Ultra-High Vacuum Scanning Tunneling Microscopy:

Individual C60 Molecules on Si(100)

Xiaowei Yao, Richard K. Workman, Dong Chen, Dror Sarid

Optical Sciences Center, University of Arizona, Tucson, AZ 85721

Figure 1 shows a 30 nm x 30 nm surface of Si(100)-2x1 imaged with the UHV-STM after a two-step C60 deposition obtained at a sample bias (V_s) of -1.9 V and a tunneling current (I_t) of 1.0 nA ⁽¹⁾.

First, a 0.02 monolayer of C60 molecules was deposited on the silicon surface at room temperature, after which the sample was annealed to 600 deg for several seconds. After the sample cooled back to room temperature, a second deposition of a 0.01 monolayer of C60 molecules was made. This procedure provided both physisorbed (pre-annealed) and chemisorbed (post-annealed) C60 molecules on the same Si(100) surface. ^(2,3)

The STM measurements were performed in-situ using a modified McAllister STM, equipped with control electronics by Digital Instruments (Nanoscope III). One observes the Si(100) surface reconstruction characterized by a pairing of top-layer atoms to form dimer rows on the surface. Note that the "zigzag" buckled dimers appear only in the vicinity of defects or adsorbates ⁽²⁾. Each C60 molecule appears as a bright round shape against the 2x1 background. The physisorbed molecules (labelled as A) are found at the four-dimer sites between the dimer rows, while the chemisorbed molecules (labelled as B and C) are bonded either to one-dimer sites on the top of the dimer rows or to missing dimer sites, respectively. Note that the size of the molecules is larger than their van der Waals diameter due to tip-sample convolution.

One can rule out the possibility of tip artifacts in these images since every observed pattern of intramolecular features is reproducible and has its own orientation relative to the Si(100)-(2x1)surface. Note that the intramolecular features, in particular those of type C, can be observed without contrast enhancement or filtering.

References

1. X. Yao, T. G. Ruskell, R. K. Workman, D. Sarid, and D. Chen, "Intramolecular Features of Individual C60 Molecules on Si(100) Observed by STM", Surf. Sci. Lett., (in print 1996).
2. X. Yao, D. Chen, T. G. Ruskell, R. K. Workman, and D. Sarid, "Thermally-Induced Changes in Bonding Properties of C60 on Si(100)-(2x1) Surfaces" Israel Journal of Chemistry 36, (1996) 1.
3. D. Chen and D. Sarid, "An STM study of C60 adsorption on Si(100)-(2x1) surfaces: from physisorption to chemisorption" Surf. Sci. 329 (1995) 261.

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