Fig. 3. (a–e) XPS spectra of CDs showing nitrogen and sulfur doping. The use of Allium fistulosum in synthesis gave access to an abundant and diverse set of biomolecules contained within the plant resulting in an elementally diverse CD with a high sulfur content. Adapted from RSC ref. 27. Copyright 2019 the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry. (f) Plot showing the diverse range of the O : C and N : C ratios of green-synthesized CDs reported in the literature. Counts are shown next to their respective bins and indicate the number of publications reporting CDs with properties in that bin's range. N : C and O : C are atomic ratios. XPS data from publications were assumed to be in atomic percent whereas elemental data from other types of instrumentation were either explicitly stated as atomic ratios or converted from their mass ratios. A reference list can be found in Table S2 of the ESI.†.