Antti Kaskela is working as a researcher and pursuing his D.Sc. in NanoMaterials Group (NMG) (Group's web pages) lead by professor Esko I. Kauppinen at the Department of Applied Physics. He studies synthesis and applications of single-walled carbon nanotubes (SWCNTs), mainly as optically transparent electrodes. He received his M.Sc. degree in Physics from the University of Helsinki in 2007. Prior to joining the NMG, he worked in Helsinki Institute of Physics and in CERN with microelectronics manufacturing and quality control development. Besides materials research he's also interested in business and economics: He's participating in the Masters Program at the Department of Industrial Economics and Management, he has co-founded several successful web-stores and worked as management consultant for several Finnish companies.

Macroscopic components out of nano-scale materials. Transparent SWCNT electrode on a flexible and transparent polymer substrate is shown against white background. The aerosol based synthesis method allows fabrication of both uniform and patterned large area conductive coatings, which can be used for variety of applications.

Single-walled carbon nanotubes (SWCNT) are high aspect ratio cylindrical structures consisting of sp2-hybridized carbon atoms. Carbon atom orientation (chirality) determines whether a tube is semi-conductive, semi-metallic or metallic. SWCNTs have high current carrying capacity and charge carrier mobility and exceptional optical, mechanical and thermal properties. As controlling the tube chirality is still a largely unsolved challenge for the research community, random networks have been considered as the first route to applications of SWCNTs as the required level of control over the nanotube characteristics is less stringent.

During my research in the NMG we have been able to significantly improve the performance of carbon nanotube networks, developed a processing method to deposit these networks onto a variety of substrates and also studied several applications of the nanotube electrodes. The major performance improvements were achieved as we learned to gradually increase the length of the carbon nanotube bundles, which are the building blocks of network electrodes, from 1 µm to 10 µm. The length control is important as the resistance of the network is mostly due to highly resistive bundle-bundle connections and increasing bundle length reduces the number of contacts and the network resistance, while the optical absorption of the network remains unaffected. The novel aerosol-based carbon nanotube synthesis, developed in the NMG, allows collecting of nanotubes directly into optically and electrically uniform assemblies by simple membrane filtration. The as-synthesized SWCNTs are of high quality and require no harsh wet processing like chemical purification and dispersion steps, which are commonly used with other SWCNT synthesis methods.

Dry processing of SWCNT-networks allows simple and fast room-temperature press transfer directly from the membrane filter to target substrates and is compatible with industrial roll-to-roll processing. The transfer process can be used with a wide array of substrates ranging from flexible polymers, like PET or PEN, to metals and to rigid substrates such as glass and silicon. The dry deposited network electrodes can be functionalized on the substrate by rapid exposure to acids or reactive gasses to further enhance the electrode performance. Chemically functionalized SWCNT-networks with sheet resistance as low as 110 Ω/□ at T=90% have been successfully fabricated. The performance matches typical ITO on flexible polymer substrates and is the lowest reported sheet resistances for SWCNT-based transparent electrodes. This is a very significant result as the currently used transparent metal oxides, such as indium-tin oxide (ITO), have several drawbacks, including high refractive index and haze, spectrally non-uniform optical transmission, limited flexibility, restricted chemical robustness and limited raw material supply.

The flexibility of the synthesis and deposition methods allows using of SWCNT electrodes in several device applications. We have fabricated extremely transparent touch sensors by using capacitive touch sensing technology, the same as used in most modern smart-phones. We have demonstrated SWCNT-electrodes in Organic Light Emitting Diodes (OLEDs) as a step towards display applications with collaborators from the University of Texas in Dallas. Furthermore, as the dry transfer method is also compatible with very thin semi-conductive SWCNT-networks, we have used it to fabricate thin film transistors with SWCNT-network channels. We have also studied utilization of SWCNT-electrodes in energy generation applications by using the networks as flexible and transparent counter electrodes in dye sensitized solar cells in collaboration with the New Energy Technologies Group at the Department of Applied Physics. The aerosol synthesis and dry transfer method combined is a powerful toolkit, which enables fabrication of multifunctional and high performance device components.

The equipment and infrastructure provided by the Department of Applied Physics, the National Nanomicroscopy Center and the Center for New Materials has been very beneficial for my research. Nanomaterials characterization and processing requires a wide array of equipment and competent collaborators which are available within the collaborative network. We have also worked closely with a spin-off company of our group, Canatu Ltd., which is working with commercialization of the carbon nanostructure based electrodes. I have really enjoyed the collaboration and the quick feedback between research findings and the company's industrial R&D. The possibility to contribute to the development of a high-potential company is a unique opportunity for a materials scientist as the true justification of applied materials research lies in the industrial applications, which enable growth of new businesses and by that contribute back to the Finnish society in general.

References

[1] Kaskela, A. et al. Aerosol synthesized SWCNT networks with tuneable conductivity and transparency by dry transfer technique. Nano Lett. DOI: 10.1021/nl101680s (2010).

[2] Moisala, A. et al. Single-walled carbon nanotube synthesis using ferrocene and iron pentacarbonyl in a laminar flow reactor. Chem. Eng. Sci. 61, 4393-4402 (2006).