| New nanomaterials have been synthesized continuously in recent decades because of the nanotechnology's development, which show greatly different properties, compared with the bulk materials, and so have a great potential application in nanotechnology area, attracting so many researchers working in this field.In this dissertation, we have mainly studied the several subjects:1) Confined growing of the gold nanotubes in the carbon nanotubes.2) The coherent phonons of the semiconducting carbon nanotubes in the low excited-states.3) The superelongation of carbon nanotubes by a double Stone-Wales mechanism.In chapter 2, we mainly introduce the calculation methods we have used in this dissertation:1) the basic principles of classical molecular dynamics; 2) the tight-binding molecular dynamics; 3) the basic ideas of the Mento Carlo method; 4) the force constant method to calculate the phonons, and the related lattice's dynamical theory.Gold nanotube is a kind of tubular metal nanomaterial, showing an excellent electric transport ability. And the single-walled gold nanotube (SWGNT) has been experimentally synthesized by an ultrahigh vacuum transmission electron microscope, which, however, could hardly be used in practice to produce the SWGNT efficiently, especially the longer ones. At the same time, the characteristic hollow tubular structure of the carbon nanotube make it possibly used as a template to synthesize other nanomaterials. So, we can imagine that if we put some gold atoms into the selected carbon nanotubes with proper diameters, the SWGNTs could be formed in them. In chaper 3, we choose the (9,2), (10,3) and (11,3) carbon nanotubes as templates in which some gold atoms are put, and make the classic MD simulations of a cooling process. Finally, the (3,3), (4,3) and (5,3) SWGNTs are indeed formed, respectively, in those carbon nanotubes. It is found that the chiral structures of the SWGNTs depend less on the chiral angles of outside carbon nanotubes, but mainly on the diameter of carbon nanotubes. The SWGNT's solidification temperature is found to increase with its length. At the same length, the smallest diameter (3,3) gold nanotube among the three gold nanotubes we obtained has the highest solidification temperature, in contrast to the lowest solidification temperature of (5,3) gold nanotube, which has the biggest diameter among the three gold nanotubes. We hope our simulation results can be helpful for producing efficiently the SWGNTs on a large-scale in future experiments.In chapter 4, we have studied the coherent phonons of semiconducting carbon nanotubes in low excited-state. It is well known that the resonant Raman spectrum is often used to detect the structures of carbon nanotubes, which is well performed for the ground-state carbon nanotubes, but is less suitable for the excited-state ones. Recently, the coherent phonon (CP) spectrum has been developed to investigate the structure of carbon nanotube in the excited-states. A train of femtosecond optical pulses is pumped to carbon nanotubes for exciting the coherent phonon oscillations, which induce the corresponding bandgap oscillation. The CP intensity could be obtained by measuring change of the differential transmission coefficient. We have used the tight-binding MD to simulate the coherent phonon excitations in the semiconducting carbon nanotubes and their relationship with the electronic bandgap, which is expected to be helpful for understanding the CP experiments more easily. It is worthy mentioning that our results about the radial breathing phonon mode (RBM) in different semiconducting carbon nanotubes are in good agreement with the experiment. Besides the RBM, the longitudinal optical mode (LO) is also found in our simulation results, which was also observed in experiments. Additionally, the transverse optical mode (TO) with small amplitude was excited in non-zigzag semiconducting carbon nanotube, which is hoped to be able to be observed in future more accurate experiment.In chapter 5, we have discussed the microscopic mechanism about carbon naotube's superelongation. It has been found in experiment that the carbon nanotube could be stretched at 2000K to 280% of its initial length, stimulating a great interest in it. The formation of Stone-Wales defect and the gliding of the 5-7 defects split from Stone-Wales defect were considered as one of the possible mechanisms for the carbon nanotube's superelongation in the previous theoretical works. Ding Feng et al. have simulated the carbon nanotube's superelongation by the one Stone-Wales defect mechanism. However, we have noticed that there were several kinks on carbon nanotube's wall, observed in the experimental superelongation. So, it is reasonable to assume that existence of several Stone-Wales defects on the tube's wall during its superelongation. So, we have proposed in our simulations a simplest carbon nanotube's superelongation mechanism with many defects, i.e., the two Stone-Wales defect mechanism, which is different from previous one Stone-Wales defect mechanism as follows:1) The carbon nanotube could be more easily stretched longer than that of one Stone-Wales defect mechanism.2) Two more 5-7 defects, split from Stone-Wale defects, can glide along the carbon nanotube's wall in this situation, so reducing more efficiently the system's strain energy.3) In our mechanism, new defects can be formed by 5-7 defects'collision, whose dynamic evolution combined with the 5-7 defect gliding contribute to carbon nanotube's superelongation. Our simulation results revealed that one of the possible mechanisms for carbon nanotube's superelongation is the formation of a few defects and their motions on carbon nanotube's wall.In a word, we hope our MD simulation results can be helpful for understanding more easily the related experiments and also synthesizing other new tubular nanomaterials. For example, our work about gold nanotube's formation in carbon nanotubes presents a new possible way to experimentally synthesize the SWGNTs more efficiently, and also other tubular nanomaterials.
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