Flattened Nanotubes - The Gateway To Huge Opportunities In Nanotechnology?

Researchers at Rice University’s Richard E. Smalley Institute for Nanoscale Science and Technology have found that nanotubes of a reasonably large diameter can spontaneously collapse into closed-edge graphene nanoribbons when atoms on the inside wall get close enough to attract each other. They have come up with a set of facts and figures about carbon nanotubes that seem to be collapsing during the growth process. They have found out that these unique configurations have the best of both worlds - properties of both nanotubes and graphene nanoribbons. These “closed-edge graphene nanoribbons” (that is how the researchers call them) may find many applications and research into this structure is all set to develop full pace.
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The pioneering work led by Robert Hauge, a distinguished faculty fellow in chemistry at Rice, is detailed in a paper that appeared online this month in the American Chemical Society journal ACS Nano. According to him, a collapsed nanotube looks like graphene in the middle, sandwiched between what looks exactly like buckyballs (carbon-60 molecules, a Nobel Prize-winning discovery at Rice) on the sides. Naturally this implies that the structure will follow the chemistry of graphene in the middle and the chemistry of buckyballs on the edges. The separation of the two electronically is possible by putting functional groups on the sides to isolate the top and bottom layers.

If you turn the sides into insulators, then the top doesn’t communicate with the bottom electronically, except through some van der Waals-type or excited-state interaction, then that’s where the new properties will come from.

According to him, the graphene world is searching for ways to make well-defined ribbons and end up cutting graphene and as a result they get ill-defined sides, something that will affect its properties. By using this structure it is possible to produce grown-to-order, two- or four-layer graphene nanoribbons with perfect edges.

Hauge’s awareness of earlier work on nanotube collapse led him to study the phenomenon. The team thought the tubes could collapse, so they started looking for the evidence. When the team observed nanotubes through a transmission electron microscope, they noticed folds, twists and kinks which were good indicators of collapsed nanotubes. These nanotubes were about 0.7 nanometers in height along the middle and a little more at what the researchers called the “highly strained bulbs” at the edges. Now that they knew that the collapse was happening, they started researching as to how the process of collapse happened.

Hauge approached Rice theoretical physicist Boris Yakobson to see how the intrinsic energy of atoms in graphene would allow such a collapse to happen. Then Ksenia Bets got included in the team. Using molecular dynamic simulation, the team fit data from the experimentalists to atomistic models of single-wall nanotubes. Later using the same parameters, the team produced results for double walls and they also fit exactly with the experimental data. The results confirmed the probability that at growth temperature – 750 degrees Celsius – flexible nanotubes fluttering in the gas breeze inside a furnace could indeed be induced to collapse. That can happen if two atoms on either side of the inner wall get close enough to each other, they can start a van der Walls cascade that flattens the nanotube.

Though at first, it takes energy to press the nanotube, when you reach a point where the two sides begin to gain the energy of attraction, the Van der Waals force takes over and collapse becomes a natural progress from that point. Also bigger the diameter of the tube, lesser the required energy to collapse a tube i.e., it becomes easier to distort. There is a trade off between the strain energy on the edges and the van der Walls interaction in the center.

The discovery has implications for bundles of nanotubes beginning to see use in fibers for electrical applications or as strengthening elements in advanced materials. The question that is bound to arise is whether a layer of collapsed tubes in a bundle is actually more energetically favorable than that same bundle of hexagonally shaped tubes – a question that hasn’t been answered so far.

And that’s not the only unanswered question. There are more. For instance, will a nanotube collapse along its entire length, whether pressure from outside could start a chain reaction leading to collapse. It’s possible that you could apply pressure to force everything to collapse, and it would stay that way because that’s what it wants to be. But still this discovery will kindle a wide range of research based on larger-diameter nanotubes and what they might offer. Let’s hope that these questions are answered soon so that all can get a clear idea about the phenomenon.

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