David L. Azevedo

Anhydrous crystals of DNA bases are wide gap semiconductors.

We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results.

Scattering of low-energy electrons by TiCl4, GeCl4, SiCl4 and CCl4: a comparison of elastic cross sections

We report results of calculations of low-energy electron scattering by TiCl4. Our calculations employed the Schwinger multichannel method implemented with norm-conserving pseudopotentials (Bettega et al 1993 Phys. Rev. A 47 1111) at the fixed-nuclei static-exchange approximation. We compare the elastic integral, differential and momentum transfer cross sections of TiCl4 with those of CCl4, SiCl4 and GeCl4. Similarities and differences in the cross sections for these molecules are discussed. We also discuss the role of the inner atom and the chlorine atom in the scattering process. To the best of our knowledge, this is the first ab initio calculation on electron scattering by TiCl4 and GeCl4 molecules. We have also calculated the ionization cross section for these molecules using the binary-encounter Bethe model and estimated the inelastic cross section for CCl4, SiCl4 and GeCl4, where measured total cross sections are available.

Four-level levodopa adsorption on C60 fullerene for transdermal and oral administration: a computational study

After more than fifty years, the administration of levodopa (LDOPA), a prodrug that crosses the blood-brain barrier and is metabolized to dopamine in the central nervous system, remains the most effective treatment for Parkinson’s disease, despite the manifestation of significant side effects. The development of carrier systems to increase the rate of LDOPA crossing the blood-brain barrier, to achieve stable therapeutic plasma levels and minimize side effects, has been a challenge. Innovative nanosystems for delivering LDOPA are being tested for improved Parkinsons disease therapy. In particular, buckminsterfullerene C60 is promising, due to its ability to penetrate through the skin and the gastrointestinal tract, as well as its biomedical applications to enhance drug delivery. Aiming to give theoretical support to attempts in developing levodopa preparations for transdermal and oral administration that may provide more continuous dopamine stimulation and fewer side effects, we present a computational study of levodopa adsorption on C60 fullerene in the 2–8 pH range. The LDOPA state with COO− and NH3+ protonated (LDOPAc) is investigated, with classical molecular dynamics (CMD) and density functional theory (DFT) simulations being undertaken to describe the LDOPAc adsorption onto C60 fullerene, LDOPAc@C60. Annealing calculations were performed to explore the space of molecular configurations of LDOPAc@C60 to obtain optimal geometries. From the DFT simulations, we found a four-level adsorption pattern, which is in agreement with the shell distribution of LDOPAc around C60 that we have obtained from our CMD simulations. Four van der Waals-like interaction potentials, characteristic of the LDOPAc@C60 adsorption levels were estimated, each one related to an –OH group, with energy minima varying from −0.35 eV to −0.73 eV, and centroid–centroid distances in the 6.5–8.8 A range. Infrared absorption and Raman scattering spectra of the four adsorption configurations were evaluated, allowing us to determine vibrational signatures, which can be very useful in probing the existence of the four adsorption levels.

B and N-doped double walled carbon nanotube: a theoretical study

The structural and electronic properties of boron and nitrogen atom substitutional doping in (8,0)@(13,0) (semiconductor@semiconductor) and (6,0)@(13,0) (metallic@semiconductor) double walled carbon nanotubes, were obtained by using the first-principle calculations based on the density functional theory. In this framework, the electronic density plays a central role and it was obtained from a self-consistent field form. When boron or nitrogen substitutes a carbon atom the structure remains practically the same with negligible deformation observed around defects in all configurations considered. The electronic band structure results indicate that the boron doped systems behave as a p-type impurity, however, the nitrogen doped systems behave as an n-type impurity. In all the systems investigated here, we found that, in the cases of semiconductor@semiconductor tubes, they were the easiest to incorporate a B atom in the outer-wall and an N atom in the inner-wall of the nanotube.

Electronic properties of Fibonacci and random Si-Ge chains.

In this paper we address a theoretical calculation of the electronic spectra of an Si-Ge atomic chain that is arranged in a Fibonacci quasi-periodic sequence, by using a semi-empirical quantum method based on the Hückel extended model. We apply the Fibonacci substitutional sequences in the atomic building blocks A(Si) and B(Ge) through the inflation rule or a recursion relation. In our ab initio calculations we use only a single point, which is sufficient for considering all the orbitals and charge distribution across the entire system. Although the calculations presented here are more complete than the models adopted in the literature which take into account the electronic interaction only up to the second and third neighbors, an interesting property remains in their electronic spectra: the fractality (which is the main signature of this kind of system). We discuss this fractality of the spectra and we compare them with the random arrangement of the Si-Ge atomic chain, and with previous results based on the tight-binding approximation of the Schrödinger equation considering up to the nearest neighbor.

Effective configurations in positron–molecule scattering

In this work, we extend the effective configurations (EC) approach (Azevedo D L, da Silva A J R and Lima M A 2000 Phys. Rev. A 61 042702) to positron scattering. EC are pseudo-eigenstates of the (N + 1)-body collision Hamiltonian. They provide a much more efficient description of polarization effects in scattering calculations by allowing considerable contraction of configuration spaces. As a result, significant computational time is saved.

van der Waals potential barrier for cobaltocene encapsulation into single-walled carbon nanotubes: classical molecular dynamics and ab initio study

In this work, we carried out geometry optimisations and classical molecular dynamics for the problem of cobaltocene (CC) encapsulation into different carbon nanotubes (CNTs) ((7,7), (8,8), (13,0) and (14,0) tubes were used). CCs are molecules composed of two aromatic pentagonal rings (C5H5) sandwiching one cobalt atom. From our simulation results, we observed that CC was encapsulated into CNTs (8,8), (13,0) and (14,0). However, for CNT (7,7), the encapsulation could not occur, in disaggrement with some previous works in the literature. Our results show that the encapsulation process is mainly governed by van der Waals potential barriers.

β-Carotene encapsulation into single-walled carbon nanotubes: a theoretical study

Recently, the encapsulation of β-carotene molecules into carbon nanotubes has been achieved. In this work, we report molecular dynamics simulations and tight-binding density functional-based results for a theoretical study of the encapsulation processes. Our results show that the molecules undergo geometrical deformations when encapsulated with significant changes in their electronic structure. Based on these results, we propose a new interpretation to the changes associated with the β-carotene absorption bands experimentally observed.

Neon atoms oscillating inside carbon and boron nitride nanotubes: a fully atomistic molecular dynamics investigation

In the present work, based on extensive fully atomistic molecular dynamics simulations, we discuss the dynamics of neon atoms oscillating inside (5,5) single-walled carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). Our results show that sustained high-frequency oscillatory regimes are possible for a large range of temperatures. Our results also show that the general features of the oscillations are quite similar to those observed in CNT and BNNT, in contrast with some speculations in previous works in the literature about the importance of broken symmetry and chirality exhibited by BNNTs.

Theoretical study of NO3− interacting with carbon nanotube

This paper deals with quantum mechanical interaction of no3− with (5,5) and (8,0) swcnts. To perform this we have made an ab initio calculation based on the density functional theory. In these framework the electronic density plays a central role and it was obtained of a self-consistent field form. It was observed through binding energy that NO3− molecule interacts with each nanotube in a physisorption regime. We propose these swcnts as a potential filter device due to reasonable interaction with NO3− molecule. Besides this type of filter could be reusable, therefore after the filtering, the swcnts could be separated from NO3− molecule.