Single Molecular Spectroscopy: Identification of Individual Fullerene Molecules

We report the molecule-by-molecule spectroscopy of individual fullerenes by means of electron spectroscopy based on scanning transmission electron microscopy. Electron energy-loss fine structure analysis of carbon $1s$ absorption spectra is used to discriminate carbon allotropes with known symmetries. ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ molecules randomly stored inside carbon nanotubes are successfully identified at a single-molecular basis. We show that a single molecule impurity is detectable, allowing the recognition of an unexpected contaminant molecule with a different symmetry. Molecules inside carbon nanotubes thus preserve their intact molecular symmetry. In contrast, molecules anchored at or sandwiched between atomic BN layers show spectral modifications possibly due to a largely degraded structural symmetry. Moreover, by comparing the spectrum from a single ${\mathrm{C}}_{60}$ molecule and its molecular crystal, we find hints of the influence of solid-state effects on its electronic structure.

Figure 1

[(a) and (b)] Model and annular dark field (ADF) images of ${\mathrm{C}}_{60}$ (blue, dark grey in print) and ${\mathrm{C}}_{70}$ (orange, light grey in print) molecules stored inside a SWNT. [(c) and (d)] Model and ADF images of ${\mathrm{C}}_{60}$ (blue) molecules stored between two layers of BN. The ADF image was taken at 30 kV for (b) and 60 kV for (d).

Figure 2

(a) ELNES carbon $K$ edge spectra extracted from five aligned molecules shown in the inset on the right. Note that the contribution of carbon nanotube to the carbon $K$ edge spectra has been extracted by recording separately the carbon $K$ edge of an empty nanotube. The solid lines are spectra after Richardson-Lucy deconvolution [18], and the dots are the raw data after background subtraction. Blue and orange arrows point to common features of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, respectively (see text). From these spectra peak configurations, the discrimination of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ molecules is, indeed, possible. The same common features can be seen in (b), where reference spectra from crystalline ${\mathrm{C}}_{60}$, ${\mathrm{C}}_{70}$, and ${\mathrm{C}}_{84}$ are shown. The ${\mathrm{C}}_{84}$ spectra were extracted from Kuzuo et al. [19] and obtained at 60 kV, while the others were obtained at 30 kV, as in the other experiments described here. Reference spectra for ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ crystals have been deconvoluted using the maximum entropy method [20]. [(c) and (d)] Comparisons of the bulk crystalline spectrum (dashed lines) with a single molecule spectrum (solid lines) for ${\mathrm{C}}_{60}$ (blue) and ${\mathrm{C}}_{70}$ (orange), respectively.

Figure 3

(a) Representative ADF image of fullerenes in a SWNT. (b) Spectra of four molecules compared to experimental spectra from bulk. Two of the molecules are identified as ${\mathrm{C}}_{60}$ and one as ${\mathrm{C}}_{70}$. The fourth molecule (red) spectrum shows broader peaks indicating a lower symmetry and closely matches that of ${\mathrm{C}}_{84}$ [shown in Fig. 2], though not definitely assigned.

Figure 4

Quantification of irradiation damage under the electron beam with increasing dose. (a) Definition of a spectrum contrast $R$ as the ratio between $m-n$ (288.2 eV peak height) to $h$ (284.3 eV peak height). (b) Spectrum contrast $R$ as a function of dose for experiments using 30 keV (black) and 60 keV (red) electrons derived from the chronospectroscopy of ${\mathrm{C}}_{60}$ bulk crystals (see Supplemental Material Fig. S5 [14]). The dashed lines mark the contrast measured from data in the literature [19, 21, 24]