The role of NiOx overlayers on spontaneous growth of NiSix nanowires from Ni seed layers.

We report a controllably reproducible and spontaneous growth of single-crystalline NiSix nanowires using NiOx/Ni seed layers during SiH4 chemical vapor deposition (CVD). We provide evidence that upon the reactions of SiH4 (vapor)-Ni seed layers (solid), the presence of the NiOx overlayer on Ni seed layers plays the key role to promote the spontaneous one-dimensional growth of NiSix single crystals without employing catalytic nanocrystals. Specifically, the spontaneous nanowire formation on the NiOx overlayer is understood within the frame of the SiH4 vapor-phase reaction with out-diffused Ni from the Ni underlayers, where the Ni diffusion is controlled by the NiOx overlayers for the limited nucleation. We show that single-crystalline NiSix nanowires by this self-organized fashion in our synthesis display a narrow diameter distribution, and their average length is set by the thickness of the Ni seed layers. We argue that our simple CVD method employing the bilayers of transition metal and their oxides as the seed layers can provide implication as the general synthetic route for the spontaneous growth of metal-silicide nanowires in large scales.