Using NEDO’s technology, the researchers continuously grow nanotubes in an atmospheric pressure chemical-vapor deposition process. The nanotubes are then collected on the filter and subsequently transferred onto a polymer substrate using simple gas-phase filtration and transfer processes to achieve clean, uniform carbon nanotube films. It takes only a few seconds to deposit the carbon nanotubes. This process may be adaptable to high-speed roll-to-roll manufacturing systems in the near future (hoping for commercialization by 2016).
Carbon nanotube thin-film transistors are expected to enable the fabrication of high-performance, flexible and transparent devices using relatively simple techniques. However, as-grown nanotube networks usually contain both metallic and semiconducting nanotubes, which leads to a trade-off between charge-carrier mobility (which increases with greater metallic tube content) and on/off ratio (which decreases). Many approaches to separating metallic nanotubes from semiconducting nanotubes have been investigated but most lead to contamination and shortening of the nanotubes, thus reducing performance. Here, we report the fabrication of high-performance thin-film transistors and integrated circuits on flexible and transparent substrates using floating-catalyst chemical vapour deposition followed by a simple gas-phase filtration and transfer process. The resulting nanotube network has a well-controlled density and a unique morphology, consisting of long (~10 µm) nanotubes connected by low-resistance Y-shaped junctions. The transistors simultaneously demonstrate a mobility of 35 cm2 V–1 s–1 and an on/off ratio of 6 × 10^6. We also demonstrate flexible integrated circuits, including a 21-stage ring oscillator and master–slave delay flip-flops that are capable of sequential logic. Our fabrication procedure should prove to be scalable, for example, by using high-throughput printing techniques.
CDRInfo – As part of NEDO’s Industrial Technology Research Grant Japan-Finland collaborative project, Professors Yutaka Ohno from Nagoya University in Japan and Esko I. Kauppinen from Aalto University in Finland along with their colleagues have developed a simple and fast process to manufacture high quality carbon nanotube-based thin film transistors (TFT) on a plastic substrate. They used this technology to manufacture the world’s first sequential logic circuits using carbon nanotubes.
According to the researchers, the gas-phase filtration and transfer processes can be applied to manufacture devices on any substrate material.
The researchers tested the new technology by integrated the high-performance carbon nanotube TFTs on plastic substrates. They claim thet they achieved successful operations of ring oscillators and flip-flops.
“High-speed operations have been achieved with a delay time of 12 microseconds per logic gate. The flip-flops that have been manufactured through these processes are the world’s first carbon nanotube-based synchronous sequential logic circuits,” the researchers added.
As the researchers explained in their study, nanotube networks contain both metallic and semiconducting nanotubes. While a greater amount of metallic nanotubes increases the transistor’s charge-carrier mobility, it also decreases the on/off ratio.
Since both of these characteristics are important for overall transistor performance, the researchers in the new study found a way to optimize both characteristics by fabricating a nanotube network with certain unique properties. For instance, the network’s morphology consists of straight, relatively long (10 micrometers) nanotubes (30% of which are metallic) compared to other nanotube networks. The new network also uses more Y-junctions than X-junctions between nanotubes. Since Y-junctions have a larger junction area than X-junctions, they also have lower junction resistance.
Using this nanotube network, the researchers fabricated TFTs that simultaneously demonstrate a high charge-carrier mobility and on/off ratio, offering significantly better performance than previous nanotube-based transistors. The researchers explained that the high mobility is due to the nanotube network’s unique morphology, while the high on/off ratio can be attributed to the lower density of metallic nanotubes, which can be controlled during the fabrication process.
After building the transistors, the researchers fabricated an IC capable of sequential logic – the first such circuit based on carbon nanotube transistors to date. In sequential logic circuits, the output depends on both the present input as well as the history of the input, so that these circuits have storage or memory functions.
The researchers predict that, by scaling up the fabrication process and using improved printing techniques, these nanotube-based TFTs could lead to the development of large-scale, inexpensive, and flexible electronics.
“Our near-future plan is to demonstrate roll-to-roll fabrication of CNT-based TFT arrays and ICs,” Ohno said. “To do so, we need to replace all the lithographic techniques by high-throughput printing techniques. For commercialization, we have to improve the uniformity of TFT characteristics more, but we are aiming at commercializing within five years.”
Sheet resistance and optical transmittance distribution on carbon nanotube film transferred to PET substrate from filter with diameter of 110 mm.