Kinetically-induced hexagonality in chemically grown silicon nanowires
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Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically synthesized nanowires. The structures are typified by mixed stacking mismatches of closely packed Si dimers. Instead of viewing them as defects, we define the concept of hexagonality and describe these structures as Si polymorphs. The small transverse dimensions of a nanowire make this approach meaningful. Unique among the polymorphs are cubic symmetry diamond and hexagonal symmetry wurtzite structures. Electron diffraction studies conducted with Au as an internal reference unambiguously confirm the existence of the hexagonal symmetry Si nanowires.
Cohesive energy calculations suggest that the wurtzite polymorph is the least stable and the diamond polymorph is the most stable. Cohesive energies of intermediate polymorphs follow a linear trend with respect to their structural hexagonality. We identify the driving force in the polymorph formations as the growth kinetics. Fast longitudinal elongation during the growth freezes stacking mismatches and thus leads to a variety of Si polymorphs. The results are expected to shed new light on the importance of growth kinetics in nanomaterial syntheses and may open up ways to produce structures that are uncommon in bulk materials.
KeywordsSilicon nanowires hexagonality polytypes cohesive energy chemical vapor deposition kinetics
- Raffy, C.; Furthmüller, J.; Bechstedt, F. Properties of hexagonal polytypes of group-IV elements from first-principles calculations. Phys. Rev. B: Condens. Matter 2002, 66, 075201.Google Scholar
- Zhang, D. B.; Hua, M.; Dumitrică, T. Stability of polycrystalline and wurtzite Si nanowires via symmetry-adapted tight-binding objective molecular dynamics. J. Chem. Phys. 2008, 128, 084104.Google Scholar
- Arbiol, J.; Morral, A. F. I.; Estradé, S.; Peiró, F.; Kalache, B.; Cabarrocas, P. R. I.; Morante, J. R. Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires. J. Appl. Phys. 2008, 104, 064312.Google Scholar
- Arbiol, J.; Kalache, B.; Cabarrocas, P. R. I.; Morante, J. R.; Morral, A. F. I. Influence of Cu as a catalyst on the properties of silicon nanowires synthesized by the vapor-solid-solid mechanism. Nanotechnology 2007, 18, 305606.Google Scholar
- Parthé, E. Crystal Chemistry of Tetrahedral Structures; Taylor & Francis, US, 1964.Google Scholar
- Glas, F. A simple calculation of energy changes upon stacking fault formation or local crystalline phase transition in semiconductors. J. Appl. Phys. 2008, 104, 093520.Google Scholar