Structural Evolution And Electronic Transport Properties Of Nanowires In Confined Space

One-dimensional nanowires and carbon nanotubes, which hold great application prospect in nano-electronics, optoelectronics, new generation of very large scale intergration and so on,have been the research hotspots in recent years. Nanowires attract great attention because of their novel structures and physical properties different from bulk materials. Researches show that nanowires are quite sensitive to oxygens and vapours. To solve this problem, carbon nanotubes are proposed to be used as templates to synthesize nanowires. Due to their excellent mechanical, thermal and chemical stability, carbon nanotubes have been widely exploited as templates to fabricate one-dimensional nano-materials. For this method, nanowires may possess special structures and properties because of the spatial confinement effect and the interaction between nanowires and nanotube wall. Hence, using carbon nanotubes as nano-moulds to fabricate nanowires is of great promise for various applications.In this thesis, we mainly studied the structures and properties of Ni, Ni-C and Si nanowires. Molecular dynamics simulations are performed to research their structural evolution in confined space, while the electronic transport properties of these nanowires are calculated through the non-equilibrium Green function methods in combination with the density functional theory. The primary contents and results are given as follows:(1) The structures and electronic transport properties of ultra-thin Ni and Ni-C nanowires obtained from carbon nanotube templates are investigated. C atoms tend to locate at the central positions of nanowires and are surrounded by Ni atoms when C concentration is quite low. As the concentration increases, C atoms no longer occupy the center strictly and C-C bonds start to form. Interestingly, spin polarization at the Fermi level is not responsible for the spin filtration of these nanowires. Increasing C concentration can improve the resistance of nanowires by decreasing the number of electronic transmission channels and abating the coupling of electron orbitals between Ni atoms. Moreover, with the increase of diameter, the conductance of these nanowires increases as well.(2) We studied the structural evolution process of Si nanowires confined in nanoscale space from 3000K to 300K and discussed the effects of diameter of carbon nanotube, cooling rate, geometry of the template etc. on structures. The freezing Si structure at 300 K with a cooling rate of 1 K/ps is stratification which is composed of a stable crystalline shell whose arrangement is similar to the (100) face of Si crystal and a glassy core. It is revealed that the statistical average of this ordered shell and disordered core gives rise to the split of the second peak of the pair distribution function curves of the Si nanowires. Due to the spatial restriction effect, the confined structure consists of more five-coordinated clusters compared to the bulk Si. Increasing the cavity size is detrimental to the stability of the layered configuration and increasing the cooling rate mainly decreases the order degree of Si atoms adjacent to the nanotube wall. Interestingly, we also find that the cylindric cavity is superior to the square one in inducing the formation of long-range crystalline order because of its uniform curvature. Moreover, the obtained Si nanowires are metallic, and the presence of carbon nanotubes leads to the weakening of the electrical electronic transmission of Si nanowires to some extent. Increasing the cooling rate can also recede the electrical electronic abilities of obtained Si nanowires.