Orientation of sp2 carbon nanoclusters in ion-implanted polymethylmethacrylate as revealed by polarized Raman spectroscopy

Raman spectroscopy is widely used for the characterization of bonding type in carbon-based materials, including carbonized surface layer in ion-implanted polymers. Studies of the polarization properties of Raman scattering from amorphous carbonaceous materials, however, are very scarce. In this paper, we investigate the polarized Raman spectra of polymethylmethacrylate (PMMA) implanted with 50-keV Si+ ions at fluences in the range 3.2 x 10(14)-1.0 x 10(17) ions/cm(2) and for different visible excitation wavelengths. The spectra of the implanted samples are dominated by the D- and G-bands of sp(2) carbon, which evidence strong carbonization of the ion-modified layer. The multiwavelength excitation allowed us to resonantly probe the depolarization ratios for sp(2) clusters of different sizes. We established that the depolarization ratio rho(G) of the G-band correlates with the sp(2) cluster size approaching the random orientation limit of 0.75 for the smallest clusters and a limiting value of 0.41 for the largest clusters. The experimental findings give evidence for a preferable orientation of the larger size clusters with their hexagonal planes perpendicular to the surface of the sample. A plausible explanation for such an arrangement is that the sp(2) clusters form tile-like arrangements along the ion tracks. This finding may give clues for understanding of the strong transconductance of the ion-modified layer, and open prospects for the application of polarized Raman spectroscopy as a characterization tool for surface morphology in ion-implanted materials. Copyright (C) 2010 John Wiley & Sons, Ltd.