Monolayer transition-metal dichalcogenides (TMDCs) such as MoS2, WS2, WSe2 and MoSe2 are atomically thin, direct-gap semiconductors whose room-temperature photoluminescence quantum yield (QY) is normally far below unity. Lien et al. (Science 364, 468, 2019) showed that the QY of as-processed, chemically untreated MoS2 and WS2 monolayers reaches near unity (75% +/- 10% and ~90% respectively) once the samples are made electrostatically intrinsic by gate bias, without any defect passivation, and that neutral exciton recombination is essentially entirely radiative despite native charged-defect densities of 10^10-10^13 cm^-2. Three mechanistic questions that the authors themselves left unresolved remain open and jointly define the missing microscopic understanding: (i) why tightly bound, charge-neutral excitons are largely insensitive to native charged defects, and whether this is a general property of strongly excitonic systems (those in which the exciton binding energy greatly exceeds the thermal energy kT and dielectric screening is weak); (ii) which microscopic channel produces the very fast nonradiative decay of trions (charged excitons; fitted lifetime of order 50 ps) - defect-assisted Shockley-Read-Hall-like decay, a geminate Auger-like process involving the extra carrier, or a combination; and (iii) why selenide-based monolayers (WSe2, MoSe2) saturate at only ~8% QY when intrinsic - because of a near-degenerate indirect band gap, optically dark exciton states, or both. None of the three questions, as posed for as-processed material under the original conditions, has been definitively answered.

Background

Monolayers of layered transition-metal dichalcogenides (TMDCs, e.g. MoS2, WS2, WSe2, MoSe2) are two-dimensional direct-gap semiconductors in which an optically created electron and hole bind into an exciton with a binding energy of hundreds of meV, far larger than in conventional semiconductors, because Coulomb screening is weak in an atomically thin sheet. Because of strong multiparticle Coulomb interactions, a monolayer hosts a family of quasiparticles: the neutral exciton (X), the charged exciton or trion (X+ or X-, an exciton bound to an extra hole or electron), the biexciton (two bound excitons), and free carriers. The photoluminescence (PL) quantum yield (QY) - the fraction of absorbed photons re-emitted - is the key figure of merit for light emitters and is set by the competition between radiative and nonradiative recombination. In conventional bulk semiconductors, native point defects act as efficient nonradiative recombination centers via Shockley-Read-Hall (SRH) capture, which is why even small defect densities are detrimental; the standard 'ABC' phenomenology (SRH at low generation, bimolecular recombination at intermediate, Auger recombination (a three-particle process in which the recombination energy is transferred to a third carrier instead of being emitted as light) at high generation) predicts characteristic power laws of QY versus optical generation rate G. As-processed TMDC monolayers typically show QY below 1%, which was long attributed to high native defect densities; chemical treatments such as the organic superacid bis(trifluoromethane)sulfonimide (TFSI), a Lewis acid that withdraws electrons from the monolayer, were known to restore near-unity QY in MoS2 (M. Amani et al., Science 350, 1065-1068, 2015), although the mechanism was unclear. Lien, Uddin, Yeh, Amani, Kim, Ager, Yablonovitch and Javey (Science 364, 468-471, 2019) swept gate voltage Vg (which sets the background charge density and thus the Fermi level) together with G in monolayer devices encapsulated in PMMA (poly(methyl methacrylate), a transparent polymer film), and described the data with a kinetic model over excitons, trions, biexcitons and free carriers with a mass-action trion formation law. They found that QY approaches unity when the monolayer is driven intrinsic, that exciton recombination requires no nonradiative channel at all to fit the data (entirely radiative, with radiative lifetime ~8 ns in MoS2), that trions instead recombine almost entirely nonradiatively (extracted radiative lifetime ~110 ns versus nonradiative ~50 ps), that biexciton (exciton-exciton) annihilation dominates at high density, and that the TFSI effect is simply electrostatic electron counter-doping, not chemical defect passivation. Sulfide monolayers (MoS2, WS2), which are n-type from chalcogen vacancies, brighten under negative gate bias; the same group later showed that sub-percent mechanical strain additionally suppresses exciton-exciton annihilation through the van Hove singularity (a sharp peak in the joint optical density of states at the band edge), giving near-unity QY at all exciton densities (H. Kim et al., Science 373, 448-452, 2021, DOI 10.1126/science.abi9193). Two further pieces of vocabulary are needed. First, in W-based TMDCs the lowest-energy exciton can be optically 'dark' (spin- or momentum-forbidden, i.e. unable to recombine radiatively without a spin flip or a momentum-supplying phonon or defect), which can lower the observed QY. Second, some monolayers have an 'indirect' band gap (minimum energy separation between valence- and conduction-band extrema at different crystal momenta) lying close in energy to the direct gap at the K point (a corner of the hexagonal Brillouin zone where the direct gap sits), so carriers can relax into momentum-dark states. WSe2 and MoSe2 monolayers in the 2019 work remained at a maximum QY of only ~8% even when their Fermi levels sat near mid-gap, which the authors attributed to either (or both) of these two possibilities. The 2019 paper explicitly flags three unresolved microscopic issues, which form the present problem: why neutral excitons are insensitive to native charged defects; whether the fast trion nonradiative decay is defect-assisted (SRH-like) or a geminate Auger-like process in which the extra carrier supplies the third particle needed for energy and momentum conservation; and the origin of the sub-unity QY of selenide-based monolayers.

Problem Statement

Establish the microscopic mechanisms that underlie the electrostatic suppression of nonradiative recombination in monolayer TMDC semiconductors, by answering the three linked questions posed by the 2019 experiment, whose common objective is a complete, predictive microscopic picture of exciton, trion and dark-state recombination pathways in real (defect-containing) monolayers, of the kind needed to generalize electrostatic brightening beyond MoS2 and WS2: (1) Defect-exciton interaction: determine the microscopic reason that tightly bound, charge-neutral excitons recombine essentially entirely radiatively in monolayers with native charged-defect densities of 10^10-10^13 cm^-2, and establish or refute the conjecture that this insensitivity to charged defects is a general property of strongly excitonic systems (as in quantum dots) rather than specific to these samples and dielectric environments. (2) Trion nonradiative decay channel: identify the microscopic mechanism responsible for the very fast nonradiative recombination of trions (effective nonradiative lifetime of order 50 ps extracted in MoS2, versus a radiative lifetime of order 100 ns) - specifically, whether it is dominated by defect-assisted Shockley-Read-Hall-like decay, by a geminate Auger-like process in which the extra (third) carrier supplies the momentum and energy balance, by phonon-assisted variants of these, or by a combination, and how the balance depends on defect density, doping, dielectric environment and temperature. (3) Intrinsic quantum-yield ceiling of selenide-based monolayers: determine the origin of the far-below-unity QY (about 8% at best) of monolayer WSe2 and MoSe2 even when their Fermi levels are near mid-gap - whether it stems from an indirect band gap lying near the direct gap (momentum-dark transitions), from optically dark exciton states below the bright exciton, from defect- and environment-mediated nonradiative channels, or from a combination - and distinguish these possibilities experimentally or theoretically.

References

  1. Der-Hsien Lien, Shiekh Zia Uddin, Matthew Yeh, Matin Amani, Hyungjin Kim, Joel W. Ager III, Eli Yablonovitch, Ali Javey, 'Electrical suppression of all nonradiative recombination pathways in monolayer semiconductors', Science 364, 468-471 (2019), DOI 10.1126/science.aaw8053, https://doi.org/10.1126/science.aaw8053
  2. Bumho Kim, Yue Luo, Daniel Rhodes, Yusong Bai, Jue Wang, Song Liu, Abraham Jordan, Baili Huang, Zhaochen Li, Takashi Taniguchi, Kenji Watanabe, Jonathan Owen, Stefan Strauf, Katayun Barmak, Xiaoyang Zhu, James Hone, 'Free Trions with Near-Unity Quantum Yield in Monolayer MoSe2', ACS Nano 16, 140-147 (2022), DOI 10.1021/acsnano.1c04331, https://doi.org/10.1021/acsnano.1c04331
  3. Hyungjin Kim, Shiekh Zia Uddin, Naoki Higashitarumizu, Eran Rabani, Ali Javey, 'Inhibited nonradiative decay at all exciton densities in monolayer semiconductors', Science 373, 448-452 (2021), DOI 10.1126/science.abi9193, https://doi.org/10.1126/science.abi9193
  4. Raul Perea-Causin, Samuel Brem, Ole Schmidt, Ermin Malic, 'Trion Photoluminescence and Trion Stability in Atomically Thin Semiconductors', Physical Review Letters 132, 036903 (2024), DOI 10.1103/PhysRevLett.132.036903, https://doi.org/10.1103/PhysRevLett.132.036903
  5. Benedikt Scharf, Vasili Perebeinos, 'Phonon-assisted Auger decay of excitons in doped transition metal dichalcogenide monolayers', The Journal of Chemical Physics 161, 134709 (2024), DOI 10.1063/5.0230578, https://doi.org/10.1063/5.0230578
  6. Suman Chatterjee, Garima Gupta, Sarthak Das, Kenji Watanabe, Takashi Taniguchi, Kausik Majumdar, 'Trion-trion annihilation in monolayer WS2', Physical Review B 105, L121409 (2022), DOI 10.1103/PhysRevB.105.L121409, https://doi.org/10.1103/PhysRevB.105.L121409
  7. Erfu Liu, Jeremiah van Baren, Ching-Tarng Liang, Takashi Taniguchi, Kenji Watanabe, Nathaniel M. Gabor, Yia-Chung Chang, Chun Hung Lui, 'Multipath optical recombination of intervalley dark excitons and trions in monolayer WSe2', Physical Review Letters 124, 196802 (2020), DOI 10.1103/PhysRevLett.124.196802, https://doi.org/10.1103/PhysRevLett.124.196802
  8. Takashi Kuroda, Yusuke Hoshi, Satoru Masubuchi, Mitsuhiro Okada, Ryo Kitaura, Kenji Watanabe, Takashi Taniguchi, Tomoki Machida, 'Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-timescale pump-probe spectroscopy', Physical Review B 102, 195407 (2020), DOI 10.1103/PhysRevB.102.195407, https://doi.org/10.1103/PhysRevB.102.195407
  9. Raquel Esteban-Puyuelo, Biplab Sanyal, 'Role of defects in ultrafast charge recombination in monolayer MoS2', Physical Review B 103, 235433 (2021), DOI 10.1103/PhysRevB.103.235433, https://doi.org/10.1103/PhysRevB.103.235433
  10. Mengfan Zhou, Wenhui Wang, Junpeng Lu, Zhenhua Ni, 'How defects influence the photoluminescence of TMDCs', Nano Research 14, 29-39 (2021), DOI 10.1007/s12274-020-3037-9, https://doi.org/10.1007/s12274-020-3037-9
  11. Yongjun Lee, Johnathas D'arf Severo Forte, Andrey Chaves, Anshuman Kumar, Trang Thu Tran, Youngbum Kim, Shrawan Roy, Takashi Taniguchi, Kenji Watanabe, Alexey Chernikov, Joon I. Jang, Tony Low, Jeongyong Kim, 'Boosting quantum yields in two-dimensional semiconductors via proximal metal plates', Nature Communications 12, 7095 (2021), DOI 10.1038/s41467-021-27418-x, https://doi.org/10.1038/s41467-021-27418-x
  12. V. Jindal, K. Mourzidis, A. Balocchi, C. Robert, P. Li, D. Van Tuan, L. Lombez, D. Lagarde, P. Renucci, T. Taniguchi, K. Watanabe, H. Dery, X. Marie, 'Brightened emission of dark trions in transition-metal dichalcogenide monolayers', Physical Review B 111, 155409 (2025), DOI 10.1103/PhysRevB.111.155409, https://doi.org/10.1103/PhysRevB.111.155409
  13. Sebastien Roux, Tilly Guyot, Abraao Cefas Torres-Dias, Delphine Lagarde, Laurent Lombez, Dinh Van Tuan, Junghwan Kim, Kenji Watanabe, Xavier Marie, Takashi Taniguchi, Hanan Dery, Cedric Robert, 'High luminescence efficiency of multi-valley excitonic complexes in heavily doped WSe2 monolayer', arXiv:2511.04306 (2025), https://arxiv.org/abs/2511.04306
  14. Theresa Kuechle, Sebastian Klimmer, Margarita Lapteva, Tarlan Hamzayev, Antony George, Andrey Turchanin, Torsten Fritz, Carsten Ronning, Marco Gruenewald, Giancarlo Soavi, 'Tuning exciton recombination rates in doped transition metal dichalcogenides', Optical Materials: X 12, 100097 (2021), DOI 10.1016/j.omx.2021.100097, https://doi.org/10.1016/j.omx.2021.100097
  15. Matin Amani, Der-Hsien Lien, Daisuke Kiriya, Jun Xiao, Angelica Azcatl, Jiyoung Noh, Surabhi R. Madhvapathy, Rafik Addou, Santosh KC, Madan Dubey, Kyeongjae Cho, Robert M. Wallace, Si-Chen Lee, Jr-Hau He, Joel W. Ager, Xiang Zhang, Eli Yablonovitch, Ali Javey, 'Near-unity photoluminescence quantum yield in MoS2', Science 350, 1065-1068 (2015), DOI 10.1126/science.aad2114, https://doi.org/10.1126/science.aad2114