X-Ray Probe Finds New Organic Transistors Do Well in Hot Water
Recent years have seen advances in organic microelectronics that replace rigid crystalline materials such as silicon with flexible polymeric materials. Engineers are eyeing a long list of potential applications such as lightweight computer displays that could be printed on a film and rolled up or folded. But, flexible organic circuits also could have broad application in medical devices, especially implantable devices like soft pacemakers.
However, such devices would have to be sterilized at high temperatures, and organic electronics that do not break down under such temperatures have been hard to make. A particular problem is the "gate insulation" layer in an organic transistor, which has to be extremely thin to hold down the operating voltage to a reasonable level while maintaining electrical integrity under heating. When heated to sterilizing temperatures, the thin films have tended to develop multiple "pinholes" that wreck performance.
To solve this, the researchers from the University of Tokyo and participants from the Japan Science and Technology Agency, Princeton University, the Max Planck Institute for Solid State Research, Hiroshima University and Nippon Kayaku Co., Ltd. of Tokyo set about research that put to use the NIST low-energy X-ray beam line at the National Synchrotron Light Source (NSLS) in Brookhaven, N.Y.
The Tokyo-based team proposed a novel gate material made of alkylphosphonic acids that "self-assembles" into an ultrathin single layer of densely packed linear molecules that line up at a slight angle to the surface rather like the hairs on a retriever. The thickness of this self-assembled monolayer (SAM) can be as small as 2 nm, according to the research team.
Making accurate structural measurements of such a thin film is difficult. To check the molecular orientation and thermal stability of the SAM, samples from before and after heat treatment were examined on the NIST beamline using a technique called "near-edge X-ray absorption fine-structure spectroscopy" (NEXAFS). The technique essentially detects chemical bonds both at the surface of a sample and in the interior, and is extremely sensitive - capable of telling the difference between a single and double carbon bond in a molecule, for instance. Pinholes in the SAM are visible because NEXAFS sees through them to the underlying substrate. The NEXAFS measurements demonstrated that the new SAM thin films maintained their stability and integrity at temperatures in excess of 300°F (150°C). This is believed to be the first time such high thermal stability has been observed in such a thin film.
For more details, see www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=1396.