Effect of hydrogen flow during cooling phase to achieve uniform and repeatable growth of bilayer graphene on copper foils over large area 5 Richard Gulotty a,b , Saptarshi Das b , Yuzi Liu b , Anirudha V. Sumant b, * a Materials Science and Engineering Program, Bourns College of Engineering, University of California – Riverside, Riverside, CA 92521, USA b Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA ARTICLE INFO Article history: Received 17 January 2014 Accepted 17 May 2014 Available online 24 May 2014 ABSTRACT The growth of single-layer graphene on copper foil by chemical vapor deposition (CVD) method has been investigated extensively by several groups, however, achieving the same for the bilayer graphene, using a fast and reproducible process, is proven to be difficult and most of the efforts in this direction so far have been on controlling the nucleation phase during active growth regime. In this article we show that by regulating the gases introduced during the cooling phase, uniform and continuous growth of both the single and bilayer graphene can be obtained on copper foils with growth phase duration reduced to 3 min (i.e., 5–60 times faster than previous methods). We demonstrated growth of bilayer graph- ene on 30 · 30 cm copper foils. We show that the use of vacuum cooling enhanced the growth of single-layer graphene while the introduction of hydrogen gas during the cooling phase promoted the growth of bilayer graphene. We explain observed results elucidating a crucial role of hydrogen leading to a growth of bilayer graphene. The characterization of single and bilayer graphene have been supported by extensive statistical analysis of Raman spectroscopy, selected area electron diffraction measurements as well as fabrication of graphene field effect transistors. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Graphene, a monolayer of hexagonally packed carbon atoms, discovered by Novoselov and Geim [1], has demonstrated remarkable electrical, mechanical, optical, and thermal prop- erties. Technological implementation of graphene is not far from reality. It is, therefore, important to investigate control- lable and sustainable processes for large-scale growth of high-quality graphene. To that end, chemical vapor deposi- tion (CVD) has been studied extensively and demonstrated to yield polycrystalline graphene from centimeter to near- meter area on metallic thin films and foils [2–5]. Studies have http://dx.doi.org/10.1016/j.carbon.2014.05.037 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved. 5 The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (‘‘Argonne’’). Argonne, a U.S. Department of Energy Office of Science Laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. * Corresponding author. E-mail address: sumant@anl.gov (A.V. Sumant). CARBON 77 (2014) 341 – 350 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon