There are many other 2D materials than graphene that exhibit a hexagonal array and are uni-atomic. A team of researchers from Brazil and Germany have used theoretical ab-initio methods to investigate how other group IV 2D materials, the so-called X-enes, interact when deposited onto a graphene-silicon carbide substrate.
Aside from graphene, there are many 2D materials that are made up of only one type of atom and are coined the X-enes. The more well-known examples of these 2D materials include silicene (silicon), germanene (germanium) and stanene (tin). There are, of course, other common 2D materials such as transition metal dichalcogenides and hexagonal boron nitride, but these are multiatomic.
2D materials are of great interest as a general class of materials. Whilst graphene has hit the headlines as the wonder material that can be commercialized, experts have already stated that other 2D materials possess greater electronic properties than graphene but currently lack in-depth research. Whilst graphene can be produced easily from the exfoliation of graphite, the other X-enes must be synthesized from the bottom-up due to a lack of material equivalent to graphite. This has limited their use at the commercial level currently, but nevertheless, their presence is still relevant as teams are now working on trying to bring these materials to the level that graphene is currently at.
Many 2D materials possess a low-buckled geometry composed of a combination of both sp2 and sp3 bonds, but with a similar lateral symmetry that graphene possesses. This is in addition to excellent optical properties, electronic properties, an almost zero band gap, high carrier mobility and the presence of Dirac cones near the K point of the Brillouin Zone.
The epitaxial growth of graphene onto graphene/SiC substrates has been well documented and the passivation of the SiC from the graphene overlayers has opened an interest for other X-ene growth approaches to be studied.
Researchers from the Instituto Tecnológico de Aeronáutica (ITA), Brazil, and the Institut fur Festkorpertheorie und -optik, Friedrich-Schiller-Universitat, Germany, have now used computational ab-initio methods to observe the growth of common uni-atomic 2D materials onto a graphene-silicon carbide (SiC) substrate.
The researchers investigated the deposition using first principle calculations from density functional theory (DFT) approaches, namely projector-augmented wave (PAW), Vienna ab-initio simulation package (VASP), exchange and correlation (XC) and Perdew-Burke-Ernzerhof (PBE) methods. The researchers mainly tested for the topological character of the X-ene/graphene bilayers and whether quantum spin Hall phases were present.
The X-ene materials bonded to the graphene layer through van der Waals interactions and quasiparticle effects on top of the C-terminated Silicon carbide substrate. The deposition of the X-enes were found to stable. The bonded overlayers also exhibited folded band structures that showed the presence of Dirac cones and the topological character showed the existence of a quantum spin Hall phase.
The interactions between the substrate were found to be reduced and small fundamental gaps (0.1 eV) were opened. The van der Waals forces between the X-ene and graphene layers were found to be weak and not strong enough to open a graphene gap.
However, a small gap between the Dirac cones was observed at 96 meV for silicene, 116 meV for germanene and 146 meV for stanene. The overall bilayer system was found to possess a half-metallic character resulting from the charge transfer between the X-enes and the graphene.
The germanene-graphene bilayer was also found to be a topological system, whereas the silicene-graphene bilayer was a trivial system. The topology of the graphene-stanene bilayer remains a mystery at this point.
The formation of covalent bonds between the graphene and silicon carbide caused the surface levels to mix with the graphene bands and produce a new substrate energy level. A similar effect was shown with the X-ene overlayers and was especially true for silicene, which was the strongest bound X-ene. Germanene on the other hand, encountered structural distortions and interlayer bonding issues which affected the electronic properties of the overlayer more so than it did silicene.
In short, it was shown theoretically that passivated graphene on top of a silicon carbide substrate could be a feasible route for the epitaxial growth of other group IV 2D materials with a low-buckled geometry.
James A Isbell/ Shutterstock.com
“Deposition of topological silicene, germanene and stanene on graphene-covered SiC substrates”- Matusalem F., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-15610-3