Move over, silicon. In a breakthrough in the quest for the next generation of computers and materials, researchers at USC have solved a long-standing challenge with carbon nanotubes: how to actually build them with specific, predictable atomic structures.
“We are solving a fundamental problem of the carbon nanotube,” said Chongwu Zhou, professor in the Ming Hsieh Department of Electrical Engineering at the USC Viterbi School of Engineering and corresponding author of the study published on Aug. 23 in the journal Nano Letters. “To be able to control the atomic structure, or chirality, of nanotubes has basically been our dream, a dream in the nanotube field.”
If this is an age built on silicon, then the next one may be built on carbon nanotubes, which have shown promise in everything from optics to energy storage to touch screens. Not only are nanotubes transparent, but this research discovery on how to control the atomic structure of nanotubes will pave the way for computers that are smaller, faster and more energy efficient than those reliant on silicon transistors.
“We are now working on scale up the process,” Zhou said. “Our method can revolutionize the field and significantly push forward the real applications of nanotube in many fields.”
Until now, scientists were unable to “grow” carbon nanotubes with specific attributes — say metallic rather than semiconducting — instead getting mixed, random batches and then sorting them. The sorting process also shortened the nanotubes significantly, making the material less practical for many applications.
For more than three years, the USC team has been working on the idea of using these short, sorted nanotubes as “seeds” to grow longer nanotubes, extending them at high temperatures to get the desired atomic structure.
A paper last year by the same team in Nature Communications outlined the technique, and in the current Nano Letters paper, the researchers report on their latest major success: identifying the “growth recipes” for building carbon nanotubes with specific atomic structures.
“We identify the mechanisms required for mass amplification of nanotubes,” said co-lead author Jia Liu, a doctoral student in chemistry at the USC Dornsife College of Letters, Arts and Sciences, recalling the moment when, alone in a dark room, she finally saw the spectral data supporting their method. “It was my eureka moment.”
She added, “To understand nanotube growth behaviors allows us to produce larger amounts of nanotubes and better control that growth.”
Each defined type of carbon nanotube has a frequency at which it expands and contracts. The researchers showed that the newly grown nanotubes had the same atomic structure by matching the Raman frequency.
“This is a very exciting field, and this was the most difficult problem,” said co-lead author Bilu Liu, a postdoctoral research associate at USC Viterbi. “I met Professor Zhou [senior author of the paper] at a conference, and he said he wanted to tackle the challenge of controlling the atomic structure of nanotubes. That’s what brought me to his lab because it was the biggest challenge.”
In addition, the study found that nanotubes with different structures also behave very differently during their growth, with some nanotube structures growing faster and others growing longer under certain conditions.
“Previously it was very difficult to control the chirality, or atomic structure, of nanotubes, particularly when using metal nanoparticles,” Bilu Liu said. “The structures may look quite similar, but the properties are very different. In this paper we decode the atomic structure of nanotubes and show how to control precisely that atomic structure.”
Additional authors of the study are Jialu Zhang of USC and Xiaomin Tu and Ming Zheng of the National Institute of Standards and Technology.
The research was funded by the Office of Naval Research and the Defense Threat Reduction Agency of the U.S. Department of Defense.
This press release is available on the USC Press Room website.
About the USC Viterbi School of Engineering
Engineering studies began at the University of Southern California in 1905. Nearly a century later, the School of Engineering received a naming gift in 2004 from alumnus Andrew J. Viterbi, inventor of the Viterbi algorithm now key to cell phone technology and numerous data applications. Consistently ranked among the top graduate programs in the world, the school enrolls more than 5,000 undergraduate and graduate students, taught by 177 tenured and tenure-track faculty, with 60 endowed chairs and professorships. http://viterbi.usc.edu
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|Figure 1. Chirality-dependent growth and length distribution of semiconducting (9, 1) and (6, 5) SWCNTs. (a) Chirality map and chiral angle, diameter, and metallicity information of seven nanotubes we studied. (b) Edge structure comparisons of (9, 1) and (6, 5) nanotubes. (c–e) SEM images of VPE-grown (9, 1) SWCNTs with growth time of 40 s, 60 s, and 15 min, respectively. (f–h) SEM images of VPE-grown (6, 5) SWCNTs with growth time of 20 s, 40 s, and 15 min, respectively. (i,j) Length distribution of nanotubes from (c–e) and (f–h), respectively. Scale bars are 50 μm for all images.|
|Figure 2. Length evolution profiles and chirality-dependent growth rate (R0) and lifetime (τ) of semiconducting SWCNTs. (a) Length evolution profiles and fitted curves based on eq 3 for (6, 5), (8, 3), (9, 1), (7, 6), and (10, 2) SWCNTs with growth times of 20 s, 40 s, 60 s, 2 min, and 15 min. Inset, chiral angle versus diameter for the above five semiconducting SWCNTs, showing that they belong to two subgroups with similar diameters in each one as highlighted by different colors. (b) Zoom-in plot of panel a shows the initial growth period. (c,d) Chiral-angle-dependent growth rate (R0) and lifetime (τ) of the above five kinds of semiconducting SWCNTs. The vertical error bars in c and d correspond to the errors of parameters extracted based on eq 3.|
|Figure 3. Atomic illustration of chirality-dependent SWCNT growth via Diels–Alders cycloaddition processes. (a–c) Cycloaddition of C2Hx species to a (6, 5) SWCNT and the formation of six-membered rings, leading to the continuous growth of this nanotube. (d) Addition of CHy species leads to the formation of five-membered ring and consequently nanotube growth stops. (e–g) Addition of C2Hx species to a (9, 1) SWCNT for its continuous growth. (h) Addition of CHy species leads to the growth stops. Multiple arrows from panels b to c and panels f to g represent multiple addition reactions.|
|Figure 4. Chirality-dependent growth of armchair metallic SWCNTs. (a,b) Representative SEM images of cloned (6, 6) and (7, 7) SWCNTs with a growth time of 20 s. (c,d) Length evolution of (6, 6) and (7, 7) SWCNTs with growth time of 20, 40, and 60 s.|