Characterization of the sp 2 bonds network in a-C:H layers with nuclear magnetic resonance, electron energy loss spectroscopy and electron spin resonance

Abstract

a-C:H layers prepared at different ion energies have been investigated by several methods including 13C nuclear magnetic resonance (NMR), electron energy loss spectroscopy (EELS) and electron spin resonance (ESR). The sp 2 fraction of the samples rose from 27% to about 60 at.% with increasing ion energies from 30 eV to 170 eV. In the EELS spectra of these layers the intensity of the π → π ∗ transition between 4 and 7 eV showed no significant variation. But a shift of the peak is observed from 7 eV to lower energy losses with increasing ion energies indicating an enhanced formation of larger sp 2 cluster sizes. This shift is accompanied by a broadening of the energy loss peak, suggesting a broadening of the cluster size distribution. The ESR spectra showed an increase of the spin density by more than one order of magnitude with increasing ion energies. Simultaneously the linewidth of the ESR signal gets narrower. This can also be interpreted as an increasing cluster size from single benzene rings to three and four fused six-fold rings. Hence, the EELS and ESR spectra lead to the same conclusions with respect to the microstructure of the a-C:H network.