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Materials scientists from the National Institute of Standards and Technology (NIST), working with an international research team, have helped prove the stability of a novel—and rugged—thin-film membrane that could prove key to a new class of sterilizable, flexible organic electronics for medical applications.
The work at the NIST low-energy X-ray beam line at the National Synchrotron Light Source (NSLS) in Brookhaven, N.Y., supported an international team led by researchers from the University of Tokyo and including 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.*
Recent years have seen significant 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 as the study's authors point out, flexible organic circuits also could have broad application in medical devices—especially implantable devices, like soft pacemakers.
But such devices would have to be sterilized at high temperatures, and organic electronics that don't break down under such temperatures have been hard to make. A particular problem is the all-important "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 Tokyo-based team proposed a novel gate material** 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 nanometers, 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 150º Celsius. This is believed to be the first time such high thermal stability has been observed in such a thin film.
* K. Kuribara, H. Wang, N. Uchiyama, K. Fukuda, T. Yokota, U. Zschieschang, C. Jaye, D. Fischer, H. Klauk, T. Yamamoto, K. Takimiya, M. Ikeda, H. Kuwabara, T. Sekitani, Y-L. Loo and T. Someya. Organic transistors with high thermal stability for medical applications. Nature Communications. 3, 723. Mar. 6, 2012. doi:10.1038/ncomms1721
** alkylphosphonic acids
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