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Ordered monolayer and multilayer thin films of perylene-3,4:9,1O-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 (4d2x5d2)R = 45", whose dimensions (bl = 14.5 A, b2 = 18.1 A) are significant different from those seen on the (0001) cleavage faces of single-crystal MoS2, where a coincident rectangular lattice with bl = 13.1 A and b2 = 21.2 A 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 (XF'S) 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-3,4:9,1O-bis(dicarboximide) (Me-PTCDI) on the Cu(100) surface. Me-PTCDI forms a rectangular (6 x 8) structure on Cu(lOO), with bl = 15.3 8, and b2 = 20.4 A. 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-FTCDI. Me-PTCDI thin films appear to grow as islands, starting with the second monolayer, in contrast to PTCDA, which appears to adopt a layerby-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.

We review the packing structures for a series of aromatic hydrocarbons, deposited by vacuum deposition methods as ordered monolayers→→multilayers, on a variety of metal, semiconductor and insulator surfaces. New results are presented for the adsorption of monolayers of perylenes, phthalocyanines, coronene, and pentacenes on the Cu(100) surface, along with the implications of these studies for the formation of ordered multilayers of these molecular systems. Aromatic molecules without heteroatoms appear to pack in a flat-lying motif, and exhibit approximately hexagonal close packing, even on a substrate with four-fold symmetry such as Cu(100). In general, aromatic systems whose bulk structures lend themselves to layer-by-layer growth during vacuum deposition appear to be the best candidates for ordered multilayer growth.

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