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Van der Waals interactions in advanced materials, in memory of David C Langreth.
MedLine Citation:
PMID:  23032091     Owner:  NLM     Status:  In-Data-Review    
The past decade has seen a dramatic rise in interest in exploring the role that van der Waals (vdW) or dispersion forces play in materials and in material behavior. Part of this stems from the obvious fact that vdW interactions (and other weak forces, such as Casimir) underpin molecular recognition, i.e., nature's approach to search for a match between genes and anti-genes and hence enable biological function. Less obvious is the recognition that vdW interactions affect a multitude of properties of a vast variety of materials in general, some of which also have strong technological applications. While for two atom- or orbital-sized material fragments the dispersive contributions to binding are small compared to those from the better known forms (ionic, covalent, metallic), those between sparse materials (spread over extended areas) can be of paramount importance. For example, an understanding of binding in graphite cannot arise solely from a study of the graphene layers individually, but also requires insight from inter-sheet graphene vdW bonding. It is the extended-area vdW bonding that provides sufficient cohesion to make graphite a robust, naturally occurring material. In fact, it is the vdW-bonded graphite, and not the all-covalently bonded diamond, that is the preferred form of pure carbon under ambient conditions. Also important is the understanding that vdW attraction can attain a dramatic relevance even if the material fragments, the building blocks, are not necessarily parallel from the outset or smooth when viewed in isolation (such as a graphene sheet or a carbon nanotube). This can happen if the building blocks have some softness and flexibility and allow an internal relative alignment to emerge. The vdW forces can then cause increasingly larger parts of the interacting fragments to line up at sub-nanometer separations and thus beget more areas with a sizable vdW bonding contribution. The gecko can scale a wall because it can bring its flexible hairs sufficiently close to any corrugated-and/or any smooth-surface and thus enforce a strong vdW-type adhesion; it exploits what is then essentially a contact force (dominated by the attraction exerted in the near-surface regions) to defy the pull of gravity on its own bulk. This Journal of Physics: Condensed Matter special issue is dedicated to the memory of David C Langreth. David is a dearly missed friend and mentor who inspired many of us. He was an outstanding condensed matter theorist and a scholar who greatly influenced us through his many-particle-physics based insights into density functional theory (DFT), surface science and related areas. His seminal works range from conserving formulations of interacting nonequilibrium transport [1] and formal-scattering theory [2] to an explicit formulation [3] of the exact DFT exchange-correlation energy in the adiabatic connection formula (ACF), the latter also being derived independently by Gunnarsson and Lundqvist [4]. David's portfolio also includes an analysis [5] that helped catalyze and guide the development of DFT from the local-density approximation (LDA) to the formulations of generalized gradient approximations (GGAs). Another salient contribution of David's is in the area of vdW interactions in materials. He was a key architect of the vdW density functional (vdW-DF) method [6, 7]. This method was developed in a long-standing Rutgers-Chalmers collaboration between David's group and that of Bengt I Lundqvist, later extending to a wider group of researchers on both sides of the Atlantic. Plasmons are collective excitations that depend on electron-density variation. The plasmon response can be seen as defining the nature of the LDA [4] and their description can thus also be seen as contributing to the success of GGA. The vdW-DF method is a regular constraint-based density functional (for ground-state DFT) which is derived within the ACF framework and which emphasizes the electrodynamical nature of the coupling between these collective plasmon excitations. The vdW-DF method thus seeks to utilise the implicit plasmon nature of the LDA/GGA success to also provide a nonempirical account of the fully nonlocal correlations that underpin the vdW bond [8]. The method retains a seamless integration [6] with the semilocal density functional components. We know that David was pleased to see how this quantitative, material-specific theory helped address problems in a broad class of materials that are sparse, i.e., that contain low electron density regions which are of significant consequence in determining material characteristics [9]. In a DFT framework, the vdW-DF method supplements other nonlocal functional descriptions [10] and the DFT-dispersion class of methods, extending GGA-DFT with an atom-based asymptotic description which relies on the vdW interaction coefficients [11]. The recent quest to understand the role of vdW forces in materials has certainly been facilitated and intensified by the set of recent developments in DFT itself. This is because these vdW-aware methods provide an opportunity to deliver a computationally effective account of the quantum-physical behavior even when the materials are sparse. It is the expectation that with such new-found theoretical capability, one may further extend the immense progress already attained by traditional (GGA) DFT calculations in reliably predicting the characteristics of materials and phenomena in materials, nanoscience, chemistry and other related fields [12]. This special issue contains a wealth of exciting contributions, mostly on vdW forces in materials. The special issue was suggested following the well-attended first focused vdW and materials session arranged by the Division of Materials Physics (DMP) of the American Physical Society (APS) at the March Meeting in 2011. The sad and unexpected passing of David in the spring of 2011 caused some rethinking and at the memorial symposium, held by David's colleagues at Rutgers in November 2011, we proposed to dedicate this special issue to David's contributions to the field. We are delighted with the overwhelmingly positive reaction that we received in response to our call for papers. We are also delighted to now be able to bring so many exciting contributions to you. The papers included in this special issue focus, in general, either through experiments or through theoretical characterization, on material properties in which vdW forces represent a central component. These articles contain arguments for and against a broader usage of DFT with vdW to account for material description, as well as establishing benchmarks which measure progress in the field. This special issue also includes fundamental theoretical analysis and suggestions for sensitive experiments that can resolve outstanding issues underlying the nature and role of the interaction. We believe that these papers will help stimulate further material-theory developments and, even more importantly, more discussions and feedback between theory and experiment. Physisorption is important to enable lubrication. This is because physisorption keeps the lubricating molecules at the interface at which a pair of internal surfaces must be free to move relative to each other. In turn, physisorbed (or perhaps weakly chemisorbed) lubricants prevent, for example, the formation of stronger bonding that will impede mechanical operation. However, it is also important to further characterize the nature and dynamics of the vdW bonding of lubricants: while physisorption means a weak binding further from the surface, there are still important friction effects. Walker et al present an experimental study, along with theoretical analysis, that directly determines the frictional heating of a Kr overlayer on graphene through quartz crystal microbalance measurements. The properties of materials reflect their atomic structure and hence indirectly their bonding nature and character. We can explore the role of dispersion forces by examining the impact their inclusion has on predictions of material properties. On the other hand, the experimental and theoretical study of Casimir forces also plays a vital role in the exploration of material behavior. vdW forces are related to the Casimir force but lack retardation effects. While the vdW bonding depends on additional effects (for example, the multipole contributions that reflect image-plane effects), the study of Casimir forces provides direct (and not indirect) measurements of the nature of interactions. Klimchitskaya et al note that to reconcile explicit measurements of the Casmir forces between semiconductor fragments within the Lifshitz description, it is relevant to question the Drude-like description of the contributions from free carriers and instead proceed with a formulation based only on optical observations of the permittivity. This optical response can be seen as a reflection of a more plasmon-like behavior. The authors suggest an experiment involving the study of the impact on the Casimir forces by a Mott transition in doped semiconductors. Such an experiment would permit explicit testing of the validity of the present model for Casimir forces and hence provide additional perspectives on the nature of dispersive interactions. There is an effort to store energy, e.g. H(2) or CO(2), inside a range of open cage-like structures, such as metal organics frameworks (MOF) or clathrates. The fact that the internal molecular adhesion is dominated by vdW forces suggests that the storage and retrieval costs could perhaps be lowered as compared to an approach that involves more traditional chemical compounds. Nijem et al have provided spectroscopic characterization of vdW interactions of both hydrogen molecules and CO in a specific MOF. The study includes a vdW-DF calculation of structure and a theoretical prediction of expected infrared activity. (ABSTRACT TRUNCATED)
Martin Weinelt; Felix von Oppen
Publication Detail:
Type:  Journal Article     Date:  2012-10-03
Journal Detail:
Title:  Journal of physics. Condensed matter : an Institute of Physics journal     Volume:  24     ISSN:  1361-648X     ISO Abbreviation:  J Phys Condens Matter     Publication Date:  2012 Oct 
Date Detail:
Created Date:  2012-10-04     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101165248     Medline TA:  J Phys Condens Matter     Country:  England    
Other Details:
Languages:  eng     Pagination:  420201     Citation Subset:  IM    
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