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Carlos Alvarado Chavarin

Bio: Carlos Alvarado Chavarin is an academic researcher from Leibniz Institute for Neurobiology. The author has contributed to research in topics: Graphene & Materials science. The author has an hindex of 7, co-authored 13 publications receiving 254 citations. Previous affiliations of Carlos Alvarado Chavarin include University of Duisburg-Essen & Innovations for High Performance Microelectronics.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the effect of carbon to oxygen ratio (C/O) on the electrical resistance of functionalized graphene sheets prepared by thermal exfoliation and reduction of graphite oxide at various temperatures was studied.
Abstract: We study the effect of carbon to oxygen ratio (C/O) on the electrical resistance of functionalized graphene sheets prepared by thermal exfoliation and reduction of graphite oxide at various temperatures. Using a 2-probe technique in conjunction with Kelvin probe force microscopy, we observe a transition from high-resistance (>400 kΩ/sq) nonlinear current/voltage characteristics at low C/O to low-resistance (<10 kΩ/sq) linear behavior at high C/O, indicating a transition from hopping to diffusive electron transport. Simultaneously, the metal-graphene contacts change from high-resistance Schottky-type behavior to nearly non-invasive metal-metal contact characteristics.

117 citations

Journal ArticleDOI
TL;DR: In this article, the Schottky barrier height was estimated in the framework of the thermionic emission theory using Cheung's and Norde's methods, and a band model was proposed to fully explain the experimental currentvoltage features, including a plateau observed in reverse current at low temperatures.
Abstract: We fabricate graphene/p-Si heterojunctions and characterize their current–voltage properties in a wide temperature range. The devices exhibit Schottky diode behaviour with a modest rectification factor up to . The Schottky parameters are estimated in the framework of the thermionic emission theory using Cheung's and Norde's methods. At room temperature, we obtain an ideality factor of about 2.5 and a Schottky barrier height of 0.18 eV, which reduces at lower temperatures. We shed light on the physical mechanisms responsible for the low barrier, discussing the p-doping of graphene caused by the transfer process, the exposure to air and the out-diffusion of boron from the Si substrate. We finally propose a band model that fully explains the experimental current–voltage features, included a plateau observed in reverse current at low temperatures.

61 citations

Journal ArticleDOI
TL;DR: In this article, the fabrication and characterization of edge contacts to large area CVD-grown monolayer graphene by means of optical lithography using CMOS compatible metals, i.e., Nickel and Aluminum is reported.
Abstract: The exploitation of the excellent intrinsic electronic properties of graphene for device applications is hampered by a large contact resistance between the metal and graphene. The formation of edge contacts rather than top contacts is one of the most promising solutions for realizing low ohmic contacts. In this paper the fabrication and characterization of edge contacts to large area CVD-grown monolayer graphene by means of optical lithography using CMOS compatible metals, i.e. Nickel and Aluminum is reported. Extraction of the contact resistance by Transfer Line Method (TLM) as well as the direct measurement using Kelvin Probe Force Microscopy demonstrates a very low width specific contact resistance down to 130 Ωμm. The contact resistance is found to be stable for annealing temperatures up to 150°C enabling further device processing. Using this contact scheme for edge contacts, a field effect transistor based on CVD graphene with a high transconductance of 0.63 mS/μm at 1 V bias voltage is fabricated.

34 citations

Journal ArticleDOI
TL;DR: In this article, the influence of optical lithography resists on the contact resistance in graphene devices was studied using a combined approach of atomic force microscopy patterning and micro-Raman mapping.
Abstract: The contact resistance is a key bottleneck limiting the performance of graphene-based electronic and optoelectronic devices. Using a combined approach of atomic force microscopy patterning, Kelvin probe force microscopy and micro-Raman mapping, we study the influence of optical lithography resists on the contact resistance in graphene devices. We find that devices fabricated by optical lithography show a significantly larger contact resistance compared to devices produced by electron beam lithography using polymethylmethacrylate as resist. This difference is attributed to a 3–4-nm-thick residual layer remaining in between the contact metal and the graphene after optical lithography.

22 citations

Journal ArticleDOI
TL;DR: A graphene/silicon junction with rectifying behaviour and remarkable photo-response was fabricated by transferring a graphene monolayer on a pillar-patterned Si substrate that shows photovoltaic effect with 0.7% power conversion efficiency and achieves 88 A/W photoresponsivity when used as photodetector.
Abstract: A graphene/silicon junction with rectifying behaviour and remarkable photo-response was fabricated by transferring a graphene monolayer on a pillar-patterned Si substrate. The device forms a 0.11 eV Schottky barrier with 2.6 ideality factor at room temperature and exhibits strongly bias- and temperature-dependent reverse current. Below room temperature, the reverse current grows exponentially with the applied voltage because the pillar-enhanced electric field lowers the Schottky barrier. Conversely, at higher temperatures, the charge carrier thermal generation is dominant and the reverse current becomes weakly bias-dependent. A quasi-saturated reverse current is similarly observed at room temperature when the charge carriers are photogenerated under light exposure. The device shows photovoltaic effect with 0.7% power conversion efficiency and achieves 88 A/W photoresponsivity when used as photodetector.

22 citations


Cited by
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Claudia Backes1, Claudia Backes2, Amr M. Abdelkader3, Concepción Alonso4, Amandine Andrieux-Ledier5, Raul Arenal6, Raul Arenal7, Jon Azpeitia6, Nilanthy Balakrishnan8, Luca Banszerus9, Julien Barjon5, Ruben Bartali10, Sebastiano Bellani11, Claire Berger12, Claire Berger13, Reinhard Berger14, M.M. Bernal Ortega15, Carlo Bernard16, Peter H. Beton8, André Beyer17, Alberto Bianco18, Peter Bøggild19, Francesco Bonaccorso11, Gabriela Borin Barin20, Cristina Botas, Rebeca A. Bueno6, Daniel Carriazo21, Andres Castellanos-Gomez6, Meganne Christian, Artur Ciesielski18, Tymoteusz Ciuk, Matthew T. Cole, Jonathan N. Coleman1, Camilla Coletti11, Luigi Crema10, Huanyao Cun16, Daniela Dasler22, Domenico De Fazio3, Noel Díez, Simon Drieschner23, Georg S. Duesberg24, Roman Fasel25, Roman Fasel20, Xinliang Feng14, Alberto Fina15, Stiven Forti11, Costas Galiotis26, Costas Galiotis27, Giovanni Garberoglio28, Jorge M. Garcia6, Jose A. Garrido, Marco Gibertini29, Armin Gölzhäuser17, Julio Gómez, Thomas Greber16, Frank Hauke22, Adrian Hemmi16, Irene Hernández-Rodríguez6, Andreas Hirsch22, Stephen A. Hodge3, Yves Huttel6, Peter Uhd Jepsen19, I. Jimenez6, Ute Kaiser30, Tommi Kaplas31, HoKwon Kim29, Andras Kis29, Konstantinos Papagelis32, Konstantinos Papagelis26, Kostas Kostarelos33, Aleksandra Krajewska34, Kangho Lee24, Changfeng Li35, Harri Lipsanen35, Andrea Liscio, Martin R. Lohe14, Annick Loiseau5, Lucia Lombardi3, María Francisca López6, Oliver Martin22, Cristina Martín36, Lidia Martínez6, José A. Martín-Gago6, José I. Martínez6, Nicola Marzari29, Alvaro Mayoral7, Alvaro Mayoral37, John B. McManus1, Manuela Melucci, Javier Méndez6, Cesar Merino, Pablo Merino6, Andreas Meyer22, Elisa Miniussi16, Vaidotas Miseikis11, Neeraj Mishra11, Vittorio Morandi, Carmen Munuera6, Roberto Muñoz6, Hugo Nolan1, Luca Ortolani, A. K. Ott3, A. K. Ott38, Irene Palacio6, Vincenzo Palermo39, John Parthenios26, Iwona Pasternak40, Amalia Patanè8, Maurizio Prato21, Maurizio Prato41, Henri Prevost5, Vladimir Prudkovskiy12, Nicola M. Pugno42, Nicola M. Pugno43, Nicola M. Pugno44, Teófilo Rojo45, Antonio Rossi11, Pascal Ruffieux20, Paolo Samorì18, Léonard Schué5, Eki J. Setijadi10, Thomas Seyller46, Giorgio Speranza10, Christoph Stampfer9, I. Stenger5, Wlodek Strupinski40, Yuri Svirko31, Simone Taioli47, Simone Taioli28, Kenneth B. K. Teo, Matteo Testi10, Flavia Tomarchio3, Mauro Tortello15, Emanuele Treossi, Andrey Turchanin48, Ester Vázquez36, Elvira Villaro, Patrick Rebsdorf Whelan19, Zhenyuan Xia39, Rositza Yakimova, Sheng Yang14, G. Reza Yazdi, Chanyoung Yim24, Duhee Yoon3, Xianghui Zhang17, Xiaodong Zhuang14, Luigi Colombo49, Andrea C. Ferrari3, Mar García-Hernández6 
Trinity College, Dublin1, Heidelberg University2, University of Cambridge3, Autonomous University of Madrid4, Université Paris-Saclay5, Spanish National Research Council6, University of Zaragoza7, University of Nottingham8, RWTH Aachen University9, Kessler Foundation10, Istituto Italiano di Tecnologia11, University of Grenoble12, Georgia Institute of Technology13, Dresden University of Technology14, Polytechnic University of Turin15, University of Zurich16, Bielefeld University17, University of Strasbourg18, Technical University of Denmark19, Swiss Federal Laboratories for Materials Science and Technology20, Ikerbasque21, University of Erlangen-Nuremberg22, Technische Universität München23, Bundeswehr University Munich24, University of Bern25, Foundation for Research & Technology – Hellas26, University of Patras27, Center for Theoretical Studies, University of Miami28, École Polytechnique Fédérale de Lausanne29, University of Ulm30, University of Eastern Finland31, Aristotle University of Thessaloniki32, University of Manchester33, Polish Academy of Sciences34, Aalto University35, University of Castilla–La Mancha36, ShanghaiTech University37, University of Exeter38, Chalmers University of Technology39, Warsaw University of Technology40, University of Trieste41, University of Trento42, Instituto Politécnico Nacional43, Queen Mary University of London44, University of the Basque Country45, Chemnitz University of Technology46, Charles University in Prague47, University of Jena48, University of Texas at Dallas49
29 Jan 2020
TL;DR: In this article, the authors present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures, adopting a 'hands-on' approach, providing practical details and procedures as derived from literature and from the authors' experience, in order to enable the reader to reproduce the results.
Abstract: © 2020 The Author(s). We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.

330 citations

Journal ArticleDOI
TL;DR: A new type of graphene aerogel-nickel foam (GA@NF) hybrid material prepared through a facile two-step approach and explored its energy storage application as a binder-free supercapacitor electrode with improved electrochemical performance was reported.
Abstract: We reported a new type of graphene aerogel–nickel foam (GA@NF) hybrid material prepared through a facile two-step approach and explored its energy storage application as a binder-free supercapacitor electrode. By simple freeze-drying and the subsequent thermal annealing of graphene oxide hydrogel–NF hybrid precursor, three-dimensional graphene aerogels with high mass, hierarchical porosity, and high conductivity were deposited on a NF framework. The resulting binder-free GA@NF electrode exhibited satisfactory double-layer capacitive behavior with high rate capability, good electrochemical cyclic stability, and a high specific capacitance of 366 F g–1 at a current density of 2 A g–1. The versatility of this approach was further verified by the successful preparation of 3D graphene/carbon nanotube hybrid aerogel–NF as a supercapacitor electrode, also with improved electrochemical performance. With advantageous features, such a facile and versatile fabrication technique shows great promise in the preparation...

264 citations

Journal ArticleDOI
20 Feb 2017-ACS Nano
TL;DR: The graphite material is stabilized without centrifugation at concentrations up to 100 g/L using carboxymethylcellulose sodium salt to formulate conductive printable inks, which is a simple and scalable production route for conductive inks for large-area printing in flexible electronics.
Abstract: We report the exfoliation of graphite in aqueous solutions under high shear rate [∼ 108 s–1] turbulent flow conditions, with a 100% exfoliation yield. The material is stabilized without centrifugation at concentrations up to 100 g/L using carboxymethylcellulose sodium salt to formulate conductive printable inks. The sheet resistance of blade coated films is below ∼2Ω/□. This is a simple and scalable production route for conductive inks for large-area printing in flexible electronics.

248 citations

Journal ArticleDOI
TL;DR: In this article, the authors report the results of a fundamental study on the remarkable conversion between n- and p-type reduced graphene oxide (rGO) with changes in the thermal annealing temperature.
Abstract: To allow for the use of graphene in various nanoelectronic applications, the methods for the large-scale production of graphene with controllable electrical properties need to be developed. Here, we report the results of a fundamental study on the remarkable conversion between n- and p-type reduced graphene oxide (rGO) with changes in the thermal annealing temperature. It was found that the charge carriers in rGO for temperatures of 300–450 °C and 800–1000 °C are electrons (n-type), whereas for temperatures of 450–800 °C, they are holes (p-type). This is because the individual oxygen functional groups present on rGO are determined by the annealing temperature. We found that the predominance of electron-withdrawing groups (i.e., carboxyl, carbonyl, and sp3-bonded hydroxyl, ether, and epoxide groups) resulted in p-type rGO, although that of electron-donating groups (sp2-bonded hydroxyl, ether and epoxide groups) lead to n-type rGO. In addition, as a proof of concept, a flexible thermoelectric device consist...

170 citations