Nucleation and 3D growth of para-sexiphenyl nanostructures from an oriented 2D liquid layer investigated by photoemission electron microscopy

The deposition in an ultrahigh vacuum of prototypical linear para-sexiphenyl (6P) molecules onto the anisotropic reconstructed surface of Cu(110) 2 x 1-O presents an ideal system with reduced symmetry for investigation. A dynamic photoemission electron microscopy (PEEM) study of the nucleation and growth of 6P, combined with data obtained from static techniques, is shown to facilitate our understanding of the requirements for 6P nuclei formation and self-assembly into long anisotropic needles. High-rate image acquisitions in PEEM are shown to reveal dynamic phenomena, such as meta-stable layer de-wetting and nanostructure growth in real time, that are the result of nucleation and self-assembly processes. Furthermore, time dependent studies of the relaxation of the meta-stable layer give insights into the molecular diffusion kinetics, whereas temperature dependent studies allow nucleation energies and molecular binding energies to be quantitatively measured. The deposition of the first monolayer of material is found to assemble without the formation of islands until full coverage (1 ML) is achieved. The second layer fills homogeneously and remains in a liquid smectic phase until a total deposition of 1.95 +/- 0.07 ML is reached, whereupon critical nuclei of 6P crystallize out of the 2D liquid layer. The maximum of the diffusion coefficient is estimated to be 2 x 10(-9) cm(2) s(-1). The resulting de-wetting of the meta-stable second layer rapidly increases the size of the nuclei while maintaining the anisotropic needle nanostructure shape. Probing the de-wetting layer reveals that 6P diffusion is 1D up to 100 degrees C. The nucleation energy and intermolecular binding energy are measured to be 675 meV and 2.1 eV, respectively.