Recently, several research groups have reported the growth of germanene and silicene, new members of the graphene family. Germanene and silicene are in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene and silicene honeycomb lattices are buckled and composed of two vertically displaced sub-lattices.
Scanning tunneling microscopy image of the buckled honeycomb lattice of germanene with an artist’s impression of the quantum spin Hall effect.
Density functional theory calculations have revealed that free-standing germanene and silicene are two-dimensional Dirac fermion systems, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, the relativistic variant of the Schrödinger equation. Germanene and silicene are very appealing two-dimensional materials. The spin-orbit gap in germanene (24 meV) and silicene (1.5meV) are much larger than in graphene (<0.05 meV), which makes germanene and silicene the ideal candidates to exhibit the quantum spin Hall effect at experimentally accessible temperatures.
Schematic diagram of the effect of an external electric field on the electronic structure near the Dirac point.
Additionally, the germanene and silicene lattices offer the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene and silicene based field-effect devices.