The feature sizes in conventional microelectronic circuits are becoming ever smaller and they will one day inevitably hit the limit imposed by the top-down lithography techniques that are used to make integrated circuits. Bottom-up techniques rather than conventional top-down lithography can be used to make nanostructures such as carbon nanotubes or semiconducting nanowires. These structures could be real alternatives as the building blocks for nanoelectronics devices.

"A big challenge with bottom-up synthesis, however, is being able to reliably control the composition, dimensions and electrical properties of these nanomaterials, but the upside is that we can make structures that we could not possibly imagine using lithography techniques,” says Federico Panciera, who is a member of Stefan Hofmann’s team at Cambridge. “Such structures include complex 3D architectures, new types of atomic arrangements, and unusual combinations of materials.”

Controlling the structure of a wire by adjusting the self-assembly conditions

One good technique for making nanowires is Vapour-Liquid-Solid (VLS) synthesis. Here, a tiny catalytic droplet helps grow a nanowire (a long but narrow crystal), self-assembling it layer by layer from the bottom up. The VLS method has come along in leaps and bounds in recent years and researchers can now control the structure of a wire by adjusting the self-assembly conditions. “This is good news because the better we can control a nanowire’s structure, the more it becomes possible to develop new applications for these materials,” adds Panciera.

In their work, the Cambridge-IBM researchers have now improved on the VLS technique even further by using the catalytic droplets not only to grow a nanowire but also to form new materials within it. These materials can be faceted nanocrystals, or quantum dots.

Monitoring nanowire growth with an ultrafast camera

When it first grows, the nanocrystal forms and floats in the liquid catalyst droplet. “After it has formed, it then attaches itself to the nanowire and becomes embedded in it as the nanowire continues to grow,” Panciera tells nanotechweb.org. “We studied this process using two customised electron microscopes - one at IBM’s TJ Watson Research Center in New York, and the second at Brookhaven National Laboratory, also in New York.”

The team monitored how the nanowires grew with an ultrafast camera that took images every 2.5 milliseconds.

Catalyst acts as a “mixing bowl”

“The image above shows how a nickel silicide nanocrystal forms in a silicon nanowire when we add a burst of nickel to the catalyst,” says Panciera, “but we have also investigated other materials to show that our approach is more general.”

In fact, the researchers say they can use the catalyst as a “mixing bowl”, carefully controlling the order and the amount of each “ingredient” they add during the process. By doing this, they were able to grow complex heterostructures made of nanowires containing nanoscale crystals. They could control the structure and dimensions of the crystals, and even their position in the nanowire.

The work is detailed in Nature Materials doi:10.1038/nmat4352.