Abstract
Growth of Xene, referred to as the analog of graphene made of atoms other than carbon, with higher lattice quality and more intrinsic electronic structures, is an important challenge to the development of two-dimensional (2D) materials. We report an approach using Pb-atom deposition to striped-phase (SP) germanene grown on Ag(111) to trigger it to a quasifreestanding phase (QP). Using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), we demonstrate that the 2D monatomic Pb layer, instead of being on top, grows side-by-side with QP germanene, not only driving the SP-to-QP transition but further enhancing the lattice order of QP as revealed by the dramatic evolution of LEED-spot shape and uniform STM-imaged array of moir\'e patterns. Originally oriented at 30\ifmmode^\circ\else\textdegree\fi{} to Ag(111), the QP germanene transits first to a narrow distribution of rotations then to two sharp domains at \ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{} from 30\ifmmode^\circ\else\textdegree\fi{}, while the monoatomic Pb domains twist from \ifmmode\pm\else\textpm\fi{}4.3\ifmmode^\circ\else\textdegree\fi{} to \ifmmode\pm\else\textpm\fi{}9.5\ifmmode^\circ\else\textdegree\fi{} with respect to Ag(111). The analysis of higher-order coincidence for both layers on Ag(111) is in complete agreement with LEED results and reveals a picture of two insoluble monatomic layers driving each other to a more optimal configuration. Moreover, the corresponding density of states measured by scanning tunneling spectroscopy for quality-enhanced QP germanene exhibits a clear V-shape distribution at the energy \ensuremath{\sim}0.4 eV above Fermi level, indicating the Fermi-Dirac cone, which agrees with our density functional theory calculation.
Highlights
Using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), we demonstrate that the 2D monatomic Pb layer, instead of being on top, grows side-by-side with quasifreestanding phase (QP) germanene, driving the SP-to-QP transition but further enhancing the lattice order of QP as revealed by the dramatic evolution of LEED-spot shape and uniform STM-imaged array of moiré patterns
The corresponding density of states measured by scanning tunneling spectroscopy for quality-enhanced QP germanene exhibits a clear V-shape distribution at the energy ∼0.4 eV above Fermi level, indicating the Fermi-Dirac cone, which agrees with our density functional theory calculation
In all the LEED simulation results displayed, the dark, light blue, red, and green colors denote the diffractions from Ag(111), SP, QP germanene, and 1-ML Pb layer, respectively
Summary
Growth of a monatomic layer with a honeycomb lattice has been a hot topic since the discovery of graphene [1], the analogs of which belong to the family of two-dimensional (2D) Xenes [2] such as silicene [3,4], germanene [5,6,7,8], stanene [9,10], plumbene [11], borophene [12], bismuthine [13], gallenene [14], phosphorene [15], arsenene [16], antimonene [17], selenene [18], and tellurene [19]. After extra Ge atoms are deposited to the SP, Ge-Ge bonds in both symmetry directions relax to the QP form with 21% lattice mismatch but only ∼3.7% compressive strain relative to unstrained freestanding germanene [5]. This relaxation-triggering effect by extra Ge atoms is interesting since it represents a pure mechanical process which might be induced by atoms of different species to offer advantages. 1], as confirmed by low-energy electron diffraction (LEED), STM, and scanning tunneling spectroscopy (STS)
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