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.

二维过渡金属碳化物和氮化物的 MXene 家族已经包括约 50 个成员，它们具有不同数量的原子层、化学计量组成和固溶体、原子的平面内或平面外排序以及各种表面终端。MXenes 显示出的特性使其对从储能到电子和医学的应用具有吸引力。尽管这种成分可变性允许微调 MXene 的特性，但由于存在多种轻元素（例如 H、C、N、O 和 F）非常接近，因此在 MXene 的分析过程中也带来了挑战。在这里，我们展示了使用超低能量二次离子质谱法以原子分辨率对 MXenes 的单个粒子及其母体 MAX 相进行深度剖析。我们直接检测碳亚晶格中的氧，从而证明碳氧化物 MXenes 的存在。我们还确定了相邻表面终止层的成分，并展示了它们之间的相互作用。金属子晶格分析表明，Mo₂ TiAlC ₂ MAX 表现出完美的面外有序性，而 Cr ₂ TiAlC ₂ MAX 在内部过渡金属层中表现出 Cr 和 Ti 之间的一些混合。我们的结果展示了开发的二次离子质谱技术以单原子层精度探测层状和二维材料的组成的能力。