Synopsis: A Good Match for Germanene

Germanene—the germanium analog of graphene—can remain a topological insulator when placed on a molybdenum disulfide substrate.
Synopsis figure
L. Zhang et al., Phys. Rev. Lett. (2016)

Researchers have recently synthesized a number of graphene “cousins”: one-atom-thick sheets of group-IV elements, such as silicene (made of silicon) and germanene (germanium). Such materials are predicted to be topological insulators—materials that are insulating in their interior but have conducting surface states. However, germanene and silicene have so far been synthesized on metallic substrates, whose conductivity makes it impossible to study the topological-insulator properties. Two studies now indicate that germanene can be stably grown on an insulating MoS2 substrate while remaining a topological insulator.

The difference between graphene and germanene lies in the spin-orbit coupling (SOC) needed to create the bandgap necessary for topological insulators. Germanene’s SOC is much stronger than graphene’s, but it can decrease when germanene’s electrons hybridize with those of a substrate.

Calculations by Taher Amlaki and Paul Kelly at the University of Twente, Netherlands, and colleagues suggest the SOC loss can be kept at bay when a MoS2 substrate is used. The authors found that germanene-MoS2 bilayers retain a small insulating bandgap, and germanene sandwiched between two MoS2 layers keeps an even larger bandgap. In both sets of calculations germanene behaved as a topological insulator.

In an independent study, Lijie Zhang, also at the University of Twente, and colleagues grew germanene on MoS2 and characterized its electronic behavior via scanning tunneling experiments. The measured density of electronic states of the material is suggestive of a “Dirac” material, in which electrons move in two dimensions as massless particles. According to Zhang et al., germanene retains its Dirac nature because its electrons couple only weakly to the MoS2 substrate through van der Waals interactions. Planned experiments at low temperatures will assess whether germanene-on-MoS2 is a topological insulator as predicted by Kelly’s team.

This research is published in Physical Review Letters.

–Matteo Rini

Matteo Rini is the Deputy Editor of Physics.


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