Goldene the First 2D Metal

Table of Contents

On 16 April 2024, scientists from Linköping University in Sweden reported that they had produced goldene, a single layer of gold atoms 100nm wide. Lars Hultman, a materials scientist on the team behind the new research, is quoted as saying “we submit that goldene is the first free-standing 2D metal, to the best of our knowledge”, meaning that it is not attached to any other material, unlike plumbene and stanene. Researchers from New York University Abu Dhabi (NYUAD) previously reported to have synthesised Goldene in 2022, however various other scientists have contended that the NYUAD team failed to prove they made a single-layer sheet of gold, as opposed to a multi-layer sheet. Goldene is expected to be used primarily for its optical properties, with applications such as sensing or as a catalyst.
https://en.wikipedia.org/wiki/Single-layer_materials

Goldene Overview

Goldene is a groundbreaking two-dimensional (2D) material, a single-atom-thick layer of gold atoms, representing the thinnest form of gold ever synthesized. Named after its resemblance to graphene, goldene was first successfully created as a free-standing material in 2024 by researchers at Linköping University, Sweden. Its discovery marks a significant advancement in nanomaterials, offering unique properties that differ from bulk gold due to its 2D structure. Below is a comprehensive exploration of goldene, covering its synthesis, properties, applications, and challenges.

Synthesis of Goldene

The creation of goldene is a remarkable feat, as metals like gold naturally tend to cluster into nanoparticles rather than form stable 2D sheets. The synthesis process, detailed in a 2024 Nature Synthesis publication, leverages a century-old Japanese blacksmithing technique involving Murakami’s reagent. Here’s how it was achieved:

Base Material: Researchers started with a layered material, titanium silicon carbide (Ti₃SiC₂), where gold was intercalated (embedded) between layers of titanium and carbon, forming titanium gold carbide (Ti₃AuC₂).
Intercalation Process: When exposed to high temperatures, gold atoms diffused into the structure, replacing the silicon layers in a process called intercalation.
Etching: Using Murakami’s reagent, a wet chemical etching process, the titanium carbide (Ti₃C₂) was removed, leaving behind free-standing goldene sheets up to 100 nanometers wide. Surfactant molecules in the solution prevented the gold sheets from adhering to each other, akin to “cornflakes in milk.”
Challenges Overcome: Earlier attempts, such as a 2022 claim by New York University Abu Dhabi, were disputed as they produced multi-layered gold rather than a true monolayer. The Linköping team’s success relied on precise control of etching and serendipitous discovery of the gold-intercalated base material.

The team is exploring further synthesis methods using other non-van der Waals Au-intercalated phases to refine the process and scale production.

Structure and Properties

Goldene’s unique properties stem from its single-atom-thick, triangular lattice structure, which contrasts with the three-dimensional, face-centered cubic structure of bulk gold. Key characteristics include:

Structural Features:
Lattice Contraction: Goldene exhibits a 9% lattice contraction compared to bulk gold, with an Au-Au bond length of approximately 2.742 Å (close to experimental and theoretical values of 2.62 Å and 2.735 Å).
Flat Topology: It has a topologically flat arrangement, similar to α-beryllene, with each gold atom having only six neighboring atoms (compared to twelve in bulk gold), resulting in two free bonds that enhance its reactivity.
Mechanical Properties:
Elastic Modulus: Predicted to be over 226 GPa, significantly higher than bulk gold (76–80 GPa) and comparable to A36 steel, indicating exceptional rigidity for a 2D material.
Tensile Strength: Goldene shows anisotropic tensile strength, with values of 11.9 GPa (x-direction) and 19.6 GPa (y-direction) via density functional theory (DFT), and 11.4 GPa and 19.8 GPa via machine learning interatomic potential (MLIP) calculations. At room temperature, it sustains over 9 GPa.
Bond Strength: The energy per bond is 0.94 eV, higher than bulk gold’s 0.52 eV due to fewer bonds per atom.
Thermal Properties:
Thermal Stability: Goldene remains stable up to 700 K (427°C), with theoretical predictions suggesting stability up to 1400 K, comparable to bulk gold.
Low Lattice Thermal Conductivity: Approximately 10 ± 2 W/(m·K) at room temperature, significantly lower than graphene due to the higher mass of gold atoms, which suppresses phonon group velocity.
Electronic Properties:
Semiconducting Behavior: Unlike bulk gold, which is a conductor, goldene is a semiconductor with a bandgap of 0.95–2.85 eV when supported on substrates. Free-standing goldene retains a metallic nature under large tensile strains.
Binding Energy: Displays an Au 4f binding energy increase of 0.88 eV, reflecting changes in electronic structure due to its 2D nature.
Dynamical Stability: Phonon dispersion relations, calculated via DFT and MLIP, confirm goldene’s stability in both stress-free and strained states. It exhibits three acoustic and three optical phonon modes, with a narrower frequency range (~5 THz) compared to graphene (~50 THz).

These properties make goldene distinct from bulk gold, opening new possibilities for applications where traditional gold is less effective.

Potential Applications

Goldene’s unique properties, particularly its thinness, semiconducting behavior, and catalytic potential, make it a promising candidate for various high-tech applications. Key areas include:

Catalysis:
Goldene’s two free bonds per atom enhance its catalytic activity, making it suitable for carbon dioxide conversion, hydrogen-generating catalysis, and selective production of value-added chemicals.
Its high surface area and reactivity could improve efficiency in electrocatalysis and heterogeneous catalysis.
Optoelectronics:
The semiconducting properties and tunable bandgap suggest applications in solar cells, sensors, and photonic devices. Its optical characteristics are under investigation for advanced optoelectronic systems.
Energy Applications:
Hydrogen Production: Goldene’s catalytic properties could facilitate efficient hydrogen production, a critical component of renewable energy systems.
Batteries: Its thinness and electronic properties may enhance battery performance, particularly in energy storage systems requiring lightweight, high-conductivity materials.
Water Purification:
Goldene’s chemical stability and catalytic potential make it a candidate for water purification systems, potentially removing contaminants more efficiently than bulk gold.
Biomedical Applications:
Gold’s biocompatibility, combined with goldene’s enhanced properties, could lead to applications in biosensors, drug delivery, or medical imaging.
Communications:
Its electronic properties may support high-speed communication devices, leveraging its semiconducting nature for signal processing.
Material Efficiency:
Goldene’s extreme thinness (400 times thinner than commercial gold leaf, ~0.2–0.4 nm) allows for significant material savings, reducing the amount of gold needed in applications while maintaining or enhancing performance.

Challenges and Future Directions

Despite its promise, goldene faces several challenges:

Synthesis Difficulties: The isotropic bonding in metals like gold favors 3D structures (e.g., nanoparticles) over 2D sheets, making large-scale production challenging. The current wet chemical etching process is complex and requires precise control.
Scalability: Producing large, uniform goldene sheets for industrial applications remains a hurdle. Researchers are exploring alternative etching schemes and non-van der Waals Au-intercalated phases to improve scalability.
Cost: Gold is inherently expensive, and while goldene uses less material, the synthesis process’s complexity may offset cost savings unless optimized.
Limited Research: As a new material, goldene has few publications, with much of the data derived from slightly thicker gold nanostructures. Further studies are needed to fully characterize its properties.

Future research at Linköping University and beyond aims to:

Develop simpler synthesis methods for other noble metals (e.g., silver, platinum) to create similar 2D metallenes.
Investigate additional applications, particularly in catalysis and optoelectronics.
Improve the understanding of goldene’s behavior under various environmental conditions.

Comparison to Other 2D Materials

Goldene is often compared to graphene, the archetypal 2D material, due to its single-atom thickness. However, key differences include:

Composition: Graphene is carbon-based, while goldene is a metallic monolayer, leading to distinct electronic and thermal properties.
Thermal Conductivity: Graphene has a much higher thermal conductivity (~2000–5000 W/(m·K)) compared to goldene’s ~10 W/(m·K), making goldene better suited for applications requiring low thermal transport.
Electronic Behavior: Graphene is a semi-metal with a zero bandgap, while goldene can exhibit semiconducting properties, broadening its electronic applications.
Stability: Both materials show high thermal stability, but goldene’s metallic nature and heavier atoms result in unique mechanical and catalytic properties.

Goldene also belongs to the broader family of metallenes (e.g., stanene, germanene), which are 2D metal or alloy monolayers. Its development builds on the timeline of 2D materials research, starting with graphene’s discovery in 2004.

Cultural and Historical Context

The name “goldene” reflects its connection to gold, a metal with profound cultural significance as a symbol of wealth and power since antiquity. Gold’s historical use in jewelry, coinage, and ritual objects contrasts with goldene’s futuristic applications, bridging ancient allure with cutting-edge technology. The use of a traditional Japanese etching technique further ties goldene’s discovery to historical craftsmanship.

Speculative Claims

Some posts on X mention speculative ideas about producing gold via nuclear fusion from materials like mercury, potentially impacting gold sentiment. However, these claims are inconclusive and not directly related to goldene, which is synthesized through chemical means, not nuclear transmutation. The high energy costs of such processes make them impractical for now.

Conclusion

Goldene represents a paradigm shift in materials science, transforming gold into a 2D semiconductor with exceptional mechanical, thermal, and catalytic properties. Its synthesis via innovative etching techniques overcomes longstanding challenges in creating metallic monolayers. While still in early research stages, goldene’s potential in catalysis, optoelectronics, energy, and biomedical applications is immense, promising to reduce gold usage while enhancing performance. Ongoing efforts to simplify synthesis and explore other noble metals will determine its scalability and practical impact. Goldene stands as a testament to the intersection of serendipity, traditional techniques, and modern computational methods in advancing nanotechnology.