Tricking Nature

by Michael Mullaney on April 2, 2009

Here’s a Thursday afternoon conundrum: How do you get nature to do something it doesn’t necessarily want to do?

Simple: You trick it.

That’s how innovative (and now rich) Japanese farmers developed square watermelons. And that’s what professor G. Ramanath did to coax nanoparticles to grow into new shapes.

Put yourself in Ramanath’s shoes – You have a nanomaterial that naturally occurs as a cube, but you would really like to be able to grow the cube into a long, thin rod. The problem is, whenever you add atoms to the cube, it just grows symmetrically on all six sides – i.e. it just becomes a bigger cube.

Ramanath and his research team thought through the problem and decided to tackle it head-on. They needed to be trickier than the cubes were stubborn.

The cubes, made of lead telluride (PbTe), are important because the material has attractive properties. PbTe conducts electricity well, but conducts heat poorly, making it a good candidate for use in all sorts of interesting nanodevices, such as a device that harvests electrical energy from waste heat.

Ramanath thought that if he could make long, hollow PbTe nanorods, the new shape would enhance the material’s attractive properties, and in turn be a base on which researchers could create entire new device architectures. To use a mining allegory, developing a PbTe nanorod with enhanced properties would be like strking gold.

One previously explored method of making these nanorods was building nanoscale templates – very tiny molds – and using an inorganic mix of liquids and solids to prompt a chemical reaction that would fill the templates with lead telluride. A very good idea, Ramanath said, but it had a serious shortcoming – there was no good way to remove the nanorods from their molds without compromising their properties. Leaving the rods in their mold wouldn’t work, as it negatively affected the rods’ conductivity.

Ramanath’s research team took a completely different approach – the idea for which he credits to his former post-doc Arup Purkayastha.

They started with tellurium nanotubes, which are fairly easy to grow, and are already in the long cylindrical shape Ramanath was seeking. The team used these nanotubes as a template, and then treated them with lead salt.

The lead salt prompted a chemical reaction that transformed the tellurium nanotubes into long, hollow, single-crystal lead telluride nanorods – exactly what Ramanath was looking for.

So that’s how one goes about tricking tricked Mother Nature. Instead of persuading a stubborn cube to go against its wishes and grow into a rod, Ramanath’s team simply found new ingredients and a new method for growing rods that are the same exact material as the stubborn cubes.

Inquisitive materials experts can check out the full research results, which were published last summer in the Chemistry of Materials.

Below are two scanning electron microscopy (SEM) images showing the tellurium nanotubes that serve as consumable molds (left), and the newly realized lead telluride nanorods obtained by converting the nanotubes using a lead salt (right).


{ 1 comment }

Kelly 06.16.09 at 1:27 am

I love this article. It takes the very complicated research topic and scale it down to a level that the average Joe can understand.
Thank you for giving me a quick look into one area of research our campus is housing.

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