▎ 摘　　要

The fabrication of nanocrystalline films is usually accompanied by undesired surface roughening, which limits their applications in tribology, optics and electronics. Here, we report a technique to reduce the average roughness of graphene nanocrystalline carbon film from 18.3 nm to 0.66 nm by similar to 5 at.% Ti-doping. The surface of the Ti-doped graphene nanocrystalline carbon film stay ultrasmooth from few nanometer thick up to 1 mm thick. Based on the nanostructure characterization, the ultrasmooth mechanism is interpreted that Ti-doping leads to the formation of Ti-C bond, and changes the growth of graphene nanocrystallite from preferred vertical-aligned orientation to random orientation. The ultrasmooth mechanism is general, as shown by similar effects with low concentration doping of Si, Al, Cr, Zr and W. This approach paves a way for fabricating ultrasmooth nanocrystalline carbon films without a requirement of ion-impact-induced downhill current, and significantly improving the hardness of the graphene nanocrystalline carbon film. (C) 2019 Elsevier Ltd. All rights reserved.