The NanoMaterials Group at Helsinki University of Technology (HUT) has been busy developing and investigating aerosol (floating catalyst) methods of single-walled carbon nanotube (CNT) synthesis. In co-operation with the Microfabrication Group of the same university and the Nanoscience Laboratory of Nokia Research Center in Finland, the researchers have implemented their novel CNT growth and dry room-temperature deposition techniques to fabricate thin film transistors (TFTs) based on random CNT networks (CNTNs) both on rigid and flexible substrates.

A random CNTN, as a semiconducting material for TFTs, can show higher performance in comparison with its organic counterpart and allows low-cost fabrication. Using the developed aerosol methods, the synthesis and deposition of CNTNs can be accomplished in a simple single-step process without additional multi-step CNTN preparation, which can be harmful to the pristine CNTs. Room-temperature deposition of CNTNs is especially important for heat intolerant flexible substrates for applications such as e-paper, smart tags, wearable displays and artificial skin.

Two types of transistor structures (see schematic view above) were fabricated in order to evaluate the performance of dry-processed CNTNs: (a) bottom-gate transistors on conventional Si/SiO2 substrates and (b) top-gate transistors on flexible polymer (e.g. kapton) substrates. Experimental evidence reproducibly demonstrates the high performance of TFTs with on/off ratios up to 105 and field-effect mobilities up to 4 cm2/Vs.

The suppression of hysteresis in the transfer characteristics of the bottom-gate CNT TFTs was also studied. Thin alumina films prepared by atomic layer deposition (ALD) technique resulted in almost complete elimination of hysteresis.

Full details of the method can be found in Nanotechnology.