Interface energetics in zinc phthalocyanine growth on Ag(100)

Abstract

The nucleation and growth of zinc phthalocyanine (ZnPc) thin films on a Ag(100) surface are studied employing in situ, real-time low-energy electron microscopy and complementary density functional theory (DFT) calculation to elucidate the role of incorporation kinetics of planar molecules in phase selection during nucleation and apply this knowledge to the fabrication of highly crystalline ZnPc films. We show that the nucleation of crystalline ZnPc islands requires a large concentration of diffusing molecules. The required amount of nominal deposition to initiate the growth of monolayer (ML) high two-dimensional crystalline islands is dependent on both growth temperature and crystalline phase. At room temperature (RT) and slightly above (RT to \ensuremath{\sim}430 K), ZnPc crystalline islands have double-domain $R33.69$ structures with average domain sizes in the submicrometer range. At higher temperatures, a 5 \ifmmode\times\else\texttimes\fi{} 5 commensurate ZnPc structure nucleates. DFT calculations reveal significant differences in interfacial energies of an isolated ZnPc molecule on a substrate, depending on an adsorption site and azimuthal orientation of the molecule relative to the substrate atomic lattice. The observed delay in the onset of the nucleation of an island is caused by the existence of a large energy barrier for molecule incorporation into an island. At certain growth conditions it is possible to induce a structural transition from the 5 \ifmmode\times\else\texttimes\fi{} 5 to the $R33.69$ phase when the nominal coverage reaches 1 ML. The resulting film has excellent crystallinity with individual domains of hundreds of micrometers in size.