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.
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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:
- X-enes: Metal-free substances such as graphene, phosphorene, boron nitride, silicene, and graphitic carbon nitride
- Metal-X-enes: Metal-containing compounds such as transition metal carbides, chalcogenides, nitrides, oxides, hydroxides, and halides
- 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 otros materiales o hecho de capas multiatómicas. Las capas de oro de un átomo de espesor tienen una fuerte tendencia termodinámica a fusionarse en películas discontinuas hechas de islas de nanopartículas.
Un informe de 2022 sobre la síntesis de goldene ha sido cuestionado por otros científicos, quienes sugieren que se fabricó con varias capas de oro en lugar de una. Sin embargo, un informe reciente sobre la síntesis de goldene, en el que la capa de oro de un átomo de espesor fue estabilizada mediante tensioactivos y ligandos, ha atraído una atención generalizada.
Un posible inconveniente del goldene es que tiene una resistencia a la tracción de solo 12 GPa, lo cual es mucho menor que los 100-200 GPa del oro a granel. También debe considerarse el desafío de estabilizar el goldene sin tensioactivos ni ligandos, ya que la presencia de estas moléculas en la superficie puede obstaculizar significativamente las propiedades del oro dependientes de la superficie, que son muy deseadas.
¿Es brillante el futuro del goldene?
La escalabilidad del desarrollo del goldene y el alcance de sus aplicaciones aún están por verse. Sin embargo, no cabe duda de que las propiedades únicas y fascinantes de los materiales de un átomo de espesor seguirán atrayendo el interés comercial y de investigación.
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