Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2

Abstract

In recent years, there has been ongoing effort in achieving efficient transport of excitons in monolayer transition metal dichalcogenides subjected to highly non-uniform strain. Here we investigate the transport of excitons and trions in monolayer semiconductor WS2 subjected to controlled non-uniform mechanical strain. An atomic force microscope (AFM)-based setup is applied to actively control and tune the strain profiles by indenting the monolayer with an AFM tip. Optical spectroscopy is used to reveal the dynamics of the excited carriers. The non-uniform strain configuration locally changes the valence and conduction bands of WS2, giving rise to effective forces attracting excitons and trions towards the point of maximum strain underneath the AFM tip. We observe large changes in the photoluminescence spectra of WS2 under strain, which we interpret using a drift–diffusion model. We show that the transport of neutral excitons, a process that was previously thought to be efficient in non-uniformly strained two-dimensional semiconductors and termed as funnelling, is negligible at room temperature, in contrast to previous observations. Conversely, we discover that redistribution of free carriers under non-uniform strain profiles leads to highly efficient conversion of excitons to trions. Conversion efficiency reaches up to about 100% even without electrical gating. Our results explain inconsistencies in previous experiments and pave the way towards new types of optoelectronic devices.

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Fig. 1: Experimental setup.
Fig. 2: Photoluminescence spectra of samples.
Fig. 3: Comparison between the experimentally measured photoluminescence spectra and the model predictions.
Fig. 4: Spatial dependency of properties of sample A.
Fig. 5: Spatial maps of the ratio between the total trion emission and the exciton emission for sample A.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Change history

  • 27 March 2020

    In the HTML version of this Article, Fig. 5 appeared with the caption for Fig. 4 and vice versa; this has now been amended. The PDF version is correct.

References

  1. 1.

    Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically thin MoS2: a new direct-gap semiconductor. Phys. Rev. Lett. 105, 136805 (2010).

    ADS  Article  Google Scholar 

  2. 2.

    Zhao, W. et al. Evolution of electronic structure in atomically thin sheets of WS2 and WSe2. ACS Nano 7, 791–797 (2013).

    Article  Google Scholar 

  3. 3.

    Xu, X., Yao, W., Xiao, D. & Heinz, T. F. Spin and pseudospins in layered transition metal dichalcogenides. Nat. Phys. 10, 343–350 (2014).

    Article  Google Scholar 

  4. 4.

    Xiao, D., Liu, G.-B., Feng, W., Xu, X. & Yao, W. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 108, 196802 (2012).

    ADS  Article  Google Scholar 

  5. 5.

    Mak, K. F., McGill, K. L., Park, J. & McEuen, P. L. Valleytronics. The valley Hall effect in MoS transistors. Science 344, 1489–1492 (2014).

    ADS  Article  Google Scholar 

  6. 6.

    Tonndorf, P. et al. Single-photon emission from localized excitons in an atomically thin semiconductor. Optica 2, 347–352 (2015).

    ADS  Article  Google Scholar 

  7. 7.

    Chakraborty, C., Kinnischtzke, L., Goodfellow, K. M., Beams, R. & Vamivakas, A. N. Voltage-controlled quantum light from an atomically thin semiconductor. Nat. Nanotechnol. 10, 507–511 (2015).

    ADS  Article  Google Scholar 

  8. 8.

    Koperski, M. et al. Single photon emitters in exfoliated WSe2 structures. Nat. Nanotechnol. 10, 503–506 (2015).

    ADS  Article  Google Scholar 

  9. 9.

    Srivastava, A. et al. Optically active quantum dots in monolayer WSe2. Nat. Nanotechnol. 10, 491–496 (2015).

    ADS  Article  Google Scholar 

  10. 10.

    He, Y.-M. et al. Single quantum emitters in monolayer semiconductors. Nat. Nanotechnol. 10, 497–502 (2015).

    ADS  Article  Google Scholar 

  11. 11.

    Roy, T. et al. Field-effect transistors built from all two-dimensional material components. ACS Nano 8, 6259–6264 (2014).

    Article  Google Scholar 

  12. 12.

    Lee, C.-H. et al. Atomically thin p–n junctions with van der Waals heterointerfaces. Nat. Nanotechnol. 9, 676–681 (2014).

    ADS  Article  Google Scholar 

  13. 13.

    Jariwala, D., Davoyan, A. R., Wong, J. & Atwater, H. A. Van der Waals materials for atomically-thin photovoltaics: promise and outlook. ACS Photon. 4, 2962–2970 (2017).

    Article  Google Scholar 

  14. 14.

    Akama, T. et al. Schottky solar cell using few-layered transition metal dichalcogenides toward large-scale fabrication of semitransparent and flexible power generator. Sci. Rep. 7, 11967 (2017).

    ADS  Article  Google Scholar 

  15. 15.

    Liu, K. et al. Elastic properties of chemical-vapor-deposited monolayer MoS2, WS2, and their bilayer heterostructures. Nano Lett. 14, 5097–5103 (2014).

    ADS  Article  Google Scholar 

  16. 16.

    Lee, C. et al. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321, 385–388 (2008).

    ADS  Article  Google Scholar 

  17. 17.

    Roldán, R., Castellanos-Gomez, A., Cappelluti, E. & Guinea, F. Strain engineering in semiconducting two-dimensional crystals. J. Phys. Condens. Matter 27, 313201 (2015).

    ADS  Article  Google Scholar 

  18. 18.

    Feng, J., Qian, X., Huang, C.-W. & Li, J. Strain-engineered artificial atom as a broad-spectrum solar energy funnel. Nat. Photon. 6, 866–872 (2012).

    ADS  Article  Google Scholar 

  19. 19.

    Conley, H. J. et al. Bandgap engineering of strained monolayer and bilayer MoS2. Nano Lett. 13, 3626–3630 (2013).

    ADS  Article  Google Scholar 

  20. 20.

    Niehues, I. et al. Strain control of exciton–phonon coupling in atomically thin semiconductors. Nano Lett. 18, 1751–1757 (2018).

    ADS  Article  Google Scholar 

  21. 21.

    Christiansen, D. et al. Phonon sidebands in monolayer transition metal dichalcogenides. Phys. Rev. Lett. 119, 187402 (2017).

    ADS  Article  Google Scholar 

  22. 22.

    Lloyd, D. et al. Band gap engineering with ultralarge biaxial strains in suspended monolayer MoS2. Nano Lett. 16, 5836–5841 (2016).

    ADS  Article  Google Scholar 

  23. 23.

    Lazić, S. et al. Scalable interconnections for remote indirect exciton systems based on acoustic transport. Phys. Rev. B 89, 085313 (2014).

    ADS  Article  Google Scholar 

  24. 24.

    Manasevit, H. M., Gergis, I. S. & Jones, A. B. Electron mobility enhancement in epitaxial multilayer Si-Si1−xGex alloy films on (100) Si. Appl. Phys. Lett. 41, 464–466 (1982).

    ADS  Article  Google Scholar 

  25. 25.

    People, R. et al. Modulation doping in GexSi1−x/Si strained layer heterostructures. Appl. Phys. Lett. 45, 1231–1233 (1984).

    ADS  Article  Google Scholar 

  26. 26.

    Castellanos-Gomez, A. et al. Local strain engineering in atomically thin MoS2. Nano Lett. 13, 5361–5366 (2013).

    ADS  Article  Google Scholar 

  27. 27.

    Tyurnina, A. V. et al. Strained bubbles in van der Waals heterostructures as local emitters of photoluminescence with adjustable wavelength. ACS Photon. 6, 516–524 (2019).

    Article  Google Scholar 

  28. 28.

    Castellanos-Gomez, A. et al. Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Mater. 1, 011002 (2014).

    Article  Google Scholar 

  29. 29.

    Dukic, M., Adams, J. D. & Fantner, G. E. Piezoresistive AFM cantilevers surpassing standard optical beam deflection in low noise topography imaging. Sci. Rep. 5, 16393 (2015).

    ADS  Article  Google Scholar 

  30. 30.

    Castellanos-Gomez, A. et al. Elastic properties of freely suspended MoS2 nanosheets. Adv. Mater. 24, 772–775 (2012).

    Article  Google Scholar 

  31. 31.

    Zhang, R., Koutsos, V. & Cheung, R. Elastic properties of suspended multilayer WSe2. Appl. Phys. Lett. 108, 042104 (2016).

    ADS  Article  Google Scholar 

  32. 32.

    Vella, D. & Davidovitch, B. Indentation metrology of clamped, ultra-thin elastic sheets. Soft Matter 13, 2264–2278 (2017).

    ADS  Article  Google Scholar 

  33. 33.

    Bertolazzi, S., Brivio, J. & Kis, A. Stretching and breaking of ultrathin MoS2. ACS Nano 5, 9703–9709 (2011).

    Article  Google Scholar 

  34. 34.

    Kulig, M. et al. Exciton diffusion and halo effects in monolayer semiconductors. Phys. Rev. Lett. 120, 207401 (2018).

    ADS  Article  Google Scholar 

  35. 35.

    Ovchinnikov, D., Allain, A., Huang, Y.-S., Dumcenco, D. & Kis, A. Electrical transport properties of single-layer WS2. ACS Nano 8, 8174–8181 (2014).

    Article  Google Scholar 

  36. 36.

    Siviniant, J., Scalbert, D., Kavokin, A. V., Coquillat, D. & Lascaray, J.-P. Chemical equilibrium between excitons, electrons, and negatively charged excitons in semiconductor quantum wells. Phys. Rev. B 59, 1602–1604 (1999).

    ADS  Article  Google Scholar 

  37. 37.

    Ross, J. S. et al. Electrical control of neutral and charged excitons in a monolayer semiconductor. Nat. Commun. 4, 1474 (2013).

    ADS  Article  Google Scholar 

  38. 38.

    Wang, Y. et al. Strain-induced direct-indirect bandgap transition and phonon modulation in monolayer WS2. Nano Res. 8, 2562–2572 (2015).

    Article  Google Scholar 

  39. 39.

    Plechinger, G. et al. Identification of excitons, trions and biexcitons in single-layer WS2. Phys. Status Solid. Rap. Res. Lett. 9, 457–461 (2015).

    ADS  Article  Google Scholar 

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Acknowledgements

We thank R. Netz for fruitful discussions. We also thank K. Höflich for technical support. This work was supported by the European Research Council Starting grant 639739, and DFG CRC/TRR 227.

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M.G.H. and K.I.B. planned and designed the experiment and wrote the paper. M.G.H. and J.N.K. performed the nanoindentation experiments. M.G.H., J.N.K. and M.Q. fabricated the samples. The experiments, data analysis and the theoretical model were done by M.G.H. K.G. contributed to the theoretical model.

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Correspondence to Kirill I. Bolotin.

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Supplementary Figs. 1–12 and discussion.

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Harats, M.G., Kirchhof, J.N., Qiao, M. et al. Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2. Nat. Photonics 14, 324–329 (2020). https://doi.org/10.1038/s41566-019-0581-5

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