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FP7

IRENA Report Summary

Project ID: 604472
Funded under: FP7-NMP
Country: Finland

Periodic Report Summary 2 - IRENA (Indium replacement by single-walled carbon nanotube thin films)

Project Context and Objectives:
This project aims to develop high performance materials, i.e. both metallic and semiconducting single-walled carbon nanotube (SWCNT) thin films to completely eliminate the use of the critical metals in electron devices:
i) Indium in transparent conducting films (TCF, indium oxide doped by tin, ITO) and ii) Indium and Gallium as semiconductor In–Ga–Zn–O (a-IGZO) in thin film field effect transistors (TFTs). The target values for fully flexible transparent electrodes based on SWCNT thin films are 10 ohms/sq at 90% transparency, i.e. comparable to ITO-on-glass, with the midterm milestone of 40 ohms/sq at 90% transparency. The applicability of the developed SWCNT films will be further demonstrated in the high-performance TFTs on flexible and transparent polymer (Ion >1mA/mm, Ion/Ioff >10^5) and in 48 zone capacitive SWCNT touch sensor. Because of the rich resource of carbon element, recycling is not needed and the film material supports the friendly environment approach. During the growth of SWCNTs common transition metal nanoparticles will be used as catalyst. Based on the fundamental understandings, the performance and reliability of SWCNT transparent conductors and TFTs will be improved in this project, so that they can be used in highly performing products in the long term, such
as AMOLEDs and future flexible electron devices with very large commercial potential in future consumer electronics. The project contributes to reduce the European and Japanese electronics industry dependence
on the indium resources as well as the cost of manufacturing. Industrial partners from both Japan and EU will be invited to join the dissemination meetings to learn about the project results. Thus the project contributes
to increase the competitiveness of the industry, especially to the SME's developing novel flexible electronics products. Project involves 3 world class teams from both Europe and Japan, having complementary expertise in nanotube synthesis, thin film manufacturing and flexible device manufacturing, in addition to detailed modeling of nanotube growth and thin film charge transport processes. Active exchange of researchers (minimum of 12 person months from EU to Japan and vice versa) will deepen the EU-Japan collaboration. IRENA project European partners are Aalto University (Finland), AALTO, The Coordinator, DTU (Denmark) and CNRS (France).

Project Results:
We have further developed floating catalyst CVD (FC-CVD) synthesis methods using ferrocene decomposition to in-situ generate Fe catalysts using both CO and ethylene as carbon precursors. With the Fe-CO system we have focused on the effect of synthesis temperature as well as CO2 concentration to the yield, film sheet resistance, tube mean diameter and the (n,m) distribution. When operating the reactor at 880 oC, the 30 % increase of CO2 concentration increased the tube diameter from 1.3 nm to 1.8 nm, and very interestingly the fraction of the metallic tubes was increased to 39 % %, i.e. significantly above the standard 33 %. In other words, the semiconducting-to-metallic tube ration can be tuned via CO2 concentration, which is being further explored during the M37-M42. In addition we explored in more detail the effect of synthesis temperature from 680 to 920 oC, and found that the yield was highest at 820 oC, with the lowest sheet resistance of 80 ohms/sq at 90 % light transmission for the non-patterned films. At 880 oC the corresponding sheet resistance was 110 ohms/sq, which reduced to below 40 ohms/sq when patterning the film. Accordingly, with further optimizing the synthesis temperature as well as CO2 concentration, we aim to further reduce the sheet resistance of patterned film towards our goal of 10 ohms/sq at 90 % transmittance during M37-42. When using ethylene as the carbon source with ferrocene based Fe catalyst, we could similarly reach 70 ohms/sq sheet resistance for non-patterned film at 90m % transmittance. Here the synthesis temperature was 1050 oC, but very importantly for practical point of view, we used nitrogen as the carrier gas, with just below 10 volumetric % of hydrogen added. The (n,m) distribution was much broader than that of ferrocene-CO-CO2 system, with 33 % of metallic tubes observed with the electron diffraction.
Environmental TEM studies to directly observe individual tube growth mechanisms have been successfully carried out when using both CO as well as ethanol as the carbon source, and using Co catalyst nanoparticles stabilized at the MgO substrates. Both SWNTs as well as fullerene molecules are nucleated from the Co catalyst when using CO as the carbon source. In addition, the tube can detach from the catalyst particle, which can nucleate another tube or the fullerene molecule. The tube growth rate is not uniform. Very clean and long tubes were observed to grow when using ethanol as the carbon course.
We carried out conducting atomic force microscopy (C-AFM) measurements to probe the electrical conductivity of isolated junctions of both pristine and nitric acid treated between individual tubes, bundles and individual tubes as well as between bundles. Total of more than 70 junctions were analysed in detail. The resistance per unit length and the contact resistance of their junctions was found to be 3− 16 kΩ/μm and 29− 532 kΩ, respectively. In addition, the contact resistance decreased with increasing SWCNT or bundle diameter and depended on the contact morphology i.e. the angle between the tubes, reaching a value of 29 kΩ at a diameter of 10 nm. The nitric acid treatment dopes SWCNTs and reduces their average contact resistance by a factor of 3 while the resistance of the nanotubes remains largely unaltered.
In collaboration with Nagoya University, we have demonstrated up to 12*12 pixel capacitive, flexible and transparent touch sensors.

Potential Impact:
Rare metals like indium (In) and gallium (Ga) have high socio-economical and technological importance, while being prone to supply-demand fluctuations. Indium is currently used as ITO (indium-tin oxide) to provide transparent conducting films (TCF) for a wide variety of consumer electronics devices, such as displays as well as touch screens of mobile phones and ipad-style portable computers. The global annual market revenue for touch panels is about 25 B€, with annual growth rate of about 10 %. The market for flexible devices is now beginning to take off, with the forecast for the flexible display revenue to reach 20 B€ during 2020. In the flexible devices ITO cannot be used.
In order to replace indium, IRENA project aims to develop flexible single-walled carbon nanotube (SWNT) thin films. The final goal with respect to film conductivity and transmittance is to challenge ITO-on-glass i.e. 10 omhs/sq at 90 % transmittance. Now we have already reached the world record performance below 30 ohms/sq at 90 % transmittance with patterned SWNT film and 65 omhs/sq with non-patterned film, using direct dry nanotube deposition method from the floating catalyst CVD reactor for the film manufacturing. These both films surpass the properties of ITO on polymer (typically PET) thus being ready for the market adaption. More than 50 % of the TCF used in touch panels currently use ITO-on-PET. Accordingly, IRENA project results can already now contribute to really large global markets. After the final goal of 10 ohms/sq has been reached, SWNT transparent can replace indium in both flexible as well non-flexible markets. Accordingly, in the field of touch sensors, IRENA project results have potential to significantly impact the global market for ITO replacement and especially in the developing new markets of flexible touch panel products. The direct dry deposition SWNT thin film manufacturing method developed in this project is continuous, environmentally friendly yet economic one, being currently adapted by companies for flexible touch sensor development.
Polycrystalline silicon is the traditional semiconductor material used in the thin film field effect transistors (TFT-FET) of the high quality display back planes. However, silicon TFTs are not suited for the flexible devices. In–Ga–Zn–O (a-IGZO) TFT-FETs have been introduced to increase the flexibility, however the charge carrier mobility if IGZO is lower than that of polycrystalline silicon. In IRENA project we aim to develop SNWT TFT-FETs for the display backplane. We have already demonstrated SWNT network TFT-FETs with mobility comparable to that of IGZO devices, demonstrating the potential of nanotube transistors for flexible display applications. With further improvement of nanotube TFT-FET properties IRENA project will offer new solutions for the globally very large markets of backplanes in future flexible displays.

List of Websites:
http://irena.aalto.fi/

Contact

Riina Kero, (Project Manager)
Tel.: +358 50 5998599
E-mail
Record Number: 192733 / Last updated on: 2016-12-16
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