Identification of ultra-thin molecular layers atop monolayer terraces in sub-monolayer organic films with scanning probe microscopy

•A molecular layer made of face-on and/or edge-on molecules on top of the edge-on organic terraces is identified in sub-monolayer organic films.•Standard and advanced Scanning Probe Microscopy techniques, operating in different environments, have confirmed such additional molecular layer.•In situ and ex situ morphological and height measurements highlight a transition from a standard to non-standard growth mechanism in sub-monolayer films.•Topographic data, recorded in air and vacuum, prove that the water layer always present in air does not affect the height measurement. The morphology of sub-monolayer sexithiophene films has been investigated in situ and ex situ as a function of the substrate temperature of deposition. In this thickness range, monolayer terraces formed of edge-on molecules, i.e. nearly upright, are typically nucleated. Herein, the terrace height is found to be correlated to both the film morphology and the substrate surface energy. In particular, the presence of a layer of variable thickness with molecules lying face-on or side-on can be identified atop the terraces when the deposition is carried out on inert substrates. This phenomenon can be evidenced thanks to accurate height measurements made with atomic force microscopy and further data obtained with advanced scanning probe microscopy techniques operating in different environments, viz. liquid, air and vacuum. An upward displacement of molecules from the substrate to the top of the terraces is considered to be responsible of this layer formation, whose molecules weakly interact with the underlying terraces.