ENGINEERING THE STRUCTURAL AND FUNCTIONAL PROPERTIES OF TRANSITION METAL OXIDE INTERFACES BY CLUSTER ASSEMBLING.
Creators: name: BORGHI, FRANCESCA
Publisher: name: Universit� degli Studi di Milano
Year: 2015

description: 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.