Ultrafast Transient Terahertz Conductivity of Monolayer MoS<sub>2</sub> and WSe<sub>2</sub> Grown by Chemical Vapor Deposition

Docherty, Callum J.; Parkinson, Patrick; Joyce, Hannah J.; Chiu, Ming Hui; Chen, Chang-Hsiao; Lee, Ming-Yang; Li, Lain‐Jong; Herz, Laura M.; Johnston, Michael B.

doi:10.1021/nn5034746

Cited by 204 publications

(167 citation statements)

References 60 publications

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“…This indicates that photoexcited holes scatter from the Γ point to the K point within this timescale, agreeing well with our data. Finally, the negative signal observed on longer timescales for ω ≤  1.84 eV can be attributed to induced absorption from electrons that have relaxed into surface-related defect or trap states 16,23,33 , which should have an appreciable density based on the magnitude of the 1.57 eV peak in our PL data ( Fig. 1(e)) (further supported by the fluence-dependent data discussed below).…”

Section: Resultssupporting

confidence: 73%

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

et al. 2014

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We have performed ultrafast optical microscopy on single flakes of atomically thin CVD-grown molybdenum disulfide, using non-degenerate femtosecond pump-probe spectroscopy to excite and probe carriers above and below the indirect and direct band gaps. These measurements reveal the influence of layer thickness on carrier dynamics when probing near the band gap. Furthermore, fluencedependent measurements indicate that carrier relaxation is primarily influenced by surface-related defect and trap states after above-bandgap photoexcitation. The ability to probe femtosecond carrier dynamics in individual flakes can thus give much insight into light-matter interactions in these twodimensional nanosystems.There has been an explosion of recent interest in the quasi-two-dimensional (2D) transition metal dichalcogenides (TMDCs), which are layered materials with strong in-plane covalent bonding, leading to atomically thin layers within their structure 1 . Their unique 2D geometry allows for the design and fabrication of complicated structures at desirable positions within optoelectronic devices, much more efficiently than other low-dimensional nanomaterials 1,2 . In particular, molybdenum disulfide (MoS 2 ) has received much recent attention due to its excellent electronic 3 and optical properties 4,5 . Most notably, the indirect bandgap in the bulk (~1.2 eV) changes to a direct bandgap (~1.9 eV) as the number of atomic layers is reduced to one, causing MoS 2 monolayers to generate strong photoluminescence [4][5][6][7] . Therefore, unlike graphene, which does not have an intrinsic band gap 8 , TMDCs are especially promising for potential optoelectronic applications, such as photo-transistors 9 , photodetectors 10 , and electroluminescent devices 11 . Many of these applications will depend on a detailed knowledge of the optical properties and carrier relaxation dynamics, which can be elucidated by photoexciting carriers and probing their relaxation as a function of layer thickness with femtosecond time resolution.Here, we use ultrafast optical microscopy (UOM) 12,13 to measure ultrafast carrier dynamics in atomically thin single molybdenum disulfide flakes grown by chemical vapor deposition (CVD). By tuning the probe photon energy through the MoS 2 band gap (both indirect and direct), our UOM measurements show that conduction and valence band states are rapidly populated on a sub-picosecond (ps) time scale in a MoS 2 monolayer after photoexcitation at 3.1 eV, consistent with previous work [14][15][16][17][18] . Pump fluence-dependent measurements reveal that subsequent carrier relaxation in our samples is primarily due to surface-related defects and trap states, not the Auger processes observed in previous measurements on MoS 2 and other semiconductor nanosystems 12,[18][19][20][21][22][23] . We also observed an increase in the carrier relaxation time with an increase in the number of MoS 2 layers, likely due to the well known layer-dependent changes in the electronic structure 16,24 . Our UOM measurements of photon energy-a...

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Section: Resultssupporting

confidence: 73%

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

et al. 2014

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“…The presence of substrates may lower charge mobility and hence the diffusion coefficient [21]. The substrate may also lower the exciton binding energy [22,23], which could subsequently lead to a smaller R. Additionally, the substrate may facilitate defectassisted recombination that can compete with the EEA as a pathway for excitons to decay [11,24]. While the presence of defect-assisted recombination may not change the EEA rate, it could make the experimental observation of the EEA more difficult, particularly when the defect-assisted decay rate is comparable to or even faster than the EEA rate.…”

Section: Two-dimensional (2d) Transition Metal Dichalcogenide (Tmdc) mentioning

confidence: 99%

Fundamental limits of exciton-exciton annihilation for light emission in transition metal dichalcogenide monolayers

et al. 2016

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We quantitatively illustrate the fundamental limit that exciton-exciton annihilation (EEA) may impose to the light emission of monolayer transition metal dichalcogenide (TMDC) materials.The EEA in TMDC monolayers shows dependence on the interaction with substrates as its rate increases from 0.1 cm 2 /s (0.05 cm 2 /s) to 0.3 cm 2 /s (0.1 cm 2 /s) with the substrates removed for WS 2 (MoS 2 ) monolayers. It turns to be the major pathway of exciton decay and dominates the luminescence efficiency when the exciton density is beyond 10 10 cm -2 in suspended monolayers or 10 11 cm -2 in supported monolayers. This sets an upper limit on the density of injected charges in light emission devices for the realization of optimal luminescence efficiency. The strong EEA rate also dictates the pumping threshold for population inversion in the monolayers to be 12-18 MW/cm 2 (optically) or 2.5-4×10 5 A/cm 2 (electrically).

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“…Recently, a number of ultrafast experiments [17][18][19][20][21][22][23] and a theoretical study 24 have been done to investigate the exciton/carrier dynamics in two-dimensional TMDs, suggesting significant influences of defects on the exciton/carrier recombination dynamics. In these previous experiments, [17][18][19][20][21][22][23][24] exciton/carrier dynamics were detected by resonant probe or terahertz probe, and the interactions between exciton/carrier and defects were indirectly inferred from exciton/carrier population decay. One general observation of the previous studies is that the relaxation of photoexcited carriers/excitons shows a fast, few picoseconds decay followed by a slow several tens of picoseconds component.…”

Section: Introductionmentioning

confidence: 99%

“…17 Defect trapping excitons/carriers is another possible reason for the fast decay. 22,23 However, very little was known about the role of midgap defects serving as carrier traps in 2D TMDs. Even though defect trapping carriers has been found as the origin of the hysteresis phenomena in electrical I DS -V G measurement of TMD field effect transistors, 25 still, not any spectroscopic signal has been assigned as a direct defect trapping signature in timeresolved transient optical measurements.…”

Section: Introductionmentioning

confidence: 99%

Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2

et al. 2017

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In two dimensional (2D) transition metal dichalcogenides, defect-related processes can significantly affect carrier dynamics and transport properties. Using femtosecond degenerate pump-probe spectroscopy, exciton capture, and release by mid-gap defects have been observed in chemical vapor deposition (CVD) grown monolayer MoSe 2 . The observed defect state filling shows a clear saturation at high exciton densities, from which the defect density is estimated to be around 0.5 × 10 12 /cm 2 . The exciton capture time extracted from experimental data is around~1 ps, while the average fast and slow release times are 52 and 700 ps, respectively. The process of defect trapping excitons is found to exist uniquely in CVD grown samples, regardless of substrate and sample thickness. X-ray photoelectron spectroscopy measurements on CVD and exfoliated samples suggest that the oxygenassociated impurities could be responsible for the exciton trapping. Our results bring new insights to understand the role of defects in capturing and releasing excitons in 2D materials, and demonstrate an approach to estimate the defect density nondestructively, both of which will facilitate the design and application of optoelectronics devices based on CVD grown 2D transition metal dichalcogenides.

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Ultrafast Transient Terahertz Conductivity of Monolayer MoS₂ and WSe₂ Grown by Chemical Vapor Deposition

Cited by 204 publications

References 60 publications

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

Fundamental limits of exciton-exciton annihilation for light emission in transition metal dichalcogenide monolayers

Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2

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Ultrafast Transient Terahertz Conductivity of Monolayer MoS2 and WSe2 Grown by Chemical Vapor Deposition

Cited by 204 publications

References 60 publications

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

Fundamental limits of exciton-exciton annihilation for light emission in transition metal dichalcogenide monolayers

Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2

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Ultrafast Transient Terahertz Conductivity of Monolayer MoS₂ and WSe₂ Grown by Chemical Vapor Deposition