Numerical Simulations On The Structures And Physical Properties Of Low-dimensional Carbon Nanomaterials And Ice Nanotubes

The nanotechnology has been considerably developed in the recent years with people's deeply understanding the microscopic world, in which a lot of different kinds of nanomaterials have been found and investigated. Because of their low-dimensional structures, their physical and chemical properties are much different from the macroscopic materials, making them have a promising potential application in future and so attracting widespread attention.In chapter 1, we firstly introduce the nanotechnology development and then the previous theoretical and experimental works on the graphene and carbon nanotubes. At last, the water's behaviors confined in carbon nanotubes are concisely introduced.In chapter 2, we have made a special introduction of the classical molecular dynamics (MD) method. We firstly give a simple description of its basic principles and then a detailed introduction of its several essentials, such as the interaction potentials, boundary conditions, numerical integration method to solve dynamic equations of motion, initial conditions, simulation ensembles and statistical average over physical quantities.In chapter 3, we have calculated the force constant matrix and vibrational modes of single walled carbon nanotubes (SWCNTs) with divacancies, using the empirical Brenner potential, based upon which their nonresonant Raman spectra have been calculated by the empirical bond polarizability model. It is found that the SWCNT's diameter will be changed by the divacancies, depending strongly on the tube's chirality and the divacancy concentration. More importantly, it is found that the divacancy-induced Raman peaks lie out of the SWCNT's G-band and their positions depend on the tube's chirality and the divacancy's symmetry, which can be used to detect experimentally the different types of divacancies.In chapter 4, we have used the classic MD simulations to investigate the thermally-excited ripples of the AA-and AB-stacking bilayer graphenes (BLGs) at different temperatures, and compared them with those of the single layer graphene (SLG). It is found that:(1) The ripples in both AA-and AB-stacking BLGs are intrinsic with a characteristic size of about 100A at room temperature, increasing with increase of temperature. (2) The ripple's intralayer height-height correlation functions for the two types of BLGs follow a power-law behavior, Gh(q)～q-a. (3) The ripple's height of the AB-stacking BLG is larger than that of the AA-stacking one at the lower temperatures. (4) Finally, the ripple's height in the two types of BLGs is greatly smaller than that in the SLG, which is because the interlayer interaction can suppress atoms'motion in z direction.In chapter 5, using the classic MD simulation method, we have studied the formation of ice nanotubes, confined in CNTs, and the ferroelectric behaviors of the ice nanotube (INT) under an axial strain. In the first part, we simulate the growing of double-walled INT confined in a CNT at different pressures. It is found that the geometrical structure of the double-walled INT is closely related to the diameter of the outside CNT and the axial pressure. For example, the confined water in a (20,0) CNT can form a double-walled INT with the square lateral side at a smaller axial pressure, such as 300MPa, which can be regarded as the intercalation structure by two single-walled INTs. Using the same CNT, but at a high pressure of 1GPa, a quasi-double-walled INT can be formed with its lateral side in a triangular lattice, in which there is a water chain lying at its central axis. Within a (19,0) CNT, we have obtained a helical double-walled INT at an axial pressure of 1GPa, in which the lateral side also presents a triangular lattice structure. In these double-walled INTs, the inner INTs satisfy the "ice rule"; but the outer INTs only partially satisfy the "ice rule". In the second part of chapter 5, we investigate the axial strain effect on the ferroelectric behaviors of a<5,0> single-walled INT. It is found that its ferroelectricity is enhanced under the compressive deformation and weaken under the tensile deformation. We also calculate the piezoelectric coefficient of the<5,0> INT and compare it with the same diameter BaTiO3 nanowire, showing that<5,0> INT is a relatively superior ferroelectric nanomaterial, which presents a theoretical background for the ice tube used possibly as a new kind of piezoelectric nanomaterials in future.