COPPER PHTHALOCYANINE ON SI(111)-7X7 AND SI(001)-2X1 SURFACES - AN X-RAY PHOTOEMISSION SPECTROSCOPY AND SYNCHROTRON X-RAY-ABSORPTION SPECTROSCOPY STUDY

The chemical bonding and molecular orientation of copper phthalocyanine (CuN8C32H16), deposited on clean single crystal silicon substrates of (111) and (001) orientation, were studied using X-ray photoemission spectroscopy (XPS) of the Cu 2p, C 1s, and N Is core-levels and synchrotron radiation X-ray absorption spectroscopy (XAS) at the Cu L(2,3), edges. In the monolayer range, the strong interaction of the molecule with the Si(111) surface is mostly evidenced by changes in the Cu 2p XPS spectra that indicate a partial reduction (up to 30%) of the Cu(II) atoms to Cu 3d(10). The linear dichroism of the XAS spectra shows that those molecules, whose copper is still Cu(II), lie down on the surface. On the other hand, the bonding of the molecule on Si(001) is weaker, as Cu 2p XPS spectra do not exhibit Cu(II) reduction, while XAS spectroscopy indicates a random orientation of the molecules with respect to the surface. N 1s XPS spectra exhibit, on both surfaces, very intense satellites which present, on Si(111), a sharp dependence on the electron take-off angle. With the growth of thicker films, the XPS line shapes of the molecular solid are recovered, and the average tilt angle of the molecules (around 70 degrees) does not depend on the chosen Si surface. We conclude that the structure of the first deposited layer strongly depends on the specific substrate, but does not affect the successive growth of thick layers.