Single Crystal and Epitaxial Film Growth

In the first part of this chapter we discuss the methods used to grow single crystals and thin solid films of van der Waals-bonded C60 and C70. In principle, the methods described here can also be used for higher-mass fullerenes, if they can be isolated in sufficient quantity. Although crystalline fullerene material can be obtained by solution growth in saturated solvents, this method for growing pristine (undoped) fullerene films or crystals often leads to trapped solvent in the lattice, and therefore this growth method is not emphasized here. However, the low sublimation temperatures of C60 and C70 allow vapor growth methods to be applied to both film and single crystal growth. Careful studies of C60 film growth have been made, and several substrates have been found which promote epitaxy in monolayer and thicker films. A more detailed treatment of the interaction between Cgg and substrates is presented in §17.9 on surface science. The photochemical transformation of solid C60 into what appears to be a polymeric solid in which the C60 shells remain intact but are tightly coupled to neighboring C60 molecules by intermolecular C-C bonds is reviewed in §7.5.

The second part of this chapter deals with methods for introducing foreign atoms or molecules into solid C60 to produce crystalline derivatives, e.g., alkaline earths, alkali metals, NH3, and sulfur. These doped fullerene-based materials are classified into two groups: (1) charge transfer materials, in which electrons are exchanged between the host fullerite sublattice and the dopants, and (2) clathrate materials, in which neutral molecules are incorporated into the lattice and no charge exchange occurs between these dopant molecules and the fullerenes. Synthesis methods for the production of carbon nanotubes are given in §19.2.5, and electrochemical synthesis is reviewed in §10.3.2. The preparation of endohedrally doped fullerenes is discussed in §5.4.

9.1. Single-Crystal Growth

9.1.1. Synthesis of Large C60 and C10 Crystals by Vapor Growth

The low sublimation temperatures of C^ and C70 can be exploited to obtain millimeter-size single crystals of these fullerites with prominent facets by vapor growth [9.1-6], The discovery of the sublimation growth method for C60 [9.1,2] is of particular historical importance, because up to that point, C60 had been crystallized primarily by growth from saturated solvents (e.g., toluene, hexane) [9.7]. Unfortunately, the solvent molecules were also incorporated into the lattice, leading to various crystal structures [9.3,8-11] and crystals with many shapes, including whiskers [9.12] and fibers with 10-fold symmetry having 10 facets along the column axis [9.13]. It has, however, been shown subsequently [9.14] that ~(0.5 x 0.3 x 0.3 mm3) face-centered cubic (fee) single crystals of C60 could be obtained from C60-saturated boiling benzene; these crystals were reported to be free of solvent.

The vapor growth technique for synthesizing single-crystal fullerites begins in a clean, dynamically pumped, quartz tube (~ 50 cm long by 1 cm in diameter) shown schematically in Fig. 9.1. C60 powder is first extracted with toluene from carbon soot via standard liquid chromatography techniques or by passing the toluene extract over activated carbon (see §5.2.1 and §5.3.1). The powder is placed in section I of the tube and then degassed by baking at 250° C under dynamic vacuum for 6-24 h [9.4-6]. Next a series of sublimations is carried out to purify the initial charge [9.4,5]. Section I is heated above the C60 sublimation temperature to ~600°C and the C60 condenses on the colder walls of section II. Section I is then sealed off at A, with a torch. This process is continued two more times until purified C^ has been vapor transported into section IV, which is then sealed off at C and D. It is

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