Carbon nanotubes (CNTs) are the material of ever-increasing concern due to their excellent electronic and physicochemical properties. Currently, research has been focused towards their applications for use in chemical and biological sensors as well as their use in optoelectronic devices. One of the major challenges is the construction of flexible electronic devices.

Transparent and electrically conductive materials such as oxide semiconductors (indium tin oxide (ITO)) can be used for versatile applications for flexible electronic devices such as displays, solar cells, and sensors. But ITO is in short supply and is becoming very expensive. ITO also is an inorganic, brittle material so it is limited in its use with flexible substrate. Thus, a transparent, flexible, and conductive coating composite built with carbon nanotubes may offer an alternate solution.

The primary objective of this work was to find candidate CNT-based coatings to achieve high electrical conductivity along with high transparency at 400 to 700-nm wavelength range once coated on flexible transparent substrate. The secondary objective was to uncover the fundamentals associated with adhesion of the coating to the surface of the substrate. The third objective was to fabricate sample electronic devices with CNT-coated substrate to test its potential commercial applications.

Among several CNT samples tested, the best performance result was with metallic SWNT (m-SWNT) coated on PEN (poly-ethylene-naphthalate) transparent composite film. It provided conductivity of 130 Ω/sq. with 80% transmittance at 400 to 700-nm wavelength range. In contrast, commercially available single-side, inorganic, brittle ITO-coated PET film gave 88 Ω/sq. with 80% transmittance. The sample mentioned was prepared with double-side coating using a dipping method. Once one side is coated, it is estimated that transmittance can probably reach close to a 90% level. Based on this assumption, we can draw a conclusion that one-side m-SWNT coated substrate is close to meeting requirements of several electronic applications such as EMI, displays, and touchscreens.

It was discovered that NMP (n-methyl-pyrolidone) solvent disperses m-SWNTs well with no use of surfactant. Besides achieving high electrical and optical performance with m-SWNT use, it was also found that CNTs adhere well to PEN (poly-ethylene-naphthalate) film surface and it passes a tape adhesion test.

Sample electronic devices such as PLEDs, solar cells, and methanol sensors were manufactured with the transparent, flexible SWNT-coated substrate. The sample devices show promise that SWNT-coated substrate as a component has the potential to be used for the fabrication of flexible electronic devices.

This work was done by Dr. Duck J. Yang of the University of Texas at Dallas for the Air Force Office of Scientific Research.


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Novel Conductive Coatings of Carbon Nanotubes

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This article first appeared in the April, 2009 issue of Defense Tech Briefs Magazine.

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