Physical Adsorption at the Nanoscale: Towards Controllable Scaling of the Substrate-Adsorbate van der Waals Interaction

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 nanometers. Comparison with recent experimental observation 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

Thermal and electronic fluctuations of flexible adsorbed molecules : azobenzene on Ag(111)

We investigate the thermal and electronic collective fluctuations that contribute to the finite-temperature adsorption properties of flexible adsorbates on surfaces on the example of the molecular switch azobenzene C12H10N2 on the Ag(111) surface. Using first-principles molecular dynamics simulations, we obtain the free energy of adsorption that accurately accounts for entropic contributions, whereas the inclusion of many-body dispersion interactions accounts for the electronic correlations that govern the adsorbate binding. We find the adsorbate properties to be strongly entropy driven, as can be judged by a kinetic molecular desorption prefactor of 10^24 s−1 that largely exceeds previously reported estimates. We relate this effect to sizable fluctuations across structural and electronic observables. A comparison of our calculations to temperature-programed desorption measurements demonstrates that finite-temperature effects play a dominant role for flexible molecules in contact with polarizable surfaces, and that recently developed first-principles methods offer an optimal tool to reveal novel collective behavior in such complex systems

On the accuracy of DFT exchange-correlation functionals for H bonds in small water clusters II: The water hexamer and van der Waals interactions

Second order Moeller-Plesset perturbation theory (MP2) at the complete basis set (CBS) limit and diffusion quantum Monte Carlo (DMC) are used to examine several low energy isomers of the water hexamer. Both approaches predict the so-called "prism" to be the lowest energy isomer, followed by "cage", "book", and "cyclic" isomers. The energies of the four isomers are very similar, all being within 10-15 meV/H2O. This reference data is then used to evaluate the performance of several density-functional theory (DFT) exchange-correlation (xc) functionals. A subset of the xc functionals tested for smaller water clusters [I: Santra et al., J. Chem. Phys. 127, 184104 (2007)] has been considered. Whilst certain functionals do a reasonable job at predicting the absolute dissociation energies of the various isomers (coming within 10-20 meV/H2O), none predict the correct energetic ordering of the four isomers, nor does any predict the correct low total energy isomer. All xc functionals tested either predict the book or cyclic isomers to have the largest dissociation energies. A many-body decomposition of the total interaction energies within the hexamers leads to the conclusion that the failure lies in the poor description of van der Waals (dispersion) forces in the xc functionals considered. It is shown that the addition of an empirical pairwise (attractive) C6/R6 correction to certain functionals allows for an improved energetic ordering of the hexamers. The relevance of these results to density-functional simulations of liquid water is also briefly discussed.Comment: 15 pages including 4 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Elastic and vibrational properties of alpha and beta-PbO

The structure, electronic and dynamic properties of the two layered alpha (litharge) and beta (massicot) phases of PbO have been studied by density functional methods. The role of London dispersion interactions as leading component of the total interaction energy between layers has been addressed by using the Grimme's approach, in which new parameters for Pb and O atoms have been developed. Both gradient corrected and hybrid functionals have been adopted using Gaussian-type basis sets of polarized triple zeta quality for O atoms and small core pseudo-potential for the Pb atoms. Basis set superposition error (BSSE) has been accounted for by the Boys-Bernardi correction to compute the interlayer separation. Cross check with calculations adopting plane waves that are BSSE free have also been performed for both structures and vibrational frequencies. With the new set of proposed Grimme's type parameters structures and dynamical parameters for both PbO phases are in good agreement with experimental data.Comment: 8 pages, 5 figure

EELS modelling of graphitisation

The impact of graphitisation-type processes on the carbon K-edge ELNES is explored for model systems using the CASTEP density functional theory code. For c lattice direction expansion, contraction of spectral peaks occurs between 20 and 27 eV above the edge onset, for a/b lattice dimension expansion spectral peaks are heavily compressed in terms of energy separation and are shifted towards the edge-onset, consistent with a 1/a^2 relationship. For a nanotube model system, it is shown that for higher curvature, an additional feature was observed in the spectrum ~5 eV, arguably consistent with ‘fullerene’-type character

Graphite and Hexagonal Boron-Nitride Possess the Same Interlayer Distance. Why?

Graphite and hexagonal boron nitride (h-BN) are two prominent members of the family of layered materials possessing a hexagonal lattice. While graphite has non-polar homo-nuclear C-C intra-layer bonds, h-BN presents highly polar B-N bonds resulting in different optimal stacking modes of the two materials in bulk form. Furthermore, the static polarizabilities of the constituent atoms considerably differ from each other suggesting large differences in the dispersive component of the interlayer bonding. Despite these major differences both materials present practically identical interlayer distances. To understand this finding, a comparative study of the nature of the interlayer bonding in both materials is presented. A full lattice sum of the interactions between the partially charged atomic centers in h-BN results in vanishingly small monopolar electrostatic contributions to the interlayer binding energy. Higher order electrostatic multipoles, exchange, and short-range correlation contributions are found to be very similar in both materials and to almost completely cancel out by the Pauli repulsions at physically relevant interlayer distances resulting in a marginal effective contribution to the interlayer binding. Further analysis of the dispersive energy term reveals that despite the large differences in the individual atomic polarizabilities the hetero-atomic B-N C6 coefficient is very similar to the homo-atomic C-C coefficient in the hexagonal bulk form resulting in very similar dispersive contribution to the interlayer binding. The overall binding energy curves of both materials are thus very similar predicting practically the same interlayer distance and very similar binding energies.Comment: 18 pages, 5 figures, 2 table

НАУЧНОЕ ОБОСНОВАНИЕ АЛГОРИТМА ПОДГОТОВКИ БОЛЬНЫХ К ПРОТЕЗИРОВАНИЮ ПОЛОСТИ РТА

The therapeutic and diagnostic activity is a collective work that is carried out jointly by specialists in different directions of dentistry and support units. The main goal of this type of activity is to improve the dental health by preventing dental diseases. It includes the treatment and rehabilitation using various ways of replacement of the lost integrity of the dentition and restoration of the teeth using a prosthetic. In order to do this, it is necessary to prepare the teeth and the pulp of the oral cavity for prosthetics. The sequence of preparatory activities includes a series of steps carried out in sequence: preventive, hygienic, therapeutic (including periodontal), surgical and orthopedic (including orthodontics). Each of these stages includes a series of manipulations, the sequence and number of which is determined by the individual dental status of each patient. The inspection and preparation of a comprehensive plan for the treatment involves a sequence of all these actions carried out by a team of experts in agreement with the patient, evidenced by the «informed consent» subscription. After the preparatory activities, it is possible to go to the final stage, that is to make a prosthetic appliance to the patient. The total duration of all phases of preparatory activities preceding the prosthetics depends on the complexity of the steps themselves, and is determined by the severity of the related dental diseases. The treatment of these diseases is the main part of the preparatory activities.Лечебно-диагностическая деятельность представляет собой коллективную работу, выполняемую совместно специалистами различных направлений стоматологии и вспомогательных подразделений. Основная ее цель - повышение стоматологического здоровья населения путем профилактики стоматологических заболеваний, включающих их лечение и реабилитацию различными способами замещения утраченной целостности зубных рядов и их восстановления с помощью ортопедических конструкций. Для этого необходимо провести мероприятия по подготовке зубов и мягких тканей полости рта к протезированию. Подготовительные мероприятия включают в себя серию последовательно проводимых этапов: профилактического, гигиенического, терапевтического, в том числе пародонтологичес-кого, хирургического, и ортопедического, в том числе ортодонтического. На каждом из указанных этапов прово дится серия манипуляций, количество и последовательность которых определяется индивидуальным стоматологическим статусом конкретного пациента. Осмотр и составление комплексного плана лечения предусматривает последовательность всех этих мероприятий выполняемых коллективом специалистов при согласовании их выполнения с пациентом, о чем свидетельствует заключение информированного согласия. Выполнение подготовительных мероприятий позволяет перейти к заключительному этапу - протезированию пациента. Совокупная продолжительность выполнения всех этапов подготовительных мероприятий, предшествующих протезированию полости рта, зависит от степени сложности выполнения самих этапов и определяется степенью тяжести сопутствующих стоматологических заболеваний, санация которых предполагает основную направленность подготовительных мероприятий

Random-phase approximation and its applications in computational chemistry and materials science

The random-phase approximation (RPA) as an approach for computing the electronic correlation energy is reviewed. After a brief account of its basic concept and historical development, the paper is devoted to the theoretical formulations of RPA, and its applications to realistic systems. With several illustrating applications, we discuss the implications of RPA for computational chemistry and materials science. The computational cost of RPA is also addressed which is critical for its widespread use in future applications. In addition, current correction schemes going beyond RPA and directions of further development will be discussed.Comment: 25 pages, 11 figures, published online in J. Mater. Sci. (2012