Initial Charge of To Vacuum
Fig. 9.1. Schematic view of a quartz tube (50 cm xl cm OD) used to degas, purify and finally grow small single crystals by vapor transport. A, B, C, D indicate constrictions in the tube used to facilitate sealing off sections with a torch. Sections I, II, and III are used to purify successively the starting material by sublimation. Tube section IV is used to grow the crystals by vapor transport (see text).
well known that a considerable amount of the initial C60 charge [9.4] cannot be sublimed, although the reason(s) for this is still not clear. Raman spectroscopy indicates that the major portion of the unsublimed residual is indeed Cft0 [9.5]. The long quartz tube (IV) containing the purified C60 in one end is then placed in a tube furnace for the final sublimation, vapor transport, and crystal growth steps. In the simplest case, the tube is loaded into the furnace initially at 500° C, and then the furnace is cooled to room temperature in 4 h [9.6]. Other researchers have chosen to place the sealed quartz tube containing C60 into a furnace with a temperature gradient [9.1]. Vapor transport of the C60 occurs from the hot end of the tube (560-620° C) to the cold end (480-540°C). In this gradient method, crystals of size visible to the unaided eye form in a period of hours. It is found that C60 has the highest vapor pressure among fullerenes, and this feature has been useful for separation applications (see §5.3.2) [9.13].
Untwinned single C60 crystals with 2 mm x 2 mm facets and C70 crystals with ~0.2 mm x 0.2 mm facets have been obtained [9.5] by a vapor growth technique, employing a double-temperature-gradient furnace. The gradient and heat treatment history are shown schematically in Fig. 9.2. For the case of C60, a sealed quartz ampoule (20 cm long x 1 cm in diameter) containing the purified C60 in one end of the tube is placed in an oven whose temperature profile is shown in Fig. 9.2. Both ends of the tube are at temperature T2 (~ 590°C), and a minimum temperature Tx (~ 550°C) is obtained near the center of the tube, as shown. A sharp concave minimum in the temperature profile at an early time in the growth process was reported to yield one large fee single crystal near the position of minimum temperature. Examples of C60 and C70 crystals obtained by this method are shown in Fig. 9.3, together with calculated faceted crystal forms [9.15] for an fee material using a strong-faceting model. The configuration of the facets can be used to identify twinning in the crystals (d, i). An example of
Fig. 9.2. The time depen
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