Ordered ultrathin films of perylenetetracarboxylic dianhydride (PTCDA) and dimethylperylenebis(dicarboximide) (Me-PTCDI) on Cu(100): Characterization of structure and surface stoichiometry by LEED, TDMS, and XPS

Ordered monolayer and multilayer thin films of perylene-3,4:9,10-tetracarboxylic dianhydride (PTCDA) have been grown on Cu(100) and MoS₂(0001) single crystals. Low-energy electron diffraction (LEED) investigations have shown that PTCDA grows on Cu(100) in a commensurate rectangular lattice (4√2×5√2)R = 45°, whose dimensions (b₁ = 14.5 Å, b₂ = 18.1 Å) are significant different from those seen on the (0001) cleavage faces of single-crystal MoS₂, where a coincident rectangular lattice with b₁ = 13.1 Å and b₂ = 21.2 Å is formed. This latter structure is much closer to the packing in the (102) plane of the bulk crystal of PTCDA. X-ray photoelectron spectra (XPS) and thermal desorption mass spectroscopy (TDMS) show that the first monolayer of PTCDA is strongly chemisorbed on the Cu surface. During the reaction with the copper surface each PTCDA molecule apparently loses the two bridging oxygen atoms in the anhydride groups, leading to a molecular system with close to the same dimensions and symmetry as the parent molecule. XPS data show that the adsorption on the surface of the weakly interacting metal dichalcogenide MoS₂ is not accompanied by such a surface reaction. Support for this hypothesis was obtained by deposition of N,N′-dimethylperylene-3,4:9,10-bis(dicarboximide) (Me-PTCDI) on the Cu(100) surface. Me-PTCDI forms a rectangular (6 × 8) structure on Cu(100), with b₁ = 15.3 Å and b₂ = 20.4 Å. XPS data obtained for the deposition of this molecule on clean copper show that Me-PTCDI apparently loses both nitrogen imide groups, for the first monolayer adsorbed. TDMS data confirm this reactivity of Me-PTCDI. Me-PTCDI thin films appear to grow as islands, starting with the second monolayer, in contrast to PTCDA, which appears to adopt a layer-by-layer growth mode for the first six monolayers. Differences in the bulk crystal structures of these two molecules are believed to lead to these differences in growth mode; the layered bulk structure of PTCDA lends itself to a layered growth mode in multilayer ultrathin films.