Influence of substrate temperature on epitaxial copper phthalocyanines studied by photoemission spectroscopy

We report the formation of heteroepitaxial copper phthalocyanine (CuPc) overlayers on a 5x20 reconstructed Au(001) substrate, with thicknesses ranging from 0.8 to 26.9 Angstrom. The adsorbed CuPc at room temperature forms a quasiepitaxial overlayer, incommensurate with the substrate lattice, aligned with the <110> and <1 (1) over bar0> axes of the Au(001) surface. At elevated substrate temperatures (ET) between 200 and 250 degreesC, the deposition of CuPc results in a 13.8 Angstrom square overlayer structure commensurate with the substrate lattice. The electronic structures of CuPc overlayers grown at both RT and ET are also investigated. While the films grown at both temperatures generally exhibit lowering of the vacuum level, consistent with the presence of an interfacial dipole, it is the ET-grown CuPc overlayer that exhibits a sharp decrease of the surface vacuum level upon completion of a monolayer. The experimental CuPc valence band energies are compared with the calculated ionization potentials of a CuPc molecule. Using the tunability of the light source and the dependence of the photoionization cross sections on photon energy for various atomic shells, the CuPc molecular orbitals are assigned with either Cu d states or Pc-derived valence orbitals. (C) 2004 American Institute of Physics.