Leeds: New technique for making nanowires

Scientists at Leeds University have developed a new technique for making nanowires out a form of liquid crystal known as discotic liquid crystals (DLCs).

The findings are relevant to devices such as solar cells and low-cost biosensors.

DLCs have been recognised as one of the more promising candidates for organic electronic devices, but controlling their alignment has proved challenging. This has been a major barrier to their use in liquid crystal displays and as molecular wires.

“DLC molecules have a tendency to stack on top of each other like a pile of coins,” said researcher Stephen Evans. The difficulty comes in controlling the orientation of these columns with respect to the surface on which they lie, which is critical.

In the past researchers have tried to get DLCs to align by rubbing the surface they sit on with a cloth, creating micro grooves. While this fairly primitive method works for macroscopic areas, Evans said, “For new generations of devices we need to accurately control how liquid crystal arranges on the surface.”

The Leeds team has developed a novel technique, using patterned surfaces to selectively control alignment, making it possible to stack the piles neatly, to create molecular ‘wires’.

The technique involves printing sheets of gold or silicon with self-assembled monolayers, which can be patterned with stripes of high and low-energy. When a droplet of liquid crystal is applied to this patterned surface and heated, it spontaneously spreads out over the high-energy stripes, leaving the low-energy regions bare.

“Within the stripes we found molecules arranged into hemi-cylindrical columns each several microns long, which we believe to be the highest level of control over DLC alignment to date,” Evans said. “We also found that the narrower the stripes, the better the ordered the columns.”

It is hoped this level of control could lead to the development of a new type of biosensor, which could test for anything that alters the surface properties. “By changing the surface properties we can get switch between alignments, which is very interesting from the point of view or sensing devices,” Evans said. While most biosensors require a backlight to see when a change has occurred, it is very easy to see when a liquid crystal has changed direction by holding it up to the light. “This opens up great possibilities for the production of very simple and, more importantly, cheap biosensors.”

The team is now testing the conductivity of these wires in the hope that they could be used for energy transfer in molecular systems. They are also looking at ways to polymerise the wires to make them stronger.

References

Planar Alignment of Columnar Discotic Liquid Crystals by Isotropic Phase Dewetting on Chemically Patterned Surfaces
Bramble, J.P. et al.
Advanced Functional Materials. Volume 20, Issue 6, pages 914–920, March 24, 2010
http://dx.doi.org/10.1002/adfm.200902140