Growth, morphology and molecular orientation of controlled Indigo thin films on silica surfaces

Thin films of the natural semiconductor indigo have been grown on Si/SiO2 substrates by physical vapor deposition under ultra-high vacuum conditions, monitoring the influence of the deposition rate on structure, morphology and thickness. This was done to allow well-defined films to be made in a controlled way, suitable for the manufacture of organic devices and the investigation of charge transport phenomena. The films have been characterized by X-ray diffraction, atomic force microscopy, microRaman spectroscopy and transmission electron microscopy, finding that the growth at the solid interface of polymorph B, the only phase present in the films, is governed by the formation of strong multiple hydrogen-bonds between neighbouring molecules, which play a predominant role in the interface organization of the molecular assemblies.