The effects of several adatom dopants (H, Be, C, N, Si, and Zn) on the electronic structure of low-energy surface reconstructions of InAs-(001, 110, 111, and 112) low-index planes-were investigated using ab initio density functional theory calculations. It was observed that the local electronic structure can be controlled in such materials, either reducing or enhancing surface states depending on the dopant and the unique surface reconstruction. The local bonding environment, in particular whether the bonding has an aromatic character, was found to influence both surface reconstruction and a dopant's affinity for a surface. Beryllium and Zinc were often found to reduce the surface states around the band gap, which could improve the charge-carrier mobility in various electronic components. However, these dopants can also cause further surface reconstruction/reconfiguration inducing different surface states.

Zn was suggested to be a promising additive to Pt in the Catalysis of dehydrogenation reactions: In this Work, mixed Pt-Zn clusters deposited on two simple oxides, MgO(100) and TiO2(110), were investigated. The stability of these systems against cluster sintering, one of the major Mechanisms of catalyst deactivation, is simulated using a Metropolis Monte Carlo scheme under the assumption of the Ostwald ripening mechanism.. Particle migration, association to and dissociation from dusters, and evaporation and redeposition of monomers were all included in the simulations. Simulations are done at several high temperatures, relevant to reactions of catalytic dehydrogenation. The effect of temperature is included via both the Metropolis algorithm and the Boltzmann-weighted populations of the global and thermally accessible local minima on the density functional theory potential energy surfaces of clusters of all sizes and compositions up to tetramers. On both surfaces, clusters are shown to sinter quite rapidly. However, the resultant compositions of the clusters most resistant to sintering are quite different,on the two supports, On TiO2(110); Pt and Zn appear to phase separate, preferentially forming clusters rich in just one or the other metal. On MgO(100), Pt and Zn remain well-mixed and form a range of bimetallic clusters of various compositions that appear relatively stable. However, Zn is more easily lost from MgO through evaporation. These phenomena were rationalized by several means of chemical bonding analysis.

Immobilized Pt clusters are interesting catalysts for dehydrogenation of alkanes. However, surface-deposited Pt clusters deactivate rapidly via sintering and coke deposition. The results reported here suggest that adding boron to oxide-supported Pt clusters could be a "magic bullet" against both means of deactivation. The model systems studied herein are pure and B-doped Pt clusters deposited on MgO(100). The nonstoichiometric boride cluster obtained via such alloying is found to anchor to the support via a covalent B-O bond, and the cluster-surface binding is much stronger than in the case of pure Pt clusters. Additionally, B introduces covalency to the intracluster bonding, leading to structural distortion and stabilization. The energy required to dissociate a Pt atom from a boride cluster is significantly larger than that of pure Pt clusters. These energetic arguments lead to the proposal that sintering via both Ostwald ripening and particle coalescence would be discouraged relative to pure Pt clusters. Finally, it is shown that the affinity to C also drops dramatically for borated clusters, discouraging coking and increasing the selectivity of potential cluster catalysts.

ADVERTISEMENT RETURN TO ISSUEPREVCorrectionORIGINAL ARTICLEThis notice is a correctionCorrection to Rutile-Deposited Pt–Pd Clusters: A Hypothesis Regarding the Stability at 50/50 RatioMai-Anh Ha, Jonny Dadras, and Anastassia N. Alexandrova*Cite this: ACS Catal. 2015, 5, 5, 2910Publication Date (Web):April 6, 2015Publication History Published online6 April 2015Published inissue 1 May 2015https://pubs.acs.org/doi/10.1021/acscatal.5b00578https://doi.org/10.1021/acscatal.5b00578correctionACS PublicationsCopyright © 2015 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views558Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (118 KB) Get e-AlertscloseSUBJECTS:Atmospheric chemistry,Cluster chemistry,Stability Get e-Alerts

Mixed Pt-Pd clusters deposited on oxides have been of great interest to catalysis. Clusters containing Pt and Pd in roughly equal proportions were found to be unusually stable against sintering, one of the major mechanisms of catalyst deactivation. After aging of such catalysts, the 50/50 Pt-Pd and Pd-O clusters appeared to be the two most prevalent phases. The reason for the enhanced stability of these equally proportioned clusters has remained unclear. In the following, sintering of mixed Pt-Pd dusters on TiO2(110) for various initial atomic concentrations of Pt and Pd and at a range of catalytically relevant temperatures was simulated. It is confirmed that equally mixed clusters have the relatively highest survival rate. Surprisingly, subnanoclusters containing Pt and Pd in all proportions have very similar geometries and chemical bonding, revealing no apparent explanation for favoring the 1:1 Pt/Pd ratio. However, it was discovered that at high temperatures, the 50/50 clusters have considerably more thermally accessible isomers than dusters containing Pt and Pd in other proportions. Hence, one of the reasons for stability is entropic stabilization. Electrostatics also plays a key role as a subtle charge redistribution, and a shift of electron density to the slightly more electronegative Pt results in the partially charged atoms being further stabilized by intracluster Coulomb attraction; this effect is greatest for 1:1 mixtures.

Laboratory experiments have been used to investigate the fundamental interactions responsible for deuterium retention in lithiated graphite. Oxygen was found to be present and play a key role in experiments that simulated NSTX lithium conditioning, where the atomic surface concentration can increase to >40% when deuterium retention chemistry is observed. Quantum-classical molecular dynamic simulations elucidated this oxygen-deuterium effect and showed that oxygen retains significantly more deuterium than lithium in a simulated matrix with 20% lithium, 20% oxygen, and 60% carbon. Simulations further show that deuterium retention is even higher when lithium is removed from the matrix. Experiments artificially increased the oxygen content in graphite to ∼16% and then bombarded with deuterium. X-ray photoelectron spectroscopy showed depletion of the oxygen and no enhanced deuterium retention, thus demonstrating that lithium is essential in retaining the oxygen that thereby retains deuterium.

Pure and doped sub-nanoclusters can exhibit superb catalytic activity, which, however, strongly depends on their size, shape, composition, and the nature of the support. This work is about surface-deposited sub-nano Pt-based clusters, which are promising catalysts for the reactions of dehydrogenation. Using density functional theory and ab initio calculations, and an ab initio genetic algorithm for finding the global minima of clusters, we found a peculiar effect that Pt5 and Pt4Zn clusters exhibit upon deposition on MgO(100). Both of them change shapes from the gas phase 3-D form to a planar form, and they stand upright on the support. Several reasons are responsible for this behaviour. In part, clusters go flat due to the electron transfer from the support. Indeed, the anionic Pt5(-) and Pt4Zn(-) species are flat also in the gas phase. Charging induces the second-order Jahn-Teller effect (or partial covalency) facilitated by the recruitment of the higher-energy 6p atomic orbitals on Pt into the valence manifold, and that is the reason for the planarization of the anions. Secondly, clusters maximize interactions with the surface O atoms (resulting in further favouring of 2-D structures over 3-D), and avoid contacts with surface Mg atoms (resulting in upright morphologies).

Mixed Pt−Pd clusters deposited on oxides have been of great interest to catalysis. Clusters containing Pt and Pd in roughly equal proportions were found to be unusually stable against sintering, one of the major mechanisms of catalyst deactivation. After aging of such catalysts, the 50/50 Pt−Pd and Pd−O clusters appeared to be the two most prevalent phases. The reason for the enhanced stability of these equally proportioned clusters has remained unclear. In the following, sintering of mixed Pt−Pd clusters on TiO2(110) for various initial atomic concentrations of Pt and Pd and at a range of catalytically relevant temperatures was simulated. It is confirmed that equally mixed clusters have the relatively highest survival rate. Surprisingly, subnanoclusters containing Pt and Pd in all proportions have very similar geometries and chemical bonding, revealing no apparent explanation for favoring the 1:1 Pt/ Pd ratio. However, it was discovered that at high temperatures, the 50/50 clusters have considerably more thermally accessible isomers than clusters containing Pt and Pd in other proportions. Hence, one of the reasons for stability is entropic stabilization. Electrostatics also plays a key role as a subtle charge redistribution, and a shift of electron density to the slightly more electronegative Pt results in the partially charged atoms being further stabilized by intracluster Coulomb attraction; this effect is greatest for 1:1 mixtures.

Lithium wall conditioning has lowered hydrogenic recycling and dramatically improved plasma performance in many magnetic-fusion devices. In this Letter, we report quantum-classical atomistic simulations and laboratory experiments that elucidate the roles of lithium and oxygen in the uptake of hydrogen in amorphous carbon. Surprisingly, we show that lithium creates a high oxygen concentration on a carbon surface when bombarded by deuterium. Furthermore, surface oxygen, rather than lithium, plays the key role in trapping hydrogen.

We investigate the mechanism of deuterium retention by lithiated graphite and its relationship to the oxygen concentration through surface sensitive experiments and atomistic simulations. Deposition of lithium on graphite yielded 5%–8% oxygen surface concentration and when subsequently irradiated with D ions at energies between 500 and 1000 eV/amu and fluences over 1016 cm−2 the oxygen concentration rose to between 25% and 40%. These enhanced oxygen levels were reached in a few seconds compared to about 300 h when the lithiated graphite was allowed to adsorb oxygen from the ambient environment under equilibrium conditions. Irradiating graphite without lithium deposition, however, resulted in complete removal of oxygen to levels below the detection limit of XPS (e.g., <1%). These findings confirm the predictions of atomistic simulations, which had concluded that oxygen was the primary component for the enhanced hydrogen retention chemistry on the lithiated graphite surface.

We report results of measurements on the evolution of the surface morphology of a hot tungsten surface due to impacting low-energy (80 12,000 eV) He ions, performed at the ORNL Multicharged Ion Research Facility (MIRF). Surface-morphology changes were investigated over a broad range of fluences, energies and temperatures for both virgin and pre-damaged W-targets. At low fluences, ordered coral-like and ridge-like surface structures are observed, with great grain-to-grain variability. At the largest fluences, individual grain characteristics disappear in FIB/SEM scans, and the entire surface is covered by a multitude of near-surface bubbles with a broad range of sizes, and disordered whisker growth, while in topdown SEM imaging the surface is virtually indistinguishable from the nanofuzz produced on linear plasma devices. These features are evident at progressively lower fluences as the He-ion energy is increased. In addition, simulations were carried out of damage caused by cumulative bombardment of 1 keV W self-atoms, using LAMMPS at the Kraken supercomputing facility of the University of Tennessee. The simulations show strong defect-recombination effects that lead to a saturation of the total defect number after a few hundred impacts, while sputtering and implantation lead to an imbalance of the vacancy and interstitial numbers.

Chemistry as well as sputtering and reflection dynamics of lithiated carbon material, bombarded by slow hydrogen atoms are studied. We present a realistic method for computational simulation of the dynamics of the polar Li-C-O-H material dynamics. It is based on an approximate, semi-empirical quantum mechanics of electrons and classical mechanics of nuclei. Results are validated qualitatively by comparison with experiments and with a first principle DFT computations. In particular, we explain observed details of the hydrogen bonding chemistry in lithiated carbon, showing that incoming hydrogen interacts preferably with Li-C rather than C structures. (C) 2011 Elsevier B.V. All rights reserved.

Irradiation dynamics of a single graphene sheet bombarded by hydrogen atoms is studied in the incident energy range of 0.1 to 200 eV. Results for reflection, transmission, and adsorption probabilities, as well as effects of a single adsorbed atom to the electronic properties of graphene, are obtained by the quantum-classical Monte Carlo molecular dynamics within a self-consistent-charge-density functional tight binding formalism We compare these results with those, distinctly different, obtained by the classical molecular dynamics.PACS: 61.80.Az, 61.48.Gh, 61.80.Jh, 34.50.Dy.