Color-coded nanotubes Fabrication and Processing

May 30, 2008

Conductive, flexible carbon nanotube films on flexible plastic substrates. The films are arranged in order of increasing average diameter (clockwise starting from lower left): 0.9 nm, 1.0 nm, 1.05 nm, 1.1 nm, 1.4 nm, and 1.6 nm. The ability to control nanotube diameter leads to the visible colors that are apparent in the photograph. (Courtesy of Mark Hersam.)

Indium tin oxide is a popular choice as a transparent, conductive coating for devices. However, this material is expensive, brittle, and has limited optical tuning and chemical stability. Single-walled carbon nanotubes (SWNTs) are more promising because they exhibit very good electrical, optical, chemical, and mechanical properties. However, the synthetic methods used to prepare SWNTs often give polydisperse mixtures containing both metallic and semiconducting species.

Now, researchers from Northwestern University have overcome this problem and come up with a method to prepare thin conducting films composed primarily of metallic SWNTs [Green et al., Nano Lett. (2008) doi:10.1021/nl080302f]. These films are optically transparent, flexible, and exhibit defined colors.

SWNTs vary in helicity and diameter. The team use density gradient ultracentrifugation (DGU) to sort the SWNTs by diameter and obtain predominantly metallic tubes with uniform electrical and optical properties.

Optical absorption is strongly dependent on diameter and as a result, a series of colored samples of SWNTs are obtained. These can be processed into colored films which look like stained glass, but unlike glass they exhibit high electrical conductivity and mechanical flexibility. The colored films enable greater control over the optical absorption of any device onto which the SWNTs are coated.

The new films register a five-fold increase in electrical conductivity in the visible region and a ten-fold increase in the infrared region, when compared with films made from unsorted SWNTs.

Katerina Busuttil