Structure of copper- and H2-phthalocyanine thin films on MoS2 studied by angle-resolved ultraviolet photoelectron spectroscopy and low energy electron diffraction

Angle-resolved ultraviolet photoelectron spectra (ARUPS) of copper phthalocyanine (CuPc) and metal-free phthalocyanine $({\mathrm{H}}_{\mathrm{2}}\mathrm{Pc})$ films (thickness from monolayer to 50–80 Å) on cleaved ${\mathrm{MoS}}_2$ substrates were measured using monochromatic synchrotron radiation. Observed take-off angle (θ) and azimuthal angle (φ) dependencies of the top π band intensity were analyzed quantitatively by the single-scattering approximation theory combined with molecular orbital calculations. The analysis indicated that the molecules lie flat on the ${\mathrm{MoS}}_2$ surface in monolayer films of CuPc and ${\mathrm{H}}_{\mathrm{2}}\mathrm{Pc}.$ The azimuthal orientation of the molecules (angle between molecular axis and surface crystal axis of ${\mathrm{MoS}}_2\mathrm{),}$ was found to be about −7°, −37°, or −67° for both monolayer films of CuPc and ${\mathrm{H}}_{\mathrm{2}}\mathrm{Pc}.$ In the azimuthal orientation, the analyses indicated that there are only molecules with conterclockwise rotation, although both clockwise and counterclockwise rotations are expected. From the low energy electron diffraction, the two-dimensional lattice structure of the monolayer film was obtained. On the basis of these two kinds of experimental results, the full structure of the monolayer film, the two-dimensional lattice and the molecular orientation at the lattice points, was determined. Furthermore, for the thick films it is found from the analyses of ARUPS that CuPc and ${\mathrm{H}}_{\mathrm{2}}\mathrm{Pc}$ molecules tilt about 10° from the surface plane.