Temperature-Driven Morphological and Microstructural Changes of Gold Nanoparticles Prepared by Aggregation from the Gas Phase

The effect of annealing on the thin layers of gold and gold nanoparticles in air was studied by statistically relevant X-ray scattering methods. The nanoparticle behavior is found to depend on the substrate coverage and annealing temperature. During annealing up to 450 °C, the size of single-crystalline nanoparticles gradually increases through the process of Ostwald ripening, while the density of crystallographic defects decreases slightly. An abrupt change occurs above 450 °C, whereas no significant evolution is observed for the less covered sample; at the sample with more material, the nanoparticles coalesce, and their shape becomes more rounded by further annealing. Only after the spheroidization is completed do the sizes of crystallites follow the nanoparticle size growth. Comparison with the thin continuous gold layer shows that the healing of the crystallographic defects, i.e., microstrain and stacking faults, takes place at significantly lower temperatures if the material is evenly distributed on the silicon substrate surface. However, annealed nanoparticle layers provide a much narrower particle size distribution when compared to a dewetted gold thin layer. At around 800 °C, the alignment of the gold crystal structure toward the substrate is detected, and it changes from the random distribution of the atomic planes given by the random initial orientation of deposited nanoparticles. Another interesting phenomenon occurs for annealing above 1000 °C; for the nanoparticle layers, the smallest nanoparticles evaporate, leaving holes in the SiO₂ surface layer.