Dissociation of two-dimensional excitons in monolayer WSe2

Massicotte, Mathieu; Vialla, Fabien; Schmidt, Peter; Lundeberg, Mark B.; Latini, Simone; Haastrup, Sten; Danovich, Mark; Davydovskaya, Diana; Watanabe, Kenji; Falko, Vladimir; Thygesen, Kristian Sommer; Pedersen, Thomas Garm; Koppens, Frank H. L.

doi:10.1038/s41467-018-03864-y

Cited by 137 publications

(199 citation statements)

References 49 publications

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“…That work also showed that the photocurrent generated in fields weaker than 15 V/µm was accurately predicted by the Mott-Wannier model [16,17]. Nevertheless, these weakfield dissociation rates proved troublesome to obtain numerically [15], and they were therefore extrapolated by fitting to the rate of a two-dimensional hydrogen atom [18]. In the present paper, we introduce a numerical method capable of computing exciton dissociation rates for significantly weaker fields with no compromise on the accuracy for stronger fields.…”

Section: Introductionmentioning

confidence: 93%

Field-induced dissociation of two-dimensional excitons in transition metal dichalcogenides

Kamban

Pedersen

2019

Phys. Rev. B

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Generation of photocurrents in semiconducting materials requires dissociation of excitons into free charge carriers. While thermal agitation is sufficient to induce dissociation in most bulk materials, an additional push is required to induce efficient dissociation of the strongly bound excitons in monolayer transition-metal dichalcogenides (TMDs). Recently, static in-plane electric fields have proven to be a promising candidate. In the present paper, we introduce a numerical procedure, based on exterior complex scaling, capable of computing field-induced exciton dissociation rates for a wider range of field strengths than previously reported in literature. We present both Stark shifts and dissociation rates for excitons in various TMDs calculated within the Mott-Wannier model. Here, we find that the field induced dissociation rate is strongly dependent on the dielectric screening environment. Furthermore, applying weak-field asymptotic theory (WFAT) to the Keldysh potential, we are able to derive an analytical expression for exciton dissociation rates in the weakfield region. arXiv:1905.02571v1 [cond-mat.mes-hall]

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Section: Introductionmentioning

confidence: 93%

Field-induced dissociation of two-dimensional excitons in transition metal dichalcogenides

Kamban

Pedersen

2019

Phys. Rev. B

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“…When the exciton is subjected to an electrostatic field, it may be dissociated. This is realized in the Wannier model by the energy eigenvalue obtaining a non-vanishing imaginary part [27][28][29]68 . The field induced dissociation rate is then given by Γ = −2 Im E, where the imaginary part can be obtained efficiently by utilizing the ECS technique [51][52][53][54] .…”

Section: Field Induced Dissociationmentioning

confidence: 99%

“…Without inducing dissociation in some way, excitons will typically recombine before they are dissociated. Applying an in-plane electric field to the excitons, however, enhances generation of photocurrents for two reasons: (i) the electric field counteracts recombination by pulling electrons and holes in opposite directions, and (ii) the electric field assists dissociation of excitons [27][28][29][30] .When two TMD monolayers are brought together with a type-II band alignment, the conduction band minimum and the valence band maximum reside in two different layers. Electrons and holes in the structure will therefore prefer to reside in separate layers, provided that the loss in exciton binding energy is smaller than the energy gained by band offsets.…”

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confidence: 99%

Interlayer excitons in van der Waals heterostructures: Binding energy, Stark shift, and field-induced dissociation

Kamban

Pedersen

2020

Sci Rep

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Photoexcited intralayer excitons in van der Waals heterostructures (vdWHs) with type-II band alignment have been observed to tunnel into interlayer excitons on ultrafast timescales. Such interlayer excitons have sufficiently long lifetimes that inducing dissociation with external in-plane electric fields becomes an attractive option of improving efficiency of photocurrent devices. In the present paper, we calculate interlayer exciton binding energies, Stark shifts, and dissociation rates for six different transition metal dichalcogenide (TMD) vdWHs using a numerical procedure based on exterior complex scaling (ECS). We utilize an analytical bilayer Keldysh potential describing the interaction between the electron-hole pair, and validate its accuracy by comparing to the full multilayer Poisson equation. Based on this model, we obtain an analytical weak-field expression for the exciton dissociation rate. The heterostructures analysed are MoS 2 /MoSe 2 , MoS 2 /WS 2 , MoS 2 /WSe 2 , MoSe 2 /WSe 2 , WS 2 /MoSe 2 , and WS 2 /WSe 2 in various dielectric environments. For weak electric fields, we find that WS 2 /WSe 2 supports the fastest dissociation rates among the six structures. We, furthermore, observe that exciton dissociation rates in vdWHs are significantly larger than in their monolayer counterparts.Naturally occurring layered materials held together by van-der-Waals-like forces have been intensely studied in recent years. Peeling graphite layer-by-layer and forming graphene 1 , a task thought impossible, turned researchers on to two-dimensional materials. Realizing that these layers may be used as building blocks of artificially layered materials, so-called van der Waals heterostructures, provides an endless array of possible combinations 2, 3 . The extraordinary electronic and optical properties of TMDs 4-11 have made them one of the most interesting classes of building blocks for vdWHs. Potential electro-optical applications of TMDs include photodetectors 12-14 , light-emitting diodes 15-17 , and solar cells 16,18,19 . It is well known that the optical properties of TMD monolayers are dominated by excitons 2,3,6,8,20 , as such two-dimensional excitons can have giant binding energies 6,8,[21][22][23] . Strongly bound excitons, in turn, make generation of photocurrents difficult, as excitons must first be dissociated into free electrons and holes. This is one of the challenges facing the use of monolayer TMDs in efficient photocurrent devices. A further complication is the fast recombination rates of excitons in these monolayers [24][25][26] . Without inducing dissociation in some way, excitons will typically recombine before they are dissociated. Applying an in-plane electric field to the excitons, however, enhances generation of photocurrents for two reasons: (i) the electric field counteracts recombination by pulling electrons and holes in opposite directions, and (ii) the electric field assists dissociation of excitons [27][28][29][30] .When two TMD monolayers are brought together with a type-II band align...

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“…2 To date, the properties of free neutral excitons and exciton complexes such as charged excitons (trions) have been studied and largely understood 3,4 . Binding energies [5][6][7] , formation and dissociation mechanisms 8,9 , coherence effects 10 , and spin-valley effects [11][12][13][14][15][16][17] of these excitons have been identified. In addition to neutral and charged excitonic peaks, a feature that is often attributed to localized, rather than free excitons appears in photoluminescence (PL) spectra of many TMDC devices at low temperatures [18][19][20][21][22][23][24][25][26] .…”

Section: Introductionmentioning

confidence: 99%

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs

et al. 2020

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We investigate an excitonic peak appearing in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs), which is commonly associated with defects and disorder. First, to uncover the intrinsic origin of defect-related excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer, MoS2. We show that the entire range of behaviors of defect-related excitons can be understood in terms of a simple model, where neutral excitons are bound to ionized donor levels, likely related to sulphur vacancies, with a density of 7×10 11 cm -2 . Second, to study the extrinsic origin of defect-related excitons, we controllably deposit oxygen molecules in-situ onto the surface of MoS2 kept at cryogenic temperature. We find that in addition to trivial pdoping of 3×10 12 cm -2 , oxygen affects the formation of defect-related excitons by functionalizing the vacancy. Combined, our results uncover the origin of defect-related excitons, suggest a simple and conclusive approach to track the functionalization of TMDCs, benchmark device quality, and pave the way towards exciton engineering in hybrid organicinorganic TMDC devices.

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Dissociation of two-dimensional excitons in monolayer WSe2

Cited by 137 publications

References 49 publications

Field-induced dissociation of two-dimensional excitons in transition metal dichalcogenides

Field-induced dissociation of two-dimensional excitons in transition metal dichalcogenides

Interlayer excitons in van der Waals heterostructures: Binding energy, Stark shift, and field-induced dissociation

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs

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