Structural characterization of ion-vapor deposited hydrogenated amorphous carbon coatings by solid state 13C nuclear magnetic resonance

Abstract

In the present study, unique structural heterogeneity was observed in ion-vapor deposited a-C:H coatings by performing 13C MAS and 1H-13C CPMAS experiments on solid state nuclear magnetic resonance devices. Two distinct types of sp2 C clusters were discovered: one of them denoted as sp2 C′ in content of 3–12 at. % was non-protonated specifically localized in hydrogen-absent regions, while the other dominant one denoted as sp2 C″ was hydrogenated or at least proximate to proton spins. On basis of the notably analogous variation of sp2 C′ content and Raman parameters as function of substrate bias voltage in the whole range of 0.5 kV–3.5 kV, a model of nano-clustering configuration was proposed that the sp2 C′ clusters were embedded between sp2 C″ clusters and amorphous sp3 C matrix as trapped interfaces or boundaries where the sp2 carbon bonds were highly distorted. Continuous increase of bias voltage would promote the nano-clustering and re-ordering of dominant sp2 C″ clusters, thus results in a marked decrease of interspace and a change of the content of sp2 C′ clusters. Further investigation on the 13C magnetization recovery showed typical stretched-exponential approximation due to the prominent presence of paramagnetic centers, and the stretched power α varied within 0.6–0.9 from distinct types of sp2 C clusters. Differently, the magnetization recovery of 1H showed better bi-exponential approximation with long and short T1(H) fluctuated within 40–60 ms and 0.1–0.3 ms approximately in content of 80% ± 5% and 20% ± 5%, respectively, varying with various bias voltages. Meanwhile, the interrupted 13C saturation recovery with an interval of short T1(H) showed that most of quick-relaxing protons were localized in sp2 C″ clusters. Such a short T1(H) was only possibly resulted from a relaxation mechanism associated with electron-nuclear dipolar interaction or spinning diffusion, and its partial distribution in sp2 C″ clusters may imply more complex configurations of unpaired electrons in studied a-C:H coatings.