In 2004 Andre Geim and Konstantin Novosolev demonstrated the possibility of isolating atomically-thin single layers of carbon atoms, which originated the new field of 2D materials. Since then, many other 2D materials analogous to graphene, but with different physical properties, have been reported. These include insulators (e.g. h-BN), semiconductors (e.g. black phosphorous), metals (e.g. Fe₃GeTe₂) and superconductors (e.g. 2H-TaS₂). Still, in this growing field dominated by inorganic materials, two main challenges have remained elusive until recently: i) the inclusion of magnetic ordering; and ii) their chemical functionalization.

In fact, the detection of ferromagnetism in a magnetic monolayer has only been achieved for the first time very recently using monolayers of CrI₃, the most favorable case (an Ising ferromagnet). However, a big drawback of this material is its unstability at ambient conditions.

Regarding the latter, the covalent functionalization of a 2D material involves first the isolation of the 2D layers and second the anchoring of molecules on its surface, which typically is an uncontrolled reaction performed in exfoliated layers suspended in solvents. Thus, this post-synthetic functionalization yields a 2D material that has an uneven distribution of functionalities, which also in occasions has suffered a chemical attack causing a change in their physical properties. 

The approach we have followed in the Institute of Molecular Science (ICMol) in Valencia, Spain, consists on the use of coordination chemistry to develop new robust 2D metal-organic materials where not only magnetic properties can be implemented through a proper chemical design, but an easy chemical functionalization can be achieved in a pre-synthetic manner.

Thus, in a recently published paper (Nature Chem. 2018, DOI: 10.1038/s41557-018-0113-9), we show how the use of a solvent-free methodology allow us to prepare a family of isoreticular 2D coordination polymers, stable at ambient conditions. The use of benzimidazolate as organic ligand provides a short magnetic pathway that facilitates the presence of magnetic exchange between the Fe(II) centres. In fact, the material magnetically orders below 20 K.

In addition, the versatility of the coordination chemistry approach is demonstrated by using up to 5 different derivatives of the organic ligand, each of them providing a different functional group that changes the surface properties of the materials, ranging from hydrophilic to hydrophobic. Notably, the magnetic ordering of the 2D materials is not altered with the different functionalities. More importantly, we have a homogeneous periodic distribution in the functionalization of the 2D monolayer, as the functional groups are inherent of the ligand used in the synthesis. Thus, we have a functionalization matching perfectly the periodicity of the 2D crystal.

This family of isoreticular 2D materials can be mechanically exfoliated down to the monolayer with the “Scotch-tape” method while preserving its crystalline integrity. This has been possible thanks to the robustness of the layers, which is an unusual feature for a coordination polymer. The use of micromechanical exfoliation prevents any damage from solvent molecules, and also leads to higher lateral sizes of higher quality. Further, the monolayer can be often achieved, which allows to explore the physics in the 2D limit. Finally, we go one step beyond and have implemented these 2D materials on an optical device that can have real applicability as a potential selective gas sensor.

To read more about this story, please see our article “Isoreticular two-dimensional magnetic coordination polymers prepared through presynthetic ligand functionalization” in Nature Chemistry here.