Nanowire Optical Properties Tuned by Alloy Catalyst

A new growth catalyst gives greater control over the colors of light emitted by gallium nitride nanowires.

Rather than using a single metal, researchers at Lawrence Berkeley National Laboratory used gold-nickel alloys as a catalyst. By altering the concentrations in the alloy, the researchers could precisely manipulate the orientation of the nanowires, even on the same substrate in the same batch.

Depending on the growth direction chosen, different optical properties were observed thanks to the crystal surfaces exposed at the surface of the nanowire.

Nanowires grown using catalysts rich in gold (top) and nickel (bottom). Courtesy of Berkeley Lab.

“No one had used bimetalic catalysts to control growth direction before,” said Tevye Kuykendall, a scientist at Berkeley Lab’s Molecular Foundry.

Kuykendall said the mechanism driving the new growth process is not fully understood, but it involves the different tendencies of gold and nickel to align with various crystallographic surfaces at the point where nanowires start to grow.

In its bulk form, GaN emits light in the blue or ultraviolet range. If indium atoms are added to it, the range can be extended to include red. The problem is that adding indium atoms puts the crystal structure of gallium nitride under stress, which leads to poorly performing devices.

GaN nanowires, however, don’t experience the same sort of crystal strain, so scientists hope to use them as tunable, broad-spectrum light sources.

Scientists have made steady progress in cultivating nanowires since the early 2000s. Early nanowire samples resembled “tangled noodles or wildfire-ravaged forests,” the researchers said. More recently, scientists have found that nanowire growth orientation is dictated by the substrate’s crystalline structure. While this can lead to more orderly nanowires, it isn’t foolproof, and some nanowires still go rogue.

Moreover, there is no simple way to grow different types of nanowires in the same environment and on the same substrate.

“For years we were searching for cleverer ways to grow nanostructures with different optical properties in identical growth conditions,” said staff scientist Dr. Shaul Aloni. “Engineering the catalyst brings us closer to achieving this goal.”

The new approach could be applied to a variety of materials and be used for making next-generation devices such as solar cells, LEDs, high-power electronics and more, Aloni said.

The research was published in Nano Letters (doi: 10.1021/nl502079v).

For more information, visit www.lbl.gov.