Enhanced Infrared Absorption of C60 on Thin Evaporated Pd Island Films
Infrared transmission spectra of C 60 * multilayers on thin Pd films deposited onto surface-oxidized Si(100) and hydrogen-terminated Si(111) substrates are reported. In both cases, the spectra in the 1500-1100 cm-' region exhibited bands at 1444, 1429, and 1182 cm-' due, respectively, to the A 8 (2), T 1u (4), and T 1u (3) modes. The appearance of the A 8 (2) mode, which is originally infrared inactive (Raman active), reveals electron transfer from the metal to chemisorbed C 60. Indeed, increasing the thickness of C 60, the A g (2) mode intensity saturated more rapidly than the T 1u (4) and T 1u (3) modes. The originally infrared active T 1u (4) and T 1u (3) modes were enhanced in intensity depending upon the Pd thickness. Actually, while both substrates gave nearly the same magnitude of enhancement, the optimum Pd thickness was smaller on the hydrogen-terminated surface than on the surface-oxidized surface. On the other hand, the A g (2) mode, was less intense on the hydrogen-terminated surface than on the oxidized surface, suggestive of a shortage of chemisorbed C 60 and thus pointing out the importance of the metal film morphology. Indeed, Pd films deposited on the two substrates gave rise to quite different AFM images. We also show that, regardless of the substrate, the A g (2) mode is an order of magnitude smaller than for Ag deposition, though no remarkable intensity differences were observed with respect to the T 1u (4) and T 1u (3) modes.
Infrared spectroscopy of fullerenes
This review summarizes the present status of the application of IR spectroscopy to the field of fullerene science. In accord with the amount of published work the report concentrates to a large extent on the C60 molecule in its neutral and charged form in a crystalline environment. After a short introduction to the basic terminologies and properties of fullerenic materials a detailed description of the response of pristine C60 the IR radiation is given. It includes a review on the vibrational fine structure, the isotope effects, and the influence of the structural phase transitions. For the case of the doped material the response of the free carriers and their interaction with the vibrational modes as observed in the IR spectra is reviewed, including the case of high-temperature superconductivity. Special attention is paid to the colourful behaviour of the phases AC60 and to the air stability of this compounds. In the final part of the review selected examples are given for the application of IR spectroscopy to higher fullerenes and to fullerene-derived materials.
Characterization of superconducting alkali and alkaline-earth fullerides prepared by thermal decomposition of azides
An alkali-metal doping process through the thermal decomposition of alkali azides was applied to the preparation of alkali-metal binary-alloys doped C60. Usually we achieved a substantial improvement in the superconducting volume fraction compared to materials doped by the standard gas-phase reaction. The decomposition of alkali azides other than sodium azide takes place in the liquid phase or melt, which is presumably responsible for a more uniform doping and improvement of the superconducting properties. For pure cesium and pure sodium, however, no indication of superconductivity has been detected so far. A systematic study on the superconducting properties of C60 doped with alkali-metal binary-alloys including Cs (Cs-Na, Cs-K, Cs-Rb) is described and compared with previous results. In addition, a similar C60 doping process via the thermal decomposition of alkaline-earth azides was found to give superconductivity for the nominal compositions Ba60C60 and Sr6C60.