Test tube trick

Physical changes in a nanoscale system can be accurately measured by restricting the reaction in a carbon 'nano test tube'.

Credit: © 2009 AAAS

It can be difficult to observe physical changes in a nanostructure, because when one varies environmental conditions such as temperature, the nanostructure will inevitably change size or shape. This introduces hard-to-measure changes in volume and surface area, and effectively destroys the original nanostructure. Now, Brian Korgel and colleagues at the University of Texas at Austin have found a solution — confining the reaction within a nanosize test tube¹.

The researchers deposited carbon on a germanium nanowire with a spherical gold nanocrystal attached to one end. This created a stiff carbon shell shaped like a test tube, with a bulb at the bottom surrounding the gold nanocrystal.

On heating the structure under a transmission electron microscope, the researchers observed the gold nanocrystal melting at the eutectic point — the lowest possible melting point of a gold–germanium mixture. The resulting gold–germanium melt mixture diffused up the neck of the nano test tube owing to capillary effects, with germanium recrystallizing in the bulb behind.

Interestingly, the rate of diffusion of gold in the nanowire was much slower than in bulk germanium, because the atoms move by different mechanisms. Therefore the nano test tube technique presents a unique controlled environment for learning why physical transformations are markedly different on the nanoscale.