A study on adatom transport through (√3 × âˆš3)-R30°-CH3S self-assembled monolayers on Au(111) using first principles calculations.

Self-assembled monolayers on Au(111) have outstanding chemical, electrical, and optical properties, and Au adatoms seem to play a key role in these properties. Still, the fundamental understanding of adatom transport inside the self-assembled structure is very thin. In this paper we use first-principles calculations to reveal new details about the migration mechanism of Au adatoms in the presence of a CH3S self-assembled structure on Au(111). We study the inclusion of Au adatoms inside a well-packed (√3 × âˆš3)-R30°-CH3S self-assembled lattice and present atomistic models supporting adatom migration by means of a hopping mechanism between pairs of CH3S species. Our calculations reveal that the transport of Au adatoms is slowed down inside the molecular network where the kinetic barrier for adatom migration is larger than on the clean Au surface. We attribute the hindered mobility of Au adatoms to the fact that adatom transport involves the breaking and making of Au-S bonds. Our results form a basis for further understanding the role played by defect transport in the properties of molecular assemblies.

Long range coupling between defect centres in inorganic nanostructures: valence alternation pairs in nanoscale silica.

Valence alternation pair (VAP) states are formed by a closed-shell combination of two space- and charge-separated topological defect centres. These pairs of defects, although historically invoked to explain the electronic properties of bulk inorganic glassy materials (e.g., amorphous silicon dioxide) via the concept of negative-U defects, have more recently been found in a number of theoretical studies of silica surfaces and nanoscale silica clusters. Using density functional theory we systematically probe the structure and internal stability of VAPs in a number of silica nanoclusters with respect to the separation of the two constituent defect centres. We find that VAP states in nanosilica are strongly stabilised by the attractive electrostatic interaction between their separated oppositely charged component defects such that VAPs can persist up to an internal separation of a least 1.5 nanometres. Beyond this distance VAPs become unstable with respect to an open-shell combination of topological defects, virtually indistinguishable from two isolated open-shell defect centres. Finally, we theoretically analyse the possibility of experimental observation of VAP states through their infra-red vibrational spectra.

Role of kinetics in the selective surface oxidations of transition metal carbides.

The different oxidation behavior of TiC and VC(100) surfaces by molecular oxygen has been investigated by density functional theory with a slab model. From the thermodynamic stability of the final states that involve dissociated O(2), one cannot well explain the experimental observations. Two different oxidation pathways of TiC and VC(100) surfaces have been explored in this work, and the results indicate that two channels share the same precursor state. However, from the precursor, only the pathway leading to the formation of a C-O bond is energetically feasible for the TiC(100) surface, while on VC(100) the O atoms tend to occupy the metal surface sites due to a smaller energy barrier for this channel. Further band structure calculations reveal that the additional d electron of V atom favors the stability of the molecularly adsorbed species. The oxidation mechanism unveiled from the present calculations clearly evidences that the kinetic effects introduced by one additional d electron of the V atom play a crucial role in explaining the different surface chemistry between TiC and VC (100) surfaces.

Electronic-structure-based material descriptors: (in)dependence on self-interaction and Hartree-Fock exchange.

Rational design of improved transition metal based materials mostly relies on their electronic structure descriptors, typically estimated by density functional theory and so unduly affected by self-interaction or static correlation errors. Here we show for all 30 transition metals that original or width-corrected d-band centers, and Hilbert transform highest peak descriptors are unaffected by self-interaction, while poor treatment of static correlation by hybrid functionals leads to an unbalanced description. Thus, descriptors have a general validity unbiased by a specific computational method.

Accurate prediction of large antiferromagnetic interactions in high- T(c) HgBa2Ca(n-1)Cu(n)O(2n+2+delta) ( n = 2,3) superconductor parent compounds

The in-plane nearest-neighbor Heisenberg magnetic coupling constant, J, of La2CuO4, Nd2CuO4, Sr2CuO2Cl2, YBa2Cu3O6, and undoped HgBa(2)Ca(n-1)Cu(n)O(2n+2+delta) ( n = 1,2,3) is calculated from accurate ab initio configuration interaction calculations. For the first four compounds, the theoretical J values are in quantitative agreement with experiment. For the Hg-based compounds the predicted values are -135 meV ( n = 1) and approximately -160 meV ( n = 2,3), the latter being much larger than in previous cases and, for n = 3, increasing with pressure. Nevertheless, the physics governing J in all these layered cuprates appears to be the same. Moreover, calculations suggest a possible relationship between J and T(c).

Comparative density functional theory based study of the reactivity of Cu, Ag, and Au nanoparticles and of (111) surfaces toward CO oxidation and NO2 reduction.

The reactivity of Cu, Ag, and Au nanoparticles and of the corresponding (111) surfaces of these elements toward CO oxidation and NO(2) reduction has been investigated by means of DFT and DFT-D calculations. The co-adsorption energies of CO and O on Ag and Au surfaces are smaller than that corresponding to Cu surface but the oxidation reaction is energetically more favored for the heavier metals. The adsorption energy of NO(2), E(ads), is about 50 % larger on nanoparticles than on the metal perfect surfaces, following the almost general rule stating that the lower coordinated sites are those where the interaction is the largest. Interestingly for the co-adsorption and oxidation of CO an increase of reactivity is found for the Au nanoparticles, which is attributed to the large number of low coordinated sites due to the specific shape of this nanoparticle induced by the adsorbates.

Mechanisms of Defect Generation and Clustering in CH3S Self-Assembled Monolayers on Au(111).

Periodic density functional calculations probe that step edges play a key role as source of defects during self-assembly. It is shown that the self-assembly process strongly reduces the energy required to strip an atom from the gold surface, locally increasing the concentration of surface defects. The thermodynamic driving force for the atom stripping is considerably more favorable along step-edge lines within the self-assembly than on the higher-coordinated terrace sites. Furthermore, the clustering of surface defects is considered, and we probe that the formation of aggregates of vacancies in the form of vacancy pits significantly stabilizes the self-assembly on the terraces of gold, where the role of the step edges is expected to be less significant. The high stability of pit-like structures arises from a balance between the corrugation and the enhanced bonding of defect-rich substrates. Our results demonstrate the important role that step edges play during assembly and could be very valuable for discovering defect-free assembled structures.