The intricate pattern above is made of carbon nanotubes grown on a silicon wafer patterned with a catalyst. The wafer is placed inside a hot chamber that’s then filled with ethylene or another carbon-containing gas. On the parts of the wafer coated with the catalyst, the pure tubes of carbon shoot up at great speed; a tree developing at an equivalent rate, Hart says, would be growing at 500 miles per hour.In this greatly magnified image, small groups of nanotubes, each tube only 5 to 10 nanometers in diameter, can be seen bridging cracks in the structure.
Intramolecular forces cause carbon nanotubes to stick to each other. As the nanotubes shoot up, they may tug on their neighbors, speeding their growth. But if reaction conditions aren’t optimal–if too much or too little of the catalyst is activated, for example–this stickiness (among other factors) may cause the nanotubes to form tangles, curlicues, fault lines and other structures.By exploiting these different tendencies, Hart can make more complex structures like the curved “fingers” above, which might be used as sensing probes.