Fernando de Brito Mota

Exploring hydrogenation and fluorination in curved 2D carbon systems: a density functional theory study on corannulene.

Corannulene has been a useful prototype for studying C-based nanostructures as well as surface chemistry and reactivity of sp(2)-hybridized carbon-based materials. We have investigated fluorination and hydrogenation of corannulene carrying out density functional theory calculations. In general, the fluorination is energetically more favorable than hydrogenation of corannulene. The substitution of the peripheral H atoms in the corannulene molecule by F atoms leads to a larger cohesive energy gain than when F (or H) atoms are bonded to the hub carbon and bridge carbon sites of this molecule. As expected for doped C-based nanostructures, the hydrogenation or fluorination significantly changes the HOMO-LUMO gap of the system. We have obtained HOMO-LUMO gap variations of 0.13-3.46 eV for F-doped and 0.38-1.52 eV for H-doped systems. These variations strongly depend on the concentration and position of the incorporated F/H atoms, instead of the structural stability of the doped systems. Considering these calculations, we avoid practical difficulties associated with the addition/substitution reactions of larger curved two-dimensional (2D) carbon nanostructures, and we obtain a comprehensive and systematic understanding of a variety of F/H 2D doped systems.

Ab initio electronic and structural properties of clean and hydrogen saturated β-SiC(100)(3 × 2) surfaces

This paper deals with ab initio calculations relating to the atomic and electronic structure of the β-SiC(100)(3 × 2)–H surface. The results lead to the interpretation that electronic states associated with H atoms are responsible for a metallic behaviour, when saturation in the H deposition on the clean β-SiC(100)(3 × 2) is effected, a feature observed experimentally by Derycke et al. Although confirming the experimentally observed electronic behaviour, the present results differ as regards the H atom positions. Atomic structural properties were calculated and compared with previous ones available in the literature.

Field and electric potential of conductors with fractal geometry

In this study, the behavior of the electric field and its potential are investigated in a region bounded by a rough fractal surface and a distant plane. Both boundaries, maintained at distinct potential values, are assumed to be conductors and, as such, the electric potential is obtained by numerically solving Laplaces equation subject to the appropriate Dirichlets condition. The rough boundaries, generated by the ballistic deposition and fractal Brownian motion methods, are characterized by the values of the surface roughness W and the local fractal dimension df = 3−α, where α is the usual roughness exponent. The equipotential surfaces, obtained from Laplaces equation, are characterized by these same parameters. Results presented show how df depends on the potential value, on the method used to generate the boundary and on W. The behavior of the electric field with respect to the equipotential surface is also considered. Its average intensity was found to increase as a function of the average distance from the equipotential to the fractal boundary; however, its intensity reaches a maximum before decreasing towards an asymptotic constant value, an effect that increases as the value of W increases.

Roughness of equipotential lines due to a self-affine boundary

In this work, the characterization of the roughness of a set of equipotential lines , due to a rough surface held at a nonzero voltage bias, is investigated. The roughness of the equipotential lines reflects the roughness of the profile, and causes a rapid variation in the electric field close to the surface. An ideal situation was considered, where a well known self-affine profile mimics the surface, while the equipotential lines are numerically evaluated using Liebmanns method. The use of an exact scale invariant profile helps to understand the dependency of the line roughness exponent on both the value of the potential (or on the average distance to the profile) and the profiles length. Results clearly support previous indications that: (a) for a system of fixed size, higher values of α characterize less corrugated lines far away from the profile; (b) for a fixed value of the potential, α decreases with the length of the profile towards the value of the boundary. This suggests that, for a system of infinite size, all equipotential lines share the same value of α.

Difração de elétrons de baixa energia (LEED) e a determinação da estrutura atômica de superfícies ordenadas

We present a brief summary of the steps that lead to the demonstration that electron scattering can result in a diffraction pattern. We include a description of the intrinsic processes in low energy electron diffraction, a technique that, at present, is routinely used for surface structural determination in an atomic scale. We also present some illustrative examples.