Small Carbon Nano-Onions: An Ion Mobility Mass Spectrometric Study

Structures and charges of nanocarbon cluster ions, C_n^z+ (100 ≤ n ≤ 800, z = 1 and 2), have been determined using ion mobility mass spectrometry. For singly charged ions, a compact cluster ion series was observed in addition to monolayer fullerene ions for n = 260-700 continuously. Previous electron microscopic observations indicated that the compact clusters were bilayer fullerenes (nano-onions), in which the inner and outer layers grow from a structure close to [C₃₀@C₂₃₀]⁺ at n = 260. The present study also suggests that several combinations of inner and outer layer fullerenes were produced. The results indicated that the interlayer distance depended on different combinations of inner and outer layers and that the observed lower limit of the interlayer distance agreed well with that of graphite (3.35 Å). The upper limit corresponded to bilayer structures in which the number of atoms of the inner layer was constant at about 30, the smallest fullerene size observed in this study. Series of monolayers and compact bilayers of doubly charged ions with cross sections that coincided with those of monocations were observed in nearly the same size region as monocations.