SYNTHESIS OF CRYSTALLINE, NANOMETER-SCALE, -(CH2)(X)- CLUSTERS AND FILMS ON GOLD SURFACES

Stable, nanometer-scale thickness films of -(CH2)(x)(-) have been obse rved to form by the surface-catalyzed decomposition of CH2N2 on evapor ated Au film substrates. In the early stages, growth occurs in the for m of isolated clusters at defect regions in the {111} textured surface s. As the average thickness increases beyond similar to 20 nm, growth spills out onto the {111} terraces with eventual coverage of the entir e surface. At all coverages, the dominant structure is highly trans, e xtended polymethylene chains packed in an orthorhombic lattice, simila r to the typical structure of crystalline, bulk-phase polyethylene but containing more conformational defects than well-formed bulk crystals . Chain melting occurs at similar to 135 degrees C, and cooling to roo m temperature results in differing extents of ordering as a function o f the total film coverage, an indication that the structures of the gr owing films are constrained in metastable forms by the presence of adj acent gold surface defect features. The polymerization mechanism appea rs to involve surface-catalyzed decomposition of the diazomethane at g old defect sites to produce methylidene adsorbate species, which subse quently initiate the formation of linear polymers via a free radical p ropagation process. This process provides a useful limiting case of th e surface-catalyzed formation of linear hydrocarbons from C-1 intermed iates on transition metals.