Semiconductor CNTs Separated With Low Energy Consumption

Nov 6, 2013
Tetsuo Nozawa, Nikkei Electronics
A voltage of 4.5V was applied by using dry-cell batteries. The diameters of the CNTs are 1.0, 1.3 and 1.7nm. The upper layers are metallic CNTs, and the lower layers are semiconductor CNTs.
A voltage of 4.5V was applied by using dry-cell batteries. The diameters of the CNTs are 1.0, 1.3 and 1.7nm. The upper layers are metallic CNTs, and the lower layers are semiconductor CNTs.
[Click to enlarge image]
The carrier mobilities and on/off ratios of the CNT transistors prototyped by TASC, etc. Though there are variations in the values, lower values are caused by the variation in the application of CNT ink.
The carrier mobilities and on/off ratios of the CNT transistors prototyped by TASC, etc. Though there are variations in the values, lower values are caused by the variation in the application of CNT ink.
[Click to enlarge image]

Japan's National Institute of Advanced Industrial Science and Technology (AIST) organized "AIST Open Lab," an event to show its laboratory, which took place from Oct 31 to Nov 1, 2013, in Tsukuba City, Ibaraki Prefecture, Japan.

At the event, AIST, NEC Corp, etc demonstrated "electric-field-induced layer formation (ELF)," a technology of separating carbon nanotubes (CNTs) into semiconductor type and metal type.

The basic principle of ELF is electrophoresis. After putting single-wall CNTs, water and a polymer surface-active agent that does not contain ions such as Na+ as dispersants in a glass container, a voltage of 4.5V is applied by using electrodes that have different lengths.

Then, semiconductor and metallic CNTs are separated, forming two layers like oil and water, in one to 19 hours. The purity of the semiconductor CNTs reaches 95-98% or even higher. The method can deal with CNTs whose diameters are from 0.9 to 2.1nm.

To separate single-wall CNTs into semiconductor type and metal type, tremendous amounts of energy and cost have been required to use an ultracentrifuge for 10 hours or longer, etc. On the other hand, the ELF technology can be used only with three dry-cell batteries or so, drastically saving energy and cost. So, it realizes a high separation accuracy with a method so simple that it brings us back to basics.

Because ELF uses a surface-active agent that does not contain ions such as Na+ as dispersants, it is very suited for forming CNT transistors.

NEC, the Technology Research Association for Single Wall Carbon Nanotubes (TASC), AIST, etc prototyped a CNT transistor with a semiconductor CNT ink made by using the ELF technology. Its carrier mobility is 10cm2/Vs. The on/off ratio of current is 104.