Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A5: Focus Session: Van der Waals Bonding in Advanced Materials - Materials Behavior |
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Sponsoring Units: DMP Chair: Roberto Car, Princeton University Room: 301 |
Monday, March 18, 2013 8:00AM - 8:12AM |
A5.00001: Beyond RPA correlation energies: Evaluation of model exchange-correlation kernels Deyu Lu The adiabatic-connection fluctuation-dissipation theorem (ACFDT) has drawn considerable attention in describing van der Waals (vdW) dispersion interactions. Under the random phase approximation (RPA), the EXX/RPA method yields the correct asymptotic behavior at large distances. However, for many advanced materials, e.g., organic/inorganic interfaces and molecular crystals, it is important to capture the short-range dispersion interaction within several angstrom. Because RPA pair distribution function is incorrect at short distances, the contribution of the exchange-correlation kernel has to be included properly. In this work, we implemented several model exchange-correlation kernels in the framework of ACFDT. Special attention was paid to develop non-local kernels suitable for inhomogeneous electronic systems. The performance of the exchange-correlation kernels were evaluated for both bulk and molecular systems. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A5.00002: Long-range van der Waals interaction between nanoclusters Jianmin Tao, John Perdew, Adrienn Ruzsinszky van der Waals (vdW) interaction is an important long-range correlation that affects many properties of materials. However, this effect cannot be accurately accounted for by first-principles calculations, due to computational challenges. Recently, we have developed a model for the vdW coefficients between quasispherical clusters such as fullerenes, sodium and silicon clusters. Our study shows that the widely-used atom-pairwise interaction picture surprisingly breaks down. A quick remedy of this problem leads to a counterintuitive scaling law of the vdW coefficients for caged molecules and clusters. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A5.00003: Binding and Diffusion of Lithium in Graphite: Quantum Monte-Carlo benchmarks and validation of van der Waals density functional methods Paul Kent, Panchapakesan Ganesh, Mina Yoon, Jeongnim Kim, Fernando Reboredo Benchmark diffusion quantum monte-carlo (DMC) studies of the adsorption and diffusion of atomic lithium in graphite are compared with density functional theory (DFT) calculations using several van der Waals methods. The charge transfer is captured adequately with conventional local density functionals. At fixed geometries, these yield surprisingly accurate energetics. In unconstrained geometries, van der Waals corrections are required to correctly reproduce graphite and lithium binding. We find that the empirical method of Grimme et al. only gives correct diffusion barriers when the Li polarizability is reduced to nearly zero, consistent with the charge transfer in the solid-state environment. The Tkatchenko-Scheffler scheme captures the polarizability reduction, yielding accurate results at low computational cost. The self-consistent vdw-DF2 functional yields the best overall results but at increased cost. Slight differences in barrier heights remain with all the DFT approaches compared to the DMC. These results establish a hierarchy of modeling approaches for the lithium-carbon system. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A5.00004: Probing van der Waals Forces at the Single-Molecule Level Invited Speaker: Latha Venkataraman Single molecule junctions represent an attractive platform to understand and control functionality of materials and devices at the nanoscale. While their electronic transport properties have received tremendous attention thus far, measurements of mechanics are new and allow for a more complete understanding of the structure-function relationship of these atomic scale devices. Here we report simultaneous measurement of force and electrical conductance across Au-Bipyridine-Au junctions using a conducting atomic force microscope (AFM). We show that these junctions have two distinct structures each with a characteristic conductance. Using statistically relevant analysis, these two structures are found to have very different mechanical properties. Specifically, we find that the higher conductance junctions have a significantly larger rupture force and stiffness than the lower conducting junctions. They also have a larger rupture force than Au single-atom point contacts, suggesting multiple points of contact. Using density functional theory simulations we show that van der Waals (vdW) interactions between the pyridine ring and Au electrodes plays a key role in the junction mechanics. These measurements thus provide a quantitative characterization of vdW interactions at metal/organic interfaces at the single-molecule level~[1].\\[4pt] [1] Aradhya, S. V., Frei, M., Hybertsen, M. S. {\&} Venkataraman, L., Nature Materials, 11, 872-876 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A5.00005: Liquid water from first principles: The importance of exact exchange, dispersion interactions, and nuclear quantum effects Robert DiStasio, Zhaofeng Li, Biswajit Santra, Xifan Wu, Roberto Car Quantitative agreement between theory and experiment on the structure of liquid water at ambient conditions has been quite difficult to achieve to date. In this work, we report that highly accurate {\it ab initio} molecular dynamics simulations of liquid water that account for exact exchange (via the hybrid PBE0 functional [PRB {\bf 79}, 085102 (2009)]), dispersion interactions [PRL {\bf 102}, 073005 (2009)], and nuclear quantum effects (presently approximated by a 30K increase in the simulation temperature) result in excellent agreement with experiments [PRL {\bf 101}, 065502 (2008)]. The importance of each of these effects in the theoretical prediction of the structure of liquid water will be demonstrated by a detailed comparative analysis of the predicted and experimental oxygen-oxygen radial distribution functions. In addition, we will discuss the connection between the experimentally observed scattering intensity, I(k), and the final radial distribution function, g(r), via the structure and form factors. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A5.00006: Spatially Resolved Raman Spectroscopy of Single- and Few-Layered WS$_2$ Ayse Berkdemir, Humberto R. Gutierrez, Andres R. Botello-Mendez, Nestor Perea-Lopez, Ana L. El\'Ias, Cheng-Ing Chia, Bei Wang, Vincent H. Crespi, Florentino Lopez-Urias, Jean-Christophe Charlier, Humberto Terrones, Mauricio Terrones We systematically investigated the Raman scattering of single- and few-layered WS$_{2}$ as a function of the number of S-W-S layers and the excitation laser wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A$_{1g}(\Gamma )$ phonon mode monotonically decreases with the number of layers, while the E$^{1}_{2g}(\Gamma )$ frequency increases. For single-layer WS$_{2}$, 514.5 nm excitation generates a second-order Raman resonance for the longitudinal acoustic mode at the M point. This 2LA(M) resonance results from a double-resonant Raman coupling between the electronic band structure and lattice vibrations, an effect not previously seen in any single-layered metal dichalcogenide. We performed ab initio calculations to determine the electronic and phonon band structures of single-layer and bulk WS$_{2}$, these results were used to compute the reduced intensity of the 2LA mode from the fourth order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS$_{2}$ islands. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A5.00007: Extraordinary room-temperature photoluminescence in WS$_{2}$ monolayers Humberto Rodriguez Gutierrez, Nestor Perea-L\'opez, Ana Laura El\'Ias, Ayse Berkdemir, Bei Wang, Ruitao Lv, Florentino L\'opez-Ur\'Ias, Vincent Crespi, Humberto Terrones, Mauricio Terrones Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from their bulk layered counterpart. Here we describe the direct synthesis of WS$_{2}$ monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S-W-S layers is also reported. The PL becomes weaker with the increase of S-W-S layers number due to a transition from direct (in a monolayer) to indirect band gap (in multilayers). The edges of WS$_{2}$ monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than the platelets center. The structure and composition of the platelet edges appear to be critical for the PL enhancement effect. These novel 2D nanoscale light sources could find diverse applications including the fabrication of flexible/transparent/low-energy optoelectronic devices [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A5.00008: How van der Waals Interactions Influence Cohesive Properties of Non-Metallic Solids Guo-Xu Zhang, Anthony M. Reilly, Alexandre Tkatchenko, Matthias Scheffler Standard semilocal and hybrid density functionals are widely used for studying cohesive properties of covalent, metallic, and ionic materials. Only recently it has been recognized that long-range van der Waals (vdW) interactions, that are missing in all semilocal and hybrid functionals, are important for an accurate description of cohesion in solids. Here we construct a database of 64 solids where reference cohesive properties are obtained from a critical revision of the available experimental data. All-electron DFT calculations with explicit treatment of zero-point vibrations for all cohesive properties are performed using the LDA, PBE, and the empirical meta-GGA M06-L [1] functionals. For 23 semiconductors, we carry out PBE and M06-L calculations with the inclusion of screened long-range vdW energy [2]. We find that PBE is the most systematic from the three employed functionals, and its accuracy is improved by a factor of two after the inclusion of vdW interactions. The LDA functional considerably overbinds for all the studied solids. The M06-L functional describes middle-range correlation better for certain semiconductors and ionic crystals, but fails for heavier semiconductors and metals.\\[4pt] [1] Zhao and Truhlar, JCP (2006).\\[0pt] [2] Tkatchenko, DiStasio, Car, Scheffler, PRL (2012). [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A5.00009: Analyzing the vdW-DF description of binding mechanisms: Comparison of C60 and benzene adsorption on graphene Kristian Berland, Per Hyldgaard There has been several efforts to improve the accuracy of the description of sparse matter problems like molecular adsorption on surfaces using non-local correlation functionals. We have explored the vdW-DF~[PRL 92, 246401 (2004)] interaction at different length scales, density regimes, density gradients, and for different system. As test calculations, we compare the potential energy curves of benzene and C60 on graphene and related systems like boron nitride because these reveal the role of geometry and band gap on the functional components. Our analysis is facilitated by explicit control of cutoff parameters in our real-space evaluation of the non-local correlation. We find that vdW-DF is very sensitive to the low density regions, but more so for the original version than in the newer one, vdW-DF2 [PRB 82, 081101 (2010)]. Our results also illustrate that a transferable account of many different geometries requires an accurate account of all length scales involved in the problem. These results are discussed in light of the functional form of vdW-DF. We also show how functional choices greatly affects corrugation. Finally, we examine the role of induced dipoles on the adsorption. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A5.00010: Bi-layer excitons in two-dimensional layered materials Mahesh Neupane, Gen Yin, Darshana Wickramaratne, Roger Lake Following the prediction of exciton condensation in closely spaced two-dimensional electron-hole bilayer systems [1], there has been a sustained theoretical and experimental investigation of this condensation phase in coupled quantum well material systems. The electron-hole pairs are bound by the interlayer Coulomb interaction, which is tuned by electrostatic gating of the charge density [2]. The magnitude of this interaction is determined by the binding energy between the electron and the hole. Improvements in the exciton binding energy can be achieved by an appropriate choice of materials. The family of van der Walle materials is considered in this study, and the effect of material choice and insulating layer thickness on the excitonic properties will be discussed and compared to experimental investigations using traditional GaAs-AlGaAs coupled quantum wells.\\[4pt] [1] Y. Lozovik and V. Yudson, JETP Letters, vol. 22, 1975\\[0pt] [2] J. Shumway and M.J. Gilbert, Phys. Rev. B., vol. 85, no. 3, 2012 [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A5.00011: Effects of interatomic potentials on mechanical deformation of glasses Wei-Ren Chen, Takuya Iwashita, Takeshi Egami Apparently glasses behave like an elastic solid, which shows a linear relationship between stress and strain in mechanical deformation. However the understanding of the mechanical response of glasses remains elusive because of structural disorder. Mechanical deformation of monatomic model glasses was studied using athermal quasi-static shear (AQS) simulation and with three different potentials. As the interatomic potentials we employed the 12-6 Lennard-Jones (LJ) potential, modified Johnson (mJ) potential, and Dzugutov (Dz) potential, respectively. For mJ and Dz glasses the shear modulus keeps constant below a critical strain, below which it decreases rapidly or discontinuously with strain. Such changes in shear modulus were mostly related to the change in local topology of atomic connectivity or anelasticity. In contrast LJ glass shows a gradual decrease in shear modulus in a continuous manner. The results indicated that the difference arises from the nature of the potentials if the topology of atomic connectivity can be clearly defined. [Preview Abstract] |
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