In a remarkable feat of nanoscale engineering, scientists have created the world’s thinnest gold sheets at just one atom thick. This new material, dubbed “goldene,” could revolutionize fields from electronics to medicine, offering unique properties that bulk gold simply can’t match.
The research team, led by scientists from Linköping University in Sweden, managed to isolate single-atom layers of gold by cleverly manipulating the metal’s atomic structure. Their findings, published in the journal Nature Synthesis, represent a significant breakthrough in the rapidly evolving field of two-dimensional (2D) materials.
Since the discovery of graphene — single-atom-thick sheets of carbon — in 2004, researchers have been racing to create 2D versions of other elements. While 2D materials made from carbon, boron, and even iron have been achieved, gold has proven particularly challenging. Previous attempts resulted in gold sheets several atoms thick or required the gold to be supported by other materials.
The Swedish team’s achievement is particularly noteworthy because they created free-standing sheets of gold just one atom thick. This ultra-thin gold, or goldene, exhibits properties quite different from its three-dimensional counterpart. For instance, the atoms in goldene are packed more tightly together, with about 9% less space between them compared to bulk gold. This compressed structure leads to changes in the material’s electronic properties, which could make it useful for a wide range of applications.
One of the most exciting potential uses for goldene is in catalysis, which is the process of speeding up chemical reactions. Gold nanoparticles are already used as catalysts in various industrial processes, from converting harmful vehicle emissions into less dangerous gases to producing hydrogen fuel. The researchers believe that goldene’s extremely high surface-area-to-volume ratio could make it an even more efficient catalyst.
The creation of goldene also opens up new possibilities in fields like electronics, photonics, and medicine. For example, the material’s unique optical properties could lead to improved solar cells or new types of sensors. In medicine, goldene might be used to create ultra-sensitive diagnostic tools or to deliver drugs more effectively within the body.
How They Did It: Peeling Gold Atom by Atom
The process of creating goldene is almost as fascinating as the material itself. The researchers used a technique that might be described as atomic-scale sculpting, carefully removing unwanted atoms to leave behind a single layer of gold.
They started with a material called Ti3AuC2, which is part of a family of compounds known as MAX phases. These materials have a layered structure, with sheets of titanium carbide (Ti3C2) alternating with layers of gold atoms. The challenge was to remove the titanium carbide layers without disturbing the gold.
To accomplish this, the team used a chemical etching process. They immersed the Ti3AuC2 in a carefully prepared solution containing potassium hydroxide and potassium ferricyanide, known as Murakami’s reagent. This solution selectively attacks the titanium carbide layers, gradually dissolving them away.
However, simply etching away the titanium carbide wasn’t enough. Left to their own devices, t
he freed gold atoms would quickly clump together, forming 3D nanoparticles instead of 2D sheets. To prevent this, the researchers added surfactants — molecules that help keep the gold atoms spread out in a single layer.
Two key surfactants were used: cetrimonium bromide (CTAB) and cysteine. These molecules attach to the surface of the gold, creating a protective barrier that prevents the atoms from coalescing. The entire process took about a week, with the researchers carefully controlling the concentration of the etching solution and surfactants to achieve the desired result.
Results: A New Form of Gold Emerges
The team’s efforts resulted in sheets of gold just one atom thick, confirmed through high-resolution electron microscopy. These goldene sheets showed several interesting properties:
- Compressed structure: The gold atoms in goldene are packed about 9% closer together than in bulk gold. This compression changes how the electrons in the material behave, potentially leading to new electronic and optical properties.
- Increased binding energy: X-ray photoelectron spectroscopy revealed that the electrons in goldene are more tightly bound to their atoms compared to bulk gold. This shift in binding energy could affect the material’s chemical reactivity.
- Rippling and curling: Unlike perfectly flat sheets, the goldene layers showed some rippling and curling, especially at the edges. This behavior is common in 2D materials and can influence their properties.
- Stability: Computer simulations suggested that goldene should be stable at room temperature, although the experimental samples showed some tendency to form blobs or clump together over time.
The researchers also found that they could control the thickness of the gold sheets by adjusting their process. Using slightly different conditions, they were able to create two- and three-atom-thick sheets of gold as well.
Limitations and Challenges
- Scale: The current process produces relatively small sheets of goldene, typically less than 100 nanometers across. Scaling up production to create larger sheets will be crucial for many potential applications.
- Stability: Although computer simulations suggest goldene should be stable, the experimental samples showed some tendency to curl and form blobs, especially at the edges. Finding ways to keep the sheets flat and prevent them from clumping together over time will be important.
- Substrate dependence: The goldene sheets were most stable when still partially attached to the original Ti3AuC2 material or when supported on a substrate. Creating large, free-standing sheets of goldene remains a challenge.
- Purity: The etching process leaves some residual titanium and carbon atoms mixed in with the gold. While these impurities are minimal, they could affect the material’s properties in some applications.
- Reproducibility: The process of creating goldene is quite sensitive to the exact conditions used. Ensuring consistent results across different batches and scaling up production will require further refinement of the technique.
