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. 2021 Nov 25;12:6856. doi: 10.1038/s41467-021-27071-4

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© The Author(s) 2021

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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Fig. 7 — a−c full-scale ultraviolet photoelectron spectroscopy (UPS), near Fermi energy (low binding energy) and secondary electron cut-off (high binding energy) regions of CDs before the protonation; d−f UPS, near Fermi energy and secondary electron cut-off regions of CDs after the protonation. g UV−Vis spectra, h one-photon fluorescence spectra; i two-photon fluorescence spectra of CDs before and after the protonation. (P-CDs: CDs after the protonation treatment).