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|Autori: ||Bagiante, Salvatore|
|Titolo: ||Carbon Nanotubes: Synthesis, Characterization and Integration|
|Abstract: ||Today many different techniques are used for CNT synthesis, and the main issues concern the production yield/cost ratio and the determination and control of the produced material and its properties. Moreover, still a lot of improvement on the CNT synthesis remains to be achieved in order to have CNT grown on selected areas, with a given diameter and chirality, thereby enabling the production of well defined devices.
The main aim of this work is the study of CNT synthesis by one of the techniques that allows high yield/cost ratio: arc discharge in liquid nitrogen. This method still needs to be fully understood in order to achieve a good control of the produced nanomaterials and their properties by the use of suitable experimental parameters. This thesis work gives a contribution towards the comprehension of the arcing process for CNT production. The second aspect that has been considered in this work is the use of CNTs for the fabrication of new and innovative devices. The unconventional approach considered here is the use of ultra-clean nanotubes, grown by chemical vapor deposition (CVD) in the last fabrication step, after all the device structure has already been completed.
In summary, the outline of the thesis is the following: after an introductive chapter on the basic properties of CNTs, with a discussion of theoretical concepts as well as fundamental experimental results (chapter 2), the next chapter (chapter 3) is devoted to the study of MWCNTs produced by arc-discharge in liquid nitrogen (LN2). In this technique, liquid nitrogen substitutes both vacuum and cooled systems. A morphological and structural characterization of the C nanostructures obtained as the experimental parameters are varied, has been performed, by electron microscopy techniques (SEM, TEM) and Raman spectroscopy. MWCNTs produced by this technique, have a high-quality, small innermost tube diameter and, for particular synthesis conditions, hybrid system, formed by linear C chains inserted in MWCNT (LCC@MWCNTs), can be produced. LCCs have attracted considerable fundamental and practical interest in material science and astrophysics. Two different kinds of linear carbon chains exist: polyyne, with alternate single-triple bonds on the chain, and cumulene, with double bonds only. Polynnes, in particular, can be considered as precursors for carbon materials such as carbine, fullerenes and nanotubes; they are candidates for molecular wires, molecular devices for electron transport through bonds or electronic communication, and the interpretation of some IR bands from the interstellar powder could be related to the presence of sp-coordinated carbon. In the last chapter (chapter 4), we present the fabrication steps of a device based on ultra-clean suspended nanotubes and preliminary results obtained by electrical and optical measurements. The use of ultra-clean carbon nanotubes gives many advantages such as: (i) nanotube not exposed to electron beam, (ii) in the optical measurement the brightness is 10-100 times higher of the conventional device, (iii) using suspended nanotubes we remove all possible interaction with the substrate, and (iv) in the electrical measurement the signal/noise ratio is very high. The resistance of different conductive materials (metals or alloys) during the CVD growth of CNTs (T=900Ã Ã Ã ÂºC with Ar, H2 and CH4 gasses) has been evaluated, in order to choose the most suitable ones to create the contacts with the nanotube. Moreover, we have demonstrated that not only it is possible to have electrostatic doping of a CNT by using different gates, but even that it is possible to have an extremely precise control of the spatial electrostatic doping and therefore the p-n junction position along the CNT, by changing the voltage applied to each gate.|
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