagonal ring in C60 binds closer to the graphene sheet than what a flat molecule such as adenine does. The role played by the difference in geometry between the flat (adenine) and spherical (C60) shape is discussed.