Meet the MXenes; the youngest family of layered materials

The goal of MORE-MXenes, a FLAG-ERA project of the Joint Transnational Call 2017, was to understand the electronic and magnetic properties of an interesting family of layered materials known as Rare Earth-i-MXenes. This young and fast-growing family of layered materials is an interesting option for electronic and spintronic devices.

To find out more, we have a chat with MORE-MXenes coordinator Thierry Ouisse of former Graphene Flagship Associate Member Laboratoire des Matériaux et du Génie Physique in France, who worked on this project together with researchers of Institut Neel (part of Graphene Flagship Partner CNRS in France), Graphene Flagship Partner Université Catholique de Louvain in Belgium and former Graphene Flagship Associated Member Linköping University in Sweden.

What are Rare Earth MXenes? 

The word MXenes (pronounced “maxenes”) comes from Mn+1AXn (or MAX) phases. They are formed by a stack of successive atomic layers of transition metals (M), elements from groups 13-15 of the periodic table (A) and carbon or nitrogen (X). The A layers are less strongly bonded than the others and can be removed. As a result, the MX layers are separated from one another and form what are called MXenes.

Rare Earth MXenes also incorporate magnetic, rare-earth elements, such as Cerium (Ce) in the M and/or A layers. For example, we have worked a lot with Mo₄Ce₄Al₇C₃, a ferromagnetic Rare Earth MXene that was discovered by our collaborators in Graphene Flagship former Associated Member Linköping University.

Why are you interested in MXenes? 

MXenes contain dangling bonds on their outer surface, which can be functionalized by chemical elements or moieties. This can boost their tunability and keeps us busy with more challenges.

These magnetic systems are academically interesting, and one day could become the building blocks of spin electronics devices, which store information relying on electrons’ spin, rather than charge.

How did the MORE-MXenes project begin? 

Everything started in Johanna Rosen's group in former Graphene Flagship Associated Member Linköping University. They discover new MAX and MXene phases with a very efficient approach. They systematically investigate the stability of hypothetical compounds with numerical calculations. This allowed them to pioneer several MAX phases, in particular Rare-Earth-based MAX phases. We found a lot of common ground for collaboration and that is how MORE-MXenes was born.

How did it go? 

Everybody brought their specific and complementary know-how to the project. For instance, researchers at Graphene Flagship former Associated Member Linköping University focused on powder precursor synthesis and MXene processing from those powders, my colleagues at the former Graphene Flagship Associate Member Laboratoire des Matériaux et du Génie Physique worked on single crystal growth and mechanical exfoliation of the crystals, scientists at NEEL provided their know-how in low dimensional device processing, and the team of Graphene Flagship Partner Louvain University focused on the numerical calculation of the mechanical, electronic and magnetic properties.

What is unique about your approach?

The MORE-MXenes was the first to focus on Rare Earth-i-MXene. We were also the first to try to produce MXenes from single crystals.

What is your future vision for MXenes? 

Our project was academic, mainly because the Curie transition temperatures of rare-earth elements are too low for widespread applications. For example, the Curie transition temperature of Mo₄Ce₄Al₇C₃ is -262^oC. Above this temperature, the material loses its ferromagnetic properties. Ideally, magnetic elements having transition temperatures above room temperature and displaying a large enough magnetism should be identified, but this has yet to be accomplished. MXenes could become part of other applications, such as electromagnetic shielding, volumetric capacitors and sensors.

References:

• Barbier, Maxime, et al. "Mo4Ce4Al7C3: A nanolamellar ferromagnetic Kondo lattice." Physical Review B 102.15 (2020): 155121.
• Tao, Quanzheng, et al. "Rare-earth (RE) nanolaminates Mo4RE4Al7C3 featuring ferromagnetism and mixed-valence states." Physical Review Materials 2.11 (2018): 114401.
• Champagne, Aurélie, et al. "Insights into the elastic properties of RE-i-MAX phases and their potential exfoliation into two-dimensional RE-i-MXenes." Physical Review Materials 4.1 (2020): 013604.
• Gkountaras, Athanasios, et al. "Mechanical exfoliation of select MAX phases and Mo4Ce4Al7C3 single crystals to produce MAXenes." Small 16.4 (2020): 1905784.