WEST LAFAYETTE, Ind. Nanotubes measuring just 100 atoms in diameter have been created from designer molecules that were customized to self-assemble into angstrom-sized circuit elements, according to researchers at Purdue University.
Professor Hicham Fenniri's research group developed the nanotube "parent" molecules, which self-assemble in water first into tiny rings. The rings then snap together into long tubes. The outside of the seed molecules harbor "hooks" on which to hang other molecules, which functionalize the resulting nanotube for a specific electronic application.
Thus far, Fenniri has demonstrated two parent molecules: one that grows conventional wires, for electricity, and one for growing photonic devices that process light. Electronic components are next on his list.
"There are several electronic applications [for nanotubes], but there are synthesis issues about how to control the chemical properties," said Fenniri. By controlling the nanotubes' chemical synthesis, the researchers hope to tailor them for applications in electronics.
Fenniri was assisted by fellow Purdue researchers Bo-Liang Deng, Alexander Ribbe and Klaas Hallenga, and by Argonne National Laboratory researchers Jaby Jacob and Pappannan Thiyagarajan.
The devices are called rosette nanotubes by Fenniri, who has applied to patent them as a new class of self-assembling organic structures. Their shape permits a hollow central interior channel that runs the length of the nanotube with tunable inner and outer diameters. On the outside, smaller hollow channels are custom-tailored to the given application.
Unlike other nanotubes, Fenniri's rosettes become tempered by higher temperatures because they are based on hydrophobic attractions between molecules that increase in strength as temperature rises. In fact, according to Fenniri, his nanotubes attain a higher level of self-organization as temperatures climb.
"We have designed a system that can be synthesized and modified almost at will. It is similar to biological systems. Our nanotubes are produced by a process of self-organization and self-assembly," said Fenniri.
The molecules self-assemble into rosettes because their edges have been chemically coded so that they can only form hydrogen bonds with each other in the correct orientation. The rosettes are hydrophobic (repelled by water) on the inside and hydrophilic (attracted to water) on the outside. Consequently, the rosettes self-assemble by stacking into tubes in an attempt to protect the inside of the tube from water. Since those "programmed" self-assembly operations, by design, can only occur in a "one and only one" manner, automatic self-correction is a free side effect.
"Our process is self-correcting, because when you design pieces that come together, it is only the pieces that have the right information that are going to come together," said Fenniri. "Pieces with the erroneous information are naturally eliminated from the system."
"We are not making carbon nanotubes, but nanotubes that are a mixture of carbon, nitrogen, hydrogen and oxygen," said Fenniri. "It's not a covalent system, like carbon nanotubes; [instead] the atoms are connected non-permanently in a dynamic system that has the ability to evolve toward the most stable structure. We have created a thermodynamic system that responds to the conditions in which you put it. In a way, it is an adaptive material."
Fenniri programmed the parent molecule so that it will self-assemble into nanotubes by borrowing the tools that DNA uses to form long organic strands. He modified two of the four bases from DNA, combining cytosine (C) and guanine (G) into a single "parent" molecule. Ordinarily the "left" edge of C links with the "right" edge of G, so by combining the molecules, the left edge of the pair links only with the right edge of another parent molecule. And because the paired molecules contain an angle of 60° between the C and G components, it takes six of them to snap together into a complete, 360° ring.
"From the DNA bases cytosine and guanine, we synthesized a small molecule, only about 350 atomic units, that has the ability to recognize itself," said Fenniri. "And when it does, it produces a hexagonal assembly that has a circular shape, with a cavity in the middle."
After the rings form, thousands automatically stack onto each other to form the nanotubes that protect their hydrophobic inside from the water in which the whole assembly is submerged. By changing the shape and size of the parent molecule, it is possible to control the size of the inner channel. As the rings stack into a tube, electrical charges on the outside of the tube create an electrostatic "belt" that holds it together and provides an anchor to which chemicals can be attached to custom-tailor the physical properties of the nanotube.
The tube's outside channels, formed by attaching molecules tailored to a specific application, are smaller but more numerous. In biological applications, the central channel can be used to pump charged ions in one direction, while the outside channels transport smaller ions with an equal amount of charge in the opposite direction, thereby maintaining electrical neutrality.
"We use chemistry to maintain near-complete control over the physical properties of the nanotube. You can introduce functional molecules at any position along the outside of the nanotube to control its physical properties with chemistry. For instance, if you want something that is photo-active that absorbs photons and transports them to a new location in the assembly then you can introduce components in the parent molecule that absorb light. If you want it to conduct electricity, then you introduce a component into the parent that absorbs an electron and transfers it from one rosette to the next," said Fenniri.
Thus far, Fenniri has verified the ability to control the diameter and length of the nanotubes, as well as how to introduce metallic molecules that conduct electricity and photonic molecules that process light. Electronic components, such as transistors, are in development.
The next phase will explore a method of precisely controlling the dimensions of the nanotube, Fenniri said. The current one has an 11-angstrom inner diameter, but he already has designed a 6-angstrom version. He is also shooting for a self-replicating process that will enable billions of parent molecules to be automatically self-assembled from free-floating chemicals in solution.
Fenniri's group hopes to grow working circuits on electronic substrates by first prepping the wafer by "writing" nanotube structures onto it via nanolithography. Self-organization could then be enlisted to grow the correct structures at precisely specified locations by self-assembly techniques.
"We have one component of a circuit now the wire but the other components, like transistors and transducers, are not all going to be made from the same material," said Fenniri. Instead, "we will use nanolithography to basically write on a substrate, but the ink will be nanotubes.
"As you write on the substrate, the nanotubes will self-assemble into the proper structures."