Temperature-tuned ferromagnetism in hydrogenated multilayer graphene

Multiferroic hydrogenated graphene bilayer

We investigated the multiferroic properties of a hydrogenated graphene bilayer using first-principles calculations. The proposed material is composed of one fully hydrogenated and one semi-hydrogenated graphene monolayer. Inside the van der Waals gap, hydrogen atoms are only adsorbed on either the top or the bottom layer of graphene, thus breaking the centrosymmetry. The calculated electric polarization is 0.137 × 10-10 C m-1, with the transition barrier of switching the polarization being 393 meV per formula unit. We showed that ferroelectricity can be preserved down to atomic thickness. We also studied the domain wall energy and its migration for various domain wall densities, and our results indicate a robust polarization configuration against room temperature thermal fluctuation. As graphene is known to be able to sustain large strain, we further explored ferroelectricity tuning via strain, and found that the polarization can be effectively tuned up to 20% without perturbing the polarization switching barrier. Our results suggest a realizable multiferroic two-dimensional material using the most used two-dimensional material, graphene.

Observation of critical magnetic behavior in 2D carbon based composites

Two dimensional (2D) carbonaceous materials such as graphene and its derivatives, e.g., graphdiyne, have enormous potential possibilities in major fields of scientific research. Theoretically, it has been proposed that the perfect atomic lattice arrangement of these materials is responsible for their outstanding physical and chemical properties, and also for their poor magnetic properties. Experimentally, it is difficult to obtain a perfect atomic lattice of carbon atoms due to the appearance of structural disorder. This structural disorder is generated during the growth or synthesis of carbon-related materials. Investigations of structural disorder reveal that it can offer both advantages and disadvantages depending on the application. For instance, disorder reduces the thermal and mechanical stability, and deteriorates the performance of 2D carbon-based electronic devices. The most interesting effect of structural disorder can be seen in the field of magnetism. Disorder not only creates magnetic ordering within 2D carbon materials but also influences the local electronic structure, which opens the door for future spintronic devices. Although various studies on the disorder induced magnetism of 2D carbon materials are available in the literature, some parts of the above field have still not been fully exploited. This review presents existing work for the future development of 2D carbon-based devices.

First principles study of semihydrogenated graphene and topological insulator heterojunction

Based on first principles calculations, we study the electronic properties of heterostructures formed by a 2D ferromagnetic insulator semihydrogenated graphene (SG) and topological insulator Bi₂Se₃ thin films of a few quintuple layers (QLs). It is found that the unsaturated C atoms in SG form bonds with Se atoms in Bi₂Se₃ thin film and the top surface states (at the interface) are strongly hybridized with SG. Due to breaking of time-reversal symmetry, the surface states open gaps of 40 meV and 150 meV for SG/3QL-Bi₂Se₃ and SG/5QL-Bi₂Se₃ heterostructures, respectively. Furthermore, a giant Rashba spin splitting is found induced by the SG layer.