HREELS Studies of K x C 60 Thin Films

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Abstract High resolution electron energy loss spectroscopy has been used to investigate the vibrational and electronic excitations of single-phase, crystalline AzC60 films. Substantial changes in intramolecular mode frequencies and intensities with alkali concentration arise from charge t transfer effects as well as free carrier screening. For x=4, splitting of the lu band is observed, resulting in an insulating phase with an estimated gap of 0.3-0.4eV. Screening in the x=3 phase results in enhanced surface sensitivity and deviations from bulk stoichiometry at the surface of metallic films are observed.

Introduction The electronic and vibrational properties of alkali-metal doped fullerenes (A1 C 60 ) have attracted considerable interest largely due to the occurrence of superconductivity in x=3 compounds. These materials also exhibit interesting metal-insulator transitions which are not yet fully understood. Of particular concern are the x=4 and x=1 phases which are predicted to be metallic by band theory arguments but exhibit insulating behavior [1, 2]. Both electron correlation and disorder effects have been invoked in order to explain the non-metallic behavior of these compounds. Electron energy loss spectroscopy in transmission (T- EELS) has been used to determine the optical constants of the alkali fullerides over a wide energy range [3]. High resolution EELS (HREELS) utilizes considerably lower energy electrons in reflection and offers additional capabilities for the study of these materials. In particular, the higher resolution permits study of intra and intermolecular vibrational modes and of low-lying electronic transitions, while the short penetration depth of low energy electrons offers the possibility of surface sensitivity. HREELS has been used previously to study undoped C 60 [4, 5, 6] as well as coadsorption of C 60 and alkali-metals in monolayer films [7, 8]. In this paper we present HREELS results for single phase multilayer (-, 200A) K1 C60 films, including the x=4 phase not previously studied by T-EELS.

Experimental Samples were prepared and measured in-situ in a multi-chamber UHV system equipped with Raman scattering, low energy electron diffraction (LEED) and photoemission in addition to the HREELS spectrometer. Epitaxial films of C 6 0 were grown by evaporation onto a clean Cu(lll) substrate. Alkali-metal doping of these films was carried out by evaporation from thoroughly de-gassed commercial SAES getter sources. The stoichiometry and phase behavior of the films was monitored with Raman scattering through the frequency of the A_(2) pentagonal pinch mode which is sensitive to the local alkali concentration. A single narrow A,(2) mode was used as the criterion for obtaining a single phase film. LEED confirmed that the films retained their crystallinity upon doping. The x=3 phase exhibited a hexagonal LEED pattern as expected from an fcc(111) surface. At 300K the spots were brighter than those from the undoped C60 film, reflecting freezing out of molecular rotations and the resulting reduction