|Abstract: ||Nanotechnologies, defined as the ensemble of disciplines aiming at manipulating matter on an atomic or molecular scale and at exploiting the corresponding properties, have recently emerged as one of the most relevant research fields due to its implications in applied science and technological applications.
Nanocrystals (NCs) with a uniformly crystalline structure and at least one dimension in the range 1÷100 nanometers, play a key role as building blocks for the assembly of innovative materials and devices for nanotechnology. A wide variety of nanocrystalline materials showing metallic, semiconducting, magnetic properties or their combinations are now available. It is well documented that nanocrystals exhibit physical properties, ranging from mechanical strength, chemical reactivity and conductivity that depend on their size and structure and differ from those of the corresponding bulk materials.
For this reason, nanocrystals are regarded as promising building blocks for the fabrication of functional materials with targeted nanotechnological application.
In particular, in semiconducting nanocrystals the formation of a set of discrete energy levels, at which the carriers can exist, results in quantum effects. Size restricts the movements of the charge carriers forcing them into a quantum confinement and is responsible for new properties. Again, magnetic materials with nanometric size may exhibit single domain structures, which reflect in unique physical properties such as superparamagnetism, enhanced coercivity, quantum tunnelling of the magnetization and giant magnetoresistance with respect to multi domain bulk magnets.
The controlled manufacture of matter at the nanoscale is considered therefore as a promising route to obtain novel materials which can be exploited in optical, electronic, photovoltaic and magnetic nanotechnological applications.
Within this framework, this PhD thesis has been focused on the design, preparation and characterization of heterostructures constituted by nanomaterials of different composition, which could be employed in two relevant areas such as energy and biomedicine.
In particular, we have addressed the preparation by chemical solution routes of heterostructures which include different domains (metal-semiconductor and metal-magnet) with the aim to develop nanocrystalline new materials with improved and combined functionalities.
A key aspect which has been taken into account is how to control the connection between the domains with different functionalities by tuning appropriate synthetic parameters of high temperature colloidal procedures and how this can be exploited to produce heterostructures with well-controlled morphology and microstructure.
In particular, metal-semiconductor heterostructures made out of platinum tips deposited at controlled sites of a chalcogenides semiconductor with a branched octapod morphology were achieved. As the semiconducting domain is able to absorb solar light whereas the noble metal tip is capable to catalyze chemical reactions, it is prospected that the developed materials, i.e. CdSe@CdS-Pt octapods, are active as solar conversion devices and photocatalysis, with particular reference to hydrogen production through optimization of the redox process behind the photocatalytic water splitting reaction. Photophysical investigation of CdSe@CdS-Pt octapods enabled us to point out the effect of heterostructure morphology-properties relationship, as it was found that two regimes for capture of photoexcited
electrons by Pt domains take place depending on the location of the metal tips onto the semiconductor octapods. When Pt is deposited at the octapod tip a slow capture takes place after energy relaxation in the semiconductor and result in large spatial separation of charges; while when Pt covers the whole octapods surface an ultrafast capture of hot electrons occurs, and charge separation happens faster than energy relaxation and Auger recombination.
As an alternative route to the fabrication of multifunctional nanostructures including different domains, the potential of high temperature polymer-mediated hydrolysis in producing magnetic clusters, which are aggregates constituted of many single crystallite approximately 10 nm in size, has been explored.
Such magnetic clusters retain the peculiar properties of the constituent nanoparticles such as superparamagnetism, while exhibiting specific features such as higher magnetization and stability which are advantageous for practical use. Here, the possibility to fine-tune the properties of the clusters by varying the composition of the primary particles has been investigated by including metal nanoparticles (Au, Ag, Pt) and through doping the iron oxide with manganese. This research has been achieved in order to perform a systematic study of the correlation between morpho-structural and magnetic and relaxometric properties, which may be employed for their potential use in biomedical field.
The development of novel magnetic doped clusters has enabled to investigate the relaxometric and heat mediator behaviour of the novel materials, in order to evaluate their potential in biomedicine as prospective novel contrast agents for detection through Magnetic Resonance Imaging and as therapeutic tools in Magnetic Fluid Hyperthermia.
In addition to contributing to the understanding of the mechanisms behind the preparation of functional and multifunctional heterostructures, this thesis also aims at elucidating the structure-properties relationship at the nanoscale in materials with highly controlled compositional and morphological features. To this end, extensive morphological and structural characterization was carried out by a multi technique approach including in particular X-Ray diffraction, conventional and advanced transmission electron microscopy techniques, X-Ray absorption spectroscopy, UV-visible spectroscopy and dynamic light scattering.
The research work is presented according to the following thesis outline:
- Chapter 1 deals with a short review on theoretical background, synthetic approaches and physical and chemical properties of semiconductor nanocrystals and magnetic nanocrystals. An overview on application of semiconductor and magnetic nanocrystals in two relevant fields such as energy and biomedicine, respectively, is also provided.
- Chapter 2 reports on the design and applications of nanometric metal-semiconductor heterostructures. Here we will discuss in detail the formation of CdSe@CdS-Pt nanocrystal hybrid materials, obtained by synthesizing Pt metal nanoparticles onto preformed CdSe@CdS octapod semiconductor obtained by the seeded growth approach. We will present the physico-chemical properties of the developed heterostructures, as obtained by a systematic structural and morphological characterization. The optical properties of these new materials, performed by ultrafast optical spectroscopy in collaboration with Prof. Michele Saba at the Department of Physics of the University of Cagliari, will also be presented and correlated to the dynamics of charges carriers of the nanocrystals, which can in turn be used to predict their prospective use in photocatalysis.
- Chapter 3 describes the synthesis and characterization of magnetic nanoclusters heterostructures based on iron oxide which were in part developed under the supervision of Dr. Antonios Kanaras during a research visit at the University of Southampton (UK). Here, a discussion on the hydrolysis approach adopted in order to produce doped magnetic
nanoclusters based on iron oxide, i.e. noble metal M-Fe3O4 (M= Au, Ag, Pt) and MnxFe3-xO4 which may used as heat mediators on hyperthermia treatment cancer and as contrast agents for magnetic resonance imaging is reported. In these samples, the morpho-structural data were correlated to their magnetic and relaxometric behaviour, which was investigated through SQUID magnetometry and diffusion curves in collaboration with Dr. Teresa Pellegrino and coworkers at the Italian Institute of Technology (IIT, Genova).
Finally, conclusive remarks and future developments arising from this work are discussed.|