Role of desorption in the growth process of molecular organic thin films

Growth studies of ultrahigh vacuum deposited thin films are often carried out ex situ, assuming the total film mass reached at the end of the deposition is preserved in the subsequent stages of film preparation. Many kinetic models commonly adopted to analyze quantitatively the mechanism of growth take into account the role of the deposition rate of molecules on the substrate surface, their diffusion, and their possible desorption. Within this framework, a strong simplification (and approximation) of the model is achieved when considering a regime of complete condensation (i.e., neglecting the possibility of re-evaporation of the deposited molecules, both during the deposition and the postdeposition stages of growth). Here, we demonstrate that, for molecular materials of relatively small organic molecules physisorbed on inert surfaces, this phenomenon may strongly affect not only the surface dynamics during deposition but also the postdeposition stage of thin film preparation. Some examples showing clearly its effects on the surface of single crystals and the thin film phase are reported and discussed. Finally, a quantitative description of desorption is provided by comparing the prediction of thermodynamics for the quaterthiophene/silica system with the experimental observation of the growth dynamics of the film and the results of approximate kinetic models. The thermodynamic model employs the surface free energies of a quaterthiophene crystal, which are evaluated by molecular simulation using a newly developed force field.