Diffusion and nucleation in multilayer growth of PTCDI-C₈ studied with in situ X-ray growth oscillations and...

We study nucleation and multilayer growth of the perylene derivative PTCDI-C₈ and find a persistent layer-by-layer growth, transformation of island shapes, and an enhancement of molecular diffusivity in upper monolayers (MLs). These findings result from the evaluation of the ML-dependent island densities, obtained by in situ real-time grazing incidence small angle X-ray scattering measurements and simultaneous X-ray growth oscillations. Complementary ex situ atomic force microscopy snapshots of different growth stages agree quantitatively with both X-ray techniques. The rate and temperature-dependent island density is analyzed using different mean-field nucleation models. Both a diffusion limited aggregation and an attachment limited aggregation model yield in the first two MLs the same critical nucleus size i, similar surface diffusion attempt frequencies in the 10¹⁹-10²⁰ s^-1 range, and a decrease of the diffusion barrier E_d in the 2nd ML by 140 meV.