Enhanced room-temperature spin-valley coupling in V-doped ${\mathrm{MoS}}_{2}$

Enhanced room-temperature spin-valley coupling in V-doped ${MoS}_{2}$

Krishna Rani Sahoo, Janmey Jay Panda, Sumit Bawari, Rahul Sharma, Dipak Maity, Ashique Lal, Raul Arenal, G. Rajalaksmi, and Tharangattu N. Narayanan

Phys. Rev. Materials 6, 085202 – Published 23 August 2022

Abstract

Achieving room-temperature valley polarization in two-dimensional (2D) atomic layers (2D materials) by substitutional doping opens new avenues of applications. Here, monolayer ${MoS}_{2}$ , when doped with vanadium at low (0.1 atomic %) concentrations, is shown to exhibit high spin-valley coupling, and hence a high degree of valley polarization at room-temperature. The atomic layers of ${MoS}_{2}$ (MS) and V-doped ${MoS}_{2}$ (VMS) are grown via the chemical vapor deposition-assisted method. The formation of new energy states near the valence band is confirmed from band gap calculations and also from the density functional theory–based band structure analyses. Time-reversal symmetry broken energy shift in the equivalent valleys is predicted in VMS, and the room-temperature chirality-controlled photoluminescent (PL) excitation measurements indicate such a shift in valley exciton energies (∼35 meV). An enhanced valley polarization in VMS (∼42%) is observed in comparison to that in MS (<12%), while in MS, the chirality-controlled excitations did not show the difference in emission energies. Spin Hall effect of light–based optical rotation measurements indicate the asymmetric absorption among the two different chiralities of the incident light, hence supporting the existence of room-temperature valley polarization. This study opens possibilities of room-temperature opto-spintronics using stable 2D materials.

Received 16 March 2022
Revised 24 June 2022
Accepted 11 August 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.085202

Physics Subject Headings (PhySH)

Density of states Spin-orbit coupling Valleytronics

Transition metal dichalcogenides

Crystal growth Density functional theory Electronic structure Photoelectron techniques Photoluminescence Raman spectroscopy Scanning electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Krishna Rani Sahoo¹, Janmey Jay Panda ¹, Sumit Bawari¹, Rahul Sharma¹, Dipak Maity¹, Ashique Lal ¹, Raul Arenal ^2,3,4, G. Rajalaksmi ¹, and Tharangattu N. Narayanan ^1,*

¹Tata Institute of Fundamental Research-Hyderabad, Sy. No. 36/P, Gopanapally Village, Serilingampally Mandal, Hyderabad-500046, India
²Instituto de Nanociencia y Materiales de Aragon (INMA), Universidad de Zaragoza, 50009 Zaragoza, Spain
³Laboratorio de Microscopias Avanzadas (LMA), Universidad de Zaragoza, 50018 Zaragoza, Spain
⁴Fundación ARAID, 50018 Zaragoza, Spain

^*tnn@tifrh.res.in

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Vol. 6, Iss. 8 — August 2022

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Figure 1
Structure and morphology study. (a) Raman spectra of VMS and MS using a laser excitation of 532 nm. The highlighted portions illustrate new modes of MS. (b) PL spectra of MS and VMS at an excitation wavelength of 532 nm. The excitation is by linear vanadium polarization, and PL detection is not chirality resolved. The inset represents a possible band structure after V doping, which is further verified in the later part using XPS. Here, DS indicates the doped states, which are vanadium states created due to doping. (c) Atomic structure of V-doped MS from STEM (HAADF-STEM) showing a hexagonal lattice structure where vanadium replaces molybdenum in some random places. Vanadium sites are highlighted with blue circles. (d) The intensity line profile at three places indicates the presence of vanadium in the MS lattice. The different-colored line profiles indicate the area (color code) mentioned in (c). Chemical information from XPS analysis: XPS spectra of (e) molybdenum $3 d$ orbital, (f) sulfur $2 p$ orbital, and (g) vanadium $2 p$ along with the molybdenum $3 s$ orbital. New bonding states are observed in molybdenum and sulfur due to V doping.
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Figure 2
PL experiments with different optical excitations $(λ = 532 nm)$ : (a) PL spectra under linear excitation and extracting two different polarizations [chirality, right ( $σ^{+}$ ) and left ( $σ^{-}$ ) circular] in emission for MS at room-temperature, (b) PL spectra of two different chirality excitations in MS at room-temperature, (c) PL spectra under linear excitation and extracting two different polarizations [right ( $σ^{+}$ ) and left ( $σ^{-}$ ) circular] in emission for VMS at room-temperature depicting valley polarization introducing ∼35 meV valley splitting, and (d) PL spectra of two different chirality excitations in VMS at room-temperature show a clear valley splitting.
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Figure 3
The SHEL-based valley polarization study. SHEL data comparison of MS and VMS in two different initial linear polarization directions: (a) vertical (V) and (b) horizontal (H). In both configurations, the difference between $σ_{MS}^{+}$ and $σ_{VMS}^{+}$ is lower than that of $σ_{MS}^{-}$ and $σ_{VMS}^{-}$ . A 633 nm laser is used for the experiment. Typical intensity profile of SHEL shift with respect to analyzer angle from (a). (c) The maximum intensity of $σ^{+}$ at position 1; (d) equal intensity at the center at position 2 showing the separation of two components of linear polarized light, i.e., $σ^{+}$ and $σ^{-}$ ; and (e) the maximum intensity of $σ^{-}$ light at position 3. A similar intensity profile is also observed for (b), which is not shown here.
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Figure 4
Understanding the origin of valley polarization in a V-doped MS system using a DFT study. (a) Electronic band structure of pristine MS with SOC (spin-orbit coupling “on”). (b) Atomic model of a V-doped MS system with local density of states. (c) Electronic band structure of VMS without SOC (SOC “off”). Degeneracy in spin-up and spin-down at the K and K′ valleys is plotted, confirming spin polarization. (d) Electronic band diagram of VMS with SOC interaction. In all the data, the Fermi level is referred to as zero. A new state formed near the Fermi level (just above the valence band) in (d), showing a $p$ doping nature. The energy difference between K and K′ is found to be 120 meV, indicating the time-reversal symmetry breaking. The SOC and spin polarization together cannot be switched on simultaneously in the DFT platform used.
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Figure 5
Schematic diagrams of the MS and VMS energy band structures. (a) MS with spin-orbit interaction and (b) VMS with spin-orbit interaction. According to the optical selection rule, the $Δ m_{j}$ value [4] should be +1 for a $σ^{+}$ transition and −1 for a $σ^{-}$ transition. The additional energy shift in K′ is induced by V doping. Magenta and green represent spin-up and spin-down states in valleys. CB, conduction band; DS, doped states; VB, valence band.
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Enhanced room-temperature spin-valley coupling in V-doped MoS2

Krishna Rani Sahoo, Janmey Jay Panda, Sumit Bawari, Rahul Sharma, Dipak Maity, Ashique Lal, Raul Arenal, G. Rajalaksmi, and Tharangattu N. Narayanan

Phys. Rev. Materials 6, 085202 – Published 23 August 2022

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Enhanced room-temperature spin-valley coupling in V-doped ${MoS}_{2}$