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. 2017 Aug 30;7:9927. doi: 10.1038/s41598-017-09811-z

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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XRD patterns taken in parts of the aged quartz tube (in the photograph, with the arrow indicating the gas flow direction) reaching different temperatures during the graphene growth process: (A) 570 °C, (B) 860 °C, (C) 1070 °C. Before the analysis the inner tube surface has been cleaned with nitric acid to remove the condensed Cu. The colors of the different tube regions (red for metallic Cu or cuprous oxide - Cu₂O -, and black for cupric oxide - CuO) demonstrate the diffusion into the bulk quartz along a temperature gradient.