Coalescence and sintering in metallic nanoparticles : in-situ transmission electron microscopy (TEM) study
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Nanoparticles possess unique physical, chemical, optical and electronic properties stemming from their nanoscale dimensions and are currently used in catalysis, microelectronics, drug delivery, as well as other applications. However, due to their large surface area-to-volume ratio, nanoparticles have a strong tendency to coalesce and sinter during processing or usage over short time scales and at low temperatures, which lead to significant changes in behavior and performance. In this work, in-situ transmission electron microscopy (TEM) heating has been used to investigate the effects of particle size, temperature and carbon capping layers on sintering in face-centered cubic (FCC) metallic nanoparticles. For the first time, we make direct and real-time measurements of nanoparticle size, neck growth, dihedral angle and grain boundary motion during sintering, which are then used to calculate fundamental material transport parameters such as surface diffusivity and grain boundary mobility. We observe that carbon surface coatings typically present on most commercial nanoparticles can significantly inhibit sintering in nanoparticles. Also, a new mechanism for coalescence in nanoparticles is shown where small clusters on the support can initiate neck growth by forming a bridge between the nanoparticles consisting of individual atoms or small clusters of atoms. In-situ TEM experiments provide critical and valuable real-time dynamic information for direct investigation of the link between the evolution of sintering and controlling mechanisms, which conventional experiments such as post-mortem TEM observations are not capable of conveying.