Structural Fluctuation in Alloy Nanoparticles Revealed by Time-Resolved High-Voltage Electron Microscopy

Kazuhisa Sato and Hidehiro Yasuda

Abstract: The development of long-range order in disordered Co-Pt alloy nanoparticles has been atomically resolved in situ with an ultra-high voltage electron microscope equipped with a direct electron detection camera. Electron-irradiation-enhanced ordering occured at 573 K with 1 MeV electrons at a dose rate of 8.9 × 10²⁴ e/m²s. High-speed (400 frames/s) imaging revealed fluctuations of the c-axis orientation of the L1₀–type ordered structure. Specifically, the c-axis orientation changes occurred at 2.5-msec intervals. Thus, the atomic ordering rate at 573 K is deduced to be 3 × 10^-17 m²/s, which is 10¹³ times higher than that estimated for interdiffusion in a bulk Co-Pt alloy. The observed kinetic ordering temperature of 573 K is significantly lower than that reported previously (> 800 K). The low-temperature ordering may be the result of enhanced atom migration via excess vacancies, 10⁶ times higher than that at thermal equilibrium, introduced by the high-energy electron irradiation. We have demonstrated that time-resolved electron imaging can directly reveal rapid spatiotemporal fluctuations.

Key words: High-voltage electron microscopy, Direct electron detection camera, Time-resolved imaging, Ultrafast in situ observation, Radiation-enhanced ordering