Researchers at the University of Southern California have demonstrated large, functional arrays of transistors made using simple methods from batches of carbon nanotubes that are relatively impure. For the first time someone has shown solution-deposited, purified semiconducting tubes for high-quality transistors,” says John Rogers, professor of materials science and engineering at the University of Illinois at Urbana-Champaign. “The accomplishment is in the integration of several promising approaches to demonstrate a full sequence for the fabrication of electronics.”
The USC researchers make large arrays of carbon nanotube transistors using solution-processing techniques at room temperature. They start by placing a silicon wafer in a chemical bath to coat its surface with a nanotube-attracting chemical, then rinse off the residue. The treated wafer is then immersed in a solution of semiconducting carbon nanotubes, which are attracted to its surface. The wafer, now coated with a carpet of nanotubes, is rinsed clean again. To make transistors from this tangled mess, the researchers put down metal electrodes at selected locations. The electrodes define where each transistor is and carry electrons into and out of the nanotubes that lie between them. Areas of silicon underlying each device act as the transistors’ gates. So far, they’ve built a prototype device on a four-inch silicon wafer and used it to control a simple organic light-emitting diode display.
The USC researchers are working to build a truly integrated organic LED display that is flexible and transparent. Such a display might be rolled up to fit in a pocket, or mounted on a car windshield to display information to the driver. The first step is eliminating the rigid silicon. Because the nanotubes may be laid down at room temperature, the USC researchers can build them on electrically active plastic sheets that can’t tolerate high temperatures. They’re also working to replace the stiff metal electrodes with a coating of indium tin oxide, a commonly used, flexible, transparent electrode material. In their prototype, the organic LED pixels are connected to the transistor array by wires; to integrate them they’ll need to come up with methods for building the LEDs on top of the control circuit.
Zhou says he is talking with display companies about commercializing these methods
Preseparated, semiconductive enriched carbon nanotubes hold great potential for thin-film transistors and display applications due to their high mobility, high percentage of semiconductive nanotubes, and room-temperature processing compatibility. Here in this paper, we report our progress on wafer-scale processing of separated nanotube thin-film transistors (SN-TFTs) for display applications, including key technology components such as wafer-scale assembly of high-density, uniform separated nanotube networks, high-yield fabrication of devices with superior performance, and demonstration of organic light-emitting diode (OLED) switching controlled by a SN-TFT. On the basis of separated nanotubes with 95% semiconductive nanotubes, we have achieved solution-based assembly of separated nanotube thin films on complete 3 in. Si/SiO2 wafers, and further carried out wafer-scale fabrication to produce transistors with high yield (>98%), small sheet resistance (25 kΩ/sq), high current density (10 μA/μm), and superior mobility (52 cm2 V−1 s−1). Moreover, on/off ratios of >10^4 are achieved in devices with channel length L > 20 μm. In addition, OLED control circuit has been demonstrated with the SN-TFT, and the modulation in the output light intensity exceeds 10^4. Our approach can be easily scaled to large areas and could serve as critical foundation for future nanotube-based display electronics.