Hard and sp2-rich amorphous carbon structure formed by ion beam irradiation of fullerene, a-C and polymeric a-C:H films

In this work, we present a comparative study of the ion irradiation effect on the mechanical and optical properties of fullerene, amorphous carbon (a-C) and polymeric hydrogenated amorphous carbon (a-C:H) films, irradiated with N ions at 400 keV in the fluence range from 10(13) to 3 x 10(16) N cm(-2). Modifications in the carbon structure, as function of the irradiation fluence, were investigated using the Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), Fourier transform infrared (FTIR), Raman spectroscopy, UV-Vis-near infrared (NIR) spectrophotometry and nanoindentation techniques. After high fluence, the three carbon samples are transformed into very similar hard (approximate to 14 GPa) and non-hydrogenated amorphous carbon layers with very low optical gap (approximate to 0.2 eV) and an unusual sp(2)-rich bonded atomic network. The mechanical properties of the irradiated films correlated with the bonding topologies of this new sp(2) carbon phase are investigated through the constraint-counting model. The results show that the structural modifications and the unusual rigidity were achieved by the distortion of the sp(2) carbon bond angles, giving origin to a constrained three-dimensional sp(2) carbon bonded network. (C) 2004 Elsevier B.V. All rights reserved.