Single-layer dual germanene phases on Ag(111)

Chung-Huang Lin, Angus Huang, Woei Wu Pai, Wei-Chuan Chen, Ting-Yu Chen, Tay-Rong Chang, Ryu Yukawa, Cheng-Maw Cheng, Chung-Yu Mou, Iwao Matsuda, T.-C. Chiang, H.-T. Jeng, and S.-J. Tang

Phys. Rev. Materials 2, 024003 – Published 9 February 2018

Abstract

Two-dimensional (2D) honeycomb lattices beyond graphene promise new physical properties such as quantum spin Hall effect. While there have been claims of growth of such lattices (silicene, germanene, stanene), their existence needs further support and their preparation and characterization remain a difficult challenge. Our findings suggest that two distinct phases associated with germanene, the analog of graphene made of germanium (Ge) instead of carbon, can be grown on Ag(111) as observed by scanning tunneling microscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy. One such germanene exhibits an atom-resolved alternatively buckled full honeycomb lattice, which is tensile strained and partially commensurate with the substrate to form a striped phase (SP). The other, a quasifreestanding phase (QP), is also consistent with a honeycomb lattice with a lattice constant incommensurate with the substrate but very close to the theoretical value for freestanding germanene. The SP, with a lower atomic density, can be driven into the QP and coexist with the QP by additional Ge deposition. Band mapping and first-principles calculations with proposed SP and QP models reveal an interface state exists only in the SP but the characteristic σ band of freestanding germanene emerges only in the QP—this leads to an important conclusion that adlayer-substrate commensurability plays a key role to affect the electronic structure of germanene. The evolution of the dual germanene phases manifests the competitive formation of Ge-Ge covalent and Ge-Ag interfacial bonds.

Received 8 May 2017
Revised 17 January 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.024003

Physics Subject Headings (PhySH)

Electronic structure First-principles calculations Growth Structural properties Surface & interfacial phenomena

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chung-Huang Lin^1,2, Angus Huang¹, Woei Wu Pai^2,3,*, Wei-Chuan Chen¹, Ting-Yu Chen¹, Tay-Rong Chang¹, Ryu Yukawa⁴, Cheng-Maw Cheng⁵, Chung-Yu Mou¹, Iwao Matsuda⁴, T.-C. Chiang^3,6, H.-T. Jeng^1,7,†, and S.-J. Tang^1,5,‡

¹Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
²Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan
³Department of Physics, National Taiwan University, Taipei 106, Taiwan
⁴Institute for Solid State Physics, the University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277–8581, Japan
⁵National Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076, Taiwan
⁶Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080, USA
⁷Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

^*wpai@ntu.edu.tw
^†jeng@phys.nthu.edu.tw
^‡sjtang@phys.nthu.edu.tw

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Vol. 2, Iss. 2 — February 2018

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Images

Figure 1
“Striped phase” germanene. (a) Atomic view of a germanene lattice grown on Ag(111) (image size, tunneling sample bias, and current: $8 \times 8 nm$ , 0.29 V, and 0.95 nA). Note that all Ge atoms are resolved and show an alternating up-and-down low buckle pattern. This SP germanene is commensurate with the Ag lattice in the stripe direction (i.e., germanene lattice zigzag direction $a_{1}$ -zz). The germanene lattice constant along the armchair (ac) direction is compressed, leading to smaller lattice spacings (cf. in the $a_{1}$ direction) along $a_{2}$ and $a_{3}$ . The incommensurability along ac leads to stripe features. (b) A line profile [line in (a)] shows a buckling height $\sim 0.1 Å$ . The inset shows a magnified view of a buckled hexagon. (c) Coexistence of three orientation striped-phase domains separated by $120^{\circ}$ (image size, tunneling sample bias, and current: $60 \times 60 nm, - 412 mV$ , and 0.91 nA). (d) Top view of the striped phase model. Dashed line rhombus indicates a $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ cell. (e) LEED diffraction pattern at 75-eV beam energy. Note that all LEED spots consist of satellite features (e.g., see inset red box). (f) Simulation of the LEED pattern of (e). See content for details.
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Figure 2
“Quasifreestanding phase” germanene. (a) Coexisting SP and QP germanene ( $15 \times 15 nm, - 0.65 V$ , and 1.0 nA). (b) QP germanene shows a slightly disordered moiré pattern with a short periodicity $\sim 9 Å$ ( $30 \times 30 nm$ , 1 V, and 1 nA). (c) Zoom-in micrograph of the moiré pattern shows adatoms (solid circles), local pinwheel-like structure (dashed circle), and a honeycomb lattice underneath: $11.6 \times 11.6 nm$ , 0.1 V, and 0.73 nA. To the right of the (a), (b) topographic panels are the fast Fourier transform (FFT) images (blue) and LEED diffraction patterns (gray) of the respective phases. SP, QP, MQ, MS, Ag, respectively, denote the periodicities of SP germanene, QP germanene periodicity, QP “moiré,” SP stripe, and Ag(111). Five-nm length scale bars, LEED beam energies, and FFT 5- $n m^{- 1}$ scales are noted in (a)–(c). See content for details. (d) Atom-resolved QP lattice ( $8 \times 4.2 nm$ , 10 mV, and 0.73 nA). A honeycomb germanene lattice with slight disorder can be discerned. Imperfections (e.g., indicated Ge pentagon) are observed in the QP phase. Adatoms atop QP show inverted contrast with this imaging condition. (e) Proposed structure of the QP. (f) Simulation of the QP LEED pattern taken with 75-eV beam energy. See content for details.
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Figure 3
Measured energy-band structures of SP germanene grown on Ag(111). (a) SBZs of Ag(111), Ag(111)- $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ , and SP, marked by blue-dashed, black-dotted, and green-solid hexagons, respectively. The symmetry points are marked in the same color as the corresponding SBZs they belong to. (b)−(e) Measured energy-band dispersions of Ag(111), and SP germanene taken at the photon energies of 27 and 35 eV in the same symmetry direction marked by $\bar{Γ} - \bar{M}$ and ${\bar{Γ}}_{S} - {\bar{K}}_{S} - {\bar{M}}_{S}$ , respectively, for their corresponding SBZs.
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Figure 4
Comparison between measured and calculated electronic structures for SP germanene. (a) Measured SP bands, $S_{1}$ and $S_{2}$ , within the Ag bulk band gap centered at common $\bar{M} ({\bar{M}}_{S})$ . (b)−(d) Measured CECs at the energies of $- 1.2, - 2.2$ , and $- 3.3 eV$ . (e) Calculated SP bands within the gap of Ag bulk projected bands (shaded gray area). (f), (g) Corresponding calculated CECs at the energies of $- 1.8$ and $- 2.3 eV$ .
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Figure 5
Comparison between measured and calculated electronic structures for QP germanene. (a) SBZs of Ag(111), Ag(111)- $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ , and QP marked by blue-dashed, black-dotted, and red-solid hexagons, respectively. (b), (c) Measured energy-band dispersions of Ag(111), and QP germanene taken by He I UV light source in the same symmetry direction marked by $\bar{Γ} - \bar{M}$ and ${\bar{Γ}}_{Q} - {\bar{K}}_{Q} - {\bar{M}}_{Q}$ . (d) The same figure as (c) with red dots superimposed to indicate QP band. (e) The calculated energy bands for a freestanding germanene superimposed with the Ag bulk band continuum (gray shade).
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