Dynamical evolution study of metal nanofilms on graphite substrates

Abstract

The dynamical evolution process of nanoscaled film on a solid substrate depends on many factors, such as the properties of thin film, the characteristics of the substrate, and the external environment. It is essential to elucidate the influences of these factors for our understanding self-organized growth of nanoparticles and the dewetting/detachment mechanism of nanofilm on a solid substrate. In the present paper, we investigate the dynamical dewetting/detachment of metal Au and Pt nanofilm on a graphene/graphite substrate at high temperature by using the molecular dynamics simulation technique. We discuss the influences of metal-substrate interaction, temperature and thickness of film on the dewetting dynamics. Our results reveal that the Au and Pt nanofilms with the same initial thickness on graphene substrates manifest different dewetting dynamical processes at high temperatures. Some nanoscale holes are formed randomly during the dewetting of Pt nanofilm with a thickness of less than 0.6 nm because of the strong interaction between the Pt films and substrate. In contrast, no hole is observed and a nanodroplet is formed directly by high temperature dewetting for Au nanofilm with the same initial thickness as that of Pt nanofilm. The resulting Au and Pt nanodroplets move in the vertical direction due to the surface tension and the constraint of the solid substrate. A high-temperature nanodroplet will be detached from the graphene substrate surface at a constant speed. Interestingly, the values of detachment velocity (vd) of nanodroplets show different dependences on initial thickness for Au and Pt nanofilm, respectively. In a thickness range of 0.2-2.3 nm, the vd of Pt nanodroplet increases and then decreases as the thickness of nanofilm increases. However, the vd of Au nanodroplet decreases gradually and then increases steeply as the Au nanofilm turns thicker. The different thickness dependences of vd for Au and Pt nanofilms are analyzed qualitatively by considering different metal-substrate viscous dissipations. In addition, the detachment time (td) of a dewetting metal film is also related to the temperature and the thickness of substrate. Our results demonstrate that the td decreases monotonically with the decrease of film thickness and the raise of temperature. These results provide a theoretical guideline for industrial production processes, such as metal coating, flotation, and the surface cleaning.