Joining the family of monoelemental crystals first set forth by graphene, Swedish researchers successfully develop the first one-atom-thick metal from gold.

Meet goldene, the latest addition to the family of xenes – a remarkable lineage of monoelemental crystals that began with graphene. Like its carbon cousin, gold was synthesized down to a thickness of one atom, which now boasts the distinction as the world's thinnest gold leaf.

Since the pioneering discovery of exfoliating graphite into graphene, the family of xenes has expanded to include a diverse array of monoelemental crystals such as silicon (silicene), germanium (germanene), tellurium (tellurene), boron (borophene), tin (stanene), bismuth (bismuthene), and lead (plumbene), and many others.

Now, researchers from Linköping University in Sweden have added a gilded member, goldene, to the roster, which they expect can help to capture light in ways that could be useful in applications such as sensor technology and catalysis.

While researchers have previously reported gold sheets sandwiched between other materials, Linköping University materials scientist Lars Hultman concludes, "We submit that goldene is the first free-standing 2D metal, to the best of our knowledge."

Importantly, the method used to make goldene is a simple chemical process that should also be amenable to larger-scale production, which the researchers reported in "Nature Synthesis" on April 16.

"I'm very excited about it," said Stephanie Reich, a solid-state physicist and materials scientist at the Free University of Berlin, who was not involved in the work. "People have been thinking for quite some time how to take traditional metals and make them into really well-ordered 2D monolayers."

In 2022, researchers at New York University Abu Dhabi (NYUAD) said they had produced goldene, but the Linköping team contends that the prior material probably contains multiple atomic layers based on the electron microscopy images and other data published in "ACS Applied Materials and Interfaces."

Reich agreed that the 2022 study failed to prove the material was single-layered goldene. In an article by Nature.com, the website wrote that the principal authors of the NYUAD study did not respond to questions about their work.

Historic technique

To prepare goldene, the Linköping researchers said they started with a material containing atomic monolayers of silicon sandwiched between titanium carbide. When gold was added to the top of this sandwich, it diffused into the structure and exchanged places with the silicon to create a trapped atom-thick layer of gold.

Upon etching the titanium carbide away, this left a free-standing golden sheet that was up to 100 nanometers wide and roughly 400 times as thin as the thinnest commercially available gold leaf, Hultman estimated.

That etching process used a solution of alkaline potassium ferricyanide, otherwise known as Murakami's reagent – typically used in analytical chemistry to find specific types of molecules – historically used to create unique patterns on the blades of Japanese swords.

"What's so fascinating is that it's a 100-year-old recipe used by Japanese smiths to decorate ironwork," said Hultman. This can be seen on swords like katanas, which bear a wave-like pattern along the length of the blade.

The researchers also added surfactant molecules – compounds that form a protective barrier between goldene and the surrounding liquid – to stop the sheets from sticking together.

Kashiwaya, S. et al. Nature Synthesis

From left to right: the Linköping team made a material containing a silicon monolayer sandwiched between sheets of titanium carbide; then added gold atoms, which diffused into the structure; replacing the silicon; and then etched away the titanium carbide with an oxidizing reagent to release the goldene sheet.

Future uses

The Linköping team suggests that goldene might be useful in applications where gold nanoparticles already show promise. Light can generate waves in the sea of electrons on a gold nanoparticle's surface, which can channel and concentrate that energy.

This strong response to light has been harnessed in gold photocatalysis to split water to produce hydrogen, for instance.

Hultman adds that goldene could open opportunities in areas such as this, but its properties need to be further investigated in more detail first.

"I think the research is really interesting," said Graham Hutchings, a chemist at the University of Cardiff in the United Kingdom, who develops gold catalysts but notes that he is concerned about residual traces of iron from Murakami's reagent that might hamper the development of goldene as a catalyst.

"I would think that potential contamination with iron is going to cause a few problems in applications," he added.

For now, the Linköping researchers are looking for better ways to sieve goldene from the solution used to produce it and to grow larger flakes of the material. They are also exploring whether their method can be used to make monolayers of other catalytic metals, including iridium, platinum, and palladium.