Multi-wavelength Raman investigation of sputtered a-C film nanostructure G. Messina, S. Santangelo * INFM, Dipartimento di Meccanica e Materiali, Facolta ` di Ingegneria, Universita ` ‘‘Mediterranea’’, Localita ` Feo di Vito, 89060 Reggio Calabria, Italy Received 23 November 2004; accepted in revised form 8 July 2005 Available online 11 August 2005 Abstract Multi-wavelength Raman spectroscopy is employed to investigate the nanostructure of amorphous carbon (a-C) films, prepared by sputtering at 20 and 400 -C, and the structural modifications produced by thermal anneling at 400 and 800 -C. The results are discussed in the light of more recent assessments on resonant Raman spectroscopy in C-based materials. High-temperature depositions and thermal annealing promote development and/or clustering of sp 2 phase, with film optical transparency reduction. In both the cases, nanoclusters of larger and more uniform dimensions are formed at higher temperatures. However, annealing process favours aromaticity, while high-temperature depositions oppositely augments distortions and promotes bond disorder. D 2005 Elsevier B.V. All rights reserved. PACS: 81.15.Cd; 78.30.Ly; 63.22.+m Keywords: Carbon; Amorphous; Sputtering; Nanostructure; Raman scattering spectroscopy; Annealing 1. Introduction Raman spectroscopy is widely employed as a fast, non- destructive tool for the post-growth material diagnostics. The analysis, aimed at the investigation of the film structural and bonding properties, is commonly pursued using 514.5 nm excitation wavelength. In disordered C-based materials, the interest is usually centred on the bands arising from the vibrational modes of the C sp 2 atoms, the well-known D- and G-bands. For visible excitation, the former, due to the A 1g breathing modes of ring-organised C-atoms, lies approximately at 1360 cm À 1 , while the latter, due to the bond stretching of all the pairs of (both ring- and chain- organised) C-atoms, is detected approximately at 1570 cm À 1 . However, owing to the resonant nature of Raman scattering from k-bonded C-atoms [1,2], there is a growing interest on multi-wavelength Raman spectroscopy (MWRS). In the last years, MWRS has been used to study the properties of both nanocrystalline and amorphous C- phases [1–6]. Alike in traditional analysis at 514.5 nm, in the most of MWRS studies attention is often focussed exclusively on the D- and G-bands. Moreover, the analysis is generally limited to deduce essential information, so as, except for a few of cases [6], only some of the indicators attainable by spectra decomposition are really considered [7–10]. In this paper, MWRS is used to study the effect of deposition and annealing temperatures on the structural properties of sputter-grown a-C films, topic whose inves- tigation, in the past years, has been faced prevailingly by single-wavelength (488 or 514.5 nm) Raman spectroscopy [11–20]. In this work, the preliminary results are presented relative to 20 and 400 -C deposition and to 400 and 800 -C annealing temperatures. Although interest is mainly focused onto the region dominated by D- and G-bands, in order to obtain complementary information about the film structure, a spectral range (from 200 to 3600 cm À 1 ) wider than that usually considered [2,13–17] is here investigated. By decomposing the spectra measured at 457.9, 514.5 and 633 nm excitation wavelengths, a large set of descriptive parameters is derived and entirely analysed. From the discussion of these results in the light of current assessments on resonant Raman spectroscopy in C-based materials, an extremely accurate picture of the film nanostructure changes is achieved. 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.07.004 * Corresponding author. Tel.: +39 0965 875305; fax: +39 0965 875201. E-mail address: santange@ing.unirc.it (S. Santangelo). Surface & Coatings Technology 200 (2006) 5427 – 5434 www.elsevier.com/locate/surfcoat