In Situ Observations of the Growth Mode of Vacuum-Deposited α-Sexithiophene

The real-time morphological evolution of vacuum-deposited a-sexithiophene (a-6T) on a weakly interacting (glass) substrate at ambient temperature is reported. In situ grazing-incidence small-angle X-ray scattering (GISAXS) enabled the observation of nanoscale aggregates, while in situ grazing-incidence wide-angle scattering (GIWAXS) allowed the study of the molecular-scale morphology. The in situ GISAXS measurements revealed that the alpha-6T growth proceeds via a StranskiKrastanov mode, whereby 24 complete monolayers are deposited, followed by subsequent layers formed via island growth. In situ GIWAXS also showed the evolution of the polymorph composition during the thin-film growth. Initially, the disordered beta-phase and the low-temperature (LT)-phase are deposited in nearly equal proportion until a thickness of 8 nm, whereby the LT-phase begins to dominate until a final alpha-6T thickness of 50 nm where the scattering intensity of the LT-phase is more than double that of the beta-phase. The change in the polymorph composition coincided with an increase in the LT-phase d-spacing, indicating a lattice strain relief as the thin film moves from surface to bulk-mediated growth. The GISAXS findings were confirmed through direct imaging using ex situ atomic force microscopy (AFM) at various thicknesses, revealing the existence of both initial the initial and intermediate monolayers and final island morphologies. The findings reveal the real-time morphological evolution of alpha-6T across both the molecular scale and the nanoscale and highlight the role of strain in polymorph growth. Due to the importance of the thin-film microstructure in device performance, it is expected that these results will aid in the development of structureproperty relationships necessary to realize the full potential of organic electronics.