Electron–hole liquid in a van der Waals heterostructure photocell at room temperature

Abstract

In semiconductors, photo-excited charge carriers exist as a gas of electrons and holes, bound electron–hole pairs (excitons), biexcitons and trions1,2,3,4. At sufficiently high densities, the non-equilibrium system of electrons (e) and holes (h+) may merge into an electronic liquid droplet5,6,7,8,9,10. Here, we report on the electron–hole liquid in ultrathin MoTe2 photocells revealed through multi-parameter dynamic photoresponse microscopy (MPDPM). By combining rich visualization with comprehensive analysis of very large data sets acquired through MPDPM, we find that ultrafast laser excitation at a graphene–MoTe2–graphene interface leads to the abrupt formation of ring-like spatial patterns in the photocurrent response as a function of increasing optical power at T = 297 K. The sudden onset to these patterns, together with extreme sublinear power dependence and picosecond-scale photocurrent dynamics, provide strong evidence for the formation of a two-dimensional electron–hole liquid droplet. The electron–hole liquid, which features a macroscopic population of correlated electrons and holes, may offer a path to room-temperature optoelectronic devices that harness collective electronic phenomena.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Multi-parameter dynamic photoresponse microscopy (MPDPM) of ultrathin MoTe2 photocells.
Fig. 2: Critical onset of ring-like photoresponse revealed through MPDPM.
Fig. 3: Room-temperature 2D electron–hole liquid and comparison to MPDPM imaging in the MoTe2 photocells.
Fig. 4: Interlayer voltage dependence of the room-temperature electron–hole liquid.
Fig. 5: Dynamic photoresponse of the 2D electron–hole liquid.

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.

References

  1. 1.

    Lampert, M. Mobile and immobile effective-mass-particle complexes in nonmetallic solids. Phys. Rev. Lett. 1, 450–453 (1958).

    ADS  Article  Google Scholar 

  2. 2.

    Kheng, K., Cox, R. & D’Aubigné, M. Observation of negatively charged excitons X in semiconductor quantum wells. Phys. Rev. Lett. 71, 1752–1755 (1993).

    ADS  Article  Google Scholar 

  3. 3.

    Scholes, G. D. & Rumbles, G. Excitons in nanoscale systems. Nat. Mater. 5, 683–696 (2006).

    ADS  Article  Google Scholar 

  4. 4.

    Mak, K. F. et al. Tightly bound trions in monolayer MoS2. Nat. Mater. 12, 207–211 (2013).

    ADS  Article  Google Scholar 

  5. 5.

    Keldysh, L. V. Proceedings of the 9th International Conference on Physics of Semiconductors 1303 (Nauka, Leningrad, 1968).

  6. 6.

    Jeffries, C. D. Electron–hole condensation in semiconductors. Science 189, 955–964 (1975).

    ADS  Article  Google Scholar 

  7. 7.

    Keldysh, L. V. The electron–hole liquid in semiconductors. Contemp. Phys. 27, 395–428 (1986).

    ADS  Article  Google Scholar 

  8. 8.

    Kaindl, R. A., Carnahan, M. A., Hägele, D., Lövenich, R. & Chemla, D. S. Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas. Nature 423, 734–738 (2003).

    ADS  Article  Google Scholar 

  9. 9.

    Almand-Hunter, A. E. et al. Quantum droplets of electrons and holes. Nature 506, 471–475 (2014).

    ADS  Article  Google Scholar 

  10. 10.

    Rustagi, A. & Kemper, A. Theoretical phase diagram for the room-temperature electron–hole liquid in photoexcited quasi-two-dimensional monolayer MoS2. Nano. Lett. 18, 455–459 (2018).

    ADS  Article  Google Scholar 

  11. 11.

    Ugeda, M. M. et al. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nat. Mater. 13, 1091–1095 (2014).

    ADS  Article  Google Scholar 

  12. 12.

    Chernikov, A., Ruppert, C., Hill, H. M., Rigosi, A. F. & Heinz, T. F. Population inversion and giant bandgap renormalization in atomically thin WS2 layers. Nat. Photon. 9, 466–470 (2015).

    ADS  Article  Google Scholar 

  13. 13.

    Sun, D. et al. Observation of rapid exciton–exciton annihilation in monolayer molybdenum disulfide. Nano. Lett. 14, 5625–5629 (2014).

    ADS  Article  Google Scholar 

  14. 14.

    Froehlicher, G., Lorchat, E. & Berciaud, S. Direct versus indirect band gap emission and exciton–exciton annihilation in atomically thin molybdenum ditelluride MoTe2. Phys. Rev. B 94, 085429 (2016).

    ADS  Article  Google Scholar 

  15. 15.

    Zhang, K. et al. Ultrasensitive near-infrared photodetectors based on a graphene–MoTe2–graphene vertical van der Waals heterostructure. ACS Appl. Mater. Interfaces 9, 5392–5398 (2017).

    Article  Google Scholar 

  16. 16.

    Wang, F. et al. Strong electrically tunable MoTe2–graphene van der Waals heterostructures for high-performance electronic and optoelectronic devices. App. Phys. Lett. 109, 193111 (2016).

    ADS  Article  Google Scholar 

  17. 17.

    Kuiri, M. et al. Enhancing photoresponsivity using MoTe2–graphene vertical heterostructures. App. Phys. Lett. 108, 063506 (2016).

    ADS  Article  Google Scholar 

  18. 18.

    Octon, T. J., Nagareddy, V. K., Russo, S., Craciun, M. F. & Wright, C. D. Fast high-responsivity few-layer MoTe2 photodetectors. Adv. Opt. Mater. 4, 1750–1754 (2016).

    Article  Google Scholar 

  19. 19.

    Ruppert, C., Aslan, O. B. & Heinz, T. F. Optical properties and band gap of single and few-layer MoTe2 crystals. Nano. Lett. 14, 6231–6236 (2014).

    ADS  Article  Google Scholar 

  20. 20.

    Lezama, I. G. et al. Indirect-to-direct band gap crossover in few-layer MoTe2. Nano. Lett. 15, 2336–2342 (2015).

    ADS  Article  Google Scholar 

  21. 21.

    Kekelidzet, G. P. & Evans, B. L. The photovoltage in single crystals of α-MoTe2. Brit. J. Appl. Phys. 2, 855–861 (1969).

    Google Scholar 

  22. 22.

    Gabor, N. M., Zhong, Z., Bosnick, K. & McEuen, P. L. Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube p−i−n photodiodes. Phys. Rev. Lett. 108, 087404 (2012).

    ADS  Article  Google Scholar 

  23. 23.

    Wang, H., Zhang, C., Chan, W., Tiwari, S. & Rana, F. Ultrafast response of monolayer molybdenum disulfide photodetectors. Nat. Commun. 6, 8831 (2015).

    ADS  Article  Google Scholar 

  24. 24.

    Ma, Q. et al. Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure. Nat. Phys. 12, 455–459 (2016).

    Article  Google Scholar 

  25. 25.

    Vogt, K. T., Shi, S., Wang, F. & Graham, M. Isolating exciton extraction pathways with electric field-dependent ultrafast photocurrent microscopy. Conference on Lasers and Electro-Optics (Optical Society of America Technical Digest, Washington, 2016).

  26. 26.

    Massicotte, M. et al. Picosecond photoresponse in van der Waals heterostructures. Nat. Nanotechnol. 11, 42–46 (2016).

    ADS  Article  Google Scholar 

  27. 27.

    Massicotte, M. et al. Photo-thermionic effect in vertical graphene heterostructures. Nat. Commun. 7, 12174 (2016).

    ADS  Article  Google Scholar 

  28. 28.

    Sun, Y. et al. The Zeeman splitting of bulk 2H-MoTe2 single crystal in high magnetic field. Appl. Phys. Lett. 110, 102102 (2017).

    ADS  Article  Google Scholar 

  29. 29.

    Chernikov, A. et al. Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2. Phys. Rev. Lett. 113, 076802 (2014).

    ADS  Article  Google Scholar 

  30. 30.

    Arora, A. et al. Valley Zeeman splitting and valley polarization of neutral and charged excitons in monolayer MoTe2 at high magnetic fields. Nano. Lett. 16, 3624–3629 (2016).

    ADS  Article  Google Scholar 

  31. 31.

    Castellanos-Gomez, A. et al. Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Mater. Lett. 1, 1–8 (2014).

    ADS  Google Scholar 

  32. 32.

    Steinmeyer, G. A review of ultrafast optics and optoelectronics. J. Opt. A 5, R1–R15 (2003).

    ADS  Article  Google Scholar 

  33. 33.

    Fushitani, M. Applications of pump-probe spectroscopy. Annu. Rep. Prog. Chem. C 104, 272 (2008).

    Article  Google Scholar 

Download references

Acknowledgements

The authors would like to acknowledge valuable discussions with C. Varma. This work was supported by the Air Force Office of Scientific Research Young Investigator Program (YIP) award number FA9550-16-1-0216, as part of the SHINES Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under award number SC0012670, and through support from the National Science Foundation Division of Materials Research CAREER award number 1651247. D.P. and N.M.G received support from SHINES. N.M.G. acknowledges support through a Cottrell Scholar Award, and through the Canadian Institute for Advanced Research (CIFAR) Azrieli Global Scholar Award. T.B.A. acknowledges support from the Fellowships and Internships in Extremely Large Data Sets (FIELDS) Program, a NASA MUREP Institutional Research Opportunity (MIRO) Program, grant number NNX15AP99A. V.A. acknowledges support from the National Science Foundation Division of Materials Research award number 1506707.

Author information

Affiliations

Authors

Contributions

N.M.G. proposed and supervised the project. T.B.A. conducted photoresponse imaging experiments. D.P. synthesized the photocell devices. V.A. supported the direction and interpretation of the experiments through theoretical understanding of the data. All authors participated in analysing the data, interpreting the experimental results and preparing the manuscript.

Corresponding author

Correspondence to Nathaniel M. Gabor.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

41566_2019_349_MOESM2_ESM.mp4

Photocurrent images versus power in the long Δt limit.

41566_2019_349_MOESM3_ESM.mp4

Nonlinearity maps versus time delay.

41566_2019_349_MOESM4_ESM.mp4

Photocurrent maps versus power in the short Δt limit.

Supplementary Information

Supplementary Text, Supplementary Figures 1–14 and captions for Supplementary Videos 1–3.

Supplementary Video 1

Photocurrent images versus power in the long Δt limit.

Supplementary Video 2

Nonlinearity maps versus time delay.

Supplementary Video 3

Photocurrent maps versus power in the short Δt limit.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Arp, T.B., Pleskot, D., Aji, V. et al. Electron–hole liquid in a van der Waals heterostructure photocell at room temperature. Nat. Photonics 13, 245–250 (2019). https://doi.org/10.1038/s41566-019-0349-y

Download citation

Further reading