Fig. 1. Schematic of photon-activated gold NP diffusion in liquid imaged by 4D-EM.

Infrared femtosecond laser pulses (1040 nm; pulse duration of 350 fs) were used to generate green femtosecond laser pulses (520 nm) via second harmonic generation, which were directed to the liquid cell integrated in the transmission electron microscope (TEM) to trigger the particle dynamics in liquid. Ultraviolet (UV) nanosecond laser pulses (266 nm; pulse duration of 10 ns) were directed to the photocathode inside the TEM to generate electron pulses, which were accelerated to 120 keV to image the particle dynamics. The femtosecond laser pump pulse and nanosecond electron probe pulse were synchronized by a digital delay generator, which controls the time delay between them. For the time-resolved imaging at long times, a continuous thermal emission electron beam with a low electron dose was present to image the NP diffusion under repetitive femtosecond laser pulse excitation, whereas for the time-resolved imaging at short time scale, a single femtosecond laser pulse triggered the NP motion and a precisely timed electron pulse was used to image the transient NP morphologies at specific time delays, where the time resolution was unlimited by the response of the charge-coupled device (CCD).