Atomic Step Flow on a Nanofacet

Crystal growth often proceeds by atomic step flow. When the surface area available for growth is limited, the nucleation and progression of the steps can be affected. This issue is particularly relevant to the formation of nanocrystals. We examine the case of Au-catalyzed GaAs nanowires, which we grow in a transmission electron microscope. Our in situ observations show that atomic layers nucleate at the periphery of the interface between the nanowire and the catalyst droplet. From this starting location, the atomic step flows within a restricted area of hexagonal shape. At specific partial coverages, the monolayer configuration changes abruptly. A simple model based on the geometry of the system and its edge energies explains these observations. In particular, we observe an inversion of the step curvature which reveals that the effective energy per unit length of monolayer edge is much lower at the interface periphery than inside the catalyst droplet.

Figure 1

Images from Video SV1 [14] showing the flow of a single ML step (arrows) during the growth of a GaAs NW with wurtzite structure. Scale bar, 5 nm.

Figure 2

Left: Bird’s eye views of a GaAs ML progressing at the liquid-solid interface, showing different successive configurations of the step edge (images extracted from Video SV2 [14]). The contour of the growing layer is schematically reproduced above the interface in color: red for the NW periphery and yellow and blue for step edges along the $⟨10\overline{1}0⟩$ and $⟨11\overline{2}0⟩$ directions. Right: Normal projection of each configuration. Note the change of concavity at high coverage. Scale bar, 10 nm.

Figure 3

ML coverage as a function of time for the four MLs of Video SV2 [14]. The concavity of each partial ML changes at coverages within the hatched zone.

Figure 4

Variation with ML coverage of the total energies of the various step configurations predicted to occur (and schematized at the bottom), calculated with optimized step energies. Dashed lines mark the shifts between configurations and arrows the ranges of coverage where these are experimentally observed. The successive configurations involve increasing numbers of hexagon sides bordered by the ML (indicated at top), except the first two.