Theoretical And Simulation Study For The Interface Properties Of Low-dimensional Nano Materials

In the last two decades, Nanoscience and nanotechnology are hot topics in material science field. For the nanometer size materials, the interface/volume ratio increases notably with the decreasing dimension sizes and the corresponding interface structures also change, which can bring unique physical and chemical properties. To study the interface of nano materials, there are two methods: top?down and bottom?up. The top?down method is the theoretical one, for which the classical thermodynamic and mechanic theory can be applied. In the bottom?up method, using first-principle density functional theory (DFT) and molecular dynamic simulation, the specific structure of nano-interface can be built and the corresponding interface properties can also be obtained. In this thesis, using both the theoretical and simulation methods, the interface energy and properties of some low-dimensional nano materials are studied. The details are as following:1. The cohesive energy of Cu nano-materials (Ec) containing different numbers of atoms (n) in the structures of pyramids, nanotubes, nanorods, films, and icosahedrons is determined using ab initio density functional theory (DFT) simulation and surface broken bonds theory. For a specific structure, Ec(n) increases monotonically as n decreases. When n > nc, Ec(n) is related to the corresponding surface volume ratio, while when n < nc, Ec(n) is related to the corresponding coordinate number Zs(n). For the whole dimensional size, Ec(n) is a function of the corresponding actual bonds number Ba. The above suggestion is in good agreement with the results obtained from the observed distributions of the density of states (DOS).2. Surface energy associated with atomic number n of Ag quasi-crystalline clusters is investigated using DFT simulation and theory for surface energy. It is found that the surface energy value is 0.55-0.66 eV/atom, which does not show obvious size dependence. This is due to the increase of cohesive energy Ec(n) and the decrease of surface coordinate number associated with the decreasing of n. As n decreases, the partial density of states (PDOS) shifts to higher energy end and there is electronic charge transfer from s/p band into d band, which leads to the increase of cohesive energy of both surface and interior atoms. In addition, the cohesive energy of Ag clusters at any site x with the magic number n [Ecx(n)] is determined through calculating the vacancy formation energy Evx(n) using DFT simulation and theoretical modeling. It is found that Ecx(n) is quite distinct at different surface and interior atomic sites within the same cluster. Ecx(n) is also structure-dependent. Ecx(n) values of the core atoms with an icosahedral structure are much larger than the corresponding bulk value due to the pressure induced d-d shells repulsion and the sp-d hybridization weakening.3. The forming abilities of monatomic chains (MC) of several fcc and bcc metals stretched in three principal crystallographic orientations of [111], [100] and [110] are analyzed in terms of a ratio between Peierls stress of a bulk crystal with dislocations and theoretical shear stress of a monatomic chain. It is found that the structure and orientation dependent stress ratios are proportional to the forming abilities of MC while the stress ratio is a function of Possion's ratio. It is found that MC is probably forming for the fcc nanowires in [111] and [100] and bcc nanowires in [110] directions. The above considerations are in agreement with known experimental and simulation results of Au. In addition, Nb as a candidate for MC formation is suggested. 4. The thickness dependent stripe structure stabilization of Ag films on Si-In substrate is thermodynamically considered. It is found that for the stability of the structure, there is a competition between the sum of elastic energy and stacking fault energy in the film and the film-substrate interface energy. The presence of equilibrium of them leads to a critical film thickness. Beyond it, the stripe structure will transform into a flat one. Our prediction for the critical layer number nc of Ag films shows reasonable agreement with experimental data. In addition, according to the established model, it is predicted that Au could also form the above stripe structure on this substrate with a similar nc value of Ag. In addition, based on the classical elastic theory and a thermodynamic model for surface energy, the critical layer number nc for Stranski-Krastanov (SK) growth mode epitaxial growth for bcc metallic thin films is calculated. nc is determined by that the sum of the surface energy of a film and the lattice mismatch elastic energy between a substrate and the film is equal to the surface energy of the substrate. When the film layer number n is larger than nc, a flat growth of the film on the substrate will transform to an island growth. Our predictions on several metallic films are in agreement with experimental results.5. The solution ability of Metal Ni in TiO and TiO2 are separately studied using alloy thermodynamics and DFT simulation. It is found that for both Ni can be meta-stably dissolved in the TiO with different Ni concentration xNi due to the small positive mixing Gibbs free energy change. While Ni can not be dissolved in the TiO2 at any xNi due to its much higher mixing Gibbs free energy change values. This simulation and theory result can help for understanding the Ni release mechanism at the surface oxide layer of NiTi alloy wires. For the incomplete oxide layer with TiO2+TiO structures, the remaining Ni can be dissolved in the TiO and the outmost TiO2 layer can act as a barrier for Ni release at the same time. For the complete oxide layer with only TiO2 structure, Since Ni can not be dissolved in the TiO2, they form clusters at the oxide layer-alloy interface, which induces much more Ni release.