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

In the present study, unique structural heterogeneity was observed in ion-vapor deposited a-C:H coatings by performing {sup 13}C MAS and {sup 1}H-{sup 13}C CPMAS experiments on solid state nuclear magnetic resonance devices. Two distinct types of sp{sup 2} C clusters were discovered: one of them denoted as sp{sup 2} C′ in content of 3–12 at. % was non-protonated specifically localized in hydrogen-absent regions, while the other dominant one denoted as sp{sup 2} C″ was hydrogenated or at least proximate to proton spins. On basis of the notably analogous variation of sp{sup 2} 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 sp{sup 2} C′ clusters were embedded between sp{sup 2} C″ clusters and amorphous sp{sup 3} C matrix as trapped interfaces or boundaries where the sp{sup 2} carbon bonds were highly distorted. Continuous increase of bias voltage would promote the nano-clustering and re-ordering of dominant sp{sup 2} C″ clusters, thus results in a marked decrease of interspace and a change of the content of sp{sup 2} C′ clusters. Further investigation on the {sup 13}C 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 sp{sup 2} C clusters. Differently, the magnetization recovery of {sup 1}H showed better bi-exponential approximation with long and short T{sub 1}(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 {sup 13}C saturation recovery with an interval of short T{sub 1}(H) showed that most of quick-relaxing protons were localized in sp{sup 2} C″ clusters. Such a short T{sub 1}(H) was only possibly resulted from a relaxation mechanism associated with electron-nuclear dipolar interaction or spinning diffusion, and its partial distribution in sp{sup 2} C″ clusters may imply more complex configurations of unpaired electrons in studied a-C:H coatings.