Dynamic Studies Of The Nanostructures On Solid Surfaces

Recently, physical vaporizing methods such as Ohm heating, laser ablation and magnetron sputtering are extensively employed to prepare quantum dots (three dimensional growth) or quantum well (layer-by-layer growth) on clean solid surfaces, and microscopy probe techniques are used to visualize the surface structures on nanometer scale, showing that under certain experimental conditions the quantum dots can take on long-range regular arrangement. However, even the most advanced instruments can hardly provide direct information about the dynamics of surface growth on single-atomic levels, and therefore, relevant theoretical studies are highly desirable for thorough understanding the dynamical processes. In this thesis molecular dynamics (MD) method is used to investigate the vaporizing of solid surfaces and the diffusion behavior of double-layer islands on solid surface, and based on the information on single-atomic levels drawn from MD simulations, a thermodynamic model is set up to describe the time-evolution of surface growth on macroscopic scale.An important subject in surface science is the investigation of coarsening and decaying of adatom islands on clean solid surfaces, which is closely relevant to the stability of quantum dots and the growth mode on surfaces. A series of experimental observations indicate that the decay of a small island on top of a large one is accelerated by several orders of magnitude when it encounters the boundary of the larger island, and many theoretical works have devoted to explore the diffusion mechanism by calculating the potential barriers for various diffusion paths supposed artificially. In the present work, a canonical molecular dynamics (MD) model is set up and used to simulate the evolution of a double-layer Cu island with nearly the same conditions as those in the previous experiments, showing that the most probable event for atoms transport from the top layer to the lower one is two-atom exchange process taking place at the boundary of the lower island; comparing the probability for various decaying mechanisms drawn from our vast MD simulations with the corresponding potential barriers obtained by stationary mechanism calculations indicates that the stationary barrier calculation is insufficient to understand the diffusion behavior. In addition, the similar MD model is used to simulate Ag islands under the same simulation conditions, showing the top layer has a stronger tendency, compared to the Cu layer, to form a compact configuration, e.g., a hexagon, which slow the decay of the Ag top layer.MD method, limited by the simulation time and the number of atoms, can not be used to simulate the dynamics evolution of a macroscopic surface over long-time scales, and thermodynamic models have been developed to explore the morphological evolution of the macroscopic islands. However, these models need many empirical parameters and can not provide structural information on atomic scale. We developed a thermodynamic model with all the macroscopic parameters drawn from corresponding MD simulations, which can provide information on both the evolution of a macroscopic surface and the atomic structure in detail.A lots of experiments showed that the angular distribution of the plume produced by laser pulses depends on the input laser spot size. In previous theoretical studies, gas dynamics equations in hydrodynamics regime, such as an adiabatic model and an isothermal model, are used to described expansion of the plumes. It is notable that all these hydrodynamics models do not deal with the initial motion of the atoms to be vaporized just after the laser energy deposits on the target, and needs an artificial assume that a gas cloud forms in a definite shape, characterized by several spatial parameters. We developed a simplified molecular dynamical model for simulating ablation of solid surfaces by laser pulses, and specifically investigated expansion of Cu cloud in vacuum vaporized on the surface, showing that the angular distributions of the plume depends on the shape of laser spot on the surface. Although only 3375 atoms are contained in our MD model, the simulations give the same adiabatic constant as the experiment measured, and show the so called flip-over effect and the angular distribution broadening with laser spot decreasing, which have been observed in previous experiments. The coincidence between the simulation results and the experimental observations indicates that MD simulations with simplified models are helpful to understand the ablations processes at atomic level. If our MD model is modified further and larger computer systems are used in the simulations, the model may provide more important information on general ablation by pulsed laser or continued electronic beams.