Atomic force microscopy as a tool for unravelling the relationship between morphology and growth dynamics of organic semiconductors

Organic semiconductors represent an emerging class of materials which recently has received a huge interest on behalf of a worldwide research community, while finding some applications in alternative and low-cost electronic devices. The performance of an organic semiconductor, in terms of, e.g., charge transport, light emission, and second harmonic generation, depends strongly on its crystallinity and purity. Defect formation in crystals is a key event making growth possible under near-equilibrium conditions. In this work, the morphological study performed by atomic force microscopy of the surface of single crystals and crystalline thin films of organic semiconductors grown under different conditions is presented. It is shown how to exploit the morphological features of defects in order to deduce the molecular orientation within the crystalline samples. Finally, comparison of the crystal structure as determined from X-ray diffraction with results obtained on real crystals is shown to allow the discovery of twins, polytypes, and polymorphs, depending on the growth conditions and mechanisms.