Investigation of the morphology of the initial growth of the aromatic molecule p-quaterphenyl on NaCl (001)

Investigations of the structural properties of the initial layers of p-quat erphenyl (p-4P) vapor deposited onto NaCl (001) have been carried out using atomic force microscopy (AFM), grazing incidence x-ray diffraction (GIXRD) , and helium atom scattering (HAS). A series of AFM images reveal accumulat ions of p-4P molecules around surface defects. Film thicknesses determined from height analyses of these images are in reasonable agreement with those found using GIXRD. The GIXRD studies indicate that p-4P films vapor-deposi ted at ambient temperature are composed of crystallites oriented with the l ong molecular axis nearly perpendicular to the NaCl (001) surface. For a no minally three-monolayer film, the b axis of the crystallites appears prefer entially oriented along the substrate's [110] azimuth, but with increasing thickness the x-ray features resemble those from two-dimensional (2D) powde rs. The diffraction peaks found in the HAS investigation at similar to 50 K for a thick film grown at similar to 200 K are similarly consistent with a 2D powder. The principal difference from the room temperature GIXRD experi ments on thick films is that the a and b lattice parameters of the monoclin ic unit cell appear doubled in length at similar to 50 K. (C) 2001 American Vacuum Society.