Structural study of thin copper phthalocyanine (CuPc) films on Au(110).

We have performed Grazing Incidence X-Ray Di_raction (GIXRD), Near Edge Xray Absorption Fine Structure spectroscopy (NEXAFS), Helium Atom Scattering (HAS) and Electron Di_raction (LEED) measurements in order to investigate the structural properties of a very thin CuPc film on Au(110). All the data have been taken at the ALOISA beamline and its branchline HASPES at ELETTRA Synchrotron (Italy). Among the organic molecules of technological interest for their optical properties, CuPc is ideally suited to UHV studies for its thermal and chemical stability. Moreover, the strong structural anisotropy of a substrate such as the missing row reconstructed (1x2)-Au(110) can drive the orientation of a growing organic overlayer[1,2]. We have observed in real time the early stages of deposition by using the HAS and LEED techniques. At a supposed coverage of 1 ML, the LEED spectra reveal a (5x3) phase, as recently reported [2]; less intense (5x7) phase features are also present. By monitoring the system at the same deposition stage with HAS, only a 5x7 pattern is measured instead. HAS is a more surface sensitive technique than electron di_raction; a model is proposed where the CuPc layer presents a x7 periodicity along the [001] direction whereas CuPc molecules locally induce a x3 reconstruction of the Au(110) substrate, whose high Z atoms cores have a larger scattering yield for electrons than low Z CuPc atoms. The x 3 substrate reconstruction has been confirmed and characterized in detail by means of GIXRD. The rodscans of six Au di_raction peaks have been taken(292 reflection measured). From the fit, the x3 Au cell model and the atom displacements in the plane perpendicular to the close-packed Au atom rows direction have been obtained. The rod simulations suggest an asymmetrical distortion of the cell structure; two di_erent domains with an opposite orientation of the cell distortion are present. From NEXAFS measurements the molecules result to be tilted by 30 degrees along the [001] direction with respect to the surface, in agreement with the two-domains, asymmetric cell found by XRD.[1] S. Prato et al., J. Phys. Chem. 103, (1999), 7788 [2] F. Evangelista, et al., Surf. Sci. 531, (2003), 123 130