Thickness-dependent spherulitic growth observed in thin films of the molecular organic radical p-nitrophenyl nitronyl nitroxide

We report on real-time measurements of the crystallization process of highl y ordered thin films of the molecular organic radical p-nitrophenyl nitrony l nitroxide (p-NPNN) grown from the vapor phase on glass substrates. The fi lms exhibit 2D spherulitic morphology with a low density of nucleation cent ers. This low density allows an accurate determination of the time evolutio n of the spherulites radii, which show a non-linear dependence with time. T he non-linearity is more pronounced for small values of the film thickness and is due to the non-negligible contribution of the interfacial energies a t the beginning of the crystallization process. As time evolves the film th ickness increases and the dependence becomes nearly linear (in-plane growth rate almost constant). (C) 2000 Elsevier Science B.V. All rights reserved.