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. 2017 May 11;7:1774. doi: 10.1038/s41598-017-01883-1

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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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THz amplitude mapping during the O₂ adsorption process. (a) THz amplitude mapping for CVD graphene/InP after annealing at 445 K in vacuum. THz mapping of the same area in sample after exposure to air for a few minutes (b) without and (c) with UV illumination. The difference in THz mapping images before and after exposure to air gives a visualization of adsorbed O₂ molecules in the graphene surface. (d) Spatial map of adsorbed O₂ molecules in graphene after exposure to air without UV illumination which reveals regions with different “natural” affinities to O₂ molecules. Regions 1 and 2 (5) show the least (highest) amount of adsorbates after exposure to air, whereas regions 3 and 4 are areas with a fair amount of adsorbates. (e) Spatial map of adsorbed O₂ molecules after exposure to air under UV illumination, showing significantly higher concentration of O₂ adsorbates in the UV exposed part. In the THz amplitude maps (a–c), the blue (red) end of the scale signifies less (more) O₂ molecules on the graphene surface, while in the ΔTHz amplitude maps (d and e), the blue (red) end of the scale signifies less (more) O₂ molecules adsorbed/added on graphene during exposure to air.