Evaluation of surface diffusion in the early stages of organic thin-film growth using artificial molecular sinks

We propose a method for evaluating surface diffusion in the early stages of organic thin-film growth by restricting the surface diffusion area of molecules. When sinks such as step edges are present on the surface, diffusing molecules are captured there, and consequently, the mobile molecular density decreases near the sink. Utilizing this phenomenon, we evaluated the molecular density distribution of pentacene sub-monolayer islands deposited on cylindrical mesa structures, where the mesa edge acts as a sink. By analyzing this distribution based on a mass balance model incorporating two-dimensional surface diffusion, the surface diffusion coefficient was estimated to be 1.3×10⁻¹⁶ m²/s at a substrate temperature of 40 °C and a deposition rate of 0.5 nm/min. This method provides an effective approach for evaluating surface diffusion during organic thin film growth, where methods developed for inorganic thin film growth are difficult to apply.