Shape of – A Thread! Form of – A Diamond!
“Wonder Twin powers—activate!” Readers of a certain age will remember the Saturday morning cartoon Super Friends being invaded by a pair of alien twins. Although, when compared to their actually super friends— Superman, Wonder Woman, Batman—who agreed to mentor the teens, Zan’s and Jayna’s powers seemed relatively tame. Zan could transform himself into water—in frozen, liquid, or vapor form, mind you!—and Jayna could shift her shape into that of any animal. Individually pretty cool, but when they worked together, they could accomplish fantastic things.
This sort of combination of superpowers, with the resulting whole being greater than the sum of its parts, happens in science, too. The Eberly College of Science fosters these kinds of interactions by encouraging cross-disciplinary research and providing the infrastructure that facilitates the collaboration of researchers with others outside of their sometimes-narrow fields of expertise. One particularly exciting example is occurring within the newly established National Science Foundation (NSF) Center for Nanothread Chemistry. Center researchers John Badding, professor of chemistry, physics, and materials science and engineering (whose superpower is solid-state materials chemistry), and Elizabeth Elacqua, assistant professor of chemistry (superpower: organic chemistry), are collaborating with a team of researchers to develop and advance our scientific understanding of carbon nanothreads, a recently developed nanomaterial that resembles a one- dimensional flexible diamond with superpower potential of its own.
Origin stories
Every good superhero tale begins with an origin story. Ours starts at Penn State, which has been a pioneer in materials research since the early 1900s. This early materials research focused on construction materials, metallurgy, and ceramics, but even then, the interdisciplinary nature of materials research was recognized. Understanding materials and optimizing their properties for some practical application extends beyond engineering and requires an understanding of the material’s chemistry and physics. “Penn State has been a leader in the area of materials science for a long time,” said Badding. “In fact, the international Materials Research Society was founded at Penn State. Because materials research was pervasive here, it filtered out into many different departments.”
In 1958 the Intercollegiate Research Program was established, which helped set the stage for the establishment of the Materials Research Lab (MRL) in 1962. In 1992 the Materials Research Institute (MRI) was created, which absorbed the MRL in 2001. The MRI now provides the infrastructure and expertise to facilitate collaborative materials research across the University and around the world. More recently, Penn State established the Center for Nanoscale Science (see page 23), an NSF-funded Materials Research Science and Engineering Center (MRSEC). The MRSEC houses four interdisciplinary research groups (Badding leads one in a different research area) and enables researchers to pursue large-scale projects that might not otherwise be feasible. With its history of interdisciplinary materials research and its massive investments in infrastructure like the MRI and the MRSEC, Penn State is one of the few places on Earth where this type of research can happen, and it’s where our heroes joined forces.
Wonder Triplets?
Members of the Badding and Elacqua labs now routinely produce nanothreads in a surprisingly small press, capable of producing massive amounts of pressure, located in the basement of the Chemistry Building on the University Park campus. But prior to 2001, the existence of nanothreads was pure science fiction. In fact, the great sci-fi writer Arthur C. Clark—author of 2001: A Space Odyssey and many other classics of the genre—imagined a similar material in his 1979 novel The Fountains of Paradise. The novel’s protagonist, Vannevar Morgan, wants to build a space elevator to transport people and goods between the Earth and spaceships in orbit. The construction of the massive tower is to be accomplished using hyperfilament—a thin, ultra-strong material described as “a continuous, pseudo-one-dimensional diamond crystal.” A nanothread!
So, how do you turn science fiction into actual science? By bringing together scientists from diverse fields in new collaborations. Another Penn State scientist from yet a third subdiscipline played a key role in getting the ball rolling. Vincent H. Crespi, Distinguished Professor of Physics, Materials Science and Engineering, and Chemistry, focuses his research in the theoretical realm. In research published in 2001, Crespi and his colleagues demonstrated the theoretical possibility of a one-dimensional carbon nanomaterial with incredible stiffness that was unlike any other previous carbon nanomaterial. Twelve years later Badding, working with Crespi and others, actually produced the first nanothreads. In order to understand the significance of this discovery, we need to know a little about the chemistry of carbon materials and how nanothreads are made.
The astonishing nanothread
“Carbon-based materials can be classified into two main categories based on the number of bonds that each of the carbon atoms in the material has,” said Badding. “In sp2 carbon materials, which can be thought of as graphitelike, each carbon atom is bonded to three other atoms. In sp3 carbon materials, which are diamondlike, each carbon atom bonds with four other atoms.”
Additionally, carbon materials can be classified by their dimensionality. Graphite, like you find in pencil “lead,” is a three-dimensional sp2 carbon material. Graphene is a two-dimensional sp2 material, and carbon nanotubes are a one- dimensional sp2 material. For sp3 materials, you have diamonds as the three-dimensional representative, graphane in two dimensions, and now nanothreads in one dimension. So, in this sense, nanothreads are like Clark’s sci-fi hyperfilament, but their superpowers could extend far beyond their strength and stiffness. Nanothreads have additional properties that are unlike other carbon nanomaterials, based on the way that they are produced. This is where organic chemistry joins forces with theoretical physics and solid-state chemistry and we can begin to see the true potential of this new material.
The production of most carbon nanomaterials relies on the addition of massive amounts of energy and produces incredible heat. The result is that no matter what your starting material, you end
up with a material that is essentially pure carbon; most other atoms get kicked out in the process.
“You can make carbon nanotubes out of nearly any carbon source, even peanut butter,” said Badding. “The production of nanothreads, on the other hand, is more akin to how organic chemists synthesize molecules. The structure of the final product is dependent on the characteristics of the starting material.”
Nanothreads are made by applying massive amounts of pressure to a starting material at room temperature. Currently, the Badding and Elacqua groups make nanothreads from benzene, an sp2 carbon molecule made up of six carbon atoms in a hexagonal ring. Each carbon atom binds to two neighboring carbon atoms around the perimeter of the ring, and its third bond is to a hydrogen atom that juts from the ring, like teeth on a six- toothed gear. Crystals of benzene are compressed at upwards of 200,000 times atmospheric pressure, which causes the atoms and bonds to rearrange, coalescing into long, one-dimensional threads.
When viewed from an end, the threads display a hexagonal, girder-like structure made up of a core of carbon atoms, which provides their strength and stiffness. Importantly, the threads maintain the hydrogen atoms from benzene, sticking out along their length. This characteristic of nanothreads, that their form and composition is dependent on the form and composition of the starting material, may be what excites Badding and Elacqua most, because it opens up the possibility to design nanothreads to perform specific functions.
“I have a background in supramolecular assembly,” said Elacqua. “It’s sort of like the LEGO-works of chemistry. It’s taking molecules that fit together by size, shape, and electronics and piecing together structures. I met John when I was interviewing at Penn State about two years ago and he told me about his nanothread research. I was extremely excited because I could see how pairing what he knows about high-pressure chemistry with my knowledge of how molecules arrange themselves and what we could do to bias and control reactivity could lead to a lot of cool design possibilities.”
The Justice League of Nanothreads
Having first demonstrated the theoretical possibility of nanothreads and then actually produced them, the next steps for Crespi, Badding, and Elacqua are to decipher what precisely is occurring at the atomic level when nanothreads are formed, further characterize their properties, ramp up production, and explore the possibilities of designing nanothreads to perform specific functions. To do that, they have assembled a team of experts from diverse scientific backgrounds from across the country to form the NSF-funded Center for Nanothread Chemistry.
“I think of nanothreads as a hybrid between a nanomaterial and a molecule,” said Badding. “They have the structural qualities of a material that could be useful on their own because they have what we call ‘pervasive connectivity in multiple dimensions,’ but because they maintain the hydrogen atoms along their perimeter, they also can behave like a molecule. So, we are exploring ways we can design nanothreads, where maybe we could replace the hydrogen atoms with other atoms or molecules that could perform specific functions.”
Elacqua and her group are exploring just these sorts of ideas. They are experimenting with different starting materials, other than benzene, to try to create nanothreads with different properties.
“One of the things were doing is trying to create nanothreads starting with co-crystals of multiple different molecules,” said Elacqua. “These co- crystals self-assemble with periodic patterns of the different starting molecules. We expect that nanothreads made from these co-crystals will maintain these periodic structures, so we can design nanothreads that have specific patterns of molecules instead of just hydrogen atoms.”
The researchers in the Center for Nanothread Chemistry also want to increase the production of nanothreads. Because the required pressure to create nanothreads is currently so high, the team can only produce small amounts of nanothreads at a time, but if they could reduce the amount of pressure required, it might be possible to produce nanothreads at an industrial scale.
“Millions of pounds of industrial diamonds are produced every year,” said Badding. “If we could decrease the amount of pressure needed to produce nanothreads, we might be able to produce them at a similar scale.”
There is still a lot of basic research to do to better understand how nanothreads form and to understand their properties before their full potential can be unlocked. But rest assured that there is no better way to ensure that potential will be reached than by assembling teams of scientists collaborating across traditional subfield boundaries, bringing their superpowers together to vanquish scientific questions—like Zan transforming into a unicycle made of ice for Jayna the octopus to ride in order to vanquish a supervillain.