Morphological Transitions in Organic Ultra-Thin Film Growth Imaged by in situ Step-by-Step Atomic Force Microscopy

In situ atomic force microscopy (AFM) allowed us to investigate the evolution atthe early stages of the growth of organic thin films. An ultrahigh-vacuum atomic force microscope,integrated with a Knudsen effusion cell for the sublimation of ?-sexithiophene (6T), continuouslyscans the same region during the deposition of sublimed molecules on native silicon oxide as afunction of the substrate temperature. Noncontact AFM images acquired sequentially providesnapshots of the time evolution of the film morphology that is monitored up to the deposition offive monolayers. At all substrate temperatures, a Stranski-Krastanov growth mode of organic filmsis observed: the first two monolayers grow layer-by-layer (two-dimensional?2D), then filmsevolve into islands (three-dimensional?3D). Despite the apparent similarity, we find an anomalous dynamic scaling characterizedby the abrupt change of the growth exponent ? vs substrate temperature. This novel transition, induced by the substratetemperature, is ascribed to the morphological transition from ziggurat islands to large terraces. The analysis of the evolution of theroot-mean-square (RMS) roughness based on the distributed growth model underlines the role of down-hill mass transport for thegrowth of the first two monolayers, transport that is progressively hindered for the next monolayers.