Thèse de doctorat en Physique
Soutenue en 2007
à l'Université de Paris-Sud. Faculté des Sciences d'Orsay (Essonne) en cotutelle avec Technische Universität Dresden .
Génération par faiseau d'électrons et structure des défauts dans les nanotubes de carbone et de nitrure de bore
The nature and role of defects is of primary importance to understand the physical properties of C and BN single walled nanotubes. TEM is a well known powerful tool to study the structure of defects in materials. However, in the case of SWNTs, the electron irradiation of the TEM may knock out atoms. This effect may alter the native structure of the tube, and has also been proposed as a potential tool for nanoengineering of nanotubular structures. Here we develop a theoretical description of the irradiation mechanism. First, the anisotropy of the emission energy threshold is obtained via density functional based calculations. Then, we numerically derive the total cross section for different emission sites of carbon and boron nitride nanotubes with different chiralities. Using a STEM microscope with experimental conditions optimised on the basis of derived cross-sections, we are able to control the generation of defects in nanotubular systems. Either point or line defects can be obtained with a spatial resolution of a few nanometers. The structure, energetics and electronics of point and line defects in BN systems have been investigated. Stability of mono- and di- vacancy defects in hexagonal boron nitride layers is investigated, and their activation energies and reaction paths for diffusion have been derived using the nudged elastic band method (NEB) combined with density functional based techniques. We show that the appearance of extended linear defects under electron irradiation is more favorable than a random distribution of point defects and this is due to the existence of preferential sites for atom emission in the presence of pre-existing defects.
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