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Dipartimento di Fisica e Scienze della Terra

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Intercalation of fullerenes

It is possible to intercalate the C60 fullerene with molecular or atomic species promoting the formation of solid state compounds, so called fullerides, with significant physical and chemical properties. Intercalation is favored by charge transfer between the intercalating species (usually electrons donors) and C60 (usually an electron acceptor), so that fulleride compounds are actually charge-transfer salts.Sicne these compounds are very moisture and oxygen sensitive, preparations always takes place in controlled atmosphere (Ar glove box or vacuum). Tipi di fulleriti di<br />
	metalli alcalini
Structure of alkaline (A) and alkaline-earth (A') intercalated C60. Metal ions are placed into interstitial sites.

Intercalated compounds Of C60 are prepared using different techniques:

  • A standard method to obtain the alkali metal fullerides involves the direct exposure of fullerenes to the metal vapors. For example, in the case of preparation of superconducting K3C60 fullerite, we have:

    3·K + C60   K3C60
    The pellets of the mixture of the two components are heat treated at 700 K under vacuum.

  • In some cases, such as lithium, alkaline earth metals, many transition elements and rare-earth, conventional methods can yield samples with low crystallinity and scarce doping uniformity, mainly due to the low intercalants vapour pressure. In this case, intecalation could be obtained by using metastable compounds containing the metal species, such as azides or some organometallic complexes, which are mixed with powder and then thermally decomposed. The thermal decomposition of alkali azides, in particular, is an efficient alternative for the preparation of lithium fullerides polymer:

    x·LiN3 + C60   LixC60 + 3x/2·N2
    The pellets are heat treated at 450 K under dynamic vacuum.

  • Depending on the solubility of the metal another possible method of preparation is the use of liquid ammonia or methylamine as solvents. Usually, in this case the solvent is incorporated in the final product. For example, in the case of production of sodium and potassium ammoniated compounds, we have:

    Na + 2·K + C60 + x·NH3   (NH3)xNaK2C60
    Alkali metals, dissolved in liquid ammonia condensed at 200 K react with the C60 powder in suspension. The excess ammonia is evaporated at 243 K.

 

 

Graphene synthesis

The first synthesis of graphene by micromechanical exfoliation of graphite (more commonly called the Scotch method) allows formation individual layers suitable for subsequent nano-manipulation. More recently, new methods based on the epitaxial deposition (eg. CVD) has been developed, which allow the growth of graphene films on various substrates. All these methods allow the production of graphene suitable for applications in nanoelectronics, but do not lead to a macroscopic production of graphene, which is required for other types of applications, like in gas absorption. For this purpose, new chemicals methods were developed, that allow the exfoliation of graphite, namely the graphene planes are separated at distances long enough to eliminate any their possible interactions. This method is based on the preliminary oxidation of graphite obtained by using the strongest known oxidizers (concentrated nitric acid + chlorates) by which -OH and =O groups bind to the graphite plane in a disordered way. A subsequent rapid heating to temperatures above 1300 K leads to the decomposition of these groups in gases, which act as a propellant for the separation of the planes.

Sintesi del grafene
Starting from pure graphite, the oxide is produced through a reaction with concentrated nitric acid and potassium chlorate (Brodie method). This allows the attack of -OH and =O groups to the planes (see picture on the left). A subsequent heating of graphite oxide at temperatures above 1300 K rapidly removes the majority of these groups which act as a propellant and allow the exfoliation of graphene planes.

For further readings:

  • K. Holczer, O. Klein, S. M. Huang, R. B. Kaner, K. J. Fu, R. L. Whetten and F. Diederich, "Alkali-fulleride superconductors: synthesis, composition and diamagnetic shielding", Science 252 (1991), 1154-1157.
  • S. Park and R. Ruoff, "Chemical methods for the production of graphenes", Nat. Nano. 4 (2009), 217-224.