The MXene family of two-dimensional transition metal carbides and nitrides already includes ~50 members with distinct numbers of atomic layers, stoichiometric compositions and solid solutions, in-plane or out-of-plane ordering of atoms, and a variety of surface terminations. MXenes have shown properties that make them attractive for applications ranging from energy storage to electronics and medicine. Although this compositional variability allows fine-tuning of the MXene properties, it also creates challenges during the analysis of MXenes because of the presence of multiple light elements (for example, H, C, N, O, and F) in close proximity. Here, we show depth profiling of single particles of MXenes and their parent MAX phases with atomic resolution using ultralow-energy secondary-ion mass spectrometry. We directly detect oxygen in the carbon sublattice, thereby demonstrating the existence of oxycarbide MXenes. We also determine the composition of adjacent surface termination layers and show their interaction with each other. Analysis of the metal sublattice shows that Mo2TiAlC2 MAX exhibits perfect out-of-plane ordering, whereas Cr2TiAlC2 MAX exhibits some intermixing between Cr and Ti in the inner transition metal layer. Our results showcase the capabilities of the developed secondary-ion mass spectrometry technique to probe the composition of layered and two-dimensional materials with monoatomic-layer precision.
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This work was supported by the National Science Centre (NCN) within SONATA 14 2018/31/D/ST5/00399 and National Centre for Research and Development (NCBR) within LIDER XII LIDER/8/0055/L-12/20/NCBR/2021 projects. MXene synthesis and characterization conducted at Drexel University were supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, grant no. DE-SC0018618. M.A. was supported by the National Science Foundation Graduate Research Fellowship under grant no. DGE-1646737 and the US Department of Education Graduate Assistance in Areas of National Need (GAANN) fellowship. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The SEM and XRD analyses were performed using instruments in the Materials Characterization Core at Drexel University. We thank B. Anasori (IUPUI, USA) for preparing the Mo2TiAlC2 and Cr2TiAlC2 MAX samples.
The authors declare no competing interests.
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Michałowski, P.P., Anayee, M., Mathis, T.S. et al. Oxycarbide MXenes and MAX phases identification using monoatomic layer-by-layer analysis with ultralow-energy secondary-ion mass spectrometry. Nat. Nanotechnol. (2022). https://doi.org/10.1038/s41565-022-01214-0