Since the discovery of graphene in 2004, there has been interest in creating 2d materials, a few atomic layers thick materials, which have applications in electronic devices.

The rhenium dichalcogenides, like graphene, have extreme properties when exfoliated to a few atomic layers, however unlike graphene they have a band gap which is required in electronic devices such as transistors.

The paper shows that the act of flipping a flake is not a symmetry operation of the system, there are two nonequivalent “ways up”. These two different vertical orientations of the flakes were identified using Raman spectroscopy. This work will be important for understanding the growth of these materials.