Thesis
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Ph. D. Thesis (Dottorato di ricerca)
Spectromicroscopy of nanostructured systems obtained by femtosecond pulsed laser ablation
Intro Nanostructured systems (NS) are at the cutting edge of material physics research, with physical and chemical properties that make them extremely promising for industrial applications in a variety of field (technology, medicine, biology).
Laser ablation (LA) at femtosecond regime is a promising but largely unexplored technique to produce NS. LA is based on the interaction between a source material and high peak intensity laser pulse inducing the melting or vaporisation of the source material. The generated plume of ablated material, expelled with high kinetic energy, is directed towards a substrate where it is deposited.
Recently it has been shown theoretically that the pulse duration (fs versus ns) and the laser fluence can determine different thermodynamic states of the ablated material. However, up to date no ordered or controlled NS have been obtained.
Our groups have recently obtained very interesting preliminary results employing fs laser pulses in various fluence regimes, depositing Ti on crystalline silicon. Atomic force microscopy (AFM) data show that it is possible to obtain film with uniform thickness (FUT) (around 3 nm) over micron squared areas, as well as fractal-like structures (FS) with the same single thickness and extension. The FS coexists with Ti clusters of few nm in diameter, with a narrow size distribution. Such titanium clusters, likely oxidized, are also an important starting point for the investigation of magnetic properties that depends on the presence of oxygen vacancies.
These results open the possibility to obtain NS with controlled thickness at the nanometer scale (films) and size distribution (clusters) over macroscopic areas with an extremely simple and scalable technique.
Aim The goal of work is to develop the laser ablation protocols to control size, thickness, and chemical state of the deposited NS. The protocols will be used to tailor the NS properties depending on the material invesigated (Ti, Ag and Au) for selected applications such as plasmonics, catalysis and bactericidal films. The student will work with SEM, AFM, XPS and other experimental techniques available at the collaborating groups.
The activity will be done at the Universita Cattolica, at Universita di Padova and at synchrotron facilities
Nanoclusters obtained by a supersonic cluster beam source: new approach to nanoscale systems
Intro Photocatalisys is a process where the energy barrier of a chemical reaction is lowered by the presence of a suitable catalysts (TiO2), that provides electron or holes to the reactants adsorbed at its surface by the absorption of a photon. It is a very promising method to reduce environmental pollution by using solar energy. Main obstacles toward employing TiO2 as efficient photoactive material are: the large optical band gap (< 400 nm), hampering visible light absorption; the recombination of electron-hole pairs, the surafce to volume ratio (svr) of the films.
Nanoparticle chemical synthesis can provide a high svr material, but with poor substitutional dopant concentations, in particular N, the presence of localized states that are detrimental for electron and hole mobility. Donors and acceptors co-doping potentially leads to a significant band gap reduction, but the effectiveness of the co-doping approach remains limited by the low solubility of dopants inside TiO2. We have shown that high concentration (few percent) of dopants (Cr and N) can be obtained for TiO2 nanoclusters thin films by Supersonic Cluster Beam Deposition (SCBD), resulting in a strongly reduced band gap and delocalized electronic states.
The co-doping is easily obtained by plasma supersonic synthesis of nanoclusters. The material to be deposited is ablated by a plasma obtained by arc discharge in presence of helium. The gas-material mixture is exiting the nozzle and becomes a supersonic beam. The source allow to tune the cluster size in the nanometer range. The source works in medium and UHV conditions, is able to deposit nanoclusters on any solid substrate and can be attached to our UHV system so that depositions under controlled conditions can be obtained.
Aim The student will develop the codoping approach for other metals (W, Mo) and will investigate different dopant concentrations to reach the optimum light absorption (XPS, SEM, AFM and synchrotron radiation experiments). At the same time, the electron-hole lifetime will be measured trough pump and probe optical technique in collaboration with other groups to correlate this information with the dopant concentrations. Moreover, efficiency validation of the matieral will be done on test pollutants by photocatalitic measurements at collaborating european groups.
The aim is to obtained films optimized for photocatalysis.
The activity will be done at the Universita Cattolica (Italy), at Universita di Padova (Italy), and at university of Twente (Netherlands).
Tailoring the properties of molecular films through metal assisted growth on 1D template
Intro Films of organic molecules have attracted much attention due to the transport properties. In particular polyacenes reveal remarkably high charge carriers mobilities in the crystalline phase. But in such molecular compounds the charge carriers mobilities are severely affected by impurities and defects like grain boundaries. Thus a significant increase of performance can be expected for organic layers with an improved structural organization and degree of order.
In contrast to inorganic semiconductors, where molecular beam epitaxy is a well established route to fabricate epitaxial heterostructures, the same approach (organic molecular beam deposition) does not allow to grow high quality and thick crystalline organic films with the desired molecular orientation. This is due to the fact that the forces driving the crystal growth are very weak (typically van der Waals) and short ranged, and reveal a pronounced anisotropy related to the shape of the molecules involved. This severely hampers the growth of crystalline organic films on inorganic substrates with a tailored molecular orientation.
For molecules deposited on metal surfaces the interaction generally favors a planar adsorption geometry for a single ordered monolayer. Unfortunately, molecules pertaining to successive layers gradually return to a standing up configuration as the thickness of the film increases and no experimental techniques have been developed to produce flat-lying pentacene films with a suitable thickness.
Aim The aim is to explore a route to drive the growth of a flat-lying film of several layers, starting from the deposition of a flat and highly ordered pentacene single layer on a copper stepped surface (a vicinal Cu(119) surface), and employing intercalating metals/molecules to direct the growth of the thick film. The student will use STM, LEED, XPS and synchrotron based techniques to investigate the film growth and its structural and chemical properties, the type of bonds involved in the film etc.
Master Thesis (Laurea Specialistica)
Nanostructures obtained by fs laser ablation: synthesis and characterization
Nanostructured systems (NS) are at the cutting edge of material physics research, with physical and chemical properties that make them extremely promising for industrial applications in a variety of field (technology, medicine, biology).
Laser ablation (LA) at femtosecond regime is a promising but largely unexplored technique to produce NS. LA is based on the interaction between a source material and high peak intensity laser pulse inducing the melting or vaporisation of the source material. The generated plume of ablated material, expelled with high kinetic energy, is directed towards a substrate where it is deposited.
Recently it has been shown theoretically that the pulse duration (fs versus ns) and the laser fluence can determine different thermodynamic states of the ablated material. However, up to date no ordered or controlled NS have been obtained.
Our group have obtained very interesting preliminary results employing fs laser pulses to deposit Ti on crystalline silicon. These results open the possibility to obtain nanostructured systems with controlled thickness at the nanometer scale (films) and size distribution (clusters) over macroscopic areas with an extremely simple and scalable technique.
Aim The goal of work is to develop some of the laser ablation protocols to control size, thickness, and chemical state of the deposited NS.
There are different metaerial of choice, depending on the possible applications: Ti for photocatalysis and substrate to direct cellular growth, Ag as bactericidal agent, Au as precursor for controlled ocide nanotube growth. Each of them will provide a subject for a single master thesis.
The candidate will work on the spectroscopic and morphologic characterization of the produced nanostructures employing scanning probe techniques like atomic force microscopy, scanning electron microscopy, scannig auger spectroscopy, XPS and other experimental techniques available at the collaborating groups.
The activity will be done at the Universita Cattolica, at Universita di Padova and at synchrotron facilietis
A flexible synthesis approach to nanoscale systems: supersonic cluster beam source
A major obstacle toward employing TiO2 as an efficient photoactive material is related to its large optical band gap, strongly limiting visible light absorption. Substitutional doping with both donors and acceptors (co-doping) potentially leads to a significant band gap reduction, but the effectiveness of the co-doping approach remains limited by the low solubility of dopants inside TiO2. We have shown that nanostructured Cr and N co-doped TiO2 thin films can be obtained by Supersonic Cluster Beam Deposition (SCBD) with a high concentration of dopants and a strongly reduced band gap.
The source is based on the supersonic expansion of the material through a small nozzle. The material to be deposited is ablated by a plasma obtained by arc discharge in presence of helium. The gas-material mixture is exiting the nozzle and becomes a supersonic beam. The source allow to tune the cluster size in the nanometer range. The source works in medium and UHV conditions, is ableto deposit nanoclusters on any solid substrate and can be attached to our UHV system so that depositions under controlled conditions can be obtained.
The candidate will work to verify the absorption and photocatalityc behavior of Cr-N co-doped nanostructured films as a function of different dopant concentrations. The films will be characterized by spectroscopy and microscopy techniques, in order to understand their chemical states and stability. The aim is to obtained films optimized for photocatalysis.
The activity will be done both at the Universita Cattolica and at Universita di Padova.
Graphene, graphite and nanotubes: control of the growth process and properties
Chemical Vapor Deposition is a powerful technique to grow carbon nanostructures. We have shown that it is possible to control the growth of planar and curved graphene sheets by using catalytic CVD of methane and acetylene on graphite. The method is promising to obtain controlled growth of graphene sheets over different substrates.
The candidate will investigate the possibility of extending the CVD method on subtrates other than graphite, studying the spectroscopic and morphologic characterization of the produced nanostructures employing scanning probe techniques like atomic force microscopy, scanning electron microscopy, scannig auger spectroscopy and XPS.
Bachelor Thesis (Laurea Triennale)
Misura delle proprieta fotocatalitiche di film di TiO2 nanostrutturati drogati Cr e N
(Tesi sperimentale)
Scopo: produrre film di cluster nanostrutturati drogati W e N e misurare l'assorbimento ottico e la dimensione delle nanoparticelle dei film prodotti.
Attivita: Deposizione di film nanostrutturati di Ti drogati W e N in condizioni di vuoto controllato e/o in condizioni standard con la sorgente supersonica di cluster. Misura dei livelli di core con fotoemissione. Misura della dimensione delle nanoparticelle col microscopio a forza atomica.
Durata: circa 2 mesi
Crescita di nanoparticelle di Ag tramite sorgente supersonica di cluster
(Tesi sperimentale)
Scopo: produrre film di cluster nanostrutturati di Ag e misurare lo stato chimico e la dimensione delle nanoparticelle dei film prodotti.
Attivita: Deposizione di film nanostrutturati di Ag in condizioni di vuoto controllato e/o in condizioni standard con la sorgente supersonica di cluster. Misura dei livelli di core con fotoemissione. Misura della dimensione delle nanoparticelle col microscopio a forza atomica.
Durata: circa 2 mesi