G O G

superimposed C60 (111) domains rotated by 23° with respect to each other. However, at thicker coverages, the LEED pattern was lost, indicating that thicker films grow in a polycrystalline form. The lattice mismatch between fee C60 and GaSe (0001) is fairly large (12%) when compared to the mismatch of Cgo to mica (4%) and MoS2 (5%) [9.30], which probably explains why these latter substrates have been reported to grow thick epitaxial films.

Highly epitaxial, thick (-1000 A) C60 films with a (111) fee orientation were obtained in studies of Cm film growth on a (100) GeS substrate at —200°C [9.20], for which only a -0.75% lattice mismatch exists. A model structure based on a lattice constant with twice the a-axis of GeS (2 x 4.30 = 8.60 A), closely matching the separation between rows of C60 molecules along the (101) direction (8.68 A), was used to model a C60 overlayer on a (001) GeS surface, as shown schematically in Fig. 9.8. It was reported that the epitaxy was achieved over millimeter square areas, despite the absence of a hexagonal surface symmetry in the substrate [9.30]. The authors attributed the good epitaxy they observed to deep corrugations of the GeS (001) surface (see Fig. 9.9). In another interesting experiment on C60 film growth on GeS (001), a film of several ML thickness was deposited onto a room temperature substrate and an amorphous film was obtained; subsequent annealing at —200° C resulted in the desorption of the upper layers of the film, leaving an epitaxial (111) single ML film still attached to the substrate, indicating clearly that the substrate interaction with C60 molecules is stronger than the intermolecular C60-C60 interaction.

Fig. 9.9. Side view of the model structure of a C60 (111) monolayer on a GeS (001) substrate, showing a large corrugation in the a-direction of the basal plane and the systematic skipping of one interchain groove by the big C60 molecules [9.30].

Fig. 9.9. Side view of the model structure of a C60 (111) monolayer on a GeS (001) substrate, showing a large corrugation in the a-direction of the basal plane and the systematic skipping of one interchain groove by the big C60 molecules [9.30].

Such phenomena have been observed on a number of surfaces with strong bonding to C60 (see §17.9).

The case of crystalline C60 growth on Si substrates is of particular interest for technological reasons. It is well established that the high density of dangling bonds on Si (111), Si (100), and Si (110) surfaces results in strong C60-Si bonding, and because of the lattice mismatch between C60 and the various Si faces, the C60 film growth usually has poor crystallinity (crystallite size of only a few nm) [9.30]. It was, however, found that by passivation of the Si surface (through hydrogenation) to tie up the dangling bonds, the bonding of C60 to the passivated Si (100) and Si (111) surfaces could be greatly reduced, so that crystalline C60 films could be prepared [9.31]. Shown in Fig. 9.10 is a comparison of an x-ray scan taken from a C60 film on an Si (100) surface without passivation (the lower trace labeled hydrophilic) and with passivation (the upper trace labeled hydrophobic). The passivation or hydrogenation of the dangling bonds was done with a 5% HF solution for the Si (100) face, and with the same 5% HF solution followed by a 40% NH4F solution for the Si (111) face. It was further found that the crystallinity of the C60 film improved with increased thickness of the film, as shown in Fig. 9.11 [9.31].

9.2.2. Free-Standing C60 Films, or Fullerene Membranes

Free-standing membranes of C60 films 2000-6000 A thick with areas as large as 6 x 6 mm2 have been fabricated successfully [9.32]. These membranes, supported on a Si frame, were found to be robust, despite the weak van der Waals bonding between C60 molecules. These membranes have been used for optical spectroscopy and mechanical properties measurements (e.g., Young's modulus) as well as for potential applications (e.g., micropore filters, x-ray windows, TEM sample supports). Figure 9.12 shows

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