Physical Properties Of Nanostructured Materials Assembled By Carbon Nanotubes

The discovery of carbon nanotube (CNTs) by Iijima et al. in 1991 initiated rapid progress in the theoretical and experimental investigation on CNTs. Very excellent electric and thermal conductivities were explored and the nanotubes were shown to be as stiff as diamond in their axial direction with Young modulus of the order of TPa. These properties make the nanotubes promising candidates for nanoelectrical and nanomechanical devices. Since Cumings and Zettl proposed that linear bearings can be made of single-walled CNTs nesting one another, a number ofl theoretical studies has been carried out in this field, particularly in double walled carbon nanotubes (DWCNTs) to address the dynamic friction as well as the energy dissipation during the translational motion of two nanotubes sliding one with respect to the other. Meanwhile, experiments were also carried out to study the rotational motion of nanotubes. Comparing with the translation motion, many questions about the character impacting the rotational motion still remain unknown. Recently, lots of works have addressed the practical process of the filling of CNTs with different metals, and suggest the encapsulation of nanowires in CNTs to be realistic, thereby fostering further studies. Encapsulated nanowires, as well as nanoparticles, may both modify the CNT properties and have their properties determined by the confining CNT. The stable structure and melting temperature of encapsulated nanowires are two interesting and arresting subjects in this respect. In this thesis, molecular dynamics simulations with the many-body Brenner potential and Lennard-Jones (LJ) pair-potential were performed to study the rotational dynamics and interwall friction of DWCNTs firstly. Based on the previous simulation and using a Finnis-Sinclair potential to describe the interaction between Au and Au, we then studied the formation of nanowires by coalescence of small gold clusters inside carbon nanotubes and the melting behavior of encapsulated Au nanowires. Our main results can be summarized as the following:We investigated the rotational motion and dynamic friction in molecular bearing composed of DWCNTs using molecular dynamics simulations. Thermal effects due to the rotational friction were mainly focused. The diameters of the bearings varied between 6 A and 16 A for inner shafts, and between 12 A and 20 A for outer sleeves. The rotation velocity varied from 0.05 rotations per picosecond to 0.25 rotations per picosecond. The simulations show that the energy dissipation, and hence the temperature of the system increases linearly with rotation time. The value of energy dissipation is around 0.59meV/atom per rotation atω= 0.05 rotations per picosecond for a (15,0)@(23,0) bearing. Correspondingly, the average friction force is around 1.75x10-5nN/atom. The dependence of the energy dissipation on the rotation velocity, interwall distance as well as the contact area of the DWCNT was also discussed. It was observed that the energy dissipation becomes the lowest when the interwall distance of the DWCNT bearing reaches about 0.34nm, the equilibrium distance of the LJ potential between carbon atoms. The low energy dissipation suggests that the DWCNT can be a good candidate as wearless rotational bearing, which supports the previous studies.The coalescence of Au13, Au55 and Au147 icosahedral clusters encapsulated inside single walled CNTs of different diameters are investigated using molecular dynamics simulation with semi-empirical potentials. Three steps needed for the formation of encapsulated nanowires are followed in detail, namely, the penetration of clusters in CNTs, the coalescence between two clusters inside CNTs and their accumulation to form wires. It is suggested that no significant energy barrier is encountered during the penetration of free clusters into CNTs provided the CNT radius is large enough, that is, about 0.3 nm larger than the cluster radius. The relative orientation of clusters imposed by the CNT favors their spontaneous coalescence. After coalescence of two clusters, the Au atoms are rearranged to form new structures of cylindrical symmetry that may be seven fold, six fold, five fold, helical or fcc depending on the CNT diameter. The thermal stability of these structures is discussed and the structural properties of nanowires formed by accumulation of many clusters in CNTs are analyzed in detail. A geometrical method is presented which allows the prediction of the structure of multi-shell helical wires, when knowing only the CNT radius. These modeling results suggest the possibility of synthesizing metallic nanowires with controlled diameter and structure by embedding clusters into nanotubes with suitable diameters.The structure and melting behavior of gold nanowires (NWs) in CNTs of different diameters were investigated using molecular dynamics simulations with semi-empirical potentials. The effect of the CNT on the melting of NWs is analysed. The simulations indicated that the predicted melting temperature of the enclosed Au-nanowires is intermediate between that of bulk gold and free standing wires. The role of the CNTs on the melting temperature is evidenced by artificially tuning the strength of the Au-C interaction. The tube diameter has a minor effect on the melting temperature in the range investigated (1.08nm< D<2.09nm). In contrast with isolated NWs, with increasing temperature, the presence of the CNT imposes the melting to initiate in the center of the NW. These findings are in good agreement with previous observations and predictions concerning the stabilizing role of the CNT on the NW structure and, in particular, of their outermost layer structure.