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.
Subscribe to Journal
Get full journal access for 1 year
We are sorry, but there is no personal subscription option available for your country.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
Lampert, M. Mobile and immobile effective-mass-particle complexes in nonmetallic solids. Phys. Rev. Lett. 1, 450–453 (1958).
Kheng, K., Cox, R. & D’Aubigné, M. Observation of negatively charged excitons X− in semiconductor quantum wells. Phys. Rev. Lett. 71, 1752–1755 (1993).
Scholes, G. D. & Rumbles, G. Excitons in nanoscale systems. Nat. Mater. 5, 683–696 (2006).
Mak, K. F. et al. Tightly bound trions in monolayer MoS2. Nat. Mater. 12, 207–211 (2013).
Keldysh, L. V. Proceedings of the 9th International Conference on Physics of Semiconductors 1303 (Nauka, Leningrad, 1968).
Jeffries, C. D. Electron–hole condensation in semiconductors. Science 189, 955–964 (1975).
Keldysh, L. V. The electron–hole liquid in semiconductors. Contemp. Phys. 27, 395–428 (1986).
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).
Almand-Hunter, A. E. et al. Quantum droplets of electrons and holes. Nature 506, 471–475 (2014).
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).
Ugeda, M. M. et al. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nat. Mater. 13, 1091–1095 (2014).
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).
Sun, D. et al. Observation of rapid exciton–exciton annihilation in monolayer molybdenum disulfide. Nano. Lett. 14, 5625–5629 (2014).
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).
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).
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).
Kuiri, M. et al. Enhancing photoresponsivity using MoTe2–graphene vertical heterostructures. App. Phys. Lett. 108, 063506 (2016).
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).
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).
Lezama, I. G. et al. Indirect-to-direct band gap crossover in few-layer MoTe2. Nano. Lett. 15, 2336–2342 (2015).
Kekelidzet, G. P. & Evans, B. L. The photovoltage in single crystals of α-MoTe2. Brit. J. Appl. Phys. 2, 855–861 (1969).
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).
Wang, H., Zhang, C., Chan, W., Tiwari, S. & Rana, F. Ultrafast response of monolayer molybdenum disulfide photodetectors. Nat. Commun. 6, 8831 (2015).
Ma, Q. et al. Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure. Nat. Phys. 12, 455–459 (2016).
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).
Massicotte, M. et al. Picosecond photoresponse in van der Waals heterostructures. Nat. Nanotechnol. 11, 42–46 (2016).
Massicotte, M. et al. Photo-thermionic effect in vertical graphene heterostructures. Nat. Commun. 7, 12174 (2016).
Sun, Y. et al. The Zeeman splitting of bulk 2H-MoTe2 single crystal in high magnetic field. Appl. Phys. Lett. 110, 102102 (2017).
Chernikov, A. et al. Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS2. Phys. Rev. Lett. 113, 076802 (2014).
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).
Castellanos-Gomez, A. et al. Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Mater. Lett. 1, 1–8 (2014).
Steinmeyer, G. A review of ultrafast optics and optoelectronics. J. Opt. A 5, R1–R15 (2003).
Fushitani, M. Applications of pump-probe spectroscopy. Annu. Rep. Prog. Chem. C 104, 272 (2008).
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.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary Text, Supplementary Figures 1–14 and captions for Supplementary Videos 1–3.
Photocurrent images versus power in the long Δt limit.
Nonlinearity maps versus time delay.
Photocurrent maps versus power in the short Δt limit.
About this article
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
Advanced Functional Materials (2020)
P-type β-Ga2O3 metal-semiconductor-metal solar-blind photodetectors with extremely high responsivity and gain-bandwidth product
Materials Today Physics (2020)
Physics Letters A (2020)
Advanced Optical Materials (2020)
Nano Letters (2020)