J. Semicond. > 2022, Volume 43 > Issue 8 > 082001
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ARTICLES

Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy

Shengmin Hu1, 2, Jialiang Ye1, 2, Ruiqi Liu1, 2 and Xinhui Zhang1, 2,

+ Author Affiliations

 Corresponding author: Xinhui Zhang, xinhuiz@semi.ac.cn

DOI: 10.1088/1674-4926/43/8/082001

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Abstract: Monolayer transition-metal dichalcogenides possess rich excitonic physics and unique valley-contrasting optical selection rule, and offer a great platform for long spin/valley lifetime engineering and the associated spin/valleytronics exploration. Using two-color time-resolved Kerr rotation and time-resolved reflectivity spectroscopy, we investigate the spin/valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy. With fine tuning of the photon energy of both pump and probe beams, the valley relaxation process for the neutral excitons and trions is found to be remarkably different—their characteristic spin/valley lifetimes vary from picoseconds to nanoseconds, respectively. The observed long trion spin lifetime of > 2.0 ns is discussed to be associated with the dark trion states, which is evidenced by the photon-energy dependent valley polarization relaxation. Our results also reveal that valley depolarization for these different excitonic states is intimately connected with the strong Coulomb interaction when the optical excitation energy is above the exciton resonance.

Key words: TMDCs, excitons, valley polarization lifetime, two-color time-resolved Kerr rotation spectroscopy



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Mai C, Barrette A, Yu Y F, et al. Many-body effects in valleytronics: Direct measurement of valley lifetimes in single-layer MoS2. Nano Lett, 2014, 14, 202 doi: 10.1021/nl403742j
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Wang G, Chernikov A, Glazov M M, et al. Colloquium: Excitons in atomically thin transition metal dichalcogenides. Rev Mod Phys, 2018, 90, 021001 doi: 10.1103/RevModPhys.90.021001
[35]
Adachi S. Luminescence spectroscopy of Cr3+ in Al2O3 polymorphs. Opt Mater, 2021, 114, 111000 doi: 10.1016/j.optmat.2021.111000
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Borghardt S, Tu J S, Winkler F, et al. Engineering of optical and electronic band gaps in transition metal dichalcogenide monolayers through external dielectric screening. Phys Rev Mater, 2017, 1, 054001 doi: 10.1103/PhysRevMaterials.1.054001
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Plechinger G, Korn T, Lupton J M. Valley-polarized exciton dynamics in exfoliated monolayer WSe2. J Phys Chem C, 2017, 121, 6409 doi: 10.1021/acs.jpcc.7b01468
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Singh A, Moody G, Wu S F, et al. Coherent electronic coupling in atomically thin MoSe2. Phys Rev Lett, 2014, 112, 216804 doi: 10.1103/PhysRevLett.112.216804
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Schmidt R, Berghäuser G, Schneider R, et al. Ultrafast coulomb-induced intervalley coupling in atomically thin WS2. Nano Lett, 2016, 16, 2945 doi: 10.1021/acs.nanolett.5b04733
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Pogna E A A, Marsili M, de Fazio D, et al. Photo-induced bandgap renormalization governs the ultrafast response of single-layer MoS2. ACS Nano, 2016, 10, 1182 doi: 10.1021/acsnano.5b06488
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Shinokita K, Wang X F, Miyauchi Y, et al. Ultrafast dynamics of bright and dark positive trions for valley polarization in monolayer WSe2. Phys Rev B, 2019, 99, 245307 doi: 10.1103/PhysRevB.99.245307
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Sanvitto D, Hogg R A, Shields A J, et al. Rapid radiative decay of charged excitons. Phys Rev B, 2000, 62, R13294 doi: 10.1103/PhysRevB.62.R13294
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Fig. 1.  (Color online) (a) AFM micrography of the monolayer WSe2 grown on sapphire substrate, the inset is the height profile of the monolayer MBE-WSe2. (b) Room temperature PL spectra of the as-grown monolayer WSe2 on sapphire and sapphire substrate itself. (c) The PL spectra of MBE-WSe2 monolayer as a function of temperature. (d) PL spectrum and Gaussian fit of MBE-WSe2 monolayer measured at 10 K.

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Fig. 2.  (Color online) (a) Normalized transient differential reflectivity (ΔR/R) and (b) TRKR signals, with respect to the peak values measured at various probe energies in resonance with A exciton (A) and trion (T), respectively, for MBE-WSe2 monolayer. Here pump energies are all set at 1.879 eV, and the measured temperature is 10 K. The inset in (b) shows the normalized TRKR response measured under excitation of right- (σ+, red trace) and left- (σ−, blue trace) circularly polarized pump beam, with the probe energy to be resonant with A exciton.

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Fig. 3.  (Color online) (a) Temperature-dependent TRKR responses and their fittings (solid lines) probed for trion, pump and probe photon energies are set to be 1.879 and 1.722 eV, respectively. (b) Temperature-dependent valley relaxation time deduced from results in (a). (c) TRKR measured at various pump energies with a fixed probe energy of 1.710 eV at 10 K. The solid lines are fitting results. (d) The extracted long valley lifetime τv3 as a function of pump wavelength at 10 K.

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Fig. 4.  (Color online) Schematics of the bright and dark trion states in WSe2. (a) A bright singlet trion and (d) a bright triplet trion; when optically excited in the K valley of n-doped WSe2. (b), (c), (e), and (f) illustrate the possible conversion channels from bright to dark trions under phonon- or defect-assisted scattering.

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[1]
Xiao D, Liu G B, Feng W X, et al. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys Rev Lett, 2012, 108, 196802 doi: 10.1103/PhysRevLett.108.196802
[2]
Sallen G, Bouet L, Marie X, et al. Robust optical emission polarization in MoS2 monolayers through selective valley excitation. Phys Rev B, 2012, 86, 081301 doi: 10.1103/PhysRevB.86.081301
[3]
Cao T, Wang G, Han W, et al. Valley-selective circular dichroism of monolayer molybdenum disulphide. Nat Commun, 2012, 3, 887 doi: 10.1038/ncomms1882
[4]
Mak K F, He K, Shan J, et al. Control of valley polarization in monolayer MoS2 by optical helicity. Nat Nanotechnol, 2012, 7, 494 doi: 10.1038/nnano.2012.96
[5]
Jones A M, Yu H, Ghimire N J, et al. Optical generation of excitonic valley coherence in monolayer WSe2. Nat Nanotechnol, 2013, 8, 634 doi: 10.1038/nnano.2013.151
[6]
Xu X, Yao W, Xiao D, et al. Spin and pseudospins in layered transition metal dichalcogenides. Nat Physics, 2014, 10, 343 doi: 10.1038/nphys2942
[7]
Shi H Y, Yan R S, Bertolazzi S, et al. Exciton dynamics in suspended monolayer and few-layer MoS2 2D crystals. ACS Nano, 2013, 7, 1072 doi: 10.1021/nn303973r
[8]
Mai C, Barrette A, Yu Y F, et al. Many-body effects in valleytronics: Direct measurement of valley lifetimes in single-layer MoS2. Nano Lett, 2014, 14, 202 doi: 10.1021/nl403742j
[9]
Wang Q S, Ge S F, Li X, et al. Valley carrier dynamics in monolayer molybdenum disulfide from helicity-resolved ultrafast pump-probe spectroscopy. ACS Nano, 2013, 7, 11087 doi: 10.1021/nn405419h
[10]
Cui Q N, Ceballos F, Kumar N, et al. Transient absorption microscopy of monolayer and bulk WSe2. ACS Nano, 2014, 8, 2970 doi: 10.1021/nn500277y
[11]
Wang G, Bouet L, Lagarde D, et al. Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2. Phys Rev B, 2014, 90, 075413 doi: 10.1103/PhysRevB.90.075413
[12]
Lagarde D, Bouet L, Marie X, et al. Carrier and polarization dynamics in monolayer MoS2. Phys Rev Lett, 2014, 112, 047401 doi: 10.1103/PhysRevLett.112.047401
[13]
Zhu C R, Zhang K, Glazov M, et al. Exciton valley dynamics probed by Kerr rotation in WSe2 monolayers. Phys Rev B, 2014, 90, 161302 doi: 10.1103/PhysRevB.90.161302
[14]
dal Conte S, Bottegoni F, Pogna E A A, et al. Ultrafast valley relaxation dynamics in monolayer MoS2 probed by nonequilibrium optical techniques. Phys Rev B, 2015, 92, 235425 doi: 10.1103/PhysRevB.92.235425
[15]
Yang L Y, Sinitsyn N A, Chen W B, et al. Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS2 and WS2. Nat Phys, 2015, 11, 830 doi: 10.1038/nphys3419
[16]
Hsu W T, Chen Y L, Chen C H, et al. Optically initialized robust valley-polarized holes in monolayer WSe2. Nat Commun, 2015, 6, 8963 doi: 10.1038/ncomms9963
[17]
Plechinger G, Nagler P, Arora A, et al. Trion fine structure and coupled spin–valley dynamics in monolayer tungsten disulfide. Nat Commun, 2016, 7, 12715 doi: 10.1038/ncomms12715
[18]
Song X L, Xie S E, Kang K, et al. Long-lived hole spin/valley polarization probed by kerr rotation in monolayer WSe2. Nano Lett, 2016, 16, 5010 doi: 10.1021/acs.nanolett.6b01727
[19]
Dey P, Yang L Y, Robert C, et al. Gate-controlled spin-valley locking of resident carriers in WSe2 monolayers. Phys Rev Lett, 2017, 119, 137401 doi: 10.1103/PhysRevLett.119.137401
[20]
Singh A, Tran K, Kolarczik M, et al. Long-lived valley polarization of intravalley trions in monolayer WSe2. Phys Rev Lett, 2016, 117, 257402 doi: 10.1103/PhysRevLett.117.257402
[21]
Volmer F, Pissinger S, Ersfeld M, et al. Intervalley dark trion states with spin lifetimes of 150 ns in WSe2. Phys Rev B, 2017, 95, 235408 doi: 10.1103/PhysRevB.95.235408
[22]
McCormick E J, Newburger M J, Luo Y K, et al. Imaging spin dynamics in monolayer WS2 by time-resolved Kerr rotation microscopy. 2D Mater, 2017, 5, 011010 doi: 10.1088/2053-1583/aa98ae
[23]
He K L, Kumar N, Zhao L, et al. Tightly bound excitons in monolayer WSe2. Phys Rev Lett, 2014, 113, 026803 doi: 10.1103/PhysRevLett.113.026803
[24]
Brem S, Ekman A, Christiansen D, et al. Phonon-assisted photoluminescence from indirect excitons in monolayers of transition-metal dichalcogenides. Nano Lett, 2020, 20, 2849 doi: 10.1021/acs.nanolett.0c00633
[25]
You Y, Zhang X X, Berkelbach T C, et al. Observation of biexcitons in monolayer WSe2. Nat Phys, 2015, 11, 477 doi: 10.1038/nphys3324
[26]
van Tuan D, Scharf B, Wang Z F, et al. Probing many-body interactions in monolayer transition-metal dichalcogenides. Phys Rev B, 2019, 99, 085301 doi: 10.1103/PhysRevB.99.085301
[27]
Feierabend M, Brem S, Ekman A, et al. Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides. 2D Mater, 2021, 8, 015013 doi: 10.1088/2053-1583/abb876
[28]
Kusaba S, Watanabe K, Taniguchi T, et al. Role of dark exciton states in the relaxation dynamics of bright 1s excitons in monolayer WSe2. Appl Phys Lett, 2021, 119, 093101 doi: 10.1063/5.0064795
[29]
Zhang X X, Cao T, Lu Z, et al. Magnetic brightening and control of dark excitons in monolayer WSe2. Nat Nanotechnol, 2017, 12, 883 doi: 10.1038/nnano.2017.105
[30]
Wang G, Robert C, Glazov M M, et al. In-plane propagation of light in transition metal dichalcogenide monolayers: Optical selection rules. Phys Rev Lett, 2017, 119, 047401 doi: 10.1103/PhysRevLett.119.047401
[31]
Ye J L, Niu B H, Li Y, et al. Exciton valley dynamics in monolayer Mo1– xW xSe2 (x = 0, 0.5, 1). Appl Phys Lett, 2017, 111, 152106 doi: 10.1063/1.4995517
[32]
Yan T F, Ye J L, Qiao X F, et al. Exciton valley dynamics in monolayer WSe2 probed by the two-color ultrafast Kerr rotation. Phys Chem Chem Phys, 2017, 19, 3176 doi: 10.1039/C6CP07208A
[33]
Arora A, Koperski M, Nogajewski K, et al. Excitonic resonances in thin films of WSe2: From monolayer to bulk material. Nanoscale, 2015, 7, 10421 doi: 10.1039/C5NR01536G
[34]
Wang G, Chernikov A, Glazov M M, et al. Colloquium: Excitons in atomically thin transition metal dichalcogenides. Rev Mod Phys, 2018, 90, 021001 doi: 10.1103/RevModPhys.90.021001
[35]
Adachi S. Luminescence spectroscopy of Cr3+ in Al2O3 polymorphs. Opt Mater, 2021, 114, 111000 doi: 10.1016/j.optmat.2021.111000
[36]
Borghardt S, Tu J S, Winkler F, et al. Engineering of optical and electronic band gaps in transition metal dichalcogenide monolayers through external dielectric screening. Phys Rev Mater, 2017, 1, 054001 doi: 10.1103/PhysRevMaterials.1.054001
[37]
Cadiz F, Courtade E, Robert C, et al. Excitonic linewidth approaching the homogeneous limit in MoS2 based van der Waals heterostructures. Phys Rev X, 2017, 7, 021026 doi: 10.1103/physrevx.7.021026
[38]
Cho Y H, Gainer G H, Fischer A J, et al. S-shaped temperature-dependent emission shift and carrier dynamics in InGaN/GaN multiple quantum wells. Appl Phys Lett, 1998, 73, 1370 doi: 10.1063/1.122164
[39]
Yan T F, Qiao X F, Liu X N, et al. Photoluminescence properties and exciton dynamics in monolayer WSe2. Appl Phys Lett, 2014, 105, 101901 doi: 10.1063/1.4895471
[40]
Courtade E, Semina M, Manca M, et al. Charged excitons in monolayer WSe2: Experiment and theory. Phys Rev B, 2017, 96, 085302 doi: 10.1103/PhysRevB.96.085302
[41]
Förste J, Tepliakov N V, Kruchinin S Y, et al. Exciton g-factors in monolayer and bilayer WSe2 from experiment and theory. Nat Commun, 2020, 11, 4539 doi: 10.1038/s41467-020-18019-1
[42]
Robert C, Lagarde D, Cadiz F, et al. Exciton radiative lifetime in transition metal dichalcogenide monolayers. Phys Rev B, 2016, 93, 205423 doi: 10.1103/PhysRevB.93.205423
[43]
Ceballos F, Cui Q N, Bellus M Z, et al. Exciton formation in monolayer transition metal dichalcogenides. Nanoscale, 2016, 8, 11681 doi: 10.1039/C6NR02516A
[44]
Yang M, Robert C, Lu Z G, et al. Exciton valley depolarization in monolayer transition-metal dichalcogenides. Phys Rev B, 2020, 101, 115307 doi: 10.1103/PhysRevB.101.115307
[45]
Schmidt D, Godde T, Schmutzler J, et al. Exciton and trion dynamics in atomically thin MoSe2 and WSe2: Effect of localization. Phys Rev B, 2016, 94, 165301 doi: 10.1103/PhysRevB.94.165301
[46]
Yu T, Wu M W. Valley depolarization due to inter- and intra-valley electron-hole exchange interactions in monolayer MoS2. Phys Rev B, 2014, 89, 205303 doi: 10.1103/PhysRevB.89.205303
[47]
Plechinger G, Korn T, Lupton J M. Valley-polarized exciton dynamics in exfoliated monolayer WSe2. J Phys Chem C, 2017, 121, 6409 doi: 10.1021/acs.jpcc.7b01468
[48]
Yu H, Liu G B, Gong P, et al. Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides. Nat Commun, 2014, 5, 3876 doi: 10.1038/ncomms4876
[49]
Singh A, Moody G, Wu S F, et al. Coherent electronic coupling in atomically thin MoSe2. Phys Rev Lett, 2014, 112, 216804 doi: 10.1103/PhysRevLett.112.216804
[50]
Schmidt R, Berghäuser G, Schneider R, et al. Ultrafast coulomb-induced intervalley coupling in atomically thin WS2. Nano Lett, 2016, 16, 2945 doi: 10.1021/acs.nanolett.5b04733
[51]
Pogna E A A, Marsili M, de Fazio D, et al. Photo-induced bandgap renormalization governs the ultrafast response of single-layer MoS2. ACS Nano, 2016, 10, 1182 doi: 10.1021/acsnano.5b06488
[52]
Shinokita K, Wang X F, Miyauchi Y, et al. Ultrafast dynamics of bright and dark positive trions for valley polarization in monolayer WSe2. Phys Rev B, 2019, 99, 245307 doi: 10.1103/PhysRevB.99.245307
[53]
Feldmann J, Peter G, Göbel E O, et al. Linewidth dependence of radiative exciton lifetimes in quantum wells. Phys Rev Lett, 1988, 60, 243 doi: 10.1103/PhysRevLett.59.2337
[54]
Sanvitto D, Hogg R A, Shields A J, et al. Rapid radiative decay of charged excitons. Phys Rev B, 2000, 62, R13294 doi: 10.1103/PhysRevB.62.R13294
[55]
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    Received: 27 January 2022 Revised: 09 March 2022 Online: Uncorrected proof: 04 July 2022Published: 01 August 2022

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      Article Navigation >  Journal of Semiconductors  > 2022  >  43(8): 082001
      Shengmin Hu, Jialiang Ye, Ruiqi Liu, Xinhui Zhang. Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy[J]. Journal of Semiconductors, 2022, 43(8): 082001. doi: 10.1088/1674-4926/43/8/082001 ****S M Hu, J L Ye, R Q Liu, X H Zhang. Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy[J]. J. Semicond, 2022, 43(8): 082001. doi: 10.1088/1674-4926/43/8/082001
      Citation:
      Shengmin Hu, Jialiang Ye, Ruiqi Liu, Xinhui Zhang. Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy[J]. Journal of Semiconductors, 2022, 43(8): 082001. doi: 10.1088/1674-4926/43/8/082001 ****
      S M Hu, J L Ye, R Q Liu, X H Zhang. Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy[J]. J. Semicond, 2022, 43(8): 082001. doi: 10.1088/1674-4926/43/8/082001
      shu

      Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy

      DOI: 10.1088/1674-4926/43/8/082001
      • 1.

        State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

      • 2.

        College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China

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      • Shengmin Hu：received his B.Sc. from North China Electric Power University in 2019. He is now a graduate student at University of Chinese Academy of Sciences under the supervision of Prof. Xinhui Zhang. His research focuses on the ultrafast valley pseudospin dynamics study in 2D semiconductors
      • Xinhui Zhang：is a professor at the Institute of Semiconductors (IOS), Chinese Academy of Sciences (CAS). She received her B.Sc. and M.Sc. degrees from Shaanxi Normal University, in 1991 and 1994 respectively, and PhD degree from the Institute of Physics, CAS in 1997. From 1997 to 2005, she worked as postdoc and research associate at Lund University (Sweden); College of William and Mary (USA); The University of Oklahoma (USA); and University of Moncton (Canada), respectively. Since 2006, she has worked as a professor at IOS, CAS. Her current research interests focus on the ultrafast spin dynamics in low-dimensional semiconductors and ferromagnet/semiconductor heterostructures
      • Corresponding author: xinhuiz@semi.ac.cn
      • Received Date: 2022-01-27
      • Revised Date: 2022-03-09
        • Available Online: 2022-07-04

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