Eduardo B. Barros

Group theory analysis of phonons in two-dimensional transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) have emerged as a new two-dimensional material’s field since the monolayer and few-layer limits show different properties when compared to each other and to their respective bulk materials. For example, in some cases when the bulk material is exfoliated down to a monolayer, an indirect-to-direct band gap in the visible range is observed. The number of layers N (N even or odd) drives changes in space-group symmetry that are reflected in the optical properties. The understanding of the space-group symmetry as a function of the number of layers is therefore important for the correct interpretation of the experimental data. Here we present a thorough group theory study of the symmetry aspects relevant to optical and spectroscopic analysis, for the most common polytypes of TMDCs, i.e., 2 Ha ,2 Hc and 1T ,a s af unction of the number of layers. Real space symmetries, the group of the wave vectors, the relevance of inversion symmetry, irreducible representations of the vibrational modes, optical activity, and Raman tensors are discussed.

Fabrication of transparent and ultraviolet shielding composite films based on graphene oxide and cellulose acetate.

Graphene oxide (GO) has been considered a promising filler material for building polymeric nanocomposites because of its excellent dispersibility and high surface area. In this work, we present the fabrication and characterization of transparent and ultraviolet (UV) shielding composite films based on GO and cellulose acetate (CA). GO sheets were found to be well-dispersed throughout the CA matrix, providing smooth and homogeneous composite films. Moreover, the GO sheets were completely embedded within the CA matrix and no presence of this nanomaterial was found at the surface. Nevertheless, CAGO composite films offered an improved high energy light-shielding capacity when compared to pristine CA films. Particularly for UVC irradiation, the CAGO film containing 0.50wt% GO displayed a UV-shielding capacity of 57%, combined with 79% optical transparency under visible light. These CAGO composite films can be potentially applied as transparent UV-protective coatings for packing biomedical, pharmaceutical, and food products.

Selection rules for one- and two-photon absorption by excitons in carbon nanotubes

Recent optical absorption and emission experiments showed that the lower energy optical transitions in carbon nanotubes are excitonic in nature, as predicted by theory. These experiments were based on the symmetry aspects of free electron-hole states and bound excitonic states. The present work shows, however, that group theory does not predict the selection rules needed to explain the two photon experiments. We obtain the symmetries and selection rules for the optical transitions of excitons in single-wall carbon nanotubes within the approach of the group of the wave vector, thus providing important information for the interpretation of theoretical and experimental optical spectra of these materials.

Environment effects on the Raman spectra of individual single-wall carbon nanotubes: Suspended and grown on polycrystalline silicon

An enhanced Raman signal is observed from individual suspended single-wall carbon nanotubes (SWNTs) and from isolated SWNTs grown on an n-doped polycrystalline silicon film used in standard silicon processing. The radial breathing modes of the Raman spectra taken from suspended SWNTs exhibit narrow linewidths, which indicate a relatively unperturbed environment for suspended SWNTs. Clear Raman signals from intermediate frequency modes in the frequency range from 520to1200cm−1 are presented, which might allow a detailed study of the phonon band structure of individual SWNTs.

Stacking-dependent shear modes in trilayer graphene

We observe distinct interlayer shear mode Raman spectra for trilayer graphene with ABA and ABC stacking order. There are two rigid-plane shear-mode phonon branches in trilayer graphene. We find that ABA trilayers exhibit pronounced Raman response from the high-frequency shear branch, without any noticeable response from the low-frequency branch. In contrast, ABC trilayers exhibit no response from the high-frequency shear branch, but significant Raman response from the low-frequency branch. Such complementary behaviors of Raman shear modes can be explained by the distinct symmetry of the two trilayer allotropes. The strong stacking-order dependence is not found in the layer-breathing modes, and thus represents a unique characteristic of the shear modes.

Microparticles of Aloe vera/vitamin E/chitosan: Microscopic, a nuclear imaging and an in vivo test analysis for burn treatment

The use of drug-loaded nanoparticles and microparticles has been increasing, especially for cosmetic and drug delivery purposes. In this work, a new microparticle formulation was developed for use in the healing process of skin burns in a composition of Aloe vera/vitamin E/chitosan. In order to observe the morphological properties, Raman and atomic force microscopy evaluation were performed. The biodistribution studies were analyzed by using a nuclear methodology, labeling the microparticles with Technetium-99m and in vivo test was procedure to analyzed the cicatrization process. The results of AFM analysis show the formation and the adherence property of the microparticles. Raman analyses show the distribution of each component in the microparticle. The nuclear method used shows that the biodistribution of the microparticles remained in the skin. The in vivo cicatrization test showed that the poloxamer gel containing the microparticles make a better cicatrization in relation to the other formulations tested.

Thickness-corrected model for nanoindentation of thin films with conical indenters

Nanoindentation of soft materials is a growing research field, demanding sophisticated models to extract accurate information from these materials. In this work we investigate the nanoindentation of thin soft films by sharp conical indenters using Finite Elements Modeling. Based on the work of Dimitriadis et al. [Biophys. J. 82, 2798 (2002)], we propose that load-displacement (F × δ) curves for conical indenters can be described by F = FHertz(δ)g[χ(δ,h)], where FHertz(δ) is the regular Hertz model, and g[χ(δ,h)] is a correction function that includes finite thickness effects. To test the applicability of the model, we analyze the elastic modulus of fibroblast cells as measured by Atomic Force Microscopy. The elastic modulus obtained with Hertz model is overestimated by 50% (when compared to our thickness-corrected model) in the thickest parts of the cell (3.67 μm), and by approximately 128% in the lamellipodia region (0.45 μm), illustrating the importance of the sample thickness for the evaluation of the mechanical properties.

Functionalization of single-wall carbon nanotubes through chloroform adsorption: theory and experiment

The interaction of chloroform (CHCl(3)) with single-wall carbon nanotubes (SWCNT) is investigated using both first principles calculations based on Density Functional Theory and vibrational spectroscopy experiments. CHCl(3) adsorption on pristine, defective, and carboxylated SWCNTs is simulated, thereby gaining a good understanding of the adsorption process of this molecule on SWCNT surfaces. The results predict a physisorption regime in all cases. These calculations point out that SWCNTs are promising materials for extracting trihalomethanes from the environment. Theoretical predictions on the stability of the systems SWCNT-CCl(2) and SWCNT-COCCl(3) are confirmed by experimental TGA data and Fourier Transform Infrared Spectroscopy (FT-IR) experiments. Results from resonance Raman scattering experiments indicate that electrons are transferred from the SWCNTs to the attached groups and these results are in agreement with the predictions made by ab initio calculations.

Effect of subinihibitory and inhibitory concentrations of Plectranthus amboinicus (Lour.) Spreng essential oil on Klebsiella pneumoniae.

We evaluated the antimicrobial activity and some mechanisms used by subinhibitory and inhibitory concentrations of the essential oil, obtained from leaves of Plectranthus amboinicus, against a standard strain of Klebsiella pneumoniae and 5 multiresistant clinical isolates of the bacteria. The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC), the rate of kill and the pH sensitivity of the essential oil were determined by microdilution tests performed in 96-well plates. Subinhibitory and inhibitory concentrations of the essential oil were tested in order to check its action on K. pneumoniae membrane permeability, capsule expression, urease activity and cell morphology. The MIC and MBC of the essential oil were 0.09±0.01%. A complete inhibition of the bacterial growth was observed after 2 h of incubation with twice the MIC of the essential oil. A better MIC was found when neutral or alkaline pH broth was used. Alteration in membrane permeability was found by the increase of crystal violet uptake when the bacteria were incubated with twice the MIC levels of the essential oil. The urease activity could be prevented when all the subinhibitory concentrations were tested in comparison to the untreated group (p<0.001). Alteration of the bacterial morphology besides inhibition of the capsule expression was verified by atomic force microscopy, and Anthonys stain method, respectively. Our data allow us to conclude that the essential oil of P. amboinicus can be a good candidate for future research.

Synthesis and Characterization of Selenium-Carbon Nanocables

In this letter, we report the synthesis and characterization of a novel Se-C hybrid nanostructure. X-ray diffraction data indicates a high degree of crystallinity for the nanostructured Se shell. High resolution transmission electron microscopy images show that the Se-C nanostructures consist of coaxial nanocables made of single wall carbon nanotubes, as the core, surrounded by a trigonal Selenium shell. Resonance Raman spectroscopy was used to access the properties of both the carbon nanotubes and selenium. The behavior of the radial breathing mode and the G-band indicates that the Se shell primarily covers semiconducting nanotubes. X-ray photoelectron spectroscopy show that the nanocables have a thin coverage of selenium oxide. We envisage that this system could be used in the fabrication of photonic devices as an interface between electronic and photonic materials.