Compared to graphite, graphene possesses remarkable improvements in properties such as tensile strength, flexibility, electrical conductivity, and thermal conductivity. The discovery  of this two-dimensional (2D) nanomaterial made of a single layer of carbon atoms has driven increasing interest in the potential application of atom-thin layers of other materials.    

Building on this foundation, researchers have been working to develop other atom-thin materials each with unique properties and potential applications.  

What is graphene?

The development of graphene, an atom-thin layer of carbon atoms organized in a hexagonal lattice, was first reported in 2004. Revolutionary 2D materials such as this possess a high surface-to-volume ratio and are much thinner, stronger, lighter, and more flexible than traditional carbon-based materials. In addition, graphene’s extremely low dimensions and quantum confinement effects give it novel properties not present in bulk materials, such as transparency and semiconductivity. Atom-thin materials also strongly interact with other materials in composites, leading to beneficial synergistic effects.  

Three categories of atom-thin layers  

Encouraged by the success of graphene, scientists began studying other atom-thin layers of materials. Current research is focused on three material types that are categorized based on their composition:  

1. X-enes: Metal-free substances such as graphene, phosphorene, boron nitride, silicene, and graphitic carbon nitride

2. Metal-X-enes: Metal-containing compounds such as transition metal carbides, chalcogenides, nitrides, oxides, hydroxides, and halides
   

3. Metallenes: Metals and alloys such as goldene, stanene, and germanene that can be further categorized into bimetallene, trimetallene, etc.  

Figure 1 traces the history of research on 2D materials since the discovery of graphene.    

Figure 2 presents the subcategories and individual atom-thin materials within them, their important properties, and their relationship to various applications.

The unique properties of atom-thin layers of gold

Of the metallenes in active development, goldene—a single layer of gold atoms—introduces remarkable new scientific possibilities. Expected to exhibit unique catalytic properties and optical characteristics, goldene holds promise for applications in catalysis and optoelectronics.  

What makes goldene such a potentially important candidate for use in solar cells, sensors, batteries, and other electronics?

• Thinness: Goldene is 400 times thinner than commercial gold leaf and is expected to have a thickness of 0.2-0.4 nm. Earlier reports on atom-thin layers of gold supported on substrates show them to be semiconducting with a bandgap of 0.95-2.85 eV.  

• Thermal stability: Theoretical predictions indicate that goldene is thermally stable up to 1400 K, which is similar to bulk gold and gold nanoparticles  

• Bond strength: The calculated energy per bond of goldene is 0.94 eV, which is higher than the 0.52 eV of bulk gold due to the fewer number of bonds per atom.

• Elasticity: Goldene’s elastic (Young’s) modulus is predicted to be 226 GPa, which is higher than the 76-80 GPa of bulk gold and the 60 GPa of gold nanosheets, and similar to A36 steel.  

Considering the high cost of gold, goldene’s mechanical properties may be less important, as those applications generally require large amounts of materials. However, the enhancements in electrical, catalytic, optical, and biomedical properties in goldene are very important, as they are in demand for many emerging and crucial technologies.

Potential applications of goldene

The opportunities for atom-thin layers of gold are even more exciting. Apart from its ornamental and trade use, gold is already an important metal in science and technology with applications in electronics, heterogeneous catalysis, electrocatalysis, sensors, photonics, and biomedicine. As this technology is new, there are a limited number of publications related to atom-thin gold layers to date. Thus, we chose to review the publications containing the slightly thicker gold nanostructures such as nanosheets, nanoplates, and films for an analysis of the reported applications of 2D gold.  

We analyzed the indexed keywords within the CAS Content Collection™ from publications reporting metallenes and 2D gold (Figure 3). The prevalent concepts in publications reporting metallenes  are related to electrocatalysis and electrochemical energy storage, predominantly due to metallenes made of noble metals other than gold, such as palladium, rhodium, platinum, and their alloys.  

In contrast, publications related to 2D gold nanostructures highlight applications such as sensors, drug delivery, photothermal therapy, and heterogeneous catalysis. This is primarily due to the unique surface plasmon resonance exhibited by only a few metals such as gold and silver. The inert and biocompatible nature of gold combined with its electrical and unique optical properties has made it a trusted material for biomedical applications.

Challenges hindering goldene development

To capture the potential of this innovative material, researchers are working to overcome many challenges to make goldene viable for broad applications.

Among the three categories of atom-thin materials, the synthesis of metallenes is the most challenging. This is due to the isotropic nature of the bonds in most of the metals which cause them to prefer closely packed structures, such as nanoparticles and one-dimensional (1D) nanostructures. In contrast, many non-metal and transition metal compounds are composed of layered structures held together by van der Waals forces, making them easier to separate into individual layers in a top-down approach.    

A vast number of nanoparticles, 1D, and 2D nanostructures of gold have been synthesized and have shown high performance in various applications. However, the preparation of atom-thin 2D gold remained elusive. Most of the 2D gold nanostructures reported so far are either an atom-thin layer supported on other materials or made of multiatom layers. Atom-thin layers of gold have a strong thermodynamic tendency to coalesce into discontinuous films made of islands of nanoparticles.  

A 2022 report on goldene synthesis has been contested by other scientists suggesting that it was made with several gold layers instead of one. However, a recent report on the synthesis of goldene where the atom-thin layer of gold was stabilized by surfactants and ligands has attracted widespread attention.  

One potential downside of goldene is that it has a tensile strength of just 12 GPa, which is much less than bulk gold’s 100-200 GPa. The challenge of stabilizing goldene without surfactants and ligands must also be considered, as the presence of these molecules on the surface can significantly hinder gold’s much-desired surface-dependent properties.

Is the future of goldene bright?

The scalability of goldene development and the breadth of its application are yet to be seen. However, there is no question that the unique and fascinating properties of atom-thin materials will continue to attract research and commercial interest.

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