ENGINEERING THE STRUCTURAL AND FUNCTIONAL PROPERTIES OF TRANSITION METAL OXIDE INTERFACES BY CLUSTER ASSEMBLING.

Nanostructured materials are defined as systems composed of single or multiple phases such that at least one of them has characteristic dimensions in the nanometer range (1-100 nm). The strategic importance of nanostructured materials rely on the fact that their structural, electronic, magnetic, catalytic, and optical properties can be tuned and controlled by a careful choice and assembling of their nanoscale elemental building blocks. Clusters, aggregations of a few atoms to a few thousands of atoms, are the building blocks used to synthetize nanostructured materials. Low-Energy Cluster Beam Deposition (LECBD) is a technique of choice for the fabrication of nanostructured systems, since it allows the deposition on a substrate of neutral particles produced in the gas phase and maintaining their properties even after deposition. This has been proven to be a powerful bottom-up approach for the engineering of nanostructured thin films with tailored properties, since it allows in principle the control of the physical and chemical characteristics of the building blocks. Among different approaches to LECBD, supersonic cluster beam deposition (SCBD) present several advantages in terms of deposition rate, lateral resolution compatible with planar microfabrication technologies and neutral particle mass selection by exploiting aerodynamic focusing effects. All these features make SCBD a superior tool to synthesize nanostructured films and their integration on microfabricated platforms. The morphology of cluster-assembled materials is characterized by a hierarchical arrangements of small units in larger and larger features up to a certain critical length-scale, in general determined by the duration of the deposition process. The cluster-assembled film morphology is characterized by high specific area and porosity at the nano and sub-nanometer scale, extending in the bulk of the film. Surface pores and surface specific area, as well as rms roughness, depend on film thickness, and increase with it. All these morphological properties is of great relevance for the use of cluster-assembled film in devices as gas sensor, (photo) catalysis, solar energy conversion and as biocompatible substrates.Recently it has been recognized that nanoscale surface morphology and nanopores play an important role in processes involving the interaction of biological entities (protein, viruses, enzymes) with nanostructured surfaces, via the modulation of electric interfacial properties. In particular, when the nanostructured material is used to produce electrodes and substrates for operation in liquid electrolytes, with given pH and ionic strength, double layer phenomena take place. An important parameter to describe these electrostatic phenomena is the IsoElectric Point (IEP), which corresponds to the pH value at which the net charge of the compact layer is zero. When two interacting surfaces approach to a distance comparable or smaller than the typical screening length of the electrolytic solution (the Debye length, determined by the ionic strength of the solution), the overlap of the charged layers determines complex regulation phenomena that are difficult to describe theoretically. While significant insights have been obtained on the properties of the electric double layers formed between flat smooth surfaces, the case of rough surfaces still represents a severe challenge, hampering analytical, yet approximate, solutions of the double layer equations to be reliably obtained. Anyway, these phenomena have been recently shown to be strongly influenced by the morphological properties of the surface. The quantitative characterization of all these interfacial properties requires imaging and force spectroscopy techniques with a resolution in and beyond the nanometer-scale. Atomic Force Spectroscopy (AFM) is an excellent candidate, since it couples the possibility of scanning with a z-resolution lower than fraction of nanometer and x-y resolution of 1 nm and also of performing very accurate force spectroscopy measurements. The first aim of my PhD work is to characterize by AFM the evolution of morphological properties of transition-metal oxides cluster-assembled materials (in particular nanostructured Titania (ns-TiOx) and nanostructured Zirconia (ns-ZrOx), starting from sub-monolayer regime to thin film, and especially to describe the influence of the building-blocks dimensions on the growth mechanisms and on the final surface morphology and topography. With this information, I have explored the influence of nanoscale morphology on double layer interactions which takes place on these nanostructured interfaces and on the wettability behaviour. The results have been used to highlight the role of morphological and structural surface properties as biophysical signal mediators for protein adsorption processes and cellular adhesion.