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Dynamics and control of gold-encapped gallium arsenide nanowires imaged by 4D electron microscopy

  1. Ahmed H. Zewaila2
  1. aPhysical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125;
  2. bAustralian National Fabrication Facility, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia;
  3. cDepartment of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia
  1. Edited by Charles M. Lieber, Harvard University, Cambridge, MA, and approved October 26, 2017 (received for review May 26, 2017)

  1. Fig. 2.

    Images and diffraction patterns of the Au/GaAs NWs. (Top) Stroboscopic image of the NW and its diffraction pattern (marked circle in the image). The diffraction pattern indicates a hexagonal WZ structure of GaAs. (Middle) Image and diffraction pattern of the tip region. Additional diffraction spots appeared besides the same ones as that in Top. These extra spots are from the cubic ZB segments, which can be seen from an enlarged image in Middle Left, Inset. (Scale bar, 50 nm.) (Bottom) Images of the NW at different tilting angles ranging from ?10° to 50°. The dotted lines indicate the length measured from the tip to the reference point (marked by an arrow). At all angles, the NW is straight, suggesting that neither inclination nor breakage of the NW occurred after the laser pulse excitation.

  2. Fig. 3.

    Single-pulse imaging of transient changes of the NW and the calculated temperatures. (Upper) Single-pulse imaging of the NW length reduction due to eutectic-related reactions. The NW shrank along its axial direction, while its lateral width had no obvious change. Approximately 70% length reduction of the NW (without the top bead) was observed after a total of 46 laser shots. The rulers (white lines) indicate the NW length. The electric field polarization of the laser pulse was parallel to the long axis of the NW. (Lower Left) Volume change of the NW. The volume of GaAs NW reduces, while that of top bead increases. The total volume slightly decreases. (Lower Right) Calculated temperatures at different laser fluences. The temperature of Au or GaAs bead was estimated by assuming 100% of the component, while the effective temperature of the bead was calculated by considering the Au:GaAs ratio in the bead. The dotted line represents the lowest temperature to trigger the reaction. Because of the large latent heat of GaAs, the GaAs NW has not melted, although the temperatures (without taking account of the latent heat) are higher than its melting point (dashed line) at high fluences. The solid red line indicates the actual temperature of the GaAs NW when its latent heat is considered.

  3. Fig. 4.

    Time-resolved dynamics of the NW. (Upper) Normalized diffraction intensity vs. delay time, indicating cooling dynamics with a time constant of 123 ± 12 ns. The fitting is shown in the red line. (Lower) Single-pulse images of the NW at a specific delay time. The images in Lower Left, Center, and RIght correspond to the states of the NW before, at specific delays, and after the process ended (the incident laser pulse has been removed), respectively. Three rows of images are associated with the NW states at the delay time of 20, 80, and 100 ns, respectively. Besides the length shrinkage, the transient shape changes of the top bead (marked with a circle) at different delays were also captured. The white rulers indicate the NW length. For easy comparison, the dotted yellow lines are the top reference lines in the initial states before laser pulse excitation (first column images). The electric field of the pump laser pulse was parallel to the long axis of the NW.

  4. Fig. 5.

    Polarization-dependent dynamics. (A) Electron micrographs of a NW before and after a laser pulse whose E-field polarization is parallel to the long axis of the NW. (Scale bar, 100 nm.) (B) Electron micrographs of a NW before and after a laser pulse, but with a perpendicular E-field polarization. (Scale bar, 100 nm.) A and B, Center and Right correspond to the images at different laser fluences (unit is mJ/cm2). The circles indicate the morphological change of the top bead. (C) Length reduction of the GaAs NWs at different polarization directions. For each polarization, the laser fluence is 13 mJ/cm2. The length reduction was determined from the NW length change after each laser shot. (C, Inset) Schematic diagram showing a NW excited by a laser pulse with its E-field polarization parallel or perpendicular to the long axis of the NW.

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