Individual C₆₀ Molecules

The topography of isolated C₆₀ molecules on the Ag(100) surface is shown in Figure 1a. Each C₆₀ molecule assumes one of four orientations. In unoccupied state images (positive sample voltage) C₆₀ molecules appear to be composed of pentagonal segments. The appearance of the C₆₀ topography is determined by the nature of the C₆₀ electronic states probed by the STM tip. By changing the sign of the tunneling voltage from positive to negative, the STM switches from probing unoccupied states to probing occupied states. The occupied C₆₀ states give the C₆₀ molecule a “dimpled golf ball” appearance, as seen in Figure 1b.

Figure 1
a) Orientations of C₆₀ molecules on Ag(100) step edges.
b) Imaging voltage dependence of C₆₀ topography.

The electronic states that produce the C₆₀ topography can be directly measured using scanning tunneling spectroscopy. Measuring the C₆₀ electronic spectrum at various points on the C₆₀ molecule (Figure 2) reveals four main spectroscopic features, which are attributed to the C₆₀ HOMO (Highest Occupied Molecular Orbital), split LUMO (Lowest Unoccupied Molecular Orbital), and LUMO+1. These spectroscopic measurements further show that the C₆₀ electronic structure is highly spatially inhomogeneous.

Figure 2. C₆₀ electronic structure.

This spatial variation is due to inhomogeneity of the C₆₀ molecular orbitals and is best visualized by mapping the height of each spectroscopic resonance at each point on the molecule. The resulting images are shown in Figure 3. These STS maps represent energy-resolved images of C₆₀ molecular orbitals.

Figure 3. dI/dV maps of C₆₀ molecular orbitals.

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References

Energy Resolved Imaging of Fullerene Molecular Orbitals
M. Grobis, Xinghua Lu, K.H. Khoo, Steven G. Louie, and M. F. Crommie
Proceedings of the 12th International Conference on Scanning Tunneling Microsocopy/Spectroscopy and Related Techniques. AIP CP# 696, 20 (2003)

Spatially Mapping the Spectral Density of a Single C₆₀ Molecule
Xinghua Lu, M. Grobis, K.H. Khoo, Steven G. Louie, and M. F. Crommie
Phys. Rev. Lett. 90, 096802 (2003)