After considerable research efforts, carbon nanotube (CNT) products are coming to the market. After two decades of extensive research, single-walled carbon nanotubes (SWNT) are at last reaching multi-ton industrial production levels. Due to their excellent optoelectrical performance, processability, stability, and high conductivity, CNT-based transparent electrode films have been put forward as a candidate to replace indium tin oxide (ITO) currently used in touchscreens and displays. CNTs are deposited in thin films, leading to a conducting layer which can also be transparent. In relation to ITO they are more cost effective, have higher resistivity and greater flexibility. CNT electronic display applications at varying stages of commercial development include large area CNT flat screen color field emission displays, large area surface conduction colour field emission displays, backlights for displays and PETS for medium resolution large area electronic billboards.
COMMERCIALIZATION
First generation multi-walled carbon nanotube (MWNT) products include composites for automotive parts, sporting good and boat hulls and water filters. Field and thermionic emission electron sources, high strength CNT yarns, electrodes for batteries and supercapacitors, loudspeakers, displays, SERS substrate, IR detector etc. have also been demonstrated. CNT TEM grids and CNT touch panels have already been commercialized.
CNTs have the potential to be the cornerstone of future electronics due to a superior combination of properties:
– 1000 times more electrically conductive than ITO
– 100 times stronger than steel while being 10 times lighter
– transparent and can be bent and flexed without reduction in these values. These materials have not yet reached significant commercial application due to complexities in forming realistic commercial devices that take advantage of the promising properties of the nanotubes. Most importantly, CNTs are abundant, and already cheaper than ITO, making them an excellent candidate material for ITO replacement in transparent electrodes.
SWNT PRODUCTION INCREASES
Single wall carbon nanotubes (SWNTs) have shown extraordinary electronic and optical properties since their discovery in 1991 by Iijima etal. Currently, they several major issues including high levels of impurities, low levels of productivity and high manufacturing costs, which are about several tens of thousands to hundreds of thousands of dollars per gram.
In May 2014, OCSiAl unveiled a breakthrough technology for the production of SWNTs, which enables large scale, commercial production. Estimated production of the SWNTs is 1 ton for the first year, which doubles current global production levels according to different estimates. Potential annual capacity of OCSiAl production is currently 10 tons. This method is easily scalable and capable of producing an unlimited quantity of SWNTs at a very low cost.
Zeon Corporation also announced in May that they will launch a commercial SWNT production plant based on super-growth technology. This method, discovered in 2004 by a team led by Dr. Kenji Hata at the Incorporated Administrative Agency National Institute of Advanced Industrial Science and Technology (AIST), is used to synthesize SWNTs. In 2011, ZEON and AIST built a demonstration plant for mass production funded by the Ministry of Economy, Trade and Industry’s supplementary budget for fiscal 2009, and have been operating the plant and disseminating technology by providing samples.
CNTs produced with the Super Growth Method demonstrate unique properties, including a high aspect ratio, high purity and highly specific surface area, and they hold promise for applications such as new specialty materials with non-conventional functionality and characteristics, and next-generation devices.
The NEDO project has already devised potential ways of incorporating carbon nanotubes into innovative materials and devices such as high-performance capacitors, highly functional rubber materials and materials with high thermal conductivity, and demand for these products is expected to grow.
The technology necessary for mass production called super growth process is close to being established. Currently, the
Tsukuba validation plant is able to produce 600 grams per day, and a decision to construct new facilities within the Tokuyama Plant in May 2014 has been made. Mass production is expected to start from the second half of 2015.
FLEXIBLE ELECTRONICS
Flexible and stretchable electronics are attracting great attention because of the variety of potential applications from flexible e-papers to wearable healthcare devices. The development of future flexible and transparent electronics relies on novel materials, which are mechanically flexible, lightweight and low-cost, in addition to being electrically conductive and optically transparent.
CNTs are highly promising for application to flexible electronics not only as thin film transistors (TFTs) but also passive devices such as transparent conductive films and trace, as CNTs have a high mobility of 10-100cm2/Vs at room temperature in addition to mechanical stability (high mechanical strength and elasticity) and chemical stability (thermal and chemical resistance).
CNT thin films have advantages in flexibility, stretchability, and performance because of these excellent electronic and mechanical properties.
Low cost manufacturing of flexible devices is also possible with good processability of CNT films. Their optical transparency is also attractive for transparent electronics applications. Applications based on CNT thin films include capacitive touch sensors, high-mobility CNT-TFTs and integrated circuits (ICs) on a transparent plastic film, all-carbon ICs demonstrating excellent stretchability and mouldability, and high-mobility TFTs fabricated with high-speed flexographic printing technique.
Transparent conductive films
The demand for transparent conductive films (TCFs) is expected to grow rapidly as electronic devices, such as touch screens, displays, solid-state lighting and photovoltaics become ubiquitous. Indium tin oxide (ITO) is the dominant material for transparent electrode used in touch screens, LCD displays, solar cells, and solid state (OLED) lighting due to its relatively high transparency at high conductivities. However, ITO is brittle, lacks flexibility and the fabrication process involves high temperatures and vacuum and therefore is relatively slow and not cost effective. Other conductive metal oxides used include antimony-tin oxide, fluorine-doped tin oxide and aluminum-doped zinc oxide.
CNTs and a variety of other nanomaterials provide cheaper alternatives that also allow devices to become flexible. CNT transparent conductive films have demonstrated great potential in various optoelectronic devices and have already been used in touch panels for smartphones. Main companies developing CNTs-TCFs are Canatu, Dow, Eikos, Linde, Toray, and Unidym.
C3 Nano, Inc. (www.c3nano.com) is a recent start-up developing TCFs comprising CNT inks. These conductive inks can comprise a polar solvent-based solvent system, such as water-based, alcohol-based or solvent combinations thereof, carbon nanotubes, and one or more dopants, generally including an ionic dopant. The company states that the primary issue concerning nanotubes as ITO replacements to date is inferior conductivity, and they have addressed this issue by using an effective combination of a multifunctional dispersant that also acts as a p-type dopant to the CNTs in the TCF.
Besides C3 Nano, Inc. CNT inks are marketed by a number of companies, such as Eikos, Inc. (www.eikos.com) for printed electronics applications. These inks can be deposited easily onto a variety of rigid and flexible substrates with standard coating techniques including spray-coating and Aerosol Jet printing. Inkjet printing of CNTs is a more recent method that shows promise.
The method is currently being used to deposit various types of conductive nanomaterials such as gold and silver. Although these metals are excellent conductors, CNTs are cheaper and more versatile as they can behave as both a semiconductor and a conductor.
In July 2013, Linde Electronics launched SEERe- ink, licensed from the London Centre for Nanotechnology (LCN). SWNTs are difficult to isolate, and hence purify, due to their strong tendency to ‘bundle’ together.
Figure 1: Nanotube inks (Nanointegris)
They are also expensive to produce. The LCN approach provides a scalable, bulk technique that avoids the limiting sonication/centrifugation steps associated with other methods (https://www.london-nano.com/sites/default/files/uploads/research/highlights/Dissolution%20and%20separation%20of%20single-walled%20carbon%20nanotubes.pdf).
In July 2013, CNTouch announced they had received significant orders for their nanotube-based touch panels for entry-level and mid-range smartphones. The company is a subsidiary of Foxconn and the technology has been developed with Tsinghua University.
Thin-film transistors
Main materials currently used for TFTs are amorphous silicon and poly-silicon. However, these materials are processed under high temperature and vacuum conditions. As a result it is difficult with conventional technologies to manufacture TFTs on low-cost plastic film substrates. Therefore CNTs have emerged as a prime candidate for manufacturing TFTs on plastic films at low temperatures.
In September 2013, NEC and the Technology Research Association for Single Wall Carbon Nanotubes (TASC) demonstrated a CNT-TFT. In addition to suppressing noise emitted during high-speed operation to one-tenth or less than that of ordinary printed transistors, this newly developed printed CNT-TFT realizes an operating speed of 500kHz, which is 10 to 50 times that of ordinary printed transistors, thanks to an output current dozens of times higher than existing printed transistors. As a result, the level of performance necessary for control circuits can be attained, thereby making it possible to apply this printed CNT-TFT to new devices in the future, including flexible large displays and sheets mounted with multiple sensors.
Toray has also developed a spray-coated CNT-TFT demonstrating the world’s highest performance levels: mobility of 2.5cm2/Vs and on/off ratio of 106, by combining its proprietary semiconductor polymer and single wall carbon nanotubes.
In June 2014, Aneeve Nanoechnologies LLC announced they had overcome a major issue in carbon nanotube technology by developing a flexible, energy-efficient hybrid circuit combining carbon nanotube thin film transistors with other thin film transistors. The company stated that this hybrid could take the place of silicon as the traditional transistor material used in electronic chips, since carbon nanotubes are more transparent, flexible, and can be processed at a lower cost.
Further information on the market for Carbon Nanotubes is available in the report “The Global Market for Carbon Nanotubes to 2024”, published by Future Markets, Inc., July 2014 (https://www.futuremarketsinc.com/index.php/nanoreports-63/nanotubes).
