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Some spectroscopic studies related to atmospheric chemistry and the thermal decomposition of organic azides
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Effects of Water Intercalation and Tribochemistry on MoS2 Lubricity: An Ab Initio Molecular Dynamics Investigation
Full text linkTo date, no clear conclusion has been reached on the atomistic mechanisms that govern the observed decrease of lubricating capabilities of MoS2 in humid environments. Based on ab initio molecular dynamic calculations, we show that intercalated water molecules hinder the sliding motion of both regular and defective layers considerably, with the velocities decaying exponentially with time. However, in the presence of an applied load and exposed edge terminations, water is rapidly removed from the interface and is adsorbed on the edges either in undissociated form or as OH/H fragments. These outcomes suggest that the interlayer slipperiness can be reduced by the presence of water even in the absence of any chemical oxidation. Our work could help to set up more dedicated experiments to further tackle a technologically relevant issue for the applications of MoS2-based lubricants
Water adsorption on diamond (111) surfaces: an ab initio study
No full textThe physical and chemical adsorption of water at the (1 Ã 1) and (2 Ã 1)-reconstructed C(111) surfaces are investigated by means of first principles calculations and compared to hydrogen adsorption. The study aims at filling a gap of knowledge about the interaction of water with the most stable diamond surface. The calculated reaction energies and barriers indicate that the Pandey-reconstructed surface is almost inert towards water and hydrogen chemisorption in comparison with the unreconstructed surface and other low-index diamond surfaces. We also show that by increasing the amount of chemisorbed hydroxyl or hydrogen groups the stability of the Pandey reconstruction is progressively reduced with respect to the unreconstructed (111) surface, which becomes energetically more favourable above about 40% of adsorbate coverage. Our results provide a microscopic description of diamond surface passivation, which is very important for controlling macroscale phenomena, such as the friction reduction of diamond coatings in humid environments
Graphene and MoS2 interacting with water: A comparison by ab initio calculations
Full text linkAlthough very similar in many technological applications, graphene and MoS2 bear significant differences if exposed to humid environments. As an example, lubrication properties of graphene are reported to improve while those of MoS2 to deteriorate: it is unclear whether this is due to oxidation from disulfide to oxide or to water adsorption on the sliding surface. By means of ab initio calculations we show here that these two layered materials have similar adsorption energies for water on the basal planes. They both tend to avoid water intercalation between their layers and to display only mild reactivity of defects located on the basal plane. It is along the edges where marked differences arise: graphene edges are more reactive at the point that they immediately prompt water splitting. MoS2 edges are more stable and consequently water adsorption is much less favoured than in graphene. We also show that water-driven oxidation of MoS2 layers is unfavoured with respect to adsorption
Ideal adhesive and shear strengths of solid interfaces: A high throughput ab initio approach
No full textWe release a computational protocol to calculate two intrinsic tribological properties of solid interfaces from first principles, namely the adhesion energy, γ and the ideal interfacial shear strength, τ. These properties, which correspond to the energy required to separate two surfaces from contact and to the static friction force per unit area, respectively, are ruled by physical/chemical interactions between the surfaces in contact. First principles calculations based on Density Functional Theory (DFT) can accurately describe surface-surface interactions, offering the possibility to characterize the adhesive and shear strengths of materials in silico. We implemented the computational protocol as an AiiDA workflow (WF) that allows to obtain the γ and τ figures of merits in a high throughput manner. The software we produced uses a simple input file and most computational parameters determined automatically. To our best knowledge, this is the first time a high throughput approach has been used in tribology
A spectroscopic and ab initio study of the formation of graphite and carbon nanotubes from thermal decomposition of silicon carbide
No full textFirst-principles comparative study on the interlayer adhesion and shear strength of transition-metal dichalcogenides and graphene
No full textDue to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers (R0 ) bears more similarities to graphene while that with antiparallel orientation (R180 ) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrie
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Influence of Subsurface Layers on the Adsorption of Large Organic Molecules on Close-Packed Metal Surfaces
No full textThe asymmetric molecule 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid (PVBA) adsorbed
on Cu(111) is characterized by scanning tunneling microscopy (STM) and density functional theory
(DFT) to determine the influence of subsurface atomic layers on the adsorption. In contrast to the
6-fold symmetry of the first atomic layer of close-packed surfaces, we find that the arrangement of
the isolated molecules follows predominantly a 3-fold symmetry. This reduction in symmetry, where
the molecule selects a specific orientation along the Æ-211æ axes, reveals the contribution of lower lying
Cu layers to the molecular arrangement. Our calculations rationalize the interaction of the
substrate with the molecule in terms of electrostatic screening and local relaxation phenomena
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