Carbon nanotube arrays could be the basis of high-density energy storage devices and efficient chip cooling systems. The performance of such devices, however, depends on the quality of the nanotubes and the precise structure of the array. So researchers including Anastasios John Hart, assistant professor of mechanical engineering at the University of Michigan, are honing techniques for growing carefully structured forests of high-quality carbon nanotubes. Hart made these images with a scanning electron microscope; all show vertically grown nanotubes.
This is a composite of many images of carbon nanotubes grown on silicon wafers or in cavities etched in the wafers. Each stalklike structure is made up of thousands of nanotubes or more. The catalyst that starts off the nanotubes’ growth is visible under some of them as a dark, shadowlike spot. Structures that appear withered were dipped in liquid after they grew; as the liquid evaporated, the nanotubes shriveled.
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.