Crystal growth is inherently an out-of-equilibrium process, but it remains surprising when the final crystal shape deviates significantly from equilibrium. A well-known example is water
forming snowflakes under atmospheric conditions. Similarly, the silicidation of nickel by heating the metal in a silane-bearing atmosphere at approximately 400°C also produces out-
of-equilibrium shapes, specifically high-aspect-ratio nanowires [1, 2]. While the growth of dendrites in snowflakes is well understood [3], the mechanism of formation of Ni-silicide
nanowires has remained a mystery. Here, using in-situ atomic-resolution environmental transmission electron microscopy, we demonstrate that the birth of such a nanowire follows an instability comparable to the nucleation of the first dendrite in a growing snowflake.
However, unlike snowflakes, where the instability is associated with the supersaturation gradient in the vicinity of the advancing surface [3], in the present case, it arises from the Ni-concentration dependence of the surface chemical reactivity and the finite nature of the Ni atom flux. The silicide starts growing in the form of a spherical amorphous particle; above a certain size, the particle surface cannot be fully covered by the available Ni flux; the surface ability for silane decomposition concentrates in a restricted region, which progressively
transforms into a crystalline nanowire. This way of creating a metastable shape appears quite different from the well-known dendritic growth.
1. C. A. Decker, R. Solanki, J. L. Freeouf, J. R. Carruthers and D. R. Evans, Directed growth of nickel silicide nanowires, Appl. Phys. Lett. 84, 1389-1391 (2004).
2. T. Le Duc, E. Moyen, M. R. Zamfir, J. Joe, X. Yan, Y. Zhang and D. Pribat, New schottky-type wire-based solar cell with NiSix nanowire contacts, ACS Applied Materials & Interfaces 12, 37464-37469 (2020).
3. J. S. Langer, Instabilities and pattern formation in crystal growth, Rev. Mod. Phys. 52, 1-28 (1980).