How to make tiny metal snowflakes

Look closely at a snowflake, and you’ll observe a one-of-a-kind gossamer lattice, its growth influenced by ambient conditions like temperature and humidity. Turns out, this sort of intricate self-assemblage can also occur in metals, researchers report in the Dec. 9 Science.

In pools of molten gallium, physicist Nicola Gaston and colleagues grew zinc nanostructures with symmetrical, hexagonal crystal frameworks. Such metal snowflakes could be useful for catalyzing chemical reactions and constructing electronics, says Gaston, of the McDermid Institute for Advanced Materials and Nanotechnology at the University of Auckland in New Zealand.

“Self-assembly is the way nature makes nanostructures,” she says. “We’re trying to learn to do the same things.” Figuring out how to craft tiny, complex metal shapes in fewer steps and with less energy could be a boon for manufacturers.

The researchers chose gallium as a growth medium, due to its relatively low melting point, ability to dissolve many other metals and the tendency for its atoms to loosely organize while in a liquid state.

After mixing zinc into the gallium, the team subjected the alloy to elevated temperatures and different pressures, and then let the mixture cool to room temperature. The loose ordering of gallium atoms appeared to coax the crystallizing zinc to bloom into symmetrical, hexagonal structures resembling natural snowflakes and other shapes, the team found. It’s somewhat like how a fruit tray imparts order on the fruits stacked within, Gaston says.

The future may be bright for research into applications of gallium and other low-temperature liquid metals. “Not to take that snowflake metaphor too far, but [this work] really hints at new branches for scientific discovery,” Gaston says.

What we are learning is that the structure of the liquid gallium is very important,''says Gaston. “That’s novel because we usually think of liquids as lacking structure or being only randomly structured.”

Interactions between the atomistic structures of the different metals and the liquid gallium cause differently shaped crystals to emerge, the scientists showed.

The crystals included cubes, rods, hexagonal plates and the zinc snowflake shapes. The six-branched symmetry of zinc, with each atom surrounded by six neighbors at equivalent distances, accounts for the snowflake design.

“In contrast to top-down approaches to forming nanostructure – by cutting away material – this bottom-up approaches relies on atoms self-assembling,” says Gaston. “This is how nature makes nanoparticles, and is both less wasteful and much more precise than top-down methods.”

The New Zealand team, part of the McDermid Institute for Advanced Materials and Nanotechnology, a national Center of Research Excellence, carried out simulations of molecular dynamics to explain why differently shaped crystals emerge from different metals. (The government’s Mars den Fund supported the research.)

Professor Nicola Gaston and research fellow Dr Steph Lamb, both of Waipahu Tamara Ray, University of Auckland, and Dr Krista Sternberger of Te Hangar WADA, Victoria University of Wellington, collaborated with colleagues in Australia led by Professor Nourish at the University of New South Wales.
The Australian team worked in the lab with nickel, copper, zinc, tin, platinum, bismuth, silver and aluminum. Metals were dissolved in gallium at high temperatures. Once cooled, the metallic crystals emerged while the gallium remained liquid.

She says the research has opened up a new, unexplored pathway for metallic nanostructures. “There’s also something very cool in creating a metallic snowflake!”