Transmission Electron Microscopy Imaging of Individual Functional Groups of Fullerene Derivatives

Mobility and reactivity of the functionalized fullerenes with pyrrolidine (${\mathrm{C}}_{60}\mathrm{\text{−}}{\mathrm{C}}_3{\mathrm{NH}}_7$) incorporated in single-wall carbon nanotubes were examined by high-resolution transmission electron microscopy. An individual functional group attached to each fullerene cage is unambiguously visualized. This provides a direct evidence for the functionalized structure on a single-molecular basis. A rotational motion of the incorporated molecules tends to occur during the observation and, consequently, each fullerene molecule is likely to stand facing its functionalized group towards the nanotube wall. A fine structure analysis of electron energy-loss spectra for the nitrogen $K$($1s$) edge shows a considerable change in the nitrogen chemical state and suggests a strong tube-fullerene interaction.

Figure 1

(a) A scheme of the used fullerene derivatives functionalized with a pyrrolidine (${\mathrm{C}}_{60}\mathrm{\text{−}}{\mathrm{C}}_3{\mathrm{NH}}_7$, N-methyle-3,4-fulleropyrrolidine). HR-TEM images of a functionalized fullerene peapod (b) and (c) the nonfunctionalized fullerene peapod.

Figure 2

A sequential HR-TEM image of $({\mathrm{C}}_{60}\mathrm{\text{−}}{\mathrm{C}}_3{\mathrm{NH}}_7{)}_n@\mathrm{SWNTs}$. The functional group attached to each fullerene cage is indicated by arrows and arrowheads. (a) 0 sec (b) 2 sec, and (c) 4 sec. Some of the fullerenes make rotations inside the SWNTs. Two of the adjacent fullerenes begin to fuse each other [(c)]. See also the provided movie file [10].

Figure 3

(a),(b) EELS spectra taken from a crystalline particle of the fullerene derivatives (${\mathrm{C}}_{60}\mathrm{\text{−}}{\mathrm{C}}_3{\mathrm{NH}}_7$) before being encapsulated and from the peapods, respectively. The fine structures of carbon represent for the fullerenes with degraded symmetry (a) and for the main contribution of SWNTs (b). The nitrogen $K$ edge in (a) resembles that of nitrogen atoms in the pyrrolidine, but it completely evolves after the encapsulation in the peapod (b) because of the strong interaction with the tube wall.

Figure 4

A comparison of the experimental HR-TEM image (a) with the simulated image (b). We have made several kinds of image simulations for the molecular structure: intact (i), with the detached hydrogen atoms (from 1–3 H atoms) (ii), or without the methyl group (iii). The latter two [(ii) and (iii)] fit reasonably well with the observed images. Note that most of the observed functional groups appearing to attach to the tube wall, suggesting the strong interaction between each functional group and the tube wall.