Pb-induced growth of bilayer germanene on Ag(111)

Germanene, the graphene analogue of germanium, has been discovered to grow on various substrates, such as Ag(111), Al(111), Pt(111), Cu(111) and MoS₂. It was predicted to be a robust 2D topological insulator resulting from the effective spin–orbit coupling, leading to the quantum spin Hall effect. For bilayer germanene, covalent bonding between two monolayer germanene sheets can cause the integration of intrinsic magnetism and band gap opening that implies many potential applications in future nanoelectronics, spin-based computation and data storage.

We report an approach of growing bilayer germanene using a Pb layer as a template deposited on a quasi-freestanding phase (QP) germanene grown on Ag(111), and then further depositing extra germanium atoms on the surface to form bilayer germanene. Using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we confirm the existence of bilayer germanene and propose a structural model which is: a 5-layer stack consisting of Ag(111) at the bottom, QP germanene with the same orientation at the second layer, a Pb(111) layer with 30-degree rotation at the third layer, and AB-stacking bilayer germanene on the top. The energy band structure measured by ARPES agrees reasonably with our density function theory (DFT) calculation based on the model of AB-staking bilayer germanene on Pb(111).