Investigation of electron-phonon coupling in epitaxial silicene by in situ Raman spectroscopy

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Investigation of electron-phonon coupling in epitaxial silicene by in situ Raman spectroscopy

Jincheng Zhuang, Xun Xu, Yi Du, Kehui Wu, Lan Chen, Weichang Hao, Jiaou Wang, Wai Kong Yeoh, Xiaolin Wang, and Shi Xue Dou

Phys. Rev. B 91, 161409(R) – Published 23 April 2015

Abstract

We report that the special coupling between Dirac fermion and lattice vibrations, in other words, electron-phonon coupling (EPC), in silicene layers on an Ag(111) surface was probed by in situ Raman spectroscopy. We find the EPC is significantly modulated due to tensile strain, which results from the lattice mismatch between silicene and the substrate, and the charge doping from the substrate. The special phonon modes corresponding to two-dimensional electron gas scattering at the edge sites in the silicene were identified. Detecting the relationship between EPC and Dirac fermions through Raman scattering will provide a direct route to investigate the exotic property in buckled two-dimensional honeycomb materials.

Received 30 October 2014
Revised 4 April 2015

DOI:https://doi.org/10.1103/PhysRevB.91.161409

Authors & Affiliations

Jincheng Zhuang¹, Xun Xu¹, Yi Du^1,*, Kehui Wu², Lan Chen², Weichang Hao^1,3, Jiaou Wang⁴, Wai Kong Yeoh¹, Xiaolin Wang¹, and Shi Xue Dou¹

¹Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong, Wollongong, NSW 2525, Australia
²Institute of Physics, Chinese Academy of Sciences, Haidian District, Beijing 100080, People's Republic of China
³Center of Materials Physics and Chemistry, and Department of Physics, Beihang University, Beijing 100191, People's Republic of China
⁴Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People's Republic of China

^*Author to whom correspondence should be addressed: yi_du@uow.edu.au

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Issue

Vol. 91, Iss. 16 — 15 April 2015

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Images

Figure 1
STM images of silicene layers in different phases: (a) mixed $\sqrt 13 \times \sqrt 13 / 4 \times 4$ , (b) $\sqrt 3 \times \sqrt 3$ silicene grown on a $\sqrt 13 \times \sqrt 13 / 4 \times 4$ buffer layer, (c) $\sqrt 3 \times \sqrt 3$ silicene grown on an Ag(111) substrate $(30 nm \times 30 nm, V_{bias} = - 1.0 V, I = 1 nA)$ , and (d) the enlarged view of $\sqrt 3 \times \sqrt 3$ silicene. The rhombus stands for the unit cell of $\sqrt 3 \times \sqrt 3$ silicene, which is used to calculate the lattice parameter of $\sqrt 3 \times \sqrt 3$ silicene $(5 nm \times 5 nm, V_{bias} = 0.5 V, I = 4 nA)$ .
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Figure 2
(a) Raman spectra of the Ag(111) substrate, Si(111), and a $\sqrt 13 \times \sqrt 13 / 4 \times 4$ . buffer layer. (b) Raman spectra of $\sqrt 3 \times \sqrt 3$ silicene grown on a $\sqrt 13 \times \sqrt 13 / 4 \times 4$ buffer layer with different coverage. “SL” denotes the coverage of the $\sqrt 3 \times \sqrt 3$ silicene layer. (c) Raman spectra of a $\sqrt 3 \times \sqrt 3$ silicene layer (coverage of 0.3 ML) grown on an Ag(111) surface and $\sqrt 13 \times \sqrt 13 / 4 \times 4$ buffer layer, respectively.
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Figure 3
(a) The enlarged view ranging from 500 to $600 {cm}^{- 1}$ of the Raman spectrum of the $E_{2 g}$ peak for different coverage samples. The $E_{2 g}$ mode frequency of FS silicene is plotted as a reference [9]. (b) Fitted results of the $E_{2 g}$ peak for 0.3 and 0.8 SL samples. The dashed lines are used to mark the position of the $E_{2 g}$ mode. (c) ARPES results of an epitaxial $\sqrt 3 \times \sqrt 3$ silicene layer. The left part shows that two faint linear dispersed bands cross at the BZ center Γ point. The right part displays constant-energy cuts of the spectral function at different binding energies, verifying that both bands originate from a Dirac cone structure, which can be assigned to linear π and $π^{*}$ states of $\sqrt 3 \times \sqrt 3$ silicene. The Dirac point is about 0.33 eV below the Fermi level, which indicates the electron-doping effect is attributed to the Ag(111) substrate. (d) $E_{2 g}$ mode frequency as a function of coverage. Both (d) and the inset of (d) are sketched to illustrate the strain and doping effects on the Raman peak position of the $E_{2 g}$ mode in $\sqrt 3 \times \sqrt 3$ silicene, in which the strain effect softens the $E_{2 g}$ mode, while charge doping upshifts the $E_{2 g}$ mode.
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Figure 4
(a) Fitted Raman spectra of $\sqrt 3 \times \sqrt 3$ silicene layers with a coverage of 0.3 SL in the frequency range of $220 – 470 c m^{- 1}$ , in which the Raman peaks due to edges are marked as $D_{1} – D_{5}$ . (b) STM image of two typical arrangements of the edges: armchair/zigzag and armchair/armchair, resulting in two edge angles of 150° and 120°, respectively. (c), (d) Atomic structures of armchair and zigzag edges. Only the armchair edge supports elastic intervalley scattering of electrons in the Brillouin zone, as indicated in the inset of (a).
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