Application of dynamic scaling to the surface properties of organic thin films: Energetic materials

The surface evolution of pentaerythritol tetranitrate (PETN) organic films, formed by room-temperature thermal evaporation on 16-mercaptohexadecanoic acid self-assembly monolayer, has been investigated using atomic force microscopy (AFM). Results are analyzed using dynamic scaling theory. High values are obtained for dynamic scaling coefficients β = 0.56 ± 0.02 and 1/z = 0.35 ± 0.01, where β is the growth exponent and 1/z is the dynamic exponent. Using mound height and in-plane correlation length the dependence of mound slope m on PETN thickness is obtained. Two thickness regimes are identified. For thickness between 40 and 300 nm, m grows rapidly due to high bulk surface diffusion with smooth PETN morphology. For thickness from 300 to 800 nm, m continues to grow but at a reduced rate. Facets are evident and crevices form in this thickness range. The surface evolution behavior is attributed to bulk surface diffusion and the presence of an Ehrlich-Schwoebel barrier, which preferentially enhances step-up diffusion.