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.