Quasi two-dimensional (2D) semiconductor materials are desirable for electronic, photonic, and energy conversion applications and have stimulated extensive research on their synthesis^1-3 and applications^4-6.  One such example is the growth of indium phosphide flag-like nanostructures (Fig. 1) by epitaxial growth on a nanowire template⁷. By the intentional incorporation of defects, control of the nano-flag location along the nanowire was achieved. Long InP nanowires with inherent high defect density in the top area of the flagpole, which pins the catalyst, resulted in top flag geometry (Fig. 1a). Intentional defect creation in the middle section of the flagpole produced middle nanoflags (Fig. 1b). Bottom flags were created by growing defect free primary nanowires under high phosphine flow (Fig. 1c).

Detailed investigation, using aberration corrected high resolution TEM (FEI Titan 80-300), high angle annular dark field STEM and electron diffraction, of the flag-like nano-structures at different stages of the growth provided valuable details about the growth mechanism and the resulting morphology of such nano-structures.

The main conclusion from the TEM observations is that the growth process involves a few stages. The first stage consists of asymmetrical dissolution of the nanowire (NW) tip into the catalyst (Fig. 2a), followed by the Au catalyst unpinning from the top of the nanowire, and its induced migration along the nanowire sidewall. The final position of the catalyst is determined by the position of structural defects (such as stacking faults – Fig. 2b) along the NW (i.e. the “flagpole”). The next stage involves the epitaxial growth of a pure Wurtzite nano-flag on one of the facets of the “flagpole” resulting in a two dimensional nano-membrane on a one dimensional NW sidewall template (Fig. 2c-d).

The comprehensive understanding of the growth process may help improve the synthesis of similar nano-structures. It may allow for better control on the growth process of more complex structures and also help achieving similar nanostructures in a variety of III-V semiconductor material systems with potential applications such as active nanophotonics.

References:

1. Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415 (February), 617–620.
2. Algra, R. E.; Verheijen, M. a; Borgström, M. T.; Feiner, L.-F.; Immink, G.; van Enckevort, W. J. P.; Vlieg, E.; Bakkers, E. P. a M. Nature 2008, 456 (7220), 369–372.
3. Gorji Ghalamestani, S.; Ek, M.; Ganjipour, B.; Thelander, C.; Johansson, J.; Caroff, P.; Dick, K. a. Nano Letters 2012, 12 (111), 4914–4919.
4. Tomioka, K.; Tanaka, T.; Hara, S.; Hiruma, K.; Fukui, T. IEEE J. Sel. Top. Quantum Electron. 2011, 17 (4), 1112–1129.
5. Nakayama, Y.; Pauzauskie, P. J.; Radenovic, A.; Onorato, R. M.; Saykally, R. J.; Liphardt, J.; Yang, P. Nature 2007, 447 (June), 1098–1101.
6. Mourik, V.; Zuo, K.; Frolov, S. M.; Plissard, S. R.; Bakkers, E. P. a. M.; Kouwenhoven, L. P. Science (80-. ). 2012, 336, 1003.
7. A. Kelrich, O. Sorias, Y. Calahorra, Y. Kauffmann, R. Gladstone, S. Cohen, M. Orenstein and D. Ritter, Nano Letters 2016, 10.1021/acs.nanolett.6b00648.

Figures:

Figure 1: Control of the nano-flag location along the nanowire by the intentional incorporation of defects. (a-c) are SEM micrographs of InP nanoflag arrays. (a) Top flag geometry. (b) Middle nanoflags. (c) Bottom flags. (d) HAADF-STEM image of a single InP middle flag demonstrating the catalyst is anchored to the polytypic segment. (e) TEM image demonstrating the transitions between the polytypic segment and pure Wurtzite parts of the nanoflag (the arrow indicates the growth direction of the flagpole).

Figure 2: HR-TEM micrographs showing the nano-flag growth process. (a) Asymmetrical dissolution of the nanowire tip into the catalyst. (b) Final position of the catalyst is determined by the position of the stacking faults (marked by the arrow). (c) A pure Wurtzite nano-membrane is produced with the Au tip at the edge with a single stacking fault running across. (d) A micrograph showing the resulting structure in case of the presence of multiple stacking faults.

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