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Functionalized Fullerenes and Heterofullerenes

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The best method to alter the electronic properties of nanostructures is to dope them via the addition and removal of electrons, in this case, from the fullerene molecules or nanotubes. There are various methods by which one can achieve this. Here we briefly show the most commonly used methods. 

Functionalization of Fullerenes

For instance, alkali metal intercalated C60 compounds exhibit metallic conductivity and superconductivity at transition temperatures only bettered by those of the high temperature superconductors. A pre-requisite to the understanding of these phenomena is a detailed knowledge of the electronic structure of the compound in question.Solid C60 is an insulator - this means that there is an energy gap between the occupied electronic states and the lowest lying unoccupied states. However, C60 is no ordinary solid - it is built up of discrete molecular C60 units, which interact only weakly with one another. In this respect it is similar to other van der Waals solids such as solidified rare gases.

The molecular nature of solid C60, which makes itself felt in the spectroscopic investigation of pristine fullerenes, has important consequences for the fullerene's electronic states lying close to the energy gap. Considering that the archetypical fullerene C60 forms a solid with a face centered cubic (fcc) structure, we have at our disposal two tetrahedral and one octahedral interstitial site per C60 ball. Metal ions can be inserted into these interstitial sites forming a host-guest system which is referred to as an intercalation compound, thus leading to a controlled doping of the fullerene electron system. The alkali metals (Na, K, Rb, Cs) are typical electron donors and much of the work on doped fullerenes to date has been carried out on the alkali metal intercalation compounds of C60.

Superconducting A3C60

  • anomalous lineshape of the high resolution photoemission spectra of these phases understood as sign of coupling of the electron system to vibrons and the charge carrier plasmon
  • non-dispersive nature charge carrier plasmon in K3C60 understood

Insulating A4C60

  • dependence of low-lying transitions between the occupied and unoccupied states on the lattice constant and number of nearest neighbors shows that the energy gap in the A4 materials has its origin in electron correlation (these compounds are Mott-Hubbard insulators)

'Superdoping' - NaxC60 x>6

  • special 'electron pockets' formed in the center of the Na4 aggregates located at the octahedral site 'soak up' the electrons donated from the Na between x = 6 and 8. Between these intercalation stages, the charge on the C60 ball doesn't change, although the Na ions are practically fully ionized.

 

 

 

 

 

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Electronic Properties of Materials
Faculty of Physics
University of Vienna

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