RASSEGNA SCIENTIFICA Articolo ricevuto in redazione nel mese di Ottobre 2002
INTERACTION OF CARBON NANOTUBES WITH ADSORBATES STUDIED BY HIGH RESOLUTION PHOTOEMISSION SPECTROSCOPY R. Larciprete1,2, A. Goldoni1,3, S. Lizzit1, L. Petaccia1, A. Laurita4 1 Sincrotrone Trieste, S.S. 14, Km.163,5, 34012 Basovizza (TS)- Italy 2 CNR-IMIP, Zona Industriale - 85050 - Tito Scalo (PZ) Italy
3 Laboratorio Nazionale TASC-INFM, , S.S. 14, Km.163,5, 34012 Basovizza (TS)- Italy 4 Centro di Servizi Interdipartimentale di Microscopia, UniversitĂ degli Studi della Basilicata, Potenza - Italy
Abstract We have used high-resolution photoemission spectroscopy to study the interaction of single wall nanotubes with gas phase molecules as O2 and NO2. The first aim of our investigation was to state the purity of commercial nanotubes, in the form of a bucky paper, and to follow the removal of the contaminants (Na, S, Si, Ni) present in the sample by annealing at increasing temperatures. Rapid annealing treatments of a few minutes up to 1800 K determined Ni evaporation and elimination of Na only from the near surface layer, whereas an Na-free clean sample could be obtained only after a prolonged annealing of a couple of hours at 1250 K. We have compared the interaction between the single wall nanotubes and O2 for the Na-contaminated and clean bucky paper. In the first case the adsorption was strongly altered by the Na traces, which simulated an intense sample oxidation leading to a modification of the electronic properties of the nanotubes. On the contrary, for clean single wall nanotubes, the lack of oxygen detection and the inertness of the C1s core level to large O2 doses demonstrated the absence of any chemical interaction between nanotubes and O2, up to pressures of the order of 10-6 mbar and temperatures between 150 and 300 K. Instead, a significant charge transfer was observed for NO2 adsorption. In this case the hole doping induced in the tubes by the adsorbed molecules led to a dramatic change in the C1s line shape and energy position, which corresponds macroscopically to a decrease of the resistance of the bucky paper sample.
its equator and lengthened by introducing further C hexagons. Microscopically a NT consists of one (singlewall nanotube, SWNT) or more (multi-wall nanotube, MWNT) graphene sheets, rolled-up to form coaxial cylinders, separated by approximately the intralayer distance of graphite (0.34 nm). In the latter case, the layers can be as many as fifty. In the rolled-up graphene layers the carbon lattice remains continuous around the circumference and the atoms nearly maintain the sp2 hybridization of graphite (obviously the degree of sp2 hybridization depends on the diameter of the nanotube, i.e. the curvature of the C-C bonds). The constraints imposed by the continuity of the lattice determine the formation of different chiral structure resulting from the helical twist around the tube axis. The only two types of non-helical tubes are arranged with the sides of the hexagonal carbon rings parallel or perpendicular to the axis of the tube, and are called zig-zag or armchair, respectively [1-4]. Depending on their diameter and chirality SWNTs can behave as semiconductors or metals [1,5,6] and from the transport point of view are described as the realization of one dimensional quantum objects. Therefore, SWNTs appear promising candidates for nano-scaled electronic devices or quantum wires. Moreover, the interest for NTs is increased by the evidence that their electronic structure can be easily tuned by distortion, introducing defects and functionalization of the tips and/or sidewalls with foreign atoms. In addition to these unique electronic characteristics, NTs exhibit excellent mechanical properties as they can be buckled without breaking and, on the basis of their very low weight and very high elastic modulus, are predicted to be the strongest fiber which can be fabricated, ideal to reinforce polymer and composites materials. Moreover the NT nanocapillarity properties, resulting from the large specific surface and hollow geometry, make them prime materials for gas and energy storage. The characteristics of adsorbing easily gas phase molecules can be also exploited to use NTs as high sensitive
1. Carbon nanotubes and their properties Carbon nanotubes (NTs) [1-2] are fullerene-related structures, consisting of long, thin, hollow cylinders of carbon, discovered in 1991 by S. Iijima [3,4]. These cylindrical molecules can have diameter as little as 1 nm and a length of many microns and can be visualized as an extended fullerene, where the C60 molecule has been cut at
Vol. 8 n. 1 Febbraio 2003
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NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE
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