Morphological Transitions in Organic Ultrathin Film Growth Imaged by In Situ Step-by-Step Atomic Force Microscopy

In situ atomic force microscopy (AFM) allowed us to investigate the evolution at the 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 alpha-sexithiophene (6T), continuously scans the same region during the deposition of sublimed molecules on native silicon oxide as a function of the substrate temperature. Noncontact AFM images acquired sequentially provide snapshots of the time evolution of the film morphology that is monitored up to the deposition of five monolayers. At all substrate temperatures, a Stranski-Krastanov growth mode of organic films is observed: the first two monolayers grow layer-by-layer (two-dimensional-2D), then films evolve into islands (three-dimensional-3D). Despite the apparent similarity, we find an anomalous dynamic scaling characterized by the abrupt change of the growth exponent beta vs substrate temperature. This novel transition, induced by the substrate temperature, is ascribed to the morphological transition from ziggurat islands to large terraces. The analysis of the evolution of the root-mean-square (RMS) roughness based on the distributed growth model underlines the role of down-hill mass transport for the growth of the first two monolayers, transport that is progressively hindered for the next monolayers.