Studies of Alloy Nanoclusters and Their Influence on Growth of Carbon Nanotubes (2012)
AuthorsBelic, Domagojshow all
In this work we examine Ag-Au and Ni-Cu nanoclusters: their structural,compositional, and morphological characteristics are investigated in detail. The clusters are produced by the inert gas aggregation (IGA) method from magnetron sputtered alloy targets, in an UHV compatible system. The design of the system is optimized for production and deposition of the clusters with size in the range 5 nm < D < 10 nm. In order to increase the flux of sub-5 nm clusters in the system, we conducted modeling and experimental studies of cluster motion: the simulations showed that skimmers with wider internal angles might significantly improve the flux of smaller nanoclusters; however, the experimental study revealed a major influence of the background gas on scattering of such nanoclusters which consequently led to the loss of their flux. A comprehensive study of Ag0:85Au0:15 nanoclusters was conducted over a period of more than 2 years. Nanoclusters with sizes in the range 3 nm < D < 10 nm were deposited onto a-C films at various surface coverages and systematically investigated by transmission electron microscopy. We found that Ag-Au nanoclusters initially exhibited icosahedral and decahedral structural motifs, with a very small fraction of face centered cubic nanoclusters present. This may suggest that the source conditions used in the experiments (primarily Ar flow) left Ag-Au nanoclusters kinetically trapped in structures which correspond to local thermodynamic minima, rather than global energetically favoured atomic configurations. When left exposed to ambient conditions, over time Ag-Au nanoclusters exhibited structural, morphological, and compositional changes: core-shell and Janus nanoclusters were observed in aged samples, as well as fragmentation of bigger particles. We attribute these changes to oxidation of the Ag component and increased diffusion of Ag₂O over the substrates. The final morphology of aged nanocluster-based thin films is governed by a combination of diffusion, Ostwald ripening, and the Plateau-Rayleigh instability. High resolution transmission electron microscopy confirmed the presence of fivefold symmetric structures in Ni-Cu nanoclusters; however, their higher oxidation rate may have influenced the structures from the outset. In addition, when these nanoclusters were exposed to the electron beam, crystalline artifacts (nanochimneys)started to grown on them, with a structure corresponding to the NiO structure. Ni-Cu nanoclusters are subsequently used as catalysts in a pilot study of carbon nanotube synthesis which confirmed that such alloy nanoclusters are catalytically active for single-wall and multi-wall carbon nanotube growth.