Striped-phase germanene confirmed by photoemission final-state effect

Abstract

The formation of germanene on Ag(111) has been extensively studied and confirmed. However, the nature of the transition stage between the initial Ag2Ge alloy phase at 1/3 monolayer (ML) and the final quasifreestanding phase germanene phase at coverage ≥1 ML still remains elusive. Using angle-resolved photoemission spectroscopy, scanning-tunneling microscopy (STM), and low-energy electron diffraction, we confirm the existence of striped-phase (SP) germanene in the transition stage. Along the armchair direction of SP, the period of higher-order coincidence corresponds to 11aAC=31aAg(111), which accommodates four stripes on the bridge sites of the underlying Ag(111) lattice, as manifested in STM topography. On the electronic-structure side, the splitlike surface band of Ag2Ge centered at the surface zone boundary M¯Ag(111) evidently evolves to a steep, V-shaped interfacial-state band, characteristic of SP germanene. Moreover, an anomalous broad Ag bulk-state band was observed to fully develop upon formation of SP germanene. The bulk-state band originates from the direct transition to the final-state bands of originally d character at ∼22-23eV above the Fermi level. Because of the strong hybridization between germanene π and Ag d orbitals, the unoccupied Ag bulk final-state d bands near the interface take on pz character, inducing the direct transition from the Ag bulk occupied pz-valence band, following the selection rule for the photoemission cross section. A germanene identity of SP is thus confirmed. Our result indicates a very important missing segment of the evolution from the initial surface alloy phase to the final Xene layer, which is the coexistence of both in a common stripe lattice during a continuous transition.