Impeding Exciton-Exciton Annihilation in Monolayer WS2 by Laser

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Impeding Exciton-Exciton Annihilation in Monolayer WS by Laser Irradiation Yongjun Lee, Ganesh Ghimire, Shrawan Roy, Youngbum Kim, Changwon Seo, Ajay K. Sood, Joon I. Jang, and Jeongyong Kim ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00249 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Impeding Exciton-Exciton Annihilation in Monolayer WS2 by Laser Irradiation Yongjun Lee,1,2 Ganesh Ghimire, 1,2 Shrawan Roy,1,2 Youngbum Kim, 1,2 Changwon Seo, 1,2 Ajay K. Sood,3 Joon I. Jang,4 and Jeongyong Kim1,2 1

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of

Korea 2

Center for Integrated Nanostructure Physics, Institute for Basic Science, Suwon 16419,

Republic of Korea 3

Department of Physics, Indian Institute of Science, Bangalore-560012, India

4

Department of Physics, Sogang University, Seoul 04107, Republic of Korea

AUTHOR INFORMATION Corresponding Author *J. Kim ([email protected])

KEYWORDS. Transition metal dichalcogenides, EEA, photoluminescence, TRPL, sulfur vacancy

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ABSTRACT

Monolayer (1L) transition metal dichalcogenides (TMDs) are two-dimensional directbandgap semiconductors with promising applications of quantum light emitters. Recent studies have shown that intrinsically low quantum yields (QY) of 1L-TMDs can be greatly improved by chemical treatments. However, nonradiative exciton-exciton annihilation (EEA) appears to significantly limit light emission of 1L-TMDs at a nominal density of photoexcited excitons due to strong Coulomb interaction. Here we show that the EEA rate constant (γ) can be reduced by laser irradiation treatment in mechanically exfoliated monolayer tungsten disulfide (1L-WS2), causing significantly improved light emission at the saturating optical pumping level. Time-resolved photoluminescence (PL) measurements showed that γ reduced from 0.66±0.15 cm2/s to 0.20±0.05 cm2/s simply using our laser irradiation. The laserirradiated region exhibited lower PL response at low excitation levels, however at the high excitation level displayed 3 times higher PL intensity and QY than the region without laser treatment. The shorter PL lifetime and lower PL response at low excitation levels suggested that laser irradiation increased the density of sulfur vacancies of 1L-WS2, but we attribute these induced defects, adsorbed by oxygen in air, to the origin for reduced EEA by hindering exciton diffusion. Our laser irradiation was likewise effective for reducing EEA and increasing PL of chemically-treated 1L-WS2 with high QY, exhibiting the general applicability of our method. Our results suggest that exciton-exciton interaction in 1L-TMDs may be conveniently controlled by the laser treatment, which may lead to unsaturated exciton emission at high excitation levels.


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INTRODUCTION Monolayer transition metal dichalcogenides (1L-TMDs) are two-dimensional (2D) semiconductors with direct bandgap. Because optical properties of 1L-TMDs are predominated by excitonic transitions, 1L-TMDs are ideal platforms for designing quantum emitters based on exciton emissions.

1,2

The spectral characteristics of 1L-TMDs have been

extensively studied, and doping states and defect formation are found to be the most crucial factors that determine spectral emission properties of 1L-TMDs. 3-5 For instance, the spectral weight between excitons and trions of 1L-TMDs can be fine tuned by controlling carrier densities of 1L-TMDs by means of chemical doping.

3,4

On the other hand, studies showed

that intrinsic defects of 1L-TMDs can be chemically treated to greatly improve the photoluminescence (PL) of 1L-TMDs. But the exact mechanism of such defect healing is yet to be understood. 6,7 The effect of exciton-exciton annihilation (EEA) on light emission of 1L-TMDs is relatively less studied. EEA is a nonradiative Auger process that occurs between two excitons close to each other.

8,9

This process is a very serious exciton decay mechanism involving the

nonradiative recombination of one exciton ionizing the other. Because of strong Coulomb interaction in 1L-TMDs, EEA is much more active in 1L-TMDs than bulk semiconductors and thus observable even under nomial photo-excitation. 10-12 For instance, an exciton density of ~1010 cm-2 or inter-exciton distance of ~100 nm was found to trigger EEA in 1L-WS2. 13,14 Such routine occurrence of EEA in 1L-TMDs suggests that EEA is a major limitation in achieving strong light emission in 1L-TMDs, which should be reduced to maximize the PL efficiency within the intrinsic monolayer thickenss of 1L-TMDs. A number of studies using optical spectroscopy such as intensity-dependent PL, time-resolved PL (TRPL) and transient absorption spectroscopy have revealed widely varying values of the EEA constant (γ) of 1L3 ACS Paragon Plus Environment

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TMDs due to different conditions of synthesis and preparation of samples.

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6,7,10-16

This

renders systematic investigation of the correlation between sample condition and EEA rather difficult. So far, significantly less γ values for 1L-WS2 suspended or encapsulated with hexagonal boron nitride than those of 1L-WS2 on SiO2 substrate were measured, which was attributed to different exciton diffusion or localization characteristics depending on the dielectric environment.

11-13

To the best of our knowledge, no active modulation of EEA of

1L-TMDs has been carried out. In this work, for the first time, we report that EEA of 1L-WS2 can be significnatly reduced to yield bright PL at high excitation levels using simple laser irradiation, which resulted in substantially enhanced light emission at the saturating photocarrier density. The fundamental mechanism for the reduced EEA process arises most likely from the suppression of exciton diffusion due to defects directly introduced by the light treatment.

EXPERIMENTAL METHODS 1L-WS2 samples were prepared by mechanical exfoliation (ME) from bulk crystal (2D Semiconductors) on the 300-nm-thick SiO2 layer on top of typical Si substrates. For laser irradiation treatment, the frequency-doubled output (532 nm) of an Nd:YAG laser (WITec Instrument GmbH) with the input intensity from 150 µW/µm2 to 600 µW/µm2 was irradiated to ME 1L-WS2 samples with a spot size of 500 nm in diameter. The PL images and spectra were obtainted by a commercial microscope, Alpha-300S (WITec Instrument GmbH) with the excitation source of 532 nm and 488 nm lasers. The Arion laser (488 nm) was used for Raman measurement. The same confocal microscope and a vacuum chamber (MicrostatHires, Oxford instruments.) were used with basal pressure of 10-3 4 ACS Paragon Plus Environment

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Torr for vaccum condition. Laser light was focused with a 0.85 numerical aperture (NA) objective lens with a coverglass (thickeness of ~150 µm) in which a proper correction was made to compensate the chamber window thickness. The PL measurement in Ar environment was carried out with the microscope (Alpha-300R (WITec Instrument GmbH)) placed in a glove box with the O2 concentration of 1.5 ppm. For TRPL measurements, the sample was excited by 375 nm (BDL-375, Becker-Hickl GmbH) and 488 nm (BDL-488, Becker-Hickl GmbH) laser light having the pulse width of ~70 ps with a repetition rate of 80 MHz. The laser light was focused by a 0.9 NA objective lens. The PL signal was collected by the same objective lens, and then guided through an optical fiber with the core diameter of 100 mm to the photodetector (HPM-100-40, BeckerHickl GmbH). The manufactured time-correlated single photon counting system (Simple-Tau 150, Becker-Hickl GmbH) was used to synchronize the laser and the photodetector to obtain the time profile of PL decay. For

the

chemical

treatment

of

ME

1L-WS2,

10-3

M

solutions

of

bis(trifluoromethane)sulfonimide (TFSI, Sigma Aldrich) was prepared in a 9:1 mixture of 1,2-dichlorobenzene and 1,2-dichloromethane. 1L-WS2 was immersed in TFSI solution in tightly sealed vial and heated at 100 °C for 15 min on a hot plate.17 The sample was then taken out from the vial and dried under a stream of nitrogen gas.

RESULTS AND DISCUSSION We first discuss the effect of focused laser irradiation on the PL and Raman spectra of mechanically exfoliated (ME) 1L-WS2. Laser light (wavelength = 532 nm) with the input intensity of 600 µW/µm2 was focused onto ME 1L-WS2 with beam diameter of ~500 nm 5 ACS Paragon Plus Environment

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(inset of Figure 1b). Figure 1a displays the PL spectra of 1L-WS2 obtained with laser irradiation time up to 200 s. The PL spectra of 1L-WS2 did not show much change until 100 s, then the PL intensity and the peak position showed gradual increase and redshift, respectively, resulting in the 3-fold enhancement of the PL intensity after 200 s laser irradiation. The observed PL spectra were fitted with three peaks of neutral exciton (A0), negatively charged trion (A-) and defect (D) emissions, around at 2.015 eV, 1.977 eV and 1.925 eV, respectively 3,18 (see Figure S1). In Figs. 1b and 1c, variation of peak positions and the separate intensities of A0 , A- and D peaks are displayed. The energy difference between A0 and A- peaks corresponds to trion dissociation energy, which is the sum of the trion binding energy and the Fermi level in the conduction band. Trion dissociation energy was reduced from 42 meV to 27 meV by p-doping effects due to a lowered Fermi level in the conduction band.

18,19

The p-doping effects were also confirmed by the enhancement of the

spectral weight of A0 from 42% to 61%. The increase of the PL intensity by p-doping of 1L-MoS2 with laser irradiation was previously observed, which was attributed to the adsorption of air molecules such as O2 and H2O onto 1L-TMDs.

20,21

In fact, oxygen-induced PL enhancement by various methods have

been reported, such as oxygen plasma treatment oxidization

24

, and laser irradiation

22

, oxygen gas environment

23

, UV-ozone

20,21,25

. TMDs usually have large amount of chalcogen

vacancies (~1012 cm-2), 26-28 and O2 and other p-doping molecules in air can bind at chalcogen vacancy regions.

20-23,29

In this binding region of air molecules, excess charge carriers in

TMDs can be transferred to p-type dopant molecules, resulting in increase of the PL intensity and the spectral weight of neutral excitons. In our experiment as well, we believe that laser irradiation prompted the adsorption of O2 to 1L-WS2, causing p-doping effects and PL enhancement.

20-22

Another sample under laser irradiation also showed a similar behavior of 6 ACS Paragon Plus Environment

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enhancement in the PL intensity and the exciton spectral weight with redshifted peak positions (see Figure S2). The reduced energy of A0 emission (i.,e., redshift) with increasing time of laser irradiation is also consistent with the previous studies regarding to the oxygen adsorption on 1L-WS2. Liu et al. calculated the electronic band structure of oxidized 1L-WS2 using a density functional theory, indicating that the bandgap of defective 1L-WS2 with oxygen adsorption was smaller than that of pristine 1L-WS2.30 Also, the PL enhancement and redshift of the A exciton peak have been observed in UV oxidized 1L-MoS2.24 We found that the peak position of A0 emission of our sample was 10 meV higher in vacuum than in the air confirming that oxygen adsoprtion was the cause of redshift of the A0 peak (Figure S3). In order to further confirm our main thesis, we employed laser irradiation to one of our samples in a glove box (Ar environment) and found the PL intensity gradually decreased with an increased spectral weight of trion emission as the irradiation treatment continued (Figure S4). Once this sample was exposed to air and irradiated by the focused laser beam, it indeed exhibited gradual increase of the PL intensity and 6 meV redshift of the A0 peak; in this case, the sample was excited by the low excitation intensity of ~0.3 µW/µm2, which is four orders of magnitude lower than the irradiation intensity used in Figure 1 to avoid PL modification during laser excitation. This result again strongly suggests that oxygen adsorption is the cause of the observed PL enhancement and the redshift of the A0 peak with laser irradiation in air. The Raman spectra obtained from the irradiated region and the non-irradiated region also indicated the p-doping effect in 1L-WS2 with laser irradiation. 1L-TMDs have two main peaks of in-plane mode (E2g) and out-of-plane mode (A1g). mode is related to the in-plane strain

32,33

31

The peak position of the E2g

and the A1g mode is sensitive to the electron

concentration 34,35. We therefore used a 488 nm laser to measure the peak position of the A1g 7 ACS Paragon Plus Environment

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mode to examine the doping effect.

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Figure 1d shows the Raman spectra of 1L-WS2

measured before and after laser irradiation in which the latter case caused the PL enhancement. Two main peaks of 2LA+E2g and A1g modes were observed at ~357 cm-1 and ~417 cm-1 respectively. 31 We found that the A1g mode blue-shifted by 1.3 cm-1 after the laser treatment, which also indicated the p-doping effect by laser irradiation. 34,35 After local laser irradiation, we obtained PL images of 1L-WS2 with various laser excitation levels. Figs. 2a and 2b display PL images and PL spectra of laser-irradiated 1LWS2 obtained with intensities ranging from 0.3 - 600 µW/µm2. It is interesting to observe that the PL intensity of laser-irradiated region (dotted circle) was considerably lower than that of non-irradiated region when the excitation intensity was less than 1 µW/µm2. With the lowest intensity of 0.3 µW/µm2 the circled area displayed only a half of the PL intensity of the nonirradiated region. Then with increasing the excitation intensity, the PL intensity of the irradiated region exhibited a steeper increase than that of the non-irradiated region, displaying the 3 times higher PL yield at 56 µW/µm2. Here we assert that stronger PL from the laserirradiated region at high excitation levels is not due to additional p-doping during laser excitation, because the normalized PL spectra of the laser-irradiated region obtained at different excitation levels, shown in Figure 2c, displayed identical spectral shapes. This indicates that there is no further change in the doping state of the laser-irradiated region of 1L-WS2. Figure 2d displays the plot of the integrated PL intensity with varying excitation levels. The critical exponent in the intensity slope (m) is about 1 and 0.85 at lower excitation levels, and decreases to 0.63 and 0.57 at higher excitation levels in the irradiated region and the non-irradiated region, respectively. In result, the estimated quantum yield (QY) was 8.0 % and 5.2 % at 0.3 µW/µm2, then decreased to 1.2 % and 3.2 % at 56 µW/µm2 for the nonirradiated region and the irradiated region, respectively. (See the details of QY estimation in 8 ACS Paragon Plus Environment

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Supporting Information) Previously, m ~ 1 was attributed to the presence of defects, trapping carriers at low excitation levels.

6,36

On the other hand, relative reduction of the intensity

slope observed in 1L-TMDs at high excitation levels arises mostly due to the EEA process accelerated by the increasing number of photoexcited carriers for a given 15

excitation area. 6-

The onset of EEA in 1L-WS2 was reported to occur at the exciton density of ~1010 cm-2, 14

corresponding to the laser intensity of 1 µW/µm2 in our experiment, (see detailed calculation of photoexcited exciton densities in Supporting Information) suggesting that EEA may be the origin for saturation of the PL intensity starting at the range of 1-10 µW/µm2 in our study. This then suggests that the EEA process was somehow reduced at the laser-irradiated region. We conducted the same procedure of laser irradiation and subsequent PL mapping on other ME 1L-WS2 grains and observed the similar results. Two ME 1L-WS2 grains were locally irradiated with the laser intensity of 600 µW/µm2. Figure 3a shows PL intensity maps of the same laser-irradiated sample at two different photoexcitation levels of 0.08 µW/µm2 (low) and 200 µW/µm2 (high). At the low intensity, the irradiated region is relatively less in PL intensity than the non-irradiated region, which is also consistent with the representative PL spectra shown in Figure 3b. However, at the high excitation level, where EEA is expected to be significant, the PL of the laser-irradiated region was 2.5 times stronger than that from the non-irradiated region. Another example of the inverted contrast of PL images and spectra of ME 1L-WS2 grain depending on the excitation levels are provided in Figs. 3c and 3d, respectively. These repeatable results of the inverted PL contrast and their dependence on the excitation levels indicate that local EEA can be readily reduced by this simple laser irradiation treatment. To further investigate the EEA effects on PL of the irradiated region and the nonirradiated region, we conducted TRPL measurements on 1L-WS2 with varying excitation 9 ACS Paragon Plus Environment

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levels. A sample was locally irradiated with 600 µW/µm2 with the focused laser beam and the PL image and TRPL measurement were conducted. Similar to the results in Figure 2 and Figure 3, at 0.08 µW/µm2 the irradiated location (dotted circles in the Figure 4a) exhibited relatively less PL, but at 200 µW/µm2 the PL intensity at the irradiated location was higher than that of the non-irradiated region. We found that TRPL curves of the irradiated location obtained with varying excitation levels displayed different decay rates from those obtained from the non-irradiated region. Figure 4b displays TRPL curves obtained with the pump fluences ranging from 1.3 - 1280 nJ/cm2. In this range of pump fluences for TRPL, the shape of the PL spectra was not changed. We also measured the TRPL curves by moving the center wavelength of detection to separete the exciton PL and the trion PL, but found no noticeable difference in the TRPL decay curves (Figure S5), all suggesting that the formation of other excitonic matter could be neglected (Figure S6).

11

The PL decay curves were fitted using a

biexponential form to extract exction lifetimes for low excitation levels of 1.3 nJ/cm2, 6.4 nJ/cm2 and 50 nJ/cm2. At 1.3 nJ/cm2, TRPL decay was apparently faster at the irradiated region than the non- irradiated region with average lifetimes, τavg, of 468 ± 20 ps and 556 ± 25 ps, respectively. The shorter decay times of the laser-irradiated region at lower excitation levels indicates that the laser-irradiated region has a larger density of defects than the nonirradiated region.

10,37

We believe that laser irradiation treatment could have caused the

formation of defects such as S-vacancies, same as the way that S-vacancies can be introduced by heat treatment. 20,38,39 With increasing the laser pump fluence above 650 nJ/cm2, the TRPL showed faster decay and the TRPL curves were not properly fitted with the biexponential model. Instead, the bimolecular decay model based on the EEA process showed the excellent fitting results. The rate equation for EEA is given by 10 ACS Paragon Plus Environment

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dn n = − − γ n2 dt τ

(1)

where n(t) is the exciton density and τ is the PL lifetime at a low exciton density free from EEA and γ is the EEA rate constant. The solution to equation (1) is given by 14,37

n(t) =

n0 e−t /τ

(2)

1 + γτ n0 (1 − e −t /τ )

where n0 is the initial exciton density estimated with the excitation spot size of ~500 nm in diameter and the quantum efficiency for absorption of 1L-WS2 in 375 nm, which was assumed to be 3 %.

18

By assuming that γ is time independent,

14,15

the measured TRPL

curves at high pump fluences at 650 nJ/cm2 and 1280 nJ/cm2 were fitted by convoluting the instrument response function (IRF) obtained by using the laser signal reflected from the sample substrate. The best fits were obtained with ߛ values of 0.66±0.15 cm2/s and 0.20± 0.05 cm2/s for the non-irradiated region and the irradiated region, respectively. Several values of ߛ in 1L-WS2 have been reported in some theoretical and experimental studies, and the range of reported values were 0.1-2.8 cm2/s, which is in a good agreement with our results. 6,7,11-15,40,41

The smaller value of the EEA constant obtained at the laser-irradiated location

than the non-irradiated region indicates that laser irradiation somehow caused reduction of EEA. We quantified variations of decay constants, short and long components of the TRPL curves by fitting with a biexponential decay model. There are density-independent channels for exciton decay, such as defect-assisted relaxation and radiative recombination channels, 10,37

in eq. 1 when n is small enough to neglect the EEA effect. At 100 nJ/cm2 (corresponding

to an exciton density of 5.7 × 1010 cm-2) where the exciton decay dynamics should be density11 ACS Paragon Plus Environment

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independent, the decay constants of ~230 ps as a short component and ~1.5 ns as a long component were measured (Figure 5a, b), which could be related to defect-assisted relaxation and radiative recombination, respectively. 42,43 Above 100 nJ/cm2, the lifetimes of both decay channels decrease (Figure 5a, b), because non-linear decay channel of EEA became significant at high pump fluences and caused the decrease of decay constants.

6,7,10-15,37

The

gradual decreasing of τavg shown in Figure 5c indeed captures the EEA effect. Overall, longer τavg of the irradiated region indicates that EEA is less significant at the irradiated region than the pristine region. Unlike the monotonous decrease of τavg observed at the pristine region with increasing the pump fluence, an initial increase of τavg at a low pump fluence was observed at the irradiated region. We attribute this behavior to the increasing proportion of the defects-related channel due to the defect formation such as S-vacancies by laser irradiation. These defect-trap sites can have a saturation behavior with increasing the laser fluence, leading to the increasing proportion of radiative recombination and hence the average lifetime.

10,37

We believe that this interesting behavior of initial increase of the

average lifetime arises due to the relatively less effect of EEA combined with higher density of defects at the irradiated region. We attribute the enhanced PL at the laser-irradiated location to the reduced EEA rate constant at high excitation levels and now discuss the mechanism of reduced EEA by laser irradiation. We observed that at low excitation levels in which EEA is negligible, the laserirradiated region displayed relatively lower PL and faster TRPL decay than the non-irradiated region, indicating a higher density of carrier-trapping defects at the laser-irradiated region. As additional support of laser-induced defect formation we employed laser irradiation at 560 µW/µm2 in a vacuum chamber and found that the PL intensity of 1L-WS2 decreased and the trion spectral weight increased with increasing illumination time (Figure S7), which are the 12 ACS Paragon Plus Environment

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typical results of the increased number of chalcogen vacancies in 1L-TMDs.

44,45

We also

note that relative reduction of Raman signal with laser irradiation shown in Figure 1d and Figure S8 suggests the increased density of defects by laser irradiation. On the other hand, for the high excitation fluence EEA becomes dominant to limit exciton emission. It is known that the EEA rate constant is proportional to the exciton diffusion length, 11,12,46 and thus observed reduction of the EEA process implies that the exciton diffusion length has been reduced with laser irradiation. In our observation (Figure 2), the exciton density of ~1011 cm-2 that corresponds to the inter-exciton distance of ~25 nm was high enough to cause EEA. Considering that the diameter of exciton in 1L-TMDs are only 1~3 nm,

47

exciton diffusion

should have played the important role in the EEA process.11,14 Based on above arguments, we believe that laser illumination increased the number of sulfur vacancies that were responsible for the reduction of the exciton diffusion length and the EEA rate constant, which is schematically drawn in Fig. 5d. As shown in Figs. S4 and S7, if the samples were irradiated under a high excitation level in Ar environment or inside the vacuum chamber, there was no increase of PL at the laser-irradiated location. We also found that these samples showed sharply increasing PL under in-situ light irradiation/monitoring at 0.35 µW/µm2 (Figure S4d) if they are exposed to air. These control experiments indicate that the PL enhancement along with the reduced EEA rate constant requires not only the formation of sulfur vacancies, but also adsorption of p-doping oxygen molecules on defect sites of 1L-WS2 in the air environment. Therefore, we are convinced that our laser irradiation has caused the formation of additional S vacancies of 1L-WS2 and if these sulfur vacancies are adsorbed by p-doping oxygen molecules, they tend to reduce the EEA efficiency by “impeding” exciton diffusion. We found that laser irradiation was effective for reducing EEA and increasing PL and QY of chemically treated 1L-WS2 as well. Chemical treatment using TFSI is known to greatly 13 ACS Paragon Plus Environment

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improve the QY of 1L-MoS2 or 1L-WS2 by repairing S vacancies.6 We performed the laser irradiation on the TFSI-treated 1L-WS2 and measured the PL spectra and QY with the varying excitation intensity. QYs of TFSI-treated sample and the pristine sample at laser irradiated region and non-irradiated region are plotted as a function of excitation intensity in Fig. 6a. Representative PL spectra and PL images at low (0.08 µW/µm2) and high (200 µW/µm2) excitation intensities are displayed in Fig. 6b (Gray curves: pristine sample. Red curves: laser irradiated region of TFSI treated sample. Navy curves: Non-irradiated region of TFSI treated sample.). First, we note that QY of TFSI-treated samples are ~5 times greater than the pristine sample reaching 57 %, suggesting that repairing the defects of 1L-WS2 by TFSI treatment occurred in our samples. More interestingly for TFSI-treated samples, the laser irradiated region showed relatively lower QY and PL intensity than the non-irradiated region at 0.08 µW/µm2 excitation intensity, however at 200 µW/µm2 laser intensity, QY and PL intensity of the laser-irradiated region was 2.2 times higher than the non-irradiated region. By using TRPL measurements as the results are displayed in Fig. 6c, we estimated EEA rate constants of TFSI-treated 1L-WS2. Similarly to the case of pristine 1L-WS2 sample, at low laser power of 20 nJ/cm2 where EEA is negligible, the laser-irradiated region showed the faster PL decay of 584 ± 20 ps than non-irradiated region of 651 ± 15 ps. At 220 nJ/cm2 and 1040 nJ/cm2 excitation intensities, γ has been estimated by fitting TRPL decay curves to eq 2 and we obtained γ of 0.35 ± 0.07 cm2/s and 0.80 ± 0.05 cm2/s at laser-irradiated region and non-irradiated region, respectively, indicating a substantial reduction of EEA effect at laser irradiated region. Estimated γ values of TFSI treated 1L-WS2 were slightly higher than the prinstine sample with γ of 0.20 cm2/s and 0.66 cm2/s at laser-irradiated region and nonirradiated region, respectively, which could be due to the improved exciton diffusion by the reduciton of S-vancacies by TFSI treatment. Our results indicate that our laser treatment is 14 ACS Paragon Plus Environment

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effective for high QY 1L-WS2 samples in reducing EEA and enhancing the light emission of 1L-WS2 at high exciton density. CONCLUSION In summary, we emphasize that simply enhacing the quantum yield of 1L-TMDs is not sufficient to overcome the ultimate limit on the practical PL efficiency as it is essentailly limited by the EEA mechanism even at nominal exciation levels. With systematic investigation based on TRPL and PL imaging/spectroscopy, we have showed that light emission of 1L-WS2 can be significantly enhanced through the reduction of the EEA process by simple laser irradiation. We have experimentally achieved an EEA rate constant being reduced by a factor of ~3, which leads to a slow decay rate accompanied with the enhanced time-integrated PL intensity and QY up to a factor of ~3. The formation of sulfur vacancies and adsorption of oxygens by laser illumination seem to be the main mechanism for the reduction of exciton diffusion and the EEA effect. Most importantly, the proposed light treatment was equally applicable to chemically treated 1L-WS2 with enhanced QY, and therefore, our results demonstrate that exciton-exciton interaction of 1L-TMDs can be conveniently controlled, which may be utilized for diverse applications of 1L-TMDs that includes high-power light-emitting devices. Our study also potentially indicates highly luminescent TMDs by design especially when exciton diffusion can be further frozen by defect control via light and/or electronic doping. ACKNOWLEDGMENT This work was supported by IBS-R011-D1. J. I. Jang acknowledges support by Basic Science Research Program (2017R1D1A1B03035539) through the National Research Foundation of Korea (NRF), funded by Korean governmet..


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ASSOCIATED CONTENT Supporting Information The Supporting information is available free of charge on the ACS Publications website at DOI: Spectral fitting results for spectra shown in Figure 1, PL results of laser irradiation in Ar and vacuum conditions, estimation of QY, calculation of the photo-excited exciton densities and the discussion for spectral contribution of other excitonic matters.


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Figure 1. PL enhancement and p-doping of 1L-WS2 by laser irradiation. (a) PL spectra of 1L-WS2 under continuous laser irradiation with the input intensity of 600 µW/µm2 in air. (b) Plot of PL intensities of excitonic matter as a function of laser irradiation time. The PL spectra were deconvoluted into neutral exciton (A0), trion (A-) and defect-bound exciton (D). Inset shows our experimental schematic. (c) Plot of peak positions of excitonic matter as a function of laser irradiation time. (d) Raman spectra of 1L-WS2 after and before the laser treatment. Lorentzian fittings of three peaks for 2LA (orange trace), E2g (blue trace) and A1g at 417 cm-1. Red traces represent the overall fit.


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Figure 2. PL mapping images and spectra of laser-irradiated 1L-WS2 with various excitation levels. (a) PL intensity images of 1L-WS2 with various excitation levels. The dotted circle in each image indicates the laser-irradiated region. (b) PL spectra of the laser-irradiated region (red) and non-irradiated regions (black). (c) Normalized PL spectra from the laser-irradiated region. (PL curves were made 70 % transparent for overlap). (d) Logarithmic plot of the PL intensity as a function of excitation intensity. Fitted slope values of m are shown for different excitation ranges.


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Figure 3. (a) PL intensity images of ME 1L-WS2 obtained with 0.08 µW/µm2 and 200 µW/µm2 (b) PL spectra obtained from the laser-irradiated region (each indicated by the dotted circle in (a)) and the non-irradiated region. (c) PL intensity images of another ME 1LWS2 grain obtained with 0.08 µW/µm2 and 38 µW/µm2. (d) PL spectra obtained from the laser-irradiated region (each indicated by the dotted circle in (c)) and the non-irradiated region.


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Figure 4. (a) PL intensity images of 1L-WS2 obtained with low (0.08 µW/µm2) and high (200 µW/µm2) excitation levels. The irradiated region is indicated by the dotted circle. (b) TRPL curves of the laser-irradiated region (red) and non-irradiated region (navy) obtained with varying excitation levels. The TRPL curves obtained with 1.3, 6.4 and 50 nJ/cm2 were fitted with the biexponential decay model and the TRPL curves obtained with 650 nJ/cm2 and 1280 nJ/cm2 were fitted with the EEA model (equations 1 and 2).


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Figure 5. Decay constants of long and short components (a, b) and averaged lifetimes (c) of TRPL curves fitted with biexponential functions vs. laser pump fluence, obtained from laserirradiated (orange) and non-irradiated (black) regions. The dashed lines are guide to the eye. (d) Schematic depicting laser-induced sulfur vacancies that hinders the diffusion of excitons to reduce the EEA rate.


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Figure 6. (a) Plot of quantum yields of pristine and TFSI-treated samples (at laser irradiated and non-irradiated regions) as function of excitation intensity. (b) PL spectra of TFSI treated 1L-WS2 (navy curves) and TFSI & laser treated 1L-WS2 (red curves) with excitation intensities of 0.08 and 200 µW/µm2. PL spectra of pristine 1L-WS2 (gray curves) are included for comparison. The insets show PL mapping images. White dotted circles indicate the location of laser irradiation. Scale bar is 2 µm. (c) TRPL curves of the TFSI treated sample (navy) and TFSI & laser irradiated region (red) obtained with varying excitation intensities. The TRPL curves obtained with 8 and 20 nJ/cm2 were fitted with the


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biexponential decay model and the TRPL curves obtained with 220 nJ/cm2 and 1040 nJ/cm2 were fitted with the EEA model (eq 1 and 2).


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For Table of Contents Use Only Manuscript title: Impeding Exciton-Exciton Annihilation in Monolayer WS2 by Laser Irradiation

Authors: Yongjun Lee, Ganesh Ghimire, Shrawan Roy, Youngbum Kim, Changwon Seo, Ajay K. Sood, Joon I. Jang, and Jeongyong Kim

Figure for TOC

Brief synopsis of the TOC graphic: Nonradiative exciton-exciton annihilation (EEA) significantly limits light emission of monolayer (1L) transition metal dichalcogenides (TMDs) at high exciton densities. We show that the EEA rate constant (γ) can be reduced by laser irradiation treatment in mechanically exfoliated monolayer tungsten disulfide (1L-WS2), causing significantly improved light emission at the saturating optical pumping level. The laser irradiation increases the density of sulfur vacancies of 1L-WS2, which hinders exciton diffusion to reduce EEA effect. Our results suggest that exciton-exciton interaction in 1L-TMDs may be conveniently controlled that leads to unsaturated exciton emission.


Impeding Exciton-Exciton Annihilation in Monolayer WS2 by Laser

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