Coalescence in the Thermal Annealing of Nanoparticles: An in Situ STEM Study of the Growth Mechanisms of Ordered Pt–Fe Nanoparticles in a KCl Matrix

Abstract

Thermal annealing is essential for achieving ultrasmall size ferromagnetic properties in next-generation high performance nanocomposite magnetic materials. However, during the annealing process, growth and agglomeration of nanoparticles normally occurs, which destroys the narrow size distributions. Thus, the materials become less suitable for application in high-density magnetic recording. The mechanism of nanoparticle growth and sintering has been difficult to determine because of the lack of suitable in situ tools to probe subnanometer changes at the local level. Here we report a study using high-resolution scanning transmission electron microscopy (STEM) coupled with an in situ thermal annealing stage of surfactant-free, monodispersed superparamagnetic PtFe (cubic) alloy nanoparticles (≈2 nm in diameter) stabilized in or on a KCl matrix. Ex situ experiments confirmed that annealing produces PtFe (tetragonal) ordered intermetallic nanoparticles with a mean diameter of 5 nm, and the in situ study revealed that the mechanism of nanoparticle growth is dominated by particle–particle coalescence, although Ostwald ripening is also implicated in a few regions. In addition, to determine the time dependent evolution of the size distribution of an ensemble of over 400 nanoparticles, analysis of the in situ data also allows tracking of individual nanoparticles, distinguishing coalescence from Ostwald ripening, nanoparticle by nanoparticle. This approach has provided valuable insights into changes in crystal structure and sintering that occur during the thermal annealing of Pt–Fe nanoparticles.