Controlling Lattice Defects and Inter-Exciton Interactions in Monolayer

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Controlling Lattice Defects and Inter-Exciton Interactions in Monolayer Transition Metal Dichalcogenides for Efficient Light Emission Yongjun Lee, and Jeongyong Kim ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00645 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Controlling Lattice Defects and Inter-Exciton Interactions in Monolayer Transition Metal Dichalcogenides for Efficient Light Emission Yongjun Lee and Jeongyong Kim* Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea AUTHOR INFORMATION Corresponding Author *J. Kim ([email protected])

ABSTRACT

Monolayer transition metal dichalcogenides (1L-TMDs) have attracted tremendous attention as two-dimensional (2D) light-emitting semiconductors because the optical properties of 1L-TMDs are primarily determined by excitonic transitions, providing an ideal platform for the study or practical uses of 2D confined exciton systems. While pristine 1LTMDs experience a low quantum yield (QY) due to the high density of lattice defects, significant advances have been made for increasing the QY based on various experimental approaches and theoretical understanding of modulating defect states of 1L-TMDs. Under the high exciton density condition, the strong inter-exciton interaction observed in 1L-TMDs was determined to be the primary limiting factor of QY. Here, we outline recent discoveries and efforts to overcome the lattice defects and exciton-exciton annihilation (EEA) with the aim of improving the efficiency of light emission of 1L-TMDs. Along with perspectives on future

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researches to realize the defect-free and highly luminescent TMDs, we propose a simple scheme of suppressing the EEA effect to maintain the QY of 1L-TMDs in high exciton densities.

KEYWORDS. TMD, quantum yield, photoluminescence, exciton diffusion, surface plasmon


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INTRODUCTION Transition metal dichalcogenide (TMD) materials with the compound formula MX2, where M = Mo, W, Ta, Re, etc., and X = S, Se, Te, are layered structures with van der Waals interlayer interaction. Monolayer (1L-) TMDs have been extensively studied as ultrathin twodimensional (2D) light-emitting semiconductors with a direct bandgap ranging from visible to infrared wavelengths.1-4 Because of their confined 2D structures and low dielectric screening, excitons in 1L-TMDs are highly stable with high binding energies of a few hundred meV.5-7 Furthermore, other exciton matters, such as trions (charged excitons formed with an extra hole or electron) or bi-excitons (excitons consisting of two electrons and two holes), are also routinely observed at room temperature with binding energies of 20–60 meV.8,9 These excitons and exciton matter in 1L-TMDs predominate optical transitions, such as absorption and photoluminescence (PL), 1L-TMDs being an ideal platform to study and exploit 2D confined excitons for nanophotonic and optoelectronic applications. Carrier density is a major factor that determines the PL spectral weights among this exciton matter and the overall PL intensity, both of which are conveniently controlled by chemical doping or backgate

bias.10-13

For

light-emitting

device

applications

using

1L-TMDs,

electroluminescence (EL) was first demonstrated using 1L-MoS2 (Fig. 1a).14 Improvement of the EL quantum efficiency (QE) was achieved using 1L-WSe23 or heterostructuring with graphene and hexagonal boron nitride (hBN) where a QE as high as ~10% was reported.15 However, one of the remaining obstacles to achieving high QE in PL or EL by 1L-TMD is a low quantum yield (QY). The low QY of 1L-TMDs originates primarily from the abundant defects in pristine 1L-TMDs, where chalcogen vacancies, which tend to quench PL by trapping charge carriers, are most frequently observed.16 Studies have reported a QY of less


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than 1% for pristine 1L-MoS21 and ~3% for 1L-WSe2.17 These values are significantly lower than those of conventional 3D semiconductors.18,19 Another important factor limiting the QY of 1L-TMDs is the exciton-exciton annihilation (EEA) process, an Auger process in which excitons decay non-radiatively. The strong Coulomb interaction between excitons inherent in 1L-TMDs causes a much stronger EEA effect than in 3D semiconductors, thereby significantly limiting the QY even in the nominal density of excitons.20 EEA is also expected to dictate the optical gain in a lasing operation.21 In view of these findings, improving the QY of 1L-TMDs by controlling the defects and EEA would be expected to greatly promote the practical uses of 1L-TMDs in light-emitting applications. Thus far, numerous studies have been conducted to improve the QY of 1L-TMDs. Various chemical treatments that modulate the charge density10,11 or passivate the lattice defects22-24 have been determined to be effective—among these the use of bis(trifluoromethane) sulfonimide (TFSI) is noted for the achievement of near unity QY for 1L-MoS2 or 1L-WS2.23 EEA rate constants have been reported for various 1L-TMDs using time-resolved optical spectroscopy measurements.20,21,25-29 In addition, for the same sample, these rate constants have been demonstrated to vary depending on the conditions of the dielectric environment and the formation of defects,21,28,29 suggesting that active control or suppression of EEA for improved light emission could be possible. In this perspective, we outline recent studies to enhance the light-emission of 1L-TMDs by modulating the defect states and charge densities. We then summarize several important results on EEA phenomena observed in 1L-TMDs and present the perspectives to control EEA to achieve high QY even at saturating exciton densities. CHARGE TRANSFER. The exciton formation and recombination of 1L-TMDs are highly modulated by the excess charge density. Chemical doping by molecular adsorption has


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been frequently used to effectively transfer the charges to or from 1L-TMDs. For example, PL intensity of n-type 1L-TMDs, such as MoS2, MoSe2, and WS2, was significantly enhanced by chemical p-doping,10-13 which was attributed to the reduced charge screening and Auger recombination. Incidental adsorption of atmospheric O2 or H2O molecules to these n-type TMDs, often accelerated by laser irradiation,29,30 also results in the large enhancement of PL due to a highly effective charge transfer (p-doping) on sulfur vacancies. Charge transfer to 1L-TMDs can also be facilitated by forming heterostructures using TMDs.31-38 Theoretical and experimental studies have revealed ultrafast interlayer charge transfer resulting in spatially separated interlayer excitons with an ultra-long lifetime at the TMD interface.31-34 Control of the charge transfer rate and formation of interlayer excitons has been achieved by hBN insertion layers (Fig. 1b and c).37,38 These TMD heterostructures have displayed promising performances of photonic detection, light harvesting, emission enhancement, and tunable energy.15,35,36 Surrente et al. reported that Se vacancies can be healed by S atoms in heterostructure of MoS2/MoSe2/MoS2.39 The presence of interlayer excitons has been suggested in bilayer MoS2 as well, the emission of which can be significant with an applied electric field.40 The charge transfer process for TMDs usually accompanies the modification of defect states; thus, the independent effect of charge transfer to light emission of TMDs has not been clearly identified. For example, the concurrent increase of trion emission and neutral exciton emission and overall PL enhancement have been observed in chemical n-doping (Fig. 1d) and laser-induced p-doping of 1L-MoS2.41,42 These results suggest that doping processes, especially molecular adsorption, could have incidentally caused the passivation of defects of 1L-TMDs enhancing the exciton emission.


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LATTICE DEFECTS. Monolayer TMDs are characterized by a high density of chalcogenide vacancies (Cv, ~1013/cm2)43-45; these lattice defects act as electron donors and induce localized states in the bandgap.43 Owing to the charged nature of Cv and the formation of a midgap state induced by Cv, the electrons or holes are captured by lattice defects, significantly limiting the PL efficiency of 1L-TMDs.16 Sulfur vacancies (Sv) are primarily observed in exfoliated MoS2 because of the lowest formation energy of Sv among the lattice defects in MoS2;45-47 however, MoS2 grown by chemical vapor deposition (CVD) that has lower QY than exfoliated MoS2 contains other types of lattice defects other than Sv as well, such as Mo vacancies, antisites, and voids (Fig. 2a).45,48,49 Therefore, reducing these defects can cause an increase in the QY of 1L-TMDs. In 3D semiconductors, thermal annealing has been the most common method for improving the crystallinity and the QY.50 Unfortunately, annealing has been determined to negatively affect 1L-TMDs by creating Svs.51 It has been reported that these defects can be “healed” by adsorbed molecules.52 For example, oxygen molecules in the atmosphere can be adsorbed at chalcogen vacancy sites and remove the midgap states by charge transfer.52-54 Repairing the defects by the substitution of atoms has also been reported, where the chemical treatment of damaged 1L-MoSe2 by hydrohalic acid (e.g., HBr) resulted in considerable enhancement of PL.22 Moreover, scanning transmission electron microscopy (STEM) imaging and density functional theory (DFT) calculations have been used to support the suggestion that Se vacancies were substituted by bromine ions and localized states were eliminated.22 Amani et al. have reported that the immersion of exfoliated 1L-MoS2 in TFSI solution resulted in a hundredfold enhancement of the PL and the measured QY achieved near unity.23 The fact that no changes were detected in the spectral shape of the PL spectra confirmed that p-doping did not occur and the PL enhancement was exclusively due to defect elimination (Fig. 2b). Likewise, this


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method was effective for exfoliated 1L-WS2; however, it was less effective in the case of CVD-grown samples and entirely ineffective for selenide 1L-TMDs.17 These results indicate that TFSI treatment is effective only for Sv and hardly the case for other types of lattice defects in 1L-TMDs. The mechanism of defect elimination by TFSI was attributed to the strong protonating nature of TFSI and the rearrangement of the S adatoms H-S into Sv, of which the mechanism was also used to explain the PL enhancement of 1L-MoS2 treated by poly(4-styrenesulfonate).55 In contrast, the recent result obtained by the correlated use of optical characterization, the direct imaging of the atomic arrangement by STEM, and the DFT calculation suggested that S vacancies of pristine 1L-MoS2 or WS2 are directly cured by S atoms dissociated from the TFSI molecule (Fig. 2c, d).24 This work explains the particular effectiveness of TFSI treatment on sulfide 1L-TMDs and experimentally proved the promising fact that 1L-TMDs with a perfect lattice and QY can be obtained. EXCITON-EXCITON ANNIHILATION. EEA entails a many-body interaction in which one exciton is destroyed in a collision of two excitons and its energy is transferred to another exciton, i.e., a non-radiative decay mechanism that occurs at high excitation density in semiconductors.56-59 In atomically thin-layered 2D systems, such as 1L- TMDs, EEA is a major factor responsible for determining the QY and the efficiency of light emitting devices at high excitation density. This is similar to the case of EL quenching observed in carbon nanotubes (CNT), which is attributable to the EEA process and free carrier generation.60-62 Experimental observation and quantitative analysis of EEA activity in TMDs were carried out by transient absorption or time-resolved photoluminescence spectroscopy. The exciton density range studied for the investigation of EEA phenomenon of TMDs was far less than so-called Mott density (~1014 cm-2,63), above which the contribution of electron-hole plasma is not negligible 64, and thus the rate equation of exciton decay that considers only the defect


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trap, radiative recombination and the EEA contribution,

dn n = − −  n2 dt  was normally used,26,29 where n,  and  represent the time-dependent exciton density, the exciton decay lifetime (by defect trap and radiative recombination) and the EEA rate constant. According to the rate equation above, the most prominent signature of EEA contribution was the gradual reduction of decay time with the increasing excitation intensity.25-29 We also note the literatures citing that a fraction of photoexcited carriers can exist as electron-hole plasma state even below Mott density65, especially with the above-band gap excitation66. In such case, these electron-hole plasma interaction may cause the similar reduction of decay time with increasing excitation intensity67. Recent studies on 1L-TMDs have led to the observation that the EEA process occurs above an excitation density of 1010 cm-2 with the estimated EEA rate constant in the range of 0.01–0.5 cm2/s in 1L-TMDs.17,20,21,23,25-28

The EEA rate constant () is known to be

proportional to the exciton diffusion length through the equation  = 4 DR , where D is the diffusion coefficient of excitons, and R is the material characteristic annihilation radius, which represents the separation of two excitons at which the annihilation is initiated.21,59,68,69 The diffusion of an exciton can increase the probability of encountering two excitons during their lifetimes; thus, resulting in an increased probability of EEA. Diffusion-induced EEA have been observed in low-dimensional semiconductors with high exciton binding energy.58,59,66 Mouri et al. observed strong nonlinear saturation behavior for PL with increasing excitation power density in 1L-WSe2, and exciton diffusion lengths of several micrometers (Fig. 3a).25 For 1L-WS2, EEA occurred above an exciton density of 109 cm-2,


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corresponding to an average inter-exciton distance of ~600 nm.26 This value is much larger than the spatial extension of excitons in 1L-WS2 (~1 nm71). An exciton diffusion coefficient of about 2 cm2/s has been reported, which is one order of magnitude higher than the EEA rate (~0.41 cm2/s26) in ME 1L-WS2,72 strongly suggesting that exciton diffusion plays a major role in EEA.25,26 Lower values of the EEA rate in CVD-grown 1L-TMDs than those of ME 1LTMDs have been reported,20,21,25,73 consistent with the fact that CVD-grown TMDs usually possess a higher defect density value and the exciton diffusion must be less efficient than ME TMDs.72,74 Yu et al. determined that different substrates or dielectric environments of 1L-WS2 resulted in different values of the EEA rate constants due to the different diffusion coefficients and the binding energies (Fig. 3b).21 Hoshi et al. have reported that 1L-WS2 encapsulated by hBN caused the EEA rate to be greatly reduced,28 and the mechanism of suppressed EEA was attributed to the delocalization of excitons because of a decrease in interface roughness resulting from hBN encapsulation.

PRECEDING EEA. Chemical treatment by TFSI was determined to increase the QY of 1L-MoS2 and 1L-WS2 to nearly 100%; however, the 100% QY decreases rapidly due to the onset of EEA as the pumping power increases above 1 W/cm2 for 1L-WS2 or 0.01 W/cm2 for 1L-MoS2 optical pumping (Fig. 3c).17 The QY of TFSI-treated 1L-MoS2 decreases to below 1% at 100 W/cm2 pumping power, indicating that repairing the lattice defects alone cannot ensure highly luminescent 1L-TMDs at all exciton densities. EEA is also expected to be the limiting factor under lasing conditions for which Yu et al. estimated that a carrier injection density of 12–18 MW/cm2 is required for the population inversion21; at this level of the


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carrier density, EEA predominates the exciton decay process.62 Recently, Lee et al. reported that the EEA rate can be reduced and the QY can be significantly increased at high exciton density by simple laser irradiation of 1L-WS2 (Fig. 3d).29 Laser-induced formation of Svs are believed to hinder the exciton diffusion resulting in the reduction of EEA. This method was determined to be effective for the TFSI-treated sample with 57% QY; thus, demonstrating that defect repair combined with EEA suppression can lead to high-intensity light-emitting TMDs even at high exciton density.29 Preventing the QY from being depressed by EEA can also be, in principle, achieved by increasing the spontaneous emission (SE) rate of 1L-TMDs beyond the time scale of the EEA process. This approach can be viewed as accelerating the radiative recombination process of excitons before they are consumed by the EEA process, as schematically shown in Fig. 4a. The Purcell effect describes that the SE rate can be enhanced by increasing the quality factor of the cavity (Q) or decreasing the mode volume (V) according to the following equation:

(

Fp = 3

4 2

)( ) Q

3

 0  V  n  (Fp = Purcell factor,

 = wavelength of the light, n = refractive index

of medium).75 Increasing the Purcell factor can be realized using the optical cavity or plasmon coupling73-80, and a factor exceeding ~70 was realized recently.77 Considering that the timescale of EEA was measured to be ~100 ps in 1L-WS221,

28

and the radiative

recombination time in 1L-WS2 was determined to be in the range of a few nanoseconds,17, 26, 81

enhancing the SE rate by a few tens of times can cause dramatic results in terms of

suppressing the EEA effect and maintaining a high QY over an extended range of optical pumping fluences. Fig. 4b displays the calculated QY of 1L-WS2 vs. the pump fluences with various values of radiative recombination time (r) in the range of 5 ns to 100 ps, using the known rate equation.21 Here, we consider the previously known results, namely that TFSItreated 1L-WS2 with 100% QY exhibited r of 3.4 ns at low excitation power in the absence


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of the EEA effect and the EEA rate constant was measured to be 2.8 cm2/s.17 In our calculation, the effect of the enhanced SE rate on the QY in high exciton density is significant; for example, at a pump fluence of 100 W/cm2, the QY of 1L-WS2 with r = 3.4 ns decreases from an initial value of 100% to 5%. However, if the SE rate increases by 34 times to become r = 0.1 ns, the QY could be maintained at 76% indicating that QY depression due to EEA is effectively prevented by enhancing the SE rate of high QY 1L-TMDs. Here the adverse effect of an increase in Fp on the QY, such as exciton quenching due to metal proximity in a plasmon coupling configuration,79 is not considered. Thus, the configuration of plasmon coupling or the photonic cavity should be optimally designed for the practical application of this scheme. In this perspective, we overviewed the major studies on treating carrier densities and lattice defects of 1L-TMDs which caused a significant increase in QY. With the established chemical treatment to heal the defects with its mechanism identified now, ideal QY of vacancy-free lattice TMDs seems plausible. We also surveyed the recent observations of EEA occurring in TMDs and its strong influence on the light emission of TMDs. Recent studies on variation or active modulation of EEA characteristics of 1L-TMDs suggest the exciting possibility of controlling EEA to maintain QY of 1L-TMDs even with a high carrier density. As a specific experimental approach, we suggested a simple scheme of suppressing EEA by enhancing the spontaneous emission rate of 1L-TMDs.

ACKNOWLEDGMENT This work was supported by IBS-R011-D1. JK acknowledges the support of the National Research Foundation of Korea grant funded by the Korean government (NRF-


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Figure 1. (a) (top) Image of EL emission. The positions of Cr/Au contacts are highlighted by thick dashed lines (white), and the 1L-MoS2 layer is indicated by thin dashed lines (gray). (bottom) PL and EL spectra peaks of 1L-MoS2 had the same photon energies, indicating that EL corresponds to exciton emission. Adapted with permission from ref. 14. Copyright 2013 American Chemical Society. (b) Illustration of heterostructure of 1L-WSe2/1L-MoS2 with few-layer hBN as separator layers. (top left) Band diagram of 1L-WSe2/1L-MoS2 heterostructure with available optical transitions. (top right) (1) intralayer exciton transitions in 1L-WSe2 and 1L-MoS2 (2) charge transfer occurring at the interface of 1L-WSe2 and 1L-


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MoS2. (3) emission transition by spatially indirect excitons. Normalized PL spectra (solid line) and absorption (dashed line) spectra at 1L-WSe2, 1L-MoS2 and heterostructure regions (bottom). Adapted with permission from ref. 38. Copyright 2014, National Academy of Sciences of the United States of America. (c) PL intensity map image (left) of the MoSe2/MoS2 hetero-bilayer (HB) and MoSe2/hBN/MoS2 hetero-multilayer (HM). The insets show the schematics of the heterostructure where one or two hBN layers were inserted between 1L-MoSe2 and 1L-MoS2. The scale bars are 5 m. PL and absorption spectra at each position are displayed on the right. Adapted with permission from ref. 37. Copyright 2016 American Chemical Society. (d) PL intensity (top left) and A exciton peak position (bottom left) map images. PL spectra before and after 16 hour methanol treated 1L-MoS2. (top right). The scale bars indicate 10 m. Plot of PL intensity and A exciton peak position as a function of methanol treatment times (bottom right). Adapted with permission from ref. 41. Copyright 2017 American Chemical Society.


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Figure 2. (a) Histograms of various point defects in physical vapor deposition (PVD), chemical vapor deposition (CVD) and mechanical exfoliation (ME) monolayers. Adapted with permission

from ref. 45. Copyright 2015 Nature Publishing Group (b) PL spectra of as-exfoliated and TFSI-treated 1L-MoS2, which show the 190-fold enhancement of the PL intensity as a result of the TFSI treatment. The inset shows normalized PL spectra and the chemical structure of TFSI. Adapted with permission from ref. 23. Copyright 2015 AAAS. (c) STEM images of CVD grown 1L-MoS2 (left) and TFSI treated 1L-MoS2 (right) and their PL spectra (middle). DFT calculation result of energy landscape of S vacancy healing process by TFSI molecules (bottom). Adapted with permission from ref. 24. Copyright 2018 American Chemical Society.


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0.3 0.6 W/m W/m22

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Figure 3. EEA observed in 1L-TMDs. (a) Schematic of the diffusion-assisted EEA process in TMDs and PL time-decay curves of 1L-WSe2 with varying excitation intensity displaying decreasing half-decay time with increasing excitation power. Adapted with permission from ref. 25. Copyright 2014 American Physical Society. (b) PL efficiency of suspended or substrate-supported 1L-WS2 and 1L-MoS2 as a function of incident power density. EEA rate constants for suspended samples estimated to be greater than for supported samples. Adapted with permission from ref. 21. Copyright 2016 American Physical Society. (c) Quantum yield (QY) of various types of 1L-TMDs as a function of incident power. QYs of chemically treated 1L-TMDs which initially displayed near 100% QY, steeply decreased with increasing incident power with the EEA in effect. Adapted with permission from ref. 17. Copyright 2016 American Chemical Society. (d) PL images and QY of 1L-WS2 with varying excitation


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intensity. PL intensity at laser irradiated region (white dotted circle in PL image and solid lines in QY plot) is higher than non-irradiated region at high excitation intensity. Adapted with permission from ref. 29. Copyright 2018 American Chemical Society.


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Incident power (W/cm2)

Figure 4. Exciton recombination preceding EEA for efficient exciton emission. (a) Schematic of enhancing the spontaneous rate above EEA time scale by surface plasmon coupling. (b) QY of perfect 1L-TMDs (assumed to be 100% in QY when free from EEA) as a function of the excitation power calculated using the rate equation adapted from ref. 21 2 ( QY =  (1/  r ) + 4 I0 − (1/  r ) 2 I0  r , where  and I0 are the absorption and the incident





power density) with different radiative recombination time (r). Values of EEA rate constants and absorption of 1L-WS2 in ref. 17 were used in the calculation.


ACS Photonics

For Table of Contents Use Only Manuscript title: Controlling Lattice Defects and Inter-Exciton Interactions in Monolayer Transition Metal Dichalcogenides for Efficient Light Emission

Authors: Yongjun Lee and Jeongyong Kim

Figure for TOC

1

Quantum Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 271 28 29 30 31 0.1 32 33 34 35 36 0.01 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

rad < EEA rad > EEA

Diffusion

Purcell effect

0 Exciton density

Brief synopsis of the TOC graphic: Lattice defects and exciton-exciton annihilation (EEA) are the major factors limiting the quantum yield (QY) of monolayer transition metal dichalcogenides (1L-TMDs). We outline recent discoveries on and efforts to overcome the lattice defects and EEA to improve the efficiency of light emission of 1L-TMDs. We propose a simple scheme of suppressing the EEA effect to maintain the QY of 1L-TMDs in high exciton densities to realize defect-free and highly luminescent TMDs.


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Controlling Lattice Defects and Inter-Exciton Interactions in Monolayer

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