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The Properties of CVD Graphene Transferred by High-Speed Electrochemical Delamination Tymoteusz Ciuk, Iwona Pasternak, Aleksandra Krajewska, Jan Sobieski, Piotr Caban, Jan Szmidt, and Wlodek Strupinski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4032139 • Publication Date (Web): 16 Sep 2013 Downloaded from http://pubs.acs.org on September 19, 2013
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The Journal of Physical Chemistry
The Properties of CVD Graphene Transferred by High-Speed Electrochemical Delamination Tymoteusz Ciuk1,2, Iwona Pasternak1, Aleksandra Krajewska1,3, Jan Sobieski4, Piotr Caban1, Jan Szmidt2, and Wlodek Strupinski1* 1) Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland 2) Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland 3) Institute of Optoelectronics, Military University of Technology, Gen. S. Kaliskiego 2, 00-908 Warsaw, Poland 4) Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland
* corresponding author: Wlodek Strupinski, Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland, telephone number: +48 22 835 30 41,
[email protected] Abstract We report on the electrical characterisation and Raman spectroscopy of CVD copper-grown graphene transferred onto a Si/SiO2 substrate by the high-speed (1mm per second) electrochemical delamination. We determine graphene's sheet resistance, carrier mobility and concentration as well as its physical quality as a function of the electolyte concentration. Graphene's electrical properties are investigated with standard Hall measurements in van der Pauw geometry and a contactless method that employs a single-post dielectric resonator operating at microwave frequencies. These properties are related to the widely used copper etching technique. The results prove that the high-speed electrochemical delamination provides good quality graphene within a short timescale.
Keywords: Graphene, copper foil, Raman spectroscopy, microwave resonator, efficient transfer
Introduction The quest for a reliable method for graphene transfer has been on since the development of its CVD growth on copper foil. The primary technique involves the application of an etchant, typically iron(III) chloride, ammonium persulfate or hydrogen chloride. This method, albeit reliable, is time and resource-consuming. Recently, there has been a novel approach suggested1 that involves the electrochemical delamination (ED) in a low-concentration aqueous solution of potassium persulfate. When the graphene/copper cathode is negatively polarised hydrogen bubbles appear at the graphene/copper interface due to the reduction of water molecules and let graphene gently detach. In this paper we examine the influence of the high pace of detachment (1mm per second) on graphene's quality.
Experiments Monolayer CVD graphene was grown on 12µm-thick 3N JTCHTE GOULD Electronics copper foil in a horizontal chemical vapour deposition hot-wall reactor Aixtron VP508 (formerly known as Epigress system)2. This system ensures perfect temperature distribution without the thermal gradient effect, laminar gas flow, pyrometrically-controlled temperature, ultra high purity of applied hydrogen, argon and propane as well as precise control of the pressure, partial pressure and gas flow. At first, copper foil was pretreated at 1000°C under Ar gas flow and then H2 gas flow at the pressure of 100mbar. The purpose of this step was to enlarge the grain size of Cu foil and improve its quality. Next, both C3H8 and H2 gases were introduced into the reactor for 2 minutes. Finally, graphene-covered copper was cooled down to
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room temperature in Ar atmosphere and PMMA (4% in anisole) spin-coated. To ensure the back-side of copper is graphene-free it was etched in oxygen plasma.
High-resistivity silicon (>1000Ωcm) was chosen a target substrate for the transferred graphene. It was thermally oxidised to form a 300nm layer of dry silicon oxide on its surface and cut into 1cm per 1cm samples. We chose potassium chloride and sodium chloride dissolved in deionised water as the electrolyte with its concentration ranging from 2mM to 2M. A custom-made mechanism was used to steadily insert copper foil into the electrolyte at a rate of 1mm/s. Except for the lowest electrolyte concentration (2mM, 100V) copper was negatively polarised 4V to 6V with respect to a carbon anode. Prior to delamination the Cu/Gr/PMMA foil was cut to match the dimensions of the target Si/SiO2 substrates (1cm per 1cm). After a 10s delamination process Gr/PMMA was rinsed with DI water, transferred onto the dielectric substrate, heated to 130ºC and treated with acetone to remove PMMA. No post acetone treatment was applied. The insertion rate of 1mm/s was kept constant for all of the samples. A set of 10 reference graphene on Si/SiO2 samples was produced by etching copper in (NH4)2S2O8: H2O (0.07M), followed by cleaning in HCl:H2O2:H2O (volume ratio 1:1:20) and NH4OH:H2O2:H2O (volume ratio 1:1:20), respectively. The averaged sheet resistance of these 10 samples served as a reference for our high-speed ED.
The custom-made mechanism's design enabled the Cu/Gr/PMMA foil to be vertically held and steadily submerged into the electrolyte, so that the bubbling could appear at one edge only. This configuration allows highly-controlled delamination regardless of the shape and dimensions of the foil. The rate of 1mm/s has been chosen to match the natural pace of graphene detachment. We observed that if the insertion rate was increased graphene was dragged under the electrolyte surface undetached. When the pace was reduced below 1mm/s the electrolyte would spontaneously ooze up the vertically held foil and promote delamination above the electrolyte surface. We verified this rate for larger samples and affirmed its applicability for areas as big as 4cm per 4cm.
Characterisation Each of the Si/SiO2/Gr samples (1cm × 1cm) was first examined with a single-post dielectric resonator3 operating at the frequencies near 13.1GHz. This axially-symmetrical resonator utilises a quasi TE011 mode with only an azimuthal component of the electric field, assuring that no current flows between the sample and the copper cavity of the resonator. The SiPDR was custom-made for the masurements of graphene films on small low-loss dielectric substrates (1cm × 1cm)4. The presence of a conductive thin-film inside the cavity affects the resonance frequency and Q-factor of the resonator5. The Q-factor is a function of the product of the conductivity and the thin-film thickness (σh). For
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σh
