Giant and tunable anisotropy of nanoscale friction in graphene

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPERJ - FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIROFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISThe nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction forces in graphene are highly dependent on the scanning direction: under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction. In comparison, for highly-oriented pyrolitic graphite (HOPG), the friction anisotropy between armchair and zigzag directions is only 15%. This giant friction anisotropy in graphene results from anisotropies in the amplitudes of flexural deformations of the graphene sheet driven by the tip movement, not present in HOPG. The effect can be seen as a novel manifestation of the classical phenomenon of Euler buckling at the nanoscale, which provides the non-linear ingredients that amplify friction anisotropy. Simulations based on a novel version of the 2D Tomlinson model (modified to include the effects of flexural deformations), as well as fully atomistic molecular dynamics simulations and first-principles density-functional theory (DFT) calculations, are able to reproduce and explain the experimental observations.The nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction forces in graphene are highly dependent on the scanning direction: under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction. In comparison, for highly-oriented pyrolitic graphite (HOPG), the friction anisotropy between armchair and zigzag directions is only 15%. This giant friction anisotropy in graphene results from anisotropies in the amplitudes of flexural deformations of the graphene sheet driven by the tip movement, not present in HOPG. The effect can be seen as a novel manifestation of the classical phenomenon of Euler buckling at the nanoscale, which provides the non-linear ingredients that amplify friction anisotropy. Simulations based on a novel version of the 2D Tomlinson model (modified to include the effects of flexural deformations), as well as fully atomistic molecular dynamics simulations and first-principles density-functional theory (DFT) calculations, are able to reproduce and explain the experimental observations.619CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPERJ - FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIROFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPERJ - FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIROFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISSem informaçãoSem informação2013/08293-7, 2014/15521-9Sem informaçãoAll authors aknowledge the financial support from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ). R.P. acknowledges Fundação de Amparo a Pesquisa do Estado de São Paulo (Fapesp) for financial support through Grant #2014/15521-9. D.S.G. thanks the Center for Computational Engineering and Sciences at Unicamp for financial support through the FAPESP/CEPID Grant # 2013/08293-7. Computer simulations carried out during this research were supported by resources supplied by the Center for Scientific Computing (NCC/GridUNESP) of the São Paulo State University (UNESP). L.G.C. acknowledges FAPEMIG and the grant PRONAMETRO (52600.056330/2012). B.F acknowledges FAPEMIG and the grant PRONAMETRO (52600.030929/2014)

Unraveling structural and compositional information in 3D FinFET electronic devices

Non-planar Fin Field Effect Transistors (FinFET) are already present in modern devices. The evolution from the well-established 2D planar technology to the design of 3D nanostructures rose new fabrication processes, but a technique capable of full characterization, particularly their dopant distribution, in a representative (high statistics) way is still lacking. Here we propose a methodology based on Medium Energy Ion Scattering (MEIS) to address this query, allowing structural and compositional quantification of advanced 3D FinFET devices with nanometer spatial resolution. When ions are backscattered, their energy losses unfold the chemistry of the different 3D compounds present in the structure. The FinFET periodicity generates oscillatory features as a function of backscattered ion energy and, in fact, these features allow a complete description of the device dimensions. Additionally, each measurement is performed over more than thousand structures, being highly representative in a statistical meaning. Finally, independent measurements using electron microscopy corroborate the proposed methodolog

Aplicação da Calorimetria Exploratória Diferencial (DSC) para Determinação da Pureza de Fármacos

O trabalho aborda um estudo comparativo entre a técnica de Cromatografia a Líquido de Alta Eficiência e a técnica de Calorimetria Exploratória Diferencial, visando avaliar a aplicação desta para a determinação direta da pureza de fármacos candidatos a materiais de referência certificados

Effect of graphene oxide on bacteria and peripheral blood mononuclear cells

"Background Driven by the potential biological applications of graphene, many groups have studied the response of cells exposed to graphene oxide (GO). In particular, investigations of bacteria indicate that there are 2 crucial parameters, which so far have only been investigated separately: GO size and exposure methodology. Our study took into account both parameters. We carefully characterized the samples to catalog sizes and structural properties, and tested different exposure methodologies: exposure in saline solution and in the presence of growth media. Furthermore, we performed experiments with peripheral blood mononuclear cells exposed to our GO materials. Methods Atomic force microscopy, scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy were used to characterize the morphology and composition of different samples of GO: GO-H2O, GO-PBS and GO-MG. Our samples had 2D sizes of ?100 nm (GO-H2O and GO-PBS) and >2 µm (GO-MG). We tested antibacterial activity and cytotoxicity toward peripheral blood mononuclear cells of 3 different GO samples. Results A size-dependent growth inhibition of Escherichia coli (DH5 ?) in suspension was found, which proved that this effect depends strongly on the protocol followed for exposure. Hemocompatibility was confirmed by exposing peripheral blood mononuclear cells to materials for 24 hours; viability and apoptosis tests were also carried out. Conclusions Our experiments provide vital information for future applications of GO in suspension. If its antibacterial properties are to be potentiated, care should be taken to select 2D sizes in the micrometer range, and exposure should not be carried out in the presence of grow media.

Assessment of boundary lubrication in biodiesels by nanotribological tests

Nanoscale measurements using atomic force microscopy are performed in order to scrutinize the friction phenomena observed in microscale ball-on-disc tribological tests under (boundary lubrication) BL regime. Two reference biodiesels, one derived from a vegetable source (soybean) and the other from animal fat, are compared. A linear dependence of the friction coefficient (mu) with the Stribeck parameter (S = viscosity x velocity/load) is observed: mu = 0.11 - 26.54 x S for the animal fat and mu = 0.12 - 51.56 x S for the soybean biodiesel. The nanotribological tests allowed highlighting the cohesion component of friction force in the BL regime that is associated to the intrinsic characteristics of the biodiesels, the respective friction coefficients being mu = 0.0206 for the animal fat and mu = 0.0233 for the soybean biodiesel. The better lubricity of the animal fat biodiesel compared to the soybean observed in microscale is attributed to the presence of sulfur and to the higher amount of mono- and di-glycerides contaminants in it. The polarity and/or chemical affinity of the respective sulfur and OH groups facilitate them to reacting with the steel surfaces during the rubbing action. At nanoscale level, the same ranking in friction is observed among the biodiesels, being that here the friction phenomena are attributed to the cohesive forces other than those related to viscosity. (C) 2013 Elsevier Ltd. All rights reserved

Modificação da superfície de polímeros com irradiação UV na presença de oxigênio e vapor de ácido acrílico

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Modificação da superfície de polímeros com irradiação UV na presença de oxigênio e vapor de ácido acrílico

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Assessment of the lubricant behaviour of biodiesel fuels using Stribeck curves

The molecular structure (carbon chain length, degree of unsaturation and branching of the chains) and the presence of contaminants in a biodiesel have a critical influence on their lubricant behaviour. Ball-on-disc tri-bological tests were performed with two different standard reference materials for biodiesel - NIST SRM 2772 (soybean) and NIST SRM 2773 (animal fat). It is demonstrated that the Stribeck curves, the plots of coefficient of friction (p) vs. Stribeck parameter S (the product of viscosity with speed divided by the normal contact load), fit to a general polynomial equation of the type mu = mu(0) (1- K-BL x S + K-EHL X S-n). Constants K-BL and K-EHL reveal the contributions of the boundary lubrication (BL) and the elastohydrodynamic (EHL) phenomena, respectively. The animal fat biodiesel is the most effective fuel as it shows the highest KBL and the lowest K-EHL values leading to the most stable coefficient of friction regarding speed variation. This behaviour is attributed to the higher content of contaminants, namely mono- and di-glycerides, and the presence of sulphur in the animal fat biodiesel. (C) 2013 Elsevier B.V. All rights reserved