Order-Disorder Transition in the Molecular Orientation during Initial Growth of Organic Thin Film

We report on the identification of molecular orientation and its order-disorder transition during the initial growth of 1,3-bis(N-carbazolyl)benzene (mCP) thin films on a highly ordered pyrolytic graphite (HOPG) surface by using photoelectron spectroscopy (PES). Theoretical PES amplitudes using a quantum mechanical calculation that adapts independent atomic center approximation (IAC) were calculated to compare with experimental observations. At low coverage, an equilibrium orientation of isolated adsorbate was estimated. As the coverage increases, the interaction between adsorbates becomes dominant and raises the disorder, which results in changes in the PES shapes as well as the line broadening of each peak.