Single-Layer Dual Germanene Phases on Ag(111)
Chung-Huang Lin1, Angus Huang1, Woei Wu Pa2,3, Wei-Chuang Chen4, Tay-Rong Chang1, Ryu Yukawa5, Cheng-Maw Cheng4, H -T Jeng1,6, Chung-Yu Mou1, Iwao Matsuda5, T.-C. Chiang7, Shu-Jung Tang1,4*
1National Tsing Hua University,, Department of Physics, Taiwan
2Center for Condensed Matter Sciences, National Taiwan University, Taiwan
3Department of physics, National Taiwan University, Taiwan
4National Synchrotron Radiation Research Center (NSRRC),, Taiwan
5Institute for Solid State Physics, The University of Tokyo, Japan
6Institute of Physics, Academia Sinica, Taiwan
7Department of Physics, University of Illinois at Urbana-Champaign, USA
* presenting author:Shu-Jung Tang,
Two distinct phase-separated single-layer honeycomb germanene lattices were identified for germanium growth on Ag(111). The geometric and electronic structures of these two phases, and their correlations, were characterized by scanning tunneling microscopy, low energy electron diffraction, angle-resolved photoemission spectroscopy, and ab-initio calculations. We discovered that a “stripe phase” germanene, which is partially commensurate with Ag(111) and possesses significant tensile strain, exhibits the unambiguous atomic up-down buckling pattern of an ideal germanene lattice. This stripe phase emerges from the de-alloying process of the known Ag2Ge surface alloy phase and covers the whole surface at 0.84 monolayer (ML) of Ge. Up to 1.08 ML, a new strain-relaxed germanene phase, which shows an abrupt decrease of Ge-Ge bond length to that of free-standing germanene and is fully incommensurate with Ag(111),appears and coexists with the stripe phase. This denser germanene phase is “quasi-free-standing-like” because it preserves the electronic structure symmetrical at germanene Κ bar point, where a dominant band observed at -3.5 eV, corresponding to Ge-Ge σ bonding, decouples itself from the Ag bulk band continuum. In contrast, the electronic structure of the stripe-phase germanene diminishes at the germanene point and a new band coupled strongly to the substrate emerges at the Ag(111) M bar . In both cases, experimental results are consistent with theoretical calculations. Our experimental results point out several important facts. First, 2D germanene can form on a substrate, even being metallic, and remains quasi-free-standing-like. Second, strain and substrate-adlayer commensurability have strong effects on the formation and evolution of germanene phases, and on the correlation of their geometric and electronic structures. These findings can lead a better control of fabricating 2D elemental honeycomb lattices

Keywords: germanene, honeycomb lattices, stripe phase, quasi-free-standing-like