PORTLAND, Ore.—Today a great deal of energy is being expended in research organizations to refine and perfect methods of sorting nanotubes so that ink can be produced that is conducting, semiconducting and insulating. With inks in hand, transistors and other circuit elements can be printed by ink-jet printers on flexible polymer substrates.
Now Rice University researchers claim to have found a method that sidesteps the costly and difficult sorting step, enabling unsorted nanotube circuitry to be ink-jet printable without presorting.
The relatively simple method discovered by Rice faculty Fellow Robert Vajtai and professor Pulickel Ajayan, ink-jet prints unsorted nanotubes in layers. By controlling the number of layers printed on each circuit trace, the researchers demonstrated how to ink-jet print the source, drain and dielectrics necessary to create field-effect transistors (see figure).
Nanotube inks can be used to print field-effect transistors without the need for a costly and difficult presorting step, according to Rice University researchers (click on image to enlarge).
The researchers' explanation of the technique was that each added layer of nanotube ink increases the interconnection paths among the randomly placed nanotubes, smoothly increasing its conductivity from semiconductors to conductors. The researchers also pre-treated the unsorted nanotubes with the polymer like polyethylene glycol (PEG) when they wanted semiconductors and with carboxylic acid when they wanted conductors. PEG was used by itself when they wanted a dielectric.
The resulting FETs were extremely large—about a square millimeter. Their next step is to scale down the process to produce FETs in the 100 micron range, with gate lengths of about 35 microns or smaller.