Branching out: semiconducting nanotrees could boost electronics.

Future electronic devices could contain forests of nanoscale trees, suggests a new study by researchers in Sweden. The research builds on work with semiconducting nanowires that are being developed in many laboratories for applications ranging from computer circuits to biomedical sensors (SN: 2/9/02, p. 83). Devices made from versatile nanowires could be faster and more powerful than today's electronic gadgetry.

In an effort to add new capabilities to nanowires, Lars Samuelson and his colleagues at Lund University in Sweden report a technique for growing treelike structures out of semiconducting materials.

The Lund team first deposits gold particles that are 40 to 70 nanometers in diameter on a small wafer of gallium phosphide. The researchers then place the wafer inside a chamber and feed in a mixture of gases that supplies the raw materials for the trees. Gradually, vertical wires of gallium phosphide grow underneath each gold particle. These gold-tipped wires, measuring only a couple microns in length, serve as trunks.

To create the branches, the researchers spray gold particles smaller than the original ones onto the trunks and again expose the material to the gas mixture. From each of these gold particles emerges a long branch of gallium phosphide. By controlling the size and number of the small gold particles, Samuelson and his colleagues can determine the width of each branch and the density of branches on each trunk.

The researchers also report in the June Nature Materials that they could grow trees made up of different materials by simply changing the mixture of chemicals in the gases added to the growth chamber. Tree parts with different compositions could perform various functions, says Samuelson.

For instance, in one experiment, the Lund team made trunks out of gallium phosphide and parts of the branches out of gallium arsenide phosphide. The researchers expect combinations of materials such as these to produce a light-emitting diode: The trunk would carry current to the branches, where the gallium arsenide phosphide would convert it into light. Alternatively, the branches could serve as light-harvesting structures, as in a solar cell, which would then shuttle excited electrons into the trunk.

Expanding on the tree metaphor, Samuelson says his team has even used the technique to grow individual "leaves" on each branch.

"This is very nice work," says Peidong Yang of the University of California, Berkeley. Solar cells made from three-dimensional nanowire structures like these could be much more efficient than current models, he says. The branches and leaves on each trunk would increase the density of light-absorbing structures in a device.

James Ellenbogen of Mitre Corp. in McLean, Va., calls the work "clever" and "worthy of further exploration."

Samuelson's Lund-based company QuMat Technologies plans to commercialize the technology.