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Carbon nanotubes (CNTs) are hollow cylindrical tubes formed by rolling a sheet of carbon atoms arranged in a hexagonal ring, as in a sheet of graphite, either in monolayer (single-walled nanotube, SWCNT) or multilayer (multi-walled nanotube, MWCNT) form.
CNTs exhibit different electronic properties based on the way these graphene layers are rolled into a cylinder. Nanotubes could either be single-walled structures, called single-walled carbon nanotubes (SWCNTs), or could have many walls, called multi-walled carbon nanotubes (MWCNTs). Their diameter may range from 0.7 (SWCNT) to 50 nanometers (MWCNT) and few tens of micron in length.
SWCNTs can be further categorized electrically into semiconducting and metallic SWCNTs (s-SWCNTs and m-SWCNTs), while MWCNTs mainly display metallic behavior.
Carbon nanotubes (CNTs) have attracted significant interest due to their unique combination of properties including high mechanical strength, large aspect ratios, high surface area, distinct optical characteristics, high thermal and electrical conductivity, which make them suitable for a wide range of applications in areas from electronics (transistors, energy production and storage) to biotechnology (imaging, sensors, actuators and drug delivery) and other applications (displays, photonics, composites and multi-functional coatings/films). Controlled growth, assembly and integration of CNTs is essential for the practical realization of current and future nanotube applications.
The most mature technology to take over silicon is the single-walled carbon nanotube after Moore’s Law, which stated that the number of transistors on a chip will double approximately every two years expected to come to an end in the beginning of the 2020s. Carbon nanotube chips could greatly improve the capabilities of high-performance computers, enabling Big Data to be analyzed faster, increasing the power and battery life of mobile devices and the Internet of Things, and allowing cloud data centers to deliver services more efficiently and economically.
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. “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.”
In Sep 2016, it was reported that University of Wisconsin–Madison materials engineers have created carbon nanotube transistors that outperform state-of-the-art silicon transistors. The transistor is the foundation of digital logic: a switch that either allows through or blocks current. The team’s CNT transistors have successfully achieved currents that are 1.9 times higher than silicon semiconductors. While new applications for CNTs continually arise, the primary reason that CNTs have not yet fully lived up to their potential is the difficulty in manufacturing them with the “right” properties for the desired application.
But even though they exhibit better electrical and thermal properties than silicon, synthesizing high-purity tubes and integrating them into chips has been a major challenge. That’s why carbon nanotube FETs have been pushed out over the years, and are still not in mass production today.
In theory, though, carbon nanotube FETs can outperform today’s finFETs and perhaps other next-generation transistor types in R&D. Targeted for beyond the 3nm node or before, carbon nanotube FETs also are appealing because they resemble and operate like today’s conventional planar transistors, and may even extend planar to advanced nodes with immunity to short-channel effects. These devices are different than carbon nanotube RAMs, which are also in the works.
‘Twenty years ago, CNTs were viewed as the most exciting thing around,’ explains senior author Max Shulaker, Bishop’s supervisor. ‘For a decade or so, many companies were working on them and couldn’t get them to work. Then all these other materials came around and people got distracted, but since CNTs were first discovered it has been really clear that for logic and computing they represent the ideal transistor.’
Carbon nanotube transistors are finally making progress for potential use in advanced logic chips after nearly a quarter century in R&D. The question now is whether they will move out of the lab and into the fab.
In total, the carbon nanotube market is expected to grow from under $150 million in 2019/2020 to more than $500 million within the next decade, according to IDTechEx.
CNTs manufacturing
Three main methods are currently available for the production of CNTs: arc discharge, laser ablation of graphite, and chemical vapor deposition (CVD). In the first two processes, graphite is combusted electrically or by means of a laser, and the CNTs developing in the gaseous phase are separated. All three methods require the use of metals (e.g. iron, cobalt, nickel) as catalysts.
The diameter of ranges from around one nm towards several hundred, taking the CNTs from being singled-walled (SWCNT) towards multi-walled (MWCNTs) and carbon nanofibers. Similarly note that the range of available length, from a few micro meters all the way to 2 millimetres. Each of these CNTs is a different material: it is produced differently; it is priced differently, it is processed differently, and it is used differently.
MWCNTs are mainly produced using the C-CVD process (catalytic chemical vapor deposition). This approach has been scaled up in different ways, e.g., floating catalyst, fluidized bed, etc. The key ingredients are temperature, carbon-containing gas, reaction time (residence time) and catalyst. The latter, which can be made of Co, Fe, etc- is critical to determining the morphology of the CNTs and the conversion yield of the process.
SWCNT have superior performance on an individual tube basis given their higher surface-to-volume ratio. They are however more difficult and expensive to grow, come as mixed metallic and semiconducting types, and are much harder to disperse even though the wt% levels involved for the same or better effect might be much lower. These three attributes have combined to keep its market limited to some niche electronic devices.
Mattershift, an NYC-based startup with alumni from MIT and Yale has achieved a breakthrough in making carbon nanotube (CNT) membranes at large scale. The startup is developing the technology’s ability to combine and separate individual molecules to make gasoline, diesel, and jet fuel from CO2 removed from the air.
Chinese Researchers make Simple Black Film Made From Carbon Nanotubes Is Stronger Than Kevlar
Finally ultralong (several centimeter) carbon nanotube fibers have been made into stronger bundles. Carbon nanotube bundles have reached 80 GPA macroscale strength. The tensile strength of CNTBs (Carbon nanotube bundles) is at least 9–45 times that of other materials. If a more rigorous engineering definition is used, the tensile strength of macroscale CNTBs is still 5–24 times that of any other types of engineering fiber, indicating the extraordinary advantages of ultralong Carbon nanotubes in fabricating superstrong fibers. The work was done at Tsinghua University and other facilities in Beijing. Researchers were Yunxiang Bai, Rufan Zhang, Xuan Ye, Zhenxing Zhu, Huanhuan Xie, Boyuan Shen, Dali Cai, Bofei Liu, Chenxi Zhang, Zhao Jia, Shenli Zhang, Xide Li & Fei Wei.
Carbon nanotubes are super strong and stretchy at the microscopic level, But when you make a bulk material out of them, their properties are watered down because they become randomly arranged — and they need to lie in parallel to make the most of their strength. Now, a team of researchers from the East China University of Science & Technology have developed a way to create films where nanotubes are neatly aligned.
They use a technique which Chemical and Engineering News likens to glass blowing. Essentially the team uses a stream of nitrogen gas to push a layer of carbon nanotubes along the surface of tube in a furnace held at around 2,100°F. As it exits, the tubular nanotube material is wound around drum, flattening and cooling into two-layer film. The team can then compress the film by applying pressure using a system of rollers.
The film developed by them has an average tensile strength of 9.6 gigapascals. For context, kevlar fibres have a strength of about 3.7 gigapascals and carbon fibre around 7 gigapascals. It’s also relatively stretchy: it can extend by 8 per cent, which is rather more than the 2 per cent that carbon fibre can image. The properties can be exploited in multiple directions by adding layers on top of each other in different orientations.
It’s thought that the new film could be used to create strong, perhaps even structural, coatings for vehicles or aerospace parts, or new kinds of armour for military applications. Or very, very good trash bags.
Purification
Even though synthetic techniques have been improved to obtain high-purity carbon nanotubes, the formation of byproducts containing impurities such as metal encapsulated nanoparticles, metal particles in the tip of a carbon nanotube, and amorphous carbon has been an unavoidable phenomenon, because the metal nanoparticles are essential for the nanotube growth. These foreign nanoparticles, as well as structural defects that occurred during synthesis, have the unfortunate implication that they modify the physico-chemical properties of the produced carbon nanotubes. That’s why carbon nanotubes need to be purified with the help of various methods such as acid treatment or ultrasound at the end of the production process.
Researchers from University of Wisconsin-Madison isolate ultra-high purity CNTs
Researchers have also struggled to isolate purely semiconducting carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and “short” the device. Researchers from University of Wisconsin-Madison led by Associate Professor Michael Arnold and Professor Padma Gopalan, has demonstrated transistors with on-off ratios 1000 times better, and conductance 100 times better than previous cutting-edge carbon nanotube transistors. To create their transistor, the research team used polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes. Another challenge that has limited the development of high-performance carbon nanotube transistors is to control the placement and alignment of nanotubes.
UW-Madison researchers also pioneered a new technique to align the nanotubes, called floating evaporative self-assembly, or FESA, by exploiting a self-assembly phenomenon triggered by rapidly evaporating a carbon nanotube solution. Another challenge is lack of control of the threshold voltage of CNT transistors that result in poor reliability and power-efficiency compared to rigid silicon chips. Zhenan Zhenan Bao and Yi Cui of Stanford University have developed a CNT circuit by doping it a blend of P-type and N-type semiconductors that can operate reliably despite power fluctuations and maintain low power consumption. Their technique involves depositing the dopant DMBI on the CNT circuit with an inkjet printer.
Carbon nanotubes have high thermal conductivity that could carry the excess heat away from the Computer chips being used in smartphones and supercomputers that are generating more and more heat as they are getting faster and faster. Frank Ogletree, a physicist with the Lawrence Berkeley National Laboratory’s Materials Sciences Division have developed a technique that uses organic molecules as a bridge between the carbon nanotubes and metal—a method that greatly reduces the interface resistance that would otherwise prevent heat from flowing more efficiently between the materials. The new study’s success rests upon the organic molecules, including aminopropyl-trialkoxy-silane (APS) and cysteamine, create strong covalent bonds between the carbon nanotubes and the metal used in microchips.
Navy researchers refining CNT-polymers for electronics
A new patent application made in June 2020 details a commercially relevant invention that directs the prep of carbon nanotube composites for creating thin, conductive layers that enable touchscreens and other electronic devices. The Navy is seeking the patent to protect the work of Prof. Claudia Luhrs and colleagues at NPS who published a journal article on their research investigating the ‘percolation threshold’ of conductive composites that contain carbon nanotubes. They called it the development of “a practical road map for controlling CNT composites’ conductivity.”
“Electrically conductive CNT composites are in high demand due to the broad range of applications that they could enable; from anti-static materials used in fuel tanks, housing materials and containers, aerospace structures and electromagnetic interference shielding systems, to sensors and conductors used as metal replacements and thermoelectric materials among others… However, despite the existence of numerous publications in regard to CNT composites and their properties, reports aiming to explain the mechanisms that dominate the electrical conductivity at extremely low loading CNT values (% wt CNT below 0.1%) are scarce,” they wrote in 2019.
The research included the investigation of epoxy-based CNT-composite prototypes using a scanning electron microscope. A comparison was made in the resistance of composites that were mixed with and without polymeric beads, and varying curing times and temperatures.
Russia discovers cheaper Production technique
One of the challenges is mass-producing a product with consistent properties, each individual carbon nanotube’s having identical conductive properties. The greatest challenge is being able to drive scale to volume and decrease cost. Today, multi-walled nanotubes are produced by CNano (U.S.), Arkema (France), Showa Denko (Japan), and Nanocyl (Belgium). But production of single-walled carbon nanotubes, which are considered of finer quality and are more expensive, until recently were carried out only in laboratories because their cost can exceed $150,000 per kilogram.
Mikhail Predtechensky, an academician from Siberia, was the first scientist to discover technology that can reduce the price of mass-produced single-walled nanotubes by 50 to 100-times, and to $3,000 per kilogram. Predtechensky co-founded OCSiAl, and in 2013 this company launched the world’s largest industrial system for synthesizing single-walled Graphetron 1.0 nanotubes. In the near future the company plans to establish in Novosibirsk a center for prototyping technologies based on single-layered carbon nanotubes to create rubber, composites, lithium-ion batteries, and many other materials.
Carbon nanotube transistors made in chip factories
Today, carbon nanotubes are used in various industrial markets. However, they have barely made a dent in semiconductor applications.
Carbon nanotube (CNT) transistors have been produced in commercial silicon chip facilities by US researchers, marking possibly the first time a post-silicon transistor intended for use in a computer processor has been made in this way.This is a milestone for CNT transistors as it shows that they are compatible with current manufacturing processes.
The new research uses incubation – a simple solution process widely used for making other types of transistor, but which had never been taken seriously for CNT transistors as the results are not quite as good as the very best achievable by other techniques. None of these more delicate methods, however, is practicable in industry. ‘For a long time, people – including us in our lab – were thinking we needed alignment, we needed the CNTs all super close together and specific distances apart,’ explains Bishop. ‘One of the big breakthroughs was realising that what we can do today can enable a lot of progress.’ Bishop and colleagues then optimised the incubation process for depositing CNTs. Simple innovations such as dry cycling, in which the silicon wafer is periodically removed and dried to stick the attached CNTs to the surface, reduced the production time from 48 hours to 150 seconds. The researchers then demonstrated nanotube transistor production independently at two commercial chip foundries.
The CNT transistors are not as good as the best silicon transistors yet, but the researchers believe that, after further development, the CNT transistors will ultimately prove superior to their silicon counterparts. ‘There’s research in the literature that, if you make the same 2D chip side-by-side but replace the traditional transistors with CNT transistors, the benefits are astronomical,’ says Bishop. ‘But it’s one thing to know those benefits could exist one day, and then there’s all the work to make it happen.’ Until then, the researchers believe scalable production of CNT transistors should enable them to scale-up circuits they have previously made that combine CNT transistors with traditional silicon ones.2
‘It’s very difficult to scale in the third dimension using existing silicon transistors, because they take over 1000˚C to build, so you destroy your bottom layer,’ Shulaker explains. ‘But the process that the team has developed is almost at room temperature, so we can build layers of CNT transistors directly on top of an existing CMOS chip,’ he adds. ‘We don’t need to beat silicon from day one: we can supplement it.’
‘It’s a nice blueprint for those of us who work with nanomaterials on how to take one of these technologies from the lab to a real-world situation,’ says electrical engineer Eric Pop of Stanford University in California, who was not involved in the research. He cautions, however, that the long-term stability and reliability of the devices still needs investigation. A crucial step, he says, is ‘building a 3D-based circuit that has advantages over a 2D circuit at industrial scale: otherwise the cost – which will go up – will not be worth it’.
3D Integration
“In the past decade, the multi-wall market has been diverse and relatively niche,” said Richard Collins, a principal analyst at IDTechEx. “Some of the more notable applications have been in conductive polymers for the likes of automotive fuel systems and IC trays. There have been other success stories in elastomers, coatings, and energy storage, but these have been relatively small volumes.” Carbon nanotube transistors are based on single-wall technology. “There are more potential advantages and a greater stepwise change rather than the iterative improvement of multi-wall. But the costs are high with limited capacity,” Collins said.
Like traditional transistors, which act like switches in devices, a carbon nanotube FET consists of a source, drain, and gate. The big difference is the channel, which allows electrons to flow from the source to the drain. In today’s transistors, the channel is based on silicon. In contrast, a carbon nanotube FET makes use of a fixed number of tiny and parallel nanotubes for the channels, each measuring 1nm in diameter. Leveraging the properties of these materials, carbon nanotube transistors exhibit high mobilities at low power.
“It’s a very good transistor,” said H.-S. Philip Wong, a professor in the School of Engineering at Stanford. “We’ve done a lot of theoretical analysis as far as looking at isolated experiments of testing, managing transistors, and measuring performance. It does outperform conventional silicon transistors, if every piece comes together.”
In addition, carbon nanotube transistors are fabricated at lower temperatures. “That makes it possible to build things in 3D. Many memory type devices also can be made at low temperatures. So there is an opportunity to build chips in 3D with highly dense connections between the memory and the logic device,” Wong said.
On paper, this solves a major problem. In systems, data moves between the memory and a processor. But at times, this exchange causes latency and power consumption. Bringing the memory closer to the logic processing functions promises to address these issues. For this, nanotube chips aren’t the only answer. The industry is developing several different chip technologies to address the problem. Another option is to integrate memory and logic in an advanced package.
TSMC, Stanford, and the University of California at San Diego have developed a new material, paving the way toward robust carbon nanotube FETs for beyond 5nm. A project led by DARPA is developing 3D devices, which stack memory on carbon nanotube transistors. The goal is to develop 3D devices on a 90nm process in 200mm fabs, which outperforms 7nm.
References and Resources also include:
https://techlinkcenter.org/news/navy-researchers-refining-cnt-polymers-for-electronics-touchscreens
