Structure and electronic properties of Zn-tetra-phenyl-porphyrins single- and multi-layers films grown on Fe(001)-p(1×1)O
Calloni, G. Bussetti, M. Celeri, R. Yivlialin, F. Bottegoni, L. Duò and F. Ciccacci
Department of Physics, Politecnico di Milano, Milano, piazza Leonardo da Vinci 32 I-20133, Italy
Porphyrins have attracted the interest of researchers in different fields: chemistry, biology, physics, material science and technology. This is due to the enormous variations of the molecule reactivity obtained by simply changing the peripheral radical groups and/or the inner metal atom. Thin porphyrin films have been grown on metals, semiconductors or organic-compatible substrates in view of possible applications in scalable organic-based devices (sensors, solar cells, etc.). Unfortunately, the molecule-substrate interaction can significantly perturb the reactivity of the porphyrin and the properties of the hypothetical device. Trying to overcome this limit, thin metal-oxide (MO) layers can tangibly reduce the molecule-substrate interaction. In MO an ordered layer of oxygen atoms passivates the buried metal substrate, decoupling the deposited molecules from the metal bulk. Although MO thin films represent an extremely wide class of compounds, we will focus our attention on the well characterized Fe(001)-p(1×1)O system. From a structural point of view, oxygen atoms reside in the (001) surface hollow sites with a fourfold symmetry, slightly above the Fe surface layer, a structural configuration that could help decoupling the porphyrin from the substrate.
We will show results obtained when a prototypical porphyrin molecule [namely, Zn-tetra-phenyl-(meso) porphyrin (ZnTPP)] is deposited on Fe(001)-p(1×1)O. The structure and electronic properties of the samples are studied by low-energy electron diffraction (LEED) and UV-(inverse) photoemission spectroscopy (PES, IPES), respectively. The results are compared with ZnTPP films grown on Si(111)-(7×7), oxygen-free Fe(001) and Au(001)-(5×20) to evaluate the role of the oxide layer. On Fe(001)-p(1×1)O, ZnTPP molecules form a (5×5) reconstruction when the first monolayer is completed, but PES and IPES still show the characteristic HOMO and LUMO states of the molecule. These results suggest that the ultra-thin oxygen layer significantly reduces the molecule-substrate interaction and increases the porphyrin mobility, allowing the molecules to form an ordered film.