| In this dissertation, both nanostructured silver (Ag) and gold (Au) aggregates (islands) are fabricated on silicone oil surfaces by thermal evaporation method. By optical microscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM), we investigate the aggregation mechanism, topological structrure, crystallinity, morphology and microstructure evolutions of the aggregates systematically. The main results are listed below:a) The morphology and microstructure evolutions of the Ag and Au aggregates on silicone oil surfaces are measured and studied by optical microscopy. After the samples are removed out from the vacuum chamber, the apparent Ag surface coverage of the total area, namely p, decays with the magnitude up to (23.0±3.8)%in few minutes. In the following two hours, several unexpected results are detected:1) as the topological structure of the aggregates evolves, the total area of the aggregates decreases gradually and the maximum decrement measured is around20%;2) if an aggregate breaks and becomes two small pieces, the total area may decrease obviously;3) however, if two small aggregates meet and stick together, a sudden increment of the total area may be observed. The findings shown above indicate that the microstructure of the Ag aggregates may evolve automatically in the ambient atmosphere after they are disturbed by the air filling process and the atmosphere, In Au system, however, similar phenomena above have not been observed obviously in our experimental time scale.b) The AFM measurement reveals that the average height of the Ag and Au aggregates, namely HAFM, increases with nominal thickness d. However, the average diameters of the granules (φg) inside the Ag and Au aggregates remain almost unchanged as d increases. The above phenomena imply the geometrical shape of both the Ag and Au granules transferred gradually from plateau to sphere. The possible reason is that the interaction between the deposited atoms and the oil molecules is reduced as the oil surface is heated by the radiation from evaporation source during deposition. By dynamic scaling analysis, the roughness exponent α of the Ag aggregates are in the range of0.8～0.9, which is related to the boundary diffusion of the granules. By double logarithmic plot of the root mean square roughness of the Ag aggregates wrms vs d, it is found that wrms is linearly dependent of d for d＜4.0nm and thus the growth exponent β=0.48±0.05. However, wrms-d deviates obviously from this dependence as d further increases. The above anomalous kinetic behavior are determined by the balance between the shadowing and reemission processes happen on the surface of Ag granules during deposition. For the Au aggregates, the roughness exponent a decays from0.9to0.6for0.5nm≤d≤15.0nm. The dependence between Wrms and d in the double logarithmic diagram fits the linear relation well in the experimental thickness scale (d＜15.0nm) and then the growth exponent is0.53±0.05, which is mainly determined by the shadowing process.c) In the Ag system, the surface coverage of the aggregates, i.e. p, decreases gradually from (9.6±0.1)%to (6.5±0.3)%as Ts increases from285K to353K, which is resulted from the increase of the HAFM of the aggregates. The above finding suggests that the preferred growth mode for Ag aggregates transfers from two dimensional (2D) to three dimensional (3D) as Ts goes up. However, compared to the Ag system, the surface coverage for the Au system is approximately independent with Ts in the range from285K to353K. This result is confirmed by the AFM observation: the average width of the Au aggregates increases obviously while the average height HAFM remains almost unchanged. The above phenomena imply that the Au aggregates show higher structural stability than that of the Ag aggregates due to the discrepancy of their interfacial energies on the oil surface.d) By TEM experiment, it is found that the microstructure of the Ag aggregates is amorphous while the Au aggregates exhibit polycrystalline structure for Ts=285K. As Ts goes up to303K, the Ag aggregates transfer to polycrystalline structure and the crystallinity is improved as Ts further increases to333K. However, the crystallinity of the Au aggregates is approximately independent of Ts in the experimental temperature scale (285K＜Ts＜353K). The results above are also confirmed in another experiment:we find that the mean diameter of the crystal grains in the Ag aggregates decreases from12.8±1.2nm to6.5±1.4nm as Ts goes up from303K to333K while the corresponding values of the Au grains are6.4±1.7nm and8.7±1.5nm with Ts=285K and343K, respectively.The contents of the dissertation are organized as follows:In chapter1, we present a review of the growth of films and atomic aggregates (islands) on various substrates, and then the recent studies on the metallic nanoparticles are emphasized.In chapter2, both the aggregation mechanism and the perturbation induced shape transition processes of the Ag and Au aggregates are studied in real time by optical microscopy, and then the microstructure evolution behaviors of the aggregates are mirrored.In chapter3, the preferred growth mode of the Ag aggregates and its dependent behavior with the nominal thickness and substrate temperature are systematically studied by AFM. The anomalous kinetic roughening of the Ag aggregates is obtained based on the analyses of the AFM images. The crystal growth of the Ag aggregates and its dependence with substrate temperature Ts are investigated by TEM.In chapter4, we present the study on the preferred structures of the Au aggregates. By dynamic scaling analyses, the detailed kinetic process of Au aggregates is established, and then the crystal structure of the Au aggregates and its dependent manner on Ts is investigated with TEM measurement.In chapter5, based on the experimental findings and theoretical analyses, main conclusions, prospects as well as proposals are presented. |
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