Molecular-Dynamics Simulation On Structural Transition And Properties Of Nano System

In this thesis we have studied the pressure-induced structure transition of multiwalled carbon nanotube and CdSe nanocrystal by using constant pressure molecular dynamics simulations. And we have designed a nanocontainer for the storage of hydrogen based on the carbon nanotube.First, we have studied the structure of isolated DWCNTs under external pressure. We find that pressure-induced structure transition takes place in all the studied DWCNTs. The critical transition pressure is strongly dependent on the radius of the inner tube. The bond length of the carbon nanotube would increase after phase transition, and we use a elastic model to explain this physics phenomenon.Second, the radial pressure transmission behavior of MWCNTs is studied. It is found that the response pressure of inner tube is much lower than the external pressure before structural transition happens. The pressure transmission efficiency increases with tube radius. Meanwhile the pressure transmission efficiency also depends upon morphology combination of MWCNTs. With the same size, the pressure transmission efficiency of commensurate MWCNTs is higher than that of incommensurate ones. Based on the simulation results, we propose an experimental method to determine the pressure transmission efficiency of MWCNTs.Third, we have studied the possibility of carbon nanotube acting as high pressure container. Based on the results, we designed a nano-container for the storage of hydrogen. At 2.5 GPa, the storage weight ratio of the container approaches a promising 7.7%. Fourth, the structural transformation of CdSe nanocrystals under hydrostatic pressure is studied. We have found the structural transformation of CdSe nanocrystal is highly affected by the size and shape. The results show that the pressure for WZ to RS structural transformation of spherical nanocrystal decreases with nanocrystal size, while it seems to increase for facet one. For all spherical nanocrystals, the final structures have nano scale grain boundaries. All the faceted ones undergo uniform deformation, the transformed RS structure is of single domain nanocrystal.