Bilayer germanene growth as a chiral heterostructure

Growing a bilayer Xene other than the well-established bilayer graphene is a tremendous challenge, but with equally great potential toward the development of new two-dimensional materials. We have used angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) to investigate the growth of bilayer germanene (BLGE) on Ag(111) with the assistance of a Pb layer. AB-stacked BLGE was successfully stabilized atop a commensurate Pb(111) monolayer, itself commensurate to a 30\ifmmode^\circ\else\textdegree\fi{} rotated germanene layer in quasifreestanding phase (QP) below. Surprisingly, this four-layer configuration resulted in BLGE with a lattice constant of \ensuremath{\sim}3.5 \AA{}, which is 15% compressed relative to the freestanding counterpart. Relaxation of these four layers by density functional theory (DFT) naturally converges to the same lattice constants from measurement, despite the large compressive strain in the BLGE. ARPES spectra in two symmetry directions match those predicted by the same DFT model---in particular a characteristic eye-shaped feature resulting from the evolved Dirac cone in the $\overline{\mathrm{\ensuremath{\Gamma}}}{\overline{\mathrm{K}}}_{\mathrm{BLGE}}$ direction. Thus, we have demonstrated a novel method to grow a new Xene bilayer as part of a unique heterostructure which stabilizes the bilayer against substantial compressive strain and further induces chirality.