  Germanene, a two-dimensional honeycomb germanium crystal, is grown at
graphene/Ag(111) and hexagonal boron nitride (h-BN)/Ag(111) interfaces by
segregating germanium atoms. A simple annealing process in N2 or H2/Ar at
ambient pressure leads to the formation of germanene, indicating that an
ultrahigh-vacuum condition is not necessary. The grown germanene is stable in
air and uniform over the entire area covered with a van der Waals (vdW)
material. As an important finding, it is necessary to use a vdW material as a
cap layer for the present germanene growth method since the use of an Al2O3 cap
layer resulted in no germanene formation. The present study also proved that
Raman spectroscopy in air is a powerful tool for characterizing germanene at
the interfaces, which is concluded by multiple analyses including
first-principles density functional theory calculations. The direct growth of
h-BN-capped germanene on Ag(111), which is demonstrated in the present study,
is considered to be a promising technique for the fabrication of future
germanene-based electronic devices.
