Probing excitonic dark states in single-layer tungsten disulphide

Ye, Ziliang; Cao, Ting; O’Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G.; Zhang, Xiang

doi:10.1038/nature13734

Cited by 918 publications

(1,025 citation statements)

References 41 publications

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“…Unlike traditional semiconductors, the Coulomb interaction in monolayer TMDs is unusually strong because screening is greatly suppressed and spatial overlap of the interaction is much larger. This enhances the stability of a variety of excitonic quasiparticles with extremely large binding energies, including excitons [9][10][11][12][13][14][15][16][17] , trions [18][19][20] , and exciton-trion complexes 21 .…”

Section: Introductionmentioning

confidence: 99%

Intervalley biexcitons and many-body effects in monolayerMoS2

et al. 2015

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Interactions between two excitons can result in the formation of bound quasiparticles, known as biexcitons. Their properties are determined by the constituent excitons, with orbital and spin states resembling those of atoms. Monolayer transition metal dichalcogenides (TMDs) present a unique system where excitons acquire a new degree of freedom, the valley pseudospin, from which a novel intervalley biexciton can be created. These biexcitons comprise two excitons from different valleys, which are distinct from biexcitons in conventional semiconductors and have no direct analogue in atomic and molecular systems. However, their valley properties are not accessible to traditional transport and optical measurements. Here, we report the observation of intervalley biexcitons in the monolayer TMD MoS2 using ultrafast pump-probe spectroscopy. By applying broadband probe pulses with different helicities, we identify two species of intervalley biexcitons with large binding energies of 60 meV and 40 meV. In addition, we also reveal effects beyond biexcitonic pairwise interactions in which the exciton energy redshifts at increasing exciton densities, indicating the presence of many-body interactions among them.

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Section: Introductionmentioning

confidence: 99%

Intervalley biexcitons and many-body effects in monolayerMoS2

et al. 2015

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“…The bright and dark exciton character stems, respectively, from dipole-allowed and dipoleforbidden transitions at K. Because exciton thermalization occurs on a faster (∼500 fs) 18,20 time scale than radiative recombination in monolayer TMDs, the radiative recombination of the bright A exciton is the main process contributing to PL. 12 In the W-based monolayers, we find a rich series of mostly dark excitons 38 at energies between the A and B peaks (A * excitons in Figure 2b) made up by holes from the VBM and electrons from the CBM and the band above the CBM. The A * excitons are not expected to contribute significantly to PL in the W-based monolayers given that they are mostly dark and their energy difference with the A exciton is much larger than k B T at room temperature.…”

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confidence: 97%

Exciton Radiative Lifetimes in Two-Dimensional Transition Metal Dichalcogenides

Palummo¹,

2015

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Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show firstprinciples calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX 2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS 2 and in two MX 2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS 2 . The MoS 2 /WS 2 and MoSe 2 /WSe 2 heterobilayers exhibit very long-lived (∼20−30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion. KEYWORDS: Monolayer materials, transition metal dichalcogenides, luminescence, radiative lifetime, excitons, optoelectronics T wo-dimensional (2D) transition metal dichalcogenides (TMDs) are promising materials for ultrathin electronic, optoelectronic, photocatalytic, and photovoltaic devices. 1−8 Out of approximately 40 existing TMDs, 9,10 some have received particular attention due to their semiconducting nature and tunable band gap. In particular, group 6 monolayer TMDs with chemical formula MX 2 (M = Mo, W and X = S, Se) are direct gap semiconductors with relatively intense photoluminescence (PL), while bilayers and thicker multilayers exhibit indirect gap and weaker PL. 1,10−17 The peculiar nature of the excited states has stimulated intense research efforts to investigate exciton dynamics and radiative/nonradiative lifetimes in TMDs. 13,18−24 Recent time-resolved experiments found a range of characteristic times for exciton dynamics in monolayer TMDs, including fast (1−10 ps) recombination attributed to exciton trapping at defects, and slower processes on a 0.1−1 ns time scale interpreted as radiative exciton recombination. 18,20−22 However, the attribution of the observed signals to radiatiave and nonradiative processes can be ambiguous in time-resolved spectroscopies since defects and impurities can modulate the excited state dynamics. In TMDs, the interpretation of time signals and comparison among different experiments is further complicated by the use of micrometer-size flakes where the edges can play a significant role in exciton recombination. This situation has stimulated an ongoing quest for the intrinsic time scale of exciton recombinatio...

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“…They correspond to the optical transitions from the two spin-split valence bands to the corresponding conduction bands 1,3,20 [ Fig. 1(a)], modified by the strong e-h interactions [41][42][43][44][45] . Near !…”

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Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

et al. 2017

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We study the electronic band structure in the K/K' valleys of the Brillouin zone of monolayer WSe 2 and MoSe 2 by optical reflection and photoluminescence spectroscopy on dual-gated field-effect devices. Our experiment reveals the distinct spin polarization in the conduction bands of these compounds by a systematic study of the doping dependence of the A and B excitonic resonances. Electrons in the highest-energy valence band and the lowest-energy conduction band have antiparallel spins in monolayer WSe 2 , and parallel spins in monolayer MoSe 2 . The spin splitting is determined to be hundreds of meV for the valence bands and tens of meV for the conduction bands, which are in good agreement with first principles calculations. These values also suggest that both n-and ptype WSe 2 and MoSe 2 can be relevant for spin-and valley-based applications.

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Probing excitonic dark states in single-layer tungsten disulphide

Cited by 918 publications

References 41 publications

Intervalley biexcitons and many-body effects in monolayerMoS2

Intervalley biexcitons and many-body effects in monolayerMoS2

Exciton Radiative Lifetimes in Two-Dimensional Transition Metal Dichalcogenides

Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

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