Researchers at Harvard University and the University of Hawaii have taken an established thin-film fabrication technology, and developed a way of aligning nanowires and nanotubes over areas 100 times larger than is possible with existing methods. The breakthrough paves the way for the mass production of nanoscale electronic devices.
Blown film extrusion is one of the most common methods for producing thin plastic films in the packaging industry, and is used, among other things, for producing cling film and supermarket carrier bags. The process involves extrusion of a molten polymer through a circular die, and expanding it with compressed air. The resulting bubbles can then be collapsed and slit to form continuous flat films with widths of more than a metre, and at rates of around 500 kg/hour.
This high volume and efficient manufacturing method has now been adapted by Harvard chemist Charles Lieber and his colleagues Guihua Yu and Anyuan Cao to produce films of aligned nanostructures that could be used for controlling pixels on flexible displays, and building sensors to detect chemicals, viruses and biomarkers for diseases.
Other methods for preparing arrays of aligned nanowires and nanotubes are effective on small scales, but it is unclear whether they can be extended to large-scale assembly.
With the blown film technique, an epoxy polymer suspension of nanowires or nanotubes with a concentration of less than 1% is expanded with nitrogen gas to form a 25-cm wide and 50-cm tall bubble at a controlled pressure and expansion rate. A metal ring stabilises the bubble as it grows, and the polymer stretches to become a 200–500 nm thick film containing evenly spaced and aligned nanowires or nanotubes separated by around 2 μm.
Studies show that 80–90% of transferred films are defect-free, and 90% of the structures are aligned to within 5° of the average direction. “This transfer yield is sufficient for the successful fabrication of large arrays of silicon nanowire based transistors,” says Lieber. “Both transfer yield and the degree of alignment can be further improved by, for example, automating the bubble expansion and film transfer. We believe that making the process more automated, together with a better understanding of the mechanism of nanostructure alignment during bubble expansion, will help to optimise and scale up the process.”
The researchers are now exploring several areas, including fabrication of nano-systems with distinct electrical or optical properties. They are also investigating the use of different polymers to facilitate device fabrication. “For example, we have developed the bubble expansion and transfer process using the photopolymer PMMA (polymethylmethacrolate),” says Lieber. “This enables our method to be integrated directly into a modern photolithography and fabrication process for defining electrodes and other circuit elements.” Other possibilities include three-dimensional structures based on the scrolling or folding of flexible films.
Lieber believes that a number of commercial applications could be enabled by this technique. As well as those mentioned previously, bubble films coated onto highly flexible substrates could be built into flexible micro- and nano-systems. Materials such as carbon nanotubes could also be used to produce reinforced composite films with desired mechanical properties.
Zhong Lin Wang, a materials scientist at Georgia Tech, describes the work of Lieber, Yu and Cao as a breakthrough technology for large-scale integration and industrial applications of nanowires as logic gates, sensor arrays, interconnects and field effect transistors. “This research demonstrates a simple, cost effective, one-step and scalable self-assembly process that integrates billions of nanowires on wafer scale semiconductor chips, plastic substrates and polymer thin films, in a controlled orientation and distribution.”
Wang adds that the blown film technique opens a new chapter in nanowire and nanotube nanotechnology towards practical industrial applications. “The impacts will be huge and long lasting.”
Further reading: Large-area blown bubble films of aligned nanowires and carbon nanotubes, Yu et al., Nature Nanotech. 2, 372 (2007).
Article first published in Nanomaterials News.