Optical-phonon resonances with saddle-point excitons in twisted-bilayer graphene

Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the density of states that can be tuned by changing the twisting angle $\theta$. A $\theta$-defined tBLG has been produced and characterized with optical reflectivity and resonance Raman scattering. The $\theta$-engineered optical response is shown to be consistent with persistent saddle-point excitons. Separate resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the two-dimensional saddle-point excitons, similar to what has been previously observed for one-dimensional carbon nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the two processes are correlated and that they share the same phonon.Comment: 5 pages, 6 figure

Raman spectroscopy as a versatile tool for studying the properties of graphene.

Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene

Elastomer Composite Based on EPDM Reinforced with Polyaniline Coated Curaua Fibers Prepared by Mechanical Mixing

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Curaua fibers were used to reinforce elastomeric matrices with polyaniline (PAni) synthesized directly on the fiber surfaces to produce antistatic-reinforced composites. In this work, composites of poly(ethylene-co-propylene-co-diene) with curaua fibers coated with PAni were prepared by mechanical mixing in a counter-rotating twin rotor internal mixer. Then, mechanical and electrical properties of these composites were correlated to Raman and Fourier transformed infrared spectra (FTIR) using chemometric data analyze, such as principal component analysis (PCA) and hierarchical cluster analysis (HCA). Raman spectra showed correlation with electrical properties of conductive composites while FTIR spectra showed good correlation with mechanical properties. EPDM reinforced with PAni coated curaua fibers presented higher tensile strength and modulus than EPDM reinforced with pristine curaua fibers, indicating that the reinforcement effect was obtained. Chemical interaction between the phases occurs with formation of hydrogen bonding between the aminic nitrogens of PAni and the carbonyl groups of lignin of the fibers. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40056.1317Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [2010/17804-7, 2010/17871-6

Elastomer Composite Based On Epdm Reinforced With Polyaniline Coated Curauá Fibers Prepared By Mechanical Mixing

Curauá fibers were used to reinforce elastomeric matrices with polyaniline (PAni) synthesized directly on the fiber surfaces to produce antistatic-reinforced composites. In this work, composites of poly(ethylene-co-propylene-co-diene) with curauá fibers coated with PAni were prepared by mechanical mixing in a counter-rotating twin rotor internal mixer. Then, mechanical and electrical properties of these composites were correlated to Raman and Fourier transformed infrared spectra (FTIR) using chemometric data analyze, such as principal component analysis (PCA) and hierarchical cluster analysis (HCA). Raman spectra showed correlation with electrical properties of conductive composites while FTIR spectra showed good correlation with mechanical properties. EPDM reinforced with PAni coated curauá fibers presented higher tensile strength and modulus than EPDM reinforced with pristine curauá fibers, indicating that the reinforcement effect was obtained. 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Elastomer composite based on EPDM reinforced with polyaniline coated curaua fibers prepared by mechanical mixing

Curaua fibers were used to reinforce elastomeric matrices with polyaniline (PAni) synthesized directly on the fiber surfaces to produce antistatic-reinforced composites. In this work, composites of poly(ethylene-co-propylene-co-diene) with curaua fibers coated with PAni were prepared by mechanical mixing in a counter-rotating twin rotor internal mixer. Then, mechanical and electrical properties of these composites were correlated to Raman and Fourier transformed infrared spectra (FTIR) using chemometric data analyze, such as principal component analysis (PCA) and hierarchical cluster analysis (HCA). Raman spectra showed correlation with electrical properties of conductive composites while FTIR spectra showed good correlation with mechanical properties. EPDM reinforced with PAni coated curaua fibers presented higher tensile strength and modulus than EPDM reinforced with pristine curaua fibers, indicating that the reinforcement effect was obtained. Chemical interaction between the phases occurs with formation of hydrogen bonding between the aminic nitrogens of PAni and the carbonyl groups of lignin of the fibers1317CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPsem informação2010/17804-7; 2010/17871-

Sodium-Mediated Low-Temperature Synthesis of Monolayers of Molybdenum Disulfide for Nanoscale Optoelectronic Devices

Monolayers of molybdenum disulfide are of vital importance in the fabrication of optical and nanoelectronic devices. The development of thin and low-cost devices has increased the demand for synthesis processes. Usually, the synthesis of molybdenum disulfide monolayers requires temperatures of approximately 800 degrees C, which is a drawback for the applications mentioned above. Here, we propose a route using the atmospheric pressure chemical vapor deposition technique to grow monolayers of MoS2 at 550 degrees C mediated by using sodium as a catalyst. We produced single crystals and polycrystalline films by controlling the NaNO3/MoO3 catalyst/precursor ratio and the growth time. Using first-principles calculations, we determined that sodium was the nucleation site of the growth process. The precursor's ratio is crucial to decrease the formation energy and the synthesis temperature. First-principles calculations and experiments showed that the ideal precursor ratio was 0.3 and that the synthesis temperature should be decreased by 250 degrees C. We investigated the monolayers with optical microscopy, high-resolution scanning transmission electron microscopy, Xray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence spectroscopy, and transport experiments. The optical and electrical performances were comparable to those of monolayers grown at higher temperatures. We believe that a low-temperature synthesis recipe is essential to drive the fabrication of nanoscale optoelectronic devices

Aligned carbon nanotube/zinc oxide nanowire hybrids as high performance electrodes for supercapacitor applications

Abstract Carbon nanotube/metal oxide based hybrids are envisioned as high performance electrochemical energy storage electrodes since these systems can provide improved performances utilizing an electric double layer coupled with fast faradaic pseudocapacitive charge storage mechanisms. In this work, we show that high performance supercapacitor electrodes with a specific capacitance of ∼192 F/g along with a maximum energy density of ∼3.8 W h/kg and a power density of ∼28 kW/kg can be achieved by synthesizing zinc oxide nanowires (ZnO NWs) directly on top of aligned multi-walled carbon nanotubes (MWCNTs). In comparison to pristine MWCNTs, these constitute a 12-fold of increase in specific capacitance as well as corresponding power and energy density values. These electrodes also possess high cycling stability and were able to retain ∼99% of their specific capacitance value over 2000 charging discharging cycles. These findings indicate potential use of a MWCNT/ZnO NW hybrid material for future electrochemical energy storage applications