Research ArticlePHYSICAL SCIENCES

Photoinduced nanobubble-driven superfast diffusion of nanoparticles imaged by 4D electron microscopy

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Science Advances  25 Aug 2017:
Vol. 3, no. 8, e1701160
DOI: 10.1126/sciadv.1701160
  • 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).

  • Fig. 2 Typical snapshots and trajectories of photon-activated gold NP diffusion in liquid.

    (A) Typical snapshots of a gold NP diffusion under 1-kHz laser pulse (fluence of 2.3 mJ/cm2) excitation at the different times. The NP was driven to move by rapid nucleation, expansion, detachment, and collapse of the photoinduced steam NBs near the particle surface (see the circles with white contrast). (B and C) Two typical trajectories (left column) of the gold NP diffusion and the corresponding displacement distributions along x and y (right column) at different laser fluences of 2.0 and 2.3 mJ/cm2, respectively. The dashed black lines in the right column of (B) and (C) show the Gaussian fit, which indicate that the NP translates in a manner of random walk.

  • Fig. 3 Laser fluence and repetition rate dependence of the gold NP diffusion dynamics.

    (A) MSDs of the gold NP diffusion under different laser fluences (repetition rate of 1.0 kHz). (B) Variation of the diffusion constant of the photon-activated NP as a function of laser fluence, which follows a power-law dependence with a retrieved threshold fluence for explosive boiling of Jc ~ 1.25 mJ/cm2. Inset: Schematic of superfast diffusion (random walk) of a gold NP driven by the photoinduced steam NBs. (C) MSDs of a gold NP diffusion under a fixed laser fluence of 3.2 mJ/cm2 with different repetition rates. (D) Variation of the diffusion constant as a function of the laser pulse repetition rate, which follows a linear dependence.

  • Fig. 4 Single-pulse imaging of a photon-activated gold NP in the ballistic regime.

    (A) Single-pulse images of the photon-activated NP at three specific delay times. The images in the first and second columns are the states of the NP before the laser pulse and at specific delays of 25, 40, and 60 ns after the laser pulse (fluence of 7.8 mJ/cm2), respectively. The third column shows the corresponding difference images obtained by subtracting the single-pulse image before the laser pulse from that at the short delay time. The red and blue dashed circles indicate the NP positions before the laser pulse and at short delays after the laser pulse, respectively. (B) Evolution of the displacement of the NP as a function of delay time with a physical model fit. The inset shows the image of the measured gold NP.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/8/e1701160/DC1

    Supplementary Text

    movie S1. Translation of a gold NP induced by repetitive femtosecond laser pulse excitation (fluence of 2.3 mJ/cm2; repetition rate of 1 kHz) under continuous electron beam imaging mode.

  • Supplementary Materials

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.avi format). Translation of a gold NP induced by repetitive femtosecond laser pulse excitation (fluence of 2.3 mJ/cm2; repetition rate of 1 kHz) under continuous electron beam imaging mode.

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