Thermal Stability And Diffusion Mechanisms Of Au-Ag And C₆₀-Au Nanoparticales

The thermal stability and surface diffusion mechanism of nanoparticles play a critical role in their applications and the fabrication of devices.However,it is still difficult for the experiments to understand the structural evolution of nanoparticles at high temperature and the diffusion process at atomic level.Moreover,coalescence of nanoparticles is a general method of bottom-up fabrication of micro-and nanoscale electronic devices.Due to the decreasing size of devices,it is very important to understand the mechanism of the coalescence at atomic level.In this dissertation,we investigate the melting behavior and surface diffusion process of nanoparticles by means of molecular dynamics simulation at atomic level.We also study the coalescence behavior of nanoparticles at the oil-water interfaces.The conclusions include the following points:1. The core size and shell thickness on the thermal stabilities of single Au@Ag core-shell nanoparticles（CSNPs）are systematically investigated by means of all-atom molecular dynamics simulations.For Au@Ag CSNPs with the same core size,their melting points increase with increasing shell thickness.If we keep the shell thickness unchanged,the melting points increase as the core sizes increase except for the CSNP with the smallest core size and a bilayer Ag shell.Besides,the thermal stability of two-dimensional close-packed Au@Ag CSNPs arrays has also been explored for the first time.the melting point of Au@Ag CSNP array is much lower than the corresponding single NP.For both the single NPs and their arrays,the premelting processes start from the surface region.This study provides a valuable reference for the application of Au-@Ag core-shell nanoparticles.2. With the help of molecular dynamics simulation,the diffusion mechanism of mass migration of the C₆₀-Au clusters obsevered at room temperature using scanning tunneling microscopy is studied.It is found that the mass migration is caused by thermal energy fluctuations that is important for small systems such as a nano-cluster.A close-packed C₆₀-Au cluster with a regular geometric shape can transfer into a liquid cluster due to a sudden injection of thermal energy.The liquid cluster then moves to a different location where it releases the extra energy and reassembles back into its initial structure.These results provide an idea for the assembly and regulation of nanoparticles on solid surfaces.3. The attachment and coalescence processes of Au@Ag CSNP and Ag NP at oil-water interface are investigated by means of all-atom molecular dynamics simulations.By analyzing the difficulty for capping ligands desorbing from nanoparticle’s surface,the evolution of numbers of the contacted NP-pairs,the shrinkage of contacted NP-pairs,and structural order of the close-packed part of contacted NPs,we found that:a)capping ligands are easier to desorb from Au@Ag CSNP’s surface;b)the number of contacted CSNPs pairs and the shrinkage of contacted CSNPs pairs are larger;c)the close-packed Au@Ag CSNPs array is more orderly than Ag NPs array after attachment.These results reveal that Au@Ag CSNPs prefer to form a continuing close-packed array at a lower temperature,which is in accordance with the experimental observation.To our knowledge,the stability and diffusion behavior of nanoparticles capped by ligands molecules in solvent are investigated for the first time using all-atom MD simulation,which establishes the foundation to study the kinetic behavior of NPs in solvent.