rafael alencar | Universidade Federal do Ceará (original) (raw)

Papers by rafael alencar

Journal of Physical Chemistry C, 2021

ALENCAR, Rafael Silva. Estudo de fonons em nanotubos de carbono e dissulfeto de molibdenio: efeit... more ALENCAR, Rafael Silva. Estudo de fonons em nanotubos de carbono e dissulfeto de molibdenio: efeito do acoplamento entre camadas. 2016. 116 f. Tese (Doutorado em Fisica) - Programa de Pos-Graduacao em Fisica, Departamento de Fisica, Centro de Ciencias, Universidade Federal do Ceara, Fortaleza, 2016.

ACS Nano, 2021

In this work, we explain the origin and the mechanism responsible for the strong enhancement of t... more In this work, we explain the origin and the mechanism responsible for the strong enhancement of the Raman signal of sulfur chains encapsulated by single-wall carbon nanotubes by running resonance Raman measurements in a wide range of excitation energies for two nanotube samples with different diameter distributions. The Raman signal associated with the vibrational modes of the sulfur chain is observed when it is confined by small-diameter metallic nanotubes. Moreover, a strong enhancement of the Raman signal is observed for excitation energies corresponding to the formation of excited nanotube-chain-hybrid electronic states. Our hypothesis was further tested by high pressure Raman measurements and confirmed by density functional theory calculations of the electronic density of states of hybrid systems formed by sulfur chains encapsulated by different types of single-wall carbon nanotubes.

Vibrational Spectroscopy, 2020

Abstract In this study we report the temperature- and power-dependence of the out-of-plane A 1 g ... more Abstract In this study we report the temperature- and power-dependence of the out-of-plane A 1 g Raman mode of mechanically exfoliated few-layered WSe 2 suspended on Si 3 N 4 holes. The A 1 g phonon frequency is red-shifted for a temperature in the range 77-548 K, with first-order temperature coefficients estimated to be -0.0083 and -0.0079 cm − 1 /K for suspended four- and six-layer WSe 2 , respectively. The power-dependence of suspended four-layer WSe 2 sample is estimated to be -0.0079 cm − 1 / μ W and no shift is observed for six-layer WSe 2 . By using the power-dependence of the suspended four layers WSe 2 and the 1D Balandin's approach, we calculate the in-plane thermal conductivity of WSe 2 at room temperature to be ∼ 57.7 W/m.K.

Carbon, 2021

Abstract Filling carbon nanotubes with molecules is a route for the development of electronically... more Abstract Filling carbon nanotubes with molecules is a route for the development of electronically modified one-dimensional hybrid structures for which the interplay between the electronic structure of molecules and nanotubes is a key factor. Tuning these energy levels with external parameters is an interesting strategy for the engineering of new devices and materials. Here we show that the hybrid system composed by quaterthiophene (4T) molecules confined in single-walled carbon nanotubes, presents a piezo-Raman-resonance of the molecule vibrational pattern. This behavior manifests as a rapid pressure induced enhancement of the 4T Raman mode intensities compared to the tubes G-band Raman modes. Density functional theory calculations allow to explain the spectral behaviour through the pressure-enhanced quaterthiophene resonance evolution. By increasing pressure, the tube cross-section deformation leads to a reduction of the intermolecular distance, to the splitting of the molecular levels and then to an increase of resonance channels. Calculations and experiments converge to the 4T piezo-resonance scenario associated with the pressure-induced nanotube radial collapse observed at about 0.8 GPa. Our findings offer possibilities for the development of pressure transducers based on molecule-filled carbon nanotubes.

IEEE Journal of Selected Topics in Quantum Electronics, 2021

The article deals with theoretical aspects and practical results of finding the grounds for choos... more The article deals with theoretical aspects and practical results of finding the grounds for choosing directions for the entrepreneurial competence formation. It is established that the professional training of modern tourism specialists should be carried out in moral and ethical, professional and business, communicative directions. It has been made the analysis of the level of formation of the moral and ethical block of the tourism specialist' personality, the level of his professional-business and communicative training; the reasons for students' unpreparedness for active entrepreneurship have been identified. The results of the ranking of educational disciplines and didactic tools that provide the level of entrepreneurial competence formation of future tourism specialists are presented.

Carbon, 2020

Abstract Magnetic carbonaceous nanocomposites (MCN) were prepared by hydrothermal carbonization (... more Abstract Magnetic carbonaceous nanocomposites (MCN) were prepared by hydrothermal carbonization (HTC) of a carbohydrate in the presence of Fe3+, followed by thermal treatment with KOH for simultaneous activation and magnetization. The precursor formed (IOCN) in the HTC process contained iron oxide nanoparticles encapsulated in the hydrochar matrix. The thermochemical parameters of the activation (temperature and IOCN/KOH mass-ratio) were varied to achieve an increase of the specific surface area along with formation of magnetic phases in MCN compared to IOCN. Activation temperature was found to be responsible for the structural and morphological properties of the MCNs whereas the IOCN/KOH mass-ratio controlled the porosity. The magnetic properties of the MCNs originated from the formation of Fe3O4 and Fe0 phases, which are encapsulated in the carbonaceous material. The MCNs were tested for adsorption of methylene blue (MB) dye, followed by magnetic separation. The MCN, produced in the optimized conditions, showed a specific surface area of 766 m2 g−1, magnetization of 8 emu g−1 and a MB adsorption capacity of 570 mg g−1. Detailed kinetic and isotherm studies of MB adsorption were also performed. The methodology of simultaneous activation and magnetization to generate MCNs, presented here, could be extended to obtain new multifunctional carbon-based nanocomposite adsorbent starting from different biomasses.

Nano Letters, 2019

The knowledge of phonon coherence length is of great importance for two-dimensional-based materia... more The knowledge of phonon coherence length is of great importance for two-dimensional-based materials since phonons can limit the lifetime of charge carriers and heat dissipation. Here we use tip-enhanced Raman spectroscopy (TERS) to measure the spatial correlation length Lc of the A11g and A21g phonons of mono and few-layers gallium sulfide (GaS). The differences in Lc values are responsible for different enhancements of the A1g modes, with A11g always enhancing more than the A21g, independently of the number of GaS layers. For 5-layers, the results show an Lc of 64 nm and 47 nm for A11g and A21g, respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2019

In this paper we present a Raman spectroscopy study of Na3MCO3PO4 (M=Mn, Fe, Co e Ni) carbonophos... more In this paper we present a Raman spectroscopy study of Na3MCO3PO4 (M=Mn, Fe, Co e Ni) carbonophosphates. The insertion of different metals with distinct ionic radii in the MO6 octahedra leads to changes in unit cell volume, thus leading to blueshifts in the energies of the Raman active modes. The experimental data are supported by lattice dynamic calculations and the vibrational properties of the carbonophosphates Na3MCO3PO4 are properly described.

Journal of Raman Spectroscopy, 2017

The effect of pressureby definition a 3-dimensional concepton 2-dimensional systems brings a numb... more The effect of pressureby definition a 3-dimensional concepton 2-dimensional systems brings a number of fundamental questions, which have been partly answered through newly engineered samples and experimental setups. In this review of the high-pressure Raman studies of graphene, we will in particular underline the importance of the presence of a supporting substrate and its role for the production of biaxial strain conditions in high pressure experiments. Raman shifts observed during these experiments may be related to the strain induced by the substrate rather than by the applied pressure, graphene stress, and hydrostatic pressure not being straightforwardly equivalent in this case of a composite 2D-on-3D material. We will also consider the effects of the nature of the substrate surface, the nature of the pressure transmitting media, as well as the effect of the number of layers on the high-pressure response of the 2D-system, by using the phonon behavior as strain probe as pressure is applied. The effects of adhesion and of strain transfer from the substrate to the 2D-system will be at the heart of our discussion.

Carbon, 2018

Abstract High-pressure resonance Raman experiments were performed on linear carbon chains Cn conf... more Abstract High-pressure resonance Raman experiments were performed on linear carbon chains Cn confined in double-wall carbon nanotubes (Cn@DWCNT). Our results indicate that the frequency of the chain mode undergoes a non-linear decreasing as pressure increases, in agreement with previous studies of the behavior of carbon chains into multi-wall carbon nanotubes. After the pressure cycle, the Cn modes present an irreversible frequency downshift together with an irreversible upshift (downshift) of the RBM modes (G+ band) of the inner tube. These spectral changes are interpreted as evidence of cross-linking between the carbon chain and the wall of the inner tube. Density functional theory (DFT) calculations were performed in order to shed light on the interpretation of the experimental data. The theoretical calculations show a pressure-induced charge transfer from the nanotube to the carbon chain, thus resulting in an increasing of the C≡C bond length, and consequently, a softening frequency of the Cn Raman mode. Furthermore, an irreversible cross-linking between the tube and Cn is observed when the maximum applied pressure is higher than the pressure of radial tube collapse is predicted by modelling, thus supporting the experimental results.

Carbon, 2019

Abstract The radial stability and the irreversible transformation of triple-wall carbon nanotubes... more Abstract The radial stability and the irreversible transformation of triple-wall carbon nanotubes (TWCNTs) bundles are investigated at high pressure conditions both experimentally and theoretically (exp. up 72 GPa). The tubes having a mean internal diameter of 0.83 nm and graphite-like intertube distance, show an onset of the radial collapse evidenced by the evolution of optical phonons. The nanotube collapse onset is observed at ∼ 22 GPa completes for the two external tubes at ∼ 29 GPa, however the innermost tube remains stable up to ∼ 37 GPa. Molecular dynamic calculations performed on smaller diameter TWCNTs bundles, as a model system, confirmed the multiple-stage pressure-induced collapse process. An analytical expression for the collapse pressure of carbon nanotubes having an arbitrary number of walls is proposed. Our experiments and modelling show that for pressures beyond ∼ 60 GPa an irreversible structural transformation of TWCNTs takes place. Ex situ transmission electron microscopy characterization on the recovered sample from 72 GPa revealed the mechanical failure of carbon nanotubes which evolve towards ribbon-like structures as corroborated by Raman spectroscopy. Modelling the tubes evolution at high pressure and high temperature showed the formation of new structures ranging from ribbon-like to graphite-like with either different degrees of amorphization or s p 3 interlinking.

The Science of the total environment, Jan 28, 2017

In this work, industrial grade multi-walled carbon nanotubes (MWCNT) were coated with humic acid ... more In this work, industrial grade multi-walled carbon nanotubes (MWCNT) were coated with humic acid (HA) for the first time by means of a milling process, which can be considered an eco-friendly mechanochemical method to prepare materials and composites. The HA-MWCNT hybrid material was characterized by atomic force microscopy (AFM), scanning electron microscopies (SEM and STEM), X-ray photoelectron spectroscopy (XPS), termogravimetric analysis (TGA), and Raman spectroscopy. STEM and AFM images demonstrated that the MWCNTs were efficiently coated by the humic acid, thus leading to an increase of 20% in the oxygen content at the nanotube surface as observed by the XPS data. After the milling process, the carbon nanotubes were shortened as unveiled by SEM images and the values of ID/IG intensity ratio increased due to shortening of the nanotubes and increasing in the number defects at the graphitic structure of carbon nanotubes walls. The analysis of TGA data showed that the quantity of ...

The Journal of Physical Chemistry C, 2014

ABSTRACT The dependence of the radial breathing modes (RBMs) and the tangential mode (G-band) of ... more ABSTRACT The dependence of the radial breathing modes (RBMs) and the tangential mode (G-band) of triple-wall carbon nanotubes (TWCNTs) under hydrostatic pressure is reported. Pressure screening effects are observed for the innermost tubes of TWCNTs similar to what has been already found for DWCNTs. However, using the RBM pressure coefficients in conjunction with the histogram of the diameter distribution, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs. By combining Raman spectroscopy and high-pressure measurements, it was possible to identify these two categories of inner tubes even if the two tubes exhibit the same diameters because their pressure response is different. Furthermore, it was possible to observe similar RBM profiles for the innermost tubes of TWCNTs using different resonance laser energies but also under different pressure conditions. This is attributed to changes in the electronic transition energies caused by small pressure-induced deformations. By using Raman spectroscopy, it was possible to estimate the displacement of the optical energy levels with pressure.

Nanotechnology, 2020

Phonons play a fundamental role in the electronic and thermal transport of 2D materials which is ... more Phonons play a fundamental role in the electronic and thermal transport of 2D materials which is crucial for device applications. In this work, we investigate the temperature-dependence of A[Formula: see text] and A[Formula: see text] Raman modes of suspended and supported mechanically exfoliated few-layer gallium sulfide (GaS), accessing their relevant thermodynamic Grüneisen parameters and anharmonicity. The Raman frequencies of these two phonons soften with increasing temperature with different [Formula: see text] temperature coefficients. The first-order temperature coefficients θ of A[Formula: see text] mode is ∼ -0.016 cm-1/K, independent of the number of layers and the support. In contrast, the θ of A[Formula: see text] mode is smaller for two-layer GaS and constant for thicker samples (∼ -0.006 2 cm-1 K-1). Furthermore, for two-layer GaS, the θ value is ∼ -0.004 4 cm-1 K-1 for the supported sample, while it is even smaller for the suspended one (∼ -0.002 9 cm-1 K-1). The higher θ value for supported and thicker samples was attributed to the increase in phonon anharmonicity induced by the substrate surface roughness and Umklapp phonon scattering. Our results shed new light on the influence of the substrate and number of layers on the thermal properties of few-layer GaS, which are fundamental for developing atomically-thin GaS electronic devices.