Until recently, magnets that operate at room temperature conditions have typically been metallic in nature and have utilized their d-orbitals to facilitate their magnetism. Metallic room-temperature magnets have been around for centuries, but a team of European researchers has managed to make a set of organically functionalized graphene-magnet sensors that are easily tuneable.
The production of room temperature carbon-based magnets using just the s and p molecular orbitals is a challenge that has been relished across all scientific disciplines. It has now been achieved by producing a series of ratio-dependant sp3 hybridized graphene derivatives that behave as antiferromagnetic materials at room temperature.
Graphene, as many people know, is a 2-dimensional, planar, sp2 hybridized sheet of carbon atoms that arrange themselves into a hexagonal array. Another way to think of graphene is as a single sheet of graphite.
Graphene also exhibits a zero-band-gap and its electrons act as massless Dirac Fermions. Graphene is well known to exhibit high structural, electronic, transport and optical properties and is currently being tested and implanted in a wide range of applications and industries, but mainly as a composite material. It has been dubbed by many as a ‘wonder material’.
Graphene’s electronic structure defines it as a semi-metal, which exhibits a high electronic conductivity, high surface area, high charge carrier mobility, high mechanical strength, optical transparency, thermal conductivity, high flexibility and is lightweight, as well as possessing many interesting phenomena- the most prevalent being the half-integer quantum Hall effect.
However, despite this myriad of properties, some of its practical applications (especially towards magnetics) are limited by the zero-band-gap, its hydrophobicity and a lack of long-range magnetic ordering.
These issues have been overcome by functionalizing the surface of graphene where the atoms/moieties are held together chemically by covalent bonds. There are currently many derivatives on the market, such as graphene oxide, graphene, and fluorographene, but the researchers have been able to specifically tune the functionality of the surface to tailor it towards magnetic applications.
There have been previous reports that functionalizing graphene has altered the hydrophobicity/philicity and the band gap to produce a magnet, but the continuing challenge has been to produce one that can operate at room temperature.
There have been many attempts to induce spin-carrying sp3 hybridized (paramagnetic) states into graphene to form many structural changes on the graphene sheet, from defects to doping, partial fluorination and creating specific geometrical edges - all in an attempt to produce such desired magnets.
The researchers have managed to create organic graphene-based magnets with a magnetic ordering that is stable up to room temperature, through specific and suitable sp3 functionalisation. The structure is designed so that a series of ‘magnetic’ carbons are present as hydroxoflourographene molecules- a graphene sheet prepared by exchanging some of the fluorine atoms in fluorographene with hydroxyl groups.
The chemical, and therefore the structural, composition of the magnet can be controlled through specific reaction conditions and the -OH precursor, which alters the F/OH ratio on the graphene surface and is a tuneable process. Appropriate compositions have been found to produce an antiferromagnetic ordering at room temperature- a behavior not exhibited by another graphene or sp-based material.
At low temperatures, the functionalized graphene sheet undergoes a transition to a ferromagnetic state, with one of the highest recorded magnetization values for a graphene-based molecule.
Through various methods, compositions and theoretical computations, the researchers discovered that the magnetism is due to energetically stable diradical motifs. These motifs consist of sp2-conjugated islands embedded in a sp3 matrix.
The presence of hydroxyl (-OH) is a key factor in the stabilization of the diradical motifs and is one of the reasons as to why this graphene sheet has shown magnetic properties compared to many that haven’t. The -OH groups are located between the motifs, and offer a significant p-bridging contribution that stabilizes the motifs through coupling and superexchange interactions.
The theoretical model for these hydroxoflourograhene systems now has a universal character that covers aspects of ‘defect induced magnetism’ and ‘diradical motif-triggered magnetism’, depending upon the degree of sp3 functionalisation.
The work, and magnets, produced now has the potential to open doors to a wider class of graphene-based, 2-dimensional room-temperature magnets. The tunability of the process and the surface itself theoretically allows for the diradical communication superexchange interactions to be further studied in other graphene-based systems. The novel development of room temperature carbon-based magnets allows for a huge possibility to testing in various applications, including spintronics and magnetically separable nanocarriers.
Tuček J., Holá K., Bourlinos A. B., Błoński P., Bakandritsos A., Ugolotti J., Dubecký M., Karlický F., Ranc V., Čépe K., Otyepka M., Zbořil R., Room temperature organic magnets derived from sp3 functionalized graphene, Nature Communications, 2017, 8, 14525