Electronics go on a bender

The prospect of low-cost, efficient electronic circuits being applied to flexible substrates has moved a step closer with two pieces of research reported by US scientists this week.

Zhenan Bao and colleagues¹ at Stanford University have developed a novel way to pattern large arrays of organic single-crystal transistors on a range of substrates, including flexible surfaces. The technique could provide a possible route to the manufacture of high-performance electronic devices that extend over large surface areas.

Meanwhile at the University of Illinois John Rogers and co-workers² have shown how it is possible to integrate multiple types of nanoscale semiconducting materials, such as nanotubes, nanowires and ribbons, to produce novel two- and three-dimensional architectures of heterogeneous electronic systems. The researchers say that their technique provides a route to the production of unusual electronic systems that would otherwise be impossible to achieve. These, too, can be produced on flexible surfaces.

Single-crystal transistors are attractive for large-area applications because they have many of the necessary electronic and physical properties needed. However, the crystals need to be placed individually into position on an array, making large-scale manufacture unfeasible. Bao’s group circumvented this by devising a way to direct the growth of crystals on a surface in pre-determined locations. The team did this by making a ’stamp’ of the desired pattern from the polymer polydimethylsiloxane. This was then coated with octadecyltriethoxysilane (OTS), a crystal-growth agent, and pressed onto the substrate that had previously been patterned with electrodes. The OTS acts as a nucleation site for growth of various crystals, such as the semiconductor rubrene, by vapour deposition. In this way the researchers showed that arrays of functioning single crystal transistors could be grown on a range of substrates, and that the functionality was retained on flexible substrates that had been subjected to repeated bending. However, for commercial application of the technology better control of the alignment of the crystals is needed and better contact between the crystal and electrode, the researchers concede.

Rogers’ team synthesised a range of semiconducting structures, including carbon nanotubes, single crystal micro- and nanoscale wires and ribbons of gallium arsenide, silicon and gallium nitride on separate surfaces. They then lifted each class of structure from the surface upon which it had been grown using an ’elastomeric stamp’ and transferred the structures to a substrate. This process could be repeated sequentially with a variety of structures, to build up stacks of heterogeneous semiconducting layers which were shown to exhibit electronic functionality based upon their composition. The layers could be applied to flexible as well as rigid substrates, and remained robust and functional after repeated bending and thermal cycling. 

Mark Welland, head of the Nanoscience Centre at the University of Cambridge, UK, told Chemistry World that Rogers’ work on producing heterogeneous semiconducting systems was an important development in ’exploring the full palette of nanomaterials which are now available, and combining them together in structures which start to perform as real devices.’ 

Paul Beecher, of Cambridge University’s Department of Engineering, said that Bao’s Targeted fabrication of device patterns ’makes this technique, as with ink-jet printing, a credible method of large area mass production of organic device arrays.’

Welland said that both of the new techniques would, eventually, need to demonstrate significant commercial advantages over conventional semiconductor technologies. ’In the end it comes down to economics,’ he said. 

Simon Hadlington