Researchers at Georgia Tech have produced the latest nano-sized structure, adding to the existing buckyballs, nanotubes, nanobelts and even nanosprings. Nanorings are complete circles formed by a self-coiling process.

The nanorings produced by Georgia Tech are based on single crystals of zinc oxide (ZnO). Zinc oxide is also significant in that it is semiconducting and possesses piezoelectric properties. This means that the nanorings could be used as sensors, resonators and transducers.

The nanorings are made up of fine nanobelts that are rolled up as coils layer by layer with as many as a hundred loops, forming a seamless ring structure. They consist of a uniformly deformed single crystal of zinc oxide.

They are formed when a mixture of zinc oxide, indium oxide and lithium carbonate combined in the ratio 20:1:1 is heated in a horizontal tube furnace to 1400°C in flowing argon gas.

They form on an alumina substrate in a section of the furnace maintained between 200 and 400°C, and start off as nanobelts, which are long ribbon-like structures with a width and thickness of about 15nm.

The surface of the nanobelts are positive on one side and negative on the other side due to the termination of zinc ions and oxygen ions on opposite sides of the structure. This results in a spontaneous polarisation across the thickness of the nanobelt. As the structures grow they coil up to neutralise the polar charges, and minimising electrostatic energy. In this way the negatively and positively charged surfaces bind together through charge attraction and the structure continues to grow parallel to the rim, in a loop-by-loop fashion. Due to the charge attraction, the loops come together in perfect alignment.

High temperature exposure results in the sintering of the coils, forming a single crystal, where the individual loops cannot be separated.

The indium oxide and lithium carbonate aid the growth mechanism by creating a planar defect in the zinc oxide nanobelt. This in turn creates energetic conditions leading to rapid growth of the nanoring along the rim.

The nanorings vary in diameter from 1 to 4 microns and are 10 to 30 nanometres thick, composed of as few as 5 or as many 100 loops.

Nanorings offer new possibilities for fabricating unique nanoscale electromechanical systems like piezoelectric resonators for detecting single biomolecules, nanoscale elastic bearings and actuators. In addition they could also be used for building implantable sensors for real time monitoring of biomedical measures like blood pressure, flow rate and stress levels at the level of single walls.

Combining piezoelectric and semiconducting properties may also allow them to explore properties no other systems currently possess.