Raman Spectroscopy for Carbon Based Amorphous Thin Films
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Raman Spectroscopy for Carbon Based Amorphous Thin Films Giuseppe Compagnini Dipartimento di Scienze Chimiche, University of Catania, Viale A.Doria 6 Catania 95125, Italy
ABSTRACT Vibrational spectroscopies and in particular Raman spectroscopy are known to be among the most useful tools to characterize and control the properties of carbon materials. This is because of the possibility to detect and study the carbon bonding state and because of the strong correlation between shape and width of the vibrational signals and the structure of the amorphous network. The aim of this work is to present some experiments in which the formation and evolution of carbon films obtained by energetic particle irradiation of solid targets or by deposition of carbonaceous species onto suitable surfaces, are controlled and studied by these methods. Particular attention will be given to pure amorphous carbon films and carbon based binary alloys.
INTRODUCTION The vibrational properties of carbon based amorphous thin films have always been considered crowded with information for two main reasons. First of all, the position and relative intensities of the bands observed in Raman scattering or infrared absorption measurements are indicative about the bonding state of the carbon atoms and allow the study of the materials in which heteroatomic bonds are included as constituents (carbon alloys, hydrogenated and nitrated systems). Secondly, the shape and width of the bands are directly connected with structural relaxation phenomena or with the extension of the nanocrystalline domains which are typically found in the amorphous microscopic structure. If these considerations are improved with the nondestructive nature of the techniques and with their accessibilities and low cost for many laboratories, it is easy to understand why they are among the most used for carbon materials. In the case of pure amorphous carbons, the Raman technique has a dominant position[1]. This is because it is more suitable in the detection and characterization of homonuclear C-C bonds, even though the relaxation of the selection rules typical of the amorphous state, frequently give the possibility to observe also other kinds of vibrations. The fact that carbon atoms are able to form linear (sp hybridized valence orbitals), trigonal (sp2 hybridized valence orbitals), and tetrahedral (sp3 hybridized valence orbitals) bonding
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arrangement, is the reason why carbon is so flexible. First of all it is the key element in life chemistry giving rise to an enormous variety of molecules with complex structures and interactions. On the other hand this flexibility is able to create a number of different crystalline forms (diamond and graphite structures), several stable nanostructured entities (fullerenes and nanotubes) and a high variety of amorphous systems with properties ranging from soft, opaque and conductive graphite-like ones to hard, resistive and transparent diamond-like carbons (DLCs). The advent of nano-science and nanotechnology era has strongly push
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