For decades, scientists have tried to harness the unique properties of carbon nanotubes to create high-performance electronics that are faster or consume less power — resulting in longer battery life, faster wireless communication and faster processing speeds for devices like smartphones and laptops.
But a number of challenges have impeded the development of high-performance transistors made of carbon nanotubes, tiny cylinders made of carbon just one atom thick. Consequently, their performance has lagged far behind semiconductors such as silicon and gallium arsenide used in computer chips and personal electronics.
Now, for the first time, University of Wisconsin-Madison materials engineers have created carbon nanotube transistors that outperform state-of-the-art silicon transistors.
Led by Michael Arnold and Padma Gopalan, UW-Madison professors of materials science and engineering, the team’s carbon nanotube transistors achieved current that’s 1.9 times higher than silicon transistors. The researchers reported their advance in a paper published Friday (Sept. 2) in the journal Science Advances.
“This achievement has been a dream of nanotechnology for the last 20 years,” says Arnold. “Making carbon nanotube transistors that are better than silicon transistors is a big milestone. This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies.”
This advance could pave the way for carbon nanotube transistors to replace silicon transistors and continue delivering the performance gains the computer industry relies on and that consumers demand. The new transistors are particularly promising for wireless communications technologies that require a lot of current flowing across a relatively small area.
As some of the best electrical conductors ever discovered, carbon nanotubes have long been recognized as a promising material for next-generation transistors.
Carbon nanotube transistors should be able to perform five times faster or use five times less energy than silicon transistors, according to extrapolations from single nanotube measurements. The nanotube’s ultra-small dimension makes it possible to rapidly change a current signal traveling across it, which could lead to substantial gains in the bandwidth of wireless communications devices.
But researchers have struggled to isolate purely carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and disrupt their semiconducting properties — like a short in an electronic device.
The UW-Madison team used polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.
“We’ve identified specific conditions in which you can get rid of nearly all metallic nanotubes, where we have less than 0.01 percent metallic nanotubes,” says Arnold.
Placement and alignment of the nanotubes is also difficult to control.
To make a good transistor, the nanotubes need to be aligned in just the right order, with just the right spacing, when assembled on a wafer. In 2014, the UW-Madison researchers overcame that challenge when they announced a technique, called “floating evaporative self-assembly,” that gives them this control.
The nanotubes must make good electrical contacts with the metal electrodes of the transistor. Because the polymer the UW-Madison researchers use to isolate the semiconducting nanotubes also acts like an insulating layer between the nanotubes and the electrodes, the team “baked” the nanotube arrays in a vacuum oven to remove the insulating layer. The result: excellent electrical contacts to the nanotubes.
The researchers also developed a treatment that removes residues from the nanotubes after they’re processed in solution.
“In our research, we’ve shown that we can simultaneously overcome all of these challenges of working with nanotubes, and that has allowed us to create these groundbreaking carbon nanotube transistors that surpass silicon and gallium arsenide transistors,” says Arnold.
The researchers benchmarked their carbon nanotube transistor against a silicon transistor of the same size, geometry and leakage current in order to make an apples-to-apples comparison.
They are continuing to work on adapting their device to match the geometry used in silicon transistors, which get smaller with each new generation. Work is also underway to develop high-performance radio frequency amplifiers that may be able to boost a cellphone signal. While the researchers have already scaled their alignment and deposition process to 1 inch by 1 inch wafers, they’re working on scaling the process up for commercial production.
Arnold says it’s exciting to finally reach the point where researchers can exploit the nanotubes to attain performance gains in actual technologies.
“There has been a lot of hype about carbon nanotubes that hasn’t been realized, and that has kind of soured many people’s outlook,” he says. “But we think the hype is deserved. It has just taken decades of work for the materials science to catch up and allow us to effectively harness these materials.”
Technion breakthrough could replace silicon chips in the world of electronics
Technion researchers have developed a method for growing carbon nanotubes that could lead to the day when molecular electronics replace the ubiquitous silicon chip as the building block of electronics. The findings are published this week in Nature Communications.
Carbon nanotubes (CNTs) have long fascinated scientists because of their unprecedented electrical, optical, thermal and mechanical properties, and chemical sensitivity. But significant challenges remain before CNTs can be implemented on a wide scale, including the need to produce them in specific locations on a smooth substrate, in conditions that will lead to the formation of a circuit around them.
Led by Prof. Yuval Yaish of the Viterbi Faculty of Electrical Engineering and the Zisapel Nanoelectronics Center at the Technion, the researchers have developed a technology that addresses these challenges. Their breakthrough also makes it possible to study the dynamic properties of CNTs, including acceleration, resonance (vibration), and the transition from softness to hardness.
The method could serve as an applicable platform for the integration of nano-electronics with silicon technologies, and possibly even the replacement of these technologies in molecular electronics.
A system that uses a laser and electrical current to precisely position and align carbon nanotubes represents a potential new tool for creating electronic devices out of the tiny fibers.
Because carbon nanotubes have unique thermal and electrical properties, they may have future applications in electronic cooling and as devices in microchips, sensors and circuits. Being able to orient the carbon nanotubes in the same direction and precisely position them could allow these nanostructures to be used in such applications.
However, it is difficult to manipulate something so small that thousands of them would fit within the diameter of a single strand of hair, said Steven T. Wereley, a professor of mechanical engineering at Purdue University.
“One of the things we can do with this technique is assemble carbon nanotubes, put them where we want and make them into complicated structures,” he said.
New findings from research led by Purdue doctoral student Avanish Mishra are detailed in a paper that has appeared online March 24 in the journal Microsystems and Nanoengineering, published by the Nature Publishing Group.
The technique, called rapid electrokinetic patterning (REP), uses two parallel electrodes made of indium tin oxide, a transparent and electrically conductive material. The nanotubes are arranged randomly while suspended in deionized water. Applying an electric field causes them to orient vertically. Then an infrared laser heats the fluid, producing a doughnut-shaped vortex of circulating liquid between the two electrodes. This vortex enables the researchers to move the nanotubes and reposition them.
“When we apply the electric field, they are immediately oriented vertically, and then when we apply the laser, it starts a vortex, that sweeps them into little nanotube forests,” Wereley said.
The research paper was authored by Mishra; Purdue graduate student Katherine Clayton; University of Louisville student Vanessa Velasco; Stuart J. Williams, an assistant professor of mechanical engineering at the University of Louisville and director of the Integrated Microfluidic Systems Laboratory; and Wereley. Williams is a former doctoral student at Purdue.
The technique overcomes limitations of other methods for manipulating particles measured on the scale of nanometers, or billionths of a meter. In this study, the procedure was used for multiwalled carbon nanotubes, which are rolled-up ultrathin sheets of carbon called graphene. However, according to the researchers, using this technique other nanoparticles such as nanowires and nanorods can be similarly positioned and fixed in vertical orientation.
The researchers have received a U.S. patent on the system.
Researchers have developed a new and highly efficient method for gene transfer. The technique, which involves culturing and transfecting cells with genetic material on an array of carbon nanotubes, appears to overcome the limitations of other gene editing technologies.
“This platform holds the potential to make the gene transfer process more robust and decrease toxic effects, while increasing amount and diversity of genetic cargo we can deliver into cells,” said Ian Dickerson, Ph.D., an associate professor in the Department of Neuroscience at the URMC and co-author of the paper.
“This represents a very simple, inexpensive, and efficient process that is well-tolerated by cells and can successfully deliver DNA into tens of thousands of cells simultaneously,” said Michael Schrlau, Ph.D., an assistant professor in the Kate Gleason College of Engineering at RIT and co-author of the paper.
Rice researchers discover that a simple filtration technique produces wafer-scale films of highly aligned carbon nanotubes. The thin films offer possibilities for flexible electronic and photonic devices.
A simple filtration process helped Rice University researchers create flexible, wafer-scale films of highly aligned and closely packed carbon nanotubes. In the right solution of nanotubes and under the right conditions, the tubes assemble themselves by the millions into long rows that are aligned better than once thought possible, the researchers reported. A scanning electron microscope image shows highly aligned and closely packed carbon nanotubes gathered into a film by researchers at Rice.