Carbon nanotubes, despite all the technological advances they’re making possible, look pretty boring. When viewed though a microscope, they are, essentially, just straight tubes. Now scientists from the University of Michigan have used a process called “capillary forming” to create nanotubes that resemble twisting spires, concentric rings, and bending petals. It's not about aesthetics though, giving nanotubes complex 3D shapes is seen as an important breakthrough in the development of microdevices and nanomaterials.
“It’s easy to make carbon nanotubes straight and vertical like buildings,” said U Michigan mechanical engineer Prof. A. John Hart. “It hasn’t been possible to make them into more complex shapes. Assembling nanostructures into three-dimensional shapes is one of the major goals of nanotechnology and nanomanufacturing. The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing.”
The capillary forming process begins with pieces of thin metal film, which serve as both catalyst and template, being laid down on a silicon wafer. These pieces of film have various shapes, which will determine the form of the finished nanotube structures.
Next, the wafer and the film are heated in a high temperature furnace that contains a hydrocarbon gas mixture. In a chemical vapor deposition process, the gas reacts with the film/catalyst, causing the carbon from the gas to form into nanotubes, which grow upwards from the film.
After that, the wafer and nanotubes are suspended over a beaker of boiling acetone. As the acetone condenses on the nanotubes, it travels up into the spaces between them via capillary action, and causes them to bend and twist into three-dimensional structures.
“We program the formation of 3D shapes with these 2D patterns,” Hart said. “We’ve discovered that the starting shape influences how the capillary forces manipulate the nanotubes in a very specific way. Some bend, others twist, and we can combine them any way we want.”
Large batches of the 3D nanotube structures can be produced at once, and due to the fact that they are very stiff, they could be used as molds for creating duplicate structures using other materials.
Hart believes that they could make many new microdevices and materials possible, such as probes that can interface with individual cells, novel microfluidic devices, and lightweight materials for aircraft and spacecraft.
The research was recently published in the journal Advanced Materials.