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surface science / reaction dynamics / laser photochemistry / laser-surface interactions / nanotechnology / physical chemistry / chemical physics

Solidification Driven Extrusion (Nanospikes)Germanium Nanospikes

Dave Mills, who now spends some of his time reading old scrolls with X-rays, noted that in some of the scanning electron microscopy (SEM) images that he took of our laser ablation created silicon pillars, that a little spike formed on the tip of the pillar. He also thought that these might be related to ice spikes. You can get ice spikes to form in your freezer simply by freezing pure distilled water. A major question in nanoscience is: what techniques to form structures scale from the macroscopic (for instance centimeter) range into the nanoscale regime? We showed that the physics behind ice spike formation scales down to the nanoscale. It works for silicon pillars just like it works for ice cubes.

In subsequent work with germanium performed with Barada Nayak and Mool Gupta, we found that nanospikes also form on germanium pillars, as shown on the right.

So what do water, silicon and germanium all have in common? They all form tetrahedrally co-ordinated solids phases and all of them expand when they freeze. This is an unusual property but that is what makes ice less dense than liquid water and why ice cubes float. It also means that if the surface of liquid water, silicon or germanium freezes that it compresses the liquid that is trapped inside. This leads to a build up of pressure. Eventually the pressure is too much and a jet of supercooled liquid shoots out and forms the ice spike.

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