crystals

Inside droplets that won’t freeze are crystals that won’t melt

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Usually, it makes no difference whether you describe a solid–liquid transition in terms of its melting temperature or its freezing temperature. In pure, bulk substances, the two are identical. But at the nanoscale, where the effects of curvature and finite size loom large, things are different.

A case in point is the gallium nanodroplets in the electron microscope image shown here. At each droplet’s core is a tiny gallium crystal (dark green) that remains solid up to 800 K, even though the surrounding liquid (light green) freezes at 180 K. Newly discovered by Maria Losurdo (CNR-NANOTEC, Bari, Italy), April Brown (Duke University), and coworkers, the droplets are thermodynamic oddities—pure substances in which solid and liquid stably coexist over a range of temperatures. Despite repeated cycling between room temperature and 380 K over the course of a year, the droplets never completely froze, nor did they fully melt.

A team from Spain predicted in 2012 that such coexistence could occur, but the new study marks the first time it’s been seen in the lab. Losurdo and her colleagues suspect the unusual behavior has to do at least partly with the sapphire substrate on which the droplets were prepared. Because crystalline gallium and sapphire have closely matching lattices, the droplet can relieve interfacial stress—and lower its energy—by partially solidifying at the sapphire surface. Thus a solid appears at temperatures ordinarily too high for it to exist. The finding is more than just a curiosity; it could add a new wrinkle to ongoing efforts to use liquid metal nanodroplets in next-generation plasmonic devices

Crystal seen growing in slow motion one atom at a time.

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Nano builders rejoice: for the first time, scientists have watched crystals grow atom by atom, offering incredible control over their microscopic structure. The technique could lead to customisable crystals that would find uses in diverse fields, from water purifiers to cloaking technologies.

“For the first time, we can actually image the motion of individual atoms, and observe the atom-by-atom assembly of crystals,” says Nicolas Barry at the University of Warwick, UK.

In the nanoscale world, rods, spheres and dots made from the same material have dramatically different chemical and physical properties. But until now, our control over such structures has been limited because they grow too fast for even the best electron microscopes to follow.

Barry and his colleagues fired a beam of electrons at a thin film of molecules containing the metal osmium, carbon and other elements. Most molecules broke down to release single osmium atoms, and the remaining film fused into a graphene lattice that supported the free atoms. Crucially, this graphene support contained impurities.

Mix and match

“It’s doped with boron and sulphur atoms, which slow down the motion of individual metal atoms on the graphene surface,” says Barry. The sluggish atoms move at the same rate as the image-capture speed of electron microscopes, allowing the team to see crystal growth in action.

 

The team also used a mix of metal atoms to produce an alloy of osmium and ruthenium for the first time, demonstrating that the technique could conceivably create other novel materials with interesting properties.

The method should make it possible to watch how crystals grow from different chemical recipes and figure out how to make customised crystals for use in diverse fields. It could also allow us to introduce desirable defects into crystals.

Sticky problem

“The ability to watch single atoms combine one by one to form nanoparticles is a significant contribution to understanding how materials form at the atomic level,” says Thomas Chamberlain at the University of Nottingham, UK.

But the reactivity of the crystal presents a hurdle, he says. Without a stabilising shell covering a particle’s surface, the material will continue to stick to any other particle it encounters, growing larger and becoming less active. “The useful properties of these crystals will change rapidly over time and then cease quite quickly.”

Still, having uncoated “islands” of highly reactive crystals on a graphene grid could be useful, says Barry. Such a set-up could detect gases or drugs at the atomic scale, for instance. “This combination could be extremely efficient for nano-catalytic applications – but we don’t know yet,” he says.

Journal reference: Nature Communications