Early-stage growth observations of orientation-controlled vacuum-deposited naphthyl end-capped oligothiophenes

Abstract

We report on the real-time structure formation and growth of two thiophene-based organic semiconductors, 5,5′-bis(naphth-2-yl)-2,2′-bi- and 5,5''-bis(naphth-2-yl)-2,2′:5′,2''-terthiophene (NaT2 and NaT3), studied in situ during vacuum deposition by grazing-incidence x-ray diffraction and supported by atomic force microscopy and photoabsorption spectroscopy measurements on corresponding ex situ samples. On device-relevant silicon dioxide substrates, for both molecules the growth is observed to transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) growth after the formation of a few-molecule-thick wetting layer. The crystal structure of the NaT2 film is considerably more ordered than the NaT3 counterpart, and there is a significant collective change in the unit cell during the initial stage of growth, indicating strain relief from substrate induced strain as the growth transitions from two to three dimensions. In addition, the orientation of the film molecules are controlled by employing substrates of horizontally and vertically oriented few-layer molybdenum disulfide. Both molecules form needle-like crystals on horizontally oriented MoS$_2$ layers, while the NaT3 molecules form tall, isolated islands on vertically oriented MoS$_2$ layers. The molecules are standing on silicon dioxide and on vertically oriented MoS$_2$, but lying flat on horizontally oriented MoS$_2$. These results demonstrate the importance of film-substrate interactions on the thin-film growth and microstructure formation in naphthyl-terminated oligothiophenes