Fig. 5

Total XPS intensities at each oxygen dosage. The XPS intensities of a Si 2p and b metals core levels for V@Si₁₆, Nb@Si₁₆, and Ta@Si₁₆. The intensity fluctuations of the X-ray source are normalized by the XPS signal at a binding energy of 110 eV, where background signals are obtained that only vary with the light intensity. The detailed procedures and the XPS data are shown in Supplementary Figure 12. Error bars: the uncertainty in the intensity analysis is estimated to be ±10%, except for V 2p (±20%). The Si 2p signals for all M@Si₁₆ increase with oxygen exposures, and are 1.25−1.35 times stronger at >1×10¹⁴ L than that of as-deposited ones, implying Si cage collapse of M@Si₁₆. The changes of the core levels of the central metals are less significant compared to the Si 2p because M atoms are located beneath a layer of Si