Symmetry breaking induced bandgap opening in epitaxial germanene on WSe₂

Germanene has attracted much attention because the material was predicted to host Dirac fermions. However, the synthesis of germanene is still in its infancy; moreover, the predicted tiny bandgap induced by the spin–orbit coupling is far from practical applications for nanoelectronic devices. Herein, quasi-freestanding germanene with linear dispersion relation of the band structure is well grown on a WSe2/Au(100) substrate. Band structure calculations reveal that the interaction of germanene with the substrate destroys the sublattice symmetry. The energy-dependent contribution of σ orbitals responsible for band crossing at the Fermi level around the Γ point induces asymmetric density of states at the Dirac point. Upon annealing in ultra-high vacuum, we observe a bandgap opening in germanene of about ∼0.17 eV, which is attributed to a sublattice symmetry breaking in germanene and the emergence of a net electric field. This work provides an effective method to tune or tailor the electronic properties of germanene, paving the way to germanene-based field-effect applications.