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Sample records for der waals interactions

  1. Imaging van der Waals Interactions.

    PubMed

    Han, Zhumin; Wei, Xinyuan; Xu, Chen; Chiang, Chi-Lun; Zhang, Yanxing; Wu, Ruqian; Ho, W

    2016-12-15

    The van der Waals interactions are responsible for a large diversity of structures and functions in chemistry, biology, and materials. Discussion of van der Waals interactions has focused on the attractive potential energy that varies as the inverse power of the distance between the two interacting partners. The origin of the attractive force is widely discussed as being due to the correlated fluctuations of electron charges that lead to instantaneous dipole-induced dipole attractions. Here, we use the inelastic tunneling probe to image the potential energy surface associated with the van der Waals interactions of xenon atoms.

  2. Five-body van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Han, Jianing

    2017-06-01

    We report on the five-body repulsive and attractive van der Waals interactions between the strongly dipole-dipole coupled Rydberg states. Compared to four-body van der Waals interactions, five-body van der Waals interactions show more energy levels and more potential wells caused by avoided crossings. This research bridges the few-body physics and many-body physics. Other disciplines, such as chemistry, biology, and medical fields, will also benefit from better understanding van der Waals interactions.

  3. Scaling laws for van der Waals interactions in nanostructured materials.

    PubMed

    Gobre, Vivekanand V; Tkatchenko, Alexandre

    2013-01-01

    Van der Waals interactions have a fundamental role in biology, physics and chemistry, in particular in the self-assembly and the ensuing function of nanostructured materials. Here we utilize an efficient microscopic method to demonstrate that van der Waals interactions in nanomaterials act at distances greater than typically assumed, and can be characterized by different scaling laws depending on the dimensionality and size of the system. Specifically, we study the behaviour of van der Waals interactions in single-layer and multilayer graphene, fullerenes of varying size, single-wall carbon nanotubes and graphene nanoribbons. As a function of nanostructure size, the van der Waals coefficients follow unusual trends for all of the considered systems, and deviate significantly from the conventionally employed pairwise-additive picture. We propose that the peculiar van der Waals interactions in nanostructured materials could be exploited to control their self-assembly.

  4. Van der Waals Interactions Involving Proteins

    NASA Technical Reports Server (NTRS)

    Roth, Charles M.; Neal, Brian L.; Lenhoff, Abraham M.

    1996-01-01

    Van der Waals (dispersion) forces contribute to interactions of proteins with other molecules or with surfaces, but because of the structural complexity of protein molecules, the magnitude of these effects is usually estimated based on idealized models of the molecular geometry, e.g., spheres or spheroids. The calculations reported here seek to account for both the geometric irregularity of protein molecules and the material properties of the interacting media. Whereas the latter are found to fall in the generally accepted range, the molecular shape is shown to cause the magnitudes of the interactions to differ significantly from those calculated using idealized models. with important consequences. First, the roughness of the molecular surface leads to much lower average interaction energies for both protein-protein and protein-surface cases relative to calculations in which the protein molecule is approximated as a sphere. These results indicate that a form of steric stabilization may be an important effect in protein solutions. Underlying this behavior is appreciable orientational dependence, one reflection of which is that molecules of complementary shape are found to exhibit very strong attractive dispersion interactions. Although this has been widely discussed previously in the context of molecular recognition processes, the broader implications of these phenomena may also be important at larger molecular separations, e.g., in the dynamics of aggregation, precipitation, and crystal growth.

  5. Van der Waals interactions involving proteins.

    PubMed Central

    Roth, C M; Neal, B L; Lenhoff, A M

    1996-01-01

    Van der Waals (dispersion) forces contribute to interactions of proteins with other molecules or with surfaces, but because of the structural complexity of protein molecules, the magnitude of these effects is usually estimated based on idealized models of the molecular geometry, e.g., spheres or spheroids. The calculations reported here seek to account for both the geometric irregularity of protein molecules and the material properties of the interacting media. Whereas the latter are found to fall in the generally accepted range, the molecular shape is shown to cause the magnitudes of the interactions to differ significantly from those calculated using idealized models, with important consequences. First, the roughness of the molecular surface leads to much lower average interaction energies for both protein-protein and protein-surface cases relative to calculations in which the protein molecule is approximated as a sphere. These results indicate that a form of steric stabilization may be an important effect in protein solutions. Underlying this behavior is appreciable orientational dependence, one reflection of which is that molecules of complementary shape are found to exhibit very strong attractive dispersion interactions. Although this has been widely discussed previously in the context of molecular recognition processes, the broader implications of these phenomena may also be important at larger molecular separations, e.g., in the dynamics of aggregation, precipitation, and crystal growth. Images FIGURE 3 PMID:8789115

  6. Modern theory of van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Dobson, John

    2014-03-01

    van der Waals (vdW, dispersion) interactions are important in diverse areas such as colloid, surface and nano science, cohesion of molecular crystals, and biomolecular science. They also provide competition in experiments to discover the fifth fundamental force.While vdW interactions have been understood in principle for a century, their quantitative first-principles prediction and modelling down to chemical contact separations have proven stubbornly difficult because the quantal many-electron problem is involved. After some brief historical material, the current state of the art will be discussed with particular reference to several approaches: pairwise additive, perturbative quantum chemical, vdW-DF, Lifshitz-like scattering, RPA-like, Adiabatic Connection Fluctuation Dissipation / Time Dependent DFT based etc.. A potentially useful classification will be introduced to aid in understanding the physical causes of departures from pairwise additivity, that is from the usual sum of C6R-6 contributions. These departures result in non-standard power law decays of nanostructure vdW interactions as a function of separation D, as well as surprising dependences of the attraction on the number, N, of atoms within each vdW-interacting fragment. Some further recent results on non-additivity will also be presented. Work supported by an Australian Research Council Discovery Grant.

  7. Van der Waals Interactions in Aspirin

    NASA Astrophysics Data System (ADS)

    Reilly, Anthony; Tkatchenko, Alexandre

    2015-03-01

    The ability of molecules to yield multiple solid forms, or polymorphs, has significance for diverse applications ranging from drug design and food chemistry to nonlinear optics and hydrogen storage. In particular, aspirin has been used and studied for over a century, but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate in terms of lattice energy, kinetic effects have been suggested to determine the metastability of the less abundant form II. Here, first-principles calculations provide an alternative explanation based on free-energy differences at room temperature. The explicit consideration of many-body van der Waals interactions in the free energy demonstrates that the stability of the most abundant form of aspirin is due to a subtle coupling between collective electronic fluctuations and quantized lattice vibrations. In addition, a systematic analysis of the elastic properties of the two forms of aspirin rules out mechanical instability of form II as making it metastable.

  8. Materials perspective on Casimir and van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Woods, L. M.; Dalvit, D. A. R.; Tkatchenko, A.; Rodriguez-Lopez, P.; Rodriguez, A. W.; Podgornik, R.

    2016-10-01

    Interactions induced by electromagnetic fluctuations, such as van der Waals and Casimir forces, are of universal nature present at any length scale between any types of systems. Such interactions are important not only for the fundamental science of materials behavior, but also for the design and improvement of micro- and nanostructured devices. In the past decade, many new materials have become available, which has stimulated the need for understanding their dispersive interactions. The field of van der Waals and Casimir forces has experienced an impetus in terms of developing novel theoretical and computational methods to provide new insights into related phenomena. The understanding of such forces has far reaching consequences as it bridges concepts in materials, atomic and molecular physics, condensed-matter physics, high-energy physics, chemistry, and biology. This review summarizes major breakthroughs and emphasizes the common origin of van der Waals and Casimir interactions. Progress related to novel ab initio modeling approaches and their application in various systems, interactions in materials with Dirac-like spectra, force manipulations through nontrivial boundary conditions, and applications of van der Waals forces in organic and biological matter are examined. The outlook of the review is to give the scientific community a materials perspective of van der Waals and Casimir phenomena and stimulate the development of experimental techniques and applications.

  9. Materials perspective on Casimir and van der Waals interactions

    SciTech Connect

    Woods, L. M.; Dalvit, D. A. R.; Tkatchenko, A.; Rodriguez-Lopez, P.; Rodriguez, A. W.; Podgornik, R.

    2016-11-02

    Interactions induced by electromagnetic fluctuations, such as van der Waals and Casimir forces, are of universal nature present at any length scale between any types of systems. In such interactions these are important not only for the fundamental science of materials behavior, but also for the design and improvement of micro- and nanostructured devices. In the past decade, many new materials have become available, which has stimulated the need for understanding their dispersive interactions. The field of van der Waals and Casimir forces has experienced an impetus in terms of developing novel theoretical and computational methods to provide new insights into related phenomena. The understanding of such forces has far reaching consequences as it bridges concepts in materials, atomic and molecular physics, condensed-matter physics, high-energy physics, chemistry, and biology. Our review summarizes major breakthroughs and emphasizes the common origin of van der Waals and Casimir interactions. Progress related to novel ab initio modeling approaches and their application in various systems, interactions in materials with Dirac-like spectra, force manipulations through nontrivial boundary conditions, and applications of van der Waals forces in organic and biological matter are examined. Finally, the outlook of the review is to give the scientific community a materials perspective of van der Waals and Casimir phenomena and stimulate the development of experimental techniques and applications.

  10. Materials perspective on Casimir and van der Waals interactions

    DOE PAGES

    Woods, L. M.; Dalvit, D. A. R.; Tkatchenko, A.; ...

    2016-11-02

    Interactions induced by electromagnetic fluctuations, such as van der Waals and Casimir forces, are of universal nature present at any length scale between any types of systems. In such interactions these are important not only for the fundamental science of materials behavior, but also for the design and improvement of micro- and nanostructured devices. In the past decade, many new materials have become available, which has stimulated the need for understanding their dispersive interactions. The field of van der Waals and Casimir forces has experienced an impetus in terms of developing novel theoretical and computational methods to provide new insightsmore » into related phenomena. The understanding of such forces has far reaching consequences as it bridges concepts in materials, atomic and molecular physics, condensed-matter physics, high-energy physics, chemistry, and biology. Our review summarizes major breakthroughs and emphasizes the common origin of van der Waals and Casimir interactions. Progress related to novel ab initio modeling approaches and their application in various systems, interactions in materials with Dirac-like spectra, force manipulations through nontrivial boundary conditions, and applications of van der Waals forces in organic and biological matter are examined. Finally, the outlook of the review is to give the scientific community a materials perspective of van der Waals and Casimir phenomena and stimulate the development of experimental techniques and applications.« less

  11. Description of van der Waals interactions using transformation optics.

    PubMed

    Zhao, Rongkuo; Luo, Yu; Fernández-Domínguez, A I; Pendry, J B

    2013-07-19

    Exact calculation of the van der Waals interaction between closely spaced plasmonic nanoparticles is challenging due to the strong concentration of the electromagnetic fields that takes place at the nanometric gap between them. The technique of transformation optics, capable of mapping a small volume into any desired length scale, enables us to shed physical insight into the intricate behavior of electromagnetic fields in extremely small gaps. Using this theoretical tool, we obtain universal analytical expressions for the van der Waals interactions between spherical nanoparticles made of realistic metals at arbitrary separation.

  12. Van der Waals interaction in uniaxial anisotropic media.

    PubMed

    Kornilovitch, Pavel E

    2013-01-23

    Van der Waals interactions between flat surfaces in uniaxial anisotropic media are investigated in the nonretarded limit. The main focus is the effect of nonzero tilt between the optical axis and the surface normal on the strength of the van der Waals attraction. General expressions for the van der Waals free energy are derived using the surface mode method and the transfer-matrix formalism. To facilitate numerical calculations a temperature-dependent three-band parameterization of the dielectric tensor of the liquid crystal 5CB is developed. A solid slab immersed in a liquid crystal experiences a van der Waals torque that aligns the surface normal relative to the optical axis of the medium. The preferred orientation is different for different materials. Two solid slabs in close proximity experience a van der Waals attraction that is strongest for homeotropic alignment of the intervening liquid crystal for all the materials studied. The results have implications for the stability of plate-like colloids in liquid crystal hosts.

  13. Effective field theories for van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Brambilla, Nora; Shtabovenko, Vladyslav; Tarrús Castellà, Jaume; Vairo, Antonio

    2017-06-01

    Van der Waals interactions between two neutral but polarizable systems at a separation R much larger than the typical size of the systems are at the core of a broad sweep of contemporary problems in settings ranging from atomic, molecular and condensed matter physics to strong interactions and gravity. In this paper, we reexamine the dispersive van der Waals interactions between two hydrogen atoms. The novelty of the analysis resides in the usage of nonrelativistic effective field theories of quantum electrodynamics. In this framework, the van der Waals potential acquires the meaning of a matching coefficient in an effective field theory, dubbed van der Waals effective field theory, suited to describe the low-energy dynamics of an atom pair. It may be computed systematically as a series in R times some typical atomic scale and in the fine-structure constant α . The van der Waals potential gets short-range contributions and radiative corrections, which we compute in dimensional regularization and renormalize here for the first time. Results are given in d space-time dimensions. One can distinguish among different regimes depending on the relative size between 1 /R and the typical atomic bound-state energy, which is of order m α2. Each regime is characterized by a specific hierarchy of scales and a corresponding tower of effective field theories. The short-distance regime is characterized by 1 /R ≫m α2 and the leading-order van der Waals potential is the London potential. We also compute next-to-next-to-next-to-leading-order corrections. In the long-distance regime we have 1 /R ≪m α2. In this regime, the van der Waals potential contains contact terms, which are parametrically larger than the Casimir-Polder potential that describes the potential at large distances. In the effective field theory, the Casimir-Polder potential counts as a next-to-next-to-next-to-leading-order effect. In the intermediate-distance regime, 1 /R ˜m α2, a significantly more complex

  14. Cold Anisotropically Interacting van der Waals Molecule: TiHe.

    PubMed

    Quiros, Nancy; Tariq, Naima; Tscherbul, Timur V; Kłos, Jacek; Weinstein, Jonathan D

    2017-05-26

    We have used laser ablation and helium buffer-gas cooling to produce titanium-helium van der Waals molecules at cryogenic temperatures. The molecules were detected through laser-induced fluorescence spectroscopy. Ground-state Ti(a^{3}F_{2})-He binding energies were determined for the ground and first rotationally excited states from studying equilibrium thermodynamic properties, and found to agree well with theoretical calculations based on newly calculated ab initio Ti-He interaction potentials, opening up novel possibilities for studying the formation, dynamics, and nonuniversal chemistry of van der Waals clusters at low temperatures.

  15. Cold Anisotropically Interacting van der Waals Molecule: TiHe

    NASA Astrophysics Data System (ADS)

    Quiros, Nancy; Tariq, Naima; Tscherbul, Timur V.; Kłos, Jacek; Weinstein, Jonathan D.

    2017-05-01

    We have used laser ablation and helium buffer-gas cooling to produce titanium-helium van der Waals molecules at cryogenic temperatures. The molecules were detected through laser-induced fluorescence spectroscopy. Ground-state Ti (a 3F2)-He binding energies were determined for the ground and first rotationally excited states from studying equilibrium thermodynamic properties, and found to agree well with theoretical calculations based on newly calculated ab initio Ti-He interaction potentials, opening up novel possibilities for studying the formation, dynamics, and nonuniversal chemistry of van der Waals clusters at low temperatures.

  16. Scaling Laws for van der Waals Interactions in Nanostructured Materials

    NASA Astrophysics Data System (ADS)

    Gobre, Vivekanand; Tkatchenko, Alexandre

    2014-03-01

    Van der Waals (vdW) forces originate from interactions between fluctuating multipoles in matter and play a significant role in the structure and stability of nanostructured materials. Many models used to describe vdW interactions in nanomaterials are based on a simple pairwise-additive approximation, neglecting the strong electrodynamic response effects caused by long-range fluctuations in matter. We develop and utilize an efficient microscopic method to demonstrate that vdW interactions in nanomaterials act at distances greater than typically assumed, and can be characterized by different scaling laws depending on the dimensionality and size of the system. Specifically, we study the behaviour of vdW interactions in single-layer and multilayer graphene, fullerenes of varying size, single-wall carbon nanotubes and graphene nanoribbons. As a function of nanostructure size, the van der Waals coefficients follow unusual trends for all of the considered systems, and deviate significantly from the conventionally employed pairwise-additive picture. We propose that the peculiar van der Waals interactions in nanostructured materials could be exploited to control their self-assembly.

  17. The vibration of nanosprings affected by van der Waals interactions.

    PubMed

    Zhao, Junhua; Ben, Sudong; Yu, Peishi

    2016-10-01

    The vibration of tightly helical nanosprings affected by van der Waals (vdW) interactions is investigated based on continuum modelling. Explicit solutions are derived to clarify the influence of initial pitch, stiffness and the number of nanosprings on the period, frequency and amplitude of the vibration. Unlike classic linear/nonlinear springs, the waveform of the vibration is always asymmetric for tightly helical nanosprings due to the asymmetry of vdW attraction and repulsion. The at most three equilibrium positions for the nanosprings strongly depend on the deformation due to competition between the vdW interactions and the elastic energy of the nanosprings. This study provides physical insights into the origin of the novel dynamic properties of such nanosprings.

  18. Engineering Low Dimensional Materials with van der Waals Interaction

    NASA Astrophysics Data System (ADS)

    Jin, Chenhao

    Two-dimensional van der Waals materials grow into a hot and big field in condensed matter physics in the past decade. One particularly intriguing thing is the possibility to stack different layers together as one wish, like playing a Lego game, which can create artificial structures that do not exist in nature. These new structures can enable rich new physics from interlayer interaction: The interaction is strong, because in low-dimension materials electrons are exposed to the interface and are susceptible to other layers; and the screening of interaction is less prominent. The consequence is rich, not only from the extensive list of two-dimensional materials available nowadays, but also from the freedom of interlayer configuration, such as displacement and twist angle, which creates a gigantic parameter space to play with. On the other hand, however, the huge parameter space sometimes can make it challenging to describe consistently with a single picture. For example, the large periodicity or even incommensurability in van der Waals systems creates difficulty in using periodic boundary condition. Worse still, the huge superlattice unit cell and overwhelming computational efforts involved to some extent prevent the establishment of a simple physical picture to understand the evolution of system properties in the parameter space of interlayer configuration. In the first part of the dissertation, I will focus on classification of the huge parameter space into subspaces, and introduce suitable theoretical approaches for each subspace. For each approach, I will discuss its validity, limitation, general solution, as well as a specific example of application demonstrating how one can obtain the most important effects of interlayer interaction with little computation efforts. Combining all the approaches introduced will provide an analytic solution to cover majority of the parameter space, which will be very helpful in understanding the intuitive physical picture behind

  19. Tuning the van der Waals Interaction of Graphene with Molecules via Doping.

    PubMed

    Huttmann, Felix; Martínez-Galera, Antonio J; Caciuc, Vasile; Atodiresei, Nicolae; Schumacher, Stefan; Standop, Sebastian; Hamada, Ikutaro; Wehling, Tim O; Blügel, Stefan; Michely, Thomas

    2015-12-04

    We use scanning tunneling microscopy to visualize and thermal desorption spectroscopy to quantitatively measure that the binding of naphthalene molecules to graphene, a case of pure van der Waals interaction, strengthens with n and weakens with p doping of graphene. Density-functional theory calculations that include the van der Waals interaction in a seamless, ab initio way accurately reproduce the observed trend in binding energies. Based on a model calculation, we propose that the van der Waals interaction is modified by changing the spatial extent of graphene's π orbitals via doping.

  20. Enhanced Chiral Discriminatory van der Waals Interactions Mediated by Chiral Surfaces

    NASA Astrophysics Data System (ADS)

    Barcellona, Pablo; Safari, Hassan; Salam, A.; Buhmann, Stefan Yoshi

    2017-05-01

    We predict a discriminatory interaction between a chiral molecule and an achiral molecule which is mediated by a chiral body. To achieve this, we generalize the van der Waals interaction potential between two ground-state molecules with electric, magnetic, and chiral response to nontrivial environments. The force is evaluated using second-order perturbation theory with an effective Hamiltonian. Chiral media enhance or reduce the free interaction via many-body interactions, making it possible to measure the chiral contributions to the van der Waals force with current technology. The van der Waals interaction is discriminatory with respect to enantiomers of different handedness and could be used to separate enantiomers. We also suggest a specific geometric configuration where the electric contribution to the van der Waals interaction is zero, making the chiral component the dominant effect.

  1. Molecular Interactions in Particular Van der Waals Nanoclusters

    NASA Astrophysics Data System (ADS)

    Jungclas, Hartmut; Komarov, Viacheslav V.; Popova, Anna M.; Schmidt, Lothar

    2017-01-01

    A method is presented to analyse the interaction energies in a nanocluster, which is consisting of three neutral molecules bound by non-covalent long range Van der Waals forces. One of the molecules (M0) in the nanocluster has a permanent dipole moment, whereas the two other molecules (M1 and M2) are non-polar. Analytical expressions are obtained for the numerical calculation of the dispersion and induction energies of the molecules in the considered nanocluster. The repulsive forces at short intermolecular distances are taken into account by introduction of damping functions. Dispersion and induction energies are calculated for a nanocluster with a definite geometry, in which the polar molecule M0 is a linear hydrocarbon molecule C5H10 and M1 and M2 are pyrene molecules. The calculations are done for fixed distances between the two pyrene molecules. The results show that the induction energies in the considered three-molecular nanocluster are comparable with the dispersion energies. Furthermore, the sum of induction energies in the substructure (M0, M1) of the considered nanocluster is much higher than the sum of induction energies in a two-molecular nanocluster with similar molecules (M0, M1) because of the absence of an electrostatic field in the latter case. This effect can be explained by the essential intermolecular induction in the three-molecular nanocluster.

  2. Van der Waals interaction between two crossed carbon nanotubes.

    PubMed

    Zhbanov, Alexander I; Pogorelov, Evgeny G; Chang, Yia-Chung

    2010-10-26

    The analytical expressions for the van der Waals potential energy and force between two crossed carbon nanotubes are presented. The Lennard-Jones potential between pairs of carbon atoms and the smeared-out approximation suggested by L. A. Girifalco (J. Phys. Chem. 1992, 96, 858) were used. The exact formula is expressed in terms of rational and elliptical functions. The potential and force for carbon nanotubes were calculated. The uniform potential curves for single- and multiwall nanotubes were plotted. The equilibrium distance, maximal attractive force, and potential energy have been evaluated.

  3. Physisorption of benzene on a tin dioxide surface: van der Waals interaction

    NASA Astrophysics Data System (ADS)

    Viitala, Matti; Kuisma, Mikael; Rantala, Tapio T.

    2012-02-01

    An adsorption study of the benzene molecule on SnO2 (110) surfaces with the density functional theory is extended to include the van der Waals interaction. We compare adsorption onto two model surfaces of SnO2, oxygen rich (stoichiometric) and oxygen poor (reduced), considered as limiting cases of varying oxygen abundance. With the chosen van der Waals approach [M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.92.246401 92, 246401 (2004)] it is found that on the stoichiometric surface, where binding has both a covalent nature and an ionic nature, with the addition of the van der Waals interaction the adsorption energy becomes somewhat stronger. However, on the reduced surface, where binding was earlier predicted to be more physisorptionlike by using a standard generalized gradient approximation (GGA) approach, the van der Waals interaction increases the adsorption energy by a larger factor. Furthermore, three different local-density approximation and GGA functionals are compared, as corrected with the van der Waals scheme. It is found that the correction brings those three to reasonably similar descriptions of adsorption on both surfaces.

  4. Van der Waals interactions and the limits of isolated atom models at interfaces

    PubMed Central

    Kawai, Shigeki; Foster, Adam S.; Björkman, Torbjörn; Nowakowska, Sylwia; Björk, Jonas; Canova, Filippo Federici; Gade, Lutz H.; Jung, Thomas A.; Meyer, Ernst

    2016-01-01

    Van der Waals forces are among the weakest, yet most decisive interactions governing condensation and aggregation processes and the phase behaviour of atomic and molecular matter. Understanding the resulting structural motifs and patterns has become increasingly important in studies of the nanoscale regime. Here we measure the paradigmatic van der Waals interactions represented by the noble gas atom pairs Ar–Xe, Kr–Xe and Xe–Xe with a Xe-functionalized tip of an atomic force microscope at low temperature. Individual rare gas atoms were fixed at node sites of a surface-confined two-dimensional metal–organic framework. We found that the magnitude of the measured force increased with the atomic radius, yet detailed simulation by density functional theory revealed that the adsorption induced charge redistribution strengthened the van der Waals forces by a factor of up to two, thus demonstrating the limits of a purely atomic description of the interaction in these representative systems. PMID:27174162

  5. Van der Waals interactions and the limits of isolated atom models at interfaces.

    PubMed

    Kawai, Shigeki; Foster, Adam S; Björkman, Torbjörn; Nowakowska, Sylwia; Björk, Jonas; Canova, Filippo Federici; Gade, Lutz H; Jung, Thomas A; Meyer, Ernst

    2016-05-13

    Van der Waals forces are among the weakest, yet most decisive interactions governing condensation and aggregation processes and the phase behaviour of atomic and molecular matter. Understanding the resulting structural motifs and patterns has become increasingly important in studies of the nanoscale regime. Here we measure the paradigmatic van der Waals interactions represented by the noble gas atom pairs Ar-Xe, Kr-Xe and Xe-Xe with a Xe-functionalized tip of an atomic force microscope at low temperature. Individual rare gas atoms were fixed at node sites of a surface-confined two-dimensional metal-organic framework. We found that the magnitude of the measured force increased with the atomic radius, yet detailed simulation by density functional theory revealed that the adsorption induced charge redistribution strengthened the van der Waals forces by a factor of up to two, thus demonstrating the limits of a purely atomic description of the interaction in these representative systems.

  6. Many-body effects in the van der Waals-Casimir interaction between graphene layers

    NASA Astrophysics Data System (ADS)

    Sarabadani, Jalal; Naji, Ali; Asgari, Reza; Podgornik, Rudolf

    2011-10-01

    Van der Waals-Casimir dispersion interactions between two apposed graphene layers, a graphene layer and a substrate, and in a multilamellar graphene system are analyzed within the framework of the Lifshitz theory. This formulation hinges on a known form of the dielectric response function of an undoped or doped graphene sheet, assumed to be of a random-phase-approximation form. In the geometry of two apposed layers, the separation dependence of the van der Waals-Casimir interaction for both types of graphene sheets is determined and critically compared with some well-known limiting cases. In a multilamellar array, the many-body effects are quantified and shown to increase the magnitude of the van der Waals-Casimir interactions.

  7. Dynamical screening of the van der Waals interaction between graphene layers.

    PubMed

    Dappe, Y J; Bolcatto, P G; Ortega, J; Flores, F

    2012-10-24

    The interaction between graphene layers is analyzed combining local orbital DFT and second order perturbation theory. For this purpose we use the linear combination of atomic orbitals-orbital occupancy (LCAO-OO) formalism, that allows us to separate the interaction energy as the sum of a weak chemical interaction between graphene layers plus the van der Waals interaction (Dappe et al 2006 Phys. Rev. B 74 205434). In this work, the weak chemical interaction is calculated by means of corrected-LDA calculations using an atomic-like sp(3)d(5) basis set. The van der Waals interaction is calculated by means of second order perturbation theory using an atom-atom interaction approximation and the atomic-like-orbital occupancies. We also analyze the effect of dynamical screening in the van der Waals interaction using a simple model. We find that this dynamical screening reduces by 40% the van der Waals interaction. Taking this effect into account, we obtain a graphene-graphene interaction energy of 70 ± 5 meV/atom in reasonable agreement with the experimental evidence.

  8. Understanding the nanoscale local buckling behavior of vertically aligned MWCNT arrays with van der Waals interactions.

    PubMed

    Li, Yupeng; Kim, Hyung-ick; Wei, Bingqing; Kang, Junmo; Choi, Jae-boong; Nam, Jae-Do; Suhr, Jonghwan

    2015-09-14

    The local buckling behavior of vertically aligned carbon nanotubes (VACNTs) has been investigated and interpreted in the view of a collective nanotube response by taking van der Waals interactions into account. To the best of our knowledge, this is the first report on the case of collective VACNT behavior regarding van der Waals force among nanotubes as a lateral support effect during the buckling process. The local buckling propagation and development of VACNTs were experimentally observed and theoretically analyzed by employing finite element modeling with lateral support from van der Waals interactions among nanotubes. Both experimental and theoretical analyses show that VACNTs buckled in the bottom region with many short waves and almost identical wavelengths, indicating a high mode buckling. Furthermore, the propagation and development mechanism of buckling waves follow the wave damping effect.

  9. van der Waals interaction between a microparticle and a single-walled carbon nanotube

    SciTech Connect

    Blagov, E. V.; Mostepanenko, V. M.; Klimchitskaya, G. L.

    2007-06-15

    The Lifshitz-type formulas describing the free energy and the force of the van der Waals interaction between an atom (molecule) and a single-walled carbon nanotube are obtained. The single-walled nanotube is considered as a cylindrical sheet carrying a two-dimensional free-electron gas with appropriate boundary conditions on the electromagnetic field. The obtained formulas are used to calculate the van der Waals free energy and force between a hydrogen atom (molecule) and single-walled carbon nanotubes of different radii. Comparison studies of the van der Waals interaction of hydrogen atoms with single-walled and multiwalled carbon nanotubes show that depending on atom-nanotube separation distance, the idealization of graphite dielectric permittivity is already applicable to nanotubes with only two or three walls.

  10. Van der Waals interactions in density functional theory

    NASA Astrophysics Data System (ADS)

    Langreth, David C.

    2009-03-01

    The van der Waals density functional which we introduced half a decade agoootnotetextM. Dion et al. Phys. Rev. Lett. 92, 246401 (2004). and its self-consistent generalizationootnotetextT. Thonhauser et al., Phys. Rev. B 76, 125112 (2007). will be briefly reviewed. There are many collaborators in the application review that will follow, not only those who worked in the physics department at Rutgers% ootnotetextMaxime Dion, Aaron Puzder, T. Thonhauser, Valentino R. Cooper, Shen Li, Eamonn Murray, Lingzhu Kong, and Kyuho Lee. and at Chalmers,% ootnotetextHenrik Rydberg, Svetla Chakarova-K"ack, Jesper Kleis, Elsebeth Schr"oder, Per Hyldgaard, and Bengt I. Lundqvist. but also at Denmarks Technical University,% ootnotetextAndrei Kelkkanen, Poul G. Moses, Jesper Kleis, and Bengt I. Lundqvist. the chemistry department at Rutgers,% ootnotetextKonhoa Li, Jing Li, Yves Chabal, and Wilma K. Olson. and most recently at the University of Texas at Dallas.% ootnotetextNour Nijem and Yves Chabal. I will expand on our recent review article,ootnotetextD. C. Langreth et al., J. Phys. Cond. Mat. (in press). which hopefully will be published before the present talk, and include applications by other groups not listed below. If possible, I will also review results from a more recent collaboration to study nucleosomal DNA and beyond.

  11. Van der Waals Interactions Between Subsystems with Overlapping Electron Density

    NASA Astrophysics Data System (ADS)

    Pavanello, Michele

    2015-03-01

    The subsystem formulation of DFT known as Frozen Density Embedding (FDE) provides a divide-and-conquer approach to Kohn-Sham DFT for weakly bound systems. We claim that a subsystem formulation of DFT can simplify both the theoretical framework and the computational effort for calculating the electronic structure of condensed phase systems. In addition, the naturally subsystem-like form of molecular aggregates makes subsystem DFT a better descriptor of the underlying physics than regular DFT of the supersystem. As an example, we present a novel van der Waals theory based on subsystem DFT which can treat seamlessly non-overlapping as well as overlapping subsystem electron densities. The theory is amenable to sensible approximations, such as RPA, and offers natural algorithms to fold in post-RPA corrections. Application of the theory to the computation of binding energies of dimers in the S22 set, as well as computation of selected potential energy surfaces is presented. M.P. acknowledges funding by NSF IIA-1404739 and CBET-1438493.

  12. Understanding the nanoscale local buckling behavior of vertically aligned MWCNT arrays with van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Li, Yupeng; Kim, Hyung-Ick; Wei, Bingqing; Kang, Junmo; Choi, Jae-Boong; Nam, Jae-Do; Suhr, Jonghwan

    2015-08-01

    The local buckling behavior of vertically aligned carbon nanotubes (VACNTs) has been investigated and interpreted in the view of a collective nanotube response by taking van der Waals interactions into account. To the best of our knowledge, this is the first report on the case of collective VACNT behavior regarding van der Waals force among nanotubes as a lateral support effect during the buckling process. The local buckling propagation and development of VACNTs were experimentally observed and theoretically analyzed by employing finite element modeling with lateral support from van der Waals interactions among nanotubes. Both experimental and theoretical analyses show that VACNTs buckled in the bottom region with many short waves and almost identical wavelengths, indicating a high mode buckling. Furthermore, the propagation and development mechanism of buckling waves follow the wave damping effect.The local buckling behavior of vertically aligned carbon nanotubes (VACNTs) has been investigated and interpreted in the view of a collective nanotube response by taking van der Waals interactions into account. To the best of our knowledge, this is the first report on the case of collective VACNT behavior regarding van der Waals force among nanotubes as a lateral support effect during the buckling process. The local buckling propagation and development of VACNTs were experimentally observed and theoretically analyzed by employing finite element modeling with lateral support from van der Waals interactions among nanotubes. Both experimental and theoretical analyses show that VACNTs buckled in the bottom region with many short waves and almost identical wavelengths, indicating a high mode buckling. Furthermore, the propagation and development mechanism of buckling waves follow the wave damping effect. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr03581c

  13. Reactivity enhancement of ultracold O(3P)+H2 collisions by van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Weck, P. F.; Balakrishnan, N.

    2005-10-01

    The role of van der Waals forces in O(P3)+H2(υ =1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial A″3 potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandão et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.

  14. Effect of van der Waals interaction on the mode I fracture characteristics of graphene sheet

    NASA Astrophysics Data System (ADS)

    Parashar, Avinash; Mertiny, Pierre

    2013-11-01

    In this paper a study has been performed to investigate the effect of van der Waals interaction forces on the mode I (opening mode) fracture characteristics of a graphene sheet. Finite element based atomistic approach was employed to perform the investigation, where graphene structure was assumed to behave like a space frame structure. Few graphene sheets were modeled in finite element environment with different set of interlayer spacing. Modified virtual crack closure technique (VCCT) was employed to estimate the strain energy release rate (SERR) under mode I of fracture criteria. Significant effect of van der Waals forces was observed on the mode I fracture characteristics of graphene.

  15. Dynamics of Gold Nanoparticles on Carbon Nanostructures Driven by van der Waals and Electrostatic Interactions.

    PubMed

    La Torre, Alessandro; Gimenez-Lopez, Maria del Carmen; Fay, Michael W; Lucas, Carlos Herreros; Brown, Paul D; Khlobystov, Andrei N

    2015-06-01

    Transmission electron microscopy studies on the assembly and growth of gold nanoparticles on carbon nanotubes supported on few-layer graphene and amorphous carbon reveal a competition between van der Waals forces and electrostatic interactions, enabling controlled positioning and sizing of adsorbed nanoparticles at the nanochannels formed between the carbon nanotube and the few-layer graph-ene surface.

  16. Interlayer Interactions in van der Waals Heterostructures: Electron and Phonon Properties.

    PubMed

    Le, Nam B; Huan, Tran Doan; Woods, Lilia M

    2016-03-09

    Artificial van der Waals heterostructures constitute an emerging field that promises to design systems with properties on demand. Stacking patterns and the utilization of different types of chemically inert layers can deliver novel materials and devices. Despite the relatively weak van der Waals interaction, which does not affect the electronic properties around the Fermi level, our first-principles calculations show significant changes in the higher conduction and deeper valence regions in the considered graphene/silicene, graphene/MoS2, and silicene/MoS2 systems. Such changes are linked to strong out-of-plane hybridization effects and van der Waals interactions. We also find that the interface coupling significantly affects the vibrational properties of the heterostructures when compared to the individual constituents. Specifically, the van der Waals coupling is found to be a major factor for the stability of the system. The emergence of shear and breathing modes, as well as the transformation of flexural modes, are also found.

  17. Effect of van der Waals interactions on the structural and binding properties of GaSe

    SciTech Connect

    Sarkisov, Sergey Y.; Kosobutsky, Alexey V.; Shandakov, Sergey D.

    2015-12-15

    The influence of van der Waals interactions on the lattice parameters, band structure, elastic moduli and binding energy of layered GaSe compound has been studied using projector-augmented wave method within density functional theory. We employed the conventional local/semilocal exchange-correlation functionals and recently developed van der Waals functionals which are able to describe dispersion forces. It is found that application of van der Waals density functionals allows to substantially increase the accuracy of calculations of the lattice constants a and c and interlayer distance in GaSe at ambient conditions and under hydrostatic pressure. The pressure dependences of the a-parameter, Ga–Ga, Ga–Se bond lengths and Ga–Ga–Se bond angle are characterized by a relatively low curvature, while c(p) has a distinct downward bowing due to nonlinear shrinking of the interlayer spacing. From the calculated binding energy curves we deduce the interlayer binding energy of GaSe, which is found to be in the range 0.172–0.197 eV/layer (14.2–16.2 meV/Å{sup 2}). - Highlights: • Effects of van der Waals interactions are analyzed using advanced density functionals. • Calculations with vdW-corrected functionals closely agree with experiment. • Interlayer binding energy of GaSe is estimated to be 14.2–16.2 meV/Å{sup 2}.

  18. Van der Waals interactions: evaluations by use of a statistical mechanical method.

    PubMed

    Høye, Johan S

    2011-10-07

    In this work the induced van der Waals interaction between a pair of neutral atoms or molecules is considered by use of a statistical mechanical method. With use of the Schrödinger equation this interaction can be obtained by standard quantum mechanical perturbation theory to second order. However, the latter is restricted to electrostatic interactions between dipole moments. So with radiating dipole-dipole interaction where retardation effects are important for large separations of the particles, other methods are needed, and the resulting induced interaction is the Casimir-Polder interaction usually obtained by field theory. It can also be evaluated, however, by a statistical mechanical method that utilizes the path integral representation. We here show explicitly by use of this method the equivalence of the Casimir-Polder interaction and the van der Waals interaction based upon the Schrödinger equation. The equivalence is to leading order for short separations where retardation effects can be neglected. In recent works [J. S. Høye, Physica A 389, 1380 (2010); Phys. Rev. E 81, 061114 (2010)], the Casimir-Polder or Casimir energy was added as a correction to calculations of systems like the electron clouds of molecules. The equivalence to van der Waals interactions indicates that the added Casimir energy will improve the accuracy of calculated molecular energies. Thus, we give numerical estimates of this energy including analysis and estimates for the uniform electron gas. © 2011 American Institute of Physics

  19. Adjustment of Perdew-Wang Exchange Functional for Describing van der Waals and DNA Base-Stacking Interactions

    SciTech Connect

    Kurita, Noriyuki; Inoue, Hidekazu; Sekino, Hideo

    2003-03-07

    In order to accurately describe the van der Waals interaction between rare-gas atoms by the density functional theory, we adjusted the exchange-functional developed by Perdew and Wang (PW). The van der Waals interactions of He, Ne, Ar and Kr dimers were investigated. The results clarified that the adjustment improves the overestimation of the interactions by the original PW exchange-functional, providing the qualitatively accurate trend in van der Waals interactions of He, Ne, Ar and Kr dimers. However, we also found that the adjusted functional for He and Ne underestimates the DNA base-stacking interaction between cytosine monomers. This may indicate that the PW exchange-functional requires a further modification or a van der Waals correction in order to give accurate DNA base-stacking interaction.

  20. Application of diffusion Monte Carlo to materials dominated by van der Waals interactions

    SciTech Connect

    Benali, Anouar; Shulenburger, Luke; Romero, Nichols; Kim, Jeongnim; Von Lilienfeld, Anatole

    2014-01-01

    Van der Waals forces are notoriously difficult to account for from first principles. We perform extensive calculation to assess the usefulness and validity of diffusion quantum Monte Carlo when applied to van der Waals forces. We present results for noble gas solids and clusters - archetypical van der Waals dominated assemblies, as well as a relevant pi-pi stacking supramolecular complex: DNA + intercalating anti-cancer drug Ellipticine.

  1. Application of Diffusion Monte Carlo to Materials Dominated by van der Waals Interactions

    DOE PAGES

    Benali, Anouar; Shulenburger, Luke; Romero, Nichols A.; ...

    2014-06-12

    Van der Waals forces are notoriously difficult to account for from first principles. We perform extensive calculation to assess the usefulness and validity of diffusion quantum Monte Carlo when applied to van der Waals forces. We present results for noble gas solids and clusters - archetypical van der Waals dominated assemblies, as well as a relevant pi-pi stacking supramolecular complex: DNA + intercalating anti-cancer drug Ellipticine.

  2. Finite-size nanowire at a surface: Unconventional power laws of the van der Waals interaction

    NASA Astrophysics Data System (ADS)

    Makhnovets, K. A.; Kolezhuk, A. K.

    2017-09-01

    We study the van der Waals interaction of a metallic or narrow-gap semiconducting nanowire with a surface, in the regime of intermediate wire-surface distances (vF/c )L ≪d ≪L or L ≪d ≪(c /vF)L , where L is the nanowire length, d is the distance to the surface, and vF is the characteristic velocity of nanowire electrons (for a metallic wire, it is the Fermi velocity). Our approach, based on the Luttinger liquid framework, allows one to analyze the dependence of the interaction on the interplay between the nanowire length, wire-surface distance, and characteristic length scales related to the spectral gap and temperature. We show that this interplay leads to nontrivial modifications of the power law that governs van der Waals forces, in particular to a nonmonotonic dependence of the power-law exponent on the wire-surface separation.

  3. Role of the van der Waals interaction in atom-diatom reaction dynamics at low temperatures

    NASA Astrophysics Data System (ADS)

    Weck, Philippe F.; Balakrishnan, Naduvalath; Brandao, Joao; Rosa, Carla; Wang, Wenli

    2006-03-01

    Quantum-mechanical scattering calculations are reported for the O(^3P)+H2 collision at energies close to the reaction threshold with emphasis on the sensitivity of the reaction dynamics to the van der Waals interaction. The dynamics has been investigated using the lowest ^3 A'' GLDP potential energy surface developed by Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions by Brandão et al. [J. Chem. Phys. 121, 8861 (2004)] which explicitly include the van der Waals interaction. Quasiclassical trajectory calculations on all three potential energy surfaces are also reported to explore the validity of this method near the reaction threshold and to assess the importance of quantum effects at low temperatures.

  4. van der Waals interactions at the nanoscale: The effects of nonlocality

    PubMed Central

    Luo, Yu; Zhao, Rongkuo; Pendry, John B.

    2014-01-01

    Calculated using classical electromagnetism, the van der Waals force increases without limit as two surfaces approach. In reality, the force saturates because the electrons cannot respond to fields of very short wavelength: polarization charges are always smeared out to some degree and in consequence the response is nonlocal. Nonlocality also plays an important role in the optical spectrum and distribution of the modes but introduces complexity into calculations, hindering an analytical solution for interactions at the nanometer scale. Here, taking as an example the case of two touching nanospheres, we show for the first time, to our knowledge, that nonlocality in 3D plasmonic systems can be accurately analyzed using the transformation optics approach. The effects of nonlocality are found to dramatically weaken the field enhancement between the spheres and hence the van der Waals interaction and to modify the spectral shifts of plasmon modes. PMID:25468982

  5. Universal curves for the van der Waals interaction between single-walled carbon nanotubes.

    PubMed

    Pogorelov, Evgeny G; Zhbanov, Alexander I; Chang, Yia-Chung; Yang, Sung

    2012-01-17

    We report very simple and accurate algebraic expressions for the van der Waals (VDW) potentials and the forces between two parallel and crossed carbon nanotubes. The Lennard-Jones potential for two carbon atoms and the method of the smeared-out approximation suggested by Girifalco were used. It is found that the interaction between parallel and crossed tubes is described by two universal curves for parallel and crossed configurations that do not depend on the van der Waals constants, the angle between tubes, and the surface density of atoms and their nature but only on the dimensionless distance. The explicit functions for equilibrium VDW distances, well depths, and maximal attractive forces have been given. These results may be used as a guide for the analysis of experimental data to investigate the interaction between nanotubes of various natures.

  6. Dielectric response variation and the strength of van der Waals interactions.

    PubMed

    Hopkins, Jaime C; Dryden, Daniel M; Ching, Wai-Yim; French, Roger H; Parsegian, V Adrian; Podgornik, Rudolf

    2014-03-01

    Small changes in the dielectric response of a material result in substantial variations in the Hamaker coefficient of the van der Waals interactions, as demonstrated in a simplified approximate model as well as a realistic example of amorphous silica with and without an exciton peak. Variation of the dielectric response spectra at one particular frequency influences all terms in the Matsubara summation, making the total change in the Hamaker coefficient depend on the spectral changes not only at that frequency but also at the rest of the spectrum, properly weighted. The Matsubara terms most affected by the addition of a single peak are not those close to the position of the added peak, but are distributed doubly non-locally over the entire range of frequencies. A possibility of eliminating van der Waals interactions or at least drastically reducing them by spectral variation in a narrow regime of frequencies thus seems very remote.

  7. van der Waals interactions at the nanoscale: the effects of nonlocality.

    PubMed

    Luo, Yu; Zhao, Rongkuo; Pendry, John B

    2014-12-30

    Calculated using classical electromagnetism, the van der Waals force increases without limit as two surfaces approach. In reality, the force saturates because the electrons cannot respond to fields of very short wavelength: polarization charges are always smeared out to some degree and in consequence the response is nonlocal. Nonlocality also plays an important role in the optical spectrum and distribution of the modes but introduces complexity into calculations, hindering an analytical solution for interactions at the nanometer scale. Here, taking as an example the case of two touching nanospheres, we show for the first time, to our knowledge, that nonlocality in 3D plasmonic systems can be accurately analyzed using the transformation optics approach. The effects of nonlocality are found to dramatically weaken the field enhancement between the spheres and hence the van der Waals interaction and to modify the spectral shifts of plasmon modes.

  8. Study of van der Waals bonding and interactions in metal organic framework materials.

    PubMed

    Zuluaga, Sebastian; Canepa, Pieremanuele; Tan, Kui; Chabal, Yves J; Thonhauser, Timo

    2014-04-02

    Metal organic framework (MOF) materials have attracted a lot of attention due to their numerous applications in fields such as hydrogen storage, carbon capture and gas sequestration. In all these applications, van der Waals forces dominate the interaction between the small guest molecules and the walls of the MOFs. In this review article, we describe how a combined theoretical and experimental approach can successfully be used to study those weak interactions and elucidate the adsorption mechanisms important for various applications. On the theory side, we show that, while standard density functional theory is not capable of correctly describing van der Waals interactions, functionals especially designed to include van der Waals forces exist, yielding results in remarkable agreement with experiment. From the experimental point of view, we show examples in which IR adsorption and Raman spectroscopy are essential to study molecule/MOF interactions. Importantly, we emphasize throughout this review that a combination of theory and experiment is crucial to effectively gain further understanding. In particular, we review such combined studies for the adsorption mechanism of small molecules in MOFs, the chemical stability of MOFs under humid conditions, water cluster formation inside MOFs, and the diffusion of small molecules into MOFs. The understanding of these phenomena is critical for the rational design of new MOFs with desired properties.

  9. van der Waals interactions between excited-state atoms and dispersive dielectric surfaces

    NASA Astrophysics Data System (ADS)

    Fichet, M.; Schuller, F.; Bloch, D.; Ducloy, M.

    1995-02-01

    van der Waals interactions between atoms and dielectric surfaces are reinvestigated. To describe the nonretarded interaction potential between a dispersive dielectric surface and an atom in an arbitrary internal energy state, we derive a general expression in terms of an integral, over real frequency, of the combined atom and surface polarizabilities. It is shown that, for excited atoms, the expression is equivalent to the one obtained by Wylie and Sipe [Phys. Rev. A 32, 2030 (1985)]. We thus demonstrate how to extend this approach to excited atoms interacting with birefringent dielectrics. For isotropic dielectrics, a method of integration in closed form allows us to derive an approximate formula for the van der Waals interaction constant in terms of resonance frequencies and oscillator strengths of both the atom and the dielectric. Frequency-dependent ``dielectric reflection'' coefficients are introduced for virtual atomic dipole couplings either in absorption or in emission. In absorption, the reflection coefficient is always positive and smaller than unity. In emission, it may take arbitrary values, positive or negative (corresponding to van der Waals repulsion). Such a behavior is shown to be related to resonant excitation exchange between the atomic system and the dielectric medium, when an atomic transition frequency gets in resonance with a dielectric absorption band. Numerical calculations performed for the cesium-sapphire system are shown to be in good agreement with data obtained by selective-reflection spectroscopy. Finally, experimental tests of the birefringent character of the sapphire response are discussed.

  10. Thin Film Evaporation Model with Retarded Van Der Waals Interaction (Postprint)

    DTIC Science & Technology

    2013-11-01

    failing to comply with a collection of information if it does not display a currently valid OMB control number . PLEASE DO NOT RETURN YOUR FORM TO THE...April 2013 4. TITLE AND SUBTITLE THIN FILM EVAPORATION MODEL WITH RETARDED VAN DER WAALS INTERACTION (POSTPRINT) 5a. CONTRACT NUMBER In-house 5b...GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 62203F 6. AUTHOR(S) Michael Hanchak, Marlin D. Vangsness, and Jamie S. Ervin (University of Dayton

  11. Molecular recognition by van der Waals interaction between polymers with sequence-specific polarizabilities

    NASA Astrophysics Data System (ADS)

    Lu, Bing-Sui; Naji, Ali; Podgornik, Rudolf

    2015-06-01

    We analyze van der Waals interactions between two rigid polymers with sequence-specific, anisotropic polarizabilities along the polymer backbones, so that the dipole moments fluctuate parallel to the polymer backbones. Assuming that each polymer has a quenched-in polarizability sequence which reflects, for example, the polynucleotide sequence of a double-stranded DNA molecule, we study the van der Waals interaction energy between a pair of such polymers with rod-like structure for the cases where their respective polarizability sequences are (i) distinct and (ii) identical, with both zero and non-zero correlation length of the polarizability correlator along the polymer backbones in the latter case. For identical polymers, we find a novel r-5 scaling behavior of the van der Waals interaction energy for small inter-polymer separation r, in contradistinction to the r-4 scaling behavior of distinct polymers, with furthermore a pronounced angular dependence favoring attraction between sufficiently aligned identical polymers. Such behavior can assist the molecular recognition between polymers.

  12. Molecular recognition by van der Waals interaction between polymers with sequence-specific polarizabilities.

    PubMed

    Lu, Bing-Sui; Naji, Ali; Podgornik, Rudolf

    2015-06-07

    We analyze van der Waals interactions between two rigid polymers with sequence-specific, anisotropic polarizabilities along the polymer backbones, so that the dipole moments fluctuate parallel to the polymer backbones. Assuming that each polymer has a quenched-in polarizability sequence which reflects, for example, the polynucleotide sequence of a double-stranded DNA molecule, we study the van der Waals interaction energy between a pair of such polymers with rod-like structure for the cases where their respective polarizability sequences are (i) distinct and (ii) identical, with both zero and non-zero correlation length of the polarizability correlator along the polymer backbones in the latter case. For identical polymers, we find a novel r(-5) scaling behavior of the van der Waals interaction energy for small inter-polymer separation r, in contradistinction to the r(-4) scaling behavior of distinct polymers, with furthermore a pronounced angular dependence favoring attraction between sufficiently aligned identical polymers. Such behavior can assist the molecular recognition between polymers.

  13. van der Waals forces in density functional theory: Perturbational long-range electron-interaction corrections

    SciTech Connect

    Angyan, Janos G.; Gerber, Iann C.; Savin, Andreas; Toulouse, Julien

    2005-07-15

    Long-range exchange and correlation effects, responsible for the failure of currently used approximate density functionals in describing van der Waals forces, are taken into account explicitly after a separation of the electron-electron interaction in the Hamiltonian into short- and long-range components. We propose a 'range-separated hybrid' functional based on a local density approximation for the short-range exchange-correlation energy, combined with a long-range exact exchange energy. Long-range correlation effects are added by a second-order perturbational treatment. The resulting scheme is general and is particularly well adapted to describe van der Waals complexes, such as rare gas dimers.

  14. van der Waals torque

    NASA Astrophysics Data System (ADS)

    Esquivel-Sirvent, Raul; Schatz, George

    2014-03-01

    The theory of generalized van der Waals forces by Lifshtz when applied to optically anisotropic media predicts the existence of a torque. In this work we present a theoretical calculation of the van der Waals torque for two systems. First we consider two isotropic parallel plates where the anisotropy is induced using an external magnetic field. The anisotropy will in turn induce a torque. As a case study we consider III-IV semiconductors such as InSb that can support magneto plasmons. The calculations of the torque are done in the Voigt configuration, that occurs when the magnetic field is parallel to the surface of the slabs. The change in the dielectric function as the magnetic field increases has the effect of decreasing the van der Waals force and increasing the torque. Thus, the external magnetic field is used to tune both the force and torque. The second example we present is the use of the torque in the non retarded regime to align arrays of nano particle slabs. The torque is calculated within Barash and Ginzburg formalism in the nonretarded limit, and is quantified by the introduction of a Hamaker torque constant. Calculations are conducted between anisotropic slabs of materials including BaTiO3 and arrays of Ag nano particles. Depending on the shape and arrangement of the Ag nano particles the effective dielectric function of the array can be tuned as to make it more or less anisotropic. We show how this torque can be used in self assembly of arrays of nano particles. ref. R. Esquivel-Sirvent, G. C. Schatz, Phys. Chem C, 117, 5492 (2013). partial support from DGAPA-UNAM.

  15. Van der Waals interactions: Evaluations by use of a statistical mechanical method

    NASA Astrophysics Data System (ADS)

    Høye, Johan S.

    2011-10-01

    In this work the induced van der Waals interaction between a pair of neutral atoms or molecules is considered by use of a statistical mechanical method. With use of the Schrödinger equation this interaction can be obtained by standard quantum mechanical perturbation theory to second order. However, the latter is restricted to electrostatic interactions between dipole moments. So with radiating dipole-dipole interaction where retardation effects are important for large separations of the particles, other methods are needed, and the resulting induced interaction is the Casimir-Polder interaction usually obtained by field theory. It can also be evaluated, however, by a statistical mechanical method that utilizes the path integral representation. We here show explicitly by use of this method the equivalence of the Casimir-Polder interaction and the van der Waals interaction based upon the Schrödinger equation. The equivalence is to leading order for short separations where retardation effects can be neglected. In recent works [J. S. Høye, Physica A 389, 1380 (2010), 10.1016/j.physa.2009.12.003; Phys. Rev. E 81, 061114 (2010)], 10.1103/PhysRevE.81.061114, the Casimir-Polder or Casimir energy was added as a correction to calculations of systems like the electron clouds of molecules. The equivalence to van der Waals interactions indicates that the added Casimir energy will improve the accuracy of calculated molecular energies. Thus, we give numerical estimates of this energy including analysis and estimates for the uniform electron gas.

  16. Stability of an attractive bosonic cloud with van der Waals interaction

    SciTech Connect

    Biswas, Anindya; Das, Tapan Kumar; Salasnich, Luca; Chakrabarti, Barnali

    2010-10-15

    We investigate the structure and stability of Bose-Einstein condensates of {sup 7}Li atoms with realistic van der Waals interactions by using the potential harmonic expansion method. Besides the known low-density metastable solution with a contact {delta}-function interaction, we find a stable branch at a higher density which corresponds to the formation of an atomic cluster. Comparison with the results of a nonlocal effective interaction is also presented. We analyze the effect of trap size on the transition between the two branches of solutions. We also compute the loss rate of a Bose condensate due to two- and three-body collisions.

  17. Van der Waals and resonance interactions between accelerated atoms in vacuum and the Unruh effect

    NASA Astrophysics Data System (ADS)

    Lattuca, M.; Marino, J.; Noto, A.; Passante, R.; Rizzuto, L.; Spagnolo, S.; Zhou, W.

    2017-08-01

    We discuss different physical effects related to the uniform acceleration of atoms in vacuum, in the framework of quantum electrodynamics. We first investigate the van der Waals/Casimir-Polder dispersion and resonance interactions between two uniformly accelerated atoms in vacuum. We show that the atomic acceleration significantly affects the van der Waals force, yielding a different scaling of the interaction with the interatomic distance and an explicit time dependence of the interaction energy. We argue how these results could allow for an indirect detection of the Unruh effect through dispersion interactions between atoms. We then consider the resonance interaction between two accelerated atoms, prepared in a correlated Bell-type state, and interacting with the electromagnetic field in the vacuum state, separating vacuum fluctuations and radiation reaction contributions, both in the free-space and in the presence of a perfectly reflecting plate. We show that nonthermal effects of acceleration manifest in the resonance interaction, yielding a change of the distance dependence of the resonance interaction energy. This suggests that the equivalence between temperature and acceleration does not apply to all radiative properties of accelerated atoms. To further explore this aspect, we evaluate the resonance interaction between two atoms in non inertial motion in the coaccelerated (Rindler) frame and show that in this case the assumption of an Unruh temperature for the field is not required for a complete equivalence of locally inertial and coaccelerated points of views.

  18. van der Waals interactions between nanotubes and nanoparticles for controlled assembly of composite nanostructures.

    PubMed

    Rance, Graham A; Marsh, Dan H; Bourne, Stephen J; Reade, Thomas J; Khlobystov, Andrei N

    2010-08-24

    We have demonstrated that ubiquitous van der Waals forces are significant in controlling the interactions between nanoparticles and nanotubes. The adsorption of gold nanoparticles (AuNPs) on nanotubes (MWNTs) obeys a simple quadratic dependence on the nanotube surface area, regardless of the source of AuNPs and MWNTs. Changes in the geometric parameters of the components have pronounced effects on the affinity of nanoparticles for nanotubes, with larger, more polarizable nanostructures exhibiting stronger attractive interactions, the impact of which changes in the following order MWNT diameter > AuNP diameter > MWNT length.

  19. EDITORIAL: Van der Waals interactions in advanced materials, in memory of David C Langreth Van der Waals interactions in advanced materials, in memory of David C Langreth

    NASA Astrophysics Data System (ADS)

    Hyldgaard, Per; Rahman, Talat S.

    2012-10-01

    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

  20. van der Waals interaction energy and disjoining pressure at small separation.

    PubMed

    White, Lee R

    2010-03-01

    The divergence of the van der Waals interaction energy E(132)(L) between plane half-spaces 1 and 2 separated by medium 3 as the separation distance L tends to zero is naively thought of as due to the overlap of the atomic polarization centers. It follows that it may therefore be prevented by properly allowing for the finite size of the atomic species which would prevent the overlap. The distance cutoff model is a simple example of such a modification. The present paper demonstrates that this is not ultimately the origin of the divergence and, that although finite atomic dimensions would alleviate the embarrassment, non-overlap does not properly address the thermodynamic restriction that pertains to the interaction energy. By allowing in an albeit approximate way for the wavelength dependence of the material dielectric response functions epsilon(i xi, k) which arise naturally in the modern Lifshitz theory for this interaction, a form for the van der Waals energy and the corresponding disjoining pressure may be derived which obey the thermodynamic constraint and remove the divergence as L-->0. The energy and disjoining pressure in this new model are compared with the classic non-retarded results and the length cutoff model.

  1. Van der Waals interactions between polymers with sequence-specific polarizabilities: Stiff polymers and Gaussian coils

    NASA Astrophysics Data System (ADS)

    Lu, Bing-Sui; Naji, Ali; Podgornik, Rudolf

    2016-01-01

    We consider the van der Waals interaction between a pair of polymers with quenched heterogeneous sequences of local polarizabilities along their backbones, and study the effective pairwise interaction energy for both stiff polymers and flexible Gaussian coils. In particular, we focus on the cases where the pair of polarizability sequences are (i) distinct and (ii) identical. We find that the pairwise interaction energies of distinct and identical Gaussian coils are both isotropic and exhibit the same decay behavior for separations larger than their gyration radius, in contradistinction to the orientationally anisotropic interaction energies of distinct and identical stiff polymers. For both Gaussian coils and stiff polymers, the attractive interaction between identical polymers is enhanced if the polarizability sequence is more heterogeneous.

  2. Adsorption by design: Tuning atom-graphene van der Waals interactions via mechanical strain

    NASA Astrophysics Data System (ADS)

    Nichols, Nathan S.; Del Maestro, Adrian; Wexler, Carlos; Kotov, Valeri N.

    2016-05-01

    We aim to understand how the van der Waals force between neutral adatoms and a graphene layer is modified by uniaxial strain and electron correlation effects. A detailed analysis is presented for three atoms (He, H, and Na) and graphene strain ranging from weak to moderately strong. We show that the van der Waals potential can be significantly enhanced by strain, and present applications of our results to the problem of elastic scattering of atoms from graphene. In particular, we find that quantum reflection can be significantly suppressed by strain, meaning that dissipative inelastic effects near the surface become of increased importance. Furthermore, we introduce a method to independently estimate the Lennard-Jones parameters used in an effective model of He interacting with graphene, and determine how they depend on strain. At short distances, we find that strain tends to reduce the interaction strength by pushing the location of the adsorption potential minima to higher distances above the deformed graphene sheet. This opens up the exciting possibility of mechanically engineering an adsorption potential, with implications for the formation and observation of anisotropic low-dimensional superfluid phases.

  3. Thioarsenides: A case for long-range Lewis acid-base-directed van der Waals interactions

    SciTech Connect

    Gibbs, Gerald V.; Wallace, Adam F.; Downs, R. T.; Ross, Nancy L.; Cox, David F.; Rosso, Kevin M.

    2011-04-01

    Electron density distributions, bond paths, Laplacian and local energy density properties have been calculated for a number of As4Sn (n = 3,4,5) thioarsenide molecular crystals. On the basis of the distributions, the intramolecular As-S and As-As interactions classify as shared bonded interactions and the intermolecular As-S, As-As and S-S interactions classify as closed-shell van der Waals bonded interactions. The bulk of the intermolecular As-S bond paths link regions of locally concentrated electron density (Lewis base regions) with aligned regions of locally depleted electron density (Lewis acid regions) on adjacent molecules. The paths are comparable with intermolecular paths reported for several other molecular crystals that link aligned Lewis base and acid regions in a key-lock fashion, interactions that classified as long range Lewis acid-base directed van der Waals interactions. As the bulk of the intermolecular As-S bond paths (~70%) link Lewis acid-base regions on adjacent molecules, it appears that molecules adopt an arrangement that maximizes the number of As-S Lewis acid-base intermolecular bonded interactions. The maximization of the number of Lewis acid-base interactions appears to be connected with the close-packed array adopted by molecules: distorted cubic close-packed arrays are adopted for alacránite, pararealgar, uzonite, realgar and β-AsS and the distorted hexagonal close-packed arrays adopted by α- and β-dimorphite. A growth mechanism is proposed for thioarsenide molecular crystals from aqueous species that maximizes the number of long range Lewis acid-base vdW As-S bonded interactions with the resulting directed bond paths structuralizing the molecules as a molecular crystal.

  4. Van der Waals quintessence stars

    SciTech Connect

    Lobo, Francisco S. N.

    2007-01-15

    The van der Waals quintessence equation of state is an interesting scenario for describing the late universe, and seems to provide a solution to the puzzle of dark energy, without the presence of exotic fluids or modifications of the Friedmann equations. In this work, the construction of inhomogeneous compact spheres supported by a van der Waals equation of state is explored. These relativistic stellar configurations shall be denoted as van der Waals quintessence stars. Despite of the fact that, in a cosmological context, the van der Waals fluid is considered homogeneous, inhomogeneities may arise through gravitational instabilities. Thus, these solutions may possibly originate from density fluctuations in the cosmological background. Two specific classes of solutions, namely, gravastars and traversable wormholes are analyzed. Exact solutions are found, and their respective characteristics and physical properties are further explored.

  5. Encapsulation of organic molecules in carbon nanotubes: role of the van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Dappe, Y. J.

    2014-02-01

    Carbon nanotubes are fascinating nano-objects not only from a fundamental point of view but also with respect to their remarkable properties, holding great potential in new materials design. When combined with organic molecules, these properties can be enhanced or modulated in order to fulfil the demand in domains as diverse as molecular electronics, biomaterials or even construction engineering, to name a few. To adequately conceive these hybrid materials it is essential to fully appreciate the nature of molecule-carbon nanotube interactions. In this review, we will discuss some relevant fundamental and applied research done on encapsulated molecules in carbon nanotubes. We will particularly focus on the weak and van der Waals interactions which rule the molecule-tube coupling. Therefore a small state of the art on the theoretical methods used to describe these interactions is presented here. Then, we will discuss various applications of molecular encapsulation, where we will consider structural, magnetic, charge transfer and transport, and optical properties.

  6. van der Waals interaction between a moving nano-cylinder and a liquid thin film.

    PubMed

    Ledesma-Alonso, René; Raphaël, Elie; Salez, Thomas; Tordjeman, Philippe; Legendre, Dominique

    2017-05-24

    We study the static and dynamic interaction between a horizontal cylindrical nano-probe and a thin liquid film. The effects of the physical and geometrical parameters, with a special focus on the film thickness, the probe speed, and the distance between the probe and the free surface are analyzed. Deformation profiles have been computed numerically from a Reynolds lubrication equation, coupled to a modified Young-Laplace equation, which takes into account the probe/liquid and the liquid/substrate non-retarded van der Waals interactions. We have found that the film thickness and the probe speed have a significant effect on the threshold separation distance below which the jump-to-contact instability is triggered. These results encourage the use of horizontal cylindrical nano-probes to scan thin liquid films, in order to determine either the physical or geometrical properties of the latter, through the measurement of interaction forces.

  7. Electrically Tunable van der Waals Interaction in Graphene-Molecule Complex.

    PubMed

    Muruganathan, Manoharan; Sun, Jian; Imamura, Tomonori; Mizuta, Hiroshi

    2015-12-09

    van der Waals (vdW) interactions play a central role in the surface-related physics and chemistry. Tuning of the correlated charge fluctuation in a vdW complex is a plausible way of modulating the molecules interaction at the atomic surface. Here, we report the vdW interaction tunability of the graphene-CO2 complex by combining the first-principles calculations with the vdW density functionals and the time evaluation measurements of CO2 molecules adsorption/desorption on graphene under an external electric field. The field-dependent charge transfer within the complex unveils the controllable tuning of CO2 from acceptor to donor. Meanwhile, the configuration of the adsorbed molecule, the equilibrium distance from graphene and O-C-O bonding angle, is modified accordingly. The range of electrical tunability is a unique feature for each type of molecule.

  8. Wavelike charge density fluctuations and van der Waals interactions at the nanoscale.

    PubMed

    Ambrosetti, Alberto; Ferri, Nicola; DiStasio, Robert A; Tkatchenko, Alexandre

    2016-03-11

    Recent experiments on noncovalent interactions at the nanoscale have challenged the basic assumptions of commonly used particle- or fragment-based models for describing van der Waals (vdW) or dispersion forces. We demonstrate that a qualitatively correct description of the vdW interactions between polarizable nanostructures over a wide range of finite distances can only be attained by accounting for the wavelike nature of charge density fluctuations. By considering a diverse set of materials and biological systems with markedly different dimensionalities, topologies, and polarizabilities, we find a visible enhancement in the nonlocality of the charge density response in the range of 10 to 20 nanometers. These collective wavelike fluctuations are responsible for the emergence of nontrivial modifications of the power laws that govern noncovalent interactions at the nanoscale. Copyright © 2016, American Association for the Advancement of Science.

  9. Electrophoresis of two spheres: Influence of double layer and van der Waals interactions.

    PubMed

    Tseng, Shiojenn; Huang, Chih-Hua; Hsu, Jyh-Ping

    2015-08-01

    Considering recent applications of electrophoresis conduced in nanoscaled devices, where particle-particle interaction can play a role, we studied for the first time the electrophoresis of two rigid spheres along their center line, taking account of the hydrodynamic, electric, and van der Waals interactions between them. Under the conditions of constant surface potential and surface charge density, the influences of the level of surface potential/charge density, the bulk salt concentration, and the particle-particle distance on their electrokinetic behaviors are examined. Numerical simulation reveals that these behaviors are much more complicated and interesting than those of isolated particles. In particular, we show that care must be taken in choosing an appropriate particle concentration in relevant experiment to avoid obtaining unreliable mobility data.

  10. Probing interlayer interactions in WS2 -graphene van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Chung, Ting Fung; Yuan, Long; Huang, Libai; Chen, Yong P.

    Two-dimensional crystals based van der Waals coupled heterostructures are of interest owing to their potential applications for flexible and transparent electronics and optoelectronics. The interaction between the 2D layered crystals at the interfaces of these heterostructures is crucial in determining the overall performance and is strongly affected by contamination and interfacial strain. We have fabricated heterostructures consisting of atomically thin exfoliated WS2 and chemical-vapor-deposited (CVD) graphene, and studied the interaction and coupling between the WS2 and graphene using atomic force microscopy (AFM), Raman spectroscopy and femtosecond transient absorption measurement (TAM). Information from Raman-active phonon modes allows us to estimate charge doping in graphene and interfacial strain on the crystals. Spatial imaging probed by TAM can be correlated to the heterostructure surface morphology measured by AFM and Raman maps of graphene and WS2, showing how the interlayer coupling alters exciton decay dynamics quantitatively.

  11. Are non-linear C-H⋯O contacts hydrogen bonds or Van der Waals interactions?. Establishing the limits between hydrogen bonds and Van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Novoa, Juan J.; Lafuente, Pilar; Mota, Fernando

    1998-07-01

    The hydrogen bond nature of angular C-H⋯O contacts is examined to determine when these contacts are better classified as hydrogen bonds or as Van der Waals bonds. To classify the bond we propose to look at the nature of the intermolecular bond critical point present in the electron density of the complex containing the bond. The physics behind this approach is explained using a qualitative orbital overlap model aimed at describing the main changes in the electronic density of the complex produced by the C-H⋯O bending.

  12. Nano-gratings and the atom-surface Van der Waals interaction

    NASA Astrophysics Data System (ADS)

    Lonij, Vincent; Holmgren, Will; McMorran, Ben; Cronin, Alex

    2009-05-01

    Nano-gratings are used in several atom- and electron-interferometers as coherent beamsplitters. Diffraction from these nano-gratings can be studied to observe the effect of the Van der Waals atom-surface interaction. In addition, these gratings have recently been used in atom-interferometers to detect a velocity dependent VdW induced phase shift. Determination of the VdW potential strength C3 from these studies, is limited by a lack of knowledge of the geometric parameters of the grating. Measurements of these parameters by conventional methods are plagued with several systematic errors. By studying diffraction of a beam of Na atoms at different angles of incidence, we are able to determine the geometric parameters with a precision that is competitive with conventional imaging methods. The precision is great enough to be able to notice the effect of atoms deposited on the gratings by the atom-beam.

  13. Classical van der Waals interactions between spherical bodies of dipolar fluid.

    PubMed

    Stenhammar, Joakim; Trulsson, Martin

    2011-07-01

    The van der Waals interaction free energy A(int) between two spherical bodies of Stockmayer fluid across a vacuum is calculated using molecular simulations and classical perturbation theory. The results are decomposed into their electrostatic and Lennard-Jones parts, and the former is shown to agree excellently with predictions from dielectric continuum theory. A(int) is decomposed into its energetic and entropic contributions and the results are compared with analytical predictions. Finally, we expand the electrostatic part of A(int) in a multipole expansion, and show that the surprisingly good agreement between the molecular and continuum descriptions is likely due to a cancellation of errors coming from the neglect of the discrete nature of the fluid within the dielectric description.

  14. Effect of van der Waals interactions on the stability of SiC polytypes

    SciTech Connect

    Kawanishi, Sakiko Mizoguchi, Teruyasu

    2016-05-07

    Density functional theory calculations with a correction of the long-range dispersion force, namely, the van der Waals (vdW) force, are performed for SiC polytypes. The lattice parameters are in good agreement with those obtained from the experiments. Furthermore, the stability of the polytypes in the experiments, which show 3C-SiC as the most stable, is reproduced by the present calculations. The effects of the vdW force on the electronic structure and the stability of polytypes are discussed. We observe that the vdW interaction is more sensitive to the cubic site than the hexagonal site. Thus, the influence of the vdW force increases with decreasing the hexagonality of the polytype, which results in the confirmation that the most stable polytype is 3C-SiC.

  15. Ytterbium in quantum gases and atomic clocks: van der Waals interactions and blackbody shifts.

    PubMed

    Safronova, M S; Porsev, S G; Clark, Charles W

    2012-12-07

    We evaluated the C(6) coefficients of Yb-Yb, Yb-alkali, and Yb-group II van der Waals interactions with 2% uncertainty. The only existing experimental result for such quantities is for the Yb-Yb dimer. Our value, C(6)=1929(39) a.u., is in excellent agreement with the recent experimental determination of 1932(35) a.u. We have also developed a new approach for the calculation of the dynamic correction to the blackbody radiation shift. We have calculated this quantity for the Yb 6s(2) (1)S(0)-6s6p (3)P(0)(o) clock transition with 3.5% uncertainty. This reduces the fractional uncertainty due to the blackbody radiation shift in the Yb optical clock at 300 K to the 10(-18) level.

  16. Van der Waals interactions between atoms and dispersive surfaces at finite temperature

    NASA Astrophysics Data System (ADS)

    Gorza, M.-P.; Ducloy, M.

    2006-12-01

    The long-range interactions between an atomic system in an arbitrary energy level and dispersive surfaces in thermal equilibrium at non-zero temperature are revisited within the framework of the quantum-mechanical linear response theory, using generalized susceptibilities for both atom and electromagnetic field. After defining the observables of interest, one presents a general analysis of the atomic level shift valid for any number and form of dielectric surfaces. It is shown that, at zero temperature, one recovers well-known results previously obtained in the linear response regime. The case of a plane dispersive surface is elaborated on in the non-retarded regime. Calculations are given in detail for a dielectric surface exhibiting a single polariton resonance. Theoretical predictions are presented within a physical viewpoint allowing one to discriminate between the various interaction processes: on one hand, the level shift induced by non-resonant quantum fluctuations, on the other hand, two potentially resonant atom-surface couplings. The first resonant process appears for excited-state atoms and originates in an atomic de-excitation channel resonantly coupled to the surface polariton mode. It exists also at zero temperature, and has been studied and observed previously. The second physical process, which exists at non-zero temperature only, corresponds to the reverse process in which a thermal quantum excitation of a surface polariton resonantly couples to an atomic absorption channel. This novel phenomenon is predicted as well for a ground state atom, and can turn the ordinary long-range van der Waals attraction of atoms into a surface repulsion at increasing temperatures. This opens the way to the control and engineering of the sign and amplitude of van der Waals forces via surface temperature adjustment.

  17. Silicon addition to hydroxyapatite increases nanoscale electrostatic, van der Waals, and adhesive interactions.

    PubMed

    Vandiver, Jennifer; Dean, Delphine; Patel, Nelesh; Botelho, Claudia; Best, Serena; Santos, José D; Lopes, Maria A; Bonfield, William; Ortiz, Christine

    2006-08-01

    The normal intersurface forces between nanosized probe tips functionalized with COO(-)-terminated alkanethiol self-assembling monolayers and dense, polycrystalline silicon-substituted synthetic hydroxyapatite (SiHA) and phase pure hydroxyapatite (HA) were measured via a nanomechanical technique called chemically specific high-resolution force spectroscopy. A significantly larger van der Waals interaction was observed for the SiHA compared to HA; Hamaker constants (A) were found to be A(SiHA) = 35 +/- 27 zJ and A(HA) = 13 +/- 12 zJ. Using the Derjaguin-Landau-Verwey-Overbeek approximation, which assumes linear additivity of the electrostatic double layer and van der Waals components, and the nonlinear Poisson-Boltzmann surface charge model for electrostatic double-layer forces, the surface charge per unit area, sigma (C/m(2)), was calculated as a function of position for specific nanosized areas within individual grains. SiHA was observed to be more negatively charged than HA with sigma(SiHA) = -0.024 +/- 0.013 C/m(2), two times greater than sigma(HA) = -0.011 +/- 0.006 C/m(2). Additionally, SiHA was found to have increased surface adhesion (0.7 +/- 0.3 nN) compared to HA (0.5 +/- 0.3 nN). The characterization of the nanoscale variations in surface forces of SiHA and HA will enable an improved understanding of the initial stages of bone-biomaterial bonding.

  18. Density, structure, and dynamics of water: the effect of van der Waals interactions.

    PubMed

    Wang, Jue; Román-Pérez, G; Soler, Jose M; Artacho, Emilio; Fernández-Serra, M-V

    2011-01-14

    It is known that ab initio molecular dynamics (AIMD) simulations of liquid water at ambient conditions, based on the generalized gradient approximation (GGA) to density functional theory (DFT), with commonly used functionals fail to produce structural and diffusive properties in reasonable agreement with experiment. This is true for canonical, constant temperature simulations where the density of the liquid is fixed to the experimental density. The equilibrium density, at ambient conditions, of DFT water has recently been shown by Schmidt et al. [J. Phys. Chem. B, 113, 11959 (2009)] to be underestimated by different GGA functionals for exchange and correlation, and corrected by the addition of interatomic pair potentials to describe van der Waals (vdW) interactions. In this contribution 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. [Phys. Rev. Lett. 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.

  19. Energy of van der Waals and dipole-dipole interactions between atoms in Rydberg states

    NASA Astrophysics Data System (ADS)

    Kamenski, A. A.; Manakov, N. L.; Mokhnenko, S. N.; Ovsiannikov, V. D.

    2017-09-01

    The van der Waals coefficient C6(θ ;n l J M ) of two like Rydberg atoms in their identical Rydberg states |n l J M 〉 is resolved into four irreducible components called scalar Rs s, axial (vector) Ra a, scalar-tensor Rs T=RT s , and tensor-tensor RT T parts in analogy with the components of dipole polarizabilities. The irreducible components determine the dependence of C6(θ ;n l J M ) on the angle θ between the interatomic and the quantization axes of atoms. The spectral resolution for the biatomic Green's function with account of the most contributing terms is used for evaluating the components Rα β of atoms in their Rydberg series of doublet states of the low angular momenta (2S , 2P , 2D , 2F ). The polynomial presentations in powers of the Rydberg-state principal quantum number n taking into account the asymptotic dependence C6(θ ;n l J M ) ∝n11 are derived for simplified evaluations of irreducible components. Numerical values of the polynomial coefficients are determined for Rb atoms in their n 2S1 /2 , n 2P1 /2 ,3 /2 , n 2D3 /2 ,5 /2 , and n 2F5 /2 ,7 /2 Rydberg states of arbitrary high n . The transformation of the van der Waals interaction law -C6/R6 into the dipole-dipole law C3/R3 in the case of close dipole-connected two-atomic states (the Förster resonance) is considered and the dependencies on the magnetic quantum numbers M and on the angle θ of the constant C3(θ ;n l J M ) are determined together with the ranges of interatomic distances R , where the transformation appears.

  20. Twisted Van der Waals Systems

    NASA Astrophysics Data System (ADS)

    Gani, Satrio; Rossi, Enrico

    Van der Waals systems formed by two-dimensional (2D) crystals and nanostructures possess electronic properties that make them extremely interesting for basic science and for possible technological applications. By tuning the relative angle (the twist angle) between the layers, or nanostructures, forming the Van der Waals systems experimentalists have been able to control the stacking configuration of such systems. We study the dependence on the twist angle of the electronic properties of two classes of Van der Waals systems: double layers formed by two, one-atom thick, layers of a metal dichalcogenide such as molybdenum disulfide (MoS2), and graphene nanoribbons on a hexagonal boron nitride substrate. We present results that show how, for both classes of systems, the electronic properties can be strongly tuned via the twist angle. Work supported by ACS-PRF-53581-DNI5 and NSF-DMR-1455233.

  1. Van der Waals Interactions and Dipole Blockade in a Cold Rydberg Gas Probed by Microwave Spectroscopy

    NASA Astrophysics Data System (ADS)

    Nguyen, Thanh Long; Celistrino Teixeira, Raul; Hermann Avigliano, Carla; Cantat Moltrecht, Tigrane; Raimond, Jean Michel; Haroche, Serge; Gleyzes, Sebastiens; Brune, Michel

    2016-05-01

    Dipole-dipole interactions between Rydberg atoms are a flourishing tool for quantum information processing and for quantum simulation of complex many-body problems. Microwave spectroscopy of a dense Rydberg gas trapped close to a superconducting atom chip in the strong dipole blockade regime reveals directly the many-body atomic interaction spectrum. We present here a direct measurement of the interaction energy distribution in the strong dipole blockade regime, based on microwave spectroscopy. We first apply this method to the observation of the excitation dynamics of the Rydberg gas, conditioned by dipole-dipole interactions, in either the strong blockade regime or the so-called facilitation regime. We also observe with this method the atomic cloud expansion driven by the repulsive Van der Waals interaction after excitation. This measurement, in good agreement with Monte Carlo simulations of the excitation process and of the cloud dynamics, reveals the limits of the frozen gas approximation. This method can help investigate self-organization and dynamical phase transitions in Rydberg-atom based quantum simulators. This study thus opens a promising route for quantum simulation of many-body systems and quantum information transport in chains of strongly interacting Rydberg atom.

  2. Van der Waals forces and electron-electron interactions in two strained graphene layers

    NASA Astrophysics Data System (ADS)

    Sharma, Anand; Harnish, Peter; Sylvester, Alexander; Kotov, Valeri N.

    2014-03-01

    We evaluate the van der Waals (vdW) interaction energy at T=0 between two undoped graphene layers which are separated by a finite distance. Our study is carried out within the Random Phase Approximation and the interaction energy is obtained for variation in the strength of effective Coulomb interaction and anisotropy due to applied uniaxial strain. We consider the following three models for the anisotropic case: a) where one of the two layers is uniaxially strained, b) the two layers are strained in the same direction, and c) one of the layers is strained in the perpendicular direction. We find that for all the three models and any given value of the coupling, the vdW interaction energy increases with increasing anisotropy. The effect is most striking for the case when both the layers are strained in the parallel direction where we observe up to an order of magnitude increase in the strained graphene relative to the unstrained case. We also investigate the effect of intra-layer electron-electron interactions in the region of large separation between the strained graphene layers. We conclude that the many-body contributions to the correlation energy are non-negligible and the vdW interaction energy decreases as a function of increasing distance between the layers. Alexander Sylvester acknowledges financial assistance from the Research Experiences for Undergraduates (REU) Program of the National Science Foundation (NSF) focussing on complex materials.

  3. van der Waals forces and electron-electron interactions in two strained graphene layers

    NASA Astrophysics Data System (ADS)

    Sharma, Anand; Harnish, Peter; Sylvester, Alexander; Kotov, Valeri N.; Neto, A. H. Castro

    2014-06-01

    We evaluate the van der Waals (vdW) interaction energy at zero temperature between two undoped strained graphene layers separated by a finite distance. We consider the following three models for the anisotropic case: (a) where one of the two layers is uniaxially strained, (b) the two layers are strained in the same direction, and (c) one of the layers is strained in the perpendicular direction with respect to the other. We find that for all three models and given value of the electron-electron interaction coupling, the vdW interaction energy increases with increasing anisotropy. The effect is most striking for the case when both layers are strained in the same direction where we observe up to an order of magnitude increase in the strained relative to the unstrained case. We also investigate the effect of electron-electron interaction renormalization in the region of large separation between the strained graphene layers. We find that the many-body renormalization contributions to the correlation energy are non-negligible and the vdW interaction energy decreases as a function of increasing distance between the layers due to renormalization of the Fermi velocity, the anisotropy, and the effective interaction. Our analysis can be useful in designing graphene-based vdW heterostructures which, in recent times, has seen an upsurge in research activity.

  4. Unifying Microscopic and Continuum Treatments of van der Waals and Casimir Interactions.

    PubMed

    Venkataram, Prashanth S; Hermann, Jan; Tkatchenko, Alexandre; Rodriguez, Alejandro W

    2017-06-30

    We present an approach for computing long-range van der Waals (vdW) interactions between complex molecular systems and arbitrarily shaped macroscopic bodies, melding atomistic treatments of electronic fluctuations based on density functional theory in the former with continuum descriptions of strongly shape-dependent electromagnetic fields in the latter, thus capturing many-body and multiple scattering effects to all orders. Such a theory is especially important when considering vdW interactions at mesoscopic scales, i.e., between molecules and structured surfaces with features on the scale of molecular sizes, in which case the finite sizes, complex shapes, and resulting nonlocal electronic excitations of molecules are strongly influenced by electromagnetic retardation and wave effects that depend crucially on the shapes of surrounding macroscopic bodies. We show that these effects together can modify vdW interaction energies and forces, as well as molecular shapes deformed by vdW interactions, by orders of magnitude compared to previous treatments based on Casimir-Polder, nonretarded, or pairwise approximations, which are valid only at macroscopically large or atomic-scale separations or in dilute insulating media, respectively.

  5. Unusual role of epilayer–substrate interactions in determining orientational relations in van der Waals epitaxy

    PubMed Central

    Liu, Lei; Siegel, David A.; Chen, Wei; Liu, Peizhi; Guo, Junjie; Duscher, Gerd; Zhao, Chong; Wang, Hao; Wang, Wenlong; Bai, Xuedong; McCarty, Kevin F.; Zhang, Zhenyu; Gu, Gong

    2014-01-01

    Using selected-area low-energy electron diffraction analysis, we showed strict orientational alignment of monolayer hexagonal boron nitride (h-BN) crystallites with Cu(100) surface lattices of Cu foil substrates during atmospheric pressure chemical vapor deposition. In sharp contrast, the graphene–Cu(100) system is well-known to assume a wide range of rotations despite graphene’s crystallographic similarity to h-BN. Our density functional theory calculations uncovered the origin of this surprising difference: The crystallite orientation is determined during nucleation by interactions between the cluster’s edges and the substrate. Unlike the weaker B– and N–Cu interactions, strong C–Cu interactions rearrange surface Cu atoms, resulting in the aligned geometry not being a distinct minimum in total energy. The discovery made in this specific case runs counter to the conventional wisdom that strong epilayer–substrate interactions enhance orientational alignment in epitaxy and sheds light on the factors that determine orientational relation in van der Waals epitaxy of 2D materials. PMID:25385622

  6. Unifying Microscopic and Continuum Treatments of van der Waals and Casimir Interactions

    NASA Astrophysics Data System (ADS)

    Venkataram, Prashanth S.; Hermann, Jan; Tkatchenko, Alexandre; Rodriguez, Alejandro W.

    2017-06-01

    We present an approach for computing long-range van der Waals (vdW) interactions between complex molecular systems and arbitrarily shaped macroscopic bodies, melding atomistic treatments of electronic fluctuations based on density functional theory in the former with continuum descriptions of strongly shape-dependent electromagnetic fields in the latter, thus capturing many-body and multiple scattering effects to all orders. Such a theory is especially important when considering vdW interactions at mesoscopic scales, i.e., between molecules and structured surfaces with features on the scale of molecular sizes, in which case the finite sizes, complex shapes, and resulting nonlocal electronic excitations of molecules are strongly influenced by electromagnetic retardation and wave effects that depend crucially on the shapes of surrounding macroscopic bodies. We show that these effects together can modify vdW interaction energies and forces, as well as molecular shapes deformed by vdW interactions, by orders of magnitude compared to previous treatments based on Casimir-Polder, nonretarded, or pairwise approximations, which are valid only at macroscopically large or atomic-scale separations or in dilute insulating media, respectively.

  7. Spectroscopic studies of van der Waals bonding and interactions in microporous materials

    NASA Astrophysics Data System (ADS)

    Chabal, Yves

    2013-03-01

    Van der Waals interactions govern the interaction of gas phase molecules in microporous materials. New theoretical approaches, such as DF-vdW methods, have brought great insight into the results of vdW forces on the adsorption and diffusion properties of molecular guests. In this talk, we highlight the role of vibrational spectroscopies (infrared and Raman) in providing information that can directly test such theoretical approaches. Typically, vdW interactions lead to measurable shifts in molecular internal modes, which can be calculated. We also show that vdW interactions often lead to minor structural alteration or reconfiguration of the microporous hosts, which can clearly be observed by IR or Raman spectroscopy. Examples will be taken from molecular hydrogen storage and gas phase separation in Metal Organic Framework materials, which represent a versatile class of porous materials. For example, the origin of interesting ``gate opening'' phenomena in flexible MOFs, leading to highly selective adsorption, will be described. This work was supported by the Department of Energy, Basic Energy Sciences, division of Materials Sciences and Engineering (DOE grant No. DE-FG02-08ER46491).

  8. Size effects in aerosol particle interactions: the van der Waals potential and collision rates

    SciTech Connect

    Marlow, W H

    1980-01-01

    Three effects which are explicitly dependent on aerosol particle size are identified and discussed. They are focussed about the particle collision rate and how it relates to the properties of the gas, the particle, and the particle's interaction potential energy which play roles in particle-particle collision rates. By incorporating the conduction electronic free path effect for conductors into the frequency-dependent dielectric constants of silver and graphite, particle size effects in the Lifshitz-van der Waals potentials for identical pairs of 1 nm and 100 nm particles are evaluated. Water and tetradecane particle interaction potentials for the same size particles are also calculated to illustrate size effects due to the retardation of the interaction. These potentials are then used to calculate the enhancement of the particle collision rates above their values in the absence of any potential at various gas pressures. The roles of the interaction potential in collision among identical pairs of particles of differing compositions is also briefly discussed.

  9. Finite-size effects and nonadditivity in the van der Waals interaction

    SciTech Connect

    Melo e Souza, Reinaldo de; Kort-Kamp, W. J. M.; Sigaud, C.; Farina, C.

    2011-11-15

    We obtain analytically the exact nonretarded dispersive interaction energy between an atom and a perfectly conducting disk. We consider the atom in the symmetry axis of the disk and assume that the atom is predominantly polarizable in the direction of this axis. For this situation we discuss the finite-size effects on the corresponding interaction energy. We follow the recent procedure introduced by Eberlein and Zietal together with the old and powerful Sommerfeld's image method for nontrivial geometries. For the sake of clarity we present a detailed discussion of Sommerfeld's image method. Comparing our results for the atom-disk system with those recently obtained for an atom near a conducting plane with a circular aperture, we discuss the nonadditivity of the van der Waals interactions involving an atom and two complementary surfaces. We show that there is a given ratio z/a between the distance z from the atom to the center of the disk (aperture) and the radius of the disk a (aperture) for which nonadditivity effects vanish. Qualitative arguments suggest that this quite unexpected result will occur not only for a circular hole, but for any other symmetric hole.

  10. van der Waals Interactions in Hadron Resonance Gas: From Nuclear Matter to Lattice QCD

    NASA Astrophysics Data System (ADS)

    Vovchenko, Volodymyr; Gorenstein, Mark I.; Stoecker, Horst

    2017-05-01

    An extension of the ideal hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters a and b are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within the VDW-HRG model. Compared to the ideal HRG model, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of second and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region T ˜140 - 190 MeV . For many observables this behavior resembles closely the results obtained from lattice QCD simulations. This hadronic model also predicts nontrivial behavior of net-baryon fluctuations in the region of phase diagram probed by heavy-ion collision experiments. These results imply that VDW interactions play a crucial role in the thermodynamics of hadron gas. Thus, the commonly performed comparisons of the ideal HRG model with the lattice and heavy-ion data may lead to misconceptions and misleading conclusions.

  11. Electronic charge rearrangement at metal/organic interfaces induced by weak van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Ferri, Nicola; Ambrosetti, Alberto; Tkatchenko, Alexandre

    2017-07-01

    Electronic charge rearrangements at interfaces between organic molecules and solid surfaces play a key role in a wide range of applications in catalysis, light-emitting diodes, single-molecule junctions, molecular sensors and switches, and photovoltaics. It is common to utilize electrostatics and Pauli pushback to control the interface electronic properties, while the ubiquitous van der Waals (vdW) interactions are often considered to have a negligible direct contribution (beyond the obvious structural relaxation). Here, we apply a fully self-consistent Tkatchenko-Scheffler vdW density functional to demonstrate that the weak vdW interactions can induce sizable charge rearrangements at hybrid metal/organic systems (HMOS). The complex vdW correlation potential smears out the interfacial electronic density, thereby reducing the charge transfer in HMOS, changes the interface work functions by up to 0.2 eV, and increases the interface dipole moment by up to 0.3 Debye. Our results suggest that vdW interactions should be considered as an additional control parameter in the design of hybrid interfaces with the desired electronic properties.

  12. Self-Consistent Density Functional Including Long-Range van der Waals Interactions

    NASA Astrophysics Data System (ADS)

    Ferri, Nicola; Distasio, Robert A., Jr.; Car, Roberto; Scheffler, Matthias; Tkatchenko, Alexandre

    2013-03-01

    Van der Waals (vdW) interactions are significant for a wide variety of systems, from noble-gas dimers to organic/inorganic interfaces. The long-range vdW energy is a tiny fraction (0.001%) of the total energy, hence it is typically assumed not to change electronic properties. Although the vdW-DF functional includes the effect of vdW energy on electronic structure, the influence of ``true'' long-range vdW interactions is difficult to assess since a significant part of vdW-DF energy arises from short distances. Here, we present a self-consistent (SC) implementation of the long-range Tkatchenko-Scheffler (TS) functional, including its extension to surfaces. The analysis of self-consistency for rare-gas dimers allows us to reconcile two different views on vdW interactions: (i) Feynman's view that claims changes in the electron density and (ii) atoms separated by infinite barrier. In agreement with previous work, we find negligible contribution from self-consistency in the structure and stability of vdW-bound complexes. However, a closer look at organic/inorganic interfaces reveals notable modification of energy levels when using the SC-TS vdW density functional.

  13. Anisotropic Charge Distribution and Anisotropic van der Waals Radius Leading to Intriguing Anisotropic Noncovalent Interactions

    PubMed Central

    Kim, Hahn; Van Dung Doan; Cho, Woo Jong; Madhav, Miriyala Vijay; Kim, Kwang S.

    2014-01-01

    Although group (IV–VII) nonmetallic elements do not favor interacting with anionic species, there are counterexamples including the halogen bond. Such binding is known to be related to the charge deficiency because of the adjacent atom's electron withdrawing effect, which creates σ/π-holes at the bond-ends. However, a completely opposite behavior is exhibited by N2 and O2, which have electrostatically positive/negative character around cylindrical-bond-surface/bond-ends. Inspired by this, here we elucidate the unusual features and origin of the anisotropic noncovalent interactions in the ground and excited states of the 2nd and 3rd row elements belonging to groups IV–VII. The anisotropy in charge distributions and van der Waals radii of atoms in such molecular systems are scrutinized. This provides an understanding of their unusual molecular configuration, binding and recognition modes involved in new types of molecular assembling and engineering. This work would lead to the design of intriguing molecular systems exploiting anisotropic noncovalent interactions. PMID:25059645

  14. van der Waals Interactions in Hadron Resonance Gas: From Nuclear Matter to Lattice QCD.

    PubMed

    Vovchenko, Volodymyr; Gorenstein, Mark I; Stoecker, Horst

    2017-05-05

    An extension of the ideal hadron resonance gas (HRG) model is constructed which includes the attractive and repulsive van der Waals (VDW) interactions between baryons. This VDW-HRG model yields the nuclear liquid-gas transition at low temperatures and high baryon densities. The VDW parameters a and b are fixed by the ground state properties of nuclear matter, and the temperature dependence of various thermodynamic observables at zero chemical potential are calculated within the VDW-HRG model. Compared to the ideal HRG model, the inclusion of VDW interactions between baryons leads to a qualitatively different behavior of second and higher moments of fluctuations of conserved charges, in particular in the so-called crossover region T∼140-190  MeV. For many observables this behavior resembles closely the results obtained from lattice QCD simulations. This hadronic model also predicts nontrivial behavior of net-baryon fluctuations in the region of phase diagram probed by heavy-ion collision experiments. These results imply that VDW interactions play a crucial role in the thermodynamics of hadron gas. Thus, the commonly performed comparisons of the ideal HRG model with the lattice and heavy-ion data may lead to misconceptions and misleading conclusions.

  15. Quantum Transport Detected by Strong Proximity Interaction at a Graphene-WS2 van der Waals Interface.

    PubMed

    O'Farrell, E C T; Avsar, A; Tan, J Y; Eda, G; Özyilmaz, B

    2015-09-09

    Magnetotransport measurements demonstrate that graphene in a van der Waals heterostructure is a sensitive probe of quantum transport in an adjacent WS2 layer via strong Coulomb interactions. We observe a large low-field magnetoresistance (≫ e(2)/h) and a -ln T temperature dependence of the resistance. In-plane magnetic field resistance indicates the origin is orbital and nonclassical. We demonstrate a strong electron-hole asymmetry in the mobility and coherence length of graphene demonstrating the presence of localized Coulomb interactions with ionized donors in the WS2 substrate, which ultimately leads to screening as the Fermi level of graphene is tuned toward the conduction band of WS2. This leads us to conclude that graphene couples to quantum localization processes in WS2 via the Coulomb interaction and results in the observed signatures of quantum transport. Our results show that theoretical descriptions of the van der Waals interface should not ignore localized strong correlations.

  16. Towards Efficient and General Method for Many-Body van-der-Waals Interactions

    NASA Astrophysics Data System (ADS)

    Tkatchenko, Alexandre

    2012-02-01

    Van der Waals interactions are intrinsically many-body phenomena, arising from collective electron fluctuations in a given material. Adiabatic connection fluctuation-dissipation theorem (ACFDT) allows to compute the many-body vdW interactions accurately. However, the ACFDT computational cost is prohibitive for real materials, even when the random-phase approximation is employed for the response function. We show how the problem of computing the long-range many-body vdW energy for real systems can be solved efficiently by mapping the system (molecule or condensed matter) onto a collection of quantum harmonic oscillators. Currently, our method, which couples density-functional theory with the many-body dispersion energy (DFT+MBD), is developed for non-metallic system [A. Tkatchenko, R. A. DiStasio Jr., R. Car, M. Scheffler, submitted]. The DFT+MBD method includes the hybridization effects by using the Tkatchenko-Scheffler approach [PRL 102, 073005 (2009)], the long-range Coulomb screening through classical electrodynamics [B. U. Felderhof, Physica 29, 1569 (1974)], and the many-body vdW energy from the coupled-fluctuating dipole model [M. W. Cole et al., Mol. Simul. 35, 849 (2009)]. The successes of the DFT+MBD approach and the many challenges that lie ahead will be discussed.

  17. Benchmark data base for accurate van der Waals interaction in inorganic fragments

    NASA Astrophysics Data System (ADS)

    Brndiar, Jan; Stich, Ivan

    2012-02-01

    A range of inorganic materials, such as Sb, As, P, S, Se are built from van der Waals (vdW) interacting units forming the crystals, which neither the standard DFT GGA description as well as cheap quantum chemistry methods, such as MP2, do not describe correctly. We use this data base, for which have performed ultra accurate CCSD(T) calculations in complete basis set limit, to test the alternative approximate theories, such as Grimme [1], Langreth-Lundqvist [2], and Tkachenko-Scheffler [3]. While none of these theories gives entirely correct description, Grimme consistently provides more accurate results than Langreth-Lundqvist, which tend to overestimate the distances and underestimate the interaction energies for this set of systems. Contrary Tkachenko-Scheffler appear to yield surprisingly accurate and computationally cheap and convenient description applicable also for systems with appreciable charge transfer. [4pt] [1] S. Grimme, J. Comp. Chem. 27, 1787 (2006) [0pt] [2] K. Lee, et al., Phys. Rev. B 82 081101 (R) (2010) [0pt] [3] Tkachenko and M. Scheffler Phys. Rev. Lett. 102 073005 (2009).

  18. How van der Waals interactions determine the unique properties of water.

    PubMed

    Morawietz, Tobias; Singraber, Andreas; Dellago, Christoph; Behler, Jörg

    2016-07-26

    Whereas the interactions between water molecules are dominated by strongly directional hydrogen bonds (HBs), it was recently proposed that relatively weak, isotropic van der Waals (vdW) forces are essential for understanding the properties of liquid water and ice. This insight was derived from ab initio computer simulations, which provide an unbiased description of water at the atomic level and yield information on the underlying molecular forces. However, the high computational cost of such simulations prevents the systematic investigation of the influence of vdW forces on the thermodynamic anomalies of water. Here, we develop efficient ab initio-quality neural network potentials and use them to demonstrate that vdW interactions are crucial for the formation of water's density maximum and its negative volume of melting. Both phenomena can be explained by the flexibility of the HB network, which is the result of a delicate balance of weak vdW forces, causing, e.g., a pronounced expansion of the second solvation shell upon cooling that induces the density maximum.

  19. The role of van der Waals interactions in the adsorption of noble gases on metal surfaces.

    PubMed

    Chen, De-Li; Al-Saidi, W A; Johnson, J Karl

    2012-10-24

    Adsorption of noble gases on metal surfaces is determined by weak interactions. We applied two versions of the nonlocal van der Waals density functional (vdW-DF) to compute adsorption energies of Ar, Kr, and Xe on Pt(111), Pd(111), Cu(111), and Cu(110) metal surfaces. We compared our results with data obtained using other density functional approaches, including the semiempirical vdW-corrected DFT-D2. The vdW-DF results show considerable improvements in the description of adsorption energies and equilibrium distances over other DFT based methods, giving good agreement with experiments. We also calculated perpendicular vibrational energies for noble gases on the metal surfaces using vdW-DF data and found excellent agreement with available experimental results. Our vdW-DF calculations show that adsorption of noble gases on low-coordination sites is energetically favored over high-coordination sites, but only by a few meV. Analysis of the two-dimensional potential energy surface shows that the high-coordination sites are local maxima on the two-dimensional potential energy surface and therefore unlikely to be observed in experiments; this provides an explanation of the experimental observations. The DFT-D2 approach with the standard parameterization was found to overestimate the dispersion interactions, and to give the wrong adsorption site preference for four of the nine systems we studied.

  20. How van der Waals interactions determine the unique properties of water

    PubMed Central

    Morawietz, Tobias; Singraber, Andreas; Dellago, Christoph; Behler, Jörg

    2016-01-01

    Whereas the interactions between water molecules are dominated by strongly directional hydrogen bonds (HBs), it was recently proposed that relatively weak, isotropic van der Waals (vdW) forces are essential for understanding the properties of liquid water and ice. This insight was derived from ab initio computer simulations, which provide an unbiased description of water at the atomic level and yield information on the underlying molecular forces. However, the high computational cost of such simulations prevents the systematic investigation of the influence of vdW forces on the thermodynamic anomalies of water. Here, we develop efficient ab initio-quality neural network potentials and use them to demonstrate that vdW interactions are crucial for the formation of water’s density maximum and its negative volume of melting. Both phenomena can be explained by the flexibility of the HB network, which is the result of a delicate balance of weak vdW forces, causing, e.g., a pronounced expansion of the second solvation shell upon cooling that induces the density maximum. PMID:27402761

  1. The role of van der Waals interactions in the adsorption of noble gases on metal surfaces

    SciTech Connect

    Chen, De-Li; Al-Saidi, W A; Johnson, J Karl

    2012-10-03

    Adsorption of noble gases on metal surfaces is determined by weak interactions. We applied two versions of the nonlocal van der Waals density functional (vdW-DF) to compute adsorption energies of Ar, Kr, and Xe on Pt(111), Pd(111), Cu(111), and Cu(110) metal surfaces. We have compared our results with data obtained using other density functional approaches, including the semiempirical vdW corrected DFT-D2. The vdW-DF results show considerable improvements in the description of adsorption energies and equilibrium distances over other DFTbased methods, giving good agreement with experiments. We have also calculated perpendicular vibrational energies for noble gases on the metal surfaces using vdWDF data and found excellent agreement with available experimental results. Our vdW-DF calculations show that adsorption of noble gases on low-coordination sites is energetically favored over high-coordination sites, but only by a few meV. Analysis of the 2-dimensional potential energy surface shows that the high-coordination sites are local maxima on the 2-dimensional potential energy surface and therefore unlikely to be observed in experiments, which provides an explanation of the experimental observations. The DFT-D2 approach with the standard parameterization was found to overestimate the dispersion interactions, and to give the wrong adsorption site preference for four of the nine systems we studied.

  2. Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light-Matter Interactions.

    PubMed

    Niu, Lin; Liu, Xinfeng; Cong, Chunxiao; Wu, Chunyang; Wu, Di; Chang, Tay Rong; Wang, Hong; Zeng, Qingsheng; Zhou, Jiadong; Wang, Xingli; Fu, Wei; Yu, Peng; Fu, Qundong; Najmaei, Sina; Zhang, Zhuhua; Yakobson, Boris I; Tay, Beng Kang; Zhou, Wu; Jeng, Horng Tay; Lin, Hsin; Sum, Tze Chien; Jin, Chuanhong; He, Haiyong; Yu, Ting; Liu, Zheng

    2015-12-16

    High-quality organic and inorganic van der Waals (vdW) solids are realized using methylammonium lead halide (CH3 NH3 PbI3 ) as the organic part (organic perovskite) and 2D inorganic monolayers as counterparts. By stacking on various 2D monolayers, the vdW solids exhibit dramatically different light emissions. Futhermore, organic/h-BN vdW solid arrays are patterned for red-light emission.

  3. Water monomer interaction with gold nanoclusters from van der Waals density functional theory.

    PubMed

    Xue, Yongqiang

    2012-01-14

    We investigate the interaction between water molecules and gold nanoclusters Au(n) through a systematic density functional theory study within both the generalized gradient approximation and the nonlocal van der Waals (vdW) density functional theory. Both planar (n = 6-12) and three-dimensional (3D) clusters (n = 17-20) are studied. We find that applying vdW density functional theory leads to an increase in the Au-Au bond length and a decrease in the cohesive energy for all clusters studied. We classify water adsorption on nanoclusters according to the corner, edge, and surface adsorption geometries. In both corner and edge adsorptions, water molecule approaches the cluster through the O atom. For planar clusters, surface adsorption occurs in a O-up/H-down geometry with water plane oriented nearly perpendicular to the cluster. For 3D clusters, water instead favors a near-flat surface adsorption geometry with the water O atom sitting nearly atop a surface Au atom, in agreement with previous study on bulk surfaces. Including vdW interaction increases the adsorption energy for the weak surface adsorption but reduces the adsorption energy for the strong corner adsorption due to increased water-cluster bond length. By analyzing the adsorption induced charge rearrangement through Bader's charge partitioning and electron density difference and the orbital interaction through the projected density of states, we conclude that the bonding between water and gold nanocluster is determined by an interplay between electrostatic interaction and covalent interaction involving both the water lone-pair and in-plane orbitals and the gold 5d and 6s orbitals. Including vdW interaction does not change qualitatively the physical picture but does change quantitatively the adsorption structure due to the fluxionality of gold nanoclusters.

  4. Spectroscopic characterization of van der Waals interactions in a metal organic framework with unsaturated metal centers: MOF-74-Mg.

    PubMed

    Nijem, Nour; Canepa, Pieremanuele; Kong, Lingzhu; Wu, Haohan; Li, Jing; Thonhauser, Timo; Chabal, Yves J

    2012-10-24

    The adsorption energies of small molecules in nanoporous materials are often determined by isotherm measurements. The nature of the interaction and the response of the host material, however, can best be studied by spectroscopic methods. We show here that infrared absorption and Raman spectroscopy measurements together with density functional theory calculations, utilizing the novel van der Waals density functional vdW-DF, constitute a powerful approach to studying the weak van der Waals interactions associated with the incorporation of small molecules in these materials. In particular, we show how vdW-DF assists the interpretation of the vibrational spectroscopy data to uncover the binding sites and energies of these molecules, including the subtle dependence on loading of the IR asymmetric stretch mode of CO(2) when adsorbed in MOF-74-Mg. To gain a better understanding of the adsorption mechanism of CO(2) in MOF-74-Mg, the results are compared with CO within MOF-74-Mg.

  5. Gold nanoparticle assemblies stabilized by bis(phthalocyaninato)lanthanide(III) complexes through van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Noda, Yuki; Noro, Shin-Ichiro; Akutagawa, Tomoyuki; Nakamura, Takayoshi

    2014-01-01

    Gold nanoparticle assemblies possess diverse application potential, ranging from industrial nanotechnology to medical biotechnology. Because the structures and properties of assemblies are directly affected by the stabilization mechanism between the organic molecules serving as protecting ligands and the gold nanoparticle surface, it is crucial to find and investigate new stabilization mechanisms. Here, we report that π-conjugated phthalocyanine rings can serve as stabilizing ligands for gold nanoparticles. Bis(phthalocyaninato)lutetium(III) (LuPc2) or bis(phthalocyaninato)terbium(III) (TbPc2), even though complex, do not have specific binding units and stabilize gold nanoparticles through van der Waals interaction between parallel adsorbed phthalocyanine ligands and the gold nanoparticle surface. AC magnetic measurements and the electron-transport properties of the assemblies give direct evidence that the phthalocyanines are isolated from each other. Each nanoparticle shows weak electronic coupling despite the short internanoparticle distance (~1 nm), suggesting Efros-Shklovskii-type variable-range hopping and collective single-electron tunnelling behaviours.

  6. Efficient Oscillator-Based Approach for Polarizability and van der Waals Interactions

    NASA Astrophysics Data System (ADS)

    Gobre, Vivekanand; Distasio, Robert A., Jr.; Car, Roberto; Scheffler, Matthias; Tkatchenko, Alexandre

    2013-03-01

    The dynamic polarizability measures the response to an applied time-dependent electric field, and its accurate determination is crucial for van der Waals (vdW) interactions and other response properties. First-principles calculations of polarizabilities in principle require a computationally expensive explicit treatment of many-electron excitations, and are only applicable in practice to systems with less than about 100 atoms. In this work, we present an efficient parameter-free approach for calculating accurate frequency dependent polarizabilities for molecules with thousands of atoms, as well as periodic materials. This is achieved by the synergistic coupling of the Tkatchenko-Scheffler method, which accurately treats short-range hybridization effects, with the self-consistent screening equation from classical electrodynamics. Using only the electron density and free atom reference, we obtain an accuracy of 7% for both static polarizabilities and vdW coefficients for an extensive database of gas-phase molecules and crystals. We analyze the interplay of hybridization and long-range electrostatic screening effects on the polarizability.

  7. Gold nanoparticle assemblies stabilized by bis(phthalocyaninato)lanthanide(III) complexes through van der Waals interactions

    PubMed Central

    Noda, Yuki; Noro, Shin-ichiro; Akutagawa, Tomoyuki; Nakamura, Takayoshi

    2014-01-01

    Gold nanoparticle assemblies possess diverse application potential, ranging from industrial nanotechnology to medical biotechnology. Because the structures and properties of assemblies are directly affected by the stabilization mechanism between the organic molecules serving as protecting ligands and the gold nanoparticle surface, it is crucial to find and investigate new stabilization mechanisms. Here, we report that π-conjugated phthalocyanine rings can serve as stabilizing ligands for gold nanoparticles. Bis(phthalocyaninato)lutetium(III) (LuPc2) or bis(phthalocyaninato)terbium(III) (TbPc2), even though complex, do not have specific binding units and stabilize gold nanoparticles through van der Waals interaction between parallel adsorbed phthalocyanine ligands and the gold nanoparticle surface. AC magnetic measurements and the electron-transport properties of the assemblies give direct evidence that the phthalocyanines are isolated from each other. Each nanoparticle shows weak electronic coupling despite the short internanoparticle distance (~1 nm), suggesting Efros–Shklovskii-type variable-range hopping and collective single-electron tunnelling behaviours. PMID:24441566

  8. Unusual Exciton-Phonon Interactions at van der Waals Engineered Interfaces.

    PubMed

    Chow, Colin M; Yu, Hongyi; Jones, Aaron M; Yan, Jiaqiang; Mandrus, David G; Taniguchi, Takashi; Watanabe, Kenji; Yao, Wang; Xu, Xiaodong

    2017-02-08

    Raman scattering is a ubiquitous phenomenon in light-matter interactions, which reveals a material's electronic, structural, and thermal properties. Controlling this process would enable new ways of studying and manipulating fundamental material properties. Here, we report a novel Raman scattering process at the interface between different van der Waals (vdW) materials as well as between a monolayer semiconductor and 3D crystalline substrates. We find that interfacing a WSe2 monolayer with materials such as SiO2, sapphire, and hexagonal boron nitride (hBN) enables Raman transitions with phonons that are either traditionally inactive or weak. This Raman scattering can be amplified by nearly 2 orders of magnitude when a foreign phonon mode is resonantly coupled to the A exciton in WSe2 directly or via an A1(') optical phonon from WSe2. We further showed that the interfacial Raman scattering is distinct between hBN-encapsulated and hBN-sandwiched WSe2 sample geometries. This cross-platform electron-phonon coupling, as well as the sensitivity of 2D excitons to their phononic environments, will prove important in the understanding and engineering of optoelectronic devices based on vdW heterostructures.

  9. Unusual exciton–phonon interactions at van der Waals engineered interfaces

    DOE PAGES

    Chow, Colin M.; Yu, Hongyi; Jones, Aaron M.; ...

    2017-01-13

    Raman scattering is a ubiquitous phenomenon in light–matter interactions, which reveals a material’s electronic, structural, and thermal properties. Controlling this process would enable new ways of studying and manipulating fundamental material properties. Here, we report a novel Raman scattering process at the interface between different van der Waals (vdW) materials as well as between a monolayer semiconductor and 3D crystalline substrates. We find that interfacing a WSe2 monolayer with materials such as SiO2, sapphire, and hexagonal boron nitride (hBN) enables Raman transitions with phonons that are either traditionally inactive or weak. This Raman scattering can be amplified by nearly 2more » orders of magnitude when a foreign phonon mode is resonantly coupled to the A exciton in WSe2 directly or via an A1' optical phonon from WSe2. We further showed that the interfacial Raman scattering is distinct between hBN-encapsulated and hBN-sandwiched WSe2 sample geometries. Finally, this cross-platform electron–phonon coupling, as well as the sensitivity of 2D excitons to their phononic environments, will prove important in the understanding and engineering of optoelectronic devices based on vdW heterostructures.« less

  10. Role of van der Waals interactions for the intrinsic stability of polyalanine helices

    NASA Astrophysics Data System (ADS)

    Tkatchenko, Alexandre; Blum, Volker; Ireta, Joel; Scheffler, Matthias

    2009-03-01

    The helical motif is an ubiquitous conformation adopted by aminoacid residues in a protein structure and helix formation is the simplest example of the protein folding process. How stable is the folded peptide helix in comparison to a random coil structure? What are the interactions responsible for stabilizing the helical conformation? Answering these questions has thus a direct implication for understanding protein folding. In this work we use density functional theory (DFT) augmented with a non-empirical correction for van der Waals (vdW) forces to study the stability of alanine polypeptide helices in vacuo. We find a large stabilization of the native helical forms when vdW correction is used. It amounts to 121%, 157% and 83% on top of the Perdew-Burke-Ernzerhof (PBE) functional in the case of infinite α, π and 310 helices, respectively. Thus, the experimentally observed α helix is significantly stabilized by vdW forces both over the fully extended and the 310 conformations. Our findings also suggest an explanation to the remarkable stability of gas-phase alanine helices up to high temperatures [M. Kohtani et al. JACS 126, 7420 (2004)].

  11. Dynamical screening of van der Waals interactions in nanostructured solids: Sublimation of fullerenes

    SciTech Connect

    Tao, Jianmin; Yang, Jing; Rappe, Andrew M.

    2015-04-28

    Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C{sub 60}). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.

  12. Dynamical screening of van der Waals interactions in nanostructured solids: Sublimation of fullerenes

    NASA Astrophysics Data System (ADS)

    Tao, Jianmin; Yang, Jing; Rappe, Andrew M.

    2015-04-01

    Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C60). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.

  13. Dynamical screening of van der Waals interactions in nanostructured solids: Sublimation of fullerenes.

    PubMed

    Tao, Jianmin; Yang, Jing; Rappe, Andrew M

    2015-04-28

    Sublimation energy is one of the most important properties of molecular crystals, but it is difficult to study, because the attractive long-range van der Waals (vdW) interaction plays an important role. Here, we apply efficient semilocal density functional theory (DFT), corrected with the dynamically screened vdW interaction (DFT + vdW), the Rutgers-Chalmers nonlocal vdW-DF, and the pairwise-based dispersion-corrected DFT-D2 developed by Grimme and co-workers, to study the sublimation of fullerenes. We find that the short-range part, which accounts for the interaction due to the orbital overlap between fullerenes, is negligibly small. Our calculation shows that there exists a strong screening effect on the vdW interaction arising from the valence electrons of fullerenes. On the other hand, higher-order contributions can be as important as the leading-order term. The reasons are that (i) the surface of fullerene molecules is metallic and thus highly polarizable, (ii) the band gap of fullerene solids is small (less than 2 eV), and (iii) fullerene molecules in the solid phase are so densely packed, yielding the high valence electron density and small equilibrium intermolecular distances (the first nearest neighbor distance is only about 10 Å for C60). However, these two effects make opposite contributions, leading to significant error cancellation between these two contributions. We demonstrate that, by considering higher-order contributions and the dynamical screening, the DFT + vdW method can yield sublimation energies of fullerenes in good agreement with reference values, followed by vdW-DF and DFT-D2. The insights from this study are important for a better understanding of the long-range nature of vdW interactions in nanostructured solids.

  14. Engineering the van der Waals interaction in cross-linking-free hydroxide exchange membranes for low swelling and high conductivity.

    PubMed

    Gu, Shuang; Skovgard, Jason; Yan, Yushan S

    2012-05-01

    What a swell for hydroxides: The typical trade-off between swelling control and ion conductivity in ion-conducting polymer membranes is overcome by enhancement of van der Waals interactions among polymer chains. Using a quaternary phosphonium-functionalized polymer, the simple combination of high electron density of the polymer and large dipole moment of the functional group leads to low membrane swelling, high hydroxide conductivity, and excellent hydroxide exchange membrane fuel cell performance.

  15. Full CI calibration of model hamiltonian, large basis set studies of the H 2-H 2 van der Waals interaction.

    NASA Astrophysics Data System (ADS)

    Burton, P. G.

    1983-08-01

    The non-variational CEPA2 PNO ansatz, recently employed in detailed studies of the H 2-H 2 van der Waals interaction by Burton and Senff and the full CI extrapolation studies on the same system by Burton are discussed in relation to the explicit full CI study of Harrison and Handy for the planar T configuration of H 2-H 2 ( R = 6.5 ao) in a basis of 80 functions.

  16. Excitonic, vibrational, and van der Waals interactions in electron energy loss spectroscopy.

    PubMed

    Mizoguchi, T; Miyata, T; Olovsson, W

    2017-09-01

    The pioneer, Ondrej L. Krivanek, and his collaborators have opened up many frontiers for the electron energy loss spectroscopy (EELS), and they have demonstrated new potentials of the EELS method for investigating materials. Here, inspired by those achievements, we show further potentials of EELS based on the results of theoretical calculations, that is excitonic and van der Waals (vdW) interactions, as well as vibrational information of materials. Concerning the excitonic interactions, we highlight the importance of the two-particle calculation to reproduce the low energy-loss near-edge structure (ELNES), the Na-L2,3 edge of NaI and the Li-K edge of LiCl and LiFePO4. Furthermore, an unusually strong excitonic interaction at the O-K edge of perovskite oxides, SrTiO3 and LaAlO3, is shown. The effect of the vdW interaction in the ELNES is also investigated, and we observe that the magnitude of the vdW effect is approximately 0.1eV in the case of the ELNES from a solid and liquid, whereas its effect is almost negligible in the case of the ELNES from the gaseous phase owing to the long inter-molecular distance. In addition to the "static" information, the influence of the "dynamic" behavior of atoms in materials to EELS is also investigated. We show that measurements of the infrared spectrum are possible by using a modern monochromator system. Furthermore, an estimation of the atomic vibration in core-loss ELNES is also presented. We show the acquisition of vibrational information using the ELNES of liquid methanol and acetic acid, solid Al2O3, and oxygen gas. Copyright © 2017 Elsevier B.V. All rights reserved.

  17. How critical are the van der Waals interactions in polymer crystals?

    PubMed

    Liu, Chun-Sheng; Pilania, Ghanshyam; Wang, Chenchen; Ramprasad, Ramamurthy

    2012-09-20

    van der Waals (vdW) interactions play a prominent role in polymer crystallization. However, density functional theory (DFT) computations that utilize conventional (semi)local exchange-correlation functionals are unable to account for vdW interactions adequately and hence lead to poor predictions of equilibrium structures, densities, cohesive energies, and bulk moduli of polymeric crystals. This study therefore applies two forms of dispersion corrections to DFT, using either the Grimme (DFT-D3/D2) or the Tkatchenko and Scheffler (DFT-TS) approaches. We critically evaluate the relative performance of these two approaches in predicting structural, energetic, and elastic properties for a wide range of polymer crystals and also compare it with conventional electron exchange-correlation functionals (LDA, PBE, and PW91). Our results show that although the conventional functionals either systematically underestimate (e.g., LDA) or overestimate (e.g., PBE and PW91) the lattice parameters that control the polymer interchain interactions in a crystal, the dispersion-corrected functionals consistently provide a better prediction of the structural parameters. In a relative sense, however, the D3 and TS schemes are superior to the D2 approach owing to the environment-dependent atomic dispersion coefficients implicit in the D3 and TS treatments (we do note though that the D2 scheme already constitutes a significant improvement over the (semi)local functionals). Our results not only elucidate the importance of dispersion corrections in the accurate determination of the structural properties of the prototypical polymers considered but also provide a benchmark for comparing other procedures that might be used for including vdW interactions in such systems.

  18. Minimizing density functional failures for non-covalent interactions beyond van der Waals complexes.

    PubMed

    Corminboeuf, Clemence

    2014-11-18

    computationally demanding self-consistent implementation. The proposed correction is then exploited to identify the key factors at the origin of the errors in thermochemistry beyond van der Waals complexes. Particular focus is placed on charge-transfer and mixed-valence complexes, which are relevant to the field of organic electronics. These types of complexes represent insightful examples for which the delocalization error may partially counterbalance the missing dispersion. Our devised methodology reveals the true performance of standard density functional approximations and the subtle interplay between the two types of errors. The analysis presented provides guidance for future functional development that could further improve the modeling of the structures and properties of molecular materials. Overall, the proposed state-of-the-art approaches have contributed to stress the crucial role of dispersion and improve their description in both straightforward van der Waals complexes and more challenging chemical situations. For the treatment of the latter, we have also provided relevant insights into which type of density functionals to favor.

  19. Effects of truncating van der Waals interactions in lipid bilayer simulations

    SciTech Connect

    Huang, Kun; García, Angel E.

    2014-09-14

    In membrane simulations, it is known that truncating electrostatic interactions results in artificial ordering of lipids at the truncation distance. However, less attention has been paid to the effect of truncating van der Waals (VDW) interactions. Since the VDW potential decays as r{sup −6}, it is frequently neglected beyond a cutoff of around 1 nm. In some cases, analytical dispersion corrections appropriate for isotropic systems are applied to the pressure and the potential energy. In this work, we systematically study the effect of truncating VDW interactions at different cutoffs in 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine bilayers with the Berger force field. We show that the area per lipid decreases systematically when the VDW cutoff (r{sub c}) increases. This dependence persists even when dispersion corrections are applied. Since the analytical form of the dispersion correction is only appropriate for isotropic systems, we suggest that a long VDW cutoff should be used in preference over a short VDW cutoff. To determine the appropriate cutoff, we simulate liquid pentadecane with the Berger parameters and find that r{sub c} ≥ 1.4 nm is sufficient to reproduce the density and the heat of vaporization of pentadecane. Bilayers simulated with r{sub c} ≥ 1.4 nm show an improved agreement with experiments in both the form factors and the deuterium order parameters. Finally, we report that the VDW cutoff has a significant impact on the lipid flip-flop energetics and an inappropriate short VDW cutoff results in a bilayer that is prone to form water defects across the bilayer.

  20. Nano-scale displacement sensing based on van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Hu, Lin; Zhao, Jin; Yang, Jinlong

    2015-05-01

    We propose that a nano-scale displacement sensor with high resolution in weak-force systems can be realized based on vertically stacked two-dimensional (2D) atomic corrugated layer materials bound through van der Waals (vdW) interactions. Using first-principles calculations, we found that the electronic structures of bi-layer blue phosphorus (BLBP) vary appreciably with lateral and vertical interlayer displacements. The variation of the electronic structure is attributed to the change of the interlayer distance dz for both the lateral and vertical displacement. For lateral displacement, the change of dz is induced by atomic layer corrugation. Despite the different stacking configurations of BLBP, we find that the change of the indirect band gap is proportional to dz-2. Furthermore, this dz-2 dependence is found to be applicable to other graphene-like corrugated bi-layer materials such as MoS2. BLBP represents a large family of bi-layer 2D atomic corrugated materials for which the electronic structure is sensitive to the interlayer vertical and lateral displacement, and thus could be used for a nano-scale displacement sensor. This can be done by monitoring the tunable electronic structure using absorption spectroscopy. Because this type of sensor is established on atomic layers coupled through vdW interactions, it provides unique applications in the measurements of nano-scale displacement induced by tiny external forces.We propose that a nano-scale displacement sensor with high resolution in weak-force systems can be realized based on vertically stacked two-dimensional (2D) atomic corrugated layer materials bound through van der Waals (vdW) interactions. Using first-principles calculations, we found that the electronic structures of bi-layer blue phosphorus (BLBP) vary appreciably with lateral and vertical interlayer displacements. The variation of the electronic structure is attributed to the change of the interlayer distance dz for both the lateral and vertical

  1. Interaction of boron with graphite: A van der Waals density functional study

    NASA Astrophysics Data System (ADS)

    Liu, Juan; Wang, Chen; Liang, Tongxiang; Lai, Wensheng

    2016-08-01

    Boron doping has been widely investigated to improve oxidation resistance of graphite. In this work the interaction of boron with graphite is investigated by a van der Waals density-functional approach (vdW-DF). The traditional density-functional theory (DFT) is well accounted for the binding in boron-substituted graphite. However, to investigate the boron atom on graphite surface and the interstitial impurities require use of a description of graphite interlayer binding. Traditional DFT cannot describe the vdW physics, for instance, GGA calculations show no relevant binding between graphite sheets. LDA shows some binding, but they fail to provide an accurate account of vdW forces. In this paper, we compare the calculation results of graphite lattice constant and cohesive energy by several functionals, it shows that vdW-DF such as two optimized functionals optB88-vdW and optB86b-vdW give much improved results than traditional DFT. The vdW-DF approach is then applied to study the interaction of boron with graphite. Boron adsorption, substitution, and intercalation are discussed in terms of structural parameters and electronic structures. When adsorbing on graphite surface, boron behaves as π electron acceptor. The π electron approaches boron atom because of more electropositive of boron than carbon. For substitution situation, the hole introduced by boron mainly concentrates on boron and the nearest three carbon atoms. The B-doped graphite system with the hole has less ability to offer electrons to oxygen, ultimately resulted in the inhibition of carbon oxidation. For interstitial doping, vdW-DFs show more accurate formation energy than LDA. PBE functional cannot describe the interstitial boron in graphite reasonably because of the ignoring binding of graphite sheets. The investigation of electron structures of boron doped graphite will play an important role in understanding the oxidation mechanism in further study.

  2. Virtual resonance and frequency difference generation by van der Waals interaction.

    PubMed

    Tetard, L; Passian, A; Eslami, S; Jalili, N; Farahi, R H; Thundat, T

    2011-05-06

    The ability to explore the interior of materials for the presence of inhomogeneities was recently demonstrated by mode synthesizing atomic force microscopy [L. Tetard, A. Passian, and T. Thundat, Nature Nanotech. 5, 105 (2009).]. Proposing a semiempirical nonlinear force, we show that difference frequency ω_ generation, regarded as the simplest synthesized mode, occurs optimally when the force is tuned to van der Waals form. From a parametric study of the probe-sample excitation, we show that the predicted ω_ oscillation agrees well with experiments. We then introduce the concept of virtual resonance to show that probe oscillations at ω_ can efficiently be enhanced.

  3. Virtual Resonance and Frequency Difference Generation by van der Waals Interaction

    NASA Astrophysics Data System (ADS)

    Tetard, L.; Passian, A.; Eslami, S.; Jalili, N.; Farahi, R. H.; Thundat, T.

    2011-05-01

    The ability to explore the interior of materials for the presence of inhomogeneities was recently demonstrated by mode synthesizing atomic force microscopy [L. Tetard, A. Passian, and T. Thundat, Nature Nanotech. 5, 105 (2009).NNAABX1748-338710.1038/nnano.2009.454]. Proposing a semiempirical nonlinear force, we show that difference frequency ω- generation, regarded as the simplest synthesized mode, occurs optimally when the force is tuned to van der Waals form. From a parametric study of the probe-sample excitation, we show that the predicted ω- oscillation agrees well with experiments. We then introduce the concept of virtual resonance to show that probe oscillations at ω- can efficiently be enhanced.

  4. Spectral mixing formulations for van der Waals-London dispersion interactions between multicomponent carbon nanotubes.

    PubMed

    Rajter, Rick; French, Roger H; Podgornik, Rudi; Ching, W Y; Parsegian, V Adrian

    2008-09-01

    Recognition of spatially varying optical properties is a necessity when studying the van der Waals-London dispersion (vdW-Ld) interactions of carbon nanotubes (CNTs) that have surfactant coatings, tubes within tubes, andor substantial core sizes. The ideal way to address these radially dependent optical properties would be to have an analytical add-a-layer solution in cylindrical coordinates similar to the one readily available for the plane-plane geometry. However, such a formulation does not exist nor does it appear trivial to be obtained exactly. The best and most pragmatic alternative for end-users is to take the optical spectra of the many components and to use a spectral mixing formulation so as to create effective solid-cylinder spectra for use in the far-limit regime. The near-limit regime at "contact" is dominated by the optical properties of the outermost layer, and thus no spectral mixing is required. Specifically we use a combination of a parallel capacitor in the axial direction and the Bruggeman effective medium in the radial direction. We then analyze the impact of using this mixing formulation upon the effective vdW-Ld spectra and the resulting Hamaker coefficients for small and large diameter single walled CNTs (SWCNTs) in both the near- and far-limit regions. We also test the spectra of a [16,0,s+7,0,s] multiwalled CNT (MWCNT) with an effective MWCNT spectrum created by mixing its [16,0,s] and [7,0,s] SWCNT components to demonstrate nonlinear coupling effects that exist between neighboring layers. Although this paper is primarily on nanotubes, the strategies, implementation, and analysis presented are applicable and likely necessary to any system where one needs to resolve spatially varying optical properties in a particular Lifshitz formulation.

  5. Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals

    NASA Astrophysics Data System (ADS)

    Ambrosio, Francesco; Miceli, Giacomo; Pasquarello, Alfredo

    2015-12-01

    We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H+/H2 level defining the standard hydrogen electrode, the OH-/OH∗ level corresponding to the oxidation of the hydroxyl ion, and the H2O/OH∗ level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band-edge positions, and redox levels in overall

  6. Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals

    SciTech Connect

    Ambrosio, Francesco Miceli, Giacomo; Pasquarello, Alfredo

    2015-12-28

    We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H{sup +}/H{sub 2} level defining the standard hydrogen electrode, the OH{sup −}/OH{sup ∗} level corresponding to the oxidation of the hydroxyl ion, and the H{sub 2}O/OH{sup ∗} level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band

  7. Theory of coherent van der Waals matter.

    PubMed

    Kulić, Igor M; Kulić, Miodrag L

    2014-12-01

    We explain in depth the previously proposed theory of the coherent van der Waals (cvdW) interaction, the counterpart of van der Waals (vdW) force, emerging in spatially coherently fluctuating electromagnetic fields. We show that cvdW driven matter is dominated by many-body interactions, which are significantly stronger than those found in standard van der Waals (vdW) systems. Remarkably, the leading two- and three-body interactions are of the same order with respect to the distance (∝R(-6)), in contrast to the usually weak vdW three-body effects (∝R(-9)). From a microscopic theory we show that the anisotropic cvdW many-body interactions drive the formation of low-dimensional structures such as chains, membranes, and vesicles with very unusual, nonlocal properties. In particular, cvdW chains display a logarithmically growing stiffness with the chain length, while cvdW membranes have a bending modulus growing linearly with their size. We argue that the cvdW anisotropic many-body forces cause local cohesion but also a negative effective "surface tension." We conclude by deriving the equation of state for cvdW materials and propose experiments to test the theory, in particular the unusual three-body nature of cvdW.

  8. Theory of coherent van der Waals matter

    NASA Astrophysics Data System (ADS)

    Kulić, Igor M.; Kulić, Miodrag L.

    2014-12-01

    We explain in depth the previously proposed theory of the coherent van der Waals (cvdW) interaction, the counterpart of van der Waals (vdW) force, emerging in spatially coherently fluctuating electromagnetic fields. We show that cvdW driven matter is dominated by many-body interactions, which are significantly stronger than those found in standard van der Waals (vdW) systems. Remarkably, the leading two- and three-body interactions are of the same order with respect to the distance (∝R-6) , in contrast to the usually weak vdW three-body effects (∝R-9 ). From a microscopic theory we show that the anisotropic cvdW many-body interactions drive the formation of low-dimensional structures such as chains, membranes, and vesicles with very unusual, nonlocal properties. In particular, cvdW chains display a logarithmically growing stiffness with the chain length, while cvdW membranes have a bending modulus growing linearly with their size. We argue that the cvdW anisotropic many-body forces cause local cohesion but also a negative effective "surface tension." We conclude by deriving the equation of state for cvdW materials and propose experiments to test the theory, in particular the unusual three-body nature of cvdW.

  9. Role of directed van der Waals bonded interactions in the determination of the structures of molecular arsenate solids.

    PubMed

    Gibbs, G V; Wallace, A F; Cox, D F; Dove, P M; Downs, R T; Ross, N L; Rosso, K M

    2009-01-29

    Bond paths, local energy density properties, and Laplacian, L(r) = -wedge(2)rho(r), composite isosurfaces of the electron density distributions were calculated for the intramolecular and intermolecular bonded interactions for molecular solids of As(2)O(3) and AsO(2) composition, an As(2)O(5) crystal, a number of arsenate molecules, and the arsenic metalloid, arsenolamprite. The directed intermolecular van der Waals As-O, O-O, and As-As bonded interactions are believed to serve as mainstays between the individual molecules in each of the molecular solids. As-O bond paths between the bonded atoms connect Lewis base charge concentrations and Lewis acid charge depletion domains, whereas the O-O and As-As paths connect Lewis base pair and Lewis acid pair domains, respectively, giving rise to sets of intermolecular directed bond paths. The alignment of the directed bond paths results in the periodic structures adopted by the arsenates. The arrangements of the As atoms in the claudetite polymorphs of As(2)O(3) and the As atoms in arsenolamprite are similar. Like the As(2)O(3) polymorphs, arsenolamprite is a molecular solid connected by relatively weak As-As intermolecular directed van der Waals bond paths between the layers of stronger As-As intramolecular bonded interactions. The bond critical point and local energy density properties of the intermolecular As-As bonded interactions in arsenolamprite are comparable with the As-As interactions in claudetite I. As such, the structure of claudetite I can be viewed as a stuffed derivative of the arsenolamprite structure with O atoms between pairs of As atoms comprising the layers of the structure. The cubic structure adopted by the arsenolite polymorph can be understood in terms of sets of directed acid-base As-O and base-base O-O pair domains and bond paths that radiate from the tetrahedral faces of its constituent molecules, serving as face-to-face key-lock mainstays in forming a periodic tetrahedral array of molecules

  10. Two-atom interaction energies with one atom in an excited state: van der Waals potentials versus level shifts

    NASA Astrophysics Data System (ADS)

    Donaire, M.

    2016-05-01

    I revisit the problem of the interaction between two dissimilar atoms with one atom in an excited state, recently addressed by Berman [Phys. Rev. A 91, 042127 (2015), 10.1103/PhysRevA.91.042127], Donaire et al. [Phys. Rev. Lett. 115, 033201 (2015), 10.1103/PhysRevLett.115.033201], and Milonni and Rafsanjani [Phys. Rev. A 92, 062711 (2015), 10.1103/PhysRevA.92.062711], for which precedent approaches have given conflicting results. In the first place, I discuss to what extent these works provide equivalent results. I show that the phase-shift rate of the two-atom wave function computed by Berman, the van der Waals potential of the excited atom by Donaire et al., and the level shift of the excited atom by Milonni and Rafsanjani possess equivalent expressions in the quasistationary approximation. In addition, I show that the level shift of the ground-state atom computed by Milonni and Rafsanjani is equivalent to its van der Waals potential. A diagrammatic representation of all those quantities is provided. The equivalences among them are, however, not generic. In particular, it is found that for the case of the interaction between two identical atoms excited, the phase-shift rate and the van der Waals potentials differ. Concerning the conflicting results of previous approaches in regards to the spatial oscillation of the interactions, I conclude, in agreement with Berman and with Milonni and Rafsanjani, that they refer to different physical quantities. The impacts of free-space dissipation and finite excitation rates on the dynamics of the potentials are analyzed. In contrast with Milonni and Rafsanjani, the oscillatory versus monotonic spatial forms of the potentials of each atom are found not to be related to the reversible versus irreversible nature of the excitation transfer involved.

  11. Superlubricity using repulsive van der Waals forces.

    PubMed

    Feiler, Adam A; Bergström, Lennart; Rutland, Mark W

    2008-03-18

    Using colloid probe atomic force microscopy, we show that if repulsive van der Waals forces exist between two surfaces prior to their contact then friction is essentially precluded and supersliding is achieved. The friction measurements presented here are of the same order as the lowest ever recorded friction coefficients in liquid, though they are achieved by a completely different approach. A gold sphere attached to an AFM cantilever is forced to interact with a smooth Teflon surface (templated on mica). In cyclohexane, a repulsive van der Waals force is observed that diverges at short separations. The friction coefficient associated with this system is on the order of 0.0003. When the refractive index of the liquid is changed, the force can be tuned from repulsive to attractive and adhesive. The friction coefficient increases as the Hamaker constant becomes more positive and the divergent repulsive force, which prevents solid-solid contact, gets switched off.

  12. Lifshitz-type formulas for graphene and single-wall carbon nanotubes: van der Waals and Casimir interactions

    SciTech Connect

    Bordag, M.; Geyer, B.; Klimchitskaya, G. L.; Mostepanenko, V. M.

    2006-11-15

    Lifshitz-type formulas are obtained for the van der Waals and Casimir interaction between graphene and a material plate, graphene and an atom or a molecule, and between a single-wall carbon nanotube and a plate. The reflection properties of electromagnetic oscillations on graphene are governed by the specific boundary conditions imposed on the infinitely thin positively charged plasma sheet, carrying a continuous fluid with some mass and charge density. The obtained formulas are applied to graphene interacting with Au and Si plates, to hydrogen atoms and molecules interacting with graphene, and to single-wall carbon nanotubes interacting with Au and Si plates. The generalizations to more complicated carbon nanostructures are discussed.

  13. Interaction of jump-fan composite waves in a two-dimensional jet for van der Waals gases

    NASA Astrophysics Data System (ADS)

    Lai, Geng

    2015-06-01

    We consider a two-dimensional (2D) jet by van der Waals gas streaming in parallel supersonic flow out of a duct into the atmosphere. We assume that the pressure p0 of the oncoming uniform parallel flow is greater than the atmospheric pressure pA and belongs to ( p1 e , p2 i ) . Then at the corners at exit the oncoming flow expands in two symmetric jump-fan (jf) composite waves to the atmospheric pressure. These two jf composite waves interact and emerge as simple waves from their zone of penetration. We present a mathematical analysis of the interaction of the jf composite waves. To construct the flow in the interaction region, we consider a discontinuous Goursat problem for the 2D isentropic irrotational steady Euler equations. The existence of global piecewise C1 solution to the discontinuous Goursat problem is obtained constructively.

  14. Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy

    PubMed Central

    Jobst, Johannes; van der Torren, Alexander J. H.; Krasovskii, Eugene E.; Balgley, Jesse; Dean, Cory R.; Tromp, Rudolf M.; van der Molen, Sense Jan

    2016-01-01

    High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the ‘chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of. PMID:27897180

  15. Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy

    NASA Astrophysics Data System (ADS)

    Jobst, Johannes; van der Torren, Alexander J. H.; Krasovskii, Eugene E.; Balgley, Jesse; Dean, Cory R.; Tromp, Rudolf M.; van der Molen, Sense Jan

    2016-11-01

    High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the `chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of.

  16. Van der Waals interactions in density functional theory by combining the quantum harmonic oscillator-model with localized Wannier functions.

    PubMed

    Silvestrelli, Pier Luigi

    2013-08-07

    We present a new scheme to include the van der Waals (vdW) interactions in approximated Density Functional Theory (DFT) by combining the quantum harmonic oscillator model with the maximally localized Wannier function technique. With respect to the recently developed DFT/vdW-WF2 method, also based on Wannier Functions, the new approach is more general, being no longer restricted to the case of well separated interacting fragments. Moreover, it includes higher than pairwise energy contributions, coming from the dipole-dipole coupling among quantum oscillators. The method is successfully applied to the popular S22 molecular database, and also to extended systems, namely graphite and H2 adsorbed on the Cu(111) metal surface (in this case metal screening effects are taken into account). The results are also compared with those obtained by other vdW-corrected DFT schemes.

  17. Site-dependence of van der Waals interaction explains exciton spectra of double-walled tubular J-aggregates

    NASA Astrophysics Data System (ADS)

    Megow, Jörg; Röhr, Merle I. S.; Schmidt am Busch, Marcel; Renger, Thomas; Mitrić, Roland; Kirstein, Stefan; Rabe, Jürgen P.; May, Volkhard

    The simulation of the optical properties of supramolecular aggregates requires the development of methods, which are able to treat a large number of coupled chromophores interacting with the environment. Since it is currently not possible to treat large systems by quantum chemistry, the Frenkel exciton model is a valuable alternative. In this work we show how the Frenkel exciton model can be extended in order to explain the excitonic spectra of a specific double-walled tubular dye aggregate explicitly taking into account dispersive energy shifts of ground and excited states due to van der Waals interaction with all surrounding molecules. The experimentally observed splitting is well explained by the site-dependent energy shift of molecules placed at the inner or outer side of the double-walled tube, respectively. Therefore we can conclude, that inclusion of the site-dependent dispersive effect in the theoretical description of optical properties of nanoscaled dye aggregates is mandatory.

  18. Interlayer Hybridization in van der Waals Heterostructures

    NASA Astrophysics Data System (ADS)

    Le, Nam; Tran, Huan; Woods, Lilia

    Van der Waals heterostructures composed of chemically inert dissimilar layers are of great interest for fundamental science and applications. The weak interplanar interactions and orbital overlap are expected to bring modifications to the constituent materials. By using first principles simulations, we investigate the properties of several heterostructures, including graphene/silicene, graphene/MoS2, and silicene/MoS2. The calculations reveal superlattice characteristic points in the Brillouin zone associated with the different stacking patterns. Band structures projected on each of the constituents show hybridization features related to specific orbital overlap for each heterostructure. Phonon dispersion spectra for the considered heterostructures are also investigated.

  19. Investigation of the distinction between van der Waals interaction and chemical bonding based on the PAEM-MO diagram.

    PubMed

    Zhao, Dong-Xia; Yang, Zhong-Zhi

    2014-05-15

    In recent years, the basic problem of understanding chemical bonding, nonbonded, and/or van der Waals interactions has been intensively debated in terms of various theoretical methods. We propose and construct the potential acting on one electron in a molecule-molecular orbital (PAEM-MO) diagram, which draws the PAEM inserted the MO energy levels with their major atomic orbital components. PAEM-MO diagram is able to show clear distinction of chemical bonding from nonbonded and/or vdW interactions. The rule for this is as follows. Along the line connecting two atoms in a molecule or a complex, the existence of chemical bonding between these two atoms needs to satisfy two conditions: (a) a critical point of PAEM exists and (b) PAEM barrier between the two atoms is lower in energy than the occupied major valence-shell bonding MO which contains in-phase atomic components (positive overlap) of the two considered atoms. In contrast to the chemical bonding, for a nonbonded interaction or van der Waals interaction between two atoms, both conditions (a) and (b) do not be satisfied at the same time. This is demonstrated and discussed by various typical cases, particularly those related to helium atom and H-H bonding in phenanthrene. There are helium bonds in HHeF and HeBeO molecules, whereas no H-H bonding in phenanthrene. The validity and limitation for this rule is demonstrated through the investigations of the curves of the PAEM barrier top and MO energies versus the internuclear distances for He2 , H2 , and He2 (+) systems. Copyright © 2014 Wiley Periodicals, Inc.

  20. Quantitative Understanding of van der Waals Interactions by Analyzing the Adsorption Structure and Low-Frequency Vibrational Modes of Single Benzene Molecules on Silver.

    PubMed

    Yuan, Dingwang; Han, Zhumin; Czap, Gregory; Chiang, Chi-Lun; Xu, Chen; Ho, W; Wu, Ruqian

    2016-06-16

    The combination of a sub-Kelvin scanning tunneling microscope and density functional calculations incorporating van der Waals (vdW) corrections has been used successfully to probe the adsorption structure and low-frequency vibrational modes of single benzene molecules on Ag(110). The inclusion of optimized vdW functionals and improved C6-based vdW dispersion schemes in density functional theory is crucial for obtaining the correct adsorption structure and low-energy vibrational modes. These results demonstrate the emerging capability to quantitatively probe the van der Waals interactions between a physisorbed molecule and an inert substrate.

  1. Van der Waals Interactions in Pyridine and Pyridine-like Molecular Crystals: An ab initio Molecular Dynamics Study

    NASA Astrophysics Data System (ADS)

    Ko, Hsin-Yu; Distasio, Robert A., Jr.; Santra, Biswajit; Car, Roberto

    2014-03-01

    Pyridine has recently been investigated as a potentially effective material for use in artificial light harvesting.In this work, we propose the use of ab initio molecular dynamics (AIMD) to gain valuable physical insight into the artificial photosynthetic processes occurring in condensed-phase pyridine, the study of which has been limited to semi-empirical force fields to date.For this purpose, we introduce an accurate and efficient AIMD method, based on density functional theory (DFT) and a self-consistent pairwise description of van der Waals (vdW) interactions, for use in finite temperature and pressure (NPT) simulations on pyridine and several pyridine-like molecular crystals (PLMCs). Utilizing this approach, we demonstrate that vdW forces play a crucial role in the theoretical prediction of the structure and density of pyridine and PLMCs, and therefore must be accounted for in studies of these potential alternative energy materials. DOE: DE-SC0008626, NSF: DMS-1065894.

  2. Significant decrease in thermal conductivity of multi-walled carbon nanotube induced by inter-wall van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Zhang, Xue; Zhou, Wu-Xing; Chen, Xue-Kun; Liu, Yue-Yang; Chen, Ke-Qiu

    2016-05-01

    The thermal transport properties of multi-walled carbon nanotubes (MWCNTs) were investigated by using non-equilibrium molecular dynamics simulation. The results show that the thermal conductivity of MWCNTs decreases significantly comparing to that of single-walled carbon nanotubes (SWCNTs) due to the inter-wall van der Waals interactions. The more interesting is a fact that the thermal conductance of MWCNTs is significantly greater than the thermal conductance summation of each SWCNTs. This is because the thermal conductance of a carbon nanotube protected by an outer tube is much larger than that of one that is not protected. Moreover, we also studied the thermal flux distribution of MWCNTs, and found that the outer tube plays a dominant role in heat energy transfer.

  3. Compact two-electron wave function for bond dissociation and Van der Waals interactions: A natural amplitude assessment

    SciTech Connect

    Giesbertz, Klaas J. H.; Leeuwen, Robert van

    2014-05-14

    Electron correlations in molecules can be divided in short range dynamical correlations, long range Van der Waals type interactions, and near degeneracy static correlations. In this work, we analyze for a one-dimensional model of a two-electron system how these three types of correlations can be incorporated in a simple wave function of restricted functional form consisting of an orbital product multiplied by a single correlation function f (r{sub 12}) depending on the interelectronic distance r{sub 12}. Since the three types of correlations mentioned lead to different signatures in terms of the natural orbital (NO) amplitudes in two-electron systems, we make an analysis of the wave function in terms of the NO amplitudes for a model system of a diatomic molecule. In our numerical implementation, we fully optimize the orbitals and the correlation function on a spatial grid without restrictions on their functional form. Due to this particular form of the wave function, we can prove that none of the amplitudes vanishes and moreover that it displays a distinct sign pattern and a series of avoided crossings as a function of the bond distance in agreement with the exact solution. This shows that the wave function ansatz correctly incorporates the long range Van der Waals interactions. We further show that the approximate wave function gives an excellent binding curve and is able to describe static correlations. We show that in order to do this the correlation function f (r{sub 12}) needs to diverge for large r{sub 12} at large internuclear distances while for shorter bond distances it increases as a function of r{sub 12} to a maximum value after which it decays exponentially. We further give a physical interpretation of this behavior.

  4. Use of Two-Body Correlated Basis Functions with van der Waals Interaction to Study the Shape-Independent Approximation for a Large Number of Trapped Interacting Bosons

    NASA Astrophysics Data System (ADS)

    Lekala, M. L.; Chakrabarti, B.; Das, T. K.; Rampho, G. J.; Sofianos, S. A.; Adam, R. M.; Haldar, S. K.

    2017-01-01

    We study the ground-state and the low-lying excitations of a trapped Bose gas in an isotropic harmonic potential for very small (˜ 3) to very large (˜ 10^7 ) particle numbers. We use the two-body correlated basis functions and the shape-dependent van der Waals interaction in our many-body calculations. We present an exhaustive study of the effect of inter-atomic correlations and the accuracy of the mean-field equations considering a wide range of particle numbers. We calculate the ground-state energy and the one-body density for different values of the van der Waals parameter C6 . We compare our results with those of the modified Gross-Pitaevskii results, the correlated Hartree hypernetted-chain equations (which also utilize the two-body correlated basis functions), as well as of the diffusion Monte Carlo for hard sphere interactions. We observe the effect of the attractive tail of the van der Waals potential in the calculations of the one-body density over the truly repulsive zero-range potential as used in the Gross-Pitaevskii equation and discuss the finite-size effects. We also present the low-lying collective excitations which are well described by a hydrodynamic model in the large particle limit.

  5. Entropic effects of thermal rippling on van der Waals interactions between monolayer graphene and a rigid substrate

    NASA Astrophysics Data System (ADS)

    Wang, Peng; Gao, Wei; Huang, Rui

    2016-02-01

    Graphene monolayer, with extremely low flexural stiffness, displays spontaneous rippling due to thermal fluctuations at a finite temperature. When a graphene membrane is placed on a solid substrate, the adhesive interactions between graphene and the substrate could considerably suppress thermal rippling. On the other hand, the statistical nature of thermal rippling adds an entropic contribution to the graphene-substrate interactions. In this paper, we present a statistical mechanics analysis on thermal rippling of monolayer graphene supported on a rigid substrate, assuming a generic form of van der Waals interactions between graphene and substrate at T = 0 K. The rippling amplitude, the equilibrium average separation, and the average interaction energy are predicted simultaneously and compared with molecular dynamics (MD) simulations. While the amplitude of thermal rippling is reduced by adhesive interactions, the entropic contribution leads to an effective repulsion. As a result, the equilibrium average separation increases and the effective adhesion energy decreases with increasing temperature. Moreover, the effect of a biaxial pre-strain in graphene is considered, and a buckling instability is predicted at a critical compressive strain that depends on both the temperature and the adhesive interactions. Limited by the harmonic approximations, the theoretical predictions agree with MD simulations only for relatively small rippling amplitudes but can be extended to account for the anharmonic effects.

  6. Physical adsorption at the nanoscale: Towards controllable scaling of the substrate-adsorbate van der Waals interaction

    NASA Astrophysics Data System (ADS)

    Ambrosetti, Alberto; Silvestrelli, Pier Luigi; Tkatchenko, Alexandre

    2017-06-01

    The Lifshitz-Zaremba-Kohn (LZK) theory is commonly considered as the correct large-distance limit for the van der Waals (vdW) interaction of adsorbates (atoms, molecules, or nanoparticles) with solid substrates. In the standard approximate form, implicitly based on local dielectric functions, the LZK approach predicts universal power laws for vdW interactions depending only on the dimensionality of the interacting objects. However, recent experimental findings are challenging the universality of this theoretical approach at finite distances of relevance for nanoscale assembly. Here, we present a combined analytical and numerical many-body study demonstrating that physical adsorption can be significantly enhanced at the nanoscale. Regardless of the band gap or the nature of the adsorbate specie, we find deviations from conventional LZK power laws that extend to separation distances of up to 10-20 nm. Comparison with recent experimental observations of ultra-long-ranged vdW interactions in the delamination of graphene from a silicon substrate reveals qualitative agreement with the present theory. The sensitivity of vdW interactions to the substrate response and to the adsorbate characteristic excitation frequency also suggests that adsorption strength can be effectively tuned in experiments, paving the way to an improved control of physical adsorption at the nanoscale.

  7. Testing the distance-dependence of the van der Waals interaction between an atom and a surface through spectroscopy in a vapor nanocell

    NASA Astrophysics Data System (ADS)

    Laliotis, A.; Maurin, I.; Todorov, P.; Hamdi, I.; Dutier, G.; Yarovitski, A.; Saltiel, S.; Gorza, M.-P.; Fichet, M.; Ducloy, M.; Bloch, D.

    2007-03-01

    This paper presents our current measurements in a vapor nanocell aiming at a test of the distance-dependence of the atom-surface interaction, when simple asymptotic descriptions may turn to be not valid. A state-of-the-art of atom-surface interaction measurements is provided as an introduction, along with the comparison with the theory of the van der Waals (or Casimir-Polder) interaction; it is followed by a presentation of the most salient features of nanocell spectroscopy.

  8. Development of the Technique of Total Internal Reflection Microscopy and Measurement of the Van Der Waals Interaction Energy.

    NASA Astrophysics Data System (ADS)

    Walz, John Yokum

    The optical technique of Total Internal Reflection Microscopy (TIRM) has proven to be an effective tool for studying the interaction of a single, microscopic particle with a large planar surface. The technique utilizes the unique properties of an evanescent surface wave formed at the interface between two optically different media to measure the instantaneous separation distance of a particle from the interface. This thesis describes two major refinements to the TIRM technique. First, a geometric optics light scattering model was written to simulate the scattering of an evanescent wave by a microscopic dielectric sphere. The model is unique in that multiple reflections between the sphere and plate are included. The model predicts that for small polystyrene spheres (diameter <=q10 μm) in an aqueous medium, the simple exponential relationship between scattering intensity and separation distance, which was known to be valid at large distances, also holds down to contact. Experimental validation of the model with MgF_2-coated glass slides supports the model results. Second, a radiation pressure, or optical tweezers, technique was added to the TIRM apparatus. This technique involves use of a tightly focused laser beam, separate from that used to produce the evanescent wave, to exert a controllable force on the sphere. Through a linear momentum balance, this force is shown to contain both axial (parallel to incident beam) as well as radial (perpendicular to incident beam) components. The radial component allows holding a single particle at a fixed lateral position for an indefinite period against Brownian diffusion, which greatly facilitates measurements with TIRM. Finally, these two methods were used to measure the van der Waals attractive interaction energy between a 10 μm polystyrene sphere trapped in a secondary potential energy well above a sheet of cleaved mica. The measured interaction energies are found to be significantly less than those predicted with the

  9. Integrable extended van der Waals model

    NASA Astrophysics Data System (ADS)

    Giglio, Francesco; Landolfi, Giulio; Moro, Antonio

    2016-10-01

    Inspired by the recent developments in the study of the thermodynamics of van der Waals fluids via the theory of nonlinear conservation laws and the description of phase transitions in terms of classical (dissipative) shock waves, we propose a novel approach to the construction of multi-parameter generalisations of the van der Waals model. The theory of integrable nonlinear conservation laws still represents the inspiring framework. Starting from a macroscopic approach, a four parameter family of integrable extended van der Waals models is indeed constructed in such a way that the equation of state is a solution to an integrable nonlinear conservation law linearisable by a Cole-Hopf transformation. This family is further specified by the request that, in regime of high temperature, far from the critical region, the extended model reproduces asymptotically the standard van der Waals equation of state. We provide a detailed comparison of our extended model with two notable empirical models such as Peng-Robinson and Soave's modification of the Redlich-Kwong equations of state. We show that our extended van der Waals equation of state is compatible with both empirical models for a suitable choice of the free parameters and can be viewed as a master interpolating equation. The present approach also suggests that further generalisations can be obtained by including the class of dispersive and viscous-dispersive nonlinear conservation laws and could lead to a new type of thermodynamic phase transitions associated to nonclassical and dispersive shock waves.

  10. Effect of the van der Waals interaction on the electron energy-loss near edge structure theoretical calculation.

    PubMed

    Katsukura, Hirotaka; Miyata, Tomohiro; Tomita, Kota; Mizoguchi, Teruyasu

    2017-07-01

    The effect of the van der Waals (vdW) interaction on the simulation of the electron energy-loss near edge structure (ELNES) by a first-principles band-structure calculation is reported. The effect of the vdW interaction is considered by the Tkatchenko-Scheffler scheme, and the change of the spectrum profile and the energy shift are discussed. We perform calculations on systems in the solid, liquid and gaseous states. The transition energy shifts to lower energy by approximately 0.1eV in the condensed (solid and liquid) systems by introducing the vdW effect into the calculation, whereas the energy shift in the gaseous models is negligible owing to the long intermolecular distance. We reveal that the vdW interaction exhibits a larger effect on the excited state than the ground state owing to the presence of an excited electron in the unoccupied band. Moreover, the vdW effect is found to depend on the local electron density and the molecular coordination. In addition, this study suggests that the detection of the vdW interactions exhibited within materials is possible by a very stable and high resolution observation. Copyright © 2016 Elsevier B.V. All rights reserved.

  11. Brownian dynamics simulation of aerosol coagulation: effect of shear flow of fluid, Brownian motion, and van der Waals interaction

    SciTech Connect

    Gupta, D.

    1986-01-01

    The influence of shear flow, Brownian motion and interparticle forces on the coagulation coefficient are studied; and effects of many-body interactions on the coagulation coefficient for concentrated dispersions are analyzed. This study is conducted in two parts. In the first part, computer experiments are performed using Brownian Dynamics simulation methods. The relative importance of shear flow and Brownian motion, and of shear flow and van der Waals attraction, are characterized by the Peclet number, Pe, and the Flow number, FI, respectively. Results from computer experiments for FL ..-->.. infinity (i.e. no interparticle interactions) show that the principle of superposition underestimates the coagulation rate at low Pe (by as much as 100%) and overestimates the coagulation rate at large Pe (by roughly 30 to 40%). In the second part, the potential of mean force concept from dense gas kinetic theory is used to investigate the effect of particle volume fraction, Phi. It is shown that for large values of Phi, a shielding effect due to surrounding particles results in an attractive force on the particles. This leads to an overall enhancement in the coagulation rate when compared with the results based on the binary interaction potential.

  12. First-principles study of van der Waals interactions and lattice mismatch at MoS2/metal interfaces

    NASA Astrophysics Data System (ADS)

    Farmanbar, Mojtaba; Brocks, Geert

    2016-02-01

    We explore the adsorption of MoS2 on a range of metal substrates by means of first-principles density functional theory calculations. Including van der Waals forces in the density functional is essential to capture the interaction between MoS2 and a metal surface, and obtain reliable interface potential steps and Schottky barriers. Special care is taken to construct interface structures that have a mismatch between the MoS2 and the metal lattices of <1 % . MoS2 is chemisorbed on the early transition metal Ti, which leads to a strong perturbation of its (electronic) structure and a pinning of the Fermi level 0.54 eV below the MoS2 conduction band due to interface states. MoS2 is physisorbed on Au, where the bonding hardly perturbs the electronic structure. The bonding of MoS2 on other metals lies between these two extreme cases, with interface interactions for the late 3 d transition metals Co, Ni, Cu and the simple metal Mg that are somewhat stronger than for the late 4 d /5 d transition metals Pd, Ag, Pt and the simple metal Al. Even a weak interaction, such as in the case of Al, gives interface states, however, with energies inside the MoS2 band gap, which pin the Fermi level below the conduction band.

  13. On the tautomerisation of porphycene on copper (111): Finding the subtle balance between van der Waals interactions and hybridisation

    NASA Astrophysics Data System (ADS)

    Novko, Dino; Tremblay, Jean Christophe; Blanco-Rey, María

    2016-12-01

    We use density-functional theory (DFT) to analyse the interaction of trans- and cis-porphycene with Cu(111) and their interconversion by intramolecular H-transfer. This tautomerisation reaction is characterised by small values for the reaction energy and barrier, on the order of ˜0.1 eV, where the trans configuration is thermodynamically more stable upon adsorption according to the experiments [J. N. Ladenthin et al., ACS Nano 9, 7287-7295 (2015)]. To gain even a qualitatively correct description of this reaction at the DFT level, an accurate treatment of dispersion interactions and a careful choice of the exchange contribution are required in order to predict the subtle energetics. Analysis of the electronic structure shows that adsorption is contributed by a van der Waals (vdW) interaction, mainly responsible for stabilising the polyaromatic fragments, and by a significant charge redistribution localised between Cu and the unsaturated N atoms of the molecule central cavity. We find that different vdW functionals can produce qualitatively different electronic structures, while yielding small trans vs. cis energy differences. Unlike other functionals surveyed here, vdW-DF with PBE exchange satisfactorily reproduces not only the experimental energetics but also the scanning tunneling microscopy images. This gives us confidence that this functional achieves a reliable balance between the two mechanisms contributing to the adsorption of porphycene.

  14. Accurate and Efficient Calculation of van der Waals Interactions Within Density Functional Theory by Local Atomic Potential Approach

    SciTech Connect

    Sun, Y. Y.; Kim, Y. H.; Lee, K.; Zhang, S. B.

    2008-01-01

    Density functional theory (DFT) in the commonly used local density or generalized gradient approximation fails to describe van der Waals (vdW) interactions that are vital to organic, biological, and other molecular systems. Here, we propose a simple, efficient, yet accurate local atomic potential (LAP) approach, named DFT+LAP, for including vdW interactions in the framework of DFT. The LAPs for H, C, N, and O are generated by fitting the DFT+LAP potential energy curves of small molecule dimers to those obtained from coupled cluster calculations with single, double, and perturbatively treated triple excitations, CCSD(T). Excellent transferability of the LAPs is demonstrated by remarkable agreement with the JSCH-2005 benchmark database [P. Jurecka et al. Phys. Chem. Chem. Phys. 8, 1985 (2006)], which provides the interaction energies of CCSD(T) quality for 165 vdW and hydrogen-bonded complexes. For over 100 vdW dominant complexes in this database, our DFT+LAP calculations give a mean absolute deviation from the benchmark results less than 0.5 kcal/mol. The DFT+LAP approach involves no extra computational cost other than standard DFT calculations and no modification of existing DFT codes, which enables straightforward quantum simulations, such as ab initio molecular dynamics, on biomolecular systems, as well as on other organic systems.

  15. First-Principles Models for van der Waals Interactions in Molecules and Materials: Concepts, Theory, and Applications.

    PubMed

    Hermann, Jan; DiStasio, Robert A; Tkatchenko, Alexandre

    2017-03-22

    Noncovalent van der Waals (vdW) or dispersion forces are ubiquitous in nature and influence the structure, stability, dynamics, and function of molecules and materials throughout chemistry, biology, physics, and materials science. These forces are quantum mechanical in origin and arise from electrostatic interactions between fluctuations in the electronic charge density. Here, we explore the conceptual and mathematical ingredients required for an exact treatment of vdW interactions, and present a systematic and unified framework for classifying the current first-principles vdW methods based on the adiabatic-connection fluctuation-dissipation (ACFD) theorem (namely the Rutgers-Chalmers vdW-DF, Vydrov-Van Voorhis (VV), exchange-hole dipole moment (XDM), Tkatchenko-Scheffler (TS), many-body dispersion (MBD), and random-phase approximation (RPA) approaches). Particular attention is paid to the intriguing nature of many-body vdW interactions, whose fundamental relevance has recently been highlighted in several landmark experiments. The performance of these models in predicting binding energetics as well as structural, electronic, and thermodynamic properties is connected with the theoretical concepts and provides a numerical summary of the state-of-the-art in the field. We conclude with a roadmap of the conceptual, methodological, practical, and numerical challenges that remain in obtaining a universally applicable and truly predictive vdW method for realistic molecular systems and materials.

  16. On the tautomerisation of porphycene on copper (111): Finding the subtle balance between van der Waals interactions and hybridisation.

    PubMed

    Novko, Dino; Tremblay, Jean Christophe; Blanco-Rey, María

    2016-12-28

    We use density-functional theory (DFT) to analyse the interaction of trans- and cis-porphycene with Cu(111) and their interconversion by intramolecular H-transfer. This tautomerisation reaction is characterised by small values for the reaction energy and barrier, on the order of ∼0.1 eV, where the trans configuration is thermodynamically more stable upon adsorption according to the experiments [J. N. Ladenthin et al., ACS Nano 9, 7287-7295 (2015)]. To gain even a qualitatively correct description of this reaction at the DFT level, an accurate treatment of dispersion interactions and a careful choice of the exchange contribution are required in order to predict the subtle energetics. Analysis of the electronic structure shows that adsorption is contributed by a van der Waals (vdW) interaction, mainly responsible for stabilising the polyaromatic fragments, and by a significant charge redistribution localised between Cu and the unsaturated N atoms of the molecule central cavity. We find that different vdW functionals can produce qualitatively different electronic structures, while yielding small trans vs. cis energy differences. Unlike other functionals surveyed here, vdW-DF with PBE exchange satisfactorily reproduces not only the experimental energetics but also the scanning tunneling microscopy images. This gives us confidence that this functional achieves a reliable balance between the two mechanisms contributing to the adsorption of porphycene.

  17. Accurate and efficient calculation of van der Waals interactions within density functional theory by local atomic potential approach.

    PubMed

    Sun, Y Y; Kim, Yong-Hyun; Lee, Kyuho; Zhang, S B

    2008-10-21

    Density functional theory (DFT) in the commonly used local density or generalized gradient approximation fails to describe van der Waals (vdW) interactions that are vital to organic, biological, and other molecular systems. Here, we propose a simple, efficient, yet accurate local atomic potential (LAP) approach, named DFT+LAP, for including vdW interactions in the framework of DFT. The LAPs for H, C, N, and O are generated by fitting the DFT+LAP potential energy curves of small molecule dimers to those obtained from coupled cluster calculations with single, double, and perturbatively treated triple excitations, CCSD(T). Excellent transferability of the LAPs is demonstrated by remarkable agreement with the JSCH-2005 benchmark database [P. Jurecka et al. Phys. Chem. Chem. Phys. 8, 1985 (2006)], which provides the interaction energies of CCSD(T) quality for 165 vdW and hydrogen-bonded complexes. For over 100 vdW dominant complexes in this database, our DFT+LAP calculations give a mean absolute deviation from the benchmark results less than 0.5 kcal/mol. The DFT+LAP approach involves no extra computational cost other than standard DFT calculations and no modification of existing DFT codes, which enables straightforward quantum simulations, such as ab initio molecular dynamics, on biomolecular systems, as well as on other organic systems.

  18. van der Waals radii of noble gases.

    PubMed

    Vogt, Jürgen; Alvarez, Santiago

    2014-09-02

    Consistent van der Waals radii are deduced for Ne-Xe, based on the noble gas···oxygen intermolecular distances found in gas phase structures. The set of radii proposed is shown to provide van der Waals distances for a wide variety of noble gas···element atom pairs that represent properly the distribution of distances both in the gas phase and in the solid state. Moreover, these radii show a smooth periodic trend down the group which is parallel to that shown by the halogens.

  19. Computations of Lifshitz-van der Waals interaction energies between irregular particles and surfaces at all separations for resuspension modelling

    NASA Astrophysics Data System (ADS)

    Priye, Aashish; Marlow, William H.

    2013-10-01

    The phenomenon of particle resuspension plays a vital role in numerous fields. Among many aspects of particle resuspension dynamics, a dominant concern is the accurate description and formulation of the van der Waals (vdW) interactions between the particle and substrate. Current models treat adhesion by incorporating a material-dependent Hamaker's constant which relies on the heuristic Hamaker's two-body interactions. However, this assumption of pairwise summation of interaction energies can lead to significant errors in condensed matter as it does not take into account the many-body interaction and retardation effects. To address these issues, an approach based on Lifshitz continuum theory of vdW interactions has been developed to calculate the principal many-body interactions between arbitrary geometries at all separation distances to a high degree of accuracy through Lifshitz's theory. We have applied this numerical implementation to calculate the many-body vdW interactions between spherical particles and surfaces with sinusoidally varying roughness profile and also to non-spherical particles (cubes, cylinders, tetrahedron etc) orientated differently with respect to the surface. Our calculations revealed that increasing the surface roughness amplitude decreases the adhesion force and non-spherical particles adhere to the surfaces more strongly when their flatter sides are oriented towards the surface. Such practical shapes and structures of particle-surface systems have not been previously considered in resuspension models and this rigorous treatment of vdW interactions provides more realistic adhesion forces between the particle and the surface which can then be coupled with computational fluid dynamics models to improve the predictive capabilities of particle resuspension dynamics.

  20. Hyperdislocations in van der Waals Layered Materials.

    PubMed

    Ly, Thuc Hue; Zhao, Jiong; Keum, Dong Hoon; Deng, Qingming; Yu, Zhiyang; Lee, Young Hee

    2016-12-14

    Dislocations are one-dimensional line defects in three-dimensional crystals or periodic structures. It is common that the dislocation networks made of interactive dislocations be generated during plastic deformation. In van der Waals layered materials, the highly anisotropic nature facilitates the formation of such dislocation networks, which is critical for the friction or exfoliation behavior for these materials. By transmission electron microscopy analysis, we found the topological defects in such dislocation networks can be perfectly rationalized in the framework of traditional dislocation theory, which we applied the name "hyperdislocations". Due to the strong pinning effect of hyperdislocations, the state of exfoliation can be easily triggered by 1° twisting between two layers, which also explains the origin of disregistry and frictionlessness for all of the superlubricants that are widely used for friction reduction and wear protection.

  1. The CO-Ne van der Waals complex: ab initio intermolecular potential energy, interaction induced electric dipole moment and polarizability surfaces, and second virial coefficients.

    PubMed

    Baranowska, Angelika; Fernández, Berta; Rizzo, Antonio; Jansík, Branislav

    2009-11-14

    The intermolecular potential energy, interaction induced electric dipole moment and polarizability surfaces of the CO-Ne van der Waals complex are calculated using coupled cluster methods and the d-aug-cc-pVTZ basis set extended with a set of 3s3p2d1f1g midbond functions placed in the middle of the van der Waals bond. After fitting the interaction properties to appropriate analytical functions the surfaces are further used in semiclassical calculations of the pressure, the dielectric and the refractivity second virial coefficients of the system. The interaction potential energy surface has a single minimum (-49.9952 cm(-1)), which corresponds to R = 3.383 A and theta = 79.4 degrees. The computed dielectric second virial coefficient B(epsilon) approximately -0.27 cm(6) mol(-2) around the room temperature.

  2. Effects of van der Waals Interactions in the Adsorption of Isooctane and Ethanol on Fe(100) Surfaces.

    PubMed

    Bedolla, Pedro O; Feldbauer, Gregor; Wolloch, Michael; Eder, Stefan J; Dörr, Nicole; Mohn, Peter; Redinger, Josef; Vernes, András

    2014-08-07

    van der Waals (vdW) forces play a fundamental role in the structure and behavior of diverse systems. Because of development of functionals that include nonlocal correlation, it is possible to study the effects of vdW interactions in systems of industrial and tribological interest. Here we simulated within the framework of density functional theory (DFT) the adsorption of isooctane (2,2,4-trimethylpentane) and ethanol on an Fe(100) surface, employing various exchange-correlation functionals to take vdW forces into account. In particular, this paper discusses the effect of vdW forces on the magnitude of adsorption energies, equilibrium geometries, and their role in the binding mechanism. According to our calculations, vdW interactions increase the adsorption energies and reduce the equilibrium distances. Nevertheless, they do not influence the spatial configuration of the adsorbed molecules. Their effect on the electronic density is a nonisotropic, delocalized accumulation of charge between the molecule and the slab. In conclusion, vdW forces are essential for the adsorption of isooctane and ethanol on a bcc Fe(100) surface.

  3. The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface.

    PubMed

    Morbec, Juliana M; Kratzer, Peter

    2017-01-21

    Using first-principles calculations based on density-functional theory (DFT), we investigated the effects of the van der Waals (vdW) interactions on the structural and electronic properties of anthracene and pentacene adsorbed on the Ag(111) surface. We found that the inclusion of vdW corrections strongly affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding adsorption heights and energies more consistent with the experimental results than standard DFT calculations with generalized gradient approximation (GGA). For anthracene/Ag(111) the effect of the vdW interactions is even more dramatic: we found that "pure" DFT-GGA calculations (without including vdW corrections) result in preference for a tilted configuration, in contrast to the experimental observations of flat-lying adsorption; including vdW corrections, on the other hand, alters the binding geometry of anthracene/Ag(111), favoring the flat configuration. The electronic structure obtained using a self-consistent vdW scheme was found to be nearly indistinguishable from the conventional DFT electronic structure once the correct vdW geometry is employed for these physisorbed systems. Moreover, we show that a vdW correction scheme based on a hybrid functional DFT calculation (HSE) results in an improved description of the highest occupied molecular level of the adsorbed molecules.

  4. The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface

    NASA Astrophysics Data System (ADS)

    Morbec, Juliana M.; Kratzer, Peter

    2017-01-01

    Using first-principles calculations based on density-functional theory (DFT), we investigated the effects of the van der Waals (vdW) interactions on the structural and electronic properties of anthracene and pentacene adsorbed on the Ag(111) surface. We found that the inclusion of vdW corrections strongly affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding adsorption heights and energies more consistent with the experimental results than standard DFT calculations with generalized gradient approximation (GGA). For anthracene/Ag(111) the effect of the vdW interactions is even more dramatic: we found that "pure" DFT-GGA calculations (without including vdW corrections) result in preference for a tilted configuration, in contrast to the experimental observations of flat-lying adsorption; including vdW corrections, on the other hand, alters the binding geometry of anthracene/Ag(111), favoring the flat configuration. The electronic structure obtained using a self-consistent vdW scheme was found to be nearly indistinguishable from the conventional DFT electronic structure once the correct vdW geometry is employed for these physisorbed systems. Moreover, we show that a vdW correction scheme based on a hybrid functional DFT calculation (HSE) results in an improved description of the highest occupied molecular level of the adsorbed molecules.

  5. van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions

    SciTech Connect

    Berland, Kristian; Arter, Calvin A.; Thonhauser, T.; Cooper, Valentino R.; Lee, Kyuho; Lundqvist, Bengt I.; Schröder, Elsebeth; Hyldgaard, Per

    2014-05-14

    The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory. One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO{sub 3}, the adsorption of small molecules within metal-organic frameworks, the graphite/diamond phase transition, and the adsorption of an aromatic-molecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general-purpose functional that could be applied to a range of materials problems with a variety of competing interactions.

  6. van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions

    SciTech Connect

    Berland, Kristian; Arter, Calvin A; Cooper, Valentino R; Lee, Dr. Kyuho; Lundqvist, Prof. Bengt I.; Schroder, Prof. Elsebeth; Thonhauser, Prof. Timo; Hyldgaard, Per

    2014-01-01

    The theoretical description of sparse matter attracts much interest, in particular for those groundstate properties that can be described by density functional theory (DFT). One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B, in print] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO3, the adsorption of small molecules within metal-organic frameworks (MOFs), the graphite/diamond phase transition, and the adsorption of an aromaticmolecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general purpose functional that could be applied to a range of materials problems with a variety of competing interactions.

  7. Adsorption of benzene on low index surfaces of platinum in the presence of van der Waals interactions

    NASA Astrophysics Data System (ADS)

    K, Ayishabi P.; Chatanathodi, Raghu

    2017-10-01

    We have studied the adsorption of benzene on three low index surfaces of platinum using plane-wave Density Functional Theory (DFT) calculations, taking into consideration van der Waals (vdW) interaction. Experimentally, it is known that benzene adsorbs at the bridge site on the (111) surface, but in case of (110) and (100), this is not known yet. Our calculations show that benzene preferably adsorbs on bridge position on Pt(111) surface, whereas on Pt(110) and Pt(100) surfaces, the hollow position is energetically more favoured. The structural and electronic modifications of molecule and the surfaces are also examined. In all cases, adsorption-induced distortions of adsorbate-substrate complex are found to be modest in character, but relatively maximum in case of the (110) facet. The molecule is bound most strongly to the (110) surface. Importantly, we find that adsorption at bridge and atop positions are energetically feasible on the (110) surface, with the canting of benzene ring at a small angle from the metal plane. We study changes in electronic structure and the net charge transfer upon adsorption of benzene on all three low index planes. Inclusion of vdW interactions is important for obtaining realistic adsorption strengths for benzene on various Pt facets.

  8. Evolutionary design of interfacial phase change van der Waals heterostructures.

    PubMed

    Kalikka, Janne; Zhou, Xilin; Behera, Jitendra; Nannicini, Giacomo; Simpson, Robert E

    2016-10-27

    We use an evolutionary algorithm to explore the design space of hexagonal Ge2Sb2Te5; a van der Waals layered two dimensional crystal heterostructure. The Ge2Sb2Te5 structure is more complicated than previously thought. Predominant features include layers of Ge3Sb2Te6 and Ge1Sb2Te4 two dimensional crystals that interact through Te-Te van der Waals bonds. Interestingly, (Ge/Sb)-Te-(Ge/Sb)-Te alternation is a common feature for the most stable structures of each generation's evolution. This emergent rule provides an important structural motif that must be included in the design of high performance Sb2Te3-GeTe van der Waals heterostructure superlattices with interfacial atomic switching capability. The structures predicted by the algorithm agree well with experimental measurements on highly oriented, and single crystal Ge2Sb2Te5 samples. By analysing the evolutionary algorithm optimised structures, we show that diffusive atomic switching is probable by Ge atoms undergoing a transition at the van der Waals interface from layers of Ge3Sb2Te6 to Ge1Sb2Te4 thus producing two blocks of Ge2Sb2Te5. Evolutionary methods present an efficient approach to explore the enormous multi-dimensional design parameter space of van der Waals bonded heterostructure superlattices.

  9. Application of mixed-mode, solid-phase extraction in environmental and clinical chemistry. Combining hydrogen-bonding, cation-exchange and Van der Waals interactions

    USGS Publications Warehouse

    Mills, M.S.; Thurman, E.M.; Pedersen, M.J.

    1993-01-01

    Silica- and styrene-divinylbenzene-based mixed-mode resins that contain C8, C18 and sulphonated cation-exchange groups were compared for their efficiency in isolation of neutral triazine compounds from water and of the basic drug, benzoylecgonine, from urine. The triazine compounds were isolated by a combination of Van der Waals and hydrogen-bonding interactions, and benzoylecgonine was isolated by Van der Waals interactions and cation exchange. All analytes were eluted with a polar organic solvent contaning 2% ammonium hydroxide. Larger recoveries (95%) were achieved on copolymerized mixed-mode resins where C18 and sulfonic acid are in closer proximity than on 'blended' mixed-mode resins (60-70% recovery).

  10. Ab initio calculations of van der Waals interactions in one- and two-dimensional infinite periodic systems

    NASA Astrophysics Data System (ADS)

    Fink, Karin; Staemmler, Volker

    1995-08-01

    A new CEPA-PNO (coupled electron pair approach with pair natural orbitals) method for the calculation of correlation energies in infinite periodic systems is proposed and applied to one- and two-dimensional He. The method starts from a crystal orbital Hartree-Fock (COHF) wavefunction with the occupied Bloch orbitals transformed into Wannier orbitals. The coupled-cluster equations for the infinite system are simplified by CEPA-type approximations: A CEPA-0 (or linear coupled-cluster) formula is applied for the small intercell contributions to the total correlation energy while CI-SD, ACPF or other CEPA variants are used for the large intracell contributions. The enormous number of single and double excitations into the virtual space is greatly reduced by the use of pair natural orbitals (PNOs), which leads to large savings in the necessary computer time and disk storage. First applications to the van der Waals interaction in the linear chain and the hexagonal plane of He atoms, performed with medium size and large atomic basis sets, show that an accuracy can be reached for the infinite systems which is comparable to the accuracy of the corresponding calculations for small He clusters. Because of the extended use of the translational symmetry of the Wannier orbitals, the calculations for the linear infinite systems are even considerably faster than those for the oligomers He5 and He7.

  11. The effect of intertube van der Waals interaction on the stability of pristine and functionalized carbon nanotubes under compression.

    PubMed

    Kuang, Y D; Shi, S Q; Chan, P K L; Chen, C Y

    2010-03-26

    This paper investigates the effect of intertube van der Waals interaction on the stability of pristine and covalently functionalized carbon nanotubes under axial compression, using molecular mechanics simulations. After regulating the number of inner layers of the armchair four-walled (5, 5)@(10, 10)@(15, 15)@(20, 20) and zigzag four-walled (6, 0)@(15, 0)@(24, 0)@(33, 0) carbon nanotubes, the critical buckling strains of the corresponding tubes are calculated. The results show that each of the three inner layers in the functionalized armchair nanotube noticeably contributes to the stability of the outermost tube, and together increase the critical strain amplitude by 155%. However, the three inner layers in the corresponding pristine nanotube, taken together, increase the critical strain of the outermost tube by only 23%. In addition, for both the pristine and functionalized zigzag nanotubes, only the (24, 0) layer, among the three inner layers, contributes to the critical strain of the corresponding outermost tube, by 11% and 29%, respectively. The underlying mechanism of the enhanced stability related to nanotube chirality and functionalization is analyzed in detail.

  12. Detachment of compliant films adhered to stiff substrates via van der Waals interactions: role of frictional sliding during peeling

    PubMed Central

    Collino, Rachel R.; Philips, Noah R.; Rossol, Michael N.; McMeeking, Robert M.; Begley, Matthew R.

    2014-01-01

    The remarkable ability of some plants and animals to cling strongly to substrates despite relatively weak interfacial bonds has important implications for the development of synthetic adhesives. Here, we examine the origins of large detachment forces using a thin elastomer tape adhered to a glass slide via van der Waals interactions, which serves as a model system for geckos, mussels and ivy. The forces required for peeling of the tape are shown to be a strong function of the angle of peeling, which is a consequence of frictional sliding at the edge of attachment that serves to dissipate energy that would otherwise drive detachment. Experiments and theory demonstrate that proper accounting for frictional sliding leads to an inferred work of adhesion of only approximately 0.5 J m−2 (defined for purely normal separations) for all load orientations. This starkly contrasts with the interface energies inferred using conventional interface fracture models that assume pure sticking behaviour, which are considerably larger and shown to depend not only on the mode-mixity, but also on the magnitude of the mode-I stress intensity factor. The implications for developing frameworks to predict detachment forces in the presence of interface sliding are briefly discussed. PMID:24920120

  13. Linear interaction energy models for beta-secretase (BACE) inhibitors: Role of van der Waals, electrostatic, and continuum-solvation terms.

    PubMed

    Tounge, Brett A; Rajamani, Ramkumar; Baxter, Ellen W; Reitz, Allen B; Reynolds, Charles H

    2006-05-01

    Computing the binding affinity of a protein-ligand complex is one of the most fundamental and difficult tasks in computer-aided drug design. Many approaches for computing binding affinities can be classified as linear interaction energy (LIE) models as they rely on some type of linear fit of computed interaction energies between ligand and protein. We have examined the computed interaction energies of a series of beta-secretase (BACE) inhibitors in terms of van der Waals, coulombic, and continuum-solvation contributions to ligand binding. We have also systematically examined the effect of different protonation states of the protein and ligands. We find that the binding affinities are relatively insensitive to the protonation state of the protein when neutral ligands are considered. Inclusion of charged ligands leads to large deviations in the coulomb, solvation, and even van der Waals terms. The latter is due to increased repulsive van der Waals interactions in the complex due to the strong coulomb attraction found between oppositely charged functional groups in the protein and ligand. In general, we find that the best models are obtained when the protein is judiciously charged (e.g. Asp32-, Arg235+) and the potentially charged ligands are treated as neutral.

  14. Curves of growth for van der Waals broadened spectral lines

    NASA Technical Reports Server (NTRS)

    Park, C.

    1980-01-01

    Curves of growth are evaluated for a spectral line broadened by the van der Waals interactions during collisions. The growth of the equivalent widths of such lines is shown to be dependent on the product of the perturber density and the 6/10 power of the van der Waals potential coefficient. When the parameter is small, the widths grow as the 1/2 power of the optical depth as they do for the Voigt profile: but when the parameter is large, they grow as 2/3 power and, hence, faster than the Voigt profile. An approximate analytical expression for the computed growth characteristics is given.

  15. Note on a van der Waals Gas.

    ERIC Educational Resources Information Center

    Bauman, Robert P.; Harrison, Joseph G.

    1996-01-01

    Discusses the difficulties with the standard model for introduction of attractive forces into the van der Waals equation. Presents an analysis in terms of force and time delays and an alternative analysis for more advanced students in terms of energy. (JRH)

  16. Structure and stability of weakly chemisorbed ethene adsorbed on low-index Cu surfaces: performance of density functionals with van der Waals interactions.

    PubMed

    Hanke, Felix; Dyer, Matthew S; Björk, Jonas; Persson, Mats

    2012-10-24

    We have investigated the performance of popular density functionals that include van der Waals interactions for the experimentally well-characterized problem of ethene (C(2)H(4)) adsorbed on the low-index surfaces of copper. This set of functionals does not only include three van der Waals density functionals-vdwDF-PBE, vdwDF-revPBE and optB86b-vdwDF-and two dispersion-corrected functionals-Grimme and TS-but also local and semi-local functionals such as LDA and PBE. The adsorption system of ethene on copper was chosen because it is a weakly chemisorbed system for which the vdW interactions are expected to give a significant contribution to the adsorption energy. Overall the density functionals that include vdW interactions increased substantially the adsorption energies compared to the PBE density functional but predicted the same adsorption sites and very similar C-C bonding distances except for two of the van der Waals functionals. The top adsorption site was predicted almost exclusively for all functionals on the (110), (100) and (111) surfaces, which is in agreement with experiment for the (110) surface but not for the (100) surface. On the (100) surface, all functionals except two van der Waals density functionals singled out the observed cross-hollow site from the calculated C-C bonding distances and adsorption heights. On the top sites on the (110) surface and the cross-hollow site on the Cu(100) surface, the ethene molecule was found to form a weak chemisorption bond. On the (111) surface, all functionals gave a C-C bonding distance and an adsorption height more typical for physisorption, in agreement with experiments.

  17. A molecular H2 potential for heterogeneous simulations including polarization and many-body van der Waals interactions

    NASA Astrophysics Data System (ADS)

    McLaughlin, Keith; Cioce, Christian R.; Belof, Jonathan L.; Space, Brian

    2012-05-01

    A highly accurate aniostropic intermolecular potential for diatomic hydrogen has been developed that is transferable for molecular modeling in heterogeneous systems. The potential surface is designed to be efficacious in modeling mixed sorbates in metal-organic materials that include sorption interactions with charged interfaces and open metal sites. The potential parameters are compatible for mixed simulations but still maintain high accuracy while deriving dispersion parameters from a proven polarizability model. The potential includes essential physical interactions including: short-range repulsions, dispersion, and permanent and induced electrostatics. Many-body polarization is introduced via a point-atomic polarizability model that is also extended to account for many-body van der Waals interactions in a consistent fashion. Permanent electrostatics are incorporated using point partial charges on atomic sites. However, contrary to expectation, the best potentials are obtained by permitting the charges to take on values that do not reproduce the first non-vanishing moment of the electrostatic potential surface, i.e., the quadrupole moment. Potential parameters are fit to match ab initio energies for a representative range of dimer geometries. The resulting potential is shown to be highly effective by comparing to electronic structure calculations for a thermal distribution of trimer geometries, and by reproducing experimental bulk pressure-density isotherms. The surface is shown to be superior to other similarly portable potential choices even in tests on homogeneous systems without strong polarizing fields. The present streamlined approach to developing such potentials allows for a simple adaptation to other molecules amenable to investigation by high-level electronic structure methods.

  18. The Economics of van der Waals Force Engineering

    NASA Astrophysics Data System (ADS)

    Pinto, Fabrizio

    2008-01-01

    As micro-electro-mechanical system (MEMS) fabrication continues on an ever-decreasing scale, new technological challenges must be successfully negotiated if Moore's Law is to be an even approximately valid model of the future of device miniaturization. Among the most significant obstacles is the existence of strong surface forces related to quantum mechanical van der Waals interatomic interactions, which rapidly diverge as the distance between any two neutral boundaries decreases. The van der Waals force is a contributing factor in several device failures and limitations, including, for instance, stiction and oscillator non-linearities. In the last decade, however, it has been conclusively shown that van der Waals forces are not just a MEMS limitation but can be engineered in both magnitude and sign so as to enable classes of proprietary inventions which either deliver novel capabilities or improve upon existing ones. The evolution of van der Waals force research from an almost exclusively theoretical field in quantum-electro-dynamics to an enabling nanotechnology discipline represents a useful example of the ongoing paradigm shift from government-centered to private-capital funded R&D in cutting-edge physics leading to potentially profitable products. In this paper, we discuss the reasons van der Waals force engineering may lead to the creation of thriving markets both in the short and medium terms by highlighting technical challenges that can be competitively addressed by this novel approach. We also discuss some notable obstacles to the cultural transformation of the academic research community required for the emergence of a functional van der Waals force engineering industry worldwide.

  19. Precise, Self-Limited Epitaxy of Ultrathin Organic Semiconductors and Heterojunctions Tailored by van der Waals Interactions.

    PubMed

    Wu, Bing; Zhao, Yinghe; Nan, Haiyan; Yang, Ziyi; Zhang, Yuhan; Zhao, Huijuan; He, Daowei; Jiang, Zonglin; Liu, Xiaolong; Li, Yun; Shi, Yi; Ni, Zhenhua; Wang, Jinlan; Xu, Jian-Bin; Wang, Xinran

    2016-06-08

    Precise assembly of semiconductor heterojunctions is the key to realize many optoelectronic devices. By exploiting the strong and tunable van der Waals (vdW) forces between graphene and organic small molecules, we demonstrate layer-by-layer epitaxy of ultrathin organic semiconductors and heterostructures with unprecedented precision with well-defined number of layers and self-limited characteristics. We further demonstrate organic p-n heterojunctions with molecularly flat interface, which exhibit excellent rectifying behavior and photovoltaic responses. The self-limited organic molecular beam epitaxy (SLOMBE) is generically applicable for many layered small-molecule semiconductors and may lead to advanced organic optoelectronic devices beyond bulk heterojunctions.

  20. Nonperturbative calculation of the London-van der Waals interaction potential

    NASA Astrophysics Data System (ADS)

    Berman, P. R.; Ford, G. W.; Milonni, P. W.

    2014-02-01

    The so-called remarkable formula [G. W. Ford, J. T. Lewis, and R. F. O'Connell, Phys. Rev. Lett. 55, 2273 (1985), 10.1103/PhysRevLett.55.2273] for the Helmholtz free energy is applied to the problem of determining the interaction potential to all orders in the coupling strength of a pair of oscillator dipoles interacting through the familiar dipole-dipole interaction of electrodynamics. Simple, straightforward calculations lead to expressions for (1) the London short-range potential, (2) the Casimir-Polder long-range potential, and (3) the potential at high temperature. Explicit results are shown for both the temperature dependence of the interaction potential and its deviation from the weak-coupling limit. It is stressed that the interaction potential is a change in free energy, not the energy; in particular, in the high temperature case, the change of energy is zero.

  1. Van der Waals Interactions of Organic Molecules on Semiconductor and Metal Surfaces: a Comparative Study

    NASA Astrophysics Data System (ADS)

    Li, Guo; Cooper, Valentino; Cho, Jun-Hyung; Tamblyn, Isaac; Du, Shixuan; Neaton, Jeffrey; Gao, Hong-Jun; Zhang, Zhenyu

    2012-02-01

    We present a comparative investigation of vdW interactions of the organic molecules on semiconductor and metal surfaces using the DFT method implemented with vdW-DF. For styrene/H-Si(100), the vdW interactions reverse the effective intermolecular interaction from repulsive to attractive, ensuring preferred growth of long wires as observed experimentally. We further propose that an external E field and the selective creation of Si dangling bonds can drastically improve the ordered arrangement of the molecular nanowires [1]. For BDA/Au(111), the vdW interactions not only dramatically enhances the adsorption energies, but also significantly changes the molecular configurations. In the azobenzene/Ag(111) system, vdW-DF produces superior predictions for the adsorption energy than those obtained with other vdW corrected DFT approaches, providing evidence for the applicability of the vdW-DF method [2].

  2. Van der Waals Effects on semiconductor clusters.

    PubMed

    Li, Haisheng; Chen, Weiguang; Han, Xiaoyu; Li, Liben; Sun, Qiang; Guo, Zhengxiao; Jia, Yu

    2015-09-30

    Van der Waals (vdW) interactions play an important role on semiconductors in nanoscale. Here, we utilized first-principles calculations based on density functional theory to demonstrate the growth mode transition from prolate to multiunit configurations for Gen (n = 10-50) clusters. In agreement with the injected ion drift tube techniques that "clusters with n < 70 can be thought of as loosely bound assemblies of small strongly bound fragments (such as Ge7 and Ge10 )," we found these stable fragments are connected by Ge6 , Ge9 , or Ge10 unit (from bulk diamond), via strong covalent bonds. Our calculated cations usually fragment to Ge7 and Ge10 clusters, in accordance with the experiment results that the spectra Ge7 and Ge10 correspond to the mass abundance spectra. By controlling a germanium cluster with vdW interactions parameters in the program or not, we found that the vdW effects strengthen the covalent bond from different units more strikingly than that in a single unit. With more bonds between units than the threadlike structures, the multiunit structures have larger vdW energies, explaining why the isolated nanowires are harder to produce. © 2015 Wiley Periodicals, Inc.

  3. Thermal response in van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Naidu Gandi, Appala; Alshareef, Husam N.; Schwingenschlögl, Udo

    2017-01-01

    We solve numerically the Boltzmann transport equations of the phonons and electrons to understand the thermoelectric response in heterostructures of M2CO2 (M: Ti, Zr, Hf) MXenes with transition metal dichalcogenide monolayers. Low frequency optical phonons are found to occur as a consequence of the van der Waals bonding, contribute significantly to the thermal transport, and compensate for the reduced contributions of the acoustic phonons (increased scattering cross-sections in heterostructures), such that the thermal conductivities turn out to be similar to those of the bare MXenes. Our results indicate that the important superlattice design approach of thermoelectrics (to reduce the thermal conductivity) may be effective for two-dimensional van der Waals materials when used in conjunction with intercalation.

  4. Thermal response in van der Waals heterostructures.

    PubMed

    Gandi, Appala Naidu; Alshareef, Husam N; Schwingenschlögl, Udo

    2017-01-25

    We solve numerically the Boltzmann transport equations of the phonons and electrons to understand the thermoelectric response in heterostructures of M2CO2 (M: Ti, Zr, Hf) MXenes with transition metal dichalcogenide monolayers. Low frequency optical phonons are found to occur as a consequence of the van der Waals bonding, contribute significantly to the thermal transport, and compensate for the reduced contributions of the acoustic phonons (increased scattering cross-sections in heterostructures), such that the thermal conductivities turn out to be similar to those of the bare MXenes. Our results indicate that the important superlattice design approach of thermoelectrics (to reduce the thermal conductivity) may be effective for two-dimensional van der Waals materials when used in conjunction with intercalation.

  5. Third-order perturbation theory for van der Waals interaction coefficients

    SciTech Connect

    Tang Liyan; Shi Tingyun; Yan Zongchao; Mitroy, J.

    2011-11-15

    The third-order expression for the dispersion interaction between two atoms is written as a sum over lists of transition matrix elements. Particular attention is given to the C{sub 9}/R{sup 9} interaction which occurs in the homonuclear case when one atom is in an S state and the other is in a P state. Numerical values of the C{sub 9} coefficient are given for the homonuclear alkali-metal dimers. The size of the C{sub 9}:C{sub 3} dispersion coefficient ratio increases for the heavier alkali-metal atoms. The C{sub 11} and C{sub 13} coefficients between two helium atoms and lithium atoms in their ground states are also given.

  6. Effects of dielectric disorder on van der Waals interactions in slab geometries.

    PubMed

    Dean, David S; Horgan, Ron R; Naji, Ali; Podgornik, Rudolf

    2010-05-01

    We analyze the effects of disorder on the thermal Casimir interaction for the case of two semi-infinite planar slabs across an intervening homogeneous unstructured dielectric. The semi-infinite bounding layers are assumed to be composed of plane-parallel layers of random dielectric materials. We show that the effective thermal Casimir interaction at long distances is self-averaging and can be written in the same form as the one between nonrandom media but with the effective dielectric tensor of the corresponding random media. On the contrary, the behavior at short distances becomes random, and thus sample dependent, dominated by the local values of the dielectric constants proximal to each other across the central homogeneous unstructured dielectric layer. We extend these results to the regime of intermediate slab separations by using perturbation theory for weak disorder as well as by extensive numerical simulations for a number of systems where the dielectric function has a log-normal distribution. Numerical simulation completely corroborates all the main features of the disorder dependent thermal Casimir interaction deduced analytically.

  7. Importance of van der Waals interaction for organic molecule-metal junctions: adsorption of thiophene on Cu(110) as a prototype.

    PubMed

    Sony, Priya; Puschnig, Peter; Nabok, Dmitrii; Ambrosch-Draxl, Claudia

    2007-10-26

    We report ab initio calculations for the interface energetics of a weakly adsorbed organic molecule on a metal surface, which serves as a model interface relevant for organic electronics. The studied thiophene ring is found to be physisorbed on the Cu(110) surface with an adsorption energy of -0.50 eV. Nonlocal correlations, i.e., van der Waals interactions, are solely responsible for the binding in this weakly interacting system, and the choice of the proper exchange-correlation function is crucially important. The adsorption of thiophene lowers the metal work function due to the formation of surface dipoles while no sizable charge transfer is found.

  8. Van der Waals Force Assisted Heat Transfer

    NASA Astrophysics Data System (ADS)

    Sasihithlu, K.; Pendry, J. B.; Craster, R. V.

    2017-02-01

    Phonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons), it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed [J. B. Pendry, K. Sasihithlu, and R. V. Craster, Phys. Rev. B 94, 075414 (2016)] by which phonons can transport heat across a vacuum gap - through the Van der Waals interaction between two bodies with gap less than the wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modelling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro- and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps as well as the heat transfer calculation due to photon exchange.

  9. Van der Waals Layered Materials: Surface Morphology, Interlayer Interaction, and Electronic Structure

    NASA Astrophysics Data System (ADS)

    Yeh, Po-Chun

    The search for new ultrathin materials as the "new silicon" has begun. In this dissertation, I examine (1) the surface structure, including the growth, the crystal quality, and thin film surface corrugation of a monolayer sample and a few layers of MoS2 and WSe2, and (2) their electronic structure. The characteristics of these electronic systems depend intimately on the morphology of the surfaces they inhabit, and their interactions with the substrate or within layers. These physical properties will be addressed in each chapter. This thesis has dedicated to the characterization of mono- and a few layers of MoS2 and WSe2 that uses surface-sensitive probes such as low-energy electron microscopy and diffraction (LEEM and LEED). Prior to our studies, the characterization of monolayer MoS2 and WSe2 has been generally limited to optical and transport probes. Furthermore, the heavy use of thick silicon oxide layer as the supporting substrate has been important in order to allow optical microscopic characterization of the 2D material. Hence, to the best of our knowledge, this has prohibited studies of this material on other surfaces, and it has precluded the discovery of potentially rich interface interactions that may exist between MoS 2 and its supporting substrate. Thus, in our study, we use a so-called SPELEEM system (Spectroscopic Photo-Emission and Low Energy Electron Microscopy) to address these imaging modalities: (1) real-space microscopy, which would allow locating of monolayer MoS2 samples, (2) spatially-resolved low-energy diffraction which would allow confirmation of the crystalline quality and domain orientation of MoS2 samples, and, (3) spatially-resolved spectroscopy, which would allow electronic structure mapping of MoS2 samples. Moreover, we have developed a preparation procedure for samples that yield, a surface-probe ready, ultra-clean, and can be transferred on an arbitrary substrate. To fully understand the physics in MoS2 such as direct

  10. Effect of van der Waals interactions on the chemisorption and physisorption of phenol and phenoxy on metal surfaces

    NASA Astrophysics Data System (ADS)

    Peköz, Rengin; Donadio, Davide

    2016-09-01

    The adsorption of phenol and phenoxy on the (111) surface of Au and Pt has been investigated by density functional theory calculations with the conventional PBE functional and three different non-local van der Waals (vdW) exchange and correlation functionals. It is found that both phenol and phenoxy on Au(111) are physisorbed. In contrast, phenol on Pt(111) presents an adsorption energy profile with a stable chemisorption state and a weakly metastable physisorbed precursor. While the use of vdW functionals is essential to determine the correct binding energy of both chemisorption and physisorption states, the relative stability and existence of an energy barrier between them depend on the semi-local approximations in the functionals. The first dissociation mechanism of phenol, yielding phenoxy and atomic hydrogen, has been also investigated, and the reaction and activation energies of the resulting phenoxy on the flat surfaces of Au and Pt were discussed.

  11. C{sub 6}H{sub 6}/Au(111): Interface dipoles, band alignment, charging energy, and van der Waals interaction

    SciTech Connect

    Abad, E.; Martinez, J. I.; Flores, F.; Ortega, J.; Dappe, Y. J.

    2011-01-28

    We analyze the benzene/Au(111) interface taking into account charging energy effects to properly describe the electronic structure of the interface and van der Waals interactions to obtain the adsorption energy and geometry. We also analyze the interface dipoles and discuss the barrier formation as a function of the metal work-function. We interpret our DFT calculations within the induced density of interface states (IDIS) model. Our results compare well with experimental and other theoretical results, showing that the dipole formation of these interfaces is due to the charge transfer between the metal and benzene, as described in the IDIS model.

  12. Devices and applications of van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Li, Chao; Zhou, Peng; Zhang, David Wei

    2017-03-01

    Van der Waals heterostructures, composed of individual two-dimensional material have been developing extremely fast. Synthesis of van der Waals heterostructures without the constraint of lattice matching and processing compatibility provides an ideal platform for fundamental research and new device exploitation. We review the approach of synthesis of van der Waals heterostructures, discuss the property of heterostructures and thoroughly illustrate the functional van der Waals heterostructures used in novel electronic and photoelectronic device. Project supported by the National Key Research and Development Program (No. 2016YFA0203900) and the National Natural Science Foundation of China (Nos. 61376093, 61622401).

  13. Accurate van der Waals coefficients from density functional theory

    PubMed Central

    Tao, Jianmin; Perdew, John P.; Ruzsinszky, Adrienn

    2012-01-01

    The van der Waals interaction is a weak, long-range correlation, arising from quantum electronic charge fluctuations. This interaction affects many properties of materials. A simple and yet accurate estimate of this effect will facilitate computer simulation of complex molecular materials and drug design. Here we develop a fast approach for accurate evaluation of dynamic multipole polarizabilities and van der Waals (vdW) coefficients of all orders from the electron density and static multipole polarizabilities of each atom or other spherical object, without empirical fitting. Our dynamic polarizabilities (dipole, quadrupole, octupole, etc.) are exact in the zero- and high-frequency limits, and exact at all frequencies for a metallic sphere of uniform density. Our theory predicts dynamic multipole polarizabilities in excellent agreement with more expensive many-body methods, and yields therefrom vdW coefficients C6, C8, C10 for atom pairs with a mean absolute relative error of only 3%. PMID:22205765

  14. A natural orbital analysis of the long range behavior of chemical bonding and van der Waals interaction in singlet H2: the issue of zero natural orbital occupation numbers.

    PubMed

    Sheng, X W; Mentel, Ł M; Gritsenko, O V; Baerends, E J

    2013-04-28

    This paper gives a natural orbital (NO) based analysis of the van der Waals interaction in (singlet) H2 at long distance. The van der Waals interaction, even if not leading to a distinct van der Waals well, affects the shape of the interaction potential in the van der Waals distance range of 5-9 bohrs and can be clearly distinguished from chemical bonding effects. In the NO basis the van der Waals interaction can be quantitatively covered with, apart from the ground state configurations (1σ(g))(2) and (1σ(u))(2), just the 4 configurations (2σ(g))(2) and (2σ(u))(2), and (1π(u))(2) and (1π(g))(2). The physics of the dispersion interaction requires and explains the peculiar relatively large positive CI coefficients of the doubly excited electron configurations (2σ(u))(2) and (1π(g))(2) (the occupancy amplitudes of the 2σ(u) and 1π(gx, y) NOs) in the distance range 5-9 bohrs, which have been observed before by Cioslowski and Pernal [Chem. Phys. Lett. 430, 188 (2006)]. We show that such positive occupancy amplitudes do not necessarily lead to the existence of zero occupation numbers at some H-H distances.

  15. van der Waals Heterostructures Grown by MBE

    NASA Astrophysics Data System (ADS)

    Hinkle, Christopher

    In this work, we demonstrate the high-quality MBE heterostructure growth of various layered 2D materials by van der Waals epitaxy (VDWE). The coupling of different types of van der Waals materials including transition metal dichalcogenide thin films (e.g., WSe2, WTe2, HfSe2) , insulating hexagonal boron nitride (h-BN), and topological insulators (e.g., Bi2Se3) allows for the fabrication of novel electronic devices that take advantage of unique quantum confinement and spin-based characteristics. The relaxed lattice-matching criteria of van der Waals epitaxy has allowed for high-quality heterostructure growth with atomically abrupt interfaces, allowing us to couple these materials based primarily on their band alignment and electronic properties. We will discuss the impact of sample preparation, surface reactivity, and lattice mismatch of various substrates (sapphire, graphene, TMDs, Bi2Se3) on the growth mode and quality of the films and will discuss our studies of substrate temperature and flux rates on the resultant growth and grain size. Structural and chemical characterization was conducted via reflection high energy electron diffraction (RHEED, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning tunneling microscopy/spectroscopy (STM/S), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Experimentally determined band alignments have been determined and compared with first-principles calculations allowing the design of novel low-power logic and magnetic memory devices. Initial results from the electrical characterization of these grown thin films and some simple devices will also be presented. These VDWE grown layered 2D materials show significant potential for fabricating novel heterostructures with tunable band alignments and magnetic properties for a variety of nanoelectronic and optoelectronic applications.

  16. Combining density functional and incremental post-Hartree-Fock approaches for van der Waals dominated adsorbate-surface interactions: Ag{sub 2}/graphene

    SciTech Connect

    Lara-Castells, María Pilar de; Mitrushchenkov, Alexander O.; Stoll, Hermann

    2015-09-14

    A combined density functional (DFT) and incremental post-Hartree-Fock (post-HF) approach, proven earlier to calculate He-surface potential energy surfaces [de Lara-Castells et al., J. Chem. Phys. 141, 151102 (2014)], is applied to describe the van der Waals dominated Ag{sub 2}/graphene interaction. It extends the dispersionless density functional theory developed by Pernal et al. [Phys. Rev. Lett. 103, 263201 (2009)] by including periodic boundary conditions while the dispersion is parametrized via the method of increments [H. Stoll, J. Chem. Phys. 97, 8449 (1992)]. Starting with the elementary cluster unit of the target surface (benzene), continuing through the realistic cluster model (coronene), and ending with the periodic model of the extended system, modern ab initio methodologies for intermolecular interactions as well as state-of-the-art van der Waals-corrected density functional-based approaches are put together both to assess the accuracy of the composite scheme and to better characterize the Ag{sub 2}/graphene interaction. The present work illustrates how the combination of DFT and post-HF perspectives may be efficient to design simple and reliable ab initio-based schemes in extended systems for surface science applications.

  17. Van der Waals density functional: An appropriate exchange functional

    NASA Astrophysics Data System (ADS)

    Cooper, Valentino R.

    2010-04-01

    In this Rapid Communication, an exchange functional which is compatible with the nonlocal Rutgers-Chalmers correlation functional [van der Waals density functional (vdW-DF)] is presented. This functional, when employed with vdW-DF, demonstrates remarkable improvements on intermolecular separation distances while further improving the accuracy of vdW-DF interaction energies. The key to the success of this three-parameter functional is its reduction in short-range exchange repulsion through matching to the gradient expansion approximation in the slowly varying/high-density limit while recovering the large reduced gradient, s , limit set in the revised Perdew-Burke-Ernzerhof (revPBE) exchange functional. This augmented exchange functional could be a solution to long-standing issues of vdW-DF lending to further applicability of density-functional theory to the study of relatively large, dispersion bound (van der Waals) complexes.

  18. Diffusiophoretic self-propulsion of colloids driven by a surface reaction: The sub-micron particle regime for exponential and van der Waals interactions

    NASA Astrophysics Data System (ADS)

    Sharifi-Mood, Nima; Koplik, Joel; Maldarelli, Charles

    2013-01-01

    Diffusiophoresis is a mechanism for propelling colloid particles in a liquid in which the driving force for the motion derives from intermolecular interactions between solute molecules surrounding the particle and the colloid itself. When solutes are asymmetrically distributed around the particle, the solutal interactions exerted on the colloid are unbalanced, and the particle is propelled. In self-diffusiophoresis, the particle itself creates the asymmetric distribution as a means of autonomous motion (a motor). Experiments implement the asymmetric production of a solutal concentration gradient by functionalizing one side of the colloid with a catalyst, which converts a reactant solute into a product. Previous hydrodynamic models of this design have assumed the length scale L of the intermolecular interaction (typically of order 1-10 nm) to be much smaller than the colloid radius, a (order 1 μm), L/a < 1. In this limit, assuming the catalytic reaction produces a constant flux of solute, and convective effects are negligible, the self-diffusiophoretic velocity is to leading order independent of a. Anticipating future experiments on nanosized motors (a=O(10-100 nm)), numerical solutions are presented for the velocity up to order one in L/a, and an integral asymptotic approximation is constructed accurate for L/a less than 0.1. Three intermolecular interactions are examined, a hard sphere excluded volume potential, an exponential interaction and a long-range van der Waals attraction, which is computed by pairwise additivity and formulated to include the attraction of the solvent with the colloid. For each interaction, the velocity decreases as the colloid radius decreases with the interaction parameters constant. For small L/a, velocity for the exponential potential decreases with an order one correction in L/a while this correction is logarithmic for the van der Waals potential. A curve for velocity as a function of a is constructed for the van der Waals

  19. Virtual Resonant Emission and Oscillatory Long-Range Tails in van der Waals Interactions of Excited States: QED Treatment and Applications

    NASA Astrophysics Data System (ADS)

    Jentschura, U. D.; Adhikari, C. M.; Debierre, V.

    2017-03-01

    We report on a quantum electrodynamic (QED) investigation of the interaction between a ground state atom with another atom in an excited state. General expressions, applicable to any atom, are indicated for the long-range tails that are due to virtual resonant emission and absorption into and from vacuum modes whose frequency equals the transition frequency to available lower-lying atomic states. For identical atoms, one of which is in an excited state, we also discuss the mixing term that depends on the symmetry of the two-atom wave function (these evolve into either the gerade or the ungerade state for close approach), and we include all nonresonant states in our rigorous QED treatment. In order to illustrate the findings, we analyze the fine-structure resolved van der Waals interaction for n D -1 S hydrogen interactions with n =8 , 10, 12 and find surprisingly large numerical coefficients.

  20. Spherical and hyperspherical harmonics representation of van der Waals aggregates

    NASA Astrophysics Data System (ADS)

    Lombardi, Andrea; Palazzetti, Federico; Aquilanti, Vincenzo; Grossi, Gaia; Albernaz, Alessandra F.; Barreto, Patricia R. P.; Cruz, Ana Claudia P. S.

    2016-12-01

    The representation of the potential energy surfaces of atom-molecule or molecular dimers interactions should account faithfully for the symmetry properties of the systems, preserving at the same time a compact analytical form. To this aim, the choice of a proper set of coordinates is a necessary precondition. Here we illustrate a description in terms of hyperspherical coordinates and the expansion of the intermolecular interaction energy in terms of hypersherical harmonics, as a general method for building potential energy surfaces suitable for molecular dynamics simulations of van der Waals aggregates. Examples for the prototypical case diatomic-molecule-diatomic-molecule interactions are shown.

  1. Direct synthesis of van der Waals solids.

    PubMed

    Lin, Yu-Chuan; Lu, Ning; Perea-Lopez, Nestor; Li, Jie; Lin, Zhong; Peng, Xin; Lee, Chia Hui; Sun, Ce; Calderin, Lazaro; Browning, Paul N; Bresnehan, Michael S; Kim, Moon J; Mayer, Theresa S; Terrones, Mauricio; Robinson, Joshua A

    2014-04-22

    The stacking of two-dimensional layered materials, such as semiconducting transition metal dichalcogenides (TMDs), insulating hexagonal boron nitride (hBN), and semimetallic graphene, has been theorized to produce tunable electronic and optoelectronic properties. Here we demonstrate the direct growth of MoS2, WSe2, and hBN on epitaxial graphene to form large-area van der Waals heterostructures. We reveal that the properties of the underlying graphene dictate properties of the heterostructures, where strain, wrinkling, and defects on the surface of graphene act as nucleation centers for lateral growth of the overlayer. Additionally, we show that the direct synthesis of TMDs on epitaxial graphene exhibits atomically sharp interfaces. Finally, we demonstrate that direct growth of MoS2 on epitaxial graphene can lead to a 10(3) improvement in photoresponse compared to MoS2 alone.

  2. Basis sets for the evaluation of van der Waals complex interaction energies: Ne-N2 intermolecular potential and microwave spectrum.

    PubMed

    Baranowska-Łączkowska, Angelika; Fernández, Berta

    2014-01-30

    In order to obtain efficient basis sets for the evaluation of van der Waals complex intermolecular potentials, we carry out systematic basis set studies. For this, interaction energies at representative geometries on the potential energy surfaces are evaluated using the CCSD(T) correlation method and large polarized LPol-n and augmented polarization-consistent aug-pc-2 basis sets extended with different sets of midbond functions. On the basis of the root mean square errors calculated with respect to the values for the most accurate potentials available, basis sets are selected for fitting the corresponding interaction energies and getting analytical potentials. In this work, we study the Ne-N2 van der Waals complex and after the above procedure, the aug-pc-2-3321 and the LPol-ds-33221 basis set results are fitted. The obtained potentials are characterized by T-shaped global minima at distances between the Ne atom and the N2 center of mass of 3.39 Å, with interaction energies of -49.36 cm(-1) for the aug-pc-2-3321 surface and -50.28 cm(-1) for the LPol-ds-33221 surface. Both sets of results are in excellent agreement with the reference surface. To check the potentials further microwave transition frequencies are calculated that agree well with the experimental and the aV5Z-33221 values. The success of this study suggests that it is feasible to carry out similar accurate calculations of interaction energies and ro-vibrational spectra at reduced cost for larger complexes than has been possible hitherto.

  3. High resolution selective reflection spectroscopy as a probe of long-range surface interaction : measurement of the surface van der Waals attraction exerted on excited Cs atoms

    NASA Astrophysics Data System (ADS)

    Chevrollier, Martine; Fichet, Michèle; Oria, Marcos; Rahmat, Gabriel; Bloch, Daniel; Ducloy, Martial

    1992-04-01

    Selective reflection spectroscopy at an interface with a low-density resonant vapor, especially when combined with a frequency modulation technique, is a high-resolution Doppler-free tool for probing atoms interacting with a surface. We analyze different types of relevant surface interaction, emphasizing the spectral consequences of a van der Waals surface attraction associated to a z^{-3} potential dependence (z: distance to the wall). We present detailed results of two series of experiments at a Cs vapor/dielectric window interface on the 6S{1/2}-6P{3/2} (λ = 852 nm) resonance line and on the 6S{1/2}-7P second resonance line (λ = 455 nm and 459 nm). Lineshape analysis at various pressures consistently shows that a van der Waals-type surface attraction has to be considered to interpret strong lineshape distortions and resonance shift. The attractive strengths are found to be equal respectively to ≈ 2 kHz μm^3 and ≈ 20 kHz μm^3, independently of the considered hyperfine component, within the experimental accuracy. It yields also typical parameters of pressure broadening and shift, which are shown to originate in collisional processes, at densities where the medium is opaque. Theoretical expectations for the VW strength are discussed on the basis of the results of atomic theory. The predicted values are smaller, by a typical factor of 2, than those deduced from the experiments. The validity of the theory, when applied to a dielectric interface, is discussed and seems questionable when the frequency of virtual atomic transitions involved in the van der Waals attraction potential lies in the dielectric window absorption range. La spectroscopie de réflexion sélective à l'interface d'une vapeur résonnante de faible densité, combinée à une technique de modulation de fréquence, permet de sonder à haute résolution et sans effet Doppler des atomes en interaction avec une surface. On analyse différents types d'interaction de surface envisageables, en

  4. Graphene on boron-nitride: Moiré pattern in the van der Waals energy

    SciTech Connect

    Neek-Amal, M.; Peeters, F. M.

    2014-01-27

    The spatial dependence of the van der Waals (vdW) energy between graphene and hexagonal boron-nitride (h-BN) is investigated using atomistic simulations. The van der Waals energy between graphene and h-BN shows a hexagonal superlattice structure identical to the observed Moiré pattern in the local density of states, which depends on the lattice mismatch and misorientation angle between graphene and h-BN. Our results provide atomistic features of the weak van der Waals interaction between graphene and BN which are in agreement with experiment and provide an analytical expression for the size of the spatial variation of the weak van der Waals interaction. We also found that the A-B-lattice symmetry of graphene is broken along the armchair direction.

  5. Electronic structure and photoabsorption property of pseudocubic perovskites CH3NH3PbX3(X = I, Br) including van der Waals interaction

    NASA Astrophysics Data System (ADS)

    Yu, Chol-Jun; Jong, Un-Gi; Ri, Mun-Hyok; Ri, Gum-Chol; Pae, Yong-Hyon

    2016-11-01

    Using density functional theory with the inclusion of van der Waals (vdW) interaction, we have investigated electronic energy bands, density of states, effective masses of charge carriers, and photo absorption coefficients of pseudo-cubic CH$_3$NH$_3$PbX$_3$ (X=I, Br). Our results confirm the direct bandgap of 1.49 (1.92) eV for X=I (Br) in the pseudo-cubic $Pm$ phase with lattice constant of 6.324 (5.966) \\AA, being agreed well with experiment and indicating the necessity of vdW correction. The calculated photo absorption coefficients for X=I (Br) have the onset at red (orange) color and the first peak around violet (ultraviolet) color in overall agreement with the experiment.

  6. Isotope separation by photodissociation of Van der Waal's molecules

    DOEpatents

    Lee, Yuan T.

    1977-01-01

    A method of separating isotopes based on the dissociation of a Van der Waal's complex. A beam of molecules of a Van der Waal's complex containing, as one partner of the complex, a molecular species in which an element is present in a plurality of isotopes is subjected to radiation from a source tuned to a frequency which will selectively excite vibrational motion by a vibrational transition or through electronic transition of those complexed molecules of the molecular species which contain a desired isotope. Since the Van der Waal's binding energy is much smaller than the excitational energy of vibrational motion, the thus excited Van der Waal's complex dissociate into molecular components enriched in the desired isotope. The recoil velocity associated with vibrational to translational and rotational relaxation will send the separated molecules away from the beam whereupon the product enriched in the desired isotope can be separated from the constituents of the beam.

  7. Lifshitz theory of van der Waals pressure in dissipative media

    SciTech Connect

    Zheng Yi; Narayanaswamy, Arvind

    2011-04-15

    We derive a first-principles method of determining the van der Waals or Casimir pressure in a dissipative and dispersive planar multilayered system by calculating the Maxwell stress tensor in a fictitious layer of vacuum, that is eventually made to vanish, introduced in the structure. This is illustrated by calculating the van der Waals pressure in a thin film with dissipative properties embedded between two semi-infinite media.

  8. Modified Van der Waals equation and law of corresponding states

    NASA Astrophysics Data System (ADS)

    Zhong, Wei; Xiao, Changming; Zhu, Yongkai

    2017-04-01

    It is well known that the Van der Waals equation is a modification of the ideal gas law, yet it can be used to describe both gas and liquid, and some important messages can be obtained from this state equation. However, the Van der Waals equation is not a precise state equation, and it does not give a good description of the law of corresponding states. In this paper, we expand the Van der Waals equation into its Taylor's series form, and then modify the fourth order expansion by changing the constant Virial coefficients into their analogous ones. Via this way, a more precise result about the law of corresponding states has been obtained, and the law of corresponding states can then be expressed as: in terms of the reduced variables, all fluids should obey the same equation with the analogous Virial coefficients. In addition, the system of 3 He with quantum effects has also been taken into consideration with our modified Van der Waals equation, and it is found that, for a normal system without quantum effect, the modification on ideal gas law from the Van der Waals equation is more significant than the real case, however, for a system with quantum effect, this modification is less significant than the real case, thus a factor is introduced in this paper to weaken or strengthen the modification of the Van der Waals equation, respectively.

  9. Binding and Release between Polymeric Carrier and Protein Drug: pH Mediated Interplay of Coulomb forces, Hydrogen Bonding, van der Waals Interactions and Entropy.

    PubMed

    De Luca, Sergio; Chen, Fan; Seal, Prasenjit; Stenzel, Martina H; Smith, Sean C

    2017-09-07

    The accelerating search for new types of drugs and delivery strategies poses the challenge to understand the mechanism of delivery. To this end, a detailed atomistic picture of binding between the drug and the carrier is quintessential. While many studies focus on the electrostatics of drug-vector interactions, it has also been pointed out that entropic factors relating to water and counter ions can play an important role. By carrying out extensive molecular dynamics simulations and subsequently validating with experiment, we shed light herein on the binding in aqueous solution between a protein drug and a polymeric carrier. We examined the complexation between the polymer, poly (ethylene glycol) methyl ether acrylate-b-poly(carboxyethyl acrylate (PEGMEA-b-PCEA) and the protein, egg white lysozyme, a system that acts as a model for polymer-vector / protein-drug delivery systems. The complexation has been visualized and characterized using contact maps and hydrogen bonding analyses for five independent simulations of the complex, each running over 100 ns. Binding at physiological pH is, as expected, mediated by coulombic attraction between the positively charged protein and negatively charged carboxylate groups on the polymer. However, we find that consideration of electrostatics alone is insufficient to explain the complexation behaviour at low pH. Intra-complex hydrogen bonds, van der Waals interactions also water-water interactions dictate that the polymer does not release the protein at pH 4.8 or indeed at pH 3.2, even though the Coulombic attractions are largely removed as carboxylate groups on the polymer become titrated. Experiments in aqueous solution carried out at pH = 7.0, 4.5, and 3.0 confirm the veracity of the computed binding behaviour. Overall, these combined simulation and experimental results illustrate that coulomb interactions need to be complemented with consideration of other entropic forces, mediated by van der Waals interactions and hydrogen

  10. A van der Waals DFT Approach to Modeling Water

    NASA Astrophysics Data System (ADS)

    Kolb, Brian; Thonhauser, Timo

    2010-03-01

    We present density functional theory calculations for small water clusters and bulk water including van der Waals interactions via the non-local functional vdW-DF [1]. Historically, standard functionals such as LDA or GGA have been unable to accurately predict vibrational frequencies of small water clusters or the freezing point of bulk water, which has been partly attributed to the lack of van der Waals interactions [2]. We have implemented vdW-DF in the PWscf package, using an efficient convolution approach [3]. Our results for the vibrational frequencies of small water clusters show that vdW-DF gives a significant improvement compared to LDA or GGA. While the discrepancy between experiment and LDA/GGA is as much as 28% for certain modes, vdW-DF reduces this error to only about 6%. We also present results for the vibrational spectrum and Raman spectrum of periodic ice, again showing the advantages of vdW-DF. In addition, we show preliminary results for bulk water from our vdW-DF MD simulations.[4pt] [1] Thonhauser et al., Phys. Rev. B 76, 125112 (2007).[2] H. Sit and N. Marzari, J. Chem. Phys. 122, 204510 (2005). [3] G. Roman-Perez and J. Soler, Phys. Rev. Lett. 103, 096102 (2009).

  11. Br...Br and van der Waals interactions along a homologous series: crystal packing of 1,2-dibromo-4,5-dialkoxybenzenes.

    PubMed

    Suarez, Sebastián A; Muller, Federico; Gutiérrez Suburu, Matías E; Fonrouge, Ana; Baggio, Ricardo F; Cukiernik, Fabio D

    2016-10-01

    The crystalline structures of four homologues of the 1,2-dibromo-4,5-dialkoxybenzene series [Br2C6H2(OCnH2n + 1)2 for n = 2, 12, 14 and 18] have been solved by means of single-crystal crystallography. Comparison along the series, including the previously reported n = 10 and n = 16 derivatives, shows a clear metric trend (b and c essentially fixed along the series and a growing linearly with n), in spite of some subtle differences in space groups and/or packing modes. A uniform packing pattern for the aliphatic chains has been found for the n = 12 to 18 homologues, which slightly differs from that of the n = 10 derivative. The crystalline structures of all the higher homologues (n = 10-18) seem to arise from van der Waals interchain interactions and, to a lesser extent, type II Br...Br interactions. The dominant role of interchain interactions provides direct structural support for the usual interpretation of melting point trends like that found along this series. Atoms in Molecules (AIM) analysis allows a comparison of the relative magnitude of the interchain and Br...Br interactions, an analysis validated by the measured melting enthalpies.

  12. Reactivity of phosphorene with a 3d element trioxide (CrO3) considering van der Waals molecular interactions: a DFT-D2 study.

    PubMed

    Rubio-Pereda, Pamela; Cocoletzi, Gregorio H

    2017-02-01

    First-principle calculations are performed to investigate the interaction between clean black phosphorene and the CrO3 molecule which is known to be a powerful oxidizer and a suspected carcinogen. Van der Waals forces are included in all calculations through empirical corrections. Energetics studies are first done to determine the structural stability. Then charge density, Löwdin population analysis and electronic states are evaluated. Results show that the CrO3 molecule, with an acceptor electron character, is chemisorbed on the phosphorene surface inducing minimal geometrical distortions, however, after adsorption, a partial charge gradient is produced between the P atoms located at the phosphorene upper and lower planes. Furthermore, variations on the CrO3 concentration causes different interaction strengths. At high concentrations of adsorbed CrO3 molecules, the interaction with the surface becomes stronger due to an increased steric effect between neighboring molecules. Nevertheless, this effect along with the geometrical distortions produced on the phosphorene structure, due to the large number of molecules adsorbed, leads to a decrement on the adsorption energy. It is expected that the reported results may render phosphorene as a promising material for application as a gas sensor.

  13. Two-dimensional van der Waals C60 molecular crystal

    PubMed Central

    Reddy, C. D.; Gen Yu, Zhi; Zhang, Yong-Wei

    2015-01-01

    Two-dimensional (2D) atomic crystals, such as graphene and transition metal dichalcogenides et al. have drawn extraordinary attention recently. For these 2D materials, atoms within their monolayer are covalently bonded. An interesting question arises: Can molecules form a 2D monolayer crystal via van der Waals interactions? Here, we first study the structural stability of a free-standing infinite C60 molecular monolayer using molecular dynamic simulations, and find that the monolayer is stable up to 600 K. We further study the mechanical properties of the monolayer, and find that the elastic modulus, ultimate tensile stress and failure strain are 55–100 GPa, 90–155 MPa, and 1.5–2.3%, respectively, depending on the stretching orientation. The monolayer fails due to shearing and cavitation under uniaxial tensile loading. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the monolayer are found to be delocalized and as a result, the band gap is reduced to only 60% of the isolated C60 molecule. Interestingly, this band gap can be tuned up to ±30% using strain engineering. Owing to its thermal stability, low density, strain-tunable semi-conducting characteristics and large bending flexibility, this van der Waals molecular monolayer crystal presents aplenty opportunities for developing novel applications in nanoelectronics. PMID:26183501

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

    NASA Astrophysics Data System (ADS)

    Liu, Wei; Ruiz-López, Victor G.; Zhang, Guo-Xu; Ren, Xinguo; Scheffler, Matthias; Tkatchenko, Alexandre

    2012-02-01

    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\\surfcirc 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^o 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 å, 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'opez et al., submitted. [2] X. Ren et al., Phys. Rev. Lett. 106, 153003 (2011).

  15. H2O and CO2 confined in cement based materials: an ab initio molecular dynamics study with van der Waals interactions

    NASA Astrophysics Data System (ADS)

    de Almeida, James; Miranda, Caetano; Fazzio, Adalberto

    2013-03-01

    Although the cement has been widely used for a long time, very little is known regarding the atomistic mechanism behind its functionality. Particularly, the dynamics of molecular systems at confined nanoporous and water hydration is largely unknown. Here, we study the dynamical and structural properties of H2O and CO2 confined between Tobermorite 9Å(T9) surfaces with Car-Parrinello molecular dynamics with and without van der Waals (vdW) interactions, at room temperature. For H2O confined, we have observed a broadening in the intra and intermolecular bond angle distribution. A shift from an ice-like to a liquid-like infrared spectrum with the inclusion of vdW interactions was observed. The bond distance for the confined CO2 was increased, followed with the appearance of shorter (larger) intramolecular (intermolecular) angles. These structural modifications result in variations on the CO2 symmetric stretching Raman active vibration modes. The diffusion coefficient obtained for both confined H2O and CO2 were found to be lower than their bulk counterparts. Interestingly, during the water dynamics, a proton exchange between H2O and the T9 surface was observed. However, for confined CO2, no chemical reactions or bond breaking were observed.

  16. Enhanced and switchable nanoscale thermal conduction due to van der Waals interfaces.

    PubMed

    Yang, Juekuan; Yang, Yang; Waltermire, Scott W; Wu, Xiaoxia; Zhang, Haitao; Gutu, Timothy; Jiang, Youfei; Chen, Yunfei; Zinn, Alfred A; Prasher, Ravi; Xu, Terry T; Li, Deyu

    2011-12-11

    Understanding thermal transport in nanostructured materials is important for the development of energy conversion applications and the thermal management of microelectronic and optoelectronic devices. Most nanostructures interact through van der Waals interactions, and these interactions typically lead to a reduction in thermal transport. Here, we show that the thermal conductivity of a bundle of boron nanoribbons can be significantly higher than that of a single free-standing nanoribbon. Moreover, the thermal conductivity of the bundle can be switched between the enhanced values and that of a single nanoribbon by wetting the van der Waals interface between the nanoribbons with various solutions.

  17. Statistical mechanics of simple fluids - Beyond van der Waals

    NASA Astrophysics Data System (ADS)

    Lebowitz, J. L.; Waisman, E. M.

    1980-03-01

    Consideration is given to recent developments in the theory of dense fluids, based on a model fluid of hard spheres. The fluid is treated as consisting of electrically neutral particles interacting through pair potentials dependent only on the distance between their centers, a macroscopic system which can be described by classical statistical mechanics. The van der Waals equation of state and the Maxwell amendment to it for temperatures less than the critical temperature are reviewed, and subsequent rigorous derivations of the amended equation are presented. A relatively simple scheme for approximating a dense, single-component simple classical fluid whose atoms interact via the Lennard-Jones potential, based on the hard sphere model and employing computer calculations is then outlined. It is noted that the approach can be easily generalized to treat quantitatively mixtures of simple fluids, and nonuniform fluids qualitatively, and that there remains much to be done to understand why the schemes presented work as well as they do.

  18. Ab initiointermolecular potential energy surface of Ne···NCCN van der Waals complex: effect of the place of midbond function on the interaction

    NASA Astrophysics Data System (ADS)

    Solimannejad, Mohammad; Jouypazadeh, Hamidreza; Farrokhpour, Hossein

    2015-11-01

    The intermolecular potential energy surface of Ne...NCCN van der Waals complex was evaluated in the framework of the counterpoise-corrected supermolecular approach using CCSD(T) level and aug-cc-pVDZ basis set extended with a set of midbond (3s3p2d1f1g) functions. The effect of the place of midbond function on the accuracy of the calculated potential energy surface was examined and the optimised position for placing midbond function was determined. The calculated potential energy surface was fitted by an analytical function. The analytical function of intermolecular potential energy surface of Ne...NCCN demonstrated a global minimum energy of -12.024 meV related to the T-shape geometry at the distance between Ne and the centre of mass of NCCN of 3.28 Å. Finally, the interaction second virial coefficients (B12) of Ne and NCCN were calculated and used to calculate the second virial coefficients for the mixture of neon and cyanogen gases at different mole fractions of Ne gas.

  19. Understanding the role of vibrations, exact exchange, and many-body van der Waals interactions in the cohesive properties of molecular crystals.

    PubMed

    Reilly, Anthony M; Tkatchenko, Alexandre

    2013-07-14

    The development and application of computational methods for studying molecular crystals, particularly density-functional theory (DFT), is a large and ever-growing field, driven by their numerous applications. Here we expand on our recent study of the importance of many-body van der Waals interactions in molecular crystals [A. M. Reilly and A. Tkatchenko, J. Phys. Chem. Lett. 4, 1028 (2013)], with a larger database of 23 molecular crystals. Particular attention has been paid to the role of the vibrational contributions that are required to compare experiment sublimation enthalpies with calculated lattice energies, employing both phonon calculations and experimental heat-capacity data to provide harmonic and anharmonic estimates of the vibrational contributions. Exact exchange, which is rarely considered in DFT studies of molecular crystals, is shown to have a significant contribution to lattice energies, systematically improving agreement between theory and experiment. When the vibrational and exact-exchange contributions are coupled with a many-body approach to dispersion, DFT yields a mean absolute error (3.92 kJ/mol) within the coveted "chemical accuracy" target (4.2 kJ/mol). The role of many-body dispersion for structures has also been investigated for a subset of the database, showing good performance compared to X-ray and neutron diffraction crystal structures. The results show that the approach employed here can reach the demanding accuracy of crystal-structure prediction and organic material design with minimal empiricism.

  20. Understanding the role of vibrations, exact exchange, and many-body van der Waals interactions in the cohesive properties of molecular crystals

    NASA Astrophysics Data System (ADS)

    Reilly, Anthony M.; Tkatchenko, Alexandre

    2013-07-01

    The development and application of computational methods for studying molecular crystals, particularly density-functional theory (DFT), is a large and ever-growing field, driven by their numerous applications. Here we expand on our recent study of the importance of many-body van der Waals interactions in molecular crystals [A. M. Reilly and A. Tkatchenko, J. Phys. Chem. Lett. 4, 1028 (2013), 10.1021/jz400226x], with a larger database of 23 molecular crystals. Particular attention has been paid to the role of the vibrational contributions that are required to compare experiment sublimation enthalpies with calculated lattice energies, employing both phonon calculations and experimental heat-capacity data to provide harmonic and anharmonic estimates of the vibrational contributions. Exact exchange, which is rarely considered in DFT studies of molecular crystals, is shown to have a significant contribution to lattice energies, systematically improving agreement between theory and experiment. When the vibrational and exact-exchange contributions are coupled with a many-body approach to dispersion, DFT yields a mean absolute error (3.92 kJ/mol) within the coveted "chemical accuracy" target (4.2 kJ/mol). The role of many-body dispersion for structures has also been investigated for a subset of the database, showing good performance compared to X-ray and neutron diffraction crystal structures. The results show that the approach employed here can reach the demanding accuracy of crystal-structure prediction and organic material design with minimal empiricism.

  1. Capillary and van der Waals interactions on CaF2 crystals from amplitude modulation AFM force reconstruction profiles under ambient conditions

    PubMed Central

    Calò, Annalisa; Robles, Oriol Vidal; Santos, Sergio

    2015-01-01

    Summary There has been much interest in the past two decades to produce experimental force profiles characteristic of the interaction between nanoscale objects or a nanoscale object and a plane. Arguably, the advent of the atomic force microscope AFM was instrumental in driving such efforts because, in principle, force profiles could be recovered directly. Nevertheless, it has taken years before techniques have developed enough as to recover the attractive part of the force with relatively low noise and without missing information on critical ranges, particularly under ambient conditions where capillary interactions are believed to dominate. Thus a systematic study of the different profiles that may arise in such situations is still lacking. Here we employ the surfaces of CaF2, on which nanoscale water films form, to report on the range and force profiles that might originate by dynamic capillary interactions occurring between an AFM tip and nanoscale water patches. Three types of force profiles were observed under ambient conditions. One in which the force decay resembles the well-known inverse-square law typical of van der Waals interactions during the first 0.5–1 nm of decay, a second one in which the force decays almost linearly, in relatively good agreement with capillary force predicted by the constant chemical potential approximation, and a third one in which the attractive force is almost constant, i.e., forms a plateau, up to 3–4 nm above the surface when the formation of a capillary neck dominates the tip–sample interaction. PMID:25977852

  2. A cartography of the van der Waals territories.

    PubMed

    Alvarez, Santiago

    2013-06-28

    The distribution of distances from atoms of a particular element E to a probe atom X (oxygen in most cases), both bonded and intermolecular non-bonded contacts, has been analyzed. In general, the distribution is characterized by a maximum at short E···X distances corresponding to chemical bonds, followed by a range of unpopulated distances--the van der Waals gap--and a second maximum at longer distances--the van der Waals peak--superimposed on a random distribution function that roughly follows a d(3) dependence. The analysis of more than five million interatomic "non-bonded" distances has led to the proposal of a consistent set of van der Waals radii for most naturally occurring elements, and its applicability to other element pairs has been tested for a set of more than three million data, all of them compared to over one million bond distances.

  3. A crossover in anisotropic nanomechanochemistry of van der Waals crystals

    SciTech Connect

    Shimamura, Kohei; Misawa, Masaaki; Li, Ying; Kalia, Rajiv K.; Nakano, Aiichiro; Vashishta, Priya; Shimojo, Fuyuki

    2015-12-07

    In nanoscale mechanochemistry, mechanical forces selectively break covalent bonds to essentially control chemical reactions. An archetype is anisotropic detonation of layered energetic molecular crystals bonded by van der Waals (vdW) interactions. Here, quantum molecular dynamics simulations reveal a crossover of anisotropic nanomechanochemistry of vdW crystal. Within 10{sup −13} s from the passage of shock front, lateral collision produces NO{sub 2} via twisting and bending of nitro-groups and the resulting inverse Jahn-Teller effect, which is mediated by strong intra-layer hydrogen bonds. Subsequently, as we transition from heterogeneous to homogeneous mechanochemical regimes around 10{sup −12} s, shock normal to multilayers becomes more reactive, producing H{sub 2}O assisted by inter-layer N-N bond formation. These time-resolved results provide much needed atomistic understanding of nanomechanochemistry that underlies a wider range of technologies.

  4. Heterostructures based on inorganic and organic van der Waals systems

    NASA Astrophysics Data System (ADS)

    Lee, Gwan-Hyoung; Lee, Chul-Ho; van der Zande, Arend M.; Han, Minyong; Cui, Xu; Arefe, Ghidewon; Nuckolls, Colin; Heinz, Tony F.; Hone, James; Kim, Philip

    2014-09-01

    The two-dimensional limit of layered materials has recently been realized through the use of van der Waals (vdW) heterostructures composed of weakly interacting layers. In this paper, we describe two different classes of vdW heterostructures: inorganic vdW heterostructures prepared by co-lamination and restacking; and organic-inorganic hetero-epitaxy created by physical vapor deposition of organic molecule crystals on an inorganic vdW substrate. Both types of heterostructures exhibit atomically clean vdW interfaces. Employing such vdW heterostructures, we have demonstrated various novel devices, including graphene/hexagonal boron nitride (hBN) and MoS2 heterostructures for memory devices; graphene/MoS2/WSe2/graphene vertical p-n junctions for photovoltaic devices, and organic crystals on hBN with graphene electrodes for high-performance transistors.

  5. Coincident-site lattice matching during van der Waals epitaxy

    PubMed Central

    Boschker, Jos E.; Galves, Lauren A.; Flissikowski, Timur; Lopes, Joao Marcelo J.; Riechert, Henning; Calarco, Raffaella

    2015-01-01

    Van der Waals (vdW) epitaxy is an attractive method for the fabrication of vdW heterostructures. Here Sb2Te3 films grown on three different kind of graphene substrates (monolayer epitaxial graphene, quasi freestanding bilayer graphene and the SiC (6√3 × 6√3)R30° buffer layer) are used to study the vdW epitaxy between two 2-dimensionally (2D) bonded materials. It is shown that the Sb2Te3 /graphene interface is stable and that coincidence lattices are formed between the epilayers and substrate that depend on the size of the surface unit cell. This demonstrates that there is a significant, although relatively weak, interfacial interaction between the two materials. Lattice matching is thus relevant for vdW epitaxy with two 2D bonded materials and a fundamental design parameter for vdW heterostructures. PMID:26658715

  6. A Scalable Implementation of Van der Waals Density Functionals

    NASA Astrophysics Data System (ADS)

    Wu, Jun; Gygi, Francois

    2010-03-01

    Recently developed Van der Waals density functionals[1] offer the promise to account for weak intermolecular interactions that are not described accurately by local exchange-correlation density functionals. In spite of recent progress [2], the computational cost of such calculations remains high. We present a scalable parallel implementation of the functional proposed by Dion et al.[1]. The method is implemented in the Qbox first-principles simulation code (http://eslab.ucdavis.edu/software/qbox). Application to large molecular systems will be presented. [4pt] [1] M. Dion et al. Phys. Rev. Lett. 92, 246401 (2004).[0pt] [2] G. Roman-Perez and J. M. Soler, Phys. Rev. Lett. 103, 096102 (2009).

  7. Van der Waals molecular interactions in the organic functionalization of graphane, silicane, and germanane with alkene and alkyne molecules: a DFT-D2 study.

    PubMed

    Rubio-Pereda, Pamela; Takeuchi, Noboru

    2016-08-01

    Density functional theory with the addition of a semi-empirical dispersion potential was applied to the conventional Kohn-Sham energy to study the adsorption of alkene and alkyne molecules on hydrogen-terminated two-dimensional group IV systems (graphane, silicane, and germanane) by means of a radical-initiated reaction. In particular, we investigated the interactions of acetylene, ethylene, and styrene with those surfaces. Although we had studied these systems previously, we included van der Waals interactions in all of the cases examined in the present work. These forces, which are noncovalent interactions, can heavily influence different processes in molecular chemistry, such as the adsorption of organic molecules on semiconductor surfaces. This unified approach allowed us to perform a comparative study of the relative reactivities of the various organic molecule/surface systems. The results showed that the degree of covalency of the surface, the lattice size, and the partial charge distribution (caused by differences in electronegativity) are all key elements that determine the reactivity between the molecules and the surfaces tested in this work. The covalent nature of graphane gives rise to energetically favorable intermediate states, while the opposite polarities of the charge distributions of silicane and germanane with the organic molecules favor subsequent steps of the radical-initiated reaction. Finally, the lattice size is a factor that has important consequences due to steric effects present in the systems and the possibility of chain reaction continuation. The results obtained in this work show that careful selection of the substrate is very important. Calculated energy barriers, heats of adsorption, and optimized atomic structures show that the silicane system offers the best reactivity in organic functionalization.

  8. Test of a nonempirical density functional: Short-range part of the van der Waals interaction in rare-gas dimers

    NASA Astrophysics Data System (ADS)

    Tao, Jianmin; Perdew, John P.

    2005-03-01

    It is known that the nonempirical generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) provides a much more realistic description of the short-range part of the van der Waals (vdW) interaction than does the local spin density (LSD) approximation. In the present work, the ability of the higher-level nonempirical meta-GGA of Tao, Perdew, Staroverov, and Scuseria (TPSS) to describe vdW interaction is tested self-consistently in ten rare-gas dimers with Z ⩽36. The one-parameter hybrid version (TPSSh) of the TPSS exchange-correlation functional is also included in this test. Calculations show that both TPSS and TPSSh functionals correctly yield vdW bonds in these dimers and significantly improve the prediction of bond lengths, binding energies, and harmonic vibrational frequencies over LSD. The rather close agreement of TPSS with PBE for these dimers confirms a principle of the TPSS construction: preservation of the PBE large-gradient behavior. More importantly, it suggests that TPSS can serve as a platform on which to construct a still-higher level of nonempirical functionals. Compared with the PBE GGA, TPSS, and TPSSh yield a slightly weaker binding. As for normally bonded molecules, TPSSh yields the most accurate vibrational frequencies. The typically too-long bond lengths and too-small binding energies of TPSS meta-GGA suggest the need for some long-range vdW interaction correction even in this class of systems. The effect of basis-set superposition error on the calculated properties of these vdW systems is investigated. We also show that the relatively strong anharmonic effects in the rare-gas dimers are described remarkably well by the Morse potential.

  9. Periodic potentials in hybrid van der Waals heterostructures formed by supramolecular lattices on graphene

    NASA Astrophysics Data System (ADS)

    Gobbi, Marco; Bonacchi, Sara; Lian, Jian X.; Liu, Yi; Wang, Xiao-Ye; Stoeckel, Marc-Antoine; Squillaci, Marco A.; D'Avino, Gabriele; Narita, Akimitsu; Müllen, Klaus; Feng, Xinliang; Olivier, Yoann; Beljonne, David; Samorì, Paolo; Orgiu, Emanuele

    2017-03-01

    The rise of 2D materials made it possible to form heterostructures held together by weak interplanar van der Waals interactions. Within such van der Waals heterostructures, the occurrence of 2D periodic potentials significantly modifies the electronic structure of single sheets within the stack, therefore modulating the material properties. However, these periodic potentials are determined by the mechanical alignment of adjacent 2D materials, which is cumbersome and time-consuming. Here we show that programmable 1D periodic potentials extending over areas exceeding 104 nm2 and stable at ambient conditions arise when graphene is covered by a self-assembled supramolecular lattice. The amplitude and sign of the potential can be modified without altering its periodicity by employing photoreactive molecules or their reaction products. In this regard, the supramolecular lattice/graphene bilayer represents the hybrid analogue of fully inorganic van der Waals heterostructures, highlighting the rich prospects that molecular design offers to create ad hoc materials.

  10. Harris-type van der Waals density functional scheme

    NASA Astrophysics Data System (ADS)

    Berland, Kristian; Londero, Elisa; Schröder, Elsebeth; Hyldgaard, Per

    2013-07-01

    Biomolecular systems that involve thousands of atoms are difficult to address with standard density functional theory (DFT) calculations. With the development of sparse-matter methods such as the van der Waals density functional (vdW-DF) method [M. Dion , Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.92.246401 92, 246401 (2004)], it is now possible to include the dispersive forces in DFT which are necessary to describe the cohesion and behavior of these systems. vdW-DF implementations can be as efficient as those for traditional DFT. Yet, the computational costs of self-consistently determining the electron wave functions and hence the kinetic-energy repulsion still limit the scope of sparse-matter DFT. We propose to speed up sparse-matter calculations by using the Harris scheme [J. Harris, Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.31.1770 31, 1770 (1985)]; that is, we propose to perform electronic relaxations only for separated fragments (molecules) and use a superposition of fragment densities as a starting point to obtain the total energy non-self-consistently. We evaluate the feasibility of this approach for an adaption of the Harris scheme for non-self-consistent vdW-DF (sfd-vdW-DF). We study four molecular dimers with varying degrees of polarity and find that the sfd scheme accurately reproduces standard non-self-consistent vdW-DF for van der Waals dominated systems but is less accurate for those dominated by polar interactions. Results for the S22 set of typical organic molecular dimers are promising.

  11. Electron response in van der Waals density functionals

    NASA Astrophysics Data System (ADS)

    Hyldgaard, Per

    2013-03-01

    There is significant interest in density functional theory (DFT) of dispersive or van der Waals (vdW) interactions and in DFT studies of sparse systems where vdW forces contribute to the cohesion and behavior. The Rutgers-Chalmers van der Waals density functional (vdW-DF) method [PRL 92, 246401 (2004); PRB 76, 125112 (2007)] is a nonempirical approach to calculate vdW bonding and for DFT characterizations of sparse matter. The vdW-DF framework is defined by a single exchange-correlation density functional that rests on a plasmon-type description for both semilocal components and for a parameter-free evaluation of nonlocal correlation. My talk summarizes a set of vdW-DF studies that seeks to map and analyze details in the vdW-DF electron-response nature. The purpose is in part to extract consequences that can facilitate an experiment-theory comparison that goes beyond binding geometries and energies. The aim is also to seek implications that can help develop the vdW-DF framework. I present an analysis of the relative importance of morphology, screening (image-plane formation), and collective effects in the vdW-DF description of molecular systems. In addition, I compare vdW-DF results with Cu(111) experiments that tests the electron-response behavior in terms of adsorption-induced band shifts, the form of the overall light-molecule physisorption potential, and the corrugation in the kinetic-energy repulsion of molecules at surfaces. Overall, the vdW-DF studies suggest the importance of benchmarking vdW methods across different length scales and by exploring the variation that arise when related structures have a different balance between exchange repulsion and vdW attraction.

  12. A high-pressure van der Waals compound in solid nitrogen-helium mixtures

    NASA Technical Reports Server (NTRS)

    Vos, W. L.; Finger, L. W.; Hemley, R. J.; Hu, J. Z.; Mao, H. K.; Schouten, J. A.

    1992-01-01

    A detailed diamond anvil-cell study using synchrotron X-ray diffraction, Raman scattering, and optical microscopy has been conducted for the He-N system, with a view to the weakly-bound van der Waals molecule interactions that can be formed in the gas phase. High pressure is found to stabilize the formation of a stoichiometric, solid van der Waals compound of He(N2)11 composition which may exemplify a novel class of compounds found at high pressures in the interiors of the outer planets and their satellites.

  13. Van der Waals forces in pNRQED

    SciTech Connect

    Shtabovenko, Vladyslav

    2016-01-22

    We report on the calculation of electromagnetic van der Waals forces [1] between two hydrogen atoms using non-relativistic effective field theories (EFTs) of QED for large and small momentum transfers with respect to the intrinsic energy scale of the hydrogen atom. Our results reproduce the well known London and Casimir-Polder forces.

  14. Statistical complexity, virial expansion, and van der Waals equation

    NASA Astrophysics Data System (ADS)

    Pennini, F.; Plastino, A.

    2016-09-01

    We investigate the notion of LMC statistical complexity with regards to a real gas and in terms of the second virial coefficient. The ensuing results are applied to the van der Waals equation. Interestingly enough, one finds a complexity-interpretation for the associated phase transition.

  15. Nonadiabatic corrections to the van der Waals potential

    NASA Technical Reports Server (NTRS)

    Au, C. K.

    1988-01-01

    Closed-form expressions are derived for the finite-mass nonadiabatic corrections to the van der Waals potential in the electric-dipole approximations in terms of London-analog two-center atomic spectral sums. The nonadiabatic corrections to all orders at threshold energy and also those with first-order energy dependence are given.

  16. Excited nucleon as a van der Waals system of partons

    SciTech Connect

    Jenkovszky, L. L.; Muskeyev, A. O. Yezhov, S. N.

    2012-06-15

    Saturation in deep inelastic scattering (DIS) and deeply virtual Compton scattering (DVCS) is associated with a phase transition between the partonic gas, typical of moderate x and Q{sup 2}, and partonic fluid appearing at increasing Q{sup 2} and decreasing Bjorken x. We suggest the van der Waals equation of state to describe properly this phase transition.

  17. Is there a difference in van der Waals interactions between rare gas atoms adsorbed on metallic and semiconducting single-walled carbon nanotubes?

    PubMed

    Chen, De-Li; Mandeltort, Lynn; Saidi, Wissam A; Yates, John T; Cole, Milton W; Johnson, J Karl

    2013-03-29

    The differences in the polarizabilities of metallic (M) and semiconducting (S) single-walled carbon nanotubes (SWNTs) might give rise to differences in adsorption potentials. We show from experiments and van der Waals--corrected density functional theory that the binding energies of Xe adsorbed on M- and S-SWNTs are nearly identical. Temperature programed desorption experiments of Xe on purified M- and S-SWNTs give similar peak temperatures, indicating that desorption kinetics and binding energies are independent of the type of SWNT. Binding energies computed from vdW-corrected density functional theory are in good agreement with experiments.

  18. Is there a Difference in Van Der Waals Interactions between Rare Gas Atoms Adsorbed on Metallic and Semiconducting Single-Walled Carbon Nanotubes?

    SciTech Connect

    Chen, De-Li; Mandeltort, Lynn; Saidi, Wissam A.; Yates, John T.; Cole, Milton W.; Johnson, J. Karl

    2013-03-01

    Differences in polarizabilities of metallic (M) and semiconducting (S) single-walled carbon nanotubes (SWNTs) might give rise to differences in adsorption potentials. We show from experiments and van der Waals-corrected density functional theory (DFT) that binding energies of Xe adsorbed on M- and S-SWNTs are nearly identical. Temperature programmed desorption of Xe on purified M- and S-SWNTs give similar peak temperatures, indicating that desorption kinetics and binding energies are independent of the type of SWNT. Binding energies computed from vdW-corrected DFT are in good agreement with experiments.

  19. Convergence of the many-body expansion of interaction potentials: From van der Waals to covalent and metallic systems

    SciTech Connect

    Hermann, Andreas; Krawczyk, Robert P.; Lein, Matthias; Schwerdtfeger, Peter; Hamilton, I. P.; Stewart, James J. P.

    2007-07-15

    The many-body expansion of the interaction potential between atoms and molecules is analyzed in detail for different types of interactions involving up to seven atoms. Elementary clusters of Ar, Na, Si, and, in particular, Au are studied, using first-principles wave-function- and density-functional-based methods to obtain the individual n-body contributions to the interaction energies. With increasing atom number the many-body expansion converges rapidly only for long-range weak interactions. Large oscillatory behavior is observed for other types of interactions. This is consistent with the fact that Au clusters up to a certain size prefer planar structures over the more compact three-dimensional Lennard-Jones-type structures. Several Au model potentials and semiempirical PM6 theory are investigated for their ability to reproduce the quantum results. We further investigate small water clusters as prototypes of hydrogen-bonded systems. Here, the many-body expansion converges rapidly, reflecting the localized nature of the hydrogen bond and justifying the use of two-body potentials to describe water-water interactions. The question of whether electron correlation contributions can be successfully modeled by a many-body interaction potential is also addressed.

  20. Influence of the van der Waals interaction in the dissociation dynamics of N{sub 2} on W(110) from first principles

    SciTech Connect

    Martin-Gondre, L.; Juaristi, J. I.; Blanco-Rey, M.; Díez Muiño, R.; Alducin, M.

    2015-02-21

    Using ab initio molecular dynamics (AIMD) calculations, we investigate the role of the van der Waals (vdW) interaction in the dissociative adsorption of N{sub 2} on W(110). Hitherto, existing classical dynamics calculations performed on six-dimensional potential energy surfaces based on density functional theory (DFT), and the semi-local PW91 and RPBE [Hammer et al. Phys. Rev. B 59, 7413 (1999)] exchange-correlation functionals were unable to fully describe the dependence of the initial sticking coefficient on the molecular beam incidence conditions as found in experiments. N{sub 2} dissociation on W(110) was shown to be very sensitive not only to short molecule-surface distances but also to large distances where the vdW interaction, not included in semilocal-DFT, should dominate. In this work, we perform a systematic study on the dissociative adsorption using a selection of existing non-local functionals that include the vdW interaction (vdW-functionals). Clearly, the inclusion of the non-local correlation term contributes in all cases to correct the unrealistic energy barriers that were identified in the RPBE at large molecule-surface distances. Among the tested vdW-functionals, the original vdW-DF by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and the ulterior vdW-DF2 give also an adequate description of the N{sub 2} adsorption energy and energy barrier at the transition state, i.e., of the properties that are commonly used to verify the quality of any exchange-correlation functional. However, the results of our AIMD calculations, which are performed at different incidence conditions and hence extensively probe the multi-configurational potential energy surface of the system, do not seem as satisfactory as the preliminary static analysis suggested. When comparing the obtained dissociation probabilities with existing experimental data, none of the used vdW-functionals seems to provide altogether an adequate description of the N{sub 2}/W(110) interaction at

  1. Van der Waals interactions from the exchange hole dipole moment: Application to bio-organic benchmark systems

    NASA Astrophysics Data System (ADS)

    Johnson, Erin R.; Becke, Axel D.

    2006-12-01

    We have recently completed the development of a simple model of the dispersion interaction based on the dipole moment of the exchange hole [E.R. Johnson, A.D. Becke, J. Chem. Phys. 124 (2006) 174104, and references therein]. The model generates remarkably accurate dispersion coefficients, geometries, and binding energies of intermolecular complexes. In this work, the model is tested on three biochemical benchmark systems: binding energies of nucleobase pairs, relative conformational energies of the alanine dipeptide, and the anomeric effect from conformational energies of substituted tetrahydropyrans and cyclohexanes. The model gives binding energies and conformational energies in good agreement with correlated ab initio reference data.

  2. The HCO{sup +}–H{sub 2} van der Waals interaction: Potential energy and scattering

    SciTech Connect

    Massó, H.; Wiesenfeld, L.

    2014-11-14

    We compute the rigid-body, four-dimensional interaction potential between HCO{sup +} and H{sub 2}. The ab initio energies are obtained at the coupled-cluster single double triple level of theory, corrected for Basis Set Superposition Errors. The ab initio points are fit onto the spherical basis relevant for quantum scattering. We present elastic and rotationally inelastic coupled channels scattering between low lying rotational levels of HCO{sup +} and para-/ortho-H{sub 2}. Results are compared with similar earlier computations with He or isotropic para-H{sub 2} as the projectile. Computations agree with earlier pressure broadening measurements.

  3. Toward transferable interatomic van der Waals interactions without electrons: The role of multipole electrostatics and many-body dispersion

    NASA Astrophysics Data System (ADS)

    Bereau, Tristan; von Lilienfeld, O. Anatole

    2014-07-01

    We estimate polarizabilities of atoms in molecules without electron density, using a Voronoi tesselation approach instead of conventional density partitioning schemes. The resulting atomic dispersion coefficients are calculated, as well as many-body dispersion effects on intermolecular potential energies. We also estimate contributions from multipole electrostatics and compare them to dispersion. We assess the performance of the resulting intermolecular interaction model from dispersion and electrostatics for more than 1300 neutral and charged, small organic molecular dimers. Applications to water clusters, the benzene crystal, the anti-cancer drug ellipticine—intercalated between two Watson-Crick DNA base pairs, as well as six macro-molecular host-guest complexes highlight the potential of this method and help to identify points of future improvement. The mean absolute error made by the combination of static electrostatics with many-body dispersion reduces at larger distances, while it plateaus for two-body dispersion, in conflict with the common assumption that the simple 1/R6 correction will yield proper dissociative tails. Overall, the method achieves an accuracy well within conventional molecular force fields while exhibiting a simple parametrization protocol.

  4. Toward transferable interatomic van der Waals interactions without electrons: The role of multipole electrostatics and many-body dispersion

    SciTech Connect

    Bereau, Tristan; Lilienfeld, O. Anatole von

    2014-07-21

    We estimate polarizabilities of atoms in molecules without electron density, using a Voronoi tesselation approach instead of conventional density partitioning schemes. The resulting atomic dispersion coefficients are calculated, as well as many-body dispersion effects on intermolecular potential energies. We also estimate contributions from multipole electrostatics and compare them to dispersion. We assess the performance of the resulting intermolecular interaction model from dispersion and electrostatics for more than 1300 neutral and charged, small organic molecular dimers. Applications to water clusters, the benzene crystal, the anti-cancer drug ellipticine—intercalated between two Watson-Crick DNA base pairs, as well as six macro-molecular host-guest complexes highlight the potential of this method and help to identify points of future improvement. The mean absolute error made by the combination of static electrostatics with many-body dispersion reduces at larger distances, while it plateaus for two-body dispersion, in conflict with the common assumption that the simple 1/R{sup 6} correction will yield proper dissociative tails. Overall, the method achieves an accuracy well within conventional molecular force fields while exhibiting a simple parametrization protocol.

  5. Binding and Diffusion of Lithium in Graphite: Quantum Monte Carlo Benchmarks and Validation of van der Waals Density Functional Methods.

    PubMed

    Ganesh, P; Kim, Jeongnim; Park, Changwon; Yoon, Mina; Reboredo, Fernando A; Kent, Paul R C

    2014-12-09

    Highly accurate diffusion quantum Monte Carlo (QMC) studies of the adsorption and diffusion of atomic lithium in AA-stacked graphite are compared with van der Waals-including density functional theory (DFT) calculations. Predicted QMC lattice constants for pure AA graphite agree with experiment. Pure AA-stacked graphite is shown to challenge many van der Waals methods even when they are accurate for conventional AB graphite. Highest overall DFT accuracy, considering pure AA-stacked graphite as well as lithium binding and diffusion, is obtained by the self-consistent van der Waals functional vdW-DF2, although errors in binding energies remain. Empirical approaches based on point charges such as DFT-D are inaccurate unless the local charge transfer is assessed. The results demonstrate that the lithium-carbon system requires a simultaneous highly accurate description of both charge transfer and van der Waals interactions, favoring self-consistent approaches.

  6. Binding and Diffusion of Lithium in Graphite: Quantum Monte Carlo Benchmarks and Validation of van der Waals Density Functional Methods

    SciTech Connect

    Ganesh, P.; Kim, Jeongnim; Park, Changwon; Yoon, Mina; Reboredo, Fernando A.; Kent, Paul R. C.

    2014-11-03

    In highly accurate diffusion quantum Monte Carlo (QMC) studies of the adsorption and diffusion of atomic lithium in AA-stacked graphite are compared with van der Waals-including density functional theory (DFT) calculations. Predicted QMC lattice constants for pure AA graphite agree with experiment. Pure AA-stacked graphite is shown to challenge many van der Waals methods even when they are accurate for conventional AB graphite. Moreover, the highest overall DFT accuracy, considering pure AA-stacked graphite as well as lithium binding and diffusion, is obtained by the self-consistent van der Waals functional vdW-DF2, although errors in binding energies remain. Empirical approaches based on point charges such as DFT-D are inaccurate unless the local charge transfer is assessed. Our results demonstrate that the lithium carbon system requires a simultaneous highly accurate description of both charge transfer and van der Waals interactions, favoring self-consistent approaches.

  7. Aqueous gating of van der Waals materials on bilayer nanopaper.

    PubMed

    Bao, Wenzhong; Fang, Zhiqiang; Wan, Jiayu; Dai, Jiaqi; Zhu, Hongli; Han, Xiaogang; Yang, Xiaofeng; Preston, Colin; Hu, Liangbing

    2014-10-28

    In this work, we report transistors made of van der Waals materials on a mesoporous paper with a smooth nanoscale surface. The aqueous transistor has a novel planar structure with source, drain, and gate electrodes on the same surface of the paper, while the mesoporous paper is used as an electrolyte reservoir. These transistors are enabled by an all-cellulose paper with nanofibrillated cellulose (NFC) on the top surface that leads to an excellent surface smoothness, while the rest of the microsized cellulose fibers can absorb electrolyte effectively. Based on two-dimensional van der Waals materials, including MoS2 and graphene, we demonstrate high-performance transistors with a large on-off ratio and low subthreshold swing. Such planar transistors with absorbed electrolyte gating can be used as sensors integrated with other components to form paper microfluidic systems. This study is significant for future paper-based electronics and biosensors.

  8. Discrete structure of van der Waals domains in globular proteins.

    PubMed

    Berezovsky, Igor N

    2003-03-01

    Most globular proteins are divisible by domains, distinct substructures of the globule. The notion of hierarchy of the domains was introduced earlier via van der Waals energy profiles that allow one to subdivide the proteins into domains (subdomains). The question remains open as to what is the possible structural connection of the energy profiles. The recent discovery of the loop-n-lock elements in the globular proteins suggests such a structural connection. A direct comparison of the segmentation by van der Waals energy criteria with the maps of the locked loops of nearly standard size reveals a striking correlation: domains in general appear to consist of one to several such loops. In addition, it was demonstrated that a variety of subdivisions of the same protein into domains is just a regrouping of the loop-n-lock elements.

  9. Towards a more accurate van der Waals density functional

    NASA Astrophysics Data System (ADS)

    Hamada, Ikutaro

    2014-03-01

    The van der Waals density functional (vdW-DF) of Dion et al. [1] has attracted considerable attention, because the functional is able to describe intra- and intermolecular bondings with different natures, e.g., covalent and van der Waals bondings in a seamless fashion within the framework of density functional theory. However, the accuracy of the functional is yet to be improved for the applications to various systems. Here I propose an exchange functional for the second version of vdW-DF [2], which improves the accuracy of vdW-DF. The keys in the improved exchange are the matching to the gradient expansion approximation in the slowly varying limit and the large density gradient behavior set in Becke's exchange (B86b)[3]. Systematic study on gas phase molecules, solids, and molecular adsorption demonstrates the applicability of the proposed functional to a wide variety of materials.

  10. Poisson-Boltzmann Calculations: van der Waals or Molecular Surface?

    PubMed Central

    Pang, Xiaodong; Zhou, Huan-Xiang

    2012-01-01

    The Poisson-Boltzmann equation is widely used for modeling the electrostatics of biomolecules, but the calculation results are sensitive to the choice of the boundary between the low solute dielectric and the high solvent dielectric. The default choice for the dielectric boundary has been the molecular surface, but the use of the van der Waals surface has also been advocated. Here we review recent studies in which the two choices are tested against experimental results and explicit-solvent calculations. The assignment of the solvent high dielectric constant to interstitial voids in the solute is often used as a criticism against the van der Waals surface. However, this assignment may not be as unrealistic as previously thought, since hydrogen exchange and other NMR experiments have firmly established that all interior parts of proteins are transiently accessible to the solvent. PMID:23293674

  11. Poisson-Boltzmann Calculations: van der Waals or Molecular Surface?

    PubMed

    Pang, Xiaodong; Zhou, Huan-Xiang

    2013-01-01

    The Poisson-Boltzmann equation is widely used for modeling the electrostatics of biomolecules, but the calculation results are sensitive to the choice of the boundary between the low solute dielectric and the high solvent dielectric. The default choice for the dielectric boundary has been the molecular surface, but the use of the van der Waals surface has also been advocated. Here we review recent studies in which the two choices are tested against experimental results and explicit-solvent calculations. The assignment of the solvent high dielectric constant to interstitial voids in the solute is often used as a criticism against the van der Waals surface. However, this assignment may not be as unrealistic as previously thought, since hydrogen exchange and other NMR experiments have firmly established that all interior parts of proteins are transiently accessible to the solvent.

  12. Etude théorique de l'interaction de van der Waals entre une sonde métallique et une surface diélectrique : application à la microscopie par mesure de forces atomiques

    NASA Astrophysics Data System (ADS)

    Maghezzi, S.; Girard, C.; van Labeke, D.

    1991-02-01

    The van der Waals interaction force between a metallic tip and a non planar dielectric is derived from a non local formalism. A general formulation is given for a spherical tip of nanometric size and for surfaces of arbitrary shape (monocrystal adsorbed on planar surfaces). The dispersion part of the attractive force is obtained from a non local theory expressed in terms of generalized electric susceptibilities of the two partners. Implications for atomic force microscopy in attractive mode are also discussed. La force d'interaction de van der Waals entre une pointe métallique et une surface diélectrique est calculée à partir d'une théorie non-locale. Plus précisément, on considère comme point de départ que l'interaction sonde-surface est due au couplage entre les fluctuations des densités de charges des solides en présence. De plus, dans cette approche la force de van der Waals est calculée dans le cadre de la théorie de la réponse linéaire en utilisant pour chacun des milieux la fonction réponse non-locale correspondante. En particulier la sonde, de nature métallique, est caractérisée par un jeu de susceptibilités du champ décrites par un formalisme auto-cohérent. On effectue enfin une étude numérique de la force sur des surfaces présentant des corrugations de dimensions nanométriques.

  13. Structure and dynamics of small van der Waals complexes

    SciTech Connect

    Loreau, J.

    2014-10-06

    We illustrate computational aspects of the calculation of the potential energy surfaces of small (up to five atoms) van der Waals complexes with high-level quantum chemistry techniques such as the CCSD(T) method with extended basis sets. We discuss the compromise between the required accuracy and the computational time. Further, we show how these potential energy surfaces can be fitted and used in dynamical calculations such as non-reactive inelastic scattering.

  14. Grippers Based on Opposing Van Der Waals Adhesive Pads

    NASA Technical Reports Server (NTRS)

    Parness, Aaron (Inventor); Kennedy, Brett A. (Inventor); Heverly, Matthew C (Inventor); Cutkosky, Mark R. (Inventor); Hawkes, Elliot Wright (Inventor)

    2016-01-01

    Novel gripping structures based on van der Waals adhesive forces are disclosed. Pads covered with fibers can be activated in pairs by opposite forces, thereby enabling control of the adhesive force in an ON or OFF state. Pads can be used in groups, each comprising a group of opposite pads. The adhesive structures enable anchoring forces that can resist adverse forces from different directions. The adhesive structures can be used to enable the operation of robots on surfaces of space vehicles.

  15. Dynamic changes of phase in a van der Waals fluid

    NASA Astrophysics Data System (ADS)

    Hagan, R.; Serrin, J.

    1984-03-01

    This paper gives sufficient conditions to guarantee the existence of a shock layer solution connecting two different equilibrium states in a van der Waals fluid. In particular, the equilibrium states can belong to two different phases of the fluid. The constitutive laws come from a modified Korteweg theory which is compatible with the Clausius Duhem inequality. The Clausius Duhem inequality in turn gives rise to a Liapunov function. The main mathematical tool is the LaSalle invariance principle.

  16. Nano Electronics on Atomically Controlled van der Waals Quantum Heterostructures

    DTIC Science & Technology

    2015-03-30

    ES) Columbia University 538 West 120th Street New York 10027 United States 8. PERFORMING ORGANIZATION REPORT NUMBER N/A 9...dimensional (2D) van der Waals (vdW) materials for the realization of novel quantum electronic states . We employed molecular beam epitaxy (MBE) combined with...transitions between retro intraband and specular interband Andreev reflections opens a new route for future experiments that could employ the gate

  17. Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope.

    PubMed

    Govyadinov, Alexander A; Konečná, Andrea; Chuvilin, Andrey; Vélez, Saül; Dolado, Irene; Nikitin, Alexey Y; Lopatin, Sergei; Casanova, Fèlix; Hueso, Luis E; Aizpurua, Javier; Hillenbrand, Rainer

    2017-07-21

    Van der Waals materials exhibit intriguing structural, electronic, and photonic properties. Electron energy loss spectroscopy within scanning transmission electron microscopy allows for nanoscale mapping of such properties. However, its detection is typically limited to energy losses in the eV range-too large for probing low-energy excitations such as phonons or mid-infrared plasmons. Here, we adapt a conventional instrument to probe energy loss down to 100 meV, and map phononic states in hexagonal boron nitride, a representative van der Waals material. The boron nitride spectra depend on the flake thickness and on the distance of the electron beam to the flake edges. To explain these observations, we developed a classical response theory that describes the interaction of fast electrons with (anisotropic) van der Waals slabs, revealing that the electron energy loss is dominated by excitation of hyperbolic phonon polaritons, and not of bulk phonons as often reported. Thus, our work is of fundamental importance for interpreting future low-energy loss spectra of van der Waals materials.Here the authors adapt a STEM-EELS system to probe energy loss down to 100 meV, and apply it to map phononic states in hexagonal boron nitride, revealing that the electron loss is dominated by hyperbolic phonon polaritons.

  18. van der Waals Density Functional Theory vdW-DFq for Semihard Materials

    NASA Astrophysics Data System (ADS)

    Peng, Qing; de, Suvranu

    There are a large number of materials with mild stiffness, which are not as soft as tissues and not as strong as metals. These semihard materials includes energetic materials, molecular crystals, layered materials, and van der Waals crystals. The integrity and mechanical stability are mainly determined by the interactions between instantaneously induced dipoles, the so called London dispersion force or van der Waals force. It is challenging to accurately model the structural and mechanical properties of these semihard materials in the frame of density functional theory where the non-local correlation functionals are not well known. Here we propose a van der Waals density functional named vdW-DFq to accurately model the density and geometry of semihard materials. Using β-cyclotetramethylene tetranitramine as a prototype, we adjust the enhancement factor of the exchange energy functional with generalized gradient approximations. We find this method to be simple and robust over a wide tuning range when calibrating the functional on-demand with experimental data. With a calibrated value q = 1 . 05 , the proposed vdW-DFq method shows good performance in predicting the geometries of 11 common energetic material molecular crystals and 3 typical layered van der Waals crystals. The authors would like to acknowledge the generous financial support from the Defense Threat Reduction Agency (DTRA) Grant # HDTRA1-13-1-0025.

  19. Consistent van der Waals Radii for the Whole Main Group

    PubMed Central

    Mantina, Manjeera; Chamberlin, Adam C.; Valero, Rosendo; Cramer, Christopher J.; Truhlar, Donald G.

    2013-01-01

    Atomic radii are not precisely defined but are nevertheless widely used parameters in modeling and understanding molecular structure and interactions. The van der Waals radii determined by Bondi from molecular crystals and noble gas crystals are the most widely used values, but Bondi recommended radius values for only 28 of the 44 main-group elements in the periodic table. In the present article we present atomic radii for the other 16; these new radii were determined in a way designed to be compatible with Bondi’s scale. The method chosen is a set of two-parameter correlations of Bondi’s radii with repulsive-wall distances calculated by relativistic coupled-cluster electronic structure calculations. The newly determined radii (in Å) are Be, 1.53; B, 1.92; Al, 1.84; Ca, 2.31; Ge, 2.11; Rb, 3.03; Sr, 2.50; Sb, 2.06; Cs, 3.43; Ba, 2.68; Bi, 2.07; Po, 1.97; At, 2.02; Rn, 2.20; Fr, 3.48; and Ra, 2.83. PMID:19382751

  20. Consistent van der Waals radii for the whole main group.

    PubMed

    Mantina, Manjeera; Chamberlin, Adam C; Valero, Rosendo; Cramer, Christopher J; Truhlar, Donald G

    2009-05-14

    Atomic radii are not precisely defined but are nevertheless widely used parameters in modeling and understanding molecular structure and interactions. The van der Waals radii determined by Bondi from molecular crystals and data for gases are the most widely used values, but Bondi recommended radius values for only 28 of the 44 main-group elements in the periodic table. In the present Article, we present atomic radii for the other 16; these new radii were determined in a way designed to be compatible with Bondi's scale. The method chosen is a set of two-parameter correlations of Bondi's radii with repulsive-wall distances calculated by relativistic coupled-cluster electronic structure calculations. The newly determined radii (in A) are Be, 1.53; B, 1.92; Al, 1.84; Ca, 2.31; Ge, 2.11; Rb, 3.03; Sr, 2.49; Sb, 2.06; Cs, 3.43; Ba, 2.68; Bi, 2.07; Po, 1.97; At, 2.02; Rn, 2.20; Fr, 3.48; and Ra, 2.83.

  1. Van der Waals density functional study of water binding in metal-organic frameworks

    NASA Astrophysics Data System (ADS)

    Lee, Kyuho; Smit, Berend; Neaton, Jeffrey B.

    2013-03-01

    Metal-organic frameworks (MOFs) are promising candidate materials for gas storage, gas separation and catalysis. However, MOFs are vulnerable to humid air and effective surface area drops dramatically on an exposure to water. In this theoretical study, we investigate the interaction of single water molecule with MOF-74 on different binding sites by using van der Waals density functionals. We also explore how different type of metal cations affect the interaction.

  2. Van der Waals coefficients beyond the classical shell model

    SciTech Connect

    Tao, Jianmin; Fang, Yuan; Hao, Pan; Scuseria, G. E.; Ruzsinszky, Adrienn; Perdew, John P.

    2015-01-14

    Van der Waals (vdW) coefficients can be accurately generated and understood by modelling the dynamic multipole polarizability of each interacting object. Accurate static polarizabilities are the key to accurate dynamic polarizabilities and vdW coefficients. In this work, we present and study in detail a hollow-sphere model for the dynamic multipole polarizability proposed recently by two of the present authors (JT and JPP) to simulate the vdW coefficients for inhomogeneous systems that allow for a cavity. The inputs to this model are the accurate static multipole polarizabilities and the electron density. A simplification of the full hollow-sphere model, the single-frequency approximation (SFA), circumvents the need for a detailed electron density and for a double numerical integration over space. We find that the hollow-sphere model in SFA is not only accurate for nanoclusters and cage molecules (e.g., fullerenes) but also yields vdW coefficients among atoms, fullerenes, and small clusters in good agreement with expensive time-dependent density functional calculations. However, the classical shell model (CSM), which inputs the static dipole polarizabilities and estimates the static higher-order multipole polarizabilities therefrom, is accurate for the higher-order vdW coefficients only when the interacting objects are large. For the lowest-order vdW coefficient C{sub 6}, SFA and CSM are exactly the same. The higher-order (C{sub 8} and C{sub 10}) terms of the vdW expansion can be almost as important as the C{sub 6} term in molecular crystals. Application to a variety of clusters shows that there is strong non-additivity of the long-range vdW interactions between nanoclusters.

  3. A simplified implementation of van der Waals density functionals for first-principles molecular dynamics applications

    NASA Astrophysics Data System (ADS)

    Wu, Jun; Gygi, François

    2012-06-01

    We present a simplified implementation of the non-local van der Waals correlation functional introduced by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and reformulated by Román-Pérez et al. [Phys. Rev. Lett. 103, 096102 (2009)]. The proposed numerical approach removes the logarithmic singularity of the kernel function. Complete expressions of the self-consistent correlation potential and of the stress tensor are given. Combined with various choices of exchange functionals, five versions of van der Waals density functionals are implemented. Applications to the computation of the interaction energy of the benzene-water complex and to the computation of the equilibrium cell parameters of the benzene crystal are presented. As an example of crystal structure calculation involving a mixture of hydrogen bonding and dispersion interactions, we compute the equilibrium structure of two polymorphs of aspirin (2-acetoxybenzoic acid, C9H8O4) in the P21/c monoclinic structure.

  4. Van der Waals force: a dominant factor for reactivity of graphene.

    PubMed

    Lee, Jong Hak; Avsar, Ahmet; Jung, Jeil; Tan, Jun You; Watanabe, K; Taniguchi, T; Natarajan, Srinivasan; Eda, Goki; Adam, Shaffique; Castro Neto, Antonio H; Özyilmaz, Barbaros

    2015-01-14

    Reactivity control of graphene is an important issue because chemical functionalization can modulate graphene's unique mechanical, optical, and electronic properties. Using systematic optical studies, we demonstrate that van der Waals interaction is the dominant factor for the chemical reactivity of graphene on two-dimensional (2D) heterostructures. A significant enhancement in the chemical stability of graphene is achieved by replacing the common SiO2 substrate with 2D crystals such as an additional graphene layer, WS2, MoS2, or h-BN. Our theoretical and experimental results show that its origin is a strong van der Waals interaction between the graphene layer and the 2D substrate. This results in a high resistive force on graphene toward geometric lattice deformation. We also demonstrate that the chemical reactivity of graphene can be controlled by the relative lattice orientation with respect to the substrates and thus can be used for a wide range of applications including hydrogen storage.

  5. Van der Waals heterostructure of phosphorene and hexagonal boron nitride: First-principles modeling

    NASA Astrophysics Data System (ADS)

    Peng, Zhang; Jing, Wang; Xiang-Mei, Duan

    2016-03-01

    We have studied the structural and electronic properties of a hybrid hexagonal boron nitride with phosphorene nanocomposite using ab initio density functional calculations. It is found that the interaction between the hexagonal boron nitride and phosphorene is dominated by the weak van der Waals interaction, with their own intrinsic electronic properties preserved. Furthermore, the band gap of the nanocomposite is dependent on the interfacial distance. Our results could shed light on the design of new devices based on van der Waals heterostructure. Projected supported by the National Natural Science Foundation of China (Grant No. 11574167), the New Century 151 Talents Project of Zhejiang Province,China, and the K. C. Wong Magna Foundation in Ningbo University, China.

  6. Two dimensional graphene nanogenerator by coulomb dragging: Moving van der Waals heterostructure

    NASA Astrophysics Data System (ADS)

    Zhong, Huikai; Li, Xiaoqiang; Wu, Zhiqian; Zhang, Shengjiao; Xu, Zhijuan; Chen, Hongsheng; Lin, Shisheng

    2015-06-01

    Harvesting energy from environment is the current focus of scientific community. Here, we demonstrate a graphene nanogenerator, which is based on moving van der Waals heterostructure formed between graphene and two dimensional (2D) graphene oxide (GO). This nanogenerator can convert mechanical energy into electricity with a voltage output of around 10 mV. Systematic experiments reveal the generated electricity originates from the coulomb interaction induced momentum transfer between 2D GO and holes in graphene. 2D boron nitride was also demonstrated to be effective in the framework of moving van der Waals heterostructure nanogenerator. This investigation of nanogenerator based on the interaction between 2D macromolecule materials will be important to understand the origin of the flow-induced potential in nanomaterials and may have great potential in practical applications.

  7. FAST TRACK COMMUNICATION: A variational principle behind the van der Waals picture of strongly coupled plasmas

    NASA Astrophysics Data System (ADS)

    Frusawa, Hiroshi

    2009-02-01

    Various strong coupling theories of the one-component plasma have successfully predicted the thermodynamic and structural properties by separating the Coulomb potential into short- and long-ranged parts in ad hoc ways. Moreover, it has been demonstrated that the density-density correlation function in a mimic system with only short-ranged interactions resembles that of the full Coulomb system, revealing that the van der Waals picture applies to the strongly coupled Coulomb systems. Here we present a variational theory forming the basis of the van der Waals picture. Our approach provides hybrid formulations which combine both the liquid state theory and statistical field theory; essential use is made of the coarse-grained system with only the long-ranged part of Coulomb interactions as a reference system in introducing both the lower bound variational principle and strong coupling expansion.

  8. van der Waals torque and force between dielectrically anisotropic layered media.

    PubMed

    Lu, Bing-Sui; Podgornik, Rudolf

    2016-07-28

    We analyse van der Waals interactions between a pair of dielectrically anisotropic plane-layered media interacting across a dielectrically isotropic solvent medium. We develop a general formalism based on transfer matrices to investigate the van der Waals torque and force in the limit of weak birefringence and dielectric matching between the ordinary axes of the anisotropic layers and the solvent. We apply this formalism to study the following systems: (i) a pair of single anisotropic layers, (ii) a single anisotropic layer interacting with a multilayered slab consisting of alternating anisotropic and isotropic layers, and (iii) a pair of multilayered slabs each consisting of alternating anisotropic and isotropic layers, looking at the cases where the optic axes lie parallel and/or perpendicular to the plane of the layers. For the first case, the optic axes of the oppositely facing anisotropic layers of the two interacting slabs generally possess an angular mismatch, and within each multilayered slab the optic axes may either be the same or undergo constant angular increments across the anisotropic layers. In particular, we examine how the behaviors of the van der Waals torque and force can be "tuned" by adjusting the layer thicknesses, the relative angular increment within each slab, and the angular mismatch between the slabs.

  9. Van der Waals phase transition in the framework of holography

    NASA Astrophysics Data System (ADS)

    Zeng, Xiao-Xiong; Li, Li-Fang

    2017-01-01

    Phase structure of the quintessence Reissner-Nordström-AdS black hole is probed by the nonlocal observables such as holographic entanglement entropy and two point correlation function. Our result shows that, as the case of the thermal entropy, both the observables exhibit the Van der Waals-like phase transition. To reinforce this conclusion, we further check the equal area law for the first order phase transition and critical exponent of the heat capacity for the second order phase transition. We also discuss the effect of the state parameter on the phase structure of the nonlocal observables.

  10. Slidable atomic layers in van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Kobayashi, Yu; Taniguchi, Takashi; Watanabe, Kenji; Maniwa, Yutaka; Miyata, Yasumitsu

    2017-04-01

    We report the preparation and manipulation of slidable atomic layers in clean, incommensurate van der Waals (vdW) heterostructures. Monolayer and multilayer WS2 grains are grown on graphite and hexagonal boron nitride (hBN) via chemical vapor deposition, and these grains can slide smoothly on graphite and hBN surfaces by manipulation with a tip. Furthermore, this sliding process allows the suspension, tearing, stacking, and connection of the atomic layers. These results demonstrate a novel approach for developing a wide variety of atomic-layer heterostructures with tunable interlayer coupling and an ideal system for studying the superlubricity of incommensurate, highly clean vdW contacts.

  11. Exfoliation of natural van der Waals heterostructures to a single unit cell thickness

    NASA Astrophysics Data System (ADS)

    Velický, Matěj; Toth, Peter S.; Rakowski, Alexander M.; Rooney, Aidan P.; Kozikov, Aleksey; Woods, Colin R.; Mishchenko, Artem; Fumagalli, Laura; Yin, Jun; Zólyomi, Viktor; Georgiou, Thanasis; Haigh, Sarah J.; Novoselov, Kostya S.; Dryfe, Robert A. W.

    2017-02-01

    Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results.

  12. Exfoliation of natural van der Waals heterostructures to a single unit cell thickness

    PubMed Central

    Velický, Matěj; Toth, Peter S.; Rakowski, Alexander M.; Rooney, Aidan P.; Kozikov, Aleksey; Woods, Colin R.; Mishchenko, Artem; Fumagalli, Laura; Yin, Jun; Zólyomi, Viktor; Georgiou, Thanasis; Haigh, Sarah J.; Novoselov, Kostya S.; Dryfe, Robert A. W.

    2017-01-01

    Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results. PMID:28194026

  13. Measuring the thermal boundary resistance of van der Waals contacts using an individual carbon nanotube.

    PubMed

    Hirotani, Jun; Ikuta, Tatsuya; Nishiyama, Takashi; Takahashi, Koji

    2013-01-16

    Interfacial thermal transport via van der Waals interaction is quantitatively evaluated using an individual multi-walled carbon nanotube bonded on a platinum hot-film sensor. The thermal boundary resistance per unit contact area was obtained at the interface between the closed end or sidewall of the nanotube and platinum, gold, or a silicon dioxide surface. When taking into consideration the surface roughness, the thermal boundary resistance at the sidewall is found to coincide with that at the closed end. A new finding is that the thermal boundary resistance between a carbon nanotube and a solid surface is independent of the materials within the experimental errors, which is inconsistent with a traditional phonon mismatch model, which shows a clear material dependence of the thermal boundary resistance. Our data indicate the inapplicability of existing phonon models when weak van der Waals forces are dominant at the interfaces.

  14. Exfoliation of natural van der Waals heterostructures to a single unit cell thickness.

    PubMed

    Velický, Matěj; Toth, Peter S; Rakowski, Alexander M; Rooney, Aidan P; Kozikov, Aleksey; Woods, Colin R; Mishchenko, Artem; Fumagalli, Laura; Yin, Jun; Zólyomi, Viktor; Georgiou, Thanasis; Haigh, Sarah J; Novoselov, Kostya S; Dryfe, Robert A W

    2017-02-13

    Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results.

  15. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures

    PubMed Central

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H.; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E.; Dappe, Yannick J.; Bertran, François; Fèvre, Patrick Le; Johnson, A. T. Charlie; Ouerghi, Abdelkarim

    2016-01-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design. PMID:27246929

  16. Spooky correlations and unusual van der Waals forces between gapless and near-gapless molecules

    NASA Astrophysics Data System (ADS)

    Dobson, John F.; Savin, Andreas; Ángyán, János G.; Liu, Ru-Fen

    2016-11-01

    We consider the zero-temperature van der Waals (vdW) interaction between two molecules, each of which has a zero or near-zero electronic gap between a ground state and the first excited state, using a toy model molecule (equilateral H3) as an example. We show that the van der Waals energy between two ground state molecules falls off as D-3 instead of the usual D-6 dependence, when the molecules are separated by distance D. We show that this is caused by a perfect "spooky" correlation between the two fluctuating electric dipoles. The phenomenon is related to, but not the same as, the "resonant" interaction between an electronically excited and a ground state molecule introduced by Eisenschitz and London in 1930. It is also an example of "type C van der Waals non-additivity" recently introduced by one of us [J. F. Dobson, Int. J. Quantum Chem. 114, 1157 (2014)]. Our toy molecule H3 is not stable, but symmetry considerations suggest that a similar vdW phenomenon may be observable, despite Jahn-Teller effects, in molecules with a discrete rotational symmetry and broken inversion symmetry, such as certain metal atom clusters. The motion of the nuclei will need to be included for a definitive analysis of such cases, however.

  17. Spooky correlations and unusual van der Waals forces between gapless and near-gapless molecules.

    PubMed

    Dobson, John F; Savin, Andreas; Ángyán, János G; Liu, Ru-Fen

    2016-11-28

    We consider the zero-temperature van der Waals (vdW) interaction between two molecules, each of which has a zero or near-zero electronic gap between a ground state and the first excited state, using a toy model molecule (equilateral H3) as an example. We show that the van der Waals energy between two ground state molecules falls off as D(-3) instead of the usual D(-6) dependence, when the molecules are separated by distance D. We show that this is caused by a perfect "spooky" correlation between the two fluctuating electric dipoles. The phenomenon is related to, but not the same as, the "resonant" interaction between an electronically excited and a ground state molecule introduced by Eisenschitz and London in 1930. It is also an example of "type C van der Waals non-additivity" recently introduced by one of us [J. F. Dobson, Int. J. Quantum Chem. 114, 1157 (2014)]. Our toy molecule H3 is not stable, but symmetry considerations suggest that a similar vdW phenomenon may be observable, despite Jahn-Teller effects, in molecules with a discrete rotational symmetry and broken inversion symmetry, such as certain metal atom clusters. The motion of the nuclei will need to be included for a definitive analysis of such cases, however.

  18. Large area molybdenum disulphide- epitaxial graphene vertical Van der Waals heterostructures.

    PubMed

    Pierucci, Debora; Henck, Hugo; Naylor, Carl H; Sediri, Haikel; Lhuillier, Emmanuel; Balan, Adrian; Rault, Julien E; Dappe, Yannick J; Bertran, François; Fèvre, Patrick Le; Johnson, A T Charlie; Ouerghi, Abdelkarim

    2016-06-01

    Two-dimensional layered transition metal dichalcogenides (TMDCs) show great potential for optoelectronic devices due to their electronic and optical properties. A metal-semiconductor interface, as epitaxial graphene - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science, as it constitutes an outstanding platform to investigate the interlayer interaction in van der Waals heterostructures. Here, we study large area MoS2-graphene-heterostructures formed by direct transfer of chemical-vapor deposited MoS2 layer onto epitaxial graphene/SiC. We show that via a direct transfer, which minimizes interface contamination, we can obtain high quality and homogeneous van der Waals heterostructures. Angle-resolved photoemission spectroscopy (ARPES) measurements combined with Density Functional Theory (DFT) calculations show that the transition from indirect to direct bandgap in monolayer MoS2 is maintained in these heterostructures due to the weak van der Waals interaction with epitaxial graphene. A downshift of the Raman 2D band of the graphene, an up shift of the A1g peak of MoS2 and a significant photoluminescence quenching are observed for both monolayer and bilayer MoS2 as a result of charge transfer from MoS2 to epitaxial graphene under illumination. Our work provides a possible route to modify the thin film TDMCs photoluminescence properties via substrate engineering for future device design.

  19. van der Waals forces influencing adhesion of cells

    PubMed Central

    Kendall, K.; Roberts, A. D.

    2015-01-01

    Adhesion molecules, often thought to be acting by a ‘lock and key’ mechanism, have been thought to control the adhesion of cells. While there is no doubt that a coating of adhesion molecules such as fibronectin on a surface affects cell adhesion, this paper aims to show that such surface contamination is only one factor in the equation. Starting from the baseline idea that van der Waals force is a ubiquitous attraction between all molecules, and thereby must contribute to cell adhesion, it is clear that effects from geometry, elasticity and surface molecules must all add on to the basic cell attractive force. These effects of geometry, elasticity and surface molecules are analysed. The adhesion force measured between macroscopic polymer spheres was found to be strongest when the surfaces were absolutely smooth and clean, with no projecting protruberances. Values of the measured surface energy were then about 35 mJ m−2, as expected for van der Waals attractions between the non-polar molecules. Surface projections such as abrasion roughness or dust reduced the molecular adhesion substantially. Water cut the measured surface energy to 3.4 mJ m−2. Surface active molecules lowered the adhesion still further to less than 0.3 mJ m−2. These observations do not support the lock and key concept. PMID:25533101

  20. Van der Waals Epitaxy of Ultrathin Halide Perovkistes

    NASA Astrophysics Data System (ADS)

    Wang, Yiping; Shi, Yunfeng; Shi, Jian

    We present our understanding, with CH3NH3PbX3 as a model system, on the 2D van der Waals growth and kinetics of 3D parent materials. We show the successful synthesis of ultrathin (sub-10 nm), large scale (a few tens of μm) single crystalline 2D perovskite thin films on layered mica substrate by van der Waals (VDW) epitaxy. Classical nucleation and growth model explaining conventional epitaxy has been modified to interpret the unique 2D results under VDW mechanism. The generalization of our model shows that a 3D crystal with low cohesive energy tends to favor the 2D growth while the one with strong cohesive energy has less kinetic window. With Monte Carlo simulations, we show that the fractal 2D morphology in perovskite precisely manifests the kinetic competition between VDW diffusivity and thermodynamic driving force, a unique phenomenon to VDW growth, suggesting a fundamental limit on the morphology stability of the 2D form of a 3D material. On the other hand, our single crystal thin film growth results and subsequent cryogenic study in the iodide perovskite provide a perfect resource for the exploration of its complex optical and electronic properties and unveiling the origins of its popularity in the energy conversion field.

  1. Franckeite as a naturally occurring van der Waals heterostructure

    PubMed Central

    Molina-Mendoza, Aday J.; Giovanelli, Emerson; Paz, Wendel S.; Niño, Miguel Angel; Island, Joshua O.; Evangeli, Charalambos; Aballe, Lucía; Foerster, Michael; van der Zant, Herre S. J.; Rubio-Bollinger, Gabino; Agraït, Nicolás; Palacios, J. J.; Pérez, Emilio M.; Castellanos-Gomez, Andres

    2017-01-01

    The fabrication of van der Waals heterostructures, artificial materials assembled by individual stacking of 2D layers, is among the most promising directions in 2D materials research. Until now, the most widespread approach to stack 2D layers relies on deterministic placement methods, which are cumbersome and tend to suffer from poor control over the lattice orientations and the presence of unwanted interlayer adsorbates. Here, we present a different approach to fabricate ultrathin heterostructures by exfoliation of bulk franckeite which is a naturally occurring and air stable van der Waals heterostructure (composed of alternating SnS2-like and PbS-like layers stacked on top of each other). Presenting both an attractive narrow bandgap (<0.7 eV) and p-type doping, we find that the material can be exfoliated both mechanically and chemically down to few-layer thicknesses. We present extensive theoretical and experimental characterizations of the material's electronic properties and crystal structure, and explore applications for near-infrared photodetectors. PMID:28194037

  2. Franckeite as a naturally occurring van der Waals heterostructure.

    PubMed

    Molina-Mendoza, Aday J; Giovanelli, Emerson; Paz, Wendel S; Niño, Miguel Angel; Island, Joshua O; Evangeli, Charalambos; Aballe, Lucía; Foerster, Michael; van der Zant, Herre S J; Rubio-Bollinger, Gabino; Agraït, Nicolás; Palacios, J J; Pérez, Emilio M; Castellanos-Gomez, Andres

    2017-02-13

    The fabrication of van der Waals heterostructures, artificial materials assembled by individual stacking of 2D layers, is among the most promising directions in 2D materials research. Until now, the most widespread approach to stack 2D layers relies on deterministic placement methods, which are cumbersome and tend to suffer from poor control over the lattice orientations and the presence of unwanted interlayer adsorbates. Here, we present a different approach to fabricate ultrathin heterostructures by exfoliation of bulk franckeite which is a naturally occurring and air stable van der Waals heterostructure (composed of alternating SnS2-like and PbS-like layers stacked on top of each other). Presenting both an attractive narrow bandgap (<0.7 eV) and p-type doping, we find that the material can be exfoliated both mechanically and chemically down to few-layer thicknesses. We present extensive theoretical and experimental characterizations of the material's electronic properties and crystal structure, and explore applications for near-infrared photodetectors.

  3. Franckeite as a naturally occurring van der Waals heterostructure

    NASA Astrophysics Data System (ADS)

    Molina-Mendoza, Aday J.; Giovanelli, Emerson; Paz, Wendel S.; Niño, Miguel Angel; Island, Joshua O.; Evangeli, Charalambos; Aballe, Lucía; Foerster, Michael; van der Zant, Herre S. J.; Rubio-Bollinger, Gabino; Agraït, Nicolás; Palacios, J. J.; Pérez, Emilio M.; Castellanos-Gomez, Andres

    2017-02-01

    The fabrication of van der Waals heterostructures, artificial materials assembled by individual stacking of 2D layers, is among the most promising directions in 2D materials research. Until now, the most widespread approach to stack 2D layers relies on deterministic placement methods, which are cumbersome and tend to suffer from poor control over the lattice orientations and the presence of unwanted interlayer adsorbates. Here, we present a different approach to fabricate ultrathin heterostructures by exfoliation of bulk franckeite which is a naturally occurring and air stable van der Waals heterostructure (composed of alternating SnS2-like and PbS-like layers stacked on top of each other). Presenting both an attractive narrow bandgap (<0.7 eV) and p-type doping, we find that the material can be exfoliated both mechanically and chemically down to few-layer thicknesses. We present extensive theoretical and experimental characterizations of the material's electronic properties and crystal structure, and explore applications for near-infrared photodetectors.

  4. A Smart Magnetically Active Nanovehicle for on-Demand Targeted Drug Delivery: Where van der Waals Force Balances the Magnetic Interaction.

    PubMed

    Panja, Sudipta; Maji, Somnath; Maiti, Tapas K; Chattopadhyay, Santanu

    2015-11-04

    The magnetic field is a promising external stimulus for controlled and targeted delivery of therapeutic agents. Here, we focused on the preparation of a novel magnetically active polymeric micelle (MAPM) for magnetically targeted controlled drug delivery. To accomplish this, a number of superparamagnetic as well as biocompatible hybrid micelles were prepared by grafting four armed pentaerythretol poly(ε-caprolactone) (PE-PCL) onto the surface of Fe3O4 magnetic nanoparticles (MNPs) of two different ranges of size (∼5 nm and ∼15 nm). PE-PCL (four-armed) was synthesized by ring-opening polymerization, and it has been subsequently grafted onto the surface of modified MNP through urethane (-NHCO-) linkage. Polymer-immobilized MNP (5 and 15 nm) showed peculiar dispersion behavior. One displayed uniform dispersion of MNP (5 nm), while the other (15 nm) revealed associated structure. This type of size dependent contradictory dispersion behavior was realized by taking the van der Waals force as well as magnetic dipole-dipole force into consideration. The uniformly dispersed polymer immobilized MNP (5 nm) was used for the preparation of MAPM. The hydrodynamic size and bulk morphology of MAPM were studied by dynamic light scattering and high-resolution transmission electron microscopy. The anticancer drug (DOX) was encapsulated into the MAPM. The magnetic field triggers cell uptake of MAPM micelles preferentially toward targeted cells compare to untargeted ones. The cell viabilities of MAMP, DOX-encapsulated MAPM, and free DOX were studied against HeLa cell by MTT assay. In vitro release profile displayed about 51.5% release of DOX from MAPM (just after 1 h) under the influence of high frequency alternating magnetic field (HFAMF; prepared in-house device). The DOX release rate has also been tailored by on-demand application of HFAMF.

  5. Improved description of soft layered materials with van der Waals density functional theory.

    PubMed

    Graziano, Gabriella; Klimeš, Jiří; Fernandez-Alonso, Felix; Michaelides, Angelos

    2012-10-24

    The accurate description of van der Waals forces within density functional theory is currently one of the most active areas of research in computational physics and chemistry. Here we report results on the structural and energetic properties of graphite and hexagonal boron nitride, two layered materials where interlayer binding is dominated by van der Waals forces. Results from several density functionals are reported, including the optimized Becke88 van der Waals (optB88-vdW) and the optimized PBE van der Waals (optPBE-vdW) (Klimeš et al 2010 J. Phys.: Condens. Matter 22 022201) functionals. Where comparison to experiment and higher-level theory is possible, the results obtained from the two new van der Waals density functionals are in good agreement. An analysis of the physical nature of the interlayer binding in both graphite and hexagonal boron nitride is also reported.

  6. Van der Waals equation of state revisited: importance of the dispersion correction.

    PubMed

    de Visser, Sam P

    2011-04-28

    One of the most basic equations of state describing nonideal gases and liquids is the van der Waals equation of state, and as a consequence, it is generally taught in most first year undergraduate chemistry courses. In this work, we show that the constants a and b in the van der Waals equation of state are linearly proportional to the polarizability volume of the molecules in a gas or liquid. Using this information, a new thermodynamic one-parameter equation of state is derived that contains experimentally measurable variables and physics constants only. This is the first equation of state apart from the Ideal Gas Law that contains experimentally measurable variables and physics constants only, and as such, it may be a very useful and practical equation for the description of dilute gases and liquids. The modified van der Waals equation of state describes pV as the sum of repulsive and attractive intermolecular interaction energies that are represented by an exponential repulsion function between the electron clouds of the molecules and a London dispersion component, respectively. The newly derived equation of state is tested against experimental data for several gas and liquid examples, and the agreement is satisfactory. The description of the equation of state as a one-parameter function also has implications on other thermodynamic functions, such as critical parameters, virial coefficients, and isothermal compressibilities. Using our modified van der Waals equation of state, we show that all of these properties are a function of the molecular polarizability volume. Correlations of experimental data confirm the derived proportionalities.

  7. The effects of van der Waals attractions on cloud droplet growth by coalescence

    NASA Technical Reports Server (NTRS)

    Rogers, Jan R.; Davis, Robert H.

    1990-01-01

    The inclusion of van der Waals attractions in the interaction between cloud droplets has been recently shown to significantly increase the collision efficiencies of the smaller droplets. In the current work, these larger values for the collision efficiencies are used in a population dynamics model of the droplet size distribution evolution with time, in hopes of at least partially resolving the long-standing paradox in cloud microphysics that predicted rates of the onset of precipitation are generally much lower than those which are observed. Evolutions of several initial cloud droplet spectra have been tracked in time. Size evolutions are compared as predicted from the use of collision efficiencies computed using two different models to allow for droplet-droplet contact: one which considers slip flow effects only, and one which considers the combined effects of van der Waals forces and slip flow. The rate at which the droplet mass density function shifts to larger droplet sizes is increased by typically 20-25 percent, when collision efficiencies which include van der Waals forces are used.

  8. The effects of van der Waals attractions on cloud droplet growth by coalescence

    NASA Technical Reports Server (NTRS)

    Rogers, Jan R.; Davis, Robert H.

    1990-01-01

    The inclusion of van der Waals attractions in the interaction between cloud droplets has been recently shown to significantly increase the collision efficiencies of the smaller droplets. In the current work, these larger values for the collision efficiencies are used in a population dynamics model of the droplet size distribution evolution with time, in hopes of at least partially resolving the long-standing paradox in cloud microphysics that predicted rates of the onset of precipitation are generally much lower than those which are observed. Evolutions of several initial cloud droplet spectra have been tracked in time. Size evolutions are compared as predicted from the use of collision efficiencies computed using two different models to allow for droplet-droplet contact: one which considers slip flow effects only, and one which considers the combined effects of van der Waals forces and slip flow. The rate at which the droplet mass density function shifts to larger droplet sizes is increased by typically 20-25 percent, when collision efficiencies which include van der Waals forces are used.

  9. Periodic potentials in hybrid van der Waals heterostructures formed by supramolecular lattices on graphene

    PubMed Central

    Gobbi, Marco; Bonacchi, Sara; Lian, Jian X.; Liu, Yi; Wang, Xiao-Ye; Stoeckel, Marc-Antoine; Squillaci, Marco A.; D'Avino, Gabriele; Narita, Akimitsu; Müllen, Klaus; Feng, Xinliang; Olivier, Yoann; Beljonne, David; Samorì, Paolo; Orgiu, Emanuele

    2017-01-01

    The rise of 2D materials made it possible to form heterostructures held together by weak interplanar van der Waals interactions. Within such van der Waals heterostructures, the occurrence of 2D periodic potentials significantly modifies the electronic structure of single sheets within the stack, therefore modulating the material properties. However, these periodic potentials are determined by the mechanical alignment of adjacent 2D materials, which is cumbersome and time-consuming. Here we show that programmable 1D periodic potentials extending over areas exceeding 104 nm2 and stable at ambient conditions arise when graphene is covered by a self-assembled supramolecular lattice. The amplitude and sign of the potential can be modified without altering its periodicity by employing photoreactive molecules or their reaction products. In this regard, the supramolecular lattice/graphene bilayer represents the hybrid analogue of fully inorganic van der Waals heterostructures, highlighting the rich prospects that molecular design offers to create ad hoc materials. PMID:28322229

  10. Semi-empirical correlation for binary interaction parameters of the Peng–Robinson equation of state with the van der Waals mixing rules for the prediction of high-pressure vapor–liquid equilibrium

    PubMed Central

    Fateen, Seif-Eddeen K.; Khalil, Menna M.; Elnabawy, Ahmed O.

    2012-01-01

    Peng–Robinson equation of state is widely used with the classical van der Waals mixing rules to predict vapor liquid equilibria for systems containing hydrocarbons and related compounds. This model requires good values of the binary interaction parameter kij. In this work, we developed a semi-empirical correlation for kij partly based on the Huron–Vidal mixing rules. We obtained values for the adjustable parameters of the developed formula for over 60 binary systems and over 10 categories of components. The predictions of the new equation system were slightly better than the constant-kij model in most cases, except for 10 systems whose predictions were considerably improved with the new correlation. PMID:25685411

  11. Semi-empirical correlation for binary interaction parameters of the Peng-Robinson equation of state with the van der Waals mixing rules for the prediction of high-pressure vapor-liquid equilibrium.

    PubMed

    Fateen, Seif-Eddeen K; Khalil, Menna M; Elnabawy, Ahmed O

    2013-03-01

    Peng-Robinson equation of state is widely used with the classical van der Waals mixing rules to predict vapor liquid equilibria for systems containing hydrocarbons and related compounds. This model requires good values of the binary interaction parameter kij . In this work, we developed a semi-empirical correlation for kij partly based on the Huron-Vidal mixing rules. We obtained values for the adjustable parameters of the developed formula for over 60 binary systems and over 10 categories of components. The predictions of the new equation system were slightly better than the constant-kij model in most cases, except for 10 systems whose predictions were considerably improved with the new correlation.

  12. Image method in the calculation of the van der Waals force between an atom and a conducting surface

    NASA Astrophysics Data System (ADS)

    de Melo e Souza, Reinaldo; Kort-Kamp, W. J. M.; Sigaud, C.; Farina, C.

    2013-05-01

    After a brief survey of van der Waals forces, we review a method recently proposed by Eberlein and Zietal to compute the dispersion van der Waals interaction between a neutral but polarizable atom and a perfectly conducting surface of arbitrary shape. This method has the advantage of relating the quantum problem to a corresponding classical one in electrostatics in an enlightening way so that all one needs is to compute an appropriate Green function. We show how the image method of electrostatics can be conveniently used together with the Eberlein and Zietal method (when the image solution is known). We then illustrate this method in some simple but important cases, including the atom-sphere system. Finally, we present an original result for the van der Waals force between an atom and a boss hat made of a grounded conducting material.

  13. Empirical evaluation of attractive van der Waals potentials for type-purified single-walled carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Hobbie, Erik K.; Ihle, Thomas; Harris, John M.; Semler, Matthew R.

    2012-06-01

    van der Waals forces play a critical role in the structure and stability of single-wall carbon nanotube (SWCNT) materials. Thin films assembled from SWCNTs purified by electronic type show particular promise for flexible electronics applications, but mechanical durability remains an unresolved issue. Using transition resonances determined from spectroscopic measurements of type-purified SWCNTs deposited on quartz, coupled with analogous spectroscopic characterization of polydimethylsiloxane (PDMS) substrates, we use the Lifshitz theory of van der Waals dispersion interactions developed by Rajter and co-workers [Rajter , Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.76.045417 76, 045417 (2007)] to examine the influence of electronic type on van der Waals contact potentials in polymer-supported nanotube networks. Our results suggest a significantly stronger nanotube-nanotube and nanotube-polymer attraction for the semiconducting SWCNT fractions, consistent with recent measurements of the electronic durability of flexible transparent SWCNT coatings.

  14. Van der Waals Epitaxy of Functional Oxide Heterostructures

    NASA Astrophysics Data System (ADS)

    Chu, Ying-Hao

    In the diligent pursuit of low-power consumption, multifunctional, and environmentally friendly electronics, more sophisticated requirements on functional materials are on demand. Recently, the discovery of 2D layered materials has created a revolution to this field. Pioneered by graphene, these new 2D materials exhibit abundant unusual physical phenomena that is undiscovered in bulk forms. These materials are characterized with their layer form and almost pure 2D electronic behavior. The confinement of charge and heat transport at such ultrathin planes offers possibilities to overcome the bottleneck of present device development in thickness limitation, and thus push the technologies into next generation. Van der Waals epitaxy, an epitaxial growth method to combine 2D and 3D materials, is one of current reliable manufacturing processes to fabricate 2D materials by growing these 2D materials epitaxially on 3D materials. Then, transferring the 2D materials to the substrates for practical applications. In the mean time, van der Waals epitaxy has also been used to create free-standing 3D materials by growing 3D materials on 2D materials and then removing them from 2D materials since the interfacial boding between 2D and 3D materials should be weak van der Waals bonds. In this study, we intend to take the same concept, but to integrate a family of functional materials in order to open new avenue to flexible electronics. Due to the interplay of lattice, charge, orbital, and spin degrees of freedom, correlated electrons in oxides generate a rich spectrum of competing phases and physical properties. Recently, lots of studies have suggested that oxide heterostructures provide a powerful route to create and manipulate the degrees of freedom and offer new possibilities for next generation devices, thus create a new playground for researchers to investigate novel physics and the emergence of fascinating states of condensed matter. In this talk, we use a 2D layered material as

  15. Repulsive van der Waals forces in soft matter: why bubbles do not stick to walls.

    PubMed

    Tabor, Rico F; Manica, Rogerio; Chan, Derek Y C; Grieser, Franz; Dagastine, Raymond R

    2011-02-11

    Measurements of nonequilibrium hydrodynamic interactions between bubbles and solid surfaces in water provide direct evidence that repulsive van der Waals forces of quantum origin control the behavior of liquid films on solids in air. In addition to being the simplest and most universal 3-phase system, the deformable air-water interface greatly enhances the sensitivity of force measurements compared with rigid systems. The strength of the repulsive interaction, controlled by the choice of solid, is sufficient to prevent coalescence (sticking) on separation due to hydrodynamic interactions.

  16. Characterization of rarefaction waves in van der Waals fluids

    NASA Astrophysics Data System (ADS)

    Yuen, Albert; Barnard, John J.

    2015-12-01

    We calculate the isentropic evolution of an instantaneously heated foil, assuming a van der Waals equation of state with the Maxwell construction. The analysis by Yuen and Barnard [Phys. Rev. E 92, 033019 (2015), 10.1103/PhysRevE.92.033019] is extended for the particular case of three degrees of freedom. We assume heating to temperatures in the vicinity of the critical point. The self-similar profiles of the rarefaction waves describing the evolution of the foil display plateaus in density and temperature due to a phase transition from the single-phase to the two-phase regime. The hydrodynamic equations are expressed in a dimensionless form and the solutions form a set of universal curves, depending on a single parameter: the dimensionless initial entropy. We characterize the rarefaction waves by calculating how the plateau length, density, pressure, temperature, velocity, internal energy, and sound speed vary with dimensionless initial entropy.

  17. Flexible ferroelectric element based on van der Waals heteroepitaxy.

    PubMed

    Jiang, Jie; Bitla, Yugandhar; Huang, Chun-Wei; Do, Thi Hien; Liu, Heng-Jui; Hsieh, Ying-Hui; Ma, Chun-Hao; Jang, Chi-Yuan; Lai, Yu-Hong; Chiu, Po-Wen; Wu, Wen-Wei; Chen, Yi-Chun; Zhou, Yi-Chun; Chu, Ying-Hao

    2017-06-01

    We present a promising technology for nonvolatile flexible electronic devices: A direct fabrication of epitaxial lead zirconium titanate (PZT) on flexible mica substrate via van der Waals epitaxy. These single-crystalline flexible ferroelectric PZT films not only retain their performance, reliability, and thermal stability comparable to those on rigid counterparts in tests of nonvolatile memory elements but also exhibit remarkable mechanical properties with robust operation in bent states (bending radii down to 2.5 mm) and cycling tests (1000 times). This study marks the technological advancement toward realizing much-awaited flexible yet single-crystalline nonvolatile electronic devices for the design and development of flexible, lightweight, and next-generation smart devices with potential applications in electronics, robotics, automotive, health care, industrial, and military systems.

  18. Entanglement growth during Van der Waals like phase transition

    NASA Astrophysics Data System (ADS)

    Xu, Hao

    2017-09-01

    We address the problem of describing the coexistence state of two different black holes and Van der Waals like phase transition in Reissner-Nordström-AdS space-time. We start by a small charged black hole, then introduce a collapsing neutral thin-shell described by Vaidya metric to form a large one. The formation of the large black hole does not change the temperature and free energy of the initial state. We discuss the entanglement growing during the phase transition. The transition is always continuous and the saturation time is determined by the final state. It opens a possibility for studying the holography from excited states to excited states.

  19. Flexible ferroelectric element based on van der Waals heteroepitaxy

    PubMed Central

    Jiang, Jie; Bitla, Yugandhar; Huang, Chun-Wei; Do, Thi Hien; Liu, Heng-Jui; Hsieh, Ying-Hui; Ma, Chun-Hao; Jang, Chi-Yuan; Lai, Yu-Hong; Chiu, Po-Wen; Wu, Wen-Wei; Chen, Yi-Chun; Zhou, Yi-Chun; Chu, Ying-Hao

    2017-01-01

    We present a promising technology for nonvolatile flexible electronic devices: A direct fabrication of epitaxial lead zirconium titanate (PZT) on flexible mica substrate via van der Waals epitaxy. These single-crystalline flexible ferroelectric PZT films not only retain their performance, reliability, and thermal stability comparable to those on rigid counterparts in tests of nonvolatile memory elements but also exhibit remarkable mechanical properties with robust operation in bent states (bending radii down to 2.5 mm) and cycling tests (1000 times). This study marks the technological advancement toward realizing much-awaited flexible yet single-crystalline nonvolatile electronic devices for the design and development of flexible, lightweight, and next-generation smart devices with potential applications in electronics, robotics, automotive, health care, industrial, and military systems. PMID:28630922

  20. Extended version of the van der Waals capillarity theory.

    PubMed

    Baidakov, V G; Boltachev, G Sh

    2004-11-01

    An extended version of the van der Waals capillarity theory describing the liquid-vapor interface in the temperature range from the triple to the critical point is suggested. A model functional of thermodynamic potential for a two-phase Lennard-Jones system taking into account the effect of the highest degree terms of gradient expansion has been constructed. The identity of the thermodynamic and the mechanical definition of Tolman's length has been proved in the framework of the adopted form of functional. The properties of nuclei of the liquid and the vapor phase are described. The paper determines: the work of formation of a nucleus, density profiles, size dependences of the surface tension, and the parameter delta in the Gibbs-Tolman-Koenig-Buff equation.

  1. Characterization of rarefaction waves in van der Waals fluids.

    PubMed

    Yuen, Albert; Barnard, John J

    2015-12-01

    We calculate the isentropic evolution of an instantaneously heated foil, assuming a van der Waals equation of state with the Maxwell construction. The analysis by Yuen and Barnard [Phys. Rev. E 92, 033019 (2015)] is extended for the particular case of three degrees of freedom. We assume heating to temperatures in the vicinity of the critical point. The self-similar profiles of the rarefaction waves describing the evolution of the foil display plateaus in density and temperature due to a phase transition from the single-phase to the two-phase regime. The hydrodynamic equations are expressed in a dimensionless form and the solutions form a set of universal curves, depending on a single parameter: the dimensionless initial entropy. We characterize the rarefaction waves by calculating how the plateau length, density, pressure, temperature, velocity, internal energy, and sound speed vary with dimensionless initial entropy.

  2. Renormalization and Universality of Van der Waals forces

    NASA Astrophysics Data System (ADS)

    Ruiz Arriola, Enrique; Calle Cordón, Alvaro

    2010-04-01

    Renormalization ideas can profitably be exploited in conjunction with the superposition principle of boundary conditions in the description of model independent and universal scaling features of the singular and long range Van der Waals force between neutral atoms. The dominance of the leading power law is highlighted both in the scattering as well as in the bound state problem. The role of off-shell two-body unitarity and causality within the Effective Field Theory framework on the light of universality and scaling at low energies is analyzed. Presented by E. Ruiz Arriola at 19th International IUPAP Conference On Few-Body Problems In Physics (FB 19) 31 Aug - 5 Sep 2009, Bonn, Germany

  3. General van der Waals potential for common organic molecules.

    PubMed

    Qi, Rui; Wang, Qiantao; Ren, Pengyu

    2016-10-15

    This work presents a systematic development of a new van der Waals potential (vdW2016) for common organic molecules based on symmetry-adapted perturbation theory (SAPT) energy decomposition. The Buf-14-7 function, as well as Cubic-mean and Waldman-Hagler mixing rules were chosen given their best performance among other popular potentials. A database containing 39 organic molecules and 108 dimers was utilized to derive a general set of vdW parameters, which were further validated on nucleobase stacking systems and testing organic dimers. The vdW2016 potential is anticipated to significantly improve the accuracy and transferability of new generations of force fields for organic molecules. Copyright © 2016. Published by Elsevier Ltd.

  4. Thermohydrodynamics of boiling in a van der Waals fluid.

    PubMed

    Laurila, T; Carlson, A; Do-Quang, M; Ala-Nissila, T; Amberg, G

    2012-02-01

    We present a modeling approach that enables numerical simulations of a boiling Van der Waals fluid based on the diffuse interface description. A boundary condition is implemented that allows in and out flux of mass at constant external pressure. In addition, a boundary condition for controlled wetting properties of the boiling surface is also proposed. We present isothermal verification cases for each element of our modeling approach. By using these two boundary conditions we are able to numerically access a system that contains the essential physics of the boiling process at microscopic scales. Evolution of bubbles under film boiling and nucleate boiling conditions are observed by varying boiling surface wettability. We observe flow patters around the three-phase contact line where the phase change is greatest. For a hydrophilic boiling surface, a complex flow pattern consistent with vapor recoil theory is observed.

  5. SCF, IEPA, PNOCI, and CEPA2-PNO analysis of the anisotropy of (H2)2 interaction near the minimum in the van der Waals interaction

    NASA Astrophysics Data System (ADS)

    Burton, Peter G.; Senff, Ulrich E.

    1983-07-01

    The nature of the anisotropy of the H2-H2 interaction is investigated in some detail. The electron correlation effects and their influence toward the anisotropy of the (H2) dimer interaction are discussed in some detail. (AIP)

  6. Microwaves Probe Dipole Blockade and van der Waals Forces in a Cold Rydberg Gas

    NASA Astrophysics Data System (ADS)

    Teixeira, R. Celistrino; Hermann-Avigliano, C.; Nguyen, T. L.; Cantat-Moltrecht, T.; Raimond, J. M.; Haroche, S.; Gleyzes, S.; Brune, M.

    2015-07-01

    We show that microwave spectroscopy of a dense Rydberg gas trapped on a superconducting atom chip in the dipole blockade regime reveals directly the dipole-dipole many-body interaction energy spectrum. We use this method to investigate the expansion of the Rydberg cloud under the effect of repulsive van der Waals forces and the breakdown of the frozen gas approximation. This study opens a promising route for quantum simulation of many-body systems and quantum information transport in chains of strongly interacting Rydberg atoms.

  7. {InSe}/{GaSe} heterointerfaces prepared by Van der Waals epitaxy

    NASA Astrophysics Data System (ADS)

    Lang, O.; Klein, A.; Schlaf, R.; Löher, T.; Pettenkofer, C.; Jaegermann, W.; Chevy, A.

    1995-01-01

    Epitaxial films of layered substrates can be prepared onto layered substrates even for large lattice mismatch, when the growth is attempted with the Van der Waals surfaces opposing each other (Van der Waals epitaxy). Thin epitaxial InSe(GaSe) films are prepared onto ultrahigh vacuum (UHV) cleaved GaSe(InSe) Van der Waals (0001) surfaces. The films and the heterointerface are characterized by photoelectron spectroscopy, electron diffraction and scanning tunneling microscopy (STM). High quality and stoichiometric films are obtained by direct InSe(GaSe) evaporation from a Knudsen cell at sample temperatures between 520 and 620 K. Despite a 6% lattice mismatch the deposited films are oriented with their c- and α-axis to the hexagonal substrate. The growth mostly follows the Frank-Van der Merwe mode. This rather ideal growth behaviour is related to the specific properties of the Van der Waals plane which contains no dangling bonds.

  8. Influence of van der Waals forces on increasing the strength and toughness in dynamic fracture of nanofibre networks: a peridynamic approach

    NASA Astrophysics Data System (ADS)

    Bobaru, F.

    2007-07-01

    The peridynamic method is used here to analyse the effect of van der Waals forces on the mechanical behaviour and strength and toughness properties of three-dimensional nanofibre networks under imposed stretch deformation. The peridynamic formulation allows for a natural inclusion of long-range forces (such as van der Waals forces) by considering all interactions as 'long-range'. We use van der Waals interactions only between different fibres and do not need to model individual atoms. Fracture is introduced at the microstructural (peridynamic bond) level for the microelastic type bonds, while van der Waals bonds can reform at any time. We conduct statistical studies to determine a certain volume element for which the network of randomly oriented fibres becomes quasi-isotropic and insensitive to statistical variations. This qualitative study shows that the presence of van der Waals interactions and of heterogeneities (sacrificial bonds) in the strength of the bonds at the crosslinks between fibres can help in increasing the strength and toughness of the nanofibre network. Two main mechanisms appear to control the deformation of nanofibre networks: fibre reorientation (caused by deformation and breakage) and fibre accretion (due to van der Waals interaction). Similarities to the observed toughness of polymer adhesive in the abalone shell composition are explained. The author would like to dedicate this work to the 60th anniversary of Professor Subrata Mukherjee.

  9. Storage of methane and freon by interstitial van der Waals confinement.

    PubMed

    Atwood, Jerry L; Barbour, Leonard J; Jerga, Agoston

    2002-06-28

    A known host-guest assembly, organized only by means of relatively weak dispersive forces, exhibits hitherto unappreciated thermal stability. The hexagonal close-packed arrangement of calix[4]arene contains lattice voids that can occlude small, highly volatile molecules. This host-guest system can be exploited to retain a range of freons, as well as methane, not only well above their normal boiling points, but also at relatively high temperatures and low pressures. The usually overlooked van der Waals interactions in organic crystals can indeed be used in a highly stable supramolecular system for gas storage.

  10. van der Waals force between positronium and hydrogenic atoms Finite-mass corrections

    NASA Technical Reports Server (NTRS)

    Au, C. K.; Drachman, R. J.

    1986-01-01

    The Feshbach projection-operator formalism is used to derive the asymptotic effective interaction potential between two atoms. Beyond the usual van der Waals potential, falling like x exp -6, three x exp -8 terms are also obtained: an attractive dipole-quadrupole term (absent in the positronium-positronium case because of symmetry), a repulsive energy-dependent term, and a repulsive mass-dependent but energy-independent one. This last term was not obtained by Manson and Ritchie (1985) using an independent method.

  11. Application of Van Der Waals Density Functional Theory to Study Physical Properties of Energetic Materials

    NASA Astrophysics Data System (ADS)

    Conroy, M. W.; Budzevich, M. M.; Lin, Y.; Oleynik, I. I.; White, C. T.

    2009-12-01

    An empirical correction to account for van der Waals interactions based on the work of Neumann and Perrin [J. Phys. Chem. B 109, 15531 (2005)] was applied to density functional theory calculations of energetic molecular crystals. The calculated equilibrium unit-cell volumes of FOX-7, β-HMX, solid nitromethane, PETN-I, α-RDX, and TATB show a significant improvement in the agreement with experimental results. Hydrostatic-compression simulations of β-HMX, PETN-I, and α-RDX were also performed. The isothermal equations of state calculated from the results show increased agreement with experiment in the pressure intervals studied.

  12. Nonsingular van der Waals Potential and Its Contributions to Gravitational Coagulation.

    PubMed

    Wang, Hao; Wen, Ching-Sung

    2001-07-01

    The coagulation and the stability of dilute colloids at high Péclet number were studied in consideration of the universal nonsingular van der Waals interactions recently developed by J. X. Lu and W. H. Marlow (1995, Phys. Rev. Lett. 74, 1724). The capture efficiency of gravitational coagulation of uncharged colloids was found to be diminished due to the effect of finite molecular size (EFMS). The gravitational coagulation stability diagram of charged colloids was also found to be shifted when the gravitational convection was much stronger than the Brownian diffusion. Copyright 2001 Academic Press.

  13. Surface energy and wettability of van der Waals structures

    NASA Astrophysics Data System (ADS)

    Annamalai, Meenakshi; Gopinadhan, Kalon; Han, Sang A.; Saha, Surajit; Park, Hye Jeong; Cho, Eun Bi; Kumar, Brijesh; Patra, Abhijeet; Kim, Sang-Woo; Venkatesan, T.

    2016-03-01

    The wetting behaviour of surfaces is believed to be affected by van der Waals (vdW) forces; however, there is no clear demonstration of this. With the isolation of two-dimensional vdW layered materials it is possible to test this hypothesis. In this paper, we report the wetting behaviour of vdW heterostructures which include chemical vapor deposition (CVD) grown graphene, molybdenum disulfide (MoS2) and tungsten disulfide (WS2) on few layers of hexagon boron nitride (h-BN) and SiO2/Si. Our study clearly shows that while this class of two-dimensional materials are not completely wetting transparent, there seems to be a significant amount of influence on their wetting properties by the underlying substrate due to dominant vdW forces. Contact angle measurements indicate that graphene and graphene-like layered transitional metal dichalcogenides invariably have intrinsically dispersive surfaces with a dominating London-vdW force-mediated wettability.The wetting behaviour of surfaces is believed to be affected by van der Waals (vdW) forces; however, there is no clear demonstration of this. With the isolation of two-dimensional vdW layered materials it is possible to test this hypothesis. In this paper, we report the wetting behaviour of vdW heterostructures which include chemical vapor deposition (CVD) grown graphene, molybdenum disulfide (MoS2) and tungsten disulfide (WS2) on few layers of hexagon boron nitride (h-BN) and SiO2/Si. Our study clearly shows that while this class of two-dimensional materials are not completely wetting transparent, there seems to be a significant amount of influence on their wetting properties by the underlying substrate due to dominant vdW forces. Contact angle measurements indicate that graphene and graphene-like layered transitional metal dichalcogenides invariably have intrinsically dispersive surfaces with a dominating London-vdW force-mediated wettability. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr06705

  14. van der Waals screening by single-layer graphene and molybdenum disulfide.

    PubMed

    Tsoi, Stanislav; Dev, Pratibha; Friedman, Adam L; Stine, Rory; Robinson, Jeremy T; Reinecke, Thomas L; Sheehan, Paul E

    2014-12-23

    A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

  15. High broad-band photoresponsivity of mechanically formed InSe-graphene van der Waals heterostructures.

    PubMed

    Mudd, Garry W; Svatek, Simon A; Hague, Lee; Makarovsky, Oleg; Kudrynskyi, Zakhar R; Mellor, Christopher J; Beton, Peter H; Eaves, Laurence; Novoselov, Kostya S; Kovalyuk, Zakhar D; Vdovin, Evgeny E; Marsden, Alex J; Wilson, Neil R; Patanè, Amalia

    2015-07-01

    High broad-band photoresponsivity of mechanically formed InSe-graphene van der Waals heterostructures is achieved by exploiting the broad-band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near-infrared to the visible spectrum.

  16. Graphene Substrate for van der Waals Epitaxy of Layer-Structured Bismuth Antimony Telluride Thermoelectric Film.

    PubMed

    Kim, Eun Sung; Hwang, Jae-Yeol; Lee, Kyu Hyoung; Ohta, Hiromichi; Lee, Young Hee; Kim, Sung Wng

    2017-02-01

    Graphene as a substrate for the van der Waals epitaxy of 2D layered materials is utilized for the epitaxial growth of a layer-structured thermoelectric film. Van der Waals epitaxial Bi0.5 Sb1.5 Te3 film on graphene synthesized via a simple and scalable fabrication method exhibits good crystallinity and high thermoelectric transport properties comparable to single crystals.

  17. Binding and Diffusion of Lithium in Graphite: Quantum Monte Carlo Benchmarks and Validation of van der Waals Density Functional Methods

    DOE PAGES

    Ganesh, P.; Kim, Jeongnim; Park, Changwon; ...

    2014-11-03

    In highly accurate diffusion quantum Monte Carlo (QMC) studies of the adsorption and diffusion of atomic lithium in AA-stacked graphite are compared with van der Waals-including density functional theory (DFT) calculations. Predicted QMC lattice constants for pure AA graphite agree with experiment. Pure AA-stacked graphite is shown to challenge many van der Waals methods even when they are accurate for conventional AB graphite. Moreover, the highest overall DFT accuracy, considering pure AA-stacked graphite as well as lithium binding and diffusion, is obtained by the self-consistent van der Waals functional vdW-DF2, although errors in binding energies remain. Empirical approaches based onmore » point charges such as DFT-D are inaccurate unless the local charge transfer is assessed. Our results demonstrate that the lithium carbon system requires a simultaneous highly accurate description of both charge transfer and van der Waals interactions, favoring self-consistent approaches.« less

  18. Thermionic Energy Conversion Based on Graphene van der Waals Heterostructures

    PubMed Central

    Liang, Shi-Jun; Liu, Bo; Hu, Wei; Zhou, Kun; Ang, L. K.

    2017-01-01

    Seeking for thermoelectric (TE) materials with high figure of merit (or ZT), which can directly converts low-grade wasted heat (400 to 500 K) into electricity, has been a big challenge. Inspired by the concept of multilayer thermionic devices, we propose and design a solid-state thermionic devices (as a power generator or a refrigerator) in using van der Waals (vdW) heterostructure sandwiched between two graphene electrodes, to achieve high energy conversion efficiency in the temperature range of 400 to 500 K. The vdW heterostructure is composed of suitable multiple layers of transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2 and WSe2. From our calculations, WSe2 and MoSe2 are identified as two ideal TMDs (using the reported experimental material’s properties), which can harvest waste heat at 400 K with efficiencies about 7% to 8%. To our best knowledge, this design is the first in combining the advantages of graphene electrodes and TMDs to function as a thermionic-based device. PMID:28387363

  19. van der Waals forces, sum rules, and pseudostate expansions

    NASA Astrophysics Data System (ADS)

    Au, C. K.; Drachman, Richard J.

    1988-02-01

    We reexamine the pseudostate expansion used previously to evaluate van der Waals (vdW) coefficients and finite-mass corrections. In particular, we wish to understand the very rapid convergence of the expansion, which for the case of two hydrogenic atoms requires only ten terms to achieve an accuracy of 1×10-10. The analysis proceeds from the representation of the vdW coefficients as integrals of single-atom frequency-dependent polarizabilities. These in turn can be expanded in power series with coefficients of the form JnM2(nl)/(Enl-E1s)k, where M(nl) is a multipole matrix element between the ground (1s) and excited (nl) states. Although defined as sums over an infinite set of hydrogenic states, the coefficients of each order k are reproduced exactly by a finite sum over suitable pseudostates. By analytic continuation, the frequency-dependent polarizabilities computed using pseudostates are expected to be very close to the exact values, and hence the rapid convergence of the vdW coefficients is understood.

  20. Ion dip spectroscopy of van der Waals clusters

    SciTech Connect

    Stanley, R.J.; Castleman, A.W. Jr. )

    1990-05-15

    We report the implementation of ion dip spectroscopy in a supersonic molecular beam time-of-flight mass spectrometer as a powerful mass-selective method for observing ground-state vibrational levels in van der Waals clusters. Ion dip spectra of phenylacetylene and phenylacetylene-NH{sub 3} are demonstrated in the range of 900--1100 cm{sup {minus}1}, showing prominent dips at 978.0, 1002.8, and 1028.0 cm{sup {minus}1}. These dips have been tentatively assigned as 13{sup 0}{sub 1} 35{sup 1}{sub 1}, 11{sup 0}{sub 1} 35{sup 1}{sub 0}, and 35{sup 1}{sub 2}, respectively, in phenylacetylene. Shifts in the 35{sup 1}{sub 2} and 11{sup 0}{sub 1}35{sup 1}{sub 0} vibrational bands of the complex are observed while the 13{sup 0}{sub 1}35{sup 1}{sub 1} band of the complex is either shifted or attenuated.

  1. Rotational dissociation of impulsively aligned van der Waals complexes

    NASA Astrophysics Data System (ADS)

    Søndergaard, Anders A.; Zillich, Robert E.; Stapelfeldt, Henrik

    2017-08-01

    The nonadiabatic alignment dynamics of weakly bound molecule-atom complexes, induced by a moderately intense 300 fs nonresonant laser pulse, is calculated by direct numerical solution of the time-dependent Schrödinger equation. Our method propagates the wave function according to the coupled channel equations for the complex, which can be done in a very efficient and stable manner out to large times. We present results for two van der Waal complexes, CS2-He and HCCH-He, as respective examples of linear molecules with large and small moments of inertia. Our main result is that at intensities typical of nonadiabatic alignment experiments, these complexes rapidly dissociate. In the case of the CS2-He complex, the ensuing rotational dynamics resembles that of isolated molecules, whereas for the HCCH-He complex, the detachment of the He atom severely perturbs and essentially quenches the subsequent rotational motion. At intensities of the laser pulse ≲2.0 × 1012 W/cm2, it is shown that the molecule-He complex can rotate and align without breaking apart. We discuss the implications of our findings for recent experiments on iodine molecules solvated in helium nanodroplets.

  2. Infrared photodetectors based on graphene van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Ryzhii, V.; Ryzhii, M.; Svintsov, D.; Leiman, V.; Mitin, V.; Shur, M. S.; Otsuji, T.

    2017-08-01

    We propose and evaluate the graphene layer (GL) infrared photodetectors (GLIPs) based on the van der Waals (vdW) heterostructures with the radiation absorbing GLs. The operation of the GLIPs is associated with the electron photoexcitation from the GL valence band to the continuum states above the inter-GL barriers (either via tunneling or direct transitions to the continuum states). Using the developed device model, we calculate the photodetector characteristics as functions of the GL-vdW heterostructure parameters. We show that due to a relatively large efficiency of the electron photoexcitation and low capture efficiency of the electrons propagating over the barriers in the inter-GL layers, GLIPs should exhibit the elevated photoelectric gain and detector responsivity as well as relatively high detectivity. The possibility of high-speed operation, high conductivity, transparency of the GLIP contact layers, and the sensitivity to normally incident IR radiation provides additional potential advantages in comparison with other IR photodetectors. In particular, the proposed GLIPs can compete with unitravelling-carrier photodetectors.

  3. Thermionic Energy Conversion Based on Graphene van der Waals Heterostructures

    NASA Astrophysics Data System (ADS)

    Liang, Shi-Jun; Liu, Bo; Hu, Wei; Zhou, Kun; Ang, L. K.

    2017-04-01

    Seeking for thermoelectric (TE) materials with high figure of merit (or ZT), which can directly converts low-grade wasted heat (400 to 500 K) into electricity, has been a big challenge. Inspired by the concept of multilayer thermionic devices, we propose and design a solid-state thermionic devices (as a power generator or a refrigerator) in using van der Waals (vdW) heterostructure sandwiched between two graphene electrodes, to achieve high energy conversion efficiency in the temperature range of 400 to 500 K. The vdW heterostructure is composed of suitable multiple layers of transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2 and WSe2. From our calculations, WSe2 and MoSe2 are identified as two ideal TMDs (using the reported experimental material’s properties), which can harvest waste heat at 400 K with efficiencies about 7% to 8%. To our best knowledge, this design is the first in combining the advantages of graphene electrodes and TMDs to function as a thermionic-based device.

  4. Robust 2D topological insulators in van der Waals heterostructures.

    PubMed

    Kou, Liangzhi; Wu, Shu-Chun; Felser, Claudia; Frauenheim, Thomas; Chen, Changfeng; Yan, Binghai

    2014-10-28

    We predict a family of robust two-dimensional (2D) topological insulators in van der Waals heterostructures comprising graphene and chalcogenides BiTeX (X = Cl, Br, and I). The layered structures of both constituent materials produce a naturally smooth interface that is conducive to proximity-induced topological states. First-principles calculations reveal intrinsic topologically nontrivial bulk energy gaps as large as 70-80 meV, which can be further enhanced up to 120 meV by compression. The strong spin-orbit coupling in BiTeX has a significant influence on the graphene Dirac states, resulting in the topologically nontrivial band structure, which is confirmed by calculated nontrivial Z2 index and an explicit demonstration of metallic edge states. Such heterostructures offer a unique Dirac transport system that combines the 2D Dirac states from graphene and 1D Dirac edge states from the topological insulator, and it offers ideas for innovative device designs.

  5. Local Probe Spectroscopy of Two-Dimensional van der Waals Heterostructures

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew Abraham

    A large family of materials, collectively known as "van der Waals materials", have attracted enormous research attention over the past decade following the realization that they could be isolated into individual crystalline monolayers, with charge carriers behaving effectively two-dimensionally. More recently, an even larger class of composite materials has been realized, made possible by combining the isolated atomic layers of different materials into "van der Waals heterostructures", which can exhibit electronic and optical behaviors not observed in the parent materials alone. This thesis describes efforts to characterize the atomic-scale structural and electronic properties of these van der Waals materials and heterostructures through scanning tunneling microscopy measurements. The majority of this work addresses the properties of monolayer and few-layer graphene, whose charge carriers are described by massless and massive chiral Dirac Hamiltonians, respectively. In heterostructures with hexagonal boron nitride, an insulating isomorph of graphene, we observe electronic interference patterns between the two materials which depend on their relative rotation. As a result, replica Dirac cones are formed in the valence and conduction bands of graphene, with their energy tuned by the rotation. Further, we are able to dynamically drag the graphene lattice in these heterostructures, owing to an interaction between the scanning probe tip and the domain walls formed by the electronic interference pattern. Similar dragging is observed in domain walls of trilayer graphene, whose electronic properties are found to depend on the stacking configuration of the three layers. Scanning tunneling spectroscopy provides a direct method for visualizing the scattering pathways of electrons in these materials. By analyzing the scattering, we can directly infer properties of the band structures and local environments of these heterostructures. In bilayer graphene, we map the electrically

  6. Self-consistent van der Waals density functional: Development and Applications

    NASA Astrophysics Data System (ADS)

    Cooper, Valentino

    2008-03-01

    The inability of density functional theory (DFT), with standard exchange-correlation functionals, to correctly describe van der Waals/dispersion (vdW) interactions has severely limited its applicability to sparsely packed systems, such as organic and biological molecules. Numerous attempts have been made to correct these deficiencies; however, many of them either require extensive reparameterization for each new situation or scale poorly with system size. In this paper, I will discuss the development and implementation of an exchange-correlation functional which correctly incorporates non-local vdW interactions within DFT (vdW-DF)ootnotetextM. Dion, H. Rydberg, E. Schröder, B. I. Lundqvist and D. C. Langreth, Phys. Rev. Lett., 92, 246401 (2004). In addition, I will present our recent development of the corresponding exchange- correlation potential (Vxc)ootnotetextT. Thonhauser, V. R. Cooper, S. Li, A. Puzder, P. Hyldgaard, and David C. Langreth, Phys. Rev. B, 76, 125112 (2007). The Vxc gives us the ability to compute Hellmann-Feynman forces, allowing for structural relaxations and molecular dynamics simulation. Using the Vxc I will examine the nature of the van der Waals bond between molecules. Finally, to demonstrate the power of the vdW-DF, I will discuss our relatively large scale application of the functional to study the influence of stacking interactions on the structure and stability of DNA. Here, I will show how these interactions are crucial for defining the twist and base pair separation in DNA and how methyl-nucleobase and methyl-methyl interactions give additional stability to DNA.

  7. Strain-Induced Electronic Structure Changes in Stacked van der Waals Heterostructures.

    PubMed

    He, Yongmin; Yang, Yang; Zhang, Zhuhua; Gong, Yongji; Zhou, Wu; Hu, Zhili; Ye, Gonglan; Zhang, Xiang; Bianco, Elisabeth; Lei, Sidong; Jin, Zehua; Zou, Xiaolong; Yang, Yingchao; Zhang, Yuan; Xie, Erqing; Lou, Jun; Yakobson, Boris; Vajtai, Robert; Li, Bo; Ajayan, Pulickel

    2016-05-11

    Vertically stacked van der Waals heterostructures composed of compositionally different two-dimensional atomic layers give rise to interesting properties due to substantial interactions between the layers. However, these interactions can be easily obscured by the twisting of atomic layers or cross-contamination introduced by transfer processes, rendering their experimental demonstration challenging. Here, we explore the electronic structure and its strain dependence of stacked MoSe2/WSe2 heterostructures directly synthesized by chemical vapor deposition, which unambiguously reveal strong electronic coupling between the atomic layers. The direct and indirect band gaps (1.48 and 1.28 eV) of the heterostructures are measured to be lower than the band gaps of individual MoSe2 (1.50 eV) and WSe2 (1.60 eV) layers. Photoluminescence measurements further show that both the direct and indirect band gaps undergo redshifts with applied tensile strain to the heterostructures, with the change of the indirect gap being particularly more sensitive to strain. This demonstration of strain engineering in van der Waals heterostructures opens a new route toward fabricating flexible electronics.

  8. Understanding corrosion inhibition with van der Waals DFT methods: the case of benzotriazole.

    PubMed

    Gattinoni, Chiara; Michaelides, Angelos

    2015-01-01

    The corrosion of materials is an undesirable and costly process affecting many areas of technology and everyday life. As such, considerable effort has gone into understanding and preventing it. Organic molecule based coatings can in certain circumstances act as effective corrosion inhibitors. Although they have been used to great effect for more than sixty years, how they function at the atomic-level is still a matter of debate. In this work, computer simulation approaches based on density functional theory are used to investigate benzotriazole (BTAH), one of the most widely used and studied corrosion inhibitors for copper. In particular, the structures formed by protonated and deprotonated BTAH molecules on Cu(111) have been determined and linked to their inhibiting properties. It is found that hydrogen bonding, van der Waals interactions and steric repulsions all contribute in shaping how BTAH molecules adsorb, with flat-lying structures preferred at low coverage and upright configurations preferred at high coverage. The interaction of the dehydrogenated benzotriazole molecule (BTA) with the copper surface is instead dominated by strong chemisorption via the azole moiety with the aid of copper adatoms. Structures of dimers or chains are found to be the most stable structures at all coverages, in good agreement with scanning tunnelling microscopy results. Benzotriazole thus shows a complex phase behaviour in which van der Waals forces play an important role and which depends on coverage and on its protonation state and all of these factors feasibly contribute to its effectiveness as a corrosion inhibitor.

  9. Ab initio studies of the interaction potential for the Xe-NO(X2Π) van der Waals complex: bound states and fully quantum and quasi-classical scattering.

    PubMed

    Kłos, J; Aoiz, F J; Menéndez, M; Brouard, M; Chadwick, H; Eyles, C J

    2012-07-07

    Adiabatic potential energy surfaces for the ground electronic state of the Xe⋅⋅⋅NO(X(2)Π) van der Waals complex have been calculated using the spin-restricted coupled cluster method with single, double, and non-iterative triple excitations (RCCSD(T)). The scalar relativistic effects present in the Xe atom were included by an effective core potential and we extended the basis with bond functions to improve the description of the dispersion interaction. It has been found that the global minimum on the A(') adiabatic surface occurs at a T-shaped geometry with γ(e) = 94° and R(e) = 7.46 a(0), and with well depth of D(e) = 148.68 cm(-1). There is also an additional local minimum for the collinear geometry Xe-NO with a well depth of 104.5 cm(-1). The adiabat of A('') symmetry exhibits a single minimum at a distance R(e) = 7.68 a(0) and has a skewed geometry with γ(e) = 64° and a well depth of 148.23 cm(-1). Several C(nl) van der Waals dispersion coefficients are also estimated, of which C(6, 0) and C(6, 2) are in a reasonable agreement with previous theoretical results obtained by Nielson et al. [J. Chem. Phys. 64, 2055 (1976)]. The new potential energy surfaces were used to calculate bound states of the complex for total angular momentum quantum numbers up to J = 7/2. The ground state energy of Xe⋅⋅⋅NO(X(2)Π) is D(0) = 117 cm(-1), which matches the experimental value very accurately (within 3.3%). Scattering calculations of integral and differential cross sections have also been performed using fully quantum close coupling calculations and quasi-classical trajectory method at a collision energy of 63 meV. These calculations reveal the important role played by L-type rainbows in the scattering dynamics of the heavier Rg-NO(X) systems.

  10. Discriminating short-range from van der Waals forces using total force data in noncontact atomic force microscopy

    NASA Astrophysics Data System (ADS)

    Kuhn, Stefan; Rahe, Philipp

    2014-06-01

    Noncontact atomic force microscopy (NC-AFM) features the measurement of forces with highest spatial resolution and sensitivity, resolving forces of the order of pico-Newtons with submolecular resolution. However, the measured total force is a mixture composed of various interactions. While some interactions such as electrostatic or magnetic forces can be excluded by a careful design of the experiment, the subtraction of van der Waals forces, which mainly originate from London dispersion interactions between the macroscopic tip shank and the bulk sample, remains a challenge. We present the determination of the inherently present van der Waals forces in total interaction force data from fitting a suitable model, allowing for extraction of the short-range force component. We compare the applicability of several van der Waals models based on experimental interaction data from the calcite(101¯4) surface. The feasibility to fit these models to experimental data is critically discussed. We furthermore introduce criteria to assess the transition point from pure long-range interaction to mixed short- and long-range forces based on the variance of lateral and vertical force data. This determination allows us to extract the short-range interaction forces, which remained a challenge so far in NC-AFM experiments.

  11. van der Waals forces in density functional theory: a review of the vdW-DF method

    DOE PAGES

    Berland, Kristian; Cooper, Valentino R.; Lee, Kyuho; ...

    2015-05-15

    We review a density functional theory (DFT) that accounts for van der Waals (vdW) interactions in condensed matter, materials physics, chemistry, and biology. The insights that led to the construction of the Rutgers–Chalmers van der Waals density functional (vdW-DF) are presented with the aim of giving a historical perspective, while also emphasizing more recent efforts which have sought to improve its accuracy. In addition to technical details, we discuss a range of recent applications that illustrate the necessity of including dispersion interactions in DFT. This review highlights the value of the vdW-DF method as a general-purpose method, not only formore » dispersion bound systems, but also in densely packed systems where these types of interactions are traditionally thought to be negligible.« less

  12. van der Waals forces in density functional theory: a review of the vdW-DF method

    SciTech Connect

    Berland, Kristian; Cooper, Valentino R.; Lee, Kyuho; Schröder, Elsebeth; Thonhauser, T.; Hyldgaard, Per; Lundqvist, Bengt I.

    2015-05-15

    We review a density functional theory (DFT) that accounts for van der Waals (vdW) interactions in condensed matter, materials physics, chemistry, and biology. The insights that led to the construction of the Rutgers–Chalmers van der Waals density functional (vdW-DF) are presented with the aim of giving a historical perspective, while also emphasizing more recent efforts which have sought to improve its accuracy. In addition to technical details, we discuss a range of recent applications that illustrate the necessity of including dispersion interactions in DFT. This review highlights the value of the vdW-DF method as a general-purpose method, not only for dispersion bound systems, but also in densely packed systems where these types of interactions are traditionally thought to be negligible.

  13. van der Waals forces in density functional theory: a review of the vdW-DF method

    NASA Astrophysics Data System (ADS)

    Berland, Kristian; Cooper, Valentino R.; Lee, Kyuho; Schröder, Elsebeth; Thonhauser, T.; Hyldgaard, Per; Lundqvist, Bengt I.

    2015-06-01

    A density functional theory (DFT) that accounts for van der Waals (vdW) interactions in condensed matter, materials physics, chemistry, and biology is reviewed. The insights that led to the construction of the Rutgers-Chalmers van der Waals density functional (vdW-DF) are presented with the aim of giving a historical perspective, while also emphasizing more recent efforts which have sought to improve its accuracy. In addition to technical details, we discuss a range of recent applications that illustrate the necessity of including dispersion interactions in DFT. This review highlights the value of the vdW-DF method as a general-purpose method, not only for dispersion bound systems, but also in densely packed systems where these types of interactions are traditionally thought to be negligible.

  14. Structural and quantum properties of van der Waals cluster near the unitary regime

    NASA Astrophysics Data System (ADS)

    Lekala, M. L.; Chakrabarti, B.; Haldar, S. K.; Roy, R.; Rampho, G. J.

    2017-07-01

    We study the structural and several quantum properties of three-dimensional bosonic cluster interacting through van der Waals potential at large scattering length. We use Faddeev-type decomposition of the many-body wave function which includes all possible two-body correlations. At large scattering length, we observe spatially extended states which exhibit the exponential dependence on the state number. The cluster ground state energy shows universal nature at large negative scattering length. We also find the existence of generalized Tjon lines for N-body clusters. Signature of universal behaviour of weakly bound clusters can be observed in experiments of ultracold Bose gases. We also study the spectral statistics of the system. We calculate both the short-range fluctuation and long-range correlation and observe semi-Poisson distribution which interpolates the Gaussian Orthogonal Ensemble (GOE) and Poisson statistics of random matrix theory. It indicates that the van der Waal cluster near the unitary becomes highly complex and correlated. However additional study of P (r) distribution (without unfolding of energy spectrum) reveals the possibility of chaos for larger cluster.

  15. Clamping instability and van der Waals forces in carbon nanotube mechanical resonators.

    PubMed

    Aykol, Mehmet; Hou, Bingya; Dhall, Rohan; Chang, Shun-Wen; Branham, William; Qiu, Jing; Cronin, Stephen B

    2014-05-14

    We investigate the role of weak clamping forces, typically assumed to be infinite, in carbon nanotube mechanical resonators. Due to these forces, we observe a hysteretic clamping and unclamping of the nanotube device that results in a discrete drop in the mechanical resonance frequency on the order of 5-20 MHz, when the temperature is cycled between 340 and 375 K. This instability in the resonant frequency results from the nanotube unpinning from the electrode/trench sidewall where it is bound weakly by van der Waals forces. Interestingly, this unpinning does not affect the Q-factor of the resonance, since the clamping is still governed by van der Waals forces above and below the unpinning. For a 1 μm device, the drop observed in resonance frequency corresponds to a change in nanotube length of approximately 50-65 nm. On the basis of these findings, we introduce a new model, which includes a finite tension around zero gate voltage due to van der Waals forces and shows better agreement with the experimental data than the perfect clamping model. From the gate dependence of the mechanical resonance frequency, we extract the van der Waals clamping force to be 1.8 pN. The mechanical resonance frequency exhibits a striking temperature dependence below 200 K attributed to a temperature-dependent slack arising from the competition between the van der Waals force and the thermal fluctuations in the suspended nanotube.

  16. Colossal terahertz nonlinearity of tunneling van der Waals gap (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Bahk, Young-Mi; Kang, Bong Joo; Kim, Yong Seung; Kim, Joon-Yeon; Kim, Won Tae; Kim, Tae Yun; Kang, Taehee; Rhie, Ji Yeah; Han, Sanghoon; Park, Cheol-Hwan; Rotermund, Fabian; Kim, Dai-Sik

    2016-09-01

    We manufactured an array of three angstrom-wide, five millimeter-long van der Waals gaps of copper-graphene-copper composite, in which unprecedented nonlinearity was observed. To probe and manipulate van der Waals gaps with long wavelength electromagnetic waves such as terahertz waves, one is required to fabricate vertically oriented van der Waals gaps sandwiched between two metal planes with an infinite length in the sense of being much larger than any of the wavelengths used. By comparison with the simple vertical stacking of metal-graphene-metal structure, in our structure, background signals are completely blocked enabling all the light to squeeze through the gap without any strays. When the angstrom-sized van der Waals gaps are irradiated with intense terahertz pulses, the transient voltage across the gap reaches up to 5 V with saturation, sufficiently strong to deform the quantum barrier of angstrom gaps. The large transient potential difference across the gap facilitates electron tunneling through the quantum barrier, blocking terahertz waves completely. This negative feedback of electron tunneling leads to colossal nonlinear optical response, a 97% decrease in the normalized transmittance. Our technology for infinitely long van der Waals gaps can be utilized for other atomically thin materials than single layer graphene, enabling linear and nonlinear angstrom optics in a broad spectral range.

  17. Application of an empirical dispersion potential to van der Waals binding in nitromethane, pentaerythritol, and pentaerythritol tetranitrate

    NASA Astrophysics Data System (ADS)

    Slough, Wil; Perger, W. F.

    2010-09-01

    The use of an empirical dispersion potential to accurately represent the van der Waals interaction in molecular crystals is discussed. Results for nitromethane, pentaerythritol (PE), and pentaerythritol tetranitrate (PETN) are presented for equilibrium volumes using a variety of basis sets ranging in quality in the CRYSTAL program. Agreement with experiment for equilibrium volumes is found to be within a few percent when the Grimme potential and a high-quality basis set is used.

  18. Chemical sensing by band modulation of a black phosphorus/molybdenum diselenide van der Waals hetero-structure

    NASA Astrophysics Data System (ADS)

    Feng, Zhihong; Chen, Buyun; Qian, Shuangbei; Xu, Linyan; Feng, Liefeng; Yu, Yuanyuan; Zhang, Rui; Chen, Jiancui; Li, Qianqian; Li, Quanning; Sun, Chongling; Zhang, Hao; Liu, Jing; Pang, Wei; Zhang, Daihua

    2016-09-01

    We report on a new chemical sensor based on black phosphorus/molybdenum diselenide van der Waals hetero-junctions. Due to the atomically thin nature of two-dimensional (2D) materials, surface adsorption of gas molecules can effectively modulate the band alignment at the junction interface, making the device a highly sensitive detector for chemical adsorptions. Compared to sensors made of homogeneous nanomaterials, the hetero-junction demonstrates considerably lower detection limit and higher sensitivity toward nitrogen dioxide. Kelvin probe force microscopy and finite element simulations have provided experimental and theoretical explanations for the enhanced performance, proving that chemical adsorption can induce significant changes in band alignment and carrier transport behaviors. The study demonstrates the potential of van der Waals hetero-junction as a new platform for sensing applications, and provides more insights into the interaction between gaseous molecules and 2D hetero-structures.

  19. The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures.

    PubMed

    Wang, Han; Bang, Junhyeok; Sun, Yiyang; Liang, Liangbo; West, Damien; Meunier, Vincent; Zhang, Shengbai

    2016-05-10

    The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the interface. Application to the MoS2/WS2 heterostructure yields good agreement with experiments, indicating near complete charge transfer within a timescale of 100 fs.

  20. Electron transfer and coupling in graphene-tungsten disulfide van der Waals heterostructures.

    PubMed

    He, Jiaqi; Kumar, Nardeep; Bellus, Matthew Z; Chiu, Hsin-Ying; He, Dawei; Wang, Yongsheng; Zhao, Hui

    2014-11-25

    The newly discovered two-dimensional materials can be used to form atomically thin and sharp van der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements: electron transfer across the interface and interlayer coupling. Here we show, using graphene-tungsten disulfide heterostructures as an example, evidence of ultrafast and highly efficient interlayer electron transfer and strong interlayer coupling and control. We find that photocarriers injected in tungsten disulfide transfer to graphene in 1 ps and with near-unity efficiency. We also demonstrate that optical properties of tungsten disulfide can be effectively tuned by carriers in graphene. These findings illustrate basic processes required for using van der Waals heterostructures in electronics and photonics.

  1. Seebeck Coefficient of a Single van der Waals Junction in Twisted Bilayer Graphene.

    PubMed

    Mahapatra, Phanibhusan S; Sarkar, Kingshuk; Krishnamurthy, H R; Mukerjee, Subroto; Ghosh, Arindam

    2017-10-04

    When two planar atomic membranes are placed within the van der Waals distance, the charge and heat transport across the interface are coupled by the rules of momentum conservation and structural commensurability, leading to outstanding thermoelectric properties. Here we show that an effective "interlayer phonon drag" determines the Seebeck coecient (S) across the van der Waals gap formed in twisted bilayer graphene (tBLG). The cross-plane thermovoltage, which is nonmonotonic in both temperature and density, is generated through scattering of electrons by the out-of-plane layer breathing (ZO'/ZA2) phonon modes and differs dramatically from the expected Landauer-Buttiker formalism in conventional tunnel junctions. The tunability of the cross-plane Seebeck effect in van der Waals junctions may be valuable in creating a new genre of versatile thermoelectric systems with layered solids.

  2. Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride.

    PubMed

    Dai, S; Fei, Z; Ma, Q; Rodin, A S; Wagner, M; McLeod, A S; Liu, M K; Gannett, W; Regan, W; Watanabe, K; Taniguchi, T; Thiemens, M; Dominguez, G; Castro Neto, A H; Zettl, A; Keilmann, F; Jarillo-Herrero, P; Fogler, M M; Basov, D N

    2014-03-07

    van der Waals heterostructures assembled from atomically thin crystalline layers of diverse two-dimensional solids are emerging as a new paradigm in the physics of materials. We used infrared nanoimaging to study the properties of surface phonon polaritons in a representative van der Waals crystal, hexagonal boron nitride. We launched, detected, and imaged the polaritonic waves in real space and altered their wavelength by varying the number of crystal layers in our specimens. The measured dispersion of polaritonic waves was shown to be governed by the crystal thickness according to a scaling law that persists down to a few atomic layers. Our results are likely to hold true in other polar van der Waals crystals and may lead to new functionalities.

  3. The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures

    PubMed Central

    Wang, Han; Bang, Junhyeok; Sun, Yiyang; Liang, Liangbo; West, Damien; Meunier, Vincent; Zhang, Shengbai

    2016-01-01

    The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the interface. Application to the MoS2/WS2 heterostructure yields good agreement with experiments, indicating near complete charge transfer within a timescale of 100 fs. PMID:27160484

  4. van der Waals Contribution to the Relative Stability of Aqueous Zn(2+) Coordination States.

    PubMed

    Ducher, Manoj; Pietrucci, Fabio; Balan, Etienne; Ferlat, Guillaume; Paulatto, Lorenzo; Blanchard, Marc

    2017-07-11

    Many properties of aqueous cations depend on their coordination state. However, the lack of long-range order and the dynamic character of aqueous solutions make it difficult to obtain information beyond average coordination parameters. A thorough understanding of the molecular-scale environment of aqueous cations usually requires a combination of experimental and theoretical approaches. In the case of Zn(2+), significant discrepancies occur among theoretical investigations based on first-principles molecular dynamics (FPMD) or free-energy calculations, although experimental data consistently point to a dominant hexaaquo-zinc complex (Zn[H2O]6)(2+) in pure water. In the present study, the aqueous speciation of zinc is theoretically investigated by combining FPMD simulations and free-energy calculations based on metadynamics and umbrella-sampling strategies. The simulations are carried out within the density functional theory (DFT) framework using for the exchange-correlation functional either a standard generalized gradient approximation (GGA) or a nonlocal functional (vdw-DF2) which includes van der Waals interactions. The theoretical environment of Zn is confronted to experiment by comparing calculated and measured X-ray absorption spectra. It is shown that the inclusion of van der Waals interactions is crucial for the correct modeling of zinc aqueous speciation, whereas GGA incorrectly favors tetraaquo- (Zn[H2O]4)(2+) and pentaaquo-zinc (Zn[H2O]5)(2+) complexes, results obtained with the vdW-DF2 functional show that the hexaaquo-zinc complex is more stable than the tetraaquo and pentaaquo-zinc complexes by 13 and by 4 kJ mol(-1), respectively. These results highlight the critical importance of even subtle interactions for the correct balance of different coordination states in aqueous solutions. However, for a given coordination state, GGA leads to a reasonable description of the geometry of the aqueous complex.

  5. A fully consistent spin formalism for the nonempirical van der Waals density functional vdW-DF

    NASA Astrophysics Data System (ADS)

    Thonhauser, T.; Zuluaga, S.; Arter, C.; Berland, K.; Schröder, E.; Hyldgaard, P.

    We present a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the van der Waals driven adsorption of H2 and CO2 in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO2 adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO2 would suggest. We explain this counter-intuitive result, tracking the behavior to a coincidental hybridization of the O p states with the Ni d states in the down-spin channel. More generally, by providing insight on van der Waals interactions in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions. Work supported by DOE DE-FG02-08ER46491 and NSF DMR-1145968.

  6. van der Waals equation of state for a fluid in a nanopore.

    PubMed

    Zarragoicoechea, Guillermo J; Kuz, Víctor A

    2002-02-01

    A generalization of the van der Waals equation of state is presented for a confined fluid in a nanopore. The pressure in the fluid, confined in a narrow pore of infinite length, has tensorial character. From this hypothesis, the Helmholtz free energy is constructed and expressions for the axial and transversal components of the pressure tensor are obtained. The equations predict liquid-vapor equilibria, and a shift of the critical point with respect to that obtained from the van der Waals bulk equation. The results are in good agreement with recent experiments.

  7. Using computation to teach the properties of the van der Waals fluid

    NASA Astrophysics Data System (ADS)

    Swendsen, Robert H.

    2013-10-01

    The calculation of the thermodynamic properties of the van der Waals fluid is not trivial and most of its properties are rarely discussed because of mathematical difficulties. I describe a numerical approach that produces the full thermodynamic behavior of the van der Waals fluid with little effort. The numerical approach is particularly useful for showing the behavior of the specific heat, the isothermal compressibility, and the coefficient of thermal expansion at and near the critical point. The results of these computations show some surprising properties and give new insights into the mean-field description of the liquid-gas transition.

  8. Near-Unity Absorption in van der Waals Semiconductors for Ultrathin Optoelectronics.

    PubMed

    Jariwala, Deep; Davoyan, Artur R; Tagliabue, Giulia; Sherrott, Michelle C; Wong, Joeson; Atwater, Harry A

    2016-09-14

    We demonstrate near-unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (<15 nm) van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of the key criteria to enable TMDCs as potential candidates to achieve high optoelectronic efficiency.

  9. Adsorption of polyiodobenzene molecules on the Pt(111) surface using van der Waals density functional theory

    NASA Astrophysics Data System (ADS)

    Johnston, Karen; Pekoz, Rengin; Donadio, Davide

    2016-02-01

    Adsorption of aromatic molecules on surfaces is widely studied due to applications in molecular electronics. In this work, the adsorption of iodobenzene molecules on the Pt(111) surface has been studied using density functional theory. Iodobenzene molecules, with various numbers of iodine atoms, have two non-dissociative adsorption minima. One structure exhibits chemisorption between the ring and the surface (short-range) and the other structure exhibits chemisorption between the iodine ions and the surface (long-range). Both structures have a strong van der Waals interaction with the surface. In general, the adsorption energy increases as the number of iodine atoms increases. The dissociated structure of monoiodobenzene was investigated, and the dissociation barrier and the barrier between the short- and long-range states were compared.

  10. Microwave Observation of the Van Der Waals Complex O_2-CO

    NASA Astrophysics Data System (ADS)

    Marshall, Frank E.; Persinger, Thomas D.; Gillcrist, David Joseph; Moon, Nicole; Ndengue, Steve Alexandre; Dawes, Richard; Grubbs, G. S., II

    2016-06-01

    FTMW spectroscopy has long been known to be a powerful tool in characterizing van der Waals complexes. Along with this, advances in microwave technology and computing have made complicated spin-interaction systems much easier to observe and characterize. One such system, O_2-CO has been observed for the first time on a CP-FTMW spectrometer operational in the 6-18 GHz region. Preliminary observations and calculations indicate a slipped-parallel structure. High level calculations are ongoing, including the construction of a 4D potential energy surface. Rotational assignments, along with any observed fine structure due to the ^3Σ ground state of O_2 will be discussed. Stewart Novick, Bibliography of Rotational Spectra of Weakly Bound Complexes

  11. Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures.

    PubMed

    Miller, Bastian; Steinhoff, Alexander; Pano, Borja; Klein, Julian; Jahnke, Frank; Holleitner, Alexander; Wurstbauer, Ursula

    2017-09-13

    We report the observation of a doublet structure in the low-temperature photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe2 and WSe2. Both peaks exhibit long photoluminescence lifetimes of several tens of nanoseconds up to 100 ns verifying the interlayer nature of the excitons. The energy and line width of both peaks show unusual temperature and power dependences. While the low-energy peak dominates the spectra at low power and low temperatures, the high-energy peak dominates for high power and temperature. We explain the findings by two kinds of interlayer excitons being either indirect or quasi-direct in reciprocal space. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions.

  12. Van der Waals-coupled electronic states in incommensurate double-walled carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Liu, Kaihui; Jin, Chenhao; Hong, Xiaoping; Kim, Jihoon; Zettl, Alex; Wang, Enge; Wang, Feng

    2014-10-01

    Non-commensurate two-dimensional materials such as a twisted graphene bilayer or graphene on boron nitride, consisting of components that have no finite common unit cell, exhibit emerging moiré physics such as novel Van Hove singularities, Fermi velocity renormalization, mini Dirac points and Hofstadter butterflies. Here we use double-walled carbon nanotubes as a model system for probing moiré physics in incommensurate one-dimensional systems, by combining structural and optical characterizations. We show that electron wavefunctions between incommensurate inner- and outer-wall nanotubes can hybridize strongly, contrary to the conventional wisdom of negligible electron hybridization due to destructive interference. The chirality-dependent inter-tube electronic coupling is described by one-dimensional zone folding of the electronic structure of twisted-and-stretched graphene bilayers. Our results demonstrate that incommensurate van der Waals interactions can be important for engineering the electronic structure and optical properties of one-dimensional materials.

  13. Excitons in van der Waals materials: From monolayer to bulk hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Koskelo, Jaakko; Fugallo, Giorgia; Hakala, Mikko; Gatti, Matteo; Sottile, Francesco; Cudazzo, Pierluigi

    2017-01-01

    We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic Hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyze in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the ab initio solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way, we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials.

  14. Application of van der Waals density functional theory to study physical properties of energetic materials

    NASA Astrophysics Data System (ADS)

    Conroy, Michael; Lin, You; Oleynik, Ivan; White, Carter

    2009-06-01

    An empirical correction to account for van der Waals interactions based on the work of Neumann and Perrin [J. Phys. Chem. B 109, 15531 (2005)] was applied to first-principles calculations of energetic molecular crystals. The calculated equilibrium unit-cell volumes of FOX-7, β-HMX, solid nitromethane, PETN-I, α-RDX, and TATB show a significant improvement in the agreement with experimental results. Hydrostatic-compression simulations of β-HMX, PETN-I, and α-RDX were also performed. The isothermal equations of state calculated from the results show increased agreement with experiment in the pressure intervals studied. The bulk modulus and its pressure derivative were calculated by fitting to three commonly used equations of state, and the results are within the range of reported experimental values.

  15. Improving the accuracy of the nonlocal van der Waals density functional with minimal empiricism.

    PubMed

    Vydrov, Oleg A; Van Voorhis, Troy

    2009-03-14

    The nonlocal van der Waals density functional (vdW-DF) captures the essential physics of the dispersion interaction not only in the asymptotic regime but also for a general case of overlapping fragment densities. A balanced treatment of other energetic contributions, such as exchange, is crucial if we aim for accurate description of various properties of weakly bound systems. In this paper, the vdW-DF correlation functional is modified to make it better compatible with accurate exchange functionals. We suggest a slightly simplified construction of the nonlocal correlation, yielding more accurate asymptotic C(6) coefficients. We also derive a gradient correction, containing a parameter that can be adjusted to suit an exchange functional of choice. We devise a particularly apt combination of exchange and correlation terms, which satisfies many important constraints and performs well for our benchmark tests.

  16. Repulsive van der waals forces self-limit native oxide growth.

    PubMed

    Bohling, Christian; Sigmund, Wolfgang

    2015-05-05

    Silicon is one of the most studied materials, yet questions remain unanswered about its unusual property of growing a self-limiting native oxide that attains its final thickness in a matter of hours yet months later has not grown further. For the first time, we have explored this self-limiting growth in terms of repulsive van der Waals (vdW) forces generated by the combination of material properties inherent to the system. These repulsive forces represent an energy barrier preventing additional oxidizing chemicals, mainly oxygen and water, from adsorbing on the surface as well as hindering diffusion of those that do adsorb toward the interface. We have also proven that this native oxide can be increased in thickness at room temperature and without reactive species by changing the oxidation environment to one predicted by theory to result in attractive vdW forces, thus allowing oxygen/water to interact with the surface more freely.

  17. The development of two dimensional group IV chalcogenides, blocks for van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Sa, Baisheng; Sun, Zhimei; Wu, Bo

    2015-12-01

    In this work, we introduce a series of two dimensional (2D) group IV chalcogenides (AX)2 with the building block X-A-A-X (A = Si, Ge, Sn, and Pb, and X = Se and Te) on the basis of ab initio calculations. The analysis of energy evaluation, lattice vibration as well as the chemical bonding demonstrate the good stability of these 2D materials. Furthermore, the pictures for the chemical bonding and electronic features of the 2D (AX)2 are drawn. Their narrow gapped semiconducting nature is unraveled. Especially, strong interactions between the electrons and phonons as well as the topological insulating nature in (SiTe)2 are observed. The present results indicate that such remarkable artificial 2D (AX)2 are building blocks for van der Waals heterostructure engineering, which shows potential applications in nanoscaled electronics and optoelectronics.

  18. Tuning the Schottky barrier in the arsenene/graphene van der Waals heterostructures by electric field

    NASA Astrophysics Data System (ADS)

    Li, Wei; Wang, Tian-Xing; Dai, Xian-Qi; Wang, Xiao-Long; Ma, Ya-Qiang; Chang, Shan-Shan; Tang, Ya-Nan

    2017-04-01

    Using density functional theory calculations, we investigate the electronic properties of arsenene/graphene van der Waals (vdW) heterostructures by applying external electric field perpendicular to the layers. It is demonstrated that weak vdW interactions dominate between arsenene and graphene with their intrinsic electronic properties preserved. We find that an n-type Schottky contact is formed at the arsenene/graphene interface with a Schottky barrier of 0.54 eV. Moreover, the vertical electric field can not only control the Schottky barrier height but also the Schottky contacts (n-type and p-type) and Ohmic contacts (n-type) at the interface. Tunable p-type doping in graphene is achieved under the negative electric field because electrons can transfer from the Dirac point of graphene to the conduction band of arsenene. The present study would open a new avenue for application of ultrathin arsenene/graphene heterostructures in future nano- and optoelectronics.

  19. Tunable Schottky contacts in the antimonene/graphene van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Li, Wei; Wang, Xinlian; Dai, Xianqi

    2017-03-01

    Electronic structures modulation in the antimonene/graphene van der Waals(vdW) heterostructure with an external electric field(Eext) are investigated by density functional theory calculations. It is demonstrated that weak vdW interactions dominate between antimonene and graphene with their intrinsic electronic properties preserved. Furthermore, the vertical Eext can control not only the Schottky barrier but also the Schottky contacts (n-type and p-type) and Ohmic contacts (n-type) at the antimonene/graphene interface. Meanwhile, the negative Eext can shifts the Dirac point of graphene above the Fermi level, resulting in p-type doping in graphene because electrons can easily transfer from the Dirac point of graphene to the conduction band of antimonene. The present study would open a new avenue for application of ultrathin antimonene/graphene heterostructures in future nano- and optoelectronics.

  20. Experiment and computation: a combined approach to study the van der Waals complexes

    NASA Astrophysics Data System (ADS)

    Surin, L. A.

    2016-12-01

    A review of recent results on the millimetre-wave spectroscopy of weakly bound van der Waals complexes, mostly those which contain H2 and He, is presented. In our work, we compared the experimental spectra to the theoretical bound state results, thus providing a critical test of the quality of the M-H2 and M-He potential energy surfaces (PESs) which are a key issue for reliable computations of the collisional excitation and de-excitation of molecules (M = CO, NH3, H2O) in the dense interstellar medium. The intermolecular interactions with He and H2 play also an important role for high resolution spectroscopy of helium or para-hydrogen clusters doped by a probe molecule (CO, HCN). Such experiments are directed on the detection of superfluid response of molecular rotation in the He and p-H2 clusters.

  1. Self-consistent van der Waals density functional study of benzene adsorption on Si(100)

    NASA Astrophysics Data System (ADS)

    Hamamoto, Yuji; Hamada, Ikutaro; Inagaki, Kouji; Morikawa, Yoshitada

    2016-06-01

    The adsorption of benzene on the Si(100) surface is studied theoretically using the self-consistent van der Waals density functional (vdW-DF) method. The adsorption energies of two competing adsorption structures, butterfly (BF) and tight-bridge (TB) structures, are calculated with several vdW-DFs at saturation coverage. Our results show that recently proposed vdW-DFs with high accuracy all prefer TB to BF, in accord with more accurate calculations based on exact exchange and correlation within the random-phase approximation. Detailed analyses reveal the important roles played by the molecule-surface interaction and molecular deformation upon adsorption, and we suggest that their precise description is a prerequisite for accurate prediction of the most stable adsorption structure of organic molecules on semiconductor surfaces.

  2. Molecular beam studies of open-shell systems: The van der Waals interaction between O(/sup 3/P) and He(/sup 1/S)

    SciTech Connect

    Aquilanti, V.; Candori, R.; Luzzatti, E.; Pirani, F.; Volpi, G.G.

    1986-11-01

    The absolute integral cross sections for oxygeg atom collisions with helium atoms have been obtained. From these cross sections, the interaction between He and ground state oxygen was calculated.(AIP)

  3. Computational chemistry approach to protein kinase recognition using 3D stochastic van der Waals spectral moments.

    PubMed

    González-Díaz, Humberto; Saíz-Urra, Liane; Molina, Reinaldo; González-Díaz, Yenny; Sánchez-González, Angeles

    2007-04-30

    Three-dimensional (3D) protein structures now frequently lack functional annotations because of the increase in the rate at which chemical structures are solved with respect to experimental knowledge of biological activity. As a result, predicting structure-function relationships for proteins is an active research field in computational chemistry and has implications in medicinal chemistry, biochemistry and proteomics. In previous studies stochastic spectral moments were used to predict protein stability or function (González-Díaz, H. et al. Bioorg Med Chem 2005, 13, 323; Biopolymers 2005, 77, 296). Nevertheless, these moments take into consideration only electrostatic interactions and ignore other important factors such as van der Waals interactions. The present study introduces a new class of 3D structure molecular descriptors for folded proteins named the stochastic van der Waals spectral moments ((o)beta(k)). Among many possible applications, recognition of kinases was selected due to the fact that previous computational chemistry studies in this area have not been reported, despite the widespread distribution of kinases. The best linear model found was Kact = -9.44 degrees beta(0)(c) +10.94 degrees beta(5)(c) -2.40 degrees beta(0)(i) + 2.45 degrees beta(5)(m) + 0.73, where core (c), inner (i) and middle (m) refer to specific spatial protein regions. The model with a high Matthew's regression coefficient (0.79) correctly classified 206 out of 230 proteins (89.6%) including both training and predicting series. An area under the ROC curve of 0.94 differentiates our model from a random classifier. A subsequent principal components analysis of 152 heterogeneous proteins demonstrated that beta(k) codifies information different to other descriptors used in protein computational chemistry studies. Finally, the model recognizes 110 out of 125 kinases (88.0%) in a virtual screening experiment and this can be considered as an additional validation study (these proteins

  4. The potential for fast van der Waals computations for layered materials using a Lifshitz model

    NASA Astrophysics Data System (ADS)

    Zhou, Yao; Pellouchoud, Lenson A.; Reed, Evan J.

    2017-06-01

    Computation of the van der Waals (vdW) interactions plays a crucial role in the study of layered materials. The adiabatic-connection fluctuation-dissipation theorem within random phase approximation (ACFDT-RPA) has been empirically reported to be the most accurate of commonly used methods, but it is limited to small systems due to its computational complexity. Without a computationally tractable vdW correction, fictitious strains are often introduced in the study of multilayer heterostructures, which, we find, can change the vdW binding energy by as much as 15%. In this work, we employed for the first time a defined Lifshitz model to provide the vdW potentials for a spectrum of layered materials orders of magnitude faster than the ACFDT-RPA for representative layered material structures. We find that a suitably defined Lifshitz model gives the correlation component of the binding energy to within 8-20% of the ACFDT-RPA calculations for a variety of layered heterostructures. Using this fast Lifshitz model, we studied the vdW binding properties of 210 three-layered heterostructures. Our results demonstrate that the three-body vdW effects are generally small (10% of the binding energy) in layered materials for most cases, and that non-negligible second-nearest neighbor layer interaction and three-body effects are observed for only those cases in which the middle layer is atomically thin (e.g. BN or graphene). We find that there is potential for particular combinations of stacked layers to exhibit repulsive three-body van der Waals effects, although these effects are likely to be much smaller than two-body effects.

  5. Excitons in van der Waals heterostructures: The important role of dielectric screening

    NASA Astrophysics Data System (ADS)

    Latini, S.; Olsen, T.; Thygesen, K. S.

    2015-12-01

    The existence of strongly bound excitons is one of the hallmarks of the newly discovered atomically thin semiconductors. While it is understood that the large binding energy is mainly due to the weak dielectric screening in two dimensions, a systematic investigation of the role of screening on two-dimensional (2D) excitons is still lacking. Here we provide a critical assessment of a widely used 2D hydrogenic exciton model, which assumes a dielectric function of the form ɛ (q )=1 +2 π α q , and we develop a quasi-2D model with a much broader applicability. Within the quasi-2D picture, electrons and holes are described as in-plane point charges with a finite extension in the perpendicular direction, and their interaction is screened by a dielectric function with a nonlinear q dependence which is computed ab initio. The screened interaction is used in a generalized Mott-Wannier model to calculate exciton binding energies in both isolated and supported 2D materials. For isolated 2D materials, the quasi-2D treatment yields results almost identical to those of the strict 2D model, and both are in good agreement with ab initio many-body calculations. On the other hand, for more complex structures such as supported layers or layers embedded in a van der Waals heterostructure, the size of the exciton in reciprocal space extends well beyond the linear regime of the dielectric function, and a quasi-2D description has to replace the 2D one. Our methodology has the merit of providing a seamless connection between the strict 2D limit of isolated monolayer materials and the more bulk-like screening characteristics of supported 2D materials or van der Waals heterostructures.

  6. Direct observation of interlayer hybridization and Dirac relativistic carriers in graphene/MoS₂ van der Waals heterostructures.

    PubMed

    Diaz, Horacio Coy; Avila, José; Chen, Chaoyu; Addou, Rafik; Asensio, Maria C; Batzill, Matthias

    2015-02-11

    Artificial heterostructures assembled from van der Waals materials promise to combine materials without the traditional restrictions in heterostructure-growth such as lattice matching conditions and atom interdiffusion. Simple stacking of van der Waals materials with diverse properties would thus enable the fabrication of novel materials or device structures with atomically precise interfaces. Because covalent bonding in these layered materials is limited to molecular planes and the interaction between planes are very weak, only small changes in the electronic structure are expected by stacking these materials on top of each other. Here we prepare interfaces between CVD-grown graphene and MoS2 and report the direct measurement of the electronic structure of such a van der Waals heterostructure by angle-resolved photoemission spectroscopy. While the Dirac cone of graphene remains intact and no significant charge transfer doping is detected, we observe formation of band gaps in the π-band of graphene, away from the Fermi-level, due to hybridization with states from the MoS2 substrate.

  7. Hybrid, Gate-Tunable, van der Waals p-n Heterojunctions from Pentacene and MoS2.

    PubMed

    Jariwala, Deep; Howell, Sarah L; Chen, Kan-Sheng; Kang, Junmo; Sangwan, Vinod K; Filippone, Stephen A; Turrisi, Riccardo; Marks, Tobin J; Lauhon, Lincoln J; Hersam, Mark C

    2016-01-13

    The recent emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device concepts and applications. In particular, the availability of semiconducting transition metal dichalcogenides, in addition to semimetallic graphene and insulating boron nitride, has enabled the fabrication of "all 2D" van der Waals heterostructure devices. Furthermore, the concept of van der Waals heterostructures has the potential to be significantly broadened beyond layered solids. For example, molecular and polymeric organic solids, whose surface atoms possess saturated bonds, are also known to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS2. The resulting p-n heterojunction is gate-tunable and shows asymmetric control over the antiambipolar transfer characteristic. In addition, the pentacene/MoS2 heterojunction exhibits a photovoltaic effect attributable to type II band alignment, which suggests that MoS2 can function as an acceptor in hybrid solar cells.

  8. Magnetic behavior and spin-lattice coupling in cleavable van der Waals layered CrCl3 crystals

    NASA Astrophysics Data System (ADS)

    McGuire, Michael A.; Clark, Genevieve; KC, Santosh; Chance, W. Michael; Jellison, Gerald E.; Cooper, Valentino R.; Xu, Xiaodong; Sales, Brian C.

    2017-06-01

    CrCl3 is a layered insulator that undergoes a crystallographic phase transition below room temperature and orders antiferromagnetically at low temperature. Weak van der Waals bonding between the layers and ferromagnetic in-plane magnetic order make it a promising material for obtaining atomically thin magnets and creating van der Waals heterostructures. In this work we have grown crystals of CrCl3, revisited the structural and thermodynamic properties of the bulk material, and explored mechanical exfoliation of the crystals. We find two distinct anomalies in the heat capacity at 14 and 17 K confirming that the magnetic order develops in two stages on cooling, with ferromagnetic correlations forming before long-range antiferromagnetic order develops between them. This scenario is supported by magnetization data. A magnetic phase diagram is constructed from the heat capacity and magnetization results. We also find an anomaly in the magnetic susceptibility at the crystallographic phase transition, indicating some coupling between the magnetism and the lattice. First-principles calculations accounting for van der Waals interactions also indicate spin-lattice coupling, and find multiple nearly degenerate crystallographic and magnetic structures consistent with the experimental observations. Finally, we demonstrate that monolayer and few-layer CrCl3 specimens can be produced from the bulk crystals by exfoliation, providing a path for the study of heterostructures and magnetism in ultrathin crystals down to the monolayer limit.

  9. Magnetic behavior and spin-lattice coupling in cleavable van der Waals layered CrCl3 crystals

    DOE PAGES

    McGuire, Michael A.; Clark, Genevieve; KC, Santosh; ...

    2017-06-19

    CrCl3 is a layered insulator that undergoes a crystallographic phase transition below room temperature and orders antiferromagnetically at low temperature. Weak van der Waals bonding between the layers and ferromagnetic in-plane magnetic order make it a promising material for obtaining atomically thin magnets and creating van der Waals heterostructures. In this work we have grown crystals of CrCl3, revisited the structural and thermodynamic properties of the bulk material, and explored mechanical exfoliation of the crystals. We find two distinct anomalies in the heat capacity at 14 and 17 K confirming that the magnetic order develops in two stages on cooling,more » with ferromagnetic correlations forming before long-range antiferromagnetic order develops between them. This scenario is supported by magnetization data. A magnetic phase diagram is constructed from the heat capacity and magnetization results. We also find an anomaly in the magnetic susceptibility at the crystallographic phase transition, indicating some coupling between the magnetism and the lattice. First-principles calculations accounting for van der Waals interactions also indicate spin-lattice coupling, and find multiple nearly degenerate crystallographic and magnetic structures consistent with the experimental observations. Lastly, we demonstrate that monolayer and few-layer CrCl3 specimens can be produced from the bulk crystals by exfoliation, providing a path for the study of heterostructures and magnetism in ultrathin crystals down to the monolayer limit.« less

  10. Lone pair interactions with coinage metal atoms: Weak van der Waals complexes of the coinage metal atoms with water and ammonia

    NASA Astrophysics Data System (ADS)

    Antušek, Andrej; Urban, Miroslav; Sadlej, Andrzej J.

    2003-10-01

    Interaction energies between the coinage metal atoms (Cu, Ag, and Au) and lone-pair donating molecules (H2O and NH3) are calculated by the spin adapted restricted open-shell Hartree-Fock coupled cluster method with the scalar relativistic effects accounted for by the Douglas-Kroll approximation. All ammonia complexes CuNH3, AgNH3, and AuNH3 are found to be of C3v symmetry with the counterpoise corrected interaction energies equal to -16.68, -6.87, and -14.64 mH for Cu, Ag, and Au, respectively. In the case of the water molecule the complexes are much weaker with the counterpoise corrected interaction energies equal to -3.78, -1.81, and -1.77 mH, for the three metal atoms, respectively. Moreover, all complexes with the water molecule are nonplanar. For both lone-pair donating molecules the structure and energetics of their complexes with the coinage metal atoms is mostly due to electron correlation effects. The relativistic effects are found to increase the bonding energies in the series of the ammonia complexes, whereas they reduce the bonding energy in the AgOH2 complex and are essentially negligible for CuOH2 and AuOH2. The calculated complex geometries and interaction energies are discussed in terms of different models. The pattern of interaction energies is discussed in terms of the balance between long-range induction and dispersion contributions and short-range forces. Also the possibility of some charge transfer from the lone-pair donor to the metal atom is considered and supported by analysis of the ionization potential and electron affinity data. The relativistic reduction of the size of the coinage metal atoms is found to be of importance as well. The calculated structural data are used to interpret the experimental observation concerning the existence of well resolved resonantly enhanced multiphoton ionization (REMPI) spectra of the ammonia-silver complexes and the absence of the corresponding spectra of the water-silver complex. This experimental

  11. van der Waals forces and confinement in carbon nanopores: Interaction between CH4, COOH, NH3, OH, SH and single-walled carbon nanotubes

    DOE PAGES

    Weck, Philippe F.; Kim, Eunja; Wang, Yifeng

    2016-04-13

    Interactions between CH4, COOH, NH3, OH, SH and armchair (n,n)(n=4,7,14) and zigzag (n,0)(n=7,12,25) single-walled carbon nanotubes (SWCNTs) have been systematically investigated within the framework of dispersion-corrected density functional theory (DFT-D2). Endohedral and exohedral molecular adsorption on SWCNT walls is energetically unfavorable or weak, despite the use of C6/r6 pairwise London-dispersion corrections. The effects of pore size and chirality on the molecule/SWCNTs interaction were also assessed. Furthermore, chemisorption of COOH, NH3, OH and SH at SWCNT edge sites was examined using a H-capped (7,0) SWCNT fragment and its impact on electrophilic, nucleophilic and radical attacks was predicted by means of Fukuimore » functions.« less

  12. van der Waals forces and confinement in carbon nanopores: Interaction between CH4, COOH, NH3, OH, SH and single-walled carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Weck, Philippe F.; Kim, Eunja; Wang, Yifeng

    2016-05-01

    Interactions between CH4, COOH, NH3, OH, SH and armchair (n, n) (n = 4, 7, 14) and zigzag (n, 0) (n = 7, 12, 25) single-walled carbon nanotubes (SWCNTs) have been systematically investigated within the framework of dispersion-corrected density functional theory (DFT-D2). Endohedral and exohedral molecular adsorption on SWCNT walls is energetically unfavorable or weak, despite the use of C6 /r6 pairwise London-dispersion corrections. The effects of pore size and chirality on the molecule/SWCNTs interaction were also assessed. Chemisorption of COOH, NH3, OH and SH at SWCNT edge sites was examined using a H-capped (7, 0) SWCNT fragment and its impact on electrophilic, nucleophilic and radical attacks was predicted by means of Fukui functions.

  13. van der Waals interactions and dipole polarizabilities of lanthanides: Tm(2F)-He and Yb(1S)-He potentials.

    PubMed

    Buchachenko, Alexei A; Szcześniak, Małgorzata M; Chałasiński, Grzegorz

    2006-03-21

    Anisotropic dipole polarizabilities of Tm(2F), Tm+2(2F), and Yb(1S) are calculated using the finite-field multireference averaged quadratic coupled cluster (MR-AQCC) (Tm and Tm+2) and RCCSD(T) (Yb) methods with small-core relativistic pseudopotentials ECP28MWB combined with the augmented ANO basis sets. The lanthanide atoms are strongly polarizable with the scalar part originating from the 6s electrons and the tensorial part from the open 4f shells. The adiabatic interaction potentials 2Sigma+, 2Pi, 2Delta, and 2Phi of Tm(2F)-He and Tm+2(2F)-He were examined by the multireference approaches, multireference configuration interaction and MR-AQCC, using the basis sets designed in the polarizability calculations. A closed-shell lanthanide system Yb(1S)-He was included for comparison. The Tm-He 2Sigma+, 2Pi, 2Delta, and 2Phi interaction potentials are very shallow and nearly degenerate (within 0.01 cm(-1)), with the well depths in the range of 2.35-2.36 cm(-1) at R=6.17 A. The basis-set saturated well depths are expected to be larger by ca. 25%, as estimated using the bond-function augmented basis set. The interactions of lanthanide atoms with He are one order of magnitude less anisotropic than those involving first-row transition metal atoms. The suppression of anisotropy is chiefly attributed to the screening effected by the 6s shell. When these electrons are removed as in the di-cation complex Tm+2(2F)-He, the potentials deepen to a thousand wave number range and their anisotropy is enhanced 500-fold.

  14. Intermolecular dispersion interactions of normal alkanes with rare gas atoms: van der Waals complexes of n-pentane with helium, neon, and argon

    NASA Astrophysics Data System (ADS)

    Balabin, Roman M.

    2008-09-01

    Interaction energies of normal pentane with three rare gas atoms (helium, neon, and argon) were calculated using ab initio methods: the second-order Møller-Plesset (MP2), the fourth-order Møller-Plesset (MP4), and coupled cluster with single and double substitutions with noniterative triple excitation (CCSD(T)) levels of theory. Dunning's correlation consistent basis sets up to aug-cc-pVQZ were applied. Eight profiles (246 points for each rare gas atom) of potential energy surface (PES) of all-trans (anti-anti) conformation of n-pentane were scanned. Optimal distances for complex formation were found. MP2 interaction energies at the basis set limit were evaluated by three different methods (Feller's, Helgaker's, and Martin's). The MP2 interaction energy at the basis set limit for a global minimum of n-pentane complex with argon was more than 400 cm -1, so formation of a stable complex (at least at low temperature) can be expected. A comparison with previously published data on propane complexes with rare gas atoms (both computational and experimental) was done. The MP4 level of theory was found to be sufficient for a description of C 5H 12 complexes with helium, neon, and argon.

  15. Interaction between n-Alkane Chains:  Applicability of the Empirically Corrected Density Functional Theory for Van der Waals Complexes.

    PubMed

    Goursot, Annick; Mineva, Tzonka; Kevorkyants, Ruslan; Talbi, Dahbia

    2007-05-01

    The geometries, interaction energies, and vibrational frequencies of a series of n-alkane dimers up to dodecane have been calculated using density functional theory (DFT) augmented with an empirical dispersion energy term (DFT-D). The results obtained from this method for ethane to hexane dimers are compared with those provided by the MP2 level of theory and the combined Gaussian-3 approach with CCSD(T) being the highest correlation method [G3(CCSD(T))]. Two types of dimer isomers have been studied. The most stable isomers have the two carbon chains in parallel planes, whereas the second ones have the two carbon chains in the same plane. Butane is found to be the shortest carbon chain to form dimers with similar properties, that is, a constant average distance between the monomer carbon skeletons, a similar increment per CH2 unit for the dimer interaction energy, and comparable dimer symmetric stretching frequencies. The values and trends obtained from the DFT-D approach agree very well with those obtained from MP2 for the geometries and vibrational frequencies and from the G3(CCSD(T)) method for the energies, validating the use of DFT-D for the study of large hydrocarbon complexes.

  16. Separating electrons and holes by monolayer increments in van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Ceballos, Frank; Zereshki, Peymon; Zhao, Hui

    2017-09-01

    Since the discovery of graphene and its outstanding chemical, optical, and mechanical properties, other layered materials have been fiercely hunted for throughout various techniques. Thanks to their van der Waals interaction, acting as weak glue, different types of layered materials with mismatched lattices can be stacked with high quality interfaces. The properties of the resulting multilayer structures can be tuned by choice of the materials, layer thicknesses, and sequence in which they are arranged. This opens the possibility for a large array of applications across many different fields. Here we present a systematic study with two-dimensional stacked layered materials, where their properties are tailored monolayer by monolayer. By arranging WSe2,MoSe2,WS2, and MoS2 monolayers in predetermined sequences, that are predicted to have a ladder band alignment in both the conduction and valence bands, we separate electrons and holes between the two utmost layers by monolayer increments. The samples studied are a WSe2 monolayer, a WSe2-MoSe2 bilayer, a WSe2-MoSe2-WS2 trilayer, and a WSe2-MoSe2-WS2-MoS2 four-layer. We observe an increase in absorbance, a decrease in photoluminescence, a variation in interlayer charge transfer, and photocarrier lifetimes that are extended up to a few nanoseconds as additional layers were added. With these results, we demonstrate that van der Waals stacked two-dimensional materials can form effective complex stacks and are promising platforms for fabricating ultrathin and flexible optoelectronics.

  17. Resonant two-photon ionization of fluorene rare-gas van der Waals complexes

    NASA Astrophysics Data System (ADS)

    Leutwyler, Samuel; Even, Uzi; Jortner, Joshua

    1983-12-01

    Resonant two-photon ionization combined with time-of-flight mass spectrometry was applied for the interrogation of the S0 → S1 electronic-vibrational excitations of van der Waals complexes of fluorene (FL) with rare-gas atoms and N2 in supersonic jets. Energy-resolved and mass-resolved spectra of FL ṡ Ne, FL ṡ Arn (n=1-3), FL ṡ Kr, FL ṡ Xe, and FL ṡ N2 were recorded over the energy range 0-800 cm-1 above the electronic origin of S1. The red microscopic spectral shifts of the electronic origins of FL ṡ R (R=Ar, Kr, and Xe) complexes are dominated by dispersive interactions, being proportional to the polarizability of R. The vibrational level structure of FL ṡ Rn (R=Ar, Kr, and Xe) complexes exhibits intramolecular vibrational excitations of FL, as well as intermolecular vibrations, which involve the relative motion of FL and R in the complex. The spectra of FL ṡ Ne and FL ṡ N2 reveal a rich vibrational structure in the vicinity of the electronic origin, indicating a substantial change of the nuclear configuration upon electronic excitation. Upper and lower bounds on the dissociation energies of FL ṡ R (R=Ne, Kr, and Xe) and FL ṡ Ar2 were inferred from the vibrational level structure in the mass-resolved spectra, where the disappearance of the signal of the parent van der Waals ion and the appearance of the ion signal of the fragments mark the onset of the vibrational predissociation process.

  18. Electrical Transport and Thermal Expansion in van der Waals Materials: Graphene and Topological Insulator

    NASA Astrophysics Data System (ADS)

    Jing, Lei

    Novel two-dimensional materials with weak interlayer Van der Waals interaction are fantastic platforms to study novel physical phenomena. This thesis describes our investigation on two different Van der Waals materials: graphene and bismuth selenide with calcium doping (CaxBi 2-xSe3, x as the doping level) in the topological insulator family. Firstly, we characterize the electrical transport behaviors of high-quality substrate-supported bilayer graphene devices with suspended metal gates. The device exhibits a transport gap induced by external electric field with an on/off ratio of 20,000, which could be explained by variable range hoping between localized states or disordered charge puddles. At large magnetic field, the device presents quantum Hall plateau at fractional values of conductance quantum, which arises from the equilibration of edge states between differentially doped regions. Secondly, we present our study on the electronic transport of CaxBi 2-xSe3 thin films, which are three-dimensional topological insulators and coupled with superconducting leads. In these novel Josephson transistors, we observe different characteristic features by energy dispersion spectrum (EDS) and Raman spectroscopy, and the weak suppression in the critical current Ic. Thirdly, we explore the thermal expansion of suspended graphene. By in-situ scanning electron microscope (SEM), we measure the thickness-dependence of graphene's negative thermal expansion coefficient (TEC). We propose that there is a competitive relation between the intrinsic TEC and the friction from the substrate and the graphene. Lastly, in collaboration with Dr. Nikolai Kalugin from New Mexico Tech., we explore the graphene's application as a quantum Hall effect infrared photodetector. This graphene-based detector can be operated at higher temperature (liquid nitrogen) and wider frequency than the previous implementations of quantum Hall detector.

  19. Band structure engineering in van der Waals heterostructures via dielectric screening: the GΔW method

    NASA Astrophysics Data System (ADS)

    Winther, Kirsten T.; Thygesen, Kristian S.

    2017-06-01

    The idea of combining different two-dimensional (2D) crystals in van der Waals heterostructures (vdWHs) has led to a new paradigm for band structure engineering with atomic precision. Due to the weak interlayer couplings, the band structures of the individual 2D crystals are largely preserved upon formation of the heterostructure. However, regardless of the details of the interlayer hybridisation, the size of the 2D crystal band gaps are always reduced due to the enhanced dielectric screening provided by the surrounding layers. The effect can be significant (on the order of electron volts) but its precise magnitude is non-trivial to predict because of the non-local nature of the screening in quasi-2D crystals. Moreover, the effect is not captured by effective single-particle methods such as density functional theory. Here we present an efficient and general method for calculating the band gap renormalization of a 2D material embedded in an arbitrary vdWH. The method evaluates the change in the GW self-energy of the 2D material from the change in the screened Coulomb interaction. The latter is obtained using the quantum-electrostatic heterostructure (QEH) model. We benchmark the GΔW method against full first-principles GW calculations and use it to unravel the importance of screening-induced band structure renormalisation in various vdWHs. A main result is the observation that the size of the band gap reduction of a given 2D material when inserted into a heterostructure scales inversely with the polarisability of the 2D material. Our work demonstrates that dielectric engineering via van der Waals heterostructuring represents a promising strategy for tailoring the band structure of 2D materials.

  20. An exchange-Coulomb model potential energy surface for the Ne-CO interaction. I. Calculation of Ne-CO van der Waals spectra.

    PubMed

    Dham, Ashok K; McCourt, Frederick R W; Meath, William J

    2009-06-28

    Exchange-Coulomb model potential energy surfaces have been developed for the Ne-CO interaction. The initial model is a three-dimensional potential energy surface based upon computed Heitler-London interaction energies and literature results for the long-range induction and dispersion energies, all as functions of interspecies distance, the orientation of CO relative to the interspecies axis, and the bond length of the CO molecule. Both a rigid-rotor model potential energy surface, obtained by setting the CO bond length equal to its experimental spectroscopic equilibrium value, and a vibrationally averaged model potential energy surface, obtained by averaging the stretching dependence over the ground vibrational motion of the CO molecule, have been constructed from the full data set. Adjustable parameters in each model potential energy surface have been determined through fitting a selected subset of pure rotational transition frequencies calculated for the (20)Ne-(12)C(12)O isotopolog to precisely known experimental values. Both potential energy surfaces provide calculated results for a wide range of available experimental microwave, millimeter-wave, and midinfrared Ne-CO transition frequencies that are generally far superior to those obtained using the best current literature potential energy surfaces. The vibrationally averaged CO ground state potential energy surface, employed together with a potential energy surface obtained from it by replacing the ground vibrational state average of the CO stretching dependence of the potential energy surface by an average over the first excited CO vibrational state, has been found to be particularly useful for computing and/or interpreting mid-IR transition frequencies in the Ne-CO dimer.

  1. {ital Ab initio} study of the van der Waals interaction of NH(X{sup 3}{Sigma}{sup {minus}}) with Ar({sup 1}S)

    SciTech Connect

    Kendall, R.A. Bukowski, R.; Szczesniak, M.M. Cybulski, S.M.

    1998-02-01

    The potential energy surface for the Ar({sup 1}S)+NH(X{sup 3}{Sigma}{sup {minus}}) interaction is calculated using the supermolecular unrestricted Mo/ller{endash}Plesset (UMP) perturbation theory approach and analyzed via the perturbation theory of intermolecular forces. The global minimum occurs for the approximate T-shaped geometry with Ar skewed toward the H atom at about {Theta}=67{degree} and R=6.75a{sub 0}. Our UMP4 estimate of the well depth of the global minimum is D{sub e}=100.3cm{sup {minus}1} and the related ground state dissociation energy obtained by rigid-body diffusion quantum Monte Carlo calculations (RBDQMC) is D{sub 0}=71.5{plus_minus}0.1cm{sup {minus}1}. These values are expected to be accurate to within a few percent. The potential energy surface also features a wide plateau in the proximity of Ar-N-H collinear geometry, at ca. 7.0a{sub 0}. RBDQMC calculations reveal nearly a free rotation of the NH subunit in the complex. {copyright} {ital 1998 American Institute of Physics.}

  2. Surface instability of orthotropic films under surface van der Waals forces

    NASA Astrophysics Data System (ADS)

    Wang, Xu; Li, Shuxiang

    2017-08-01

    We investigate the surface instability of an orthotropic thin elastic film interacting with another flat rigid body through van der Waals forces by means of linear stability analysis. In the obtained dispersion relation, the normalized interaction coefficient can be completely determined by the normalized wavenumber, the normalized surface energy and other two dimensionless material parameters characterizing the orthotropic elasticity of the film. The influence of these two material parameters and surface energy on the critical bifurcation mode and the critical interaction coefficient is probed in detail. The dispersion relation is also obtained for degenerate orthotropic films. An effective method based on transfer matrix is employed to derive the dispersion relations for a multilayered orthotropic elastic film interacting with a rigid contactor and interacting with another multilayered orthotropic elastic film, and also to obtain the critical interaction coefficient for a simply supported mono-layered orthotropic plate attracted by a rigid flat. Some interesting phenomena are observed for a bilayered orthotropic film, for two interacting mono-layered orthotropic films and for an orthotropic plate.

  3. Maxwell's Relations for a van der Waals Gas and a Nuclear Paramagnetic System.

    ERIC Educational Resources Information Center

    Herlihy, James; And Others

    1981-01-01

    Since Maxwell's relations are derived in general form from the first to second laws, and students often wonder what they mean and how they are used, appropriate partition functions for van der Waals gas and the nuclear paramagnetic system are used to obtain entropy expressions and equations of state. (Author/SK)

  4. Generalized van der Waals Hamiltonian: periodic orbits and C1 nonintegrability.

    PubMed

    Guirao, Juan L G; Llibre, Jaume; Vera, Juan A

    2012-03-01

    The aim of this paper is to study the periodic orbits of the generalized van der Waals Hamiltonian system. The tool for studying such periodic orbits is the averaging theory. Moreover, for this Hamiltonian system we provide information on its C(1) nonintegrability, i.e., on the existence of a second first integral of class C(1).

  5. Atomically thin p-n junctions with van der Waals heterointerfaces.

    PubMed

    Lee, Chul-Ho; Lee, Gwan-Hyoung; van der Zande, Arend M; Chen, Wenchao; Li, Yilei; Han, Minyong; Cui, Xu; Arefe, Ghidewon; Nuckolls, Colin; Heinz, Tony F; Guo, Jing; Hone, James; Kim, Philip

    2014-09-01

    Semiconductor p-n junctions are essential building blocks for electronic and optoelectronic devices. In conventional p-n junctions, regions depleted of free charge carriers form on either side of the junction, generating built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes influenced by the spatial extent of this depletion region. With the advent of atomically thin van der Waals materials and their heterostructures, it is now possible to realize a p-n junction at the ultimate thickness limit. Van der Waals junctions composed of p- and n-type semiconductors--each just one unit cell thick--are predicted to exhibit completely different charge transport characteristics than bulk heterojunctions. Here, we report the characterization of the electronic and optoelectronic properties of atomically thin p-n heterojunctions fabricated using van der Waals assembly of transition-metal dichalcogenides. We observe gate-tunable diode-like current rectification and a photovoltaic response across the p-n interface. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic p-n junction between graphene layers enhances the collection of the photoexcited carriers. The atomically scaled van der Waals p-n heterostructures presented here constitute the ultimate functional unit for nanoscale electronic and optoelectronic devices.

  6. Solution phase van der Waals epitaxy of ZnO wire arrays.

    PubMed

    Zhu, Yue; Zhou, Yong; Utama, Muhammad Iqbal Bakti; de la Mata, María; Zhao, Yanyuan; Zhang, Qing; Peng, Bo; Magen, Cesar; Arbiol, Jordi; Xiong, Qihua

    2013-08-21

    As an incommensurate epitaxy, van der Waals epitaxy allows defect-free crystals to grow on substrates even with a large lattice mismatch. Furthermore, van der Waals epitaxy is proposed as a universal platform where heteroepitaxy can be achieved irrespective of the nature of the overlayer material and the method of crystallization. Here we demonstrate van der Waals epitaxy in solution phase synthesis for seedless and catalyst-free growth of ZnO wire arrays on phlogopite mica at low temperature. A unique incommensurate interface is observed even with the incomplete initial wetting of ZnO onto the substrate. Interestingly, the imperfect contacting layer does not affect the crystalline and optical properties of other parts of the wires. In addition, we present patterned growth of a well-ordered array with hexagonal facets and in-plane alignment. We expect our seedless and catalyst-free solution phase van der Waals epitaxy synthesis to be widely applicable in other materials and structures.

  7. First principles calculations of solid-state thermionic transport in layered van der Waals heterostructures.

    PubMed

    Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan

    2016-08-21

    This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m(-2) K(-1) which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices.

  8. van der Waals Metal-Organic Framework as an Excitonic Material for Advanced Photonics.

    PubMed

    Milichko, Valentin A; Makarov, Sergey V; Yulin, Alexey V; Vinogradov, Alexandr V; Krasilin, Andrei A; Ushakova, Elena; Dzyuba, Vladimir P; Hey-Hawkins, Evamarie; Pidko, Evgeny A; Belov, Pavel A

    2017-01-23

    Synergistic combination of organic and inorganic nature in van der Waals metal-organic frameworks supports different types of robust excitons that can be effectively and independently manipulated by light at room temperature, and opens new concepts for all-optical data processing and storage.

  9. Control of excitons in multi-layer van der Waals heterostructures

    SciTech Connect

    Calman, E. V. Dorow, C. J.; Fogler, M. M.; Butov, L. V.; Hu, S.; Mishchenko, A.; Geim, A. K.

    2016-03-07

    We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of MoS{sub 2} and hexagonal boron nitride. The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.

  10. Gas Temperature Determination in Argon-Helium Plasma at Atmospheric Pressure using van der Waals Broadening

    SciTech Connect

    Munoz, Jose; Yubero, Cristina; Calzada, Maria Dolores; Dimitrijevic, Milan S.

    2008-10-22

    The use of the van der Waals broadening of Ar atomic lines to determine the gas temperature in Ar-He plasmas, taking into account both argon and helium atoms as perturbers, has been analyzed. The values of the gas temperature inferred from this broadening have been compared with those obtained from the spectra of the OH molecular species in the discharge.

  11. Van der Waals like behavior of topological AdS black holes in massive gravity

    NASA Astrophysics Data System (ADS)

    Hendi, S. H.; Mann, R. B.; Panahiyan, S.; Eslam Panah, B.

    2017-01-01

    Motivated by recent developments in black hole thermodynamics, we investigate van der Waals phase transitions of charged black holes in massive gravity. We find that massive gravity theories can exhibit strikingly different thermodynamic behavior compared to that of Einstein gravity, and that the mass of the graviton can generate a range of new phase transitions for topological black holes that are otherwise forbidden.

  12. A van der Waals Equation of State for a Dilute Boson Gas

    ERIC Educational Resources Information Center

    Deeney, F. A.; O'Leary, J. P.

    2012-01-01

    An equation of state of a system is a relationship that connects the thermodynamic variables of the system such as pressure and temperature. Such equations are well known for classical gases but less so for quantum systems. In this paper we develop a van der Waals equation of state for a dilute boson gas that may be used to explain the occurrence…

  13. Van der Waals Type Model for Neutron-Proton Elastic Scattering at High Energies

    NASA Astrophysics Data System (ADS)

    Aleem, F.

    1980-12-01

    The most recent measurements of the angular distribution and total cross-section for neutron-proton elastic scattering between 70< pL <400 GeV/c with squared four momentum transfer -t ≤ 3.6 (GeV/c)2 have been explained using Van der Waals type model.

  14. A van der Waals Equation of State for a Dilute Boson Gas

    ERIC Educational Resources Information Center

    Deeney, F. A.; O'Leary, J. P.

    2012-01-01

    An equation of state of a system is a relationship that connects the thermodynamic variables of the system such as pressure and temperature. Such equations are well known for classical gases but less so for quantum systems. In this paper we develop a van der Waals equation of state for a dilute boson gas that may be used to explain the occurrence…

  15. Control of excitons in multi-layer van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Calman, E. V.; Dorow, C. J.; Fogler, M. M.; Butov, L. V.; Hu, S.; Mishchenko, A.; Geim, A. K.

    2016-03-01

    We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of MoS2 and hexagonal boron nitride. The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.

  16. Maxwell's Relations for a van der Waals Gas and a Nuclear Paramagnetic System.

    ERIC Educational Resources Information Center

    Herlihy, James; And Others

    1981-01-01

    Since Maxwell's relations are derived in general form from the first to second laws, and students often wonder what they mean and how they are used, appropriate partition functions for van der Waals gas and the nuclear paramagnetic system are used to obtain entropy expressions and equations of state. (Author/SK)

  17. Long-Range Repulsion Between Spatially Confined van der Waals Dimers

    NASA Astrophysics Data System (ADS)

    Sadhukhan, Mainak; Tkatchenko, Alexandre

    2017-05-01

    It is an undisputed textbook fact that nonretarded van der Waals (vdW) interactions between isotropic dimers are attractive, regardless of the polarizability of the interacting systems or spatial dimensionality. The universality of vdW attraction is attributed to the dipolar coupling between fluctuating electron charge densities. Here, we demonstrate that the long-range interaction between spatially confined vdW dimers becomes repulsive when accounting for the full Coulomb interaction between charge fluctuations. Our analytic results are obtained by using the Coulomb potential as a perturbation over dipole-correlated states for two quantum harmonic oscillators embedded in spaces with reduced dimensionality; however, the long-range repulsion is expected to be a general phenomenon for spatially confined quantum systems. We suggest optical experiments to test our predictions, analyze their relevance in the context of intermolecular interactions in nanoscale environments, and rationalize the recent observation of anomalously strong screening of the lateral vdW interactions between aromatic hydrocarbons adsorbed on metal surfaces.

  18. Nonlocal van der Waals functionals: The case of rare-gas dimers and solids

    NASA Astrophysics Data System (ADS)

    Tran, Fabien; Hutter, Jürg

    2013-05-01

    Recently, the nonlocal van der Waals (vdW) density functionals [M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004), 10.1103/PhysRevLett.92.246401] have attracted considerable attention due to their good performance for systems where weak interactions are important. Since the physics of dispersion is included in these functionals, they are usually more accurate and show less erratic behavior than the semilocal and hybrid methods. In this work, several variants of the vdW functionals have been tested on rare-gas dimers (from He2 to Kr2) and solids (Ne, Ar, and Kr) and their accuracy compared to standard semilocal approximations, supplemented or not by an atom-pairwise dispersion correction [S. Grimme, J. Antony, S. Ehrlich, and H. Krieg, J. Chem. Phys. 132, 154104 (2010), 10.1063/1.3382344]. An analysis of the results in terms of energy decomposition is also provided.

  19. Effect of surface roughness on van der Waals and Casimir-Polder/Casimir attraction energies

    NASA Astrophysics Data System (ADS)

    Makeev, Maxim A.

    2017-09-01

    A theoretical model is devised to assess effects of surface roughness on dispersion interactions between macroscopic bodies, bounded by self-affine fractal surfaces and separated by a vacuum gap. The rough-surface profiles are described statistically by the saturation values of surface width and the correlation lengths; i.e., in terms of experimentally measurable quantities. The model devised takes into account the separation distance-dependent nature of dispersive interactions. The case of non-retarded van der Waals interactions, known to operate at smaller separation distances between the bodies, and that of retarded attractions, operative at larger separation length-scales, are treated separately in this work. Analytical formulae for the roughness corrections are deduced for the two aforementioned types of attractions. The model is employed to compute roughness corrections to interactions between an extended body, bounded by a self-affine surface, and: a) a point-like adherent; and b) a planar half-space. Furthermore, the roughness-induced corrections to dispersive interaction energies between half-spaces, both bounded by self-affine surfaces, are obtained under assumption that the corresponding surface profiles are not correlated. The predictions of the model are compared with some previously reported theoretical studies and available experimental data on the theme of dispersive adhesion between macroscopic bodies.

  20. Versatile van der Waals Density Functional Based on a Meta-Generalized Gradient Approximation

    NASA Astrophysics Data System (ADS)

    Peng, Haowei; Yang, Zeng-Hui; Perdew, John P.; Sun, Jianwei

    2016-10-01

    A "best-of-both-worlds" van der Waals (vdW) density functional is constructed, seamlessly supplementing the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation for short- and intermediate-range interactions with the long-range vdW interaction from r VV 10 , the revised Vydrov-van Voorhis nonlocal correlation functional. The resultant SCAN +r VV 10 is the only vdW density functional to date that yields excellent interlayer binding energies and spacings, as well as intralayer lattice constants in 28 layered materials. Its versatility for various kinds of bonding is further demonstrated by its good performance for 22 interactions between molecules; the cohesive energies and lattice constants of 50 solids; the adsorption energy and distance of a benzene molecule on coinage-metal surfaces; the binding energy curves for graphene on Cu(111), Ni(111), and Co(0001) surfaces; and the rare-gas solids. We argue that a good semilocal approximation should (as SCAN does) capture the intermediate-range vdW through its exchange term. We have found an effective range of the vdW interaction between 8 and 16 Å for systems considered here, suggesting that this interaction is negligibly small at the larger distances where it reaches its asymptotic power-law decay.

  1. Van der Waals enhancement of optical atom potentials via resonant coupling to surface polaritons.

    PubMed

    Kerckhoff, Joseph; Mabuchi, Hideo

    2009-08-17

    Contemporary experiments in cavity quantum electrodynamics (cavity QED) with gas-phase neutral atoms rely increasingly on laser cooling and optical, magneto-optical or magnetostatic trapping methods to provide atomic localization with sub-micron uncertainty. Difficult to achieve in free space, this goal is further frustrated by atom-surface interactions if the desired atomic placement approaches within several hundred nanometers of a solid surface, as can be the case in setups incorporating monolithic dielectric optical resonators such as microspheres, microtoroids, microdisks or photonic crystal defect cavities. Typically in such scenarios, the smallest atom-surface separation at which the van der Waals interaction can be neglected is taken to be the optimal localization point for associated trapping schemes, but this sort of conservative strategy generally compromises the achievable cavity QED coupling strength. Here we suggest a new approach to the design of optical dipole traps for atom confinement near surfaces that exploits strong surface interactions, rather than avoiding them, and present the results of a numerical study based on (39)K atoms and indium tin oxide (ITO). Our theoretical framework points to the possibility of utilizing nanopatterning methods to engineer novel modifications of atom-surface interactions. (c) 2009 Optical Society of America

  2. In-situ epitaxial growth of graphene/h-BN van der Waals heterostructures by molecular beam epitaxy.

    PubMed

    Zuo, Zheng; Xu, Zhongguang; Zheng, Renjing; Khanaki, Alireza; Zheng, Jian-Guo; Liu, Jianlin

    2015-10-07

    Van der Waals materials have received a great deal of attention for their exceptional layered structures and exotic properties, which can open up various device applications in nanoelectronics. However, in situ epitaxial growth of dissimilar van der Waals materials remains challenging. Here we demonstrate a solution for fabricating van der Waals heterostructures. Graphene/hexagonal boron nitride (h-BN) heterostructures were synthesized on cobalt substrates by using molecular beam epitaxy. Various characterizations were carried out to evaluate the heterostructures. Wafer-scale heterostructures consisting of single-layer/bilayer graphene and multilayer h-BN were achieved. The mismatch angle between graphene and h-BN is below 1°.

  3. In-situ epitaxial growth of graphene/h-BN van der Waals heterostructures by molecular beam epitaxy

    PubMed Central

    Zuo, Zheng; Xu, Zhongguang; Zheng, Renjing; Khanaki, Alireza; Zheng, Jian-Guo; Liu, Jianlin

    2015-01-01

    Van der Waals materials have received a great deal of attention for their exceptional layered structures and exotic properties, which can open up various device applications in nanoelectronics. However, in situ epitaxial growth of dissimilar van der Waals materials remains challenging. Here we demonstrate a solution for fabricating van der Waals heterostructures. Graphene/hexagonal boron nitride (h-BN) heterostructures were synthesized on cobalt substrates by using molecular beam epitaxy. Various characterizations were carried out to evaluate the heterostructures. Wafer-scale heterostructures consisting of single-layer/bilayer graphene and multilayer h-BN were achieved. The mismatch angle between graphene and h-BN is below 1°. PMID:26442629

  4. The toluene-Ar complex: S0 and S1 van der Waals modes, changes to methyl rotation, and torsion-van der Waals vibration coupling.

    PubMed

    Gascooke, Jason R; Lawrance, Warren D

    2013-02-28

    The methyl rotor and van der Waals vibrational levels in the S1 and S0 states of toluene-Ar have been investigated by the technique of two-dimensional laser induced fluorescence (2D-LIF). The S0 van der Waals and methyl rotor levels are reported for the first time, while improved S1 values are presented. The correlations seen in the 2D-LIF images between the S0 and S1 states lead to a reassignment of key features in the S1 ← S0 excitation spectrum. This reassignment reveals that there are significant changes in the methyl rotor levels in the complex compared with those in bare toluene, particularly at low m. The observed rotor energies are explained by the introduction of a three-fold, V3, term in the torsion potential (this term is zero in toluene) and a reduction in the height of the six-fold, V6, barriers in S0 and S1 from their values in bare toluene. The V3 term is larger in magnitude than the V6 term in both S0 and S1. The constants determined are ∣V3(S1)∣ = 33.4 ± 1.0 cm(-1), ∣V3(S0)∣ = 20.0 ± 1.0 cm(-1), V6(S1) = -10.7 ± 1.0 cm(-1), and V6(S0) = -1.7 ± 1.0 cm(-1). The methyl rotor is also found to couple with van der Waals vibration; specifically, the m(") = 2 rotor state couples with the combination level involving one quantum of the long axis bend and m(") = 1. The coupling constant is determined to be 1.9 cm(-1), which is small compared with the values typically reported for torsion-vibration coupling involving ring modes.

  5. van der Waals Corrected Density Functional Theory Calculations on Zeolitic Imidazolate Frameworks

    NASA Astrophysics Data System (ADS)

    Ray, Keith George

    The van der Waals force is ubiquitous in nature, however, first principles calculations of this interaction for large systems, i.e., around 1000 atoms, have been performed only recently. In the following are presented results on the application of the van der Waals density functional (vdW-DF) to gas adsorption and transport in zeolitic imidazolate frameworks (ZIFs). Carbon dioxide and methane binding energies and positions are calculated with the vdW-DF in three distinct binding sites in a series of five rho topology ZIFs. The isostructural set of ZIFs was selected in order to isolate the effect of framework functionalization. Gas molecules are found to bind in locations with high coordination to framework atoms at distances of around 3 A. Contributions to the binding energy from induced polarization and dispersion are quantified in order to elucidate the origins of strong CO2 adsorption and selectivity over CH4. The dispersion energy is found to dominate the interactions, however, CO2 adsorption is also enhanced by electrostatic interactions with asymmetrically functionalized linkers. Steric constraints for methane molecules, that do not impede carbon dioxide binding, further contribute to selectivity. Binding energy landscapes for CO2 and CH4 are calculated using classical force fields for the same set of rho ZIFs and several other ZIFs that differ in functionalization and topology. Quantities extracted from these landscapes are used to explain the effect of framework topology on gas adsorption at low and high pressure as well as how the positions of adsorbed gas molecules evolve as a function of pressure. Materials with large surface areas have greater gas uptake at high pressure, while smaller pores, which are associated with stronger binding, adsorb more gas at low pressure. Finally, the effect of framework flexibility on CO2 transport through the double 8-ring channel of ZIF-97 is investigated with computationally intensive climbing-nudged elastic band

  6. Screened van der Waals correction to density functional theory for solids

    NASA Astrophysics Data System (ADS)

    Tao, Jianmin; Zheng, Fan; Gebhardt, Julian; Perdew, John P.; Rappe, Andrew M.

    2017-07-01

    Lattice constant and cohesive energy are basic properties in the design of materials and devices. However, due to neglect of long-range van der Waals (vdW) interactions, density functional approximations (DFAs) often yield unusually large errors for ionic solids and heavy metals. Here, we propose a model for the dynamically screened vdW correction, including the leading order as well as higher-order contributions. The striking feature of this model is that important screening effects and higher-order contributions are properly considered and that its contribution to the short-range part is removed by a novel damping function for the avoidance of double counting. As a result, the model dramatically reduces the error of the DFA-GGA in lattice constant and cohesive energy. We also find that the three-body interactions are small, due to the screening effects. These observations greatly improve our fundamental understanding of vdW interactions and enhance the applicability of efficient semilocal DFAs.

  7. Stabilization of thin liquid films by repulsive van der Waals force.

    PubMed

    Li, Er Qiang; Vakarelski, Ivan U; Chan, Derek Y C; Thoroddsen, Sigurdur T

    2014-05-13

    Using high-speed video recording of bubble rise experiments, we study the stability of thin liquid films trapped between a rising bubble and a surfactant-free liquid-liquid meniscus interface. Using different combinations of nonpolar oils and water that are all immiscible, we investigate the extent to which film stability can be predicted by attractive and repulsive van der Waals (vdW) interactions that are indicated by the relative magnitude of the refractive indices of the liquid combinations, for example, water (refractive index, n = 1.33), perfluorohexane (n = 1.23), and tetradecane (n = 1.43). We show that, when the film-forming phase was oil (perfluorohexane or tetradecane), the stability of the film could always be predicted from the sign of the vdW interaction, with a repulsive vdW force resulting in a stable film and an attractive vdW force resulting in film rupture. However, if aqueous electrolyte is the film-forming bulk phase between the rising air bubble and the upper oil phase, the film always ruptured, even when a repulsive vdW interaction was predicted. We interpret these results as supporting the hypothesis that a short-ranged hydrophobic attraction determines the stability of the thin water film formed between an air phase and a nonpolar oil phase.

  8. Stability of Complex Biomolecular Structures: van der Waals, Hydrogen Bond Cooperativity, and Nuclear Quantum Effects.

    PubMed

    Rossi, Mariana; Fang, Wei; Michaelides, Angelos

    2015-11-05

    Biomolecules are complex systems stabilized by a delicate balance of weak interactions, making it important to assess all energetic contributions in an accurate manner. However, it is a priori unclear which contributions make more of an impact. Here, we examine stacked polyglutamine (polyQ) strands, a peptide repeat often found in amyloid aggregates. We investigate the role of hydrogen bond (HB) cooperativity, van der Waals (vdW) dispersion interactions, and quantum contributions to free energies, including anharmonicities through density functional theory and ab initio path integral simulations. Of these various factors, we find that the largest impact on structural stabilization comes from vdW interactions. HB cooperativity is the second largest contribution as the size of the stacked chain grows. Competing nuclear quantum effects make the net quantum contribution small but very sensitive to anharmonicities, vdW, and the number of HBs. Our results suggest that a reliable treatment of these systems can only be attained by considering all of these components.

  9. The Scattering of Gas Phase Molecules and Van Der Waals Clusters from Surfaces

    NASA Astrophysics Data System (ADS)

    Beauregard, John Norman

    The interaction of gas phase species with surfaces plays an important role in a myriad of processes such as heterogeneous catalysis, corrosion and the etching of semiconductor surfaces in the microelectronics industry. Thus, the study of gas-surface interactions has become a field of intense research. In this dissertation, we present the results of a computational study of the scattering of gas phase molecules and van der Waals clusters from surfaces. We have used molecular dynamics calculations which allow for the examination of the microscopic details of gas-surface scattering. In this work we study four distinct systems. In Chapter I the focus is on the scattering of van der Waals clusters of N_2 from crystal surfaces. We find that the cluster-surface scattering dynamics are very different from those observed in monomer-surface scattering. Furthermore, our results are in qualitative agreement with a recent experimental study of the scattering of nitrogen clusters from metal surfaces. The focus of Chapter II is on the effect of reagent rotation and rotational alignment on the dissociative chemisorption of H_2 on metal surfaces. We find that the probability of dissociative chemisorption depends strongly on both the rotational energy and the plane of rotation of the reactant H_2. Our results suggest that such information might be useful in uncovering intricate details of the potential energy surface governing these reactions. In Chapter III we examine the dissociative trapping of HD on a tungsten surface. In dissociative trapping only one atom becomes bound to the surface while the other returns to the gas phase. We observe a novel isotope effect in this channel which is explained in terms of a simple mechanism for the dissociative trapping process. Finally, in Chapter IV we examine the effect of dissolving an H_2 molecule in an argon microcluster on the dissociative chemisorption of H_2 on a silicon surface. We find that this does, in fact, facilitate the reaction

  10. Cooperative interplay of van der Waals forces and quantum nuclear effects on adsorption: H at graphene and at coronene.

    PubMed

    Davidson, Erlend R M; Klimeš, Jiří; Alfè, Dario; Michaelides, Angelos

    2014-10-28

    The energetic barriers that atoms and molecules often experience when binding to surfaces are incredibly important to a myriad of chemical and physical processes. However, these barriers are difficult to describe accurately with current computer simulation approaches. Two prominent contemporary challenges faced by simulation are the role of van der Waals forces and nuclear quantum effects. Here we examine the widely studied model systems of hydrogen on graphene and coronene using a van der Waals inclusive density functional theory approach together with path integral molecular dynamics at 50 K. We find that both van der Waals and quantum nuclear effects work together in a cooperative manner to dramatically reduce the barriers for hydrogen atoms to adsorb. This suggests that the low temperature hydrogenation of graphene is easier than previously thought and in more general terms that the combined roles of van der Waals and quantum tunnelling can lead to qualitative changes in adsorption.

  11. Interface thermal conductance of van der Waals monolayers on amorphous substrates.

    PubMed

    Correa, Gabriela C; Foss, Cameron J; Aksamija, Zlatan

    2017-03-01

    Heterostructures based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics. Due to the single atom thickness of monolayers, their properties are strongly affected by interactions with the external environment. We develop a model for interface thermal conductance (ITC) in an atomic monolayer van der Waals bonded to a disordered substrate. Graphene on SiO2 is initially used in our model and contrasted against available experimental data; the model is then applied to monolayer molybdenum disulfide (MoS2) on SiO2 substrate. Our findings show the dominant carrier of heat in both graphene and MoS2 in the cross-plane direction is the flexural (ZA) phonon mode, owing to the large overlap between graphene ZA and substrate vibrational density of states. The rate of phonon transfer across the interface depends quadratically on the substrate coupling constant K a , but this interaction also causes a lifting of the lowest flexural phonon modes. As a result, ITC depends roughly linearly on the strength of the coupling between a monolayer and its substrate. We conclude that, in both graphene and MoS2 on SiO2, substrate adhesion plays a strong role in determining ITC, requiring further study of substrate coupling in TMDCs.

  12. Electronic Properties of Polarizable Systems with Self-Consistent Interatomic van der Waals Density Functional

    NASA Astrophysics Data System (ADS)

    Ferri, Nicola; Distasio, Robert A., Jr.; Ambrosetti, Alberto; Car, Roberto; Scheffler, Matthias; Tkatchenko, Alexandre

    2015-03-01

    Ubiquitous long-range van der Waals (vdW) interactions play a fundamental role in the structure and stability of a wide range of systems. Within the DFT framework, the vdW energy represents a crucial, but tiny part of the total energy, hence its influence on the electronic density, n (r) , and electronic properties is typically assumed to be rather small. Here, we address this question via a fully self-consistent (SC) implementation of the interatomic Tkatchenko-Scheffler vdW functional and its extension to surfaces. Self-consistency leads to large changes in the binding energies and electrostatic moments of highly polarizable alkali metal dimers. For some metal surfaces, vdW interactions increase dipole moments and induce non-trivial charge rearrangements, leading to visible changes in the metal workfunctions. Similar behavior is observed for molecules adsorbed on metals. Our study reveals a non-trivial connection between electrostatics and long-range electron correlation effects.

  13. Shaping van der Waals nanoribbons via torsional constraints: Scrolls, folds and supercoils

    PubMed Central

    Shahabi, Alireza; Wang, Hailong; Upmanyu, Moneesh

    2014-01-01

    Interplay between structure and function in atomically thin crystalline nanoribbons is sensitive to their conformations yet the ability to prescribe them is a formidable challenge. Here, we report a novel paradigm for controlled nucleation and growth of scrolled and folded shapes in finite-length nanoribbons. All-atom computations on graphene nanoribbons (GNRs) and experiments on macroscale magnetic thin films reveal that decreasing the end distance of torsionally constrained ribbons below their contour length leads to formation of these shapes. The energy partitioning between twisted and bent shapes is modified in favor of these densely packed soft conformations due to the non-local van der Waals interactions in these 2D crystals; they subvert the formation of supercoils that are seen in their natural counterparts such as DNA and filamentous proteins. The conformational phase diagram is in excellent agreement with theoretical predictions. The facile route can be readily extended for tailoring the soft conformations of crystalline nanoscale ribbons, and more general self-interacting filaments. PMID:25417759

  14. Limiting temperature of pion gas with the van der Waals equation of state

    NASA Astrophysics Data System (ADS)

    Poberezhnyuk, R. V.; Vovchenko, V.; Anchishkin, D. V.; Gorenstein, M. I.

    2016-09-01

    The grand canonical ensemble formulation of the van der Waals equation of state that includes the effects of Bose statistics is applied to the equilibrium system of interacting pions. If the attractive interaction between pions is large enough, a limiting temperature T 0 emerges, i.e., no thermodynamical equilibrium is possible at T\\gt {T}0. The system pressure p, particle number density n, and energy density ε remain finite at T={T}0, whereas for T near T 0 both the specific heat C={{d}}\\varepsilon /{{d}}T and the scaled variance of particle number fluctuations ω [N] are proportional to {({T}0-T)}-1/2 and, thus, go to infinity at T\\to {T}0. The limiting temperature also corresponds to the softest point of the equation of state, i.e., the speed of sound squared {c}s2={{d}}p/{{d}}\\varepsilon goes to zero as {({T}0-T)}1/2. Very similar thermodynamical behavior takes place in the Hagedorn model for the special choice of a power, namely {m}-4, in the pre-exponential factor of the mass spectrum ρ (m).

  15. Surface instability of an imperfectly bonded thin elastic film under surface van der Waals forces

    NASA Astrophysics Data System (ADS)

    Wang, Xu; Jing, Rong

    2017-02-01

    This paper studies surface instability of a thin elastic film imperfectly bonded to a rigid substrate interacting with a rigid contactor through van der Waals forces under plane strain conditions. The film-substrate interface is modeled as a linear spring with vanishing thickness described in terms of the normal and tangential interface parameters. Depending on the ratio of the two imperfect interface parameters, the critical value of the Poisson's ratio for the occurrence of surface wrinkling in the absence of surface energy can be greater than, equal to, or smaller than 0.25, which is the critical Poisson's ratio for a perfect film-substrate interface. The critical surface energy for the inhibition of the surface wrinkling is also obtained. Finally, we propose a very simple and effective method to study the surface instability of a multilayered elastic film with imperfect interfaces interacting with a rigid contactor or with another multilayered elastic film (or a multilayered simply supported plate) with imperfect interfaces.

  16. Interface thermal conductance of van der Waals monolayers on amorphous substrates

    NASA Astrophysics Data System (ADS)

    Correa, Gabriela C.; Foss, Cameron J.; Aksamija, Zlatan

    2017-03-01

    Heterostructures based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics. Due to the single atom thickness of monolayers, their properties are strongly affected by interactions with the external environment. We develop a model for interface thermal conductance (ITC) in an atomic monolayer van der Waals bonded to a disordered substrate. Graphene on SiO2 is initially used in our model and contrasted against available experimental data; the model is then applied to monolayer molybdenum disulfide (MoS2) on SiO2 substrate. Our findings show the dominant carrier of heat in both graphene and MoS2 in the cross-plane direction is the flexural (ZA) phonon mode, owing to the large overlap between graphene ZA and substrate vibrational density of states. The rate of phonon transfer across the interface depends quadratically on the substrate coupling constant K a , but this interaction also causes a lifting of the lowest flexural phonon modes. As a result, ITC depends roughly linearly on the strength of the coupling between a monolayer and its substrate. We conclude that, in both graphene and MoS2 on SiO2, substrate adhesion plays a strong role in determining ITC, requiring further study of substrate coupling in TMDCs.

  17. Ab initio study of the Br(2P)-HBr van der Waals complex.

    PubMed

    Toboła, R; Chałasiński, G; Kłos, J; Szcześniak, M M

    2009-05-14

    This study reports an ab initio characterization of a prereactive van der Waals complex between an open-shell atom Br((2)P) and a closed shell molecule HBr. The three adiabatic potential surfaces 1 (2)A('), 2 (2)A('), and 1 (2)A("), which result from the splitting of degenerate P state of Br are obtained from coupled cluster calculations. The coupling between same-symmetry states is calculated by multireference configuration-interaction method. A transformation to a diabatic representation and inclusion of the spin-orbit coupling effects on the interactions are also discussed. Bound states are calculated using an adiabatic bender model. The global minimum on the lowest adiabatic potential surface corresponds to a T-shaped geometry and has a well depth of D(e)=762.5 cm(-1) at R(e)=3.22 A. A secondary minimum occurs for a hydrogen-bonded geometry with D(e)=445.3 cm(-1) at R(e)=4.24 A. Upon inclusion of spin-orbit coupling the hydrogen-bonded minimum remains at the same depth, but the T-shaped minimum washes out to less than half of its spin-free value. The lowest bound state is localized in the linear minimum. The spin-orbit coupling plays a very important role in shaping of the potential energy surfaces of Br-HBr.

  18. Infrared Spectroscopy of the H2/HD/D2-O2 Van Der Waals Complexes

    NASA Astrophysics Data System (ADS)

    Raston, Paul; Bunn, Hayley

    2016-06-01

    Hydrogen is the most abundant element in the universe and oxygen is the third, so understanding the interaction between the two in their different forms is important to understanding astrochemical processes. The interaction between H2 and O2 has been explored in low energy scattering experiments and by far infrared synchrotron spectroscopy of the van der Waals complex. The far infrared spectra suggest a parallel stacked average structure with seven bound rotationally excited states. Here, we present the far infrared spectrum of HD/D2-O2 and the mid infrared spectrum of H2-O2 at 80 K, recorded at the infrared beamline facility of the Australian Synchrotron. We observed 'sharp' peaks in the mid infrared region, corresponding to the end over end rotation of H2-O2, that are comparatively noisier than analogous peaks in the far infrared where the synchrotron light is brightest. The larger reduced mass of HD and D2 compared to H2 is expected to result in more rotational bound states and narrower bands. The latest results in our ongoing efforts to explore this system will be presented. Y. Kalugina, et al., Phys. Chem. Chem. Phys. 14, 16458 (2012) S. Chefdeville et al. Science 341, 1094 (2013) H. Bunn et al. ApJ 799, 65 (2015)

  19. Holographic Van der Waals-like phase transition in the Gauss-Bonnet gravity

    NASA Astrophysics Data System (ADS)

    He, Song; Li, Li-Fang; Zeng, Xiao-Xiong

    2017-02-01

    The Van der Waals-like phase transition is observed in temperature-thermal entropy plane in spherically symmetric charged Gauss-Bonnet-AdS black hole background. In terms of AdS/CFT, the non-local observables such as holographic entanglement entropy, Wilson loop, and two point correlation function of very heavy operators in the field theory dual to spherically symmetric charged Gauss-Bonnet-AdS black hole have been investigated. All of them exhibit the Van der Waals-like phase transition for a fixed charge parameter or Gauss-Bonnet parameter in such gravity background. Further, with choosing various values of charge or Gauss-Bonnet parameter, the equal area law and the critical exponent of the heat capacity are found to be consistent with phase structures in temperature-thermal entropy plane.

  20. First principles calculations of solid-state thermionic transport in layered van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan

    2016-08-01

    This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling-dominated transport to thermionic-dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The use of layered van der Waals heterostructures has two advantages: (a) thermionic transport barriers can be tuned by changing the number of layers, and (b) thermal conductance across these non-covalent structures is very weak. The phonon thermal conductance of the present van der Waals heterostructure is found to be 4.1 MW m-2 K-1 which is one order of magnitude lower than the lowest value for that of covalently-bonded interfaces. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices. This study shows that layered van der Waals structures have great potential to be used as solid-state energy-conversion devices.This work aims at understanding solid-state energy conversion and transport in layered (van der Waals) heterostructures in contact with metallic electrodes via a first-principles approach. As an illustration, a graphene/phosphorene/graphene heterostructure in contact with gold electrodes is studied by using density functional theory (DFT)-based first principles calculations combined with real space Green's function (GF) formalism. We show that for a monolayer phosphorene, quantum tunneling dominates the

  1. Steady dynamics of exothermic chemical wave fronts in van der Waals fluids.

    PubMed

    Dumazer, G; Antoine, C; Lemarchand, A; Nowakowski, B

    2009-12-01

    We study the steady dynamics of an exothermic Fisher-Kolmogorov-Petrovsky-Piskunov chemical wave front traveling in a one-dimensional van der Waals fluid. The propagating wave is initiated by a nonuniformity in reactant concentration contrary to usual combustion ignition processes. The heat release and activation energy of the reaction play the role of control parameters. We recently proved that the propagation of an exothermic chemical wave front in a perfect gas displays a forbidden interval of stationary wave front speeds [G. Dumazer, M. Leda, B. Nowakowski, and A. Lemarchand, Phys. Rev. E 78, 016309 (2008)]. We examine how this result is modified for nonideal fluids and determine the effect of the van der Waals parameters and fluid density on the bifurcation between diffusion flames and Chapman-Jouguet detonation waves as heat release increases. Analytical predictions are confirmed by the numerical solution of the hydrodynamic equations including reaction kinetics.

  2. Pattern-free thermal modulator via thermal radiation between Van der Waals materials

    NASA Astrophysics Data System (ADS)

    Liu, Xianglei; Shen, Jiadong; Xuan, Yimin

    2017-10-01

    Modulating heat flux provides a platform for a plethora of emerging devices such as thermal diodes, thermal transistors, and thermal memories. Here, a pattern-free noncontact thermal modulator is proposed based on the mechanical rotation between two Van der Waals films with optical axes parallel to the surfaces. A modulation contrast can reach a value higher than 5 for hexagonal Boron Nitride (hBN) films separated by a nanoscale gap distance. The dominant radiative heat exchange comes from the excitation of both Type I and Type II hyperbolic surface phonon polaritons (HSPhPs) at the vacuum-hBN interface for different orientations, while the large modulation contrast is mainly attributed to the mismatching Type I HSPhPs induced by rotation. This work opens the possibility to design cheap thermal modulators without relying on nanofabrication techniques, and paves the way to apply natural Van der Waals materials in manipulating heat currents in an active way.

  3. Microwave spectra of van der Waals complexes of importance in planetary atmospheres

    NASA Technical Reports Server (NTRS)

    Suenram, R. D.; Lovas, F. J.

    1990-01-01

    The Fourier-transform Fabry-Perot pulsed-molecular-beam microwave spectrometer at NIST was used to study the microwave spectra of a number of molecular dimers and trimers that may be present in planetary atmospheres. The weak van der Waals bonds associated with these species usually give rise to rotational-tunneling splittings in the microwave spectra. The microwave spectrum of the water dimer species was used to illustrate the complications that can arise in the study of the rotational spectra of these loosely bound species. In addition to the water dimer species, the microwave spectra of the following hydrogen-bonded and van der Waals complexes were studied: (CO2)2-H2O, CO2-(H2O)2, CO2-H2S, N2-H2O, CO-H2O, SO2-H2O, and O3-H2O.

  4. Exact traveling wave solutions of the van der Waals normal form for fluidized granular matter

    NASA Astrophysics Data System (ADS)

    Abourabia, A. M.; Morad, A. M.

    2015-11-01

    Analytical solutions of the van der Waals normal form for fluidized granular media have been done to study the phase separation phenomenon by using two different exact methods. The Painlevé analysis is discussed to illustrate the integrability of the model equation. An auto-Bäcklund transformation is presented via the truncated expansion and symbolic computation. The results show that the exact solutions of the model introduce solitary waves of different types. The solutions of the hydrodynamic model and the van der Waals equation exhibit a behavior similar to the one observed in molecular dynamic simulations such that two pairs of shock and rarefaction waves appear and move away, giving rise to the bubbles. The dispersion properties and the relation between group and phase velocities of the model equation are studied using the plane wave assumption. The diagrams are drawn to illustrate the physical properties of the exact solutions, and indicate their stability and bifurcation.

  5. Electrical and optical properties of SnS2/WSe2 van der Waals Heterojunction FETs

    NASA Astrophysics Data System (ADS)

    Zubair, Ahmad; Nourbakhsh, Amirhasan; Dresselhaus, Mildred; Palacios, Tomas

    Two dimensional crystals based on atomically thin films of transition metal dichalcogenides offer an exciting platform for various optoelectronic applications. Their unique crystal properties make them particularly attractive for van der Waals heterostructures which open up an additional degree of freedom to tailor the material properties into new physics and device applications. In this work, we explore, for the first time, the optoelectronic properties of van der Waals SnS2/WSe2 heterojunction. WSe2 is an ambipolar semiconductor while SnS2 is an n-type wide bandgap semiconductor. We use the pickup and dry transfer methods to fabricate SnS2/WSe2 heterojunction transistors (hetero-FETs). We observe negative differential transconductance in the SnS2/WSe2 hetero-FET. Also, the heterostructure couples strongly to incident light and shows high photovoltaic responsivity which can find applications in nano-devices such as photo-detectors and solar cells.

  6. Van der Waals model for phase transitions in thermoresponsive surface films.

    PubMed

    McCoy, John D; Curro, John G

    2009-05-21

    Phase transitions in polymeric surface films are studied with a simple model based on the van der Waals equation of state. Each chain is modeled by a single bead attached to the surface by an entropic-Hooke's law spring. The surface coverage is controlled by adjusting the chemical potential, and the equilibrium density profile is calculated with density functional theory. The interesting feature of this model is the multivalued nature of the density profile seen at low temperature. This van der Waals loop behavior is resolved with a Maxwell construction between a high-density phase near the wall and a low-density phase in a "vertical" phase transition. Signatures of the phase transition in experimentally measurable quantities are then found. Numerical calculations are presented for isotherms of surface pressure, for the Poisson ratio, and for the swelling ratio.

  7. Production of Protonated Methanol Ions Via Intermolecular Reactions within Van der Waals Clusters of Dime Dimethyl Ether. Revision

    DTIC Science & Technology

    1990-02-02

    preparation (17) This result also suggests that the protonated methanol ion is not produced via a reaction between the DME cluster and a water impurity. In...include Security Classification) Production ol Protonated Methanol Ions via "Intermolecular" Reactions within van der Waals Clusters of Dimethyl Ether...2/90 Production of Protonated Methanol Ions via "Intermolecular" Reactions within van der Waals Clusters of Dimethyl Ether M. Todd Coolbaugh, William

  8. Quasi-1D van der Waals materials as high current-density local interconnects (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Stolyarov, Maxim; Aytan, Ece; Bloodgood, Matthew; Salguero, Tina T.; Balandin, Alexander A.

    2016-09-01

    The continuous downscaling of interconnect dimensions in combination with the introduction of low-k dielectrics has increased the number of heat dissipation, integration and reliability challenges in modern electronics. As a result, there is a strong need for new materials that have high current-carrying capacity for applications as nanoscale interconnects. In this presentation, we show that quasi-one-dimensional (1D) van der Waals metals such as TaSe3 have excellent breakdown current density exceeding that of 5 MA/cm2. This value is above that currently achievable in conventional copper or aluminum wires. The quasi-1D van der Waals materials are characterized by strong bonds along one dimension and weak van der Waals bonds along two other dimensions. The material for this study was grown by the chemical vapor transport (CVT) method. Both mechanical and chemical exfoliation methods were used to fabricate nanowires with lateral dimensions below 100 nm. The dimensions of the quasi-1D nanowires were verified with scanning electron microscopy (SEM) and atomic force microscopy (AFM). The metal (Ti/Au) contacts for the electrical characterization were deposited using electron beam evaporation (EBE). The measurements were conducted on a number of prototype interconnects with multiple electric contacts to ensure reproducibility. The obtained results suggest that quasi-1D van der Waals metals present a feasible alternative to conventional copper interconnects in terms of the current-carrying capacity and the breakdown current-density. This work was supported, in part, by the SRC and DARPA through STARnet Center for Function Accelerated nanoMaterial Engineering (FAME).

  9. The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures

    DOE PAGES

    Wang, Han; Bang, Junhyeok; Sun, Yiyang; ...

    2016-05-10

    Here, the success of van der Waals (vdW) heterostructures, made of graphene, metal dichalcogenides, and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that vdW heterostructues can exhibit ultra-fast charge transfer despite the weak binding of the heterostructure. Using time-dependent density functional theory molecular dynamics, we identify a strong dynamic coupling between the vdW layers associated with charge transfer. This dynamic coupling results in rapid nonlinear coherentmore » charge oscillations which constitute a purely electronic phenomenon and are shown to be a general feature of vdW heterostructures provided they have a critical minimum dipole coupling. Application to MoS2/WS2 heterostructure yields good agreement with experiment, indicating near complete charge transfer within a timescale of 100 fs.The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the

  10. The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures

    SciTech Connect

    Wang, Han; Bang, Junhyeok; Sun, Yiyang; West, Damien; Meunier, Vincent; Zhang, Shengbai; Liang, Linagbo

    2016-05-10

    Here, the success of van der Waals (vdW) heterostructures, made of graphene, metal dichalcogenides, and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that vdW heterostructues can exhibit ultra-fast charge transfer despite the weak binding of the heterostructure. Using time-dependent density functional theory molecular dynamics, we identify a strong dynamic coupling between the vdW layers associated with charge transfer. This dynamic coupling results in rapid nonlinear coherent charge oscillations which constitute a purely electronic phenomenon and are shown to be a general feature of vdW heterostructures provided they have a critical minimum dipole coupling. Application to MoS2/WS2 heterostructure yields good agreement with experiment, indicating near complete charge transfer within a timescale of 100 fs.The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the interface

  11. Electronic spectroscopy of large van der waals molecules by resonant two-photon ionization

    NASA Astrophysics Data System (ADS)

    Leutwyler, Samuel; Even, Uzi; Jortner, Joshua

    1982-03-01

    Tunable laser two-photon ionization of large van der Waals molecules, combined with time-of-flight mass spectroscopy. was applied to the identification of the electronic origin and of some low vibrational excitations of the S 0 — S 1 electronic transition of fluorene·Ar 1, fluorene·Ar 2 and fluorene·Kr 1 produced in supersonic expansions.

  12. A Van der Waals-like theory of plasma double layers

    NASA Technical Reports Server (NTRS)

    Katz, Ira; Davis, V. A.

    1989-01-01

    A theory describing plasma double layers in terms of multiple roots of the charge density expression is presented. The theory presented uses the fact that equilibrium plasmas shield small potential perturbations linearly; for high potentials, the shielding decreases. The approach is analogous to Van der Waals' theory of simple fluids in which inclusion of approximate expressions for both excluded volume and long range attractive forces sufficiently describes the first-order liquid-gas phase transition.

  13. Passivation of Black Phosphorus via Self-Assembled Organic Monolayers by van der Waals Epitaxy.

    PubMed

    Zhao, Yinghe; Zhou, Qionghua; Li, Qiang; Yao, Xiaojing; Wang, Jinlan

    2017-02-01

    An effective passivation approach to protect black phosphorus (BP) from degradation based on multi-scale simulations is proposed. The self-assembly of perylene-3,4,9,10-tetracarboxylic dianhydride monolayers via van der Waals epitaxy on BP does not break the original electronic properties of BP. The passivation layer thickness is only 2 nm. This study opens up a new pathway toward fine passivation of BP.

  14. Van der Waals Type Model and Structure in π-p Elastic Scattering at High Energies

    NASA Astrophysics Data System (ADS)

    Aleem, F.

    1982-10-01

    The most recent measurement of the angular distribution for π-p elastic scattering at pL =50 and 200 GeV/c which show a structure near -t ≈ 4(GeV/c)2, with squared four momentum transfer -t extended to 10(GeV/c)2, and the total cross section data for 50 ≤ pL ≤ 370 GeV/c have been simultaneously explained by using Van der Waal's type model.

  15. EXPERIMENTAL AND THEORETICAL STUDIES OF THE CN-AR VAN DER WAALS COMPLEX

    USDA-ARS?s Scientific Manuscript database

    The CN-Ar van der Waals complex has been observed using B2E+-X2E+ and A2II-X2E+ electronic transitions. The spectra yielded a dissociation energy of D0"=109+2 cm1 and a zero point rational constant of B0"=0.067+0.005 cm-1 for CN(x)-Ar. The dissociation energy for Cn(A)-Ar was found to be D0"=132+2...

  16. van der Waals epitaxy and photoresponse of two-dimensional CdSe plates

    NASA Astrophysics Data System (ADS)

    Zhu, Dan-Dan; Xia, Jing; Wang, Lei; Li, Xuan-Ze; Tian, Li-Feng; Meng, Xiang-Min

    2016-06-01

    Here we demonstrate the first growth of two-dimensional (2D) single-crystalline CdSe plates on mica substrates via van der Waals epitaxy. The as-synthesized 2D plates exhibit hexagonal, truncated triangular and triangular shapes with the lateral size around several microns. Photodetectors based on 2D CdSe plates present a fast response time of 24 ms, revealing that 2D CdSe is a promising building block for ultrathin optoelectronic devices.

  17. Direct observation of adsorption geometry for the van der Waals adsorption of a single π-conjugated hydrocarbon molecule on Au(111).

    PubMed

    Kim, Ju-Hyung; Jung, Jaehoon; Tahara, Kazukuni; Tobe, Yoshito; Kim, Yousoo; Kawai, Maki

    2014-02-21

    Weak van der Waals adsorption of π-conjugated hydrocarbon molecules onto the gold surface, Au(111), is one of the essential processes in constructing organic-metal interfaces in organic electronics. Here we provide a first direct observation of adsorption geometry of a single π-conjugated hydrocarbon molecule on Au(111) using an atomically resolved scanning tunneling microscopy study combined with van der Waals density functional methodology. For the purpose, we utilized a highly symmetric π-conjugated hydrocarbon molecule, dehydrobenzo[12]annulene (DBA), which has a definite three-fold symmetry, the same as the Au(111) surface. Interestingly, our observations on an atomically resolved scale clearly indicate that the DBA molecule has only one adsorption configuration on Au(111) in spite of the weak van der Waals adsorption system. Based on the precisely determined adsorption geometry of DBA/Au(111), our calculation results imply that even a very small contribution of the interfacial orbital interaction at the organic-metal interface can play a decisive role in constraining the adsorption geometry even in the van der Waals adsorption system of a π-conjugated hydrocarbon molecule on the noblest Au(111) surface. Our observations provide not only deeper insight into the weak adsorption process, but also new perspectives to organic electronics using π-conjugated hydrocarbon molecules on the Au surface.

  18. Direct observation of adsorption geometry for the van der Waals adsorption of a single π-conjugated hydrocarbon molecule on Au(111)

    SciTech Connect

    Kim, Ju-Hyung; Jung, Jaehoon; Kim, Yousoo E-mail: ykim@riken.jp; Tahara, Kazukuni; Tobe, Yoshito E-mail: ykim@riken.jp; Kawai, Maki E-mail: ykim@riken.jp

    2014-02-21

    Weak van der Waals adsorption of π-conjugated hydrocarbon molecules onto the gold surface, Au(111), is one of the essential processes in constructing organic-metal interfaces in organic electronics. Here we provide a first direct observation of adsorption geometry of a single π-conjugated hydrocarbon molecule on Au(111) using an atomically resolved scanning tunneling microscopy study combined with van der Waals density functional methodology. For the purpose, we utilized a highly symmetric π-conjugated hydrocarbon molecule, dehydrobenzo[12]annulene (DBA), which has a definite three-fold symmetry, the same as the Au(111) surface. Interestingly, our observations on an atomically resolved scale clearly indicate that the DBA molecule has only one adsorption configuration on Au(111) in spite of the weak van der Waals adsorption system. Based on the precisely determined adsorption geometry of DBA/Au(111), our calculation results imply that even a very small contribution of the interfacial orbital interaction at the organic-metal interface can play a decisive role in constraining the adsorption geometry even in the van der Waals adsorption system of a π-conjugated hydrocarbon molecule on the noblest Au(111) surface. Our observations provide not only deeper insight into the weak adsorption process, but also new perspectives to organic electronics using π-conjugated hydrocarbon molecules on the Au surface.

  19. The hot pick-up technique for batch assembly of van der Waals heterostructures

    PubMed Central

    Pizzocchero, Filippo; Gammelgaard, Lene; Jessen, Bjarke S.; Caridad, José M.; Wang, Lei; Hone, James; Bøggild, Peter; Booth, Timothy J.

    2016-01-01

    The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures. PMID:27305833

  20. Thermogeometric description of the van der Waals like phase transition in AdS black holes

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Krishnakanta; Majhi, Bibhas Ranjan

    2017-05-01

    It is well known that by interpreting the cosmological constant as the pressure, the anti-de Sitter black holes behave as the van der Waals thermodynamic system. In this case, like a phase transition from vapor to liquid in a usual van der Waals system, black holes also change phases about a critical point in the P -V picture, where P is the pressure and V is the thermodynamic volume. Here, we give a geometrical description of this phase transition. Defining the relevant Legendre-invariant thermogeometrics corresponding to the two criticality conditions, which determine the critical values of respective thermodynamical entities, we show that the critical point refers to the divergence of the Ricci scalars calculated from these metrics. The similar descriptions are also provided for the other two pictures of the van der Waals like phase transition: one is T -S and the other one is Y -X where T , S , X and Y are temperature, entropy, generalized force and generalized displacement (i.e. potential corresponding to external charge, respectively). The whole discussion is very general as no specific black-hole metric is being used.

  1. The hot pick-up technique for batch assembly of van der Waals heterostructures.

    PubMed

    Pizzocchero, Filippo; Gammelgaard, Lene; Jessen, Bjarke S; Caridad, José M; Wang, Lei; Hone, James; Bøggild, Peter; Booth, Timothy J

    2016-06-16

    The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.

  2. Multiple critical points and liquid liquid equilibria from the van der Waals like equations of state

    NASA Astrophysics Data System (ADS)

    Artemenko, Sergey; Lozovsky, Taras; Mazur, Victor

    2008-06-01

    The principal aim of this work is a comprehensive analysis of the phase diagram of water via the van der Waals like equations of state (EoSs) which are considered as superpositions of repulsive and attractive forces. We test more extensively the modified van der Waals EoS (MVDW) proposed by Skibinski et al (2004 Phys. Rev. E 69 061206) and refine this model by introducing instead of the classical van der Waals repulsive term a very accurate hard sphere EoS over the entire stable and metastable regions (Liu 2006 Preprint cond-mat/0605392). It was detected that the simplest form of MVDW EoS displays a complex phase behavior, including three critical points, and identifies four fluid phases (gas, low density liquid (LDL), high density liquid (HDL), and very high density liquid (VHDL)). Moreover the experimentally observed (Mallamace et al 2007 Proc. Natl Acad. Sci. USA 104 18387) anomalous behavior of the density of water in the deeply supercooled region (a density minimum) is reproduced by the MWDW EoS. An improvement of the repulsive part does not change the topological picture of the phase behavior of water in the wide range of thermodynamic variables. The new parameters set for second and third critical points are recognized by thorough analysis of experimental data for the loci of thermodynamic response function extrema.

  3. Antiferromagnetism in the van der Waals layered spin-lozenge semiconductor CrTe3

    DOE PAGES

    McGuire, Michael A.; Garlea, V. Ovidiu; KC, Santosh; ...

    2017-04-14

    We have investigated the crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound CrTe 3 on single-crystal and polycrystalline materials. Furthermore, the crystal structure contains layers made up of lozenge-shaped Cr 4 tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3 eV. The magnetic susceptibility exhibits a broad maximum near 300 K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55 K, and single-crystal neutron diffraction reveals the onset of long-range antiferromagnetic order at thismore » temperature. Strongly dispersive spin waves are observed in the ordered state. Significant magnetoelastic coupling is indicated by the anomalous temperature dependence of the lattice parameters and is evident in structural optimization in van der Waals density functional theory calculations for different magnetic configurations. The cleavability of the compound is apparent from its handling and is confirmed by first-principles calculations, which predict a cleavage energy 0.5 J / m 2 , similar to graphite. Based on our results, CrTe 3 is identified as a promising compound for studies of low dimensional magnetism in bulk crystals as well as magnetic order in monolayer materials and van der Waals heterostructures.« less

  4. The hot pick-up technique for batch assembly of van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Pizzocchero, Filippo; Gammelgaard, Lene; Jessen, Bjarke S.; Caridad, José M.; Wang, Lei; Hone, James; Bøggild, Peter; Booth, Timothy J.

    2016-06-01

    The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.

  5. Black phosphorene/monolayer transition-metal dichalcogenides as two dimensional van der Waals heterostructures: a first-principles study.

    PubMed

    You, Baiqing; Wang, Xiaocha; Zheng, Zhida; Mi, Wenbo

    2016-03-14

    The electronic structure of black phosphorene (BP)/monolayer 1H-XT2 (X = Mo, W; T = S, Se, Te) two dimensional (2D) van der Waals heterostructures have been calculated by the first-principles method. It is found that the electronic band structures of both BP and XT2 are preserved in the combined van der Waals heterostructures. The WSe2/BP van der Waals heterostructure demonstrates a type-I band alignment, but the MoS2/BP, MoSe2/BP, MoTe2/BP, WS2/BP and WTe2/BP van der Waals heterostructures demonstrate a type-II band alignment. In particular, the n-type XT2/p-type BP van der Waals heterostructures can be applied in p-n diode and logical devices. Strong spin splitting appears in all of the heterostructures when considering the spin orbital coupling. Our results play a significant role in the prediction of novel 2D van der Waals heterostructures that have potential applications in spin-filter devices, spin field effect transistors, optoelectronic devices, etc.

  6. Tkatchenko-Scheffler van der Waals correction method with and without self-consistent screening applied to solids

    NASA Astrophysics Data System (ADS)

    Bučko, Tomáš; Lebègue, S.; Hafner, Jürgen; Ángyán, J. G.

    2013-02-01

    The method proposed by Tkatchenko and Scheffler [Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.102.073005 102, 073005 (2009)] to correct density functional calculations for the missing van der Waals interactions is implemented in the Vienna ab initio simulation package (vasp) code and tested on a wide range of solids, including noble-gas crystals, molecular crystals (α-N2, sulfur dioxide, benzene, naphthalene, cytosine), layered solids (graphite, hexagonal boron nitride, vanadium pentoxide, MoS2, NbSe2), chain-like structures (selenium, tellurium, cellulose I), ionic crystals (NaCl, KI), and metals (nickel, zinc, cadmium). In addition to the original formulation expressing the van der Waals (vdW) corrections as pairwise potentials whose strength is derived from the rescaled polarizabilities of the neutral free atoms, the self-consistently screened (TS+SCS) [Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.108.236402 108, 236402 (2012)] variant of the method involving electrodynamic response effects has been examined. Analytical expressions for the forces acting on the atoms and for the components of the stress tensor needed for the relaxation of the volume and shape of the unit cell using the TS+SCS method are derived. While the calculated structures are reasonably close to experiment, the van der Waals corrections to the binding energies are often found to be overestimated in comparison with experimental data. The TS+SCS approach leads to significantly better results in some problematic cases, such as the binding energy of graphite. However, there is room for further improvements, in particular for strongly ionic systems.

  7. Flexible Multiferroic Bulk Heterojunction with Giant Magnetoelectric Coupling via van der Waals Epitaxy

    DOE PAGES

    Amrillah, Tahta; Bitla, Yugandhar; Shin, Kwangwoo; ...

    2017-05-22

    Magnetoelectric nanocomposites have been a topic of intense research due to their profound potential in the applications of electronic devices based on spintronic technology. Nevertheless, in spite of significant progress made in the growth of high-quality nanocomposite thin films, the substrate clamping effect still remains a major hurdle in realizing the ultimate magnetoelectric coupling. To overcome this obstacle, an alternative strategy of fabricating a self-assembled ferroelectric–ferrimagnetic bulk heterojunction on a flexible muscovite via van der Waals epitaxy is adopted. In this paper, we investigated the magnetoelectric coupling in a self-assembled BiFeO3 (BFO)–CoFe2O4 (CFO) bulk heterojunction epitaxially grown on a flexiblemore » muscovite substrate. The obtained heterojunction is composed of vertically aligned multiferroic BFO nanopillars embedded in a ferrimagnetic CFO matrix. Moreover, due to the weak interaction between the flexible substrate and bulk heterojunction, the interface is incoherent and, hence, the substrate clamping effect is greatly reduced. The phase-field simulation model also complements our results. The magnetic and electrical characterizations highlight the improvement in magnetoelectric coupling of the BFO–CFO bulk heterojunction. A magnetoelectric coupling coefficient of 74 mV/cm·Oe of this bulk heterojunction is larger than the magnetoelectric coefficient reported earlier on flexible substrates. Finally and therefore, this study delivers a viable route of fabricating a remarkable magnetoelectric heterojunction and yet flexible electronic devices that are robust against extreme conditions with optimized performance.« less

  8. Assessment of van der Waals inclusive density functional theory methods for layered electroactive materials.

    PubMed

    Lozano, Ariel; Escribano, Bruno; Akhmatskaya, Elena; Carrasco, Javier

    2017-04-12

    The discovery of computationally driven materials requires efficient and accurate methods. Density functional theory (DFT) meets these two requirements for many classes of materials. However, DFT-based methods have limitations. One significant shortcoming is the inadequate treatment of weak van der Waals (vdW) interactions, which are crucial for layered materials. Here we assess the performance of various vdW-inclusive DFT approaches for predicting the structure and voltage of layered electroactive materials for Li-ion batteries, considering a set of 20 different compounds. We find that the so-called optB86b-vdW density functional improves the agreement with the experimental data, closely followed by the latest generation of dispersion correction methods. These approaches yield average relative errors for the structural parameters smaller than 3%. The average deviations for redox potentials are below 0.15 V. Looking ahead, this study identifies accurate methods for Li-ion vdW bound systems, providing enhanced predictive power to DFT-assisted design for developing new types of electroactive materials in general.

  9. Optoelectronic Properties of Van Der Waals Hybrid Structures: Fullerenes on Graphene Nanoribbons

    PubMed Central

    Correa, Julián David; Orellana, Pedro Alejandro; Pacheco, Mónica

    2017-01-01

    The search for new optical materials capable of absorbing light in the frequency range from visible to near infrared is of great importance for applications in optoelectronic devices. In this paper, we report a theoretical study of the electronic and optical properties of hybrid structures composed of fullerenes adsorbed on graphene and on graphene nanoribbons. The calculations are performed in the framework of the density functional theory including the van der Waals dispersive interactions. We found that the adsorption of the C60 fullerenes on a graphene layer does not modify its low energy states, but it has strong consequences for its optical spectrum, introducing new absorption peaks in the visible energy region. The optical absorption of fullerenes and graphene nanoribbon composites shows a strong dependence on photon polarization and geometrical characteristics of the hybrid systems, covering a broad range of energies. We show that an external electric field across the nanoribbon edges can be used to tune different optical transitions coming from nanoribbon–fullerene hybridized states, which yields a very rich electro-absorption spectrum for longitudinally polarized photons. We have carried out a qualitative analysis on the potential of these hybrids as possible donor-acceptor systems in photovoltaic cells. PMID:28336904

  10. Van der Waals engineering of ferromagnetic semiconductor heterostructures for spin and valleytronics

    DOE PAGES

    Zhong, Ding; Seyler, Kyle L.; Linpeng, Xiayu; ...

    2017-05-31

    The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of the spin and valley pseudospin in WSe2, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting and polarization via flipping of the CrI3 magnetization. The WSe2 photoluminescence intensity strongly depends on the relative alignment between photoexcitedmore » spins in WSe2 and the CrI3 magnetization, because of ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.« less

  11. Electronic Properties of Surfaces and Interfaces with Self-Consistent van der Waals Density Functional

    NASA Astrophysics Data System (ADS)

    Ferri, Nicola; Distasio, Robert A., Jr.; Car, Roberto; Tkatchenko, Alexandre; Scheffler, Matthias

    2014-03-01

    The long-range van der Waals (vdW) energy is only a small part of the total energy, hence it is typically assumed to have a minor influence on the electronic properties. Here, we address this question through a fully self-consistent (SC) implementation of the Tkatchenko-Scheffler (TS) density functional. The analysis of TS-vdWSC effects on electron density differences for atomic and molecular dimers reveals quantitative agreement with correlated densities obtained from ``gold standard'' coupled-cluster quantum-chemical calculations. In agreement with previous work, we find a very small overall contribution from self-consistency in the structure and stability of vdW-bound molecular complexes. However, TS-vdWSC (coupled with PBE functional) significantly affects electronic properties of coinage metal (111) surfaces, leading to an increase of up to 0.3 eV in the workfunction in agreement with experiments. Furthermore, vdW interactions visibly influence workfunctions in hybrid organic/metal interfaces, changing Pauli push-back and charge transfer contributions.

  12. Evaluating dispersion forces for optimization of van der Waals complexes using a non-empirical functional

    NASA Astrophysics Data System (ADS)

    Arabi, Alya A.

    2016-11-01

    Modelling dispersion interactions with traditional density functional theory (DFT) is a challenge that has been extensively addressed in the past decade. The exchange-dipole moment (XDM), among others, is a non-empirical add-on dispersion correction model in DFT. The functional PW86+PBE+XDM for exchange, correlation and dispersion, respectively, compromises an accurate functional for thermochemistry and for van der Waals (vdW) complexes at equilibrium and non-equilibrium geometries. To use this functional in optimizing vdW complexes, rather than computing single point energies, it is necessary to evaluate accurate forces. The purpose of this paper is to validate that, along the potential energy surface, the distance at which the energy is minimum is commensurate with the distance at which the forces vanish to zero. This test was validated for 10 rare gas diatomic molecules using various integration grids and different convergence criteria. It was found that the use of either convergence criterion, 10-6 or 10-8, in Gaussian09, does not affect the accuracy of computed optimal distances and binding energies. An ultra-fine grid needs to be used when computing accurate energies using generalized gradient approximation functionals. This article is part of the themed issue 'Multiscale modelling at the physics-chemistry-biology interface'.

  13. Effect of van der Waals corrections on DFT-computed metallic surface properties

    NASA Astrophysics Data System (ADS)

    Chiter, Fatah; Bac Nguyen, Van; Tarrat, Nathalie; Benoit, Magali; Tang, Hao; Lacaze-Dufaure, Corinne

    2016-04-01

    State-of-the-art van der Waals (vdW) corrected density functional theory (DFT) is routinely used to overcome the failure of standard DFT in the description of molecule/surface long range interactions. However, the systematic use of dispersion forces to model metallic surfaces could lead to less accurate results than the standard DFT and the effect of these corrections on the metal properties should be properly evaluated. In this framework, the behavior of two widely used vdW corrected DFT methods (DFT-D2 and vdW-DF/optB86b) has been evaluated on six metals, i.e. Al, Cu, Au, Ni, Co and Fe, with respect to standard GGA-DFT and experiments. Regarding bulk properties, general trends are found for the lattice parameter, cohesive energy and magnetic moment variations when the vdW correction is introduced. Surface energies, work functions and interlayer distances of closed packed surfaces, Al(111), Cu(111), Au(111) and magnetic Ni(111), Co(0001) and Fe(110), are also strongly affected by the dispersion forces. These modifications suggest a systematic verification of the surface properties when a dispersion correction is included.

  14. Evaluating dispersion forces for optimization of van der Waals complexes using a non-empirical functional.

    PubMed

    Arabi, Alya A

    2016-11-13

    Modelling dispersion interactions with traditional density functional theory (DFT) is a challenge that has been extensively addressed in the past decade. The exchange-dipole moment (XDM), among others, is a non-empirical add-on dispersion correction model in DFT. The functional PW86+PBE+XDM for exchange, correlation and dispersion, respectively, compromises an accurate functional for thermochemistry and for van der Waals (vdW) complexes at equilibrium and non-equilibrium geometries. To use this functional in optimizing vdW complexes, rather than computing single point energies, it is necessary to evaluate accurate forces. The purpose of this paper is to validate that, along the potential energy surface, the distance at which the energy is minimum is commensurate with the distance at which the forces vanish to zero. This test was validated for 10 rare gas diatomic molecules using various integration grids and different convergence criteria. It was found that the use of either convergence criterion, 10(-6) or 10(-8), in Gaussian09, does not affect the accuracy of computed optimal distances and binding energies. An ultra-fine grid needs to be used when computing accurate energies using generalized gradient approximation functionals.This article is part of the themed issue 'Multiscale modelling at the physics-chemistry-biology interface'. © 2016 The Author(s).

  15. Mg (OH) 2-WS2 van der Waals heterobilayer: Electric field tunable band-gap crossover

    NASA Astrophysics Data System (ADS)

    Yagmurcukardes, M.; Torun, E.; Senger, R. T.; Peeters, F. M.; Sahin, H.

    2016-11-01

    Magnesium hydroxide [Mg (OH) 2] has a layered brucitelike structure in its bulk form and was recently isolated as a new member of two-dimensional monolayer materials. We investigated the electronic and optical properties of monolayer crystals of Mg (OH) 2 and WS2 and their possible heterobilayer structure by means of first-principles calculations. It was found that both monolayers of Mg (OH) 2 and WS2 are direct-gap semiconductors and these two monolayers form a typical van der Waals heterostructure with a weak interlayer interaction and a type-II band alignment with a staggered gap that spatially separates electrons and holes. We also showed that an out-of-plane electric field induces a transition from a staggered to a straddling-type heterojunction. Moreover, by solving the Bethe-Salpeter equation on top of single-shot G0W0 calculations, we show that the low-energy spectrum of the heterobilayer is dominated by the intralyer excitons of the WS2 monolayer. Because of the staggered interfacial gap and the field-tunable energy-band structure, the Mg (OH) 2-WS2 heterobilayer can become an important candidate for various optoelectronic device applications in nanoscale.

  16. Fluctuations of the van der Waals attraction force between macroscopic bodies

    SciTech Connect

    Kogan, A. S.

    2003-01-01

    The tliccry of the fluctuations of the van der Waals (vdW) attractive force between macroscopic bodies is developed. A general equation for the spectral density of the fluctuating surface Maxwell stress (force per unit mea.) in va,cuurn nea,r the surface of a body is derived under the assumption that, inside the bodies, the random La.ngevin sources of the electric and magnetic fields (charges, polarizations, currents) are Gaussian. This spectral density of stress is an integral over frequencies of a sum of terms each of which is a product of Fourier amplitudes of two field components' correlation functions. For metallic bodies, the contribution of free electrons to the vdW force (at frequencies up to the frequency of electron scattering) is calculated. This contribution to the force and its noise grows with temperature. Application of noiseless voltage to two interacting metals across the vacuum gap between them generates an additional force noise. This additional noise is proportional to the volta.ge squared and to the spectral density of the random electric field at the frequency of noise measurement. The theoretical qualitative conclusions are in good agreement with experirncnts.

  17. Second osmotic virial coefficient from the two-component van der Waals equation of state.

    PubMed

    Widom, B; Underwood, Robin C

    2012-08-09

    The second osmotic virial coefficient is in principle obtained from the second-order term in the expansion of the osmotic pressure Π or solute activity z(2) in powers of the solute density ρ(2) at fixed solvent activity z(1) and temperature T. It is remarked that the second-order terms in the analogous expansions at fixed pressure p or at liquid-vapor coexistence instead of at fixed z(1) also provide measures of the effective, solvent-mediated solute-solute interactions, but these are different measures. It is shown here how the function z(2)(ρ(2), z(1), T) required to obtain the second osmotic virial coefficient B from an expansion in ρ(2) may be obtained from an equation of state of the form p = p(ρ(1), ρ(2), T) with ρ(1) the solvent density, and also how the analogous coefficient B' in the fixed-p expansion may be so obtained. The magnitude of the difference B - B' is often much smaller than that of B and B' separately, so B' is sometimes an acceptable approximation to B. That is not true of the analogous coefficient in the expansion at liquid-vapor coexistence. These calculations are illustrated with the van der Waals two-component equation of state and applied to solutions of propane in water as an example.

  18. Reconfigurable van der Waals Heterostructured Devices with Metal-Insulator Transition.

    PubMed

    Heo, Jinseong; Jeong, Heejeong; Cho, Yeonchoo; Lee, Jaeho; Lee, Kiyoung; Nam, Seunggeol; Lee, Eun-Kyu; Lee, Sangyeob; Lee, Hyangsook; Hwang, Sungwoo; Park, Seongjun

    2016-11-09

    Atomically thin two-dimensional (2D) materials range from semimetallic graphene to insulating hexagonal boron nitride to semiconducting transition-metal dichalcogenides. Recently, metal-insulator-semiconductor field effect transistors built from these 2D elements were studied for flexible and transparent electronics. However, to induce ambipolar characteristics for alternative power-efficient circuitry, ion-gel gating is often employed for high capacitive coupling, limiting stable operation at ambient conditions. Here, we report reconfigurable MoTe2 optoelectronic transistors with all 2D components, where the device can be reconfigured by both drain and gate voltages. Eight different configurations for each fixed voltage are spatially resolved by scanning photocurrent microscopy. In addition, metal-insulator transitions are observed in both electron and hole carriers under 2 V due to strong Coulomb interaction in the system. Furthermore, the vertical tunneling photocurrent through multiple van der Waals layers between the gate and source contacts is measured. Our reconfigurable devices offer potential building blocks for system-on-a-chip optoelectronics.

  19. Flexible Multiferroic Bulk Heterojunction with Giant Magnetoelectric Coupling via van der Waals Epitaxy.

    PubMed

    Amrillah, Tahta; Bitla, Yugandhar; Shin, Kwangwoo; Yang, Tiannan; Hsieh, Ying-Hui; Chiou, Yu-You; Liu, Heng-Jui; Do, Thi Hien; Su, Dong; Chen, Yi-Chun; Jen, Shien-Uang; Chen, Long-Qing; Kim, Kee Hoon; Juang, Jenh-Yih; Chu, Ying-Hao

    2017-06-27

    Magnetoelectric nanocomposites have been a topic of intense research due to their profound potential in the applications of electronic devices based on spintronic technology. Nevertheless, in spite of significant progress made in the growth of high-quality nanocomposite thin films, the substrate clamping effect still remains a major hurdle in realizing the ultimate magnetoelectric coupling. To overcome this obstacle, an alternative strategy of fabricating a self-assembled ferroelectric-ferrimagnetic bulk heterojunction on a flexible muscovite via van der Waals epitaxy is adopted. In this study, we investigated the magnetoelectric coupling in a self-assembled BiFeO3 (BFO)-CoFe2O4 (CFO) bulk heterojunction epitaxially grown on a flexible muscovite substrate. The obtained heterojunction is composed of vertically aligned multiferroic BFO nanopillars embedded in a ferrimagnetic CFO matrix. Moreover, due to the weak interaction between the flexible substrate and bulk heterojunction, the interface is incoherent and, hence, the substrate clamping effect is greatly reduced. The phase-field simulation model also complements our results. The magnetic and electrical characterizations highlight the improvement in magnetoelectric coupling of the BFO-CFO bulk heterojunction. A magnetoelectric coupling coefficient of 74 mV/cm·Oe of this bulk heterojunction is larger than the magnetoelectric coefficient reported earlier on flexible substrates. Therefore, this study delivers a viable route of fabricating a remarkable magnetoelectric heterojunction and yet flexible electronic devices that are robust against extreme conditions with optimized performance.

  20. van der Waals dispersion power laws for cleavage, exfoliation, and stretching in multiscale, layered systems

    NASA Astrophysics Data System (ADS)

    Gould, Tim; Gray, Evan; Dobson, John F.

    2009-03-01

    Layered and nanotubular systems that are metallic or graphitic are known to exhibit unusual dispersive van der Waals (vdW) power laws under some circumstances. In this Brief Report we investigate the vdW power laws of bulk and finite layered systems and their interactions with other layered systems and atoms in the electromagnetically nonretarded case. The investigation reveals substantial difference between “cleavage” and “exfoliation” of graphite and metals where cleavage obeys a C2D-2 vdW power law while exfoliation obeys a C3log(D/D0)D-3 law for graphitics and a C5/2D-5/2 law for layered metals. This leads to questions of relevance in the interpretation of experimental results for these systems which have previously assumed more trivial differences. Furthermore we gather further insight into the effect of scale on the vdW power laws of systems that simultaneously exhibit macroscopic and nanoscopic dimensions. We show that, for metallic and graphitic layered systems, the known “unusual” power laws can be reduced to standard or near standard power laws when the effective scale of one or more dimension is changed. This allows better identification of the systems for which the commonly employed “sum of C6D-6 ” type vdW methods might be valid such as layered bulk to layered bulk and layered bulk to atom.

  1. Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero-Bilayers.

    PubMed

    Luong, Dinh Hoa; Lee, Hyun Seok; Neupane, Guru Prakash; Roy, Shrawan; Ghimire, Ganesh; Lee, Jin Hee; Vu, Quoc An; Lee, Young Hee

    2017-09-01

    Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe2 /MoS2 hetero-bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band-to-band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative photovoltaic diode behavior, manifested as a drift current without interlayer excitonic emissions. This study sheds light on tailoring the tunneling transport for numerous optoelectronic HB devices. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  2. Low-energy universality and scaling of van der Waals forces

    SciTech Connect

    Calle Cordon, A.; Ruiz Arriola, E.

    2010-04-15

    At long distances, interactions between neutral ground-state atoms can be described by the van der Waals potential. In the ultracold regime, atom-atom scattering is dominated by s-waves phase shifts given by an effective range expansion in terms of the scattering length {alpha}{sub 0} and the effective range r{sub 0}. We show that while the scattering length cannot be predicted for these potentials, the effective range is given by the universal low-energy theorem r{sub 0}=A+B/{alpha}{sub 0}+C/{alpha}{sub 0}{sup 2}, where A, B, and C depend on the dispersion coefficients C{sub n} and the reduced diatom mass. We confront this formula to about 100 determinations of r{sub 0} and {alpha}{sub 0} and show why the result is dominated by the leading dispersion coefficient C{sub 6}. Universality and scaling extend much beyond naive dimensional analysis estimates.

  3. Asymmetric van der Waals Forces Drive Orientation of Compositionally Anisotropic Nanocylinders within Smectic Arrays: Experiment and Simulation

    PubMed Central

    Smith, Benjamin D.; Fichthorn, Kristen A.; Kirby, David J.; Quimby, Lisa M.; Triplett, Derek A.; González, Pedro; Hernández, Darimar; Keating, Christine D.

    2014-01-01

    Understanding how micro- and nanoparticles interact is important for achieving bottom-up assembly of desired structures. Here, we examine the self-assembly of two-component, compositionally asymmetric nanocylinders that sediment from solution onto a solid surface. These particles spontaneously formed smectic arrays. Within the rows of an array, nanocylinders tended to assemble such that neighboring particles had the same orientation of their segments. As a probe of interparticle interactions, we classified nanocylinder alignments by measuring the segment orientations of many sets of neighboring particles. Monte Carlo simulations incorporating an exact expression for the van der Waals (vdW) energy indicate that differences in the vdW interactions, even when small, are the key factor in producing observed segment alignment. These results point to asymmetrical vdW interactions as a potentially powerful means of controlling orientation in multicomponent cylinder arrays, and suggest that designing for these interactions could yield new ways to control self-assembly. PMID:24308771

  4. A theoretical study of He2ICl van der Waals cluster

    NASA Astrophysics Data System (ADS)

    Valdés, Álvaro; Prosmiti, Rita; Villarreal, Pablo; Delgado-Barrio, Gerardo

    2006-07-01

    The structure, energetics, and dynamics of He2ICl complex in its ground state are studied by means of ab initio electronic structure and quantum-mechanical calculations. Interaction energies for selected He2ICl configurations are calculated at the coupled-cluster [CCSD(T)] level of theory using a large-core pseudopotential for the I atom and the aug-cc-pVTZ and aug-cc-pV5Z basis sets for the Cl and He atoms, respectively. The surface is characterized around its lower five minima and the minimum energy pathways through them. The global minimum of the potential corresponds to a "police-nightstick (1)" configuration, the second one to a linear, the next one to tetrahedral configuration, and the following two to "bifork" and "police-nightstick (2)" structures, with well depths of -99.12, -97.42, -88.32, -85.84, and -78.54cm-1, respectively. An analytical form based on the sum of the three-body parametrized HeICl interactions plus the He-He interaction is found to represent very well the tetra-atomic CSSD(T) results. The present potential expression is employed to perform variational five-dimensional quantum-mechanical calculations to study the vibrational bound states of the van der Waals He2ICl complex. Results for total angular momentum J =0 provide the binding energy D0 and the corresponding vibrationally averaged structure for different isomers of the cluster. Comparison of these results with recent experimental observations further justifies the potential used in this work.

  5. X and Rb Atoms on Helium Nanodroplets: is the Van Der Waals Attraction Strong Enough to Form a Molecule?

    NASA Astrophysics Data System (ADS)

    Poms, Johannes; Hauser, Andreas W.; Ernst, Wolfgang E.

    2013-06-01

    Chemical reactions in the cold environment of a helium nanodroplet currently attract high interest and can be spectroscopically observed with typical molecular beam techniques. In order to estimate the influence of surrounding helium on the van der Waals interaction between heliophilic and heliophobic dopants that could be investigated in our lab with ESR spectroscopy, we apply density-functional theory to simulate a double-dotation of He-clusters with Rb and Xe atoms. Simulations of a double-doped He_{N} droplet with N = 500 show that the alkali metal atom stays on the surface, whereas the Xe atom sits in the middle of the droplet. The van der Waals attraction between Rb and Xe is not strong enough to compensate the separation of the heliophilic Xe and the heliophobic Rb caused by the helium droplet: a potential barrier of 23.4 K has to be overcome, which is to be compared with the 0.4 K internal temperature of the droplet. C. Callegari and W. E. Ernst, Helium Droplets as Nanocryostats for Molecular Spectroscopy - from the Vacuum Ultraviolet to the Microwave Regime, in: Handbook of High-Resolution Spectroscopy, eds. M. Quack and F. Merkt, John Wiley & Sons, Chichester (2011) M. Koch, C. Callegari, and W. E. Ernst, Mol. Phys. 108 (7), 1005-1011 (2010) J. Poms, A. W. Hauser, and W. E. Ernst, Phys. Chem. Chem. Phys. 14, 15158-15165 (2012)

  6. Toward a better understanding of dielectric responses of van der Waals liquids: The role of chemical structures

    NASA Astrophysics Data System (ADS)

    Jedrzejowska, Agnieszka; Wojnarowska, Zaneta; Adrjanowicz, Karolina; Ngai, K. L.; Paluch, Marian

    2017-03-01

    Exhaustive analysis of dielectric relaxation data of van der Waals glass-forming liquids revealed a strong correlation between the width of the frequency dispersion of the α-relaxation and the dielectric strength Δ ɛ , originating from the dipole-dipole interaction contribution to the intermolecular potential [M. Paluch et al., Phys. Rev. Lett. 116, 025702 (2016)]. The two van der Waals liquids, 4-vinyl-1,3-dioxolan-2-one (VPC) and 4-ethyl-1,3-dioxolan-2-one (EPC), have chemical structures modified from that of propylene carbonate. All three glass-formers have very similar values of dipole moments, exactly the same dielectric strength, and hence identical frequency dispersion of the α-relaxation in all three glass-formers is expected if the correlation holds. Based on this expectation, we performed dielectric relaxation measurements of the VPC and EPC at ambient and elevated pressures. The results obtained show not only identical α-relaxation frequency dispersion for the three glass-formers but also the excess wing which is the unresolved Johari-Goldstein β-relaxation. On the other hand, the other thermodynamics related parameters of the α-relaxation dynamics, including the glass transition temperature Tg, the fragility index mP, and activation volume Δ Va c t, are not uniformly the same for all three glass-formers.

  7. Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures via van der Waals Epitaxy.

    PubMed

    Samad, Leith; Bladow, Sage M; Ding, Qi; Zhuo, Junqiao; Jacobberger, Robert M; Arnold, Michael S; Jin, Song

    2016-07-26

    The fascinating semiconducting and optical properties of monolayer and few-layer transition metal dichalcogenides, as exemplified by MoS2, have made them promising candidates for optoelectronic applications. Controllable growth of heterostructures based on these layered materials is critical for their successful device applications. Here, we report a direct low temperature chemical vapor deposition (CVD) synthesis of MoS2 monolayer/multilayer vertical heterostructures with layer-controlled growth on a variety of layered materials (SnS2, TaS2, and graphene) via van der Waals epitaxy. Through precise control of the partial pressures of the MoCl5 and elemental sulfur precursors, reaction temperatures, and careful tracking of the ambient humidity, we have successfully and reproducibly grown MoS2 vertical heterostructures from 1 to 6 layers over a large area. The monolayer MoS2 heterostructure was verified using cross-sectional high resolution transmission electron microscopy (HRTEM) while Raman and photoluminescence spectroscopy confirmed the layer-controlled MoS2 growth and heterostructure electronic interactions. Raman, photoluminescence, and energy dispersive X-ray spectroscopy (EDS) mappings verified the uniform coverage of the MoS2 layers. This reaction provides an ideal method for the scalable layer-controlled growth of transition metal dichalcogenide heterostructures via van der Waals epitaxy for a variety of optoelectronic applications.

  8. Microwave spectrum and structure of the 3,5-difluoropyridine⋯CO2 van der Waals complex

    NASA Astrophysics Data System (ADS)

    Dewberry, Christopher T.; Cornelius, Ryan D.; Mackenzie, Rebecca B.; Smith, C. J.; Dvorak, Michael A.; Leopold, Kenneth R.

    2016-10-01

    The rotational spectrum of the weakly bound complex 3,5-difluoropyridine⋯CO2 has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent and 13CO2 isotopologues. The data indicate a planar structure in which the nitrogen approaches the carbon of the CO2 with either a C2v or effectively C2v geometry in the ground vibrational state. The N⋯C van der Waals bond distance is 2.8245(16) Å and the oxygen⋯ortho-hydrogen distance is 3.091(2) Å. The N⋯C van der Waals bond length is 0.027(8) Å longer than that previously determined for pyridine-CO2, but is still considerably shorter than the 2.998 Å distance in HCN⋯CO2. M06-2X/6-311++G(3df,3pd) calculations place the binding energy of the complex at 4.3 kcal/mol (4.1 kcal/mol with counterpoise correction). The calculations further indicate that a secondary interaction between the ortho-hydrogens of the ring and the CO2 oxygens account for ∼50% of the total binding energy.

  9. Finite-Size Effects on the Behavior of the Susceptibility in van der Waals Films Bounded by Strongly Absorbing Substrates

    NASA Technical Reports Server (NTRS)

    Dantchev, Daniel; Rudnick, Joseph; Barmatz, M.

    2007-01-01

    We study critical point finite-size effects in the case of the susceptibility of a film in which interactions are characterized by a van der Waals-type power law tail. The geometry is appropriate to a slab-like system with two bounding surfaces. Boundary conditions are consistent with surfaces that both prefer the same phase in the low temperature, or broken symmetry, state. We take into account both interactions within the system and interactions between the constituents of the system and the material surrounding it. Specific predictions are made with respect to the behavior of 3He and 4He films in the vicinity of their respective liquid-vapor critical points.

  10. van der Waals Heteroepitaxy of Germanene Islands on Graphite.

    PubMed

    Persichetti, Luca; Jardali, Fatme; Vach, Holger; Sgarlata, Anna; Berbezier, Isabelle; Crescenzi, Maurizio De; Balzarotti, Adalberto

    2016-08-18

    We fabricated flat, two-dimensional germanium sheets showing a honeycomb lattice that matches that of germanene by depositing submonolayers of Ge on graphite at room temperature and subsequent annealing to 350 °C. Scanning tunneling microscopy shows that the germanene islands have a small buckling with no atomic reconstruction and does not give any hints for alloy formation and hybridization with the substrate. Our density functional theory calculations of the structural properties agree well with our experimental findings and indicate that the germanene sheet interacts only weakly with the substrate underneath. Our band structure calculations confirm that the Dirac cone of free-standing germanene is preserved for layers supported on graphite. The germanene islands show a small but characteristic charge transfer with the graphite substrate which is predicted by our ab initio simulations in excellent agreement with scanning tunneling spectroscopy measurements.

  11. Nonlocal van der Waals density functional made simple and efficient

    NASA Astrophysics Data System (ADS)

    Sabatini, Riccardo; Gorni, Tommaso; de Gironcoli, Stefano

    2013-01-01

    We present a simple revision of the VV10 nonlocal density functional by Vydrov and Van Voorhis [J. Chem. Phys.JCPSA60021-960610.1063/1.3521275 133, 244103 (2010)] for dispersion interactions. Unlike the original functional our modification allows nonlocal correlation energy and its derivatives to be efficiently evaluated in a plane wave framework along the lines pioneered by Román-Pérez and Soler [Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.103.096102 103, 096102 (2009)]. Our revised functional maintains the outstanding precision of the original VV10 in noncovalently bound complexes and performs well in representative covalent, ionic, and metallic solids.

  12. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

    NASA Astrophysics Data System (ADS)

    Gong, Cheng; Li, Lin; Li, Zhenglu; Ji, Huiwen; Stern, Alex; Xia, Yang; Cao, Ting; Bao, Wei; Wang, Chenzhe; Wang, Yuan; Qiu, Z. Q.; Cava, R. J.; Louie, Steven G.; Xia, Jing; Zhang, Xiang

    2017-06-01

    The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin-Wagner theorem; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.

  13. Spontaneous doping on high quality talc-graphene-hBN van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Mania, E.; Alencar, A. B.; Cadore, A. R.; Carvalho, B. R.; Watanabe, K.; Taniguchi, T.; Neves, B. R. A.; Chacham, H.; Campos, L. C.

    2017-09-01

    Steady doping, added to its remarkable electronic properties, would make graphene a valuable commodity in the solar cell market, as energy power conversion could be substantially increased. Here we report a graphene van der Waals heterostructure which is able to spontaneously dope graphene (p-type) up to n ~ 2.2  ×  1013 cm-2 while providing excellent charge mobility (μ ~ 25 000 cm2 V-1 s-1). Such properties are achieved via deposition of graphene on atomically flat layered talc, a natural and abundant dielectric crystal. Raman investigation shows a preferential charge accumulation on graphene-talc van der Waals heterostructures, which are investigated through the electronic properties of talc/graphene/hBN heterostructure devices. These heterostructures preserve graphene’s good electronic quality, verified by the observation of quantum Hall effect at low magnetic fields (B  =  0.4 T) at T  =  4.2 K. In order to investigate the physical mechanisms behind graphene-on-talc p-type doping, we performed first-principles calculations of their interface structural and electronic properties. In addition to potentially improving solar cell efficiency, graphene doping via van der Waals stacking is also a promising route towards controlling the band gap opening in bilayer graphene, promoting a steady n or p type doping in graphene and, eventually, providing a new path to access superconducting states in graphene, predicted to exist only at very high doping.

  14. Light generation and harvesting in a van der Waals heterostructure.

    PubMed

    Lopez-Sanchez, Oriol; Alarcon Llado, Esther; Koman, Volodymyr; Fontcuberta i Morral, Anna; Radenovic, Aleksandra; Kis, Andras

    2014-03-25

    Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p-n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.

  15. 3D van der Waals σ-model and its topological excitations

    NASA Astrophysics Data System (ADS)

    Bulgadaev, S. A.

    2001-09-01

    It is shown that the 3D vector van der Waals nonlinear σ-model (NSM) on a sphere S2 has two types of topological excitations: reminiscent vortices and instantons of 2D NSM. The first ones, the hedgehogs, are described by the homotopic group π2(S2) = Z and have logarithmic energies. They are an analog of 2D vortices. The second ones, corresponding to 2D instantons, are the hopfions. They are described by the homotopic group π3(S2) = Z, or the Hopf invariant HinZ, and have finite energy. The possibility of a topological phase transition in this model and its applications are briefly discussed.

  16. Effects of zero van der Waals and zero electrostatic forces on droplet sedimentation

    NASA Technical Reports Server (NTRS)

    Omenyi, S. N.; Snyder, R. S.; Van Oss, C. J.; Absolom, D. R.; Neumann, A. W.

    1981-01-01

    The present investigation provides a confirmation of the dependence of droplet sedimentation on particle concentration. It is shown that it is possible to determine the maximum particle concentration which can remain stable on a given liquid from droplet sedimentation experiments. Droplet sedimentation can be reduced but not totally eliminated by the addition of appropriate amounts of dimethyl sulfoxide (DMSO) to reduce the van der Waals forces to zero. It was found that, at 12% DMSO, a maximum particle concentration of 6.3 x 10 to the 8th cells/ml of glutaraldehyde-fixed human erythrocytes suspended in physiological saline can remain stable on a D2O cushion.

  17. Hydrogen bonds and van der waals forces in ice at ambient and high pressures.

    PubMed

    Santra, Biswajit; Klimeš, Jiří; Alfè, Dario; Tkatchenko, Alexandre; Slater, Ben; Michaelides, Angelos; Car, Roberto; Scheffler, Matthias

    2011-10-28

    The first principles methods, density-functional theory and quantum Monte Carlo, have been used to examine the balance between van der Waals (vdW) forces and hydrogen bonding in ambient and high-pressure phases of ice. At higher pressure, the contribution to the lattice energy from vdW increases and that from hydrogen bonding decreases, leading vdW to have a substantial effect on the transition pressures between the crystalline ice phases. An important consequence, likely to be of relevance to molecular crystals in general, is that transition pressures obtained from density-functional theory exchange-correlation functionals which neglect vdW forces are greatly overestimated.

  18. Electro-photo modulation of the fermi level in WSe2/graphene van der Waals heterojunction

    NASA Astrophysics Data System (ADS)

    Sun, Honghui; Yang, Hang; Fang, Liang; Zhang, Jiangwei; Wang, Zhiyuan; Jiang, Tian

    2017-04-01

    We report an electro-photo double modulation of the fermi level in a WSe2/graphene heterojunction. The heterojunction exhibits high ION/IOFF ratio ( 103) in transfer characteristic in dark and distinct rectification behavior in output characteristic under light illumination, respectively. Time-dependent photoresponse reveals that the heterojunction has a considerable potential in the application of photodetection. Interestingly, an exotic current peak is observed in transfer characteristic under light illumination. This novel behavior is attributed to the tunable fermi level at the WSe2/graphene heterojunction by electro-photo double modulation. The results may be helpful to develop tunable photovoltaic optoelectronics based on van der Waals heterojunctions.

  19. On the choice of inertial axes for interpreting spectroscopic properties of van der Waals complexes

    NASA Astrophysics Data System (ADS)

    Ernesti, Andreas; Hutson, Jeremy M.

    1994-10-01

    Properties such as the dipole moments and nuclear quadrupole coupling constants of van der Waals complexes are important in the determination of intermolecular potential energy surfaces from high-resolution spectra. The properties are often interpreted in terms of angular expectation values. It is shown that, when calculating such properties, it is important to use an inertial axis system that satisfies the Eckart conditions. Projections onto other axes, such as the intermolecular vector or the instantaneous principal axes, can lead to substantial errors when the individual monomers have large moments of inertia.

  20. Super-Planckian Electron Cooling in a van der Waals Stack

    NASA Astrophysics Data System (ADS)

    Principi, Alessandro; Lundeberg, Mark B.; Hesp, Niels C. H.; Tielrooij, Klaas-Jan; Koppens, Frank H. L.; Polini, Marco

    2017-03-01

    Radiative heat transfer (RHT) between macroscopic bodies at separations that are much smaller than the thermal wavelength is ruled by evanescent electromagnetic modes and can be orders of magnitude more efficient than its far-field counterpart, which is described by the Stefan-Boltzmann law. In this Letter, we present a microscopic theory of RHT in van der Waals stacks comprising graphene and a natural hyperbolic material, i.e., hexagonal boron nitride (hBN). We demonstrate that RHT between hot carriers in graphene and hyperbolic phonon polaritons in hBN is extremely efficient at room temperature, leading to picosecond time scales for the carrier cooling dynamics.

  1. Super-Planckian Electron Cooling in a van der Waals Stack.

    PubMed

    Principi, Alessandro; Lundeberg, Mark B; Hesp, Niels C H; Tielrooij, Klaas-Jan; Koppens, Frank H L; Polini, Marco

    2017-03-24

    Radiative heat transfer (RHT) between macroscopic bodies at separations that are much smaller than the thermal wavelength is ruled by evanescent electromagnetic modes and can be orders of magnitude more efficient than its far-field counterpart, which is described by the Stefan-Boltzmann law. In this Letter, we present a microscopic theory of RHT in van der Waals stacks comprising graphene and a natural hyperbolic material, i.e., hexagonal boron nitride (hBN). We demonstrate that RHT between hot carriers in graphene and hyperbolic phonon polaritons in hBN is extremely efficient at room temperature, leading to picosecond time scales for the carrier cooling dynamics.

  2. Collision-induced Raman spectra of Hg-rare gas Van der Waals complexes

    NASA Astrophysics Data System (ADS)

    Borysow, A.; Grycuk, T.

    1982-10-01

    An absolute differential scattering cross section is calculated for the Hg-rare gas and pure Hg Van der Waals quasimolecules, taking into account both the free state and bound state contributions to the depolarized Raman spectra of these systems. The calculations are performed using the Maitland-Smith (MS) potential function fitted to available experimental data and assuming the simple dipole-induced dipole (DID) model of anisotropy of the polarizability tensor of a collisional atomic pair. The obtained cross sections are about 100 times greater than those for the pure rare gas calculated and measured by Frommhold et al.

  3. Observing the interplay between surface and bulk optical nonlinearities in thin van der Waals crystals

    PubMed Central

    Deckoff-Jones, Skylar; Zhang, Jingjing; Petoukhoff, Christopher E.; Man, Michael K.L.; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Talbayev, Diyar; Madéo, Julien; Dani, Keshav M.

    2016-01-01

    Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures. PMID:26936437

  4. The effect of the London-van der Waals dispersion force on interline heat transfer

    NASA Technical Reports Server (NTRS)

    Wayner, P. C., Jr.

    1978-01-01

    A theoretical procedure to determine the heat transfer characteristics of the interline region (junction of liquid-solid-vapor) from the macroscopic optical and thermophysical properties of the system is outlined. The analysis is based on the premise that the interline transport processes are controlled by the London-van der Waals dispersion force between condensed phases (solid and liquid). Numerical values of the dispersion constant are presented. The procedure is used to compare the relative size of the interline heat sink of various systems using a constant heat flux mode. This solution demonstrates the importance of the interline heat flow number, which is evaluated for various systems.

  5. A Van Der Waals Homojunction: Ideal p-n Diode Behavior in MoSe2.

    PubMed

    Jin, Youngjo; Keum, Dong Hoon; An, Sung-Jin; Kim, Joonggyu; Lee, Hyun Seok; Lee, Young Hee

    2015-10-07

    A MoSe2 p-n diode with a van der Waals homojunction is demonstrated by stacking undoped (n-type) and Nb-doped (p-type) semiconducting MoSe2 synthesized by chemical vapor transport for Nb substitutional doping. The p-n diode reveals an ideality factor of ≈1.0 and a high external quantum efficiency (≈52%), which increases in response to light intensity due to the negligible recombination rate at the clean homojunction interface.

  6. Experimental investigations of size distribution through large van der Waals cluster beam cross-section

    NASA Astrophysics Data System (ADS)

    Yang, Shenghong; Daineka, D. V.; Châtelet, M.

    2003-08-01

    Size distributions through large van der Waals cluster beam cross-section are studied with the pick-up technique. Based on our experimental results, we observed that the larger cluster is always concentrated in the center of the beam. From the center to the periphery, the cluster size gradually decreases. The size distributions through the beam cross-section depend on incoming cluster size and incoming cluster velocity. The larger the incoming cluster size or the faster the incoming cluster velocity, the flatter the size distributions through the beam cross-section are found. These experimental results are interpreted by the Mack focusing effect.

  7. Nanoscopy of Surface-Induced van der Waals-Zeeman Transitions

    SciTech Connect

    Hamamda, M.; Grucker, J.; Dutier, G.; Perales, F.; Baudon, J.; Ducloy, M.; Bocvarski, V.

    2008-10-22

    van der Waals transitions among magnetic sub-levels of a metastable rare gas atom passing near a surface immersed in a magnetic field, are described. Related transition amplitudes are calculated using both the sudden and the Landau-Zener approximations. Experimental data for Ne*({sup 3}P{sub 2}) atoms traversing a copper grating are presented. For a pair of surfaces (e.g. the opposite edges of a slit) and a sufficiently large coherence width, Fresnel's biprism interference fringes are obtained. From this interference pattern, detailed information about the transition amplitude at a sub-nanometric scale can be derived. The effect of gravity on this pattern is examined.

  8. Observing the interplay between surface and bulk optical nonlinearities in thin van der Waals crystals

    NASA Astrophysics Data System (ADS)

    Deckoff-Jones, Skylar; Zhang, Jingjing; Petoukhoff, Christopher E.; Man, Michael K. L.; Lei, Sidong; Vajtai, Robert; Ajayan, Pulickel M.; Talbayev, Diyar; Madéo, Julien; Dani, Keshav M.

    2016-03-01

    Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures.

  9. Interlayer coupling effects on Schottky barrier in the arsenene-graphene van der Waals heterostructures

    SciTech Connect

    Xia, Congxin Xue, Bin; Wang, Tianxing; Peng, Yuting; Jia, Yu

    2015-11-09

    The electronic characteristics of arsenene-graphene van der Waals (vdW) heterostructures are studied by using first-principles methods. The results show that a linear Dirac-like dispersion relation around the Fermi level can be quite well preserved in the vdW heterostructures. Moreover, the p-type Schottky barrier (0.18 eV) to n-type Schottky barrier (0.31 eV) transition occurs when the interlayer distance increases from 2.8 to 4.5 Å, which indicates that the Schottky barrier can be tuned effectively by the interlayer distance in the vdW heterostructures.

  10. Recent progress of photodetectors based on MX2/graphene van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Yang, Hang; Qin, Shiqiao; Fang, Jinyue; Peng, Gang; Zhang, Xueao

    2016-10-01

    Recently, heterostructures, combining the unique advantages of both graphene and transition metal dichalcogenides (TMDs, also known as MX2), have exhibited extraordinary photo-electrical properties, thus attracted tremendous interests worldwide. In this paper, we overviewed recent progress of MX2/Graphene van der Waals heterostructures, including the preparation methods, relevant parameters in opto-electronic measurements, physical mechanisms, existing experimental results and encountered problems. Here, we focus to cover the development of entire field, and provide a comprehensive and accurate understanding concerning this field, which may be helpful for interested researchers in this area.

  11. Atomically thin resonant tunnel diodes built from synthetic van der Waals heterostructures.

    PubMed

    Lin, Yu-Chuan; Ghosh, Ram Krishna; Addou, Rafik; Lu, Ning; Eichfeld, Sarah M; Zhu, Hui; Li, Ming-Yang; Peng, Xin; Kim, Moon J; Li, Lain-Jong; Wallace, Robert M; Datta, Suman; Robinson, Joshua A

    2015-06-19

    Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2-WSe2-graphene and WSe2-MoS2-graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.

  12. Construction of van der Waals magnetic tunnel junction using ferromagnetic layered dichalcogenide

    NASA Astrophysics Data System (ADS)

    Arai, Miho; Moriya, Rai; Yabuki, Naoto; Masubuchi, Satoru; Ueno, Keiji; Machida, Tomoki

    2015-09-01

    We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe0.25TaS2. The vdW interlayer coupling at the Fe-intercalated plane of Fe0.25TaS2 allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material and the vdW junction.

  13. Effects of zero van der Waals and zero electrostatic forces on droplet sedimentation

    NASA Technical Reports Server (NTRS)

    Omenyi, S. N.; Snyder, R. S.; Van Oss, C. J.; Absolom, D. R.; Neumann, A. W.

    1981-01-01

    The present investigation provides a confirmation of the dependence of droplet sedimentation on particle concentration. It is shown that it is possible to determine the maximum particle concentration which can remain stable on a given liquid from droplet sedimentation experiments. Droplet sedimentation can be reduced but not totally eliminated by the addition of appropriate amounts of dimethyl sulfoxide (DMSO) to reduce the van der Waals forces to zero. It was found that, at 12% DMSO, a maximum particle concentration of 6.3 x 10 to the 8th cells/ml of glutaraldehyde-fixed human erythrocytes suspended in physiological saline can remain stable on a D2O cushion.

  14. Field Effect in Graphene-Based van der Waals Heterostructures: Stacking Sequence Matters.

    PubMed

    Stradi, Daniele; Papior, Nick R; Hansen, Ole; Brandbyge, Mads

    2017-04-12

    Stacked van der Waals (vdW) heterostructures where semiconducting two-dimensional (2D) materials are contacted by overlaid graphene electrodes enable atomically thin, flexible electronics. We use first-principles quantum transport simulations of graphene-contacted MoS2 devices to show how the transistor effect critically depends on the stacking configuration relative to the gate electrode. We can trace this behavior to the stacking-dependent response of the contact region to the capacitive electric field induced by the gate. The contact resistance is a central parameter and our observation establishes an important design rule for ultrathin devices based on 2D atomic crystals.

  15. Construction of van der Waals magnetic tunnel junction using ferromagnetic layered dichalcogenide

    SciTech Connect

    Arai, Miho; Moriya, Rai Yabuki, Naoto; Masubuchi, Satoru; Ueno, Keiji; Machida, Tomoki

    2015-09-07

    We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe{sub 0.25}TaS{sub 2}. The vdW interlayer coupling at the Fe-intercalated plane of Fe{sub 0.25}TaS{sub 2} allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material and the vdW junction.

  16. Electronic Properties of Molecules and Surfaces with a Self-Consistent Interatomic van der Waals Density Functional

    NASA Astrophysics Data System (ADS)

    Ferri, Nicola; DiStasio, Robert A.; Ambrosetti, Alberto; Car, Roberto; Tkatchenko, Alexandre

    2015-05-01

    How strong is the effect of van der Waals (vdW) interactions on the electronic properties of molecules and extended systems? To answer this question, we derived a fully self-consistent implementation of the density-dependent interatomic vdW functional of Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005 (2009)]. Not surprisingly, vdW self-consistency leads to tiny modifications of the structure, stability, and electronic properties of molecular dimers and crystals. However, unexpectedly large effects were found in the binding energies, distances, and electrostatic moments of highly polarizable alkali-metal dimers. Most importantly, vdW interactions induced complex and sizable electronic charge redistribution in the vicinity of metallic surfaces and at organic-metal interfaces. As a result, a substantial influence on the computed work functions was found, revealing a nontrivial connection between electrostatics and long-range electron correlation effects.

  17. Electronic properties of molecules and surfaces with a self-consistent interatomic van der Waals density functional.

    PubMed

    Ferri, Nicola; DiStasio, Robert A; Ambrosetti, Alberto; Car, Roberto; Tkatchenko, Alexandre

    2015-05-01

    How strong is the effect of van der Waals (vdW) interactions on the electronic properties of molecules and extended systems? To answer this question, we derived a fully self-consistent implementation of the density-dependent interatomic vdW functional of Tkatchenko and Scheffler [Phys. Rev. Lett. 102, 073005 (2009)]. Not surprisingly, vdW self-consistency leads to tiny modifications of the structure, stability, and electronic properties of molecular dimers and crystals. However, unexpectedly large effects were found in the binding energies, distances, and electrostatic moments of highly polarizable alkali-metal dimers. Most importantly, vdW interactions induced complex and sizable electronic charge redistribution in the vicinity of metallic surfaces and at organic-metal interfaces. As a result, a substantial influence on the computed work functions was found, revealing a nontrivial connection between electrostatics and long-range electron correlation effects.

  18. Unraveling the 3D Atomic Structure of a Suspended Graphene/hBN van der Waals Heterostructure

    NASA Astrophysics Data System (ADS)

    Argentero, Giacomo; Mittelberger, Andreas; Reza Ahmadpour Monazam, Mohammad; Cao, Yang; Pennycook, Timothy J.; Mangler, Clemens; Kramberger, Christian; Kotakoski, Jani; Geim, A. K.; Meyer, Jannik C.

    2017-03-01

    In this work we demonstrate that a free-standing van der Waals heterostructure, usually regarded as a flat object, can exhibit an intrinsic buckled atomic structure resulting from the interaction between two layers with a small lattice mismatch. We studied a freely suspended membrane of well aligned graphene on a hexagonal boron nitride (hBN) monolayer by transmission electron microscopy (TEM) and scanning TEM (STEM). We developed a detection method in the STEM that is capable of recording the direction of the scattered electron beam and that is extremely sensitive to the local stacking of atoms. Comparison between experimental data and simulated models shows that the heterostructure effectively bends in the out-of-plane direction, producing an undulated structure having a periodicity that matches the moir\\'e wavelength. We attribute this rippling to the interlayer interaction and also show how this affects the intralayer strain in each layer.

  19. Unraveling the 3D Atomic Structure of a Suspended Graphene/hBN van der Waals Heterostructure.

    PubMed

    Argentero, Giacomo; Mittelberger, Andreas; Reza Ahmadpour Monazam, Mohammad; Cao, Yang; Pennycook, Timothy J; Mangler, Clemens; Kramberger, Christian; Kotakoski, Jani; Geim, A K; Meyer, Jannik C

    2017-03-08

    In this work we demonstrate that a free-standing van der Waals heterostructure, usually regarded as a flat object, can exhibit an intrinsic buckled atomic structure resulting from the interaction between two layers with a small lattice mismatch. We studied a freely suspended membrane of well-aligned graphene on a hexagonal boron nitride (hBN) monolayer by transmission electron microscopy (TEM) and scanning TEM (STEM). We developed a detection method in the STEM that is capable of recording the direction of the scattered electron beam and that is extremely sensitive to the local stacking of atoms. A comparison between experimental data and simulated models shows that the heterostructure effectively bends in the out-of-plane direction, producing an undulated structure having a periodicity that matches the moiré wavelength. We attribute this rippling to the interlayer interaction and also show how this affects the intralayer strain in each layer.

  20. Phonon scattering due to van der Waals forces in the lattice thermal conductivity of Bi2Te3 thin films

    NASA Astrophysics Data System (ADS)

    Park, Kyeong Hyun; Mohamed, Mohamed; Aksamija, Zlatan; Ravaioli, Umberto

    2015-01-01

    In this work, we calculate the thermal conductivity of layered bismuth telluride (Bi2Te3) thin films by solving the Boltzmann transport equation in the relaxation-time approximation using full phonon dispersion and compare our results with recently published experimental data and molecular dynamics simulation. The group velocity of each phonon mode is readily extracted from the full phonon dispersion obtained from first-principle density-functional theory calculation and is used along with the phonon frequency to compute the various scattering terms. Our model incorporates the typical interactions impeding thermal transport (e.g., umklapp, isotope, and boundary scatterings) and introduces a new interaction capturing the reduction of phonon transmission through van der Waals interfaces of adjacent Bi2Te3 quintuple layers forming the virtual superlattice thin film. We find that this novel approach extends the empirical Klemens-Callaway relaxation model in such anisotropic materials and recovers the experimental anisotropy while using a minimal set of parameters.