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Large Area Printed Transfer of Aligned Carbon Nanotubes for Transparent Electronics Applications

Written by: 
Jeff Morse, PhD.
In an ACS Nano Article ASAP for December, Ishikawa and colleagues report several remarkable results associated with the use of aligned single wall carbon nanotubes to create transparent thin film transistor arrays.

A review by Jeff Morse, Ph.D., National Nanomanufacturing Network

Transparent electronics will find use in a range of imaging, display, and optoelectronic applications and, if fabricated on flexible substrates, enable novel ideas such as e-paper and wearable displays to be realized. Typical transparent  electronics  are thin film transistor devices formed using amorphous semiconductor materials that exhibit low mobility due to the range of materials being considered, such as amorphous Si, In2O3, and ZnO based composites. Furthermore, the low processing temperature due to the substrate on which the transparent electronic devices are fabricated limits the quality of the semiconductor material and electrode contacts formed.

In this context there continues to be a large focus on improved materials for transparent electronic devices with the goal of increased carrier mobility and low processing temperature. Single wall carbon nanotubes (SWNT) are a promising candidate due to intrinsic mobility that exceeds 100,000 cm2/V-sec along with excellent p-type semiconductor properties, optical transparency, and mechanical flexibility.

Ishikawa Figure 1
Fabrication of fully transparent aligned SWNT transistors.
Recently, work by Ishikawa, et. al., reported several remarkable results associated with their investigation of using aligned single wall carbon nanotubes to create transparent thin film transistor arrays. Typical growth temperatures of CVD carbon nanotubes are on the order of 900°C, too high for flexible plastic substrates and even some low cost glass substrates. In order to form SWNT electronic devices on flexible transparent substrates, a novel printing transfer methodology was developed that allowed aligned SWNTs grown at a high temperature to be transferred to the low temperature transparent substrate.

The transparent substrate was first coated with pre-patterned gate electrodes made from indium tin oxide (ITO), a transparent conductor. The gate electrodes were then coated with a dielectric. The aligned carbon nanotubes were coated with a thin layer of gold after growth, and the entire array was transferred using an adhesive tape that pulled them from the growth substrate. After placement on the transparent substrate, the tape was released by ramping the temperature to 130°, which is beyond the melt point for the adhesive. The thin gold layer was then chemically etched, followed by lithographic patterning and deposition of Au-ITO thin films to form the source and drain electrodes.

The performance of the thin film transparent SWNT-based transistor exhibited good transconductance, very high on/off ratios, with estimated carrier mobility in the 800-1300 cm2/V-sec range depending on process parameters and substrate on which the devices were fabricated. Furthermore, the entire processing method was conducted at very low temperatures compatible with the flexible substrates. The variation of device performance was approximately 30%, indicating a high level of uniformity for the nanotube network. 

Nanomanufacturing of aligned nanotube devices and circuits having high mobility has benefits including low voltage operation, low power consumption, and fast switching speeds, attractive attributes for applications for portable displays and other portable and even wearable consumer electronics.  Furthermore, the printing transfer method is compatible with roll-to-roll fabrication processes and holds promise for rapid scaleup manufacturing processes for these applications. While excellent performance for the aligned nanotube electronic devices has been demonstrated, further improvements in placement and alignment of nanotube structures will improve uniformity, transistor process flow, and packing density, making this technique competitive for small and medium scale integrated circuit design applications.

Image reprinted with permission from Ishikawa, F. N., et. al., "Transparent Electronics Based on Transfer Printed Aligned Carbon Nanotubes on Rigid and Flexible Substrates," ACS Nano (Web December 10) 2008. Copyright 2008 American Chemical Society. 

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