Organic semiconductors can be used in devices like light-emitting diodes, solar cells, lasers and transistors. They have the advantage of being cheap to process and they can be built on flexible substrates. However, widespread application of these materials is being held back for lack of suitable patterning techniques. This is because many of the nanolithographic methods used to pattern inorganic semiconductors (such as electron- or focused-ion beams) are too harsh for organic materials, which are easily damaged by the high-energy beams employed.

Now, Franco Cacialli and colleagues have put forward a solution to this problem. The researchers use a small source of heat, known as a modified Wollaston wire, for the patterning instead. The wire is mounted on the head of an atomic force microscope and then scanned across surfaces to chemically convert all areas that it contacts.

Sub-30 nm resolution
The technique is able to produce pattern resolutions of below 28 nm in the organic semiconductor PPV, a surprising result because the heat source has a diameter that is 200 times bigger, at 5 µm. "This is like painting a line the size of a hair with an inch-size brush," explained Cacialli.

The resolution could be further improved in the future by using nanoscale heat sources rather than micron-sized ones.

Although the technique can not yet be scaled up to large volumes, it does show that soft functional materials can be nanopatterned with simple tools. It is also much cheaper than having to buy and service expensive, sophisticated instruments, like electron beams or focused-beam lithographers. "This means that many more scientists and technologists should be able to make a range of nanostructures on demand using just a modified AFM," said Cacialli. "The technique could really pave the way to 'nanostructures for the masses'," he told

And because no master "die" is produced, the nanopattern design can be changed in a matter of hours, instead of days or weeks as is the case for other patterning methods.

Examples of nanodevices that could be patterned with the technique include nanosized LEDs, gratings, interferometers, feedback structures (for use in organic lasers, for example), insulating gates, nanoscale field-effect transistors and photodetector active layers.

The work was reported in Nature Nanotechnology.