Browsing by Author "Gadelha, Andreij de Carvalho"

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    Graphene electromechanical water sensor : the Wetristor.
    (2019) Meireles, Leonel Muniz; Silva Neto, Eliel Gomes da; Ferrari, Gustavo Arrighi; Neves, Paulo A. A.; Gadelha, Andreij de Carvalho; Almeida, Ive Silvestre de; Taniguchi, Takashi; Watanabe, Kenji; Chacham, Helio; Neves, Bernardo Ruegger Almeida; Campos, Leonardo Cristian; Lacerda, Rodrigo Gribel
    A water-induced electromechanical response in suspended graphene atop a microfluidic channel is reported. The graphene membrane resistivity rapidly decreases to ≈25% upon water injection into the channel, defining a sensi-tive “channel wetting” device—a wetristor. The physical mechanism of the wetristor operation is investigated using two graphene membrane geometries, either uncovered or covered by an inert and rigid lid (hexagonal boron nitride multilayer or poly(methyl methacrylate) film). The wetristor effect, namely the water-induced resistivity collapse, occurs in uncovered devices only. Atomic force microscopy and Raman spectroscopy indicate substantial morphology changes of graphene membranes in such devices, while covered membranes suffer no changes, upon channel water filling. The results suggest an electromechanical nature for the wetristor effect, where the resistivity reduction is caused by unwrinkling of the graphene membrane through channel filling, with an eventual direct doping caused by water being of much smaller magnitude, if any. The wetristor device should find useful sensing applications in general micro- and nanofluidics.
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    Nanomechanics of few-layer materials : do individual layers slide upon folding?
    (2020) Batista, Ronaldo Junio Campos; Dias, Rafael Freitas; Barboza, Ana Paula Moreira; Oliveira, Alan Barros de; Manhabosco, Taíse Matte; Silva, Thiago R. Gomes; Matos, Matheus Josué de Souza; Gadelha, Andreij de Carvalho; Rabelo, Cassiano; Cançado, Luiz Gustavo de Oliveira Lopes; Jorio, Ado; Chacham, Helio; Neves, Bernardo Ruegger Almeida
    Folds naturally appear on nanometrically thin materials, also called “2D materials”, after exfoliation, eventually creating folded edges across the resulting flakes. We investigate the adhesion and flexural properties of single-layered and multilayered 2D materials upon folding in the present work. This is accomplished by measuring and modeling mechanical properties of folded edges, which allows for the experimental determination of the bending stiffness (κ) of multilayered 2D materials as a function of the number of layers (n). In the case of talc, we obtain κ ∝ n 3 for n ≥ 5, indicating no interlayer sliding upon folding, at least in this thickness range. In contrast, tip-enhanced Raman spectroscopy measurements on edges in folded graphene flakes, 14 layers thick, show no significant strain. This indicates that layers in graphene flakes, up to 5 nm thick, can still slip to relieve stress, showing the richness of the effect in 2D systems. The obtained interlayer adhesion energy for graphene (0.25 N/m) and talc (0.62 N/m) is in good agreement with recent experimental results and theoretical predictions. The obtained value for the adhesion energy of graphene on a silicon substrate is also in agreement with previous results.
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    Robust nanofabrication of monolayer MoS2 islands with strong photoluminescence enhancement via local anodic oxidation.
    (2018) Fernandes, Thales Fernando Damasceno; Gadelha, Andreij de Carvalho; Barboza, Ana Paula Moreira; Paniago, Roberto Magalhães; Campos, Leonardo Cristiano; Guimarães, Paulo Sérgio Soares; Assis, Pierre Louis de; Neves, Bernardo Ruegger Almeida
    In this work, we demonstrate the nanofabrication of monolayer MoS2 islands using local anodic oxidation of few-layer and bulk MoS2 akes. The nanofabricated islands present true monolayer Raman signal and photoluminescence intensity up to two orders of magnitude larger than that of a pristine monolayer. This technique is robust enough to result in monolayer islands without the need of meticulously ne-tuning the oxidation process, thus providing a fast and reliable way of creating monolayer regions with enhanced optical properties and with controllable size, shape, and position.