Formation, evolution, and degradation of nanostructured covalent thin films deposited by low-energy cluster beam deposit
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ow-energy cluster beam deposition (LECBD) is considered an intriguing technique for obtaining thin layers with well-defined structures at the nano- and mesoscale levels, allowing novel optical, electronic, and magnetic properties. The produced layers are highly porous and extremely reactive due to the high surface to volume ratio and must be characterized with in situ techniques to study their original composition and their evolution once exposed to reactive gases. In this work, we present a general overview and some results on the formation, evolution, and deposition of silicon and carbon cluster beams produced using a laser vaporization source.
I. INTRODUCTION
The study of small atomic clusters is considered one of the most important and active areas in modern materials science for several reasons. Indeed, atomic clusters are applied in catalysis as well as in nanoelectronics and many other fields.1–3 In this frame, the production of supersonic cluster beams by using laser ablation sources has been frequently used since the works by Smalley et al. in which they suggested the possibility of obtaining cluster beams even from the most refractory materials.4 The method has also led to the discovery of fullerenes and can be applied to a large number of atomic species,5,6 in order to obtain several novel materials. Moreover ultra-cold collisionless supersonic beams are used to study highly reactive species in many aspects. When seeded supersonic beams are deposited to form a socalled “cluster assembled layer,” the process is generally termed low-energy cluster beam deposition (LECBD) because the films are built by the deposition of preformed, size-controlled clusters at low kinetic energy (0.1–1 eV/atoms). In this case, clusters reach the substrate without consistent fragmentation and retain their original structure, even in the solid state. The obtained materials are highly porous with densities as low as about one half of the corresponding bulk densities. The characteristic nanostructured morphology and the possible memory effect of the original free cluster structures are at
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Address all correspondence to this author. e-mail: [email protected] This paper was selected as the Outstanding Meeting Paper for the 2005 MRS Fall Meeting Symposium Q Proceedings, Vol. 887. DOI: 10.1557/JMR.2006.0216
the origin of their specific properties. Many examples of novel materials obtained by LECBD are offered in the literature. For instance, the formation of transition metal cluster assembled films with specific magnetic behavior resulting from the competition between the intrinsic properties of the grains and the interactions between grains7 or the deposition of metal oxides with interesting optical and thermal properties.8 Here we restrict our interest to covalent materials such as carbon and silicon, showing that the formed cluster assembled layers posses features far from those obtained using other deposition techniques.
II. EXPERIMENTAL DETAILS AND CLUSTER BEAM FEATURES
Cluster beams were generated using a laser vapori
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