| 1. |
- Macovez, Roberto, et al.
(author)
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Low band gap and ionic bonding with charge transfer, threshold in the polymeric lithium fulleride Li4C60
- 2008
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 112:8, s. 2988-2996
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Journal article (peer-reviewed)abstract
- We demonstrate the growth of crystalline Li4C60 films. The low-energy electron diffraction pattern of the films indicates the formation of polymer chains in the plane of the surface, consistent with the reported crystal structure. Electron energy loss and photoemission spectra identify the Li4C60 polymer as a low band gap semiconductor, with a relatively strong coupling of electrons to low-frequency stretching modes of the polymer bonds and alkali phonons. No evidence is found for hybridization between the Li- and fullerene-derived electronic states. Instead, a partial charge transfer takes place, which is the same for different Li concentrations. This result rationalizes the stability of the polymer phase over a wide range of stoichiometries.
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| 2. |
- Macovez, Roberto, et al.
(author)
-
Metal-to-insulator transition in thin-film polymeric AC(60)
- 2009
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In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 11, s. 023035-
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Journal article (peer-reviewed)abstract
- We present an electron spectroscopy study of phase-pure AC(60) thin films (A = Rb, Cs) in their monomer (face-centred cubic (fcc)) and polymer phases. A surface electronic reconstruction is observed in polymeric RbC60, analogous to that reported for the fcc phase. As for pristine C-60, the occupied electronic states of AC(60) fullerides are not dramatically affected by polymerization. The energy separation between the leading feature in photoemission and inverse photoemission is similar in both stable AC(60) phases. These observations suggest that electron correlation effects are similar in the two phases, and that their different electronic behaviour is mainly related to the reduction of degeneracy of the polymer frontier states. Photoemission and electron-energy loss spectroscopy data show that the thin-film form of the RbC60 polymer is metallic at room temperature, and that it undergoes a metal-insulator transition at around 100 K. This transition temperature is much higher than that reported for the corresponding bulk phase and signals a poorer screening of Coulomb interactions at the film surface.
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