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We present 10 new Spitzer detections of fullerenes in Magellanic Cloud Planetary Nebulae, including the first extragalactic detections of the C 70 molecule. These new fullerene detections together with the most recent laboratory data permit us to report an accurate determination of the C 60 and C 70 abundances in space. Also, we report evidence for the possible detection of planar C 24 in some of our fullerene sources, as indicated by the detection of very unusual emission features coincident with the strongest transitions of this molecule at ∼6.6, 9.8, and 20 μm. The infrared spectra display a complex mix of aliphatic and aromatic species such as hydrogenated amorphous carbon grains (HACs), polycyclic aromatic hydrocarbon clusters, fullerenes, and small dehydrogenated carbon clusters (possible planar C 24 ). The coexistence of such a variety of molecular species supports the idea that fullerenes are formed from the decomposition of HACs. We propose that fullerenes are formed from the destruction of HACs, possibly as a consequence of shocks driven by the fast stellar winds, which can sometimes be very strong in transition sources and young planetary nebulae (PNe). This is supported by the fact that many of our fullerene-detected PNe show altered [Ne iii]/[Ne ii] ratios suggestive of shocks as well as P-Cygni profiles in their UV lines indicative of recently enhanced mass loss.

One-dimensional carbon atomic wires displaying sp hybridization have an appealing electronic and vibrational structure which profoundly affects their optical and transport properties. Here we investigated charge transfer in alternating triple–single bond carbon atomic wires (polyynes) terminated by phenyl rings and its effects on the structure of the system. The occurrence of a charge transfer between carbon wires and metal nanoparticles (both in liquids and supported on surfaces) is evidenced by Raman and surface enhanced Raman scattering (SERS) as a softening of the vibrational stretching modes. This is interpreted, with the support of density functional theory (DFT) calculations of the Raman modes, as a modification of the bond length alternation of carbon atoms in the wire. As a consequence of the charge transfer, carbon wires rearrange their structure toward a more equalized geometry which corresponds to a tendency toward a cumulenic structure (i.e., all double bonds). These observations open potential perspectives for developing carbon-based atomic devices with tunable electronic properties.

We present a study of sixteen Planetary Nebulae (PNe) where fullerenes have been detected in their Spitzer Space Telescope spectra. This large sample of objects offers an unique opportunity to test conditions of fullerene formation and survival under different metallicity environments as we are analyzing five sources in our own Galaxy, four in the Large Magellanic Cloud, and seven in the Small Magellanic Cloud. Among the sixteen PNe under study, we present the first detection of C 60 (possibly also C 70 ) fullerenes in the PN M 1-60 as well as of the unusual ∼6.6, 9.8, and 20 µm features (attributed to possible planar C 24 ) in the PN K 3-54. Although selection effects in the original samples of PNe observed with Spitzer may play a potentially significant role in the statistics, we find that the detection rate of fullerenes in C-rich PNe increases with decreasing metallicity (∼5% in the Galaxy, ∼20% in the LMC, and ∼44 % in the SMC) and