A Model to Interpret the Raman Spectra of Disordered, Amorphous and Nanostructured Carbons
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A MODEL TO INTERPRET THE RAMAN SPECTRA OF DISORDERED, AMORPHOUS AND NANOSTRUCTURED CARBONS Andrea Carlo Ferrari Department of Engineering, University of Cambridge, Cambridge, CB2 1PZ, UK ABSTRACT Raman spectroscopy is a very popular, non-destructive tool for the structural characterisation of carbons. Raman scattering from carbons is always a resonant process, in which those configurations whose band gaps match the excitation energy are preferentially excited. The Raman spectra of carbons do not follow the vibration density of states, but consist of three basic features, the G and D peaks around 1600 and 1350 cm-1 and an extra T peak, for UV excitation, at ~980-1060 cm-1. The Raman spectra at any wavelength depend on 1) clustering of the sp2 phase, 2) bond length and bond angle disorder, 3) presence of sp2 rings or chains, and 4) the sp2/sp3 ratio. It will be shown how the basic features of the Raman spectra vary by rationalising them within a three-stage model of order of carbons. It is shown how the three-stage model can account for the vast range of experimental data available for Raman experiments at any excitation wavelength. This model can also account for apparently contradictory trends reported in literature, since the clustering of the sp2 phase and the sp3 to sp2 conversion are separately treated. INTRODUCTION Carbon is unique in the way that simple changes in its local bonding can give rise materials as diverse as diamond, graphite, fullerenes, carbon nanotubes, and disordered, nano-structured and amorphous carbons. These materials have a remarkable range of mechanical, electronic and electrochemical properties and many possible applications [1,2]. It is thus very useful to develop fast, reliable, non-destructive techniques to probe the key parameters which control their physical behaviour. We are particularly interested in amorphous carbons. We define diamond-like carbon (DLC) as an amorphous carbon (a-C) or an hydrogenated amorphous carbon (a-C:H) with a significant fraction of sp3 bonds. A-C:H often has a rather small C-C sp3 content. DLC’s with highest sp3 content (80-90%) are called tetrahedral amorphous carbon (ta-C) and its hydrogenated analogue ta-C:H. The key parameters of interest in such materials are: 1) the sp3 content; 2) the clustering of the sp2 phase; 3) the orientation or anisotropy of the sp2 phase; 4) any cross sectional structure; 5) the H content. The sp3 content alone mainly controls the elastic constants, but films with the same sp3 and H content but different sp2 clustering, sp2 orientation or cross-sectional nano-structure can have different optical, electronic and mechanical properties. Raman spectroscopy is a popular, non-destructive tool for structural characterisation of carbons [3-12]. It is traditionally carried out at the commonly available wavelengths in the blue-green spectral region (488-514.5 nm), but multi-wavelength Raman (MW Raman) studies are becoming increasingly used. Indeed, Raman scattering from carbons is always a resonant process, in which configurations whos
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