Ricardo Faccio

Is It Possible to Dope Single‐Walled Carbon Nanotubes and Graphene with Sulfur?

Herein, we investigate sulfur substitutional defects in single-walled carbon nanotubes (SWCNTs) and graphene by using first-principles calculations. The estimated formation energies for the (3,3), (5,5), and (10,0) SWCNTs and graphene lie between 0.9 and 3.8 eV, at sulfur concentrations of 1.7-4 atom %. Thus, from a thermodynamic standpoint, sulfur doping is not difficult. Indeed, these values can be compared with that of 0.7 eV obtained for a nitrogen-doped (5,5) SWCNT. We suggest that it may be possible to introduce sulfur into the SWCNT framework by employing sulfur-containing heterocycles. Our simulations indicate that sulfur doping can modify the electronic structure of the SWCNTs and graphene, depending on the sulfur content. In the case of graphene, sulfur doping can induce different effects: the doped sheet can be a small-band-gap semiconductor, or it can have better metallic properties than the pristine sheet. Thus, S-doped graphene may be a smart choice for constructing nanoelectronic devices, since it is possible to modulate the electronic properties of the sheet by adjusting the amount of sulfur introduced. Different synthetic routes to produce sulfur-doped graphene are discussed.

Mechanical properties of graphene nanoribbons

Herein, we investigate the structural, electronic and mechanical properties of zigzag graphene nanoribbons in the presence of stress by applying density functional theory within the GGA-PBE (generalized gradient approximation-Perdew-Burke-Ernzerhof) approximation. The uniaxial stress is applied along the periodic direction, allowing a unitary deformation in the range of ± 0.02%. The mechanical properties show a linear response within that range while a nonlinear dependence is found for higher strain. The most relevant results indicate that Youngs modulus is considerable higher than those determined for graphene and carbon nanotubes. The geometrical reconstruction of the C-C bonds at the edges hardens the nanostructure. The features of the electronic structure are not sensitive to strain in this linear elastic regime, suggesting the potential for using carbon nanostructures in nano-electronic devices in the near future.

Modelling magnetism of C at O and B monovacancies in graphene

The presence of defects can introduce important changes in the electronic structure of graphene, leading to phenomena such as C magnetism. In addition, vacancies are reactive and permit the incorporation of dopants. This paper discusses the electronic properties of defective graphene for O and B decoration. Phonon calculations allow us to address directly the stability of the systems under study. We show that it is possible to obtain magnetic solutions with and without dangling bonds, demonstrating that C magnetism can be achieved in the presence of B and O.

Magnetism in multivacancy graphene systems

Ab initio calculations using density functional theory (DFT) have been performed in order to study defects in graphene. The structural distortions that can be observed when multi-atom vacancies are created in graphene and the net magnetic moment that can eventually appear are characterized for a variety of vacancy sizes and shapes. We conclude that the configuration arising in the construction of multivacancies in graphene can unambiguously indicate whether a magnetic response of the defected system is to be expected. Making use of the shape of the complementary figure-i.e. the geometric figure of the atomic arrangement that is extracted from graphene when the multivacancy is created-it is possible to construct a set of rules by means of which the optimized structural and magnetic behavior can be predicted. The validity of the rules is determined through DFT calculations.

Oxidation of monovacancies in graphene by oxygen molecules

We study the oxidation of monovacancies in graphene by oxygen molecules using first principles calculations. In particular, we address the local magnetic moments which develop at monovacancies and show that they remain intact when a molecule is adsorbed such that the dangling carbon bonds are not fully saturated. However, the lowest energy configuration does not maintain dangling bonds and is found to be semiconducting. Our data can explain the experimentally observed behavior of graphene under exposure to an oxygen plasma.

Hydrogenated double wall carbon nanotubes

Herein, we investigate the chemisorption of hydrogen on double wall carbon nanotubes (DWCNT) employing density functional theory and periodic boundary conditions. In agreement with recent investigations based on Lennard-Jones potentials, we found that the (n,m)@(n+9,m) combination is favored for tubes with small diameters. The C-H binding energies determined for the (16,0) single wall carbon nanotubes (SWCNT) are nearly identical to those computed for the (7,0)@(16,0) and (8,0)@(16,0) DWCNTs. For both of the latter we found that interlayer interaction modifies the band structure of the inner tube. In the case of hydrogenated DWCNTs, the electronic structure of the inner tube experiences very small changes at high coverages (50%). However, at lower hydrogen coverages (3%-25%) changes are observed in the electronic structure of the inner tube. In agreement with recent experimental results we conclude that, for heavily functionalized DWCNTs, the electronic properties of the inner tube remain unchanged. For zigzag SWCNTs, the band gap becomes larger upon increase in hydrogen coverage; at 50% of coverage the hydrogenated (16,0) SWCNT has a band gap of 3.38 eV. Finally, based on the fact that high coverages significantly elongate C-H bond distances, we propose that the hydrogenation coverage may be determined measuring the C-H vibrational modes.

Current trends in materials for dye sensitized solar cells.

Here, we intend to review those patents related with the technology of dye sensitized solar cells. In particular we discuss patents and papers that enable metal oxide layer to be more controllable and feasible for applications, and new and innovative dyes, sensitizers and electrolytes with promising features. Finally various methods were reviewed for fabricating semiconductor layers and complete DSSC devices focusing on the mass production of photovoltaic cells.

Synthesis, characterization, microbiological evaluation, genotoxicity and synergism tests of new nano silver complexes with sulfamoxole: X-ray diffraction of [Ag2(SMX)2]·DMSO

The synthesis and microbiological evaluation of two new Ag(I) complexes with sulfamoxole (SMX), [Ag2(SMX)2]·H2O and [Ag4(SCN)3(SMX)]·H2O are described. Both were characterized by elemental analysis, thermogravimetry, powder and single crystal X-ray diffraction, NMR, Raman and experimental and theoretical IR spectroscopies. Their antibacterial and antifungal properties were evaluated by agar and broth dilution assays, respectively. In addition, synergism tests for Pseudomonas aeruginosa were performed, and genotoxicity studies were carried out employing the Allium cepa test. Both complexes displayed good activity against Escherichia coli, Staphylococcus aureus, P. aeruginosa, and 10 fungi strains, with lower minimum inhibitory concentrations (MICs) than that of free SMX in all cases. The nanometrical crystallite particle size determined from XRPD, DLS and TEM might explain the good microbiological activity in spite of the low solubility of both complexes. The fractional inhibitory concentration (FIC) calculated from the P. aeruginosa test data indicated that the activity of the complexes is not due to synergism of the free components in the concentration ratios studied. Moreover, none of the complexes displayed cytotoxic effects on onions in the concentration range tested, and chromosome aberrations were not observed.

The effect of manganite nanoparticle addition on the low field magnetoresistance of polyaniline

In this report, we studied the effect of the addition of manganite nanoparticles on the microstructure and magnetotransport properties of polyaniline polymers. We showed experimental results on the fractal dimension of the polyaniline–manganite nanocomposites by means of small angle X-ray scattering measurements. A decrease in the number of polarons is observed for the composite with a concentration of 20% manganite nanoparticles. On the other hand, for the same concentration, the presence of a low magnetic field increases the number of polarons, in relation to an enhancement of the negative low field magnetoresistance (LFMR). This enhancement was observed for a critical amount of manganite nanoparticle addition in the whole temperature regime analyzed, as envisaged from the low magnetic field dependence in the polyaniline polaron formation observed using confocal Raman spectroscopy at room temperature.

Physical properties of single-crystalline fibers of the colossal-magnetoresistance manganite La0.7Ca0.3MnO3

We have grown high-quality single crystals of the colossal-magnetoresistance (CMR) material La0.7Ca0.3MnO3 by using the laser heated pedestal growth method. Samples were grown as fibers of different diameters, and with lengths of the order of centimeters. Their composition and structure were verified through x-ray diffraction, scanning electron microcopy with energy dispersive x-ray analysis and by Rietveld analysis. The quality of the crystalline fibers was confirmed by Laue and electron backscatter diffraction patterns. Rocking curves performed along the fiber axis revealed a half-height width of 0.073°. The CMR behavior was confirmed by electrical resistivity and magnetization measurements as a function of temperature.