Università Cattolica del Sacro Cuore

NanoScience Lab

Physics at the nanoscale

Nanoscale systems are widely studied and, in some cases, already applied in industrial devices. However, the possibility of tailoring the structural and the electronic properties of these materials is still an open matter that may push their use into a variety of innovative fields.

Graphene Oxide: a flexible chemical route to Graphene

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Controlling the growth of carbon nanostructures (CNT, graphene) by catalytic Chemical Vapor Deposition

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Metals on single layer oxides: a model system to probe relevant transformation in catalysis

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Tailoring nanoparticles by supersonic pulsed cluster source for catalisys and biomedical applications

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Graphene Oxide: a flexible chemical route to Graphene

Graphene is a one-atom-thick layer of carbon with remarkable electronic properties that have focused the attention of scientists and engineers. In particular, graphene is a semiconductor with
zero band gap and high carrier mobilities and concentrations, and shows nearly ballistic transport at room temperature. A challenging aspect of graphene integration into electronic devices and of graphene-based substrates for chemical catalysis is the exfoliation of graphite into individual sheets in a controlled, scalable, and reproducible way. The chemical exfoliation of graphite oxide (using the Hummer oxidation reaction) is efficient, cheap and clean and results in high yields of single-layered graphene oxide (GO). The individual graphene oxide sheets can then be readily deposited on virtually any substrate over large areas using solution based methods. GO forms over a range of O:C stoichiometries, with the oxygen bound to the carbon in the basal plane in the form of hydroxyl and epoxy functional groups and as carbonyl and carboxyl groups at the sheet edges.
The following picture reports the Lerf-Klinowski model of GO.
The oxygen functional groups make graphene oxide sheets strongly hydrophilic and decrease the interaction energy between the graphene layers. Hence, graphite oxide can be readily exfoliated, forming a stable aqueous dispersion.

We are investigating now how to introduce nitrogen functional groups on GO films deposited by controlled-dropcasting technique on several substrates. On these systems, we are performing nitrogen implantation using a low vacuum plasma source: this procedure permits us to obtain a precise N-doping. Furthermore, using an in-situ Surface Science rigorous approach, we are able to study both the changes in the surface morphology of the films (SEM - Scanning Electron Microscopy) and the chemical modifications of the surface after the exposition to the nitrogen plasma (XPS - X-ray Photoelectron Spectroscopy). Finally, the obtained N-doped GO systems will be tested as efficient metal-less catalyst for the Oxygen Reduction Reaction (ORR).


Controlling the growth of carbon nanostructures by catalytic Chemical Vapor Deposition

Carbon based nansotructures (CNT and graphene) are nowadays widely investigated thanks to numerous applications. We are planning to investigate the behavior of grown CNT and graphene on oxide substrates with possibile applications as electron field emitters.We show that it is possible to grow controlled carbon based nanostructures, by use of catalyst assisted Chemical Vapor deposition. Depending on the Fe catalyst coverage and localization on the substrate steps and terraces, different graphene structures are obtained: curved graphene sheets (CGS) at the edges of topmost stacked graphene bilayers, laterally grown terraces (LGT) at the edges of individual graphene layers parallel to the HOPG basal plane and planar graphene islands (PGI) on the terraces of the HOPG substrate.We propose and discuss a growth mechanism taking into account the specific features of the spatial distribution of Fe catalytic nanoparticles on the substrate surface, driven by metal film-substrate interaction. Our synthesis approach is promising for the controlled growth and modification of graphene layers, as well as for engineering the edge characteristics of graphene systems at atomic scales.Before the CVD process, the size distribution of Fe and Co deposited on sputtered graphite is extremely narrow, due to the interaction with the substrate defects. Thermal treatment results in the desorption of particles from terraces but leaves the catalyst at the step edges.

STM image of the Curved Graphene Sheet obtained at double graphite steps after Fe-assisted CVD.


Metals on single layer oxides: a model system to probe relevant transformation in catalysis

The catalytic activity of metals strongly depends on the system dimensions, and is increased as a fuction of surface area. Hence, clusters are the most suitable system with high aspect ratio. In collaboration with the university of Padova, we are investigating the beahvior of nobel and transition metals deposited on a templating substrate to obtain ordered cluster arrays by a self organization mechanism. The systems behavior is also investigated after exposure to real conditions (ambient pressure) to understand the stability and changes taking places under operative conditions.Understechiometric Titanium oxide films provide a good playground, thanks to the oxigen vacancy defects that form ordere structures when TiOx is deposited on Pt(111) surface. STM data show that Au self organizes in ordered arrays of clusters with narrow size distribution around 1 nm. This is due to the interaction taking place with the substrate defects, that pin the growth of the Au cluster. XPD data also reveal te crystalline orientation of the cluster. A similar behavior is observed at low coverage for Fe and Co, but the stronger oxigen affinity of such transition metals leads to a less ordered array.

Left: STM image of the clean z’-TiOx/Pt(111) phase
Right: STM image of the Fe/z’-TiOx/Pt(111). The granular Fe cluster are visible on top of the titania templating pattern.


Tailoring nanoparticles by supersonic pulsed cluster source for catalisys and biomedical applications

The research is focused on the controlled production of nanoparticles (NP) by cluster beam source. The source is based on the supersonic expansion through a small nozzle of the material to be deposited, that is ablated by a pulsed plasma discharge obtained with a carrier gas. The clusters form in the plasma discharge chamber and becomes a supersonic beam by exiting the nozzle. The source allows a great flexibility in the material choice, can perform deposition on virtually any substrate (including textiles), and allows to tune the cluster size in the nanometer range. The source works in medium and UHV conditions, and can be attached to the present UHV system so that depositions under controlled conditions can be obtained.

TiOx NP

Titanium oxide has an incredible number of applications (form biology to energy), due to its photocatalytic activity, i.e. the promotion of chemical reaction through the photon adosprtion. One of the major limiting factor of this material for solar energy harvesting is the large band gap (3.2), that strongly hampers the photon absorption in the visible part of the spectrum. To overcome such limitation, it is necessary to introduce dopants (N and Cr), that generate band gap states decreasing the TiO band gap. A major difficult is due to the fact that the dopants has to produce delocalized states, and therefore the dopant atoms must be substitutional. In collaboration with the University of Tennessee, our lab is carrying on a strategy to obtain codoped TiO2 nanostructured films based on the production of such system by supersonic cluster beam source.

Ag NP

Ag is largely investigated as antibacterial agent for wastewater remediation, healthcare and purification. However, a number of questions still need to be solved, i.e. the actual efficacy of Ag NP, the stability and the nontoxicity. Moreover, adhesion of the Ag NP to membranes or surfaces is still under debate. We are able to directly deposit Ag NP with narrow size distribution on virtually any substrate. In collaboration with microbiologist, we are currently investigating the antibacterial effects and the stability of the Ag NP films.

AFM image of the TiOx cluster deposited by supersonic beam on a clean Si(100) surface. Inset: Auger spectroscopy showing the formation of silicides at room temperature