Robust spin-valley polarization in commensurate $\mathrm{Mo}{\mathrm{S}}_{2}$/graphene heterostructures

Robust spin-valley polarization in commensurate $Mo S_{2}$ /graphene heterostructures

Luojun Du, Qian Zhang, Benchao Gong, Mengzhou Liao, Jianqi Zhu, Hua Yu, Rui He, Kai Liu, Rong Yang, Dongxia Shi, Lin Gu, Feng Yan, Guangyu Zhang, and Qingming Zhang

Phys. Rev. B 97, 115445 – Published 27 March 2018

Abstract

The investigation and control of quantum degrees of freedom (DoFs) of carriers lie at the heart of condensed-matter physics and next-generation electronics/optoelectronics. van der Waals heterostructures stacked from distinct two-dimensional (2D) crystals offer an unprecedented platform for combining the superior properties of individual 2D materials and manipulating spin, layer, and valley DoFs. $Mo S_{2}$ /graphene heterostructures, harboring prominent spin-transport properties of graphene, giant spin-orbit coupling, and spin-valley polarization of $Mo S_{2}$ , are predicted as a perfect venue for optospintronics. Here, we report the epitaxial growth of commensurate $Mo S_{2}$ on graphene with high quality by chemical vapor deposition, and demonstrate robust temperature-independent spin-valley polarization at off-resonant excitation. We further show that the helicity of B exciton is larger than that of A exciton, allowing the manipulation of spin bits in the commensurate heterostructures by both optical helicity and wavelength. Our results open a window for controlling spin DoF by light and pave a way for taking spin qubits as information carriers in the next-generation valley-controlled optospintronics.

Received 13 August 2017
Revised 14 November 2017

DOI:https://doi.org/10.1103/PhysRevB.97.115445

Physics Subject Headings (PhySH)

Spintronics Valleytronics

Dichalcogenides Graphene Heterostructures

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Luojun Du^1,2, Qian Zhang¹, Benchao Gong¹, Mengzhou Liao², Jianqi Zhu², Hua Yu², Rui He³, Kai Liu¹, Rong Yang², Dongxia Shi^2,4, Lin Gu^2,5, Feng Yan⁶, Guangyu Zhang^2,4,5,*, and Qingming Zhang^1,7,†

¹Department of Physics, Beijing Key Laboratory of Opto-Electronic Functional Materials and Micro-nano Devices, Renmin University of China, Beijing 100872, China
²Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
³Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas 79409, USA
⁴School of Physical Sciences, University of Chinese Academy of Science, Beijing 100190, China
⁵Collaborative Innovation Center of Quantum Matter, Beijing 100190, China
⁶Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, 999077, Kowloon, Hong Kong
⁷Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240 and Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

^*gyzhang@iphy.ac.cn
^†qmzhang@ruc.edu.cn

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Vol. 97, Iss. 11 — 15 March 2018

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Figure 1
(a) Schematic illustration of the growth process. Yellow, blue, red, and black balls represent sulfur, molybdenum, oxygen, and carbon atoms, respectively. (b) Raman spectra for $Mo S_{2}$ /graphene heterostructures (blue line), mechanically exfoliated $Mo S_{2}$ (magenta line), and mechanically exfoliated graphene (red line) at 2.33-eV excitation. (c) AFM image of $Mo S_{2}$ /graphene heterostructures. A height profile was extracted along the white line. (d) STM topographic image showing 1.4-nm moiré patterns. (e) Atomic-resolution ADF-STEM image of suspended $Mo S_{2}$ /graphene heterostructures on a TEM grid. The brighter (dimmer) spots are molybdenum (sulfur) atoms. The hexagonal lattice shows alternate molybdenum and sulfur sites with the top-view schematics overlaid. (f) SAED pattern of $Mo S_{2}$ /graphene heterostructures.
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Figure 2
(a) Photoluminescence spectrum for $Mo S_{2}$ /graphene heterostructures (black) and mechanically exfoliated $Mo S_{2}$ (red) at 2.33-eV excitation and 10 K. (b) Polarized Raman spectra for $Mo S_{2}$ /graphene heterostructures at room temperature with applied magnetic field and in the cross-polarized (upper) configuration (incident light polarization $e_{i}$ and scattered light polarization $e_{s}$ are perpendicular to each), and copolarized (lower) configuration ( $e_{i}$ and $e_{s}$ are parallel to each). Inset: magnetic-field dependence of the $A_{1}^{'}$ mode intensity for $Mo S_{2}$ /graphene heterostructures.
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Figure 3
All optical excitation is left circularly polarized ( $σ_{-}$ ). (a) Electronic band structure of $Mo S_{2}$ /graphene heterostructures from DFT with SOC. (b) Zoom-in near the K valley [highlighted by the dashed box in (a)] shows graphene bands appearing gapped and spin-polarized due to proximity to $Mo S_{2}$ . The red (blue) bands are for spin-up (spin-down) states. (c) Schematics of valley-dependent optical selection rules and spin-valley locking at K/K′ valleys in the momentum space of $Mo S_{2}$ . (d), (e) Polarization-resolved PL spectra of mechanically exfoliated $Mo S_{2}$ (d) and $Mo S_{2}$ /graphene (e) versus photon energy for 1.96-eV excitation. (f) Degree of PL polarization as a function of temperature at 1.96-eV excitation. (g), (h) Polarization-resolved PL spectra of mechanically exfoliated $Mo S_{2}$ (g) and $Mo S_{2}$ /graphene (h) versus photon energy for 2.33-eV excitation. (i) Degree of PL polarization versus temperature at 2.33-eV excitation.
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Figure 4
(a), (c) Spin-down polarization and state 0 by right circularly polarized light at 633-nm excitation (a) and left circularly polarized light at 532-nm excitation (c). (b) Spin-up polarization and state 1 by left circularly polarized light at 633-nm excitation.
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Physical Review B

covering condensed matter and materials physics

Robust spin-valley polarization in commensurate $Mo S_{2}$ /graphene heterostructures

Luojun Du, Qian Zhang, Benchao Gong, Mengzhou Liao, Jianqi Zhu, Hua Yu, Rui He, Kai Liu, Rong Yang, Dongxia Shi, Lin Gu, Feng Yan, Guangyu Zhang, and Qingming Zhang

Phys. Rev. B 97, 115445 – Published 27 March 2018

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Physical Review B

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Robust spin-valley polarization in commensurate MoS2/graphene heterostructures

Luojun Du, Qian Zhang, Benchao Gong, Mengzhou Liao, Jianqi Zhu, Hua Yu, Rui He, Kai Liu, Rong Yang, Dongxia Shi, Lin Gu, Feng Yan, Guangyu Zhang, and Qingming Zhang

Phys. Rev. B 97, 115445 – Published 27 March 2018

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Robust spin-valley polarization in commensurate $Mo S_{2}$ /graphene heterostructures