Jul 30 2019
A circle can be said to be less stable than a jagged loop, but this would be true apparently when one is referring to carbon nanotubes.
Rice University researchers have determined that an odd, two-faced “Janus” edge is more common than previously thought for carbon nanotubes growing on a rigid catalyst. The conventional nanotube at left has facets that form a circle, allowing the nanotube to grow straight up from the catalyst. But they discovered the nanotube at right, with a tilted Janus edge that has segregated sections of zigzag and armchair configurations, is far more energetically favored when growing carbon nanotubes via chemical vapor deposition. (Image credit: Evgeni Penev)
Now, theoretical researchers at Rice University have found that nanotubes with isolated sections of “armchair” and “zigzag” facets, emerging from a solid catalyst, are relatively more energetically stable when compared to a circular configuration.
According to the team, the interface between an emerging nanotube and its catalyst can achieve its lowest-known energy state under the right conditions. This would occur through the two-faced “Janus” arrangement, with a zigzag half-circle opposite six armchairs. These terms point to the shape of the edge of the nanotube—the end of a zigzag nanotube looks similar to a saw tooth, whereas an armchair looks similar to a row of seats fitted with armrests.
They represent the rudimentary edge configurations of the 2D honeycomb of carbon atoms called graphene (and also other 2D materials) and establish most of the materials’ characteristics, specifically electrical conductivity.
At George R. Brown School of Engineering, a research team, that included assistant research professor Evgeni Penev, materials theorist Boris Yakobson, and lead author and researcher Ksenia Bets, published the results of the study in the American Chemical Society journal, ACS Nano.
The concept is a continuation of the researchers’ finding made last year that the two-faced Janus interfaces may possibly develop on a catalyst of cobalt and tungsten. This would result in a single chirality, referred to as (12,6), that had been grown by other laboratories in 2014.
At present, the Rice University team has demonstrated that although structures like those are not exclusive to a particular catalyst, they are a standard trait of several stiff catalysts. This is because the atoms binding themselves to the edge of the nanotube continuously search for their lowest energy states, and they happen to locate these in the Janus configuration, which incidentally was dubbed A|Z by the team.
People have assumed in studies that the geometry of the edge is a circle. That’s intuitive—it’s normal to assume that the shortest edge is the best. But we found for chiral tubes the slightly elongated Janus edge allows it to be in much better contact with solid catalysts. The energy for this edge can be quite low.
Evgeni Penev, Assistant Research Professor, George R. Brown School of Engineering, Rice University
The flat armchair bottoms, within the circle configuration, rest on the substrate, enabling the highest number of contacts between the nanotube and catalyst; the nanotube grows directly. (The nanotube is forced by the Janus edges to grow at a certain angle.)
Carbon nanotubes are elongated, rolled-up tubes of graphene that cannot be easily seen through an electron microscope. To date, no method is available to view the bottom of a nanotube as it emerges from the bottom up inside a chemical vapor deposition furnace. However, hypothetical calculations of the atom-level energy passing between the nanotube and the catalyst at the interface can reveal a great deal of information to researchers on how these nanotubes grow.
The Rice lab has been pursuing that same path for over 10 years, pulling at the thread that shows how slight alterations made during the growth of nanotubes can alter the kinetics, and how these products can be eventually utilized in applications.
Generally, the insertion of new atoms at the nanotube edge requires breaking the interface between the nanotube and the substrate. If the interface is tight, it would cost too much energy. That is why the screw dislocation growth theory proposed by Professor Yakobson in 2009 was able to connect the growth rate with the presence of kinks, the sites on the nanotube edge that disrupt the tight carbon nanotube-substrate contact.
Ksenia Bets, Study Lead Author and Researcher, George R. Brown School of Engineering, Rice University
“Curiously, even though Janus edge configuration allows very tight contact with the substrate it still preserves a single kink that would allow continuous nanotube growth, as we demonstrated last year for the cobalt tungsten catalyst,” Bets added.
To model the nanotubes growing on three solid catalysts, Bets performed elaborate computer simulations that showed the proof of the growth of Janus nanotubes and an additional “fluid” catalyst, called tungsten carbide, which did not grow.
“The surface of that catalyst is very mobile, so the atoms can move a lot,” Penev stated. “For that one, we did not observe a clear segregation.”
Yakobson compared these Janus nanotubes to the crystals’ Wulff shape.
It’s somewhat surprising that our analysis suggests a restructured, faceted edge is energetically favored for chiral tubes. Assuming that the lowest energy edge must be a minimal-length circle is like assuming that a crystal shape must be a minimal-surface sphere but we know well that 3D shapes have facets and 2D shapes are polygons, as epitomized by the Wulff construction.
Boris Yakobson, Materials Theorist, George R. Brown School of Engineering, Rice University
Yakobson continued, “graphene has by necessity several ‘sides,’ but a nanotube cylinder has one rim, making the energy analysis different. This raises fundamentally interesting and practically important questions about the relevant structure of the nanotube edges.”
The scientists believe that their latest finding will bring them along the path and toward those answers. “The immediate implication of this finding is a paradigm shift in our understanding of growth mechanisms,” Yakobson added. “That may become important in how one practically designs the catalyst for efficient growth, especially of controlled nanotube symmetry type, for electronic and optical utility.”
Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and of Chemistry. The study was funded by the National Science Foundation (NSF) and the Air Force Office of Scientific Research.
Computing resources were offered by the Department of Defense Supercomputing Resource Center; the National Energy Research Scientific Computing Center, supported by the Department of Energy Office of Science; the NSF-supported XSEDE supercomputer; and the NSF-supported DAVinCI cluster at Rice, administered by the Center for Research Computing and procured in association with Ken Kennedy Institute for Information Technology at Rice University.