300 400 500

Wavelength (nm)

§5.1, §5.2, §5.3, §9.1, §9.2), and techniques for the endohedral doping of fullerenes are reviewed in §5.4.

9.3.1. Charge Transfer Compounds Based on Alkali Metals and Alkaline Earths

One general approach to alkali metal (M) doping is to introduce a near-stoichiometric amount of M to oxygen-free C60 films, single crystals, or microcrystalline powder placed in a sealed glass tube. The tube is then evacuated and raised in temperature to 200-300°C to promote intercalation. Depending on the grain size and the longest diffusion length of the C60 starting material and the intercalation temperature, several hours to several days may typically be required to achieve homogeneous doping. Several studies of MjC60 compounds have been carried out during intercalation as x passes from zero to its maximum value. In these types of experiments it is important to exercise caution to obtain homogeneous doping and a single phase region over the depth of the physical probe used for the sample characterization. Several phases (e.g., x = 0,1,3,4,6) may coexist, which complicates (or may even invalidate) the interpretation of experimental characterization data. Alkali metal alloy compounds, e.g., Rb^Cs^Qo, have also been synthesized using a similar technique, primarily aimed at expanding the lattice to enhance the superconducting transition temperature [9.33] (see §15.1).

Doped charge transfer materials are prepared by exposing fee C60 to the vapors of alkali metals or alkaline earths. The form of the starting C60 material, microcrystalline powder, single crystals or thin films is, of course, largely dictated by the nature of the experiments to be performed. Microcrystalline powders are easiest to work with, because it is relatively easy to obtain large quantities of fullerenes and fullerene derivatives in powder form. Synthesis of the doped compounds is also simplified since a significant mass of C60 powder can be placed in the ampoule, and therefore a stoichiometric amount of M dopants (or M and M' dopants in the case of an alloy such as Rb3„iCsi) can be added easily to the ampoule by weight. In the case of reactive alkali metals (M,M'), it is often convenient to siphon molten alkali metal into the bore of a glass capillary and to break off the desired length of the capillary to introduce a stoichiometric amount of alkali metal into the reaction chamber. The ampoule containing the mixture of C60 and alkali metal is loaded into a furnace and reacted for a time range from hours to days in an attempt to achieve the desired single-phase reaction product [9.13],

The doping of C60 with alkali metals can also be achieved in a two-temperature oven, similar to the apparatus used to prepare alkali metal graphite intercalation compounds [9.34], This approach works well with powdered samples and has also been used to intercalate single crystals. The stoichiometry is controlled by the number of moles of alkali metal introduced into the sealed ampoule (quartz or Pyrex) relative to the number of moles of C60, the temperatures of the reagents (usually less than the temperature of the C60), and the duration of the reactions.

Alkali metal doping of thin (—1000 A) C60 films can be carried out similarly, using a sealed quartz or Pyrex tube (with approximate dimensions 25 cm long x 1 cm diameter) which contains the C60 film/substrate sample and the alkali metal at opposite ends. The films are maintained at a higher temperature (200°C) than the alkali metal (M) (80-150°C) to avoid condensation of the alkali metal on the film surface. The in situ progress of the reactions can be monitored qualitatively by the color change observed in the films and more quantitatively by the doping-induced downshift of the strongest C60 Raman line (see Fig. 11.24) or by changes in the electrical conductivity (see §14.1.1).

The doping reaction in C60 single crystals has been studied during the preparation of doped single crystals for electrical transport measurements as a function of temperature [9.16]. Electrical contacts to the samples are made prior to doping, by first evaporating silver onto the crystal surfaces and then attaching gold wires with conducting silver paint. The mounted samples are then sealed together with fresh potassium metal in a Pyrex glass apparatus with tungsten feedthrough leads. Uniform doping can be accomplished using a repetitive high-temperature doping-anneal cycle. First, both the sample and dopant are heated uniformly in a furnace, while the sample resistance is continuously monitored. The temperature is raised from room temperature to about 200°C at a rate of ~6°C/min. At about 150°C, the resistance of the sample drops to a measurable range (~20 Mil). As T is raised further, the resistivity of the sample drops continually to a few hundred miicm within a few min. The tube should be maintained at about 200° C for approximately one-half hour until the resistance of the sample reaches a minimum. The potassium end of the tube is now cooled to room temperature and the sample alone is annealed at about 200°C to 250°C overnight. Following this overnight anneal, the potassium end of the ampoule is reheated to ~200°C and the sample is further doped, until a lower resistivity is reached. The sample is then annealed again for several h. This doping and annealing process is repeated until the resistance reaches an equilibrium minimum resistivity value. For transport measurements, the sample cell is injected with helium exchange gas to ensure good thermal conduction, and the sample temperature is determined by a diode temperature sensor which is mounted in the cell adjacent to the crystal.

The lattice constant of MjC6(l-related compounds has in some cases been increased by the subsequent reaction with NH3 gas [9.35], Similar reactions were tried previously with alkali metal-doped graphite intercalation compounds [9.36]. The conceptual idea behind the ammoniation of the M-doped fullerene compounds is to have the ammonia complex and the alkali metal ion both occupy the same octahedral interstitial site, thereby expanding the fee lattice. Under similar experimental conditions, T = 100°C and 0.5 atm of dry NH3 gas distilled from liquid ammonia, a microcrystalline M,M^C60 powder was reacted with the ammonia for ~1—10 days in a 9-cm sealed Pyrex tube, previously evacuated to 10~5 torr [9.35], The following results were obtained: R^Qq did not react with the NH3, while K3C60 yielded NH3K3C60 [9.37,38], and Na2MC60 (M = Rb, Cs) resulted in (NH3)4Na2MC60 [9.39], where the NH3 stoichiometry was determined by weight uptake measurements. Prior to this annealing step, the powder was first exposed to 0.5 atm NH3 gas at room temperature for 1-2 days, and a substantial pressure drop was noticed during the first few min of exposure, indicating immediate ammonia uptake. The NH3 reaction was found to be thermally reversible; mild heating of the ammoniated compounds in vacuum returned the lattice parameters and Tc to their original values (see §8.5.5 and §15.1). This observation suggests that amides are not being formed and that neutral NH3 molecules are intercalating into the structure and solvating the alkali metal ions.

The preparation of the alkaline earth intercalation compounds represents more of a challenge than that faced in the synthesis of the alkali metals because of the higher reactivity of the alkaline earths with quartz and the higher reaction temperatures required to prepare the alkaline earth compounds. To prepare CafC60 [9.40], high-purity (99.99%) dendritically solidified calcium powdered metal is weighed and mixed with purified C60 powder, in high-purity tantalum tubes, using various Ca:C60 ratios to obtain the desired stoichiometrics for the end products. The tantalum tubes containing the Ca and the C60 are then loaded inside quartz tubes. Since quartz reacts with calcium, tantalum tubes are needed to avoid sample contamination by unwanted reaction products. All sample preparation is done in a controlled-atmosphere glove box where the oxygen or water vapor concentrations are maintained below a few parts per million. After the quartz tubes are loaded as described above, an ultrahigh-vacuum valve is connected to the open end of the quartz tube. The quartz tube is then taken out of the dry box and sealed under a vacuum of 10~6 torr. Heat treatments are done in tube furnaces at 550°C for 20 h, although the reaction is nearly complete after only 1 h at this temperature. Following heat treatment, the samples are removed from the tantalum tubes in the glove box and are loaded un der vacuum into appropriate sample cells for characterization (e.g., x-ray diffraction studies) and properties measurements.

A similar preparation method is used for the preparation of strontium and barium fulleride compounds. Strontium metal is intercalated at temperatures 450-600°C and for periods ranging from minutes to weeks. The best samples showing the highest superconducting volume fraction are prepared by anneals at 550° C for a few days. Ba3C60 and Ba6C60 compounds are prepared [9.41,42] at higher heat treatment temperatures (550-800°C) and for longer time periods (from hours to weeks) than the corresponding Sr^C60 compounds [9.41,42],

The reaction of C60 with the Lewis acids SbCl5, AsF5, and SbF5 were carried out in a two-arm tube furnace (~150°C for the SbCl5 reaction with C60). After removing the sample from the furnace, the arm containing the reaction products was kept at 50°C and the second arm was placed in liquid nitrogen to distill the SbCl5. A second method used for these reactions was a solution method based on SbCl5 dissolved in CC14 [9.43]. It is inferred that the reaction of C60 with these Lewis acids results in the transfer of electrons from C60 to the Lewis acids [9.43].

9.3.2. Fullerene-Based Clathrate Materials

The methods for preparing clathrate materials depend to a large extent on the nature and size of the dopant molecule. If the dopant is a small molecule, an overpressure of the molecular gas above fee solid C60 is sufficient to induce room temperature intercalation [9.44] at pressures of 1 kbar and 0.14 kbar to introduce 02 and H2, respectively, into a substantial fraction of the large, octahedral sites in the fee C60 lattice. Exposure to 1 kbar 02 for 116 h resulted in the occupancy of 25% of these interstitial spaces. By using visible light and 1 atm of 02, it is possible to fill nearly all the octahedral sites in a thin film (~2000 A) in a much shorter time (~ 1 h)

[9.45.46]. Through study of the isothermal adsorption or intercalation of N2 and 02 into microcrystalline C60 powder and from analysis of the N2 isotherm at 77 K, it was concluded that the internal nanopore surface area associated with the octahedral sites was ~24 m2/g [9.47]. It was also found

[9.44.47] that 02 adsorption (and desorption) was reversible at 303 K, but the saturation concentration of 02 was found to be higher than for N2, namely one 02 molecule per two C60 molecules, or 50% of complete saturation of the octahedral sites [9.47]. Presumably, for a large uptake of 02 and N2, there must be enough defects in the C60 crystalline lattice to link many octahedral sites effectively, otherwise the diffusion rate would be very slow. It was pointed out that lattice defects of more than 0.2 per unit cell can produce a percolating micropore structure [9.47]. Most re cently, studies of the isothermal adsorption of He into microcrystalline C60 (80%)/C70 (20%) powders [9.48] showed that 4He intercalation into these powders takes place more readily as the temperature is decreased, in contrast to previous studies of C60 films [9.49,50], where 4He was found not to intercalate C60 films at low temperatures.

Larger molecules, such as organic solvent molecules and ring sulfur (Sg) clusters, have been introduced into crystalline clathrate structures containing either C60 or C70. For example, the crystal structures which result from heating mixtures of sulfur and microcrystalline C60 and C70 powders have been investigated [9.51] and small clathrate crystals containing oriented C60 or C70 molecules and crown-shaped Sg rings were obtained.

To prepare sulfur-C60 clathrate compounds (see §8.4.2), the C60 (C70) and excess sulfur were heated in an evacuated glass ampoule to a temperature slightly above the melting point of sulfur [9.51]. At this temperature, the S8 rings are the dominant species in the melt. For the case of C60 and sulfur, the mixture was then cooled slowly (l°C/h) down to room temperature to produce intergrown clathrate crystals with composition C60(S8)2. These crystals can also be grown at room temperature from solution, using CC14 and 10% CS2 (volume) as the solvent [9.51], Using higher concentrations of CS2 in CCI4, the compound C60(S8)2CS2 was formed by slow evaporation of the solvent (CS2 evaporates preferentially). It was further shown that isolated isometric crystals of C60(S8)2 with linear dimensions up to 200 ¿im can be grown (i.e., two S8 rings per C60 unit) [9.52],

Noncubic clathrate crystals containing C60 and solvent molecules were discovered in 1991 by several groups [9.3,11,53]. More recently, C60 clathrates formed with aliphatic molecules [e.g., «-pentane (C5H12), diethyl ether (C4H10O), and 1,3 dibromopropane (C3H6Br2)] have been prepared in single-crystal form [9.54]. These clathrate crystals, containing organic solvent molecules, are grown from solutions, either containing the pure solvent to be incorporated into the structure or in a solution containing a second solvent species. For example, aliphatic molecules were incorporated into C60 clathrate crystals from a toluene solution by precipitation methods [9.55]. These C60-aliphatic clathrate molecules all exhibited an identical 0.1 eV blue shift of the optical absorption edge, indicating a nonspecific, weak interaction between C60 and the aliphatic molecules [9.54], consistent with clathrate behavior.


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