Dissociative adsorption of methane on surface oxide structures of Pd-Pt alloys

The dissociative adsorption of methane on variously oxidized Pd, Pt and Pd-Pt surfaces is investigated using density-functional theory, as a step towards understanding the combustion of methane on these materials. For Pd-Pt alloys, models of surface oxide structures are built on the basis of known oxides on Pd and Pt. The methane adsorption energy presents large variations depending on the oxide structure and composition. Adsorption is endothermic on the bare Pd(111) metal surface as well as on stable thin layer oxide structures such as the ($\sqrt{5}\times\sqrt{5}$) surface oxide on Pd(100) and the PtO$_2$-like oxide on Pt(111). Instead, large adsorption energies are obtained for the (100) surface of bulk PdO, for metastable mixed Pd$_{1-x}$Pt$_x$O$_{4/3}$ oxide layers on Pt(100), and for Pd-Pt(111) surfaces covered with one oxygen monolayer. In the latter case, we find a net thermodynamic preference for a direct conversion of methane to methanol, which remains adsorbed on the oxidized metal substrates via weak hydrogen-bond interactions

Workflow for computational characterization of PDMS cross-linked systems

The aim of this work is to demonstrate a computational workflow for the generation of cross-linkable viscoelastic polymers and the determination of elastic and hyperelastic properties by means of all-atoms classical molecular dynamics simulations, using polydimethylsiloxane (PDMS) as an example. To improve the computational efficiency of the workflow, a phenomenological description of the cross-linking process is chosen instead of a quantum mechanical description of the cross-linking mechanism. The structures produced differ in their conversion degree of cross-linking (cdc) of 60, 70, and 80 percent and their quantity ratio between polymer chains and cross-linking units of 2 to 1 and 5 to 1. In order to exclude finite size effects of the molecular systems as much as possible, large systems of about 40,000 atoms are considered. Furthermore, for each possible configuration from the combination of cdc and the ratio of polymer chains to cross-linking units, six structures different from each other are used. Tensile and compression tests are performed to determine mechanical properties. A dependence of stresses in the deformation direction on strain rate is found for strain rates 10 7 , 10 8 , and 10 9 1/s. As the cdc increases, an increase in the stress values is observed in the tensile tests. To determine the viscoelastic material properties, relaxation tests are performed following the tensile tests. Thereby, the determined relaxed stresses after the tensile test rise with the increase of the cdc. Furthermore, no large stress deviations, .34 MPa maximum, between structures differing by chain to linker ratio are observed with the Ogden model. The computational workflow shows that classical all-atom molecular dynamics simulations can be a suitable method for structure generation and subsequent characterization of elastic and hyperelastic properties of cross-linked polymers

Setting Time and Surface Microhardness of Mineral Trioxide Aggregate and 1% and 5% Fluoride-Doped Mineral Trioxide Aggregate Mixed with Water and Gel-like Polymer

Introduction: Mineral trioxide aggregate (MTA) is extensively used in endodontics. However, it has limitations such as long setting time, low compressive strength and poor handling properties. Our study aimed to compare the setting time and surface microhardness of MTA and fluoride-doped MTA (FMTA) using gel-like polymer (GLP) or distilled water (DW) as liquid. Methods and Materials: An MTA-like cement was prepared by mixing Portland cement, bismuth oxide and gypsum (75%, 20% and 5%, respectively). FMTA (1% and 5%) was made by substituting 1% and 5% of MTA powder with fluoride. GLP, composed of methyl cellulose (MC) and propylene glycol (PG), was used as the hydrating liquid and compared with distilled water. Six experimental groups (n=10) were examined for each test. The samples were subjected to Vickers surface microhardness test after 4 and 28 days. Setting time was measured using ANSI/ADA standards. Data was analyzed using two-way and repeated measured ANOVA and the Tukey HSD tests. Results: The MTA-like cement hydrated with GLP showed a significantly reduced setting time (P<0.05); 1% FMTA, mixed with GLP, had the shortest initial and final setting times. The microhardness values of all samples increased at different rates during 28 days (P<0.00001). The microhardness significantly decreased by addition of fluoride to MTA (P<0.001). Conclusions: The results of the present in vitro study suggested that using GLP as the hydrating liquid enhances the setting time with no adverse effect on the surface microhardness. However, adding fluoride can negatively affect the microhardness of MTA. Keywords: Fluoride-oped Mineral Trioxide Aggregate; Microhardness; Mineral Trioxide Aggregate; Setting Tim

The catalytic potential of high-k dielectrics for graphene formation

The growth of single and multilayer graphene nano-flakes on MgO and ZrO2 at low temperatures is shown through transmission electron microscopy. The graphene nano-flakes are ubiquitously anchored at step edges on MgO (100) surfaces. Density functional theory investigations on MgO (100) indicate C2H2 decomposition and carbon adsorption at step-edges. Hence, both the experimental and theoretical data highlight the importance of step sites for graphene growth on MgO

Understanding the UV luminescence of zinc germanate: The role of native defects

Achieving efficient and stable ultraviolet emission is a challenging goal in optoelectronic devices. Herein, we investigate the UV luminescence of zinc germanate Zn2GeO4 microwires by means of photoluminescence measurements as a function of temperature and excitation conditions. The emitted UV light is composed of two bands (a broad one and a narrow one) associated with the native defects structure. In addition, with the aid of density functional theory (DFT) calculations, the energy positions of the electronic levels related to native defects in Zn2GeO4 have been calculated. In particular, our results support that zinc interstitials are the responsible for the narrow UV band, which is, in turn, split into two components with different temperature dependence behaviour. The origin of the two components is explained on the basis of the particular location of Zn_i in the lattice and agrees with DFT calculations. Furthermore, a kinetic luminescence model is proposed to ascertain the temperature evolution of this UV emission. These results pave the way to exploit defect engineering in achieving functional optoelectronic devices to operate in the UV region

Polymerization driven monomer passage through monolayer chemical vapour deposition graphene

Mass transport through graphene is receiving increasing attention due to the potential for molecular sieving. Experimental studies are mostly limited to the translocation of protons, ions, and water molecules, and results for larger molecules through graphene are rare. Here, we perform controlled radical polymerization with surface-anchored self-assembled initiator monolayer in a monomer solution with single-layer graphene separating the initiator from the monomer. We demonstrate that neutral monomers are able to pass through the graphene (via native defects) and increase the graphene defects ratio (Raman ID/IG) from ca. 0.09 to 0.22. The translocations of anionic and cationic monomers through graphene are significantly slower due to chemical interactions of monomers with the graphene defects. Interestingly, if micropatterned initiator-monolayers are used, the translocations of anionic monomers apparently cut the graphene sheet into congruent microscopic structures. The varied interactions between monomers and graphene defects are further investigated by quantum molecular dynamics simulations

Electron transport through self-assembled monolayers of tripeptides

We report how the electron transport through a solid-state metal/Gly-Gly-His tripeptide (GGH) monolayer/metal junction and the metal/GGH work function are modified by the GGH complexation with Cu2+ ions. Conducting AFM is used to measure the current-voltage histograms. The work function is characterized by combining macroscopic Kelvin probe and Kelvin probe force microscopy at the nanoscale. We observe that the Cu2+ ions complexation with the GGH monolayer is highly dependent on the molecular surface density and results in opposite trends. In the case of a high density monolayer the conformational changes are hindered by the proximity of the neighboring peptides, hence forming an insulating layer in response to copper-complexation. Whereas the slightly lower density monolayers allow for the conformational change to a looped peptide wrapping the Cu-ion, which results in a more conductive monolayer. Copper-ion complexation to the high- and low-density monolayers systematically induces an increase of the work functions. Copper-ion complexation to the low-density monolayer induces an increase of electron transport efficiency, while the copper-ion complexation to the high-density monolayer results in a slight decrease of electron transport. Both of the observed trends are in agreement with first-principle calculations. Complexed copper to low density GGH-monolayer induces a new gap state slightly above the Au Fermi energy that is absent in the high density monolayer.Comment: Full paper with supporting informatio