Mechanistic Studies on Sintering of Silver Nanoparticles

Metal nanoparticles are known to form highly conductive films upon heating below 200 °C. We study the mechanism and morphological changes that occur as 3 nm, thiolate encapsulated, silver nanoparticles are annealed to form conductive films. We use X-ray diffraction (XRD), grazing incidence X-ray scattering (GIXS), and transmission electron microscopy (TEM) to monitor structural changes in the film. We show that the surfactant is present during the entire sintering process, that its presence greatly influences the grain size and crystallite orientation of the resulting film, and that the film becomes fully conductive in the presence of the surfactant. We show that particles that aggregate more rapidly form films that consist of smaller crystallites and are less textured. We further show that digestive ripening can lead to the degeneration of films back into particles, particularly when annealing in air versus an inert environment. Coalescence contributes to crystallite growth when particles are small but can confound both crystallite size and orientation development in the later stages of growth. The interaction of the surfactant with the particle is weakened by moisture, lowering the temperature at which the surfactant disassociates from the particle and sintering begins. Moisture also increases the rate of both aggregation and digestion, drastically changing the morphology of the films at any given temperature.