Real-time in situ fluorescence study of α-6T thin film growth on muscovite mica

In this work, the findings of an in situ fluorescence spectroscopy and microscopy investigation of the growth of alpha-sexithiophene (alpha-6T) thin films on muscovite mica are presented. The sensitive dependence of the fluorescence emission on the molecular arrangement is exploited to reveal the details of the alpha-6T thin film growth from the initial stage. A Stranski-Krastanov growth mode with a two-monolayer-thick wetting layer is observed. The molecules within the wetting layer lie flat on the mica surface and their molecular arrangement evolves as a function of the surface coverage. After the completion of the wetting layer, needlelike three-dimensional islands consisting of flat-lying molecules and flat islands containing nearly-upright-standing molecules grow competitively. Exposing the alpha-6T thin films to air leads to dewetting and this process is found to be partially reversible, i.e., a large fraction of the wetting layer recovers gradually after the sample is transferred back into vacuum. This dewetting (rewetting) process is attributed to the surface modification due to the adsorption (desorption) of water molecules.