Info

on the left and right, respectively, refer to times before and after the steady state period [9.5], an untwinned crystal is shown in (a), and the theoretical fee crystal shape is shown in (c), which exhibits (100) and (111) natural facets. The crystals and facets shown in Fig. 9.3 are large enough to carry out a wide variety of solid-state properties studies.

Another set of single-crystal growth procedures using the sublimation method consist of sealing purified Qq powder in a quartz tube containing a few hundred torr of argon gas [9.16]. The sealed tubes are placed in a gradient furnace with the powder held at 650°C. Single crystals are reported to form in the region of the tube that is at ~ 450° C [9.16].

9.2. Thin-Film Synthesis

Thin films of fullerenes can be prepared by vapor growth and are useful for many applications, such as optical studies. Thin-film synthesis is normally carried out by vacuum sublimation of microcrystalline powder. The powder should first be degassed (~ 300°C, 3-6 h, ~10"5 torr) to remove residual solvent, following standard extraction and purification steps. The powder is then placed in either a conventional evaporation boat or a Knudsen cell for vacuum sublimation. Following conventional thin-film growth practice [9.17,18], a large substrate-to-source distance is used to prepare high-quality, epitaxial films. However, for the more difficult to obtain higher-mass fullerenes (e.g., C70, C76, C84), much shorter source distances have been used to increase the amount of higher-mass fullerene material deposited on the substrates.

Several substrates have been reported to achieve epitaxial growth for fee C60 films (e.g., GaAs (110) [9.19], mica (001) [9.20-23], GeS (001) [9.24], GaSe (001) [9.24], MoS2 [9.25], and Sb [9.26]). Several of these substrates are discussed below for illustrative purposes. Other substrates (e.g., Pyrex, Suprasil, NaCl, KBr, sapphire, Si) have been reported to generate nanocrys-talline C60 films with typical grain sizes of 10-20 nm, as determined by x-ray diffraction peak widths [9.27], Several of these are also discussed below as examples of substrates that do not lead to epitaxial growth. Pristine C60 films can be deposited onto a substrate such as a clean silicon (100) surface by sublimation of the C60 powder in a vacuum of ~10~6 torr. Ellipsome-try [9.28] or dilatometry can be used to measure the thickness of the C60 films. By variation of the substrate material and crystalline orientation, and by variation of the film growth temperature, the crystallite size and microstructure of the film can be varied.

Typical of common practice in thin-film deposition, heated substrates lead to larger grain size in nanocrystalline films and better epitaxy in epitaxial films. However, because C60 sublimes at relatively low temperatures, only moderate substrate temperatures (150-200°C) can be used. For exam-

VO g

Fig. 93. Fullerite crystals prepared by the double-gradient technique [9.5] and compared to forms calculated by Marks [9.15] for fee material using a "strong faceting" model. C& (a); Qq (b); the shape of a theoretical equilibrium crystal (c); an asymmetric twin (d), with a detailed description of its faces a,e,f,b',c' octahedral {111}, b,c,d, a' hexahedral {100} (e); and its calculated form (f); multiply twinned particles (g, h); a symmetric twin (ij) [9.5].

Fig. 93. Fullerite crystals prepared by the double-gradient technique [9.5] and compared to forms calculated by Marks [9.15] for fee material using a "strong faceting" model. C& (a); Qq (b); the shape of a theoretical equilibrium crystal (c); an asymmetric twin (d), with a detailed description of its faces a,e,f,b',c' octahedral {111}, b,c,d, a' hexahedral {100} (e); and its calculated form (f); multiply twinned particles (g, h); a symmetric twin (ij) [9.5].

<Jl pie, at substrate temperatures T = 300°C, it was found that C60 sublimes from the film faster than it is being deposited (0.25 A/s) [9.29].

In general, the C60-C60 bonding is through weak van der Waals forces. For substrates where the C^-substrate interaction is even weaker than the C60-C60 interaction (such as mica), crystalline C60 films will grow in the (111) orientation normal to the substrate. Crystalline C60 film growth also occurs when the substrate is well lattice-matched to the lattice planes in fee C60. An example of such crystalline film growth occurs for C60 on GeS (001) substrates as discussed in §9.2.1.

9.2.1. Epitaxial C60 Films

Several studies have been made to determine the nature of the earliest stages of film growth for C60 which reveal equilibrium and nonequilibrium structures. Schmicker et al. [9.20], in He atom scattering studies of C60 monolayer (ML) growth on a clean (001) mica substrate at 300 K in high vacuum (10~10 mbar), found that the growth of the first C60 layer is hexagonal and in registry with the underlying hexagonal mica substrate. Mica was selected as a substrate material because of its weak bonding to C60. Nevertheless, the film was found to grow with a 10.4 A separation between C60 molecules, double the (001) mica spacing and considerably larger than the 10.18 A spacing between molecules in the fee solid lattice, indicating a significant C60-substrate interaction. The lattice mismatch is about 3.4% between the muscovite mica and fee C60. The registry between the (001) mica surface and the C60 overlayer is shown schematically in Fig. 9.4, where the Si04 tetrahedra in the (001) mica substrate are shown together with the hexagonal C60 overlayer; the C60 molecules are represented by 10 A diameter circles. He atom diffraction scans (Fig. 9.5) for 0.7 ML (b) and 1.7 ML (d) coverages of C60 on clean (100) mica are shown. The diffraction data were taken along the (110) and (100) directions as indicated, and the lines due to the hexagonal C60 overlayer can be identified by comparing the data in Fig. 9.5(b) and (d) to data for the clean substrate, (a) and (c). He atom scattering was also used to probe the vibrational properties of the 1.7 ML C60 film, and two dispersionless surface vibrational modes were observed. A mode at 1.5 meV (12 cm"1), assigned to C60 vibrations normal to the substrate surface, and its overtone were reported [9.20].

Transmission electron microscopy (TEM) studies of C60 film growth on (001) mica were carried out at higher substrate temperatures [9.29], and a similar expansion of the in-plane lattice constant of single monolayer Qq films was found [9.29,30]. Thicker films (2500 A) of C60 grown on (100) mica at 200° C were found to grow epitaxially with the anticipated (111) orientation. The bright-field image shown in Fig. 9.6 demonstrates

J at the center of each tetrahedron 1 and an oxygen at each corner. C60

, deposited. A silicon atom is located i .

molecules are depicted by the large circles [9.29], a mica layer which consists of a hexagonal array of Si04 tetrahedra, upon which a C60 monolayer is then

Fig. 9.4. Schematic view of

J at the center of each tetrahedron 1 and an oxygen at each corner. C60

, deposited. A silicon atom is located i .

molecules are depicted by the large circles [9.29], a mica layer which consists of a hexagonal array of Si04 tetrahedra, upon which a C60 monolayer is then

Fig. 9.4. Schematic view of

Was this article helpful?

0 0

Post a comment