Nanostructured materials show macroscopic properties of exceptional interest due to their unique electronic structures and particular architectures. These nanostructures find important applications in nanoelectronics, optical systems, bionanotechnology, sensing fields etc.. Therefore, scientists recently devoted their efforts to the synthesis, characterization and properties of nanosized molecular and supramolecular systems. In this field, one of the most useful and complete technique to perform advanced characterization of nanomaterials and nanostructures is the X-Ray Photoelectron Spectroscopy (XPS). It is a surface technique suitable to probe thicknesses of the same order of the photoelectron inelastic mean free path, (a few tens of Å). This technique allows to immediately obtaining qualitative and quantitative analyses as well as oxidation states. If, in addition, the XPS instrument allows a high stability, precise and reproducible adjustment of the emission angle of the sample stage, one can afford reliable angle resolved measurements (AR-XPS) that are important to obtain film thickness and surface coverage for flat samples. For these reasons, XPS revealed to be a unique technique for an in-depth chemical and structural characterization of nanostructures. In this general field we report on some recent important applications of XPS in the study of self-assembled materials.As an example we show some interesting results obtained for a dirhodium(II,II) molecular complex, a flexible compound that shows redox activity connected to the reversible one-electron oxidation dirhodium(II,II) dirhodium(II,III) process that may be controlled by varying to some extent, the bridging ligands around the dirhodium core. Moreover, a monolayer of this complex on quartz proved to be useful for the selective optical sensing of traces of CO in air, due to the CO affinity for the free Rh axial ligation sites. The XPS analysis confirmed the Rh(II)-Rh(II) complex formulation since no splitting in Rh 3d spectrum was observed. Moreover, also the molecular footprint on quartz was obtained.Another covalent monolayer of (salen)Mn(III) molecules on silica functionalized substrates was studied to shed light on the active heterogeneous catalyst for enantioselective epoxidation reactions. The system, after activation with NaClO, clearly showed a 0.8 eV shift to higher binding energy in the Mn 2p states consistent with the formation of a new oxo Mn species in which the Mn ion possesses a higher oxidation state. This result represented the very first experimental observation of the active Mn(V) catalyst.An additional example can be represented by the XPS of a luminescent monolayer of the tris(dibenzoylmethane) mono(5-amino-1,10-phenanthroline)europium (III) complex covalent-assembled on the top of a film composed of polystyrene chains that, in turn, were covalently grown perpendicular to the Si(100) surface by an atom transfer radical polymerization reaction. In this case we exploited the overall grafting mechanism that consisted in one Eu-complex molecule by three polystyrene perpendicular chains each of them, in turn, covering four siloxane moieties.  A. Gulino, F. Lupo, M. E. Fragalà and Sandra Lo Schiavo., J. Phys. Chem. C, 2009, 113, 13558-13564.  V. La Paglia Fragola, F. Lupo, A. Pappalardo, G. Trusso Sfrazzetto, R. M. Toscano, F. P. Ballistreri, G. A. Tomaselli and Antonino Gulino., J. Mater. Chem., 2012, 22, 20561. D. A. Cristaldi, S. Millesi, P. Mineo and A. Gulino., J. Phys. Chem. C, 2013, 117, 16213-16220.
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