Structural properties of self-organized organo-silicon macromolecular films investigated by scanning tunneling microscopy and X-ray diffraction

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (ST S) have been applied to investigate thin films of a model silicon-containin g macromolecule, silicon phthalocyanine dichloride (SiPcCl2), deposited fro m solution on to the H-passivated Si(111) surface. In a high coverage film, clusters of about 5.5 nm x 4.5 nm in size are observed. These clusters are believed to partially stack on each other, suggesting a tilted growth mech anism of the layers on the surface. In a low coverage film, well-ordered mo nolayers are found, with a close structural similarity to the bulk crystal structure of SiPcCl2. In this work, the bulk crystal structure of SiPcCl2 w as determined directly from powder X-ray diffraction data using the Monte C arlo method. The results of STM manipulation and the observation of well-or dered SiPcCl2 layers on the Si(111) surface indicate a weaker interaction b etween the SiPcCl2 molecules and the substrate than between the SiPcCl2 mol ecules. STS measurements show rectifying behavior which is attributed to th e H-passivated silicon surface.