Carbon nanotubes under stress

Better production methods for carbon nanotubes (CNTs) will require control of both chemical and mechanical factors in their growth, a recent report by MIT researchers shows.

Postdoctoral Associate Mostafa Bedewy, working with Associate Professor of Mechanical Engineering A. John Hart, demonstrated that competition between the chemical activation of CNTs growing on seed particles and the mechanical coupling among growing CNTs of different sizes leads to the waviness of individual strands seen in highly magnified views of CNT forests.

Building on earlier work showing that larger-diameter CNTs grow faster than smaller-diameter ones but stop growing sooner, the new work found that larger-diameter CNTs were subject to greater mechanical stress. Maximizing the number of same-sized seed nanoparticles for nanotube growth and reducing variation in the space between them could produce straighter, denser, more evenly sized nanotube forests, the research suggests.

“Because the root-cause for these loads and stresses is the mismatch of diameter-dependent growth rates, a strategy toward growth of mutually dense and high-quality CNT structures is to engineer catalyst populations that are monodisperse in size and shape,” Bedewy and Hart wrote in “Mechanical Coupling Limits the Density and Quality of Self-Organized Carbon Nanotube Growth,” published in Nanoscale in February 2013. The underlying research was done at the University of Michigan, before Hart joined the MIT faculty in July 2013 and Bedewy also came to MIT from Michigan. X-ray scattering and attenuation measurements were taken at the Cornell High-Energy Synchrotron Source (CHESS). (See related article.)

"We study how nanotubes grow as a population and study the collective aspects that govern the process; these include collective chemical aspects and collective mechanical aspects," Bedewy says. "Mechanical coupling and chemical coupling are the newest two components to our work on identifying and overcoming challenges towards manufacturing of high-performance aligned CNT structures. This paper demonstrated that there are internal forces acting on the nanotubes during growth and showed that these forces could be an important limiting mechanism. We want to take this work to the next step and understand how to overcome these effects."

Further work on chemical coupling shows variations among aligned CNT micropillars grown on the same chip. “There are spatial variations among these arrays in geometry, density, and orientation of the pillars," Bedewy says. “Some grow bending outwards, and some have a more curved top surface versus a more square top surface. Importantly, some don't lift off the substrate." Publication of that work is in progress.

These spatial variations are important because they limit the properties of the pillars and can negatively affect their performance when they are integrated into functional devices. In collaboration with Brittan Farmer from the department of mathematics at the University of Michigan, Bedewy says, "we developed a mathematical model to explain the origins of those spatial variations. We explain the generation and diffusion of active species that promote the CVD [chemical vapor deposition] process. An overall concentration profile of active species develops as a result of the interplay between local generation of these active species and their diffusion to the surroundings. Our model was able to predict how different spacing between CNT micropillars in arrays can determine whether each one grows as a pillar or not, and when it grows, whether it has uniform geometry."

“We combined experimental results from our CVD reactor growth with the mathematical model in order to explain the origins of the synergistic growth effect — because of proximity — among the growth of CNT micropillars, based on the chemical coupling between neighboring catalyst regions,” Bedewy says. "We can predict when the pillars don't lift off and you end up having only tangled mats on the surface versus lift-off pillars."

Experimental results were obtained from CVD reactors at MIT, with further studies at Brookhaven National Lab’s Center for Functional Nanomaterials, using in situ transmission electron microscopy to watch real-time nucleation of CNTs in a tiny CVD reactor. “We study the catalyst annealing process in which the catalyst thin films are converted into catalyst nanoparticles that seed the growth of nanotubes, and we study the early growth and self-organization of nanotubes,” Bedewy says. “That’s what we call the initial nucleation stage.”

The new research reveals that only a small number of the total number of nanoparticles act as CNT seeds. "More scientific studies are needed to really understand what determines the fate of a nanoparticle, whether it ends up being a seed for growing a nanotube or it ends up being a nonactive nanoparticle that is usually encapsulated by a graphitic layer but doesn't nucleate nanotube growth,” Bedewy says. “We are continuing this work in collaboration with Dr. Eric Stach and Dr. Dmitri Zakharov at Brookhaven National Lab to understand what are the fundamental mechanisms that determine the fate of the catalyst nanoparticles. Once we understand it, we can control it to achieve higher activation percentages and manufacture higher-density CNT structures.”

“One of the limitations of aligned nanotube structures for applications is we cannot get them as dense as we want them to be,” Bedewy says. For applications such as heat transport in thermal interfaces or fluid transport for novel membranes, denser well-aligned CNT structures would perform better. Density can be increased after CNT forest growth by either capillary or mechanical compression.

Bedewy also was a coauthor of a paper in the scientific journal Small on solution-based deposition of nanoparticles as an alternative to thin-film dewetting, which is typically used to form catalyst nanoparticles from thin films upon heating. Called blade casting, the technique lays down particles on any arbitrary substrate, such as a flexible metal foil, and can be scaled as a roll-to-roll process.

In a paper published in Carbon in 2012, “Diameter-dependent kinetics of activation and deactivation in carbon nanotube population growth," Bedewy and Hart demonstrated that CNT forest growth follows an S-shaped curve — mathematically modeled as a Gompertz curve — with a slow start followed by rapid increase in number of CNTs until it reaches an inflection point where growth slows until termination.

The researchers reported that growth of smaller-diameter CNTs activates more slowly and exhibits a longer lifetime than larger-diameter CNTs. With the help of Michigan colleague Eric Meshot, they took measurements of real-time forest height during growth with a non-contact laser probe. After that, spatially resolved small-angle X-ray scattering (SAXS) and X-ray attenuation measurements of the CNT forest were collected. The work also showed that mechanical coupling between growing adjacent CNTs enabled alignment of forests, but because of size differences in diameter, it could also be responsible for the wavy and bundled strands observed in CNT forests. The work was supported by the Department of Energy.

Bedewy and colleagues also previously studied population growth dynamics of CNTs. In a paper published in ACS Nano in 2011, they characterized the evolution of vertically aligned CNT forests during growth and calculated their number density and mass density.

A native of Egypt who defended his PhD at the University of Michigan in November 2013, Bedewy is interviewing for faculty positions and hopes to begin teaching following his work as a postdoctoral associate.