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Filling of Carbon Nanostructures

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Endohedrally doped (filled) nanostructures display a doping method unique to cage molecules and cylinders in which an ion (or ions) are encaged inside the carbon framework of a fullerene molecule or a nanotube.

These hybrid-systems represent a complete new type of matter. They are described by the symbol for the encapsulated atom and the fullerene molecule's formula separated by the @ sign, which signals the endohedral nature of the complex, for example La@C82. Endohedral fullerenes containing metal ions (socalled metallofulleres) could act as vehicles to transport poisonous atoms, can be used as MRI markers in medicine and can be used to stabilize and passivate reactive species. Moreover N containing metallofullerenes can be used as containers for atomic Nitrogen and can be used as ESR standard. Metal filled nanotubes can act as nonreactive passivated nanomagnets. Last but not least so-called peapods (fullerenes filled into SWCNT) e.g. C60@SWCNT) represent a hybrid system of fullerenes and single wall carbon nanotubes (SWCNT) with intriguing new properties.

 

We span a wide range of activities. On the one hand we are working on the production and purification and separation and spectroelectrochemical characterization of endohedral fullerenes as well as on the production and analysis of the magnetical properties of metal filled MWCNT and on the formation of fullerene peapods. On the other hand we concentrate on the analysis of the electronic optical and vibronic properties of these systems in our Spectroscopy Group using high energy spectroscopy (EELS, PES, XAS) and optical spectroscopy (IR, UVVIS, Raman) as probes. We have been busy investigating the electronic and vibronic structure of endohedral fullerenes and filled SWCNT. Our aim is to understand the interaction between the encapsulated ion and its host cage/tube. As mentioned above, an important parameter here is the interplay between charge transfer and covalent interaction from the metal ion to the fullerene molecule or nanotube.

 

Endohedral Fullerenes

Tm@C82: Tm is one of the rare earth metals (a lanthanide) and is normally trivalent in the solid state - i.e. it has three valence electrons and thus an electronic configuration of [Xe] 4f12 6s2 5d1. We have investigated Tm@C82 using photoemission (PES, see Figure 1), electron energy-loss (EELS) and x-ray absorption spectroscopy (XAS). All of these data taken together offer conclusive proof that the Tm in Tm@C82 has 13 4f electrons in the ground state and thus is divalent. The charge transfer is purely ionic and formally Tm2+@C822-. This shows the possibility of the stabilization of encaged metal ions in unusual valence states.

 

 

Trimetalnitride Fullerenes

This system represents another example of an hybridsystem containing two species which are both not stable alone. However, upon formation of the endofullerene the C80 cage with Ih symmetry and the trimetalnitride units such as Sc3N, Tm3N, Dy3N, ... are stabilized. We analysed the interplay between charge transfer and covalent interaction in this system using XAS. From a comparison with multiplett calculations including charge transfer it was shown that Sc is trivalent and the effective valency of Sc(III) is 2.4 leaving a charge transfer of six electrons to the C80 cage. This shows the importance of the covalent interaction between the Sc3N unit and the C80 cage. Recent results on the systems containing rare earth trimetalnitrides, such as Tm3N and Dy3N confirmed this observation.

 

 

Carbon Peapods

C60@SWCNT: C60 peapods represent new hybridsystems of fullerenes and SWCNT. One important question is guest host interaction as well as the filling rate on a bulk scale. Using bulk sensitive EELS we have shown that pristine peapods are weakly interacting van der Waals systems with a filling ratio up to 78%

 

Upon intercalation there is a competitive charge transfer to the SWCNT and to the fullerene cage. At highest intercalation level a metallic singly bonded C606- polymer is stabilized within the SWCNT.

 

 

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Electronic Properties of Materials
Faculty of Physics
University of Vienna

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