Beyond graphene: antimonene and germanium nanolayers

Felix Zamora from former Graphene Flagship Associated Member Universidad Autonoma de Madrid in Spain, in collaboration with researchers at the Graphene Flagship Partner Friedrich-Alexander-Universität Erlangen-Nürnberg in Germany and former Graphene Flagship Associated Member the Faculty of Information Studies in Novo mesto in Slovenia, were awarded the 2D-SbGe project to prepare and characterise layered materials derived from antimony and germanium. The project was funded by the FLAG-ERA Joint Transnational Call 2017 and ended in December 2021. We interview Zamora to find out more about the properties of these materials.

How did you get the idea to investigate antimonene and germanium?

We anticipate that after graphene, transition-metal dichalcogenides and phosphorene, the next wave of layered materials will be related to the so-called Xene subfamily, which includes antimonene and germanium. These are nanometric thin structures formed of a single chemical element, analogous to how pure graphene is a single sheet of carbon atoms.

Consequently, the main goal of 2D-SbGe was to develop the best fabrication methods for these materials and study their properties both experimentally and theoretically. We wanted to provide tools to employ these materials in different applications.

What intrigues you the most about antimonene?

I am intrigued by antimonene’s stability and bandgap.

We have shown that single layers of antimonene are stable in atmospheric conditions. This highlights that antimonene layers could outperform black phosphorus in some respects.

Antimonene’s predicted bandgap could lead to a variety of (opto)electronic applications. We demonstrated theoretically and experimentally that this material has robust electrical conductivity at the surface level, independent of its surface imperfections.

How do you prepare it?

The 2D-SbGe consortium exfoliated bulk antimony via micromechanical and liquid phase techniques. We have also developed synthetic routes to produce and up-scale the fabrication of high-quality antimonene hexagons in a continuous process. We also characterised the mechanical properties and electrical conductivity of antimonene synthesized through a wet-chemical approach.

In which applications could antimonene be useful?

We evaluated different application areas, including supercapacitors, electrodes, biodetectors (e.g. DNA sensing devices), and we have considered its use as a catalyst.  

What about germanium? Which form of germanium do you use?

We produced nanolayers of alpha-germanium (2D-a-Ge); not to be mistaken with germanene, the hexagonal form of germanium that only grows on metallic surfaces.

We exfoliated pure α-germanium – a narrow-bandgap semiconductor with a diamond-like structure and strong covalent bonds – with a simple one-step procedure assisted by wet ball-milling. We obtain gram-scale, high-quality layers with large lateral dimensions and nanometre thicknesses.

What are the most exciting properties of alpha-germanium?

Theoretical calculations carried out by our consortium predicted that 2D-a-Ge has exciting optical properties, like bandgap tuning as a function of its thickness. We also found this experimentally: 2D-a-Ge exhibits bandgaps that depend on both the crystallographic direction and the number of layers.

These results have been protected with a patent. We are currently evaluating the possibility of using this novel material for different applications, such as electrodes for more efficient batteries.

What are your proudest achievements within the 2D-SbGe project?

Probably the most relevant result came from the materials’ preparation side. We devised fabrication methods to produce antimonene and germanium on a gram scale. This will enable the transition to devices and applications.

What’s in store for the future?

We foresee applications of these layered materials in the context of energy storage and generation, such as in supercapacitors, water splitting, oxygen reduction, as well as in optoelectronic devices. Theoretical calculations will be needed to rationalize their physical and chemical properties and will aid in future materials design.

References

• Gibaja, Carlos, et al. "Exfoliation of Alpha‐Germanium: A Covalent Diamond‐Like Structure." Advanced Materials 33.10 (2021): 2006826.
• Torres, Iñigo, et al. "Continuous‐Flow Synthesis of High‐Quality Few‐Layer Antimonene Hexagons." Advanced Functional Materials (2021): 2101616.
• Lloret, Vicent, et al. "Few layer 2D pnictogens catalyze the alkylation of soft nucleophiles with esters." Nature communications 10.1 (2019): 1-11. 
• ZAMORA ABANADES, Felix Juan, et al. "Few-layer alpha-germanium crystal, their preparation processes and uses thereof." U.S. Patent Application No. 16/961,449. ES2721672. PCT/ES2019/070052. WO/2019/149985