We have observed the formation of nanoscale Si spikes atop mesoscopic Si pillars during laser irradiation at intensities sufficiently high to melt silicon. The spikes have a radius on the order of 100 nm (<50 nm at the tip) and up to several micrometres in length. Nanoscale clusters formed during laser ablation deposit on the surface of the melt and nucleate the solidification of a mantle. The spikes are extruded through a solid mantle that applies pressure to a molten core as the liquid fuses. The growth mechanism is shown to be the same as the mechanism for formation of centimetre-scale ice spikes formed from freezing pure water. This mechanism of spike formation scales from the macroscopic to the nanoscopic regime and should be completely general and applicable to any material that expands upon freezing.
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