C.M.C. de Castilho

DFT Studies of the Interactions of a Graphene Layer with Small Water Aggregates

We have investigated the structure, adsorption, electronic states, and charge transfer of small water aggregates on the surface of a graphene layer using density functional theory. Our calculations were focused on water adsorbates containing up to five water molecules interacting with one and both sides of a perfect freestanding sheet. Different orientations of the aggregates with respect to the graphene sites were considered. The results show that the adsorption energy of one water molecule is primarily determined by its orientation, although it is also strongly dependent on the implemented functional scheme. Despite its intrinsic difficulties with dispersion interactions, the Perdew and Wangs exchange-correlation functional may be a viable alternative to investigate the adsorption of large molecular aggregates on a graphene surface. Although water physisorption is expected to occur in the regime of droplets, we found no induced impurity states close to the Fermi level of graphene interacting with small water clusters. In order to investigate the donor/acceptor tendency of the water clusters on graphene, we have performed a Bader charge analysis. Considering the charge transfer mechanism, we have noticed that it should preferentially occur from water to graphene only when the oxygen atom is pointing toward the surface. Otherwise, and in the case of larger adsorbed clusters, charge transfers systematically occur from graphene to water.

Field ion energy deficit calculations for liquid metal ion sources

Ionisation distributions of field-ionised ions from liquid metal ion sources are calculated. Various shapes for the emitter region, a range of currents and a range of electric field strengths at the liquid surface are considered. Their effects on the energy deficit distribution of the ions is presented through a numerical calculation for gold and gallium.

Hybrid platforms of graphane–graphene 2D structures: prototypes for atomically precise nanoelectronics

First-principles calculations demonstrate that line/ribbon defects, resulting from a controlled dehydrogenation in graphane, lead to the formation of low-dimensional electron-rich tracks in a monolayer. The present simulations point out that hybrid graphane-graphene nanostructures exhibit important elements, greatly required for the fabrication of efficient electronic circuits at the atomic level.

Field emission properties of an array of pyramidal structures

The properties and efficiency of the emission current density produced by a metallic array of pyramidal structures are investigated. The theoretical results obtained by numerical integration of the corresponding Laplace equation using a finite differences scheme offer useful information for the optimization of field emission devices based on cathodes with this geometry. Our study shows that the inter-pyramidal distance strongly affects the current density, and even more important for this issue is the protrusion characteristics of these structures. Another relevant, although less important, parameter determining this density is the anode–cathode distance. The effect of the array characteristics on the maximum local electric field intensity is also discussed.

The Simulated Annealing Global Search Algorithm Applied To The Crystallography Of Surfaces By Leed

Surface structure determination by Low Energy Electron Diffraction (LEED) is based on a comparison between experimentally measured and theoretically calculated intensity versus energy I(V) curves for the diffracted beams. The level of agreement between these, for different structural models, is quantified using a correlation function, the so-called R factor. Minimizing this factor allows one to choose the best structure for which the theoretical simulations are computed. Surface structure determination thus requires an exhaustive search of structural parameter space in order to minimize the R factor. This minimization is usually performed by the use of directed search methods, although they have serious limitations, most notably their inability to distinguish between false and real structures corresponding to local and global R factor minima. In this work we present the implementation of a global search method based on the simulated annealing algorithm, as suggested earlier by Rous, using the Van Hove and Tong standard LEED code and the results of its application to the determination of the structure of the Ag(111) and CdTe(110) surfaces. Two different R factors, RP and R1, have been employed in the structural searches, and the statistical topographies of these two factors were studied. We have also implemented a variation of the simulated annealing algorithm (Fast Simulated Annealing) and applied it to these same two systems. Some preliminary results obtained with this algorithm were used to compare its performance with the original algorithm proposed by Rous.

Spin-orbit-induced gap modification in buckled honeycomb XBi and XBi3 (X = B, Al, Ga, and In) sheets

The band structure and stability of XBi and XBi3 (X  =  B, Al, Ga, and In) single sheets are predicted using first-principles calculations. It is demonstrated that the band gap values of these new classes of two-dimensional (2D) materials depend on both the spin-orbit coupling (SOC) and type of group-III elements in these hetero-sheets. Thus, topological properties can be achieved, allowing for viable applications based on coherent spin transport at room temperature. The spin-orbit effects are proved to be essential to explain the tunability by group-III atoms. A clear effect of including SOC in the calculations is lifting the spin degeneracy of the bands at the Γ point of the Brillouin zone. The nature of the band gaps, direct or indirect, is also tuned by SOC, and by the appropriate X element involved. It is observed that, in the case of XBi single sheets, band inversions naturally occur for GaBi and InBi, which exhibit band gap values around 172 meV. This indicates that these 2D materials are potential candidates for topological insulators. On the contrary, a similar type of band inversion, as obtained for the XBi, was not observed in the XBi3 band structure. In general, the calculations, taking into account SOC, reveal that some of these buckled sheets exhibit sizable gaps, making them suitable for applications in room-temperature spintronic devices.

Effect of the local morphology in the field emission properties of conducting polymer surfaces

In this work, we present systematic theoretical evidence of a relationship between the point local roughness exponent (PLRE) (which quantifies the heterogeneity of an irregular surface) and the cold field emission properties (indicated by the local current density and the macroscopic current density) of real polyaniline (PANI) surfaces, considered nowadays as very good candidates in the design of field emission devices. The latter are obtained from atomic force microscopy data. The electric field and potential are calculated in a region bounded by the rough PANI surface and a distant plane, both boundaries held at distinct potential values. We numerically solve Laplaces equation subject to appropriate Dirichlets condition. Our results show that local roughness reveals the presence of specific sharp emitting spots with a smooth geometry, which are the main ones responsible (but not the only) for the emission efficiency of such surfaces for larger deposition times. Moreover, we have found, with a proper choice of a scale interval encompassing the experimentally measurable average grain length, a highly structured dependence of local current density on PLRE, considering different ticks of PANI surfaces.

Hydrogen, oxygen and chlorine adsorption on ag(110) surface: a cluster calculation

Abstract The adsorption of atoms on Ag(110) surfaces has been widely investigated both theoretically and experimentally. The importance of the (110) face results from the much better catalytic properties of the single crystal Ag(110) compared with polycrystalline samples. The aim of this work is to study the systems Ag(110): H, O, Cl by means of rigorous ab initio quantum-chemical calculations. We have investigated several possible binding sites, geometries, elastic constants, binding energies and charge distributions for H, O and Cl on Ag(110) surfaces simulated by clusters Ag n ( n = 3,10).

Temperature dependent structure of low index copper surfaces studied by molecular dynamics simulation

The thermal behavior of the (010), (110) and (111) copper surfaces is studied by molecular dynamics simulation. We have used a many-body potential based on the tight-binding model in order to describe the Cu-Cu interaction. The calculations we have performed correspond to simulations in the temperature range between 600 and 1800 K. The observed order in the stability follows the same order as in the packing density, i. e., (110), (010) and (111). The (110) disorder results from anharmonic effects and by vacancy-adatom formation. On the other end, the (111) surface is very stable, and remains so up to temperatures of the order of the bulk melting point. The melting proceeds by a layer-by-layer mechanism.

STRUCTURAL DETERMINATION OF THE InSb(110) SURFACE BY THE AUTOMATED TENSOR LEED PROGRAM

The InSb(110) surface structure has been re-examined using the tensor LEED approach. A refinement of the structure as well as the influence of the Debye temperature on the structure determination is presented.