F. de Brito Mota

A theoretical investigation of defects in a boron nitride monolayer

We have investigated, using first-principles calculations, the energetic stability and structural properties of antisites, vacancies and substitutional carbon defects in a boron nitride monolayer. We have found that the incorporation of a carbon atom substituting for one boron atom, in an N-rich growth condition, or a nitrogen atom, in a B-rich medium, lowers the formation energy, as compared to antisites and vacancy defects. We also verify that defects, inducing an excess of nitrogen or boron, such as N(B) and B(N), are more stable in its reverse atmosphere, i.e. N(B) is more stable in a B-rich growth medium, while B(N) is more stable in a N-rich condition. In addition we have found that the formation energy of a C(N), in a N-rich medium, and C(B) in a B-rich medium, present formation energies comparable to those of the vacancies, V(N) and V(B), respectively.

Hydrogen role on the properties of amorphous silicon nitride

We have developed an interatomic potential to investigate structural properties of hydrogenated amorphous silicon nitride. The interatomic potential used the Tersoff functional form to describe the Si–Si, Si–N, Si–H, N–H, and H–H interactions. The fitting parameters for all these interactions were found with a set of ab initio and experimental results of the silicon nitride crystalline phase, and of molecules involving hydrogen. We investigated the structural properties of unhydrogenated and hydrogenated amorphous silicon nitride through Monte Carlo simulations. The results show that depending on the nitrogen content, hydrogen has a different chemical preference to bind to either nitrogen or silicon, which is corroborated by experimental findings. Besides, hydrogen incorporation reduced considerably the concentration of undercoordinated atoms in the material, and consequently the concentration of dangling bonds.

Adsorption of monovalent metal atoms on graphene: a theoretical approach

This work investigates, using first-principles calculations, electronic and structural properties of hydrogen, lithium, sodium, potassium and rubidium that are adsorbed, in a regular pattern, on a graphene surface. The results for H-graphene (graphane) and Li-graphene were compared with previous calculations. The present results do not support previous claims that the Li-C bond in such a layer would result in an sp(2) to an sp(3) transition of carbon orbitals, being more compatible with some ionic character for the covalent bond and with lithium acting as an electron acceptor in a bridging environment. Calculations were also performed for the Na, K, and Rb-graphene systems, resulting in a similar electronic behaviour but with a more pronounced ionic character than for Li-graphene. Energy calculations indicate the possible stability of such ad-graphene layers, with only the Li-graphene being possible to be spontaneously obtained.

Van der Waals stacks of few-layer h-AlN with graphene: an ab initio study of structural, interaction and electronic properties.

Graphite-like hexagonal AlN (h-AlN) multilayers have been experimentally manifested and theoretically modeled. The development of any functional electronics applications of h-AlN would most certainly require its integration with other layered materials, particularly graphene. Here, by employing vdW-corrected density functional theory calculations, we investigate structure, interaction energy, and electronic properties of van der Waals stacking sequences of few-layer h-AlN with graphene. We find that the presence of a template such as graphene induces enough interlayer charge separation in h-AlN, favoring a graphite-like stacking formation. We also find that the interface dipole, calculated per unit cell of the stacks, tends to increase with the number of stacked layers of h-AlN and graphene.

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.

Effects of hydroxyl group distribution on the reactivity, stability and optical properties of fullerenols.

We investigate the impact of hydroxyl groups on the properties of C(60)(OH)(n) systems, with n = 1, 2, 3, 4, 8, 10, 16, 18, 24, 32 and 36 by means of first-principles density functional theory calculations. A detailed analysis from the local density of states has shown that adsorbed OH groups can induce dangling bonds in specific carbon atoms around the adsorption site. This increases the tendency to form polyhydroxylated fullerenes (fullerenols). The structural stability is analyzed in terms of the calculated formation enthalpy of each species. Also, a careful examination of the electron density of states for different fullerenols shows the possibility of synthesizing single molecules with tunable optical properties.

Structural and electronic properties of silicon nitride materials

The authors developed an empirical potential for interactions between Si and N to describe silicon nitride systems using the Tersoff functional form. With this model, they explored the structural properties of amorphous silicon nitride through the Monte Carlo simulations and compared them to available experimental data. The empirical model provided a very good description of such properties for a-SiN{sub x} (0 < x < 1.5). Electronic structure of amorphous and point defects in crystalline silicon nitride were then studied using first-principles calculations. For such calculations, the configurations were created by the empirical model, with the relaxed structures used as input for the first-principles calculations. Atomic relaxation was later allowed in the first-principles calculations.

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.

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.