Atomic Observation of Filling Vacancies in Monolayer Transition Metal

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Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms Shrawan Roy, Wooseon Choi, Sera Jeon, Do-Hwan Kim, Hyun Kim, Seok Joon Yun, Yongjun Lee, Jaekwang Lee, Young-Min Kim, and Jeongyong Kim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01714 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms

Shrawan Roy,1,2† Wooseon Choi,1† Sera Jeon,3† Do-Hwan Kim, 1 Hyun Kim,1,2 Seok Joon Yun,1,2 Yongjun Lee,1,2 Jaekwang Lee,3* Young-Min Kim,1,2* and Jeongyong Kim1,2*

1

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

Korea 2

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

Republic of Korea 3

Department of Physics, Pusan National University, Busan 46241, Republic of Korea

1

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ABSTRACT

Chemical treatment using bis(trifluoromethane) sulfonimide (TFSI) was shown to be particularly effective for increasing the photoluminescence (PL) of monolayer (1L) MoS2, suggesting a convenient method for overcoming the intrinsically low quantum yield of this material. However, the underlying atomic mechanism of the PL enhancement has remained elusive. Here, we report the microscopic origin of the defect healing observed in TFSI-treated 1L-MoS2 through a correlative combination of optical characterization and atomic-scale scanning transmission electron microscopy, which showed that most of the sulfur vacancies were directly repaired by the extrinsic sulfur atoms produced from the dissociation of TFSI, concurrently resulting in a significant PL enhancement. Density functional theory calculations confirmed that the reactive sulfur dioxide molecules that dissociated from TFSI can be reduced to sulfur and oxygen gas at the vacancy site to form strongly bound S-Mo. Our results reveal how defect-mediated nonradiative recombination can be effectively eliminated by a simple chemical treatment method, thereby advancing the practical applications of monolayer semiconductors.

Keywords: Molybdenum disulfide, tungsten disulfide, sulfur vacancies, defect healing, chemical treatment, excitons

Two-dimensional monolayer transition metal dichalcogenides (1L-TMDs) such as MoS2, MoSe2, WS2, WSe2, and MoTe2 have many interesting properties, including direct optical band gaps ranging from 1.1 to 2.1 eV, 1-4 which make them potentially applicable as semiconducting materials in the field of optoelectronics devices such as lasers,5 2


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photodetectors,6 phototransistors,7,8 and light-emitting diodes (LEDs).9 These 1L-TMDs can be obtained by either mechanical exfoliation of single crystals or large-scale growth using chemical vapor deposition (CVD) methods.10-14 One of the main disadvantages of 1L-TMDs for practical applications is their low quantum yields (QYs), which range from 0.01 to 6% 15 and are caused by the presence of structural defects at which excitons are trapped and decay non-radiatively.16 Previous scanning transmission electron microscopy (STEM) studies have shown that 1L-TMDs contain an abundance of structural defects, such as chalcogen vacancies, impurities, interstitials, antisite defects and dislocations.17-19 For CVD-grown 1LTMDs, thermal strain and local variations in the concentrations of the precursors used during the growth process are the origins of such structural defects.17,19Among the types of defects, chalcogen vacancies are dominant in CVD-grown and mechanically exfoliated 1L-TMDs.18 Mono-vacancies are abundant in 1L-TMDs because they have the lowest formation energy among all the defects.18 A variety of chemical treatments have been proposed to enhance the light emission of 1L-TMDs and overcome the limitation of the low QY. For example, chemicals containing electron-withdrawing groups, such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ),20,21 7,7,8,8-tetracyanoquinodimethane (TCNQ)20 and hydrogen peroxide,22 have been used to decrease the charge (electron) density of 1L-MoS2, which resulted in enhanced photoluminescence (PL). On the other hand, certain chemicals, such as hydrohalic acid (HBr), can replace impurities like adsorbed oxygen in the selenium vacancy sites with bromine ions and repair the structure of 1L-MoSe2.23 Similarly, bis(trifluoromethane) sulfonimide (TFSI) treatment of 1L-TMDs containing sulfur (S) as the chalcogen atom (MoS2 and WS2) has been shown to be extremely effective for enhancing the QY to nearly 100%, and this enhancement was attributed to the elimination of the harmful effects of structural defects on exciton 3


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emission.24 Although S adatoms were claimed to repair the S vacancies of 1L-TMDs and a similar mechanism was proposed for the poly(4-styrenesulfonate)-induced PL enhancement of 1L-MoS2,24-29 the exact mechanism is not understood due to the lack of experimental evidence at the atomic level. In this work, we performed the correlative study of PL and Raman imaging, scanning transmission electron microscopy (STEM) measurement and density functional theory (DFT) calculation on TFSI-treated 1L-MoS2 and directly observed in atomic level that S vacancies of 1L-MoS2 were repaired by S atoms dissociated from TFSI molecules. Our finding provides an explicit explanation for why the QY of TFSI-treated 1L-TMDs is drastically enhanced, and a unique repair mechanism for monolayer semiconductors by organic molecules is demonstrated. We show in Figure 1 PL images and spectra of three grains of TFSI-treated CVDgrown 1L-MoS2 prepared on a TEM grid. The representative PL spectrum of the pristine sample (black spectrum in Figure 1b) showed a peak at approximately 657 nm, which is consistent with previous PL results for 1L-MoS2.2,10,11,12,14 As shown in Figure 1b, the intensity of the PL spectra (red curves) numbered as 1, 2 and 3 obtained from the three different grains of the CVD-grown 1L-MoS2 after TFSI treatment in Figure 1a were approximately 10 times higher than the pristine sample. TFSI is known to be particularly effective for enhancing the QY of monolayers of MoS2 or WS2, where the origin of the enhancement was attributed to the defect healing caused by TFSI.14,24,26-29 We measured the QY of our samples which were ~0.1 to 0.2 % and 1.5~15 % for the pristine and the TFSItreated 1L-MoS2, depending on the suspended or substrate condition, respectively (See Supporting Information for QY estimation). The PL peak position was observed to blueshift by ~5 nm after TFSI treatment, suggesting that a p-doping effect also occurred. TFSI 4


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molecules have been used as p-doping chemicals for graphene because of the presence of highly electronegative fluorine atoms in these molecules, which act as the strong electronwithdrawing agents,30,31 and the p-doping effect by TFSI were often observed for the PL spectra of CVD-grown 1L-MoS2 or 1L-WS226-28 whereas exfoliated samples didn’t display noticeable blueshift of PL peak with TFSI treatment 24. The Raman spectra and corresponding spectral mapping images of 1L-MoS2 before and after TFSI treatment are presented in Figure 2a. The representative Raman spectrum of pristine 1L-MoS2 shown in the top panel of Figure 2a exhibits two characteristic Raman peaks at 386.7 cm-1 and 405.8 cm-1 that correspond to the in-plane E′ mode and out-of-plane A′1 mode, respectively.32-34 The difference between these two modes is ~19.1 cm-1, which is consistent with the monolayer thickness of MoS2.10-12,14,33-36 The A′1 mode of 1L-MoS2 is highly sensitive to the electron density, and when the electron density of 1L-MoS2 decreases (increases), the band shifts towards higher (lower) wavenumbers.10,11,12,37 In our result, the A′1 mode of 1L-MoS2 blueshifted by 0.6 cm-1 after TFSI treatment, indicating the occurrence of the p-doping effect after TFSI treatment.10,37 This p-doping effect was uniformly observed throughout the 1L-MoS2 grains after TFSI treatment, as shown by the A′1 peak position images in the bottom panel of Figure 2a. The E′ mode of 1L-MoS2 is highly sensitive to strain, and compressive or tensile strain can be identified by the direction of the shift in the corresponding Raman peak.38-40 We observed that after TFSI treatment, the E′ mode of 1LMoS2 shifted towards lower wavenumbers by 1.1 cm-1, which corresponds to 0.52% tensile strain in 1L-MoS2 according to a previous study.39 The peak position mapping images for the E′ mode of 1L-MoS2 with and without TFSI treatment, shown in the inset of the top panel of Figure 2a, also reveal that a shift in the peak of the E′ mode occurred throughout the 1LMoS2 grain. To understand the origin of the tensile strain following the TFSI treatment, we 5


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performed DFT calculations and attempted to estimate the strain that developed in the crystal lattice of 1L-MoS2 by the presence of the mono-S vacancy (VS) and di-S vacancy (VS2). The results of the DFT calculations of the vacancies are schematically presented in Figure 2b. According to the calculated results, the lattice parameter is significantly reduced by the presence of a VS2 or VS in the 1L-MoS2 lattice, as shown in Figure 2b. For example, one VS in one unit cell (within the rectangular box) results in a 0.7% decrease in the lattice parameter suggesting the relaxation of compressive strain observed after TFSI treatment may be due to the removal of S vacancies from 1L-MoS2. Previously the formation of S vacancies followed by adsorption of oxygen molecules resulted in the redshift of E′ mode, indicating highly sensitive nature of E′ mode to the status of S vacancies of 1L-MoS2.34,35 The X-ray photoelectron spectroscopy (XPS) results for the pristine and TFSI-treated 1L-MoS2 also suggested a reduction in the number of defects following the TFSI treatment (see Supporting Figure S1). To understand the atomic-scale healing of the S vacancies in 1L-MoS2 by the TFSI chemical treatment, the atomic structure and chemical composition of the pristine and TFSItreated 1L-MoS2 were analyzed by atomic-resolution annular dark field STEM (ADF-STEM) combined with energy dispersive X-ray spectrometry (EDX) and electron energy loss spectroscopy (EELS). The intensity measured in the ADF-STEM image shows a Zn contrast relationship (Z: atomic number, n ~1.6 – 2) due to its incoherent nature, so direct counting of the intensity in each atomic column allows us to quantify the number of individual atoms in a sample of known thickness.41 With the help of this imaging characteristic, atom-by-atom chemical identification as well as configurative analysis of the defects and the local concentration has been demonstrated through quantitative analysis of the 1L-MoS2 ADFSTEM images.17,42 Figure 3a (top panels) shows five selected ADF-STEM images of pristine 6


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1L-MoS2 samples, which show the typical hexagonal lattice of the 1H phase of 1L-MoS2 with alternating arrangement of Mo and S2 atoms. In this imaging mode, Mo atoms appear brighter than S2 atoms. The vacancy sites of single and double S atoms are substantially darker than the Mo and S2 atoms and are clearly detected in the image. The statistical distribution of the intensity at the S sites is given as a histogram (Figure 3b). The Gaussian fit of the histogram provides two distinct peaks and one weak tail with very low intensity. The main peak represents S2 sites, while the band with half the intensity of the S2 peak and the very weak tail right before this band represent the VS and VS2 sites, respectively. Note that no noticeable change is observed in the distribution of the Mo site intensity from multiple measurements (not shown here), meaning that vacancies are preferentially formed at the S sites in our pristine MoS2 samples. Due to the imperfection of the CVD growth method even under S-rich conditions, the formation of intrinsic point defects (leading to the typical n-type conductivity) is inevitable, and VS defects are commonplace because the formation energy of a VS is known to be the lowest among the various types of defects, while antisite defects (MoS2 and S2Mo) have the highest energies, and the formation energy of a VS2 is approximately twice that of a VS.17,43,44 From quantitative intensity analysis of the ADF-STEM images at the S sites in the pristine 1L-MoS2 samples, we could measure the total S concentration and the content of VS and VS2 defects, as shown in Figure 3a (bottom panels). The resulting S vacancy maps indicate that VS defects are randomly distributed and prevalent in the pristine 1L-MoS2. The quantitative results derived from the five measurements are summarized in Figure 3c. In these CVD-grown 1L-MoS2 samples, the total S content was estimated to be ~97.09 ± 0.63%, and the rest of the S sites were mainly occupied by VS defects (~2.65 ± 0.60%), while the VS2 defects had only a small site occupation (~0.27 ± 0.21%). The concentrations of VS and VS2 7


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in the samples can be represented as the defect density per unit area and have values of ~(8.57 ± 2.43) x 1013 and ~(0.89 ± 0.77) x 1013 cm-2, respectively, which are of similar orders of magnitude as the values previously reported for CVD-grown 1L-MoS2.18,45 Note that based on the field of view of our ADF-STEM images, which defines the total number of atomic sites that can be counted, one S vacancy among all the atomic sites is the detection limit whereby we can quantify the lowest vacancy concentration in the sample. Thus, the minimum detectable vacancy density is 1.03 x 1012 cm-2. Very few of the other types of antisite defects, such as MoS2 (arrow in Figure 3a) and vacancy complexes, are detected, and much fewer of these defects than VS2 defects are observed. This trend in the observation is consistent with the previous formation energy calculations of the various aforementioned point defects.17,43,44 We verified the reliability of our vacancy quantification by simulating ADF image of 1LMoS2 with the same density of point defects being randomly distributed at the S sites and also by adding statistical noise with a Poisson distribution, which showed the excellent match with experimental observation. (Supporting Fig. S2 and S3) To investigate the microscopic origin of the PL enhancement caused by the TFSI treatment, we transferred 1L-MoS2 samples to TEM sample grids (see Methods for details), chemically treated the samples with TFSI, and confirmed the PL enhancement, as shown in Figure 1. We then performed ADF-STEM imaging on these same samples and analyzed the results by the same method described for the pristine 1L-MoS2 samples, and the five selected ADF-STEM images are shown in Figure 4. The S vacancy mapping results of the ADFSTEM images show a remarkable decrease in the S vacancy concentration relative to that in the pristine 1L-MoS2 (see the bottom panels in Figure 4a). At most, 7 to 13 S vacancies in the form of VS and VS2 are detected in each ADF-STEM image, which corresponds to ~0.42% of the total S sites and a defect density per unit area of ~1.18 x 1013 cm-2. This result reveals that 8


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the intrinsic point defects of S in the CVD-grown MoS2 can be effectively recovered by the TFSI chemical treatment without physically changing the lattice framework. A significant reduction in the intensity of the peak for VS is clearly observed in the site-assigned intensity histogram of a representative ADF-STEM image (Figure 4b). Based on quantitative counting of the S sites in the ADF-STEM images of the TFSI-treated 1L-MoS2, ~90% of the total S vacancies in the pristine 1L-MoS2 were repaired. The total contents of VS and VS2 were reduced from ~2.65% to ~0.32% and from ~0.27% to ~0.10%, respectively, by the treatment (Figure 4c). Note that antisite defects (MoS2) are detected in the TFSI-treated MoS2 at a very low concentration similar to that in the pristine 1L-MoS2 samples (arrows in Figure 3a and 4a), suggesting that TFSI has no noticeable healing effect on the antisites. The effective S healing by TFSI was again confirmed in another 1L-MoS2 sample from a different CVDgrown batch (Figure S4 and S5), further ensuring the direct filling of S atoms into the vacancies by the treatment. The energy dispersive x-ray spectroscopy (EDX) also showed the noticeable increase of S content in TFSI-treated samples; the atomic percentage of S was evaluated to be relatively increased by ~10% in the treated samples. Also, electron energy loss spectroscopy (EELS) results confirmed the identical bonding nature of S between pristine and TFSI-treated samples. (Supporting Fig. S6). To determine whether S vacancy repair occurs with other chemical treatments, we performed optical and ADF-STEM characterization of another set of 1L-MoS2 samples consisting of pristine and F4TCNQ-treated samples. F4TCNQ treatment was previously shown to increase the PL of 1L-MoS2, and the origin of the enhancement was attributed to the depletion of excess electrons and the reduction of trion emission.20,21 As shown in Figure S7a and b, F4TCNQ-treated 1L-MoS2 displayed a PL intensity ~4 times higher than that of the 9


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pristine 1L-MoS2. However, in contrast to the results from the ADF-STEM observations of the TFSI-treated sample, a density of S vacancies similar to that in the pristine sample was maintained (Figures S7 to S10), which indicates that the direct repair of the S vacancies in 1L-MoS2 occurs specifically with the TFSI treatment. In addition, we prepared pristine and TFSI-treated CVD-grown hexagonal 1L-WS2 samples and then performed PL and ADF-STEM measurements. The hexagonal 1L-WS2 grains contain multi-domains, which are populated by different types of structural defects consisting not only of S vacancies but also W vacancies and larger void defects.19 Following the TFSI treatment, the PL intensity of the hexagonal 1L-WS2 grains was uniformly enhanced across the different domains, but ADF-STEM imaging showed that only the S vacancies were repaired, while the other types of structural defects, such as the W vacancies and void defects, were not repaired, confirming the specific effect of TFSI on the S vacancies (for the PL, Raman and ADF-STEM imaging results and detailed explanation, see Figure S11 and S12). Based on the results of ADF-STEM combined with those from EDX and EELS, we propose that TFSI directly plays a key role in healing the S vacancies, giving rise to the observed enhancement in the optoelectronic properties. The direct healing mechanism has not been fully elucidated because an atomic-scale description of the healing process has not yet been presented. Previously, surface passivation by hydrogen-bonded S adatoms was proposed as a plausible mechanism since DFT calculations predicted no midgap trap state for this configuration and since XPS detected no evidence of reactive SOX molecules.24 This passivation mechanism involving S adatoms was similarly used to explain the PL increase observed for 1L-MoS2 treated by pol(4-styrenesulfonate);25 however, we note that the S atoms in the TFSI molecules are not in the form of thiol (–SH)46,47 or sulfonate (–SO3H)25. The new Mo-H-S bonds are weaker than the covalent Mo-S bonds in pristine 1L-MoS2, 10


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which thus might readily induce breakage of the Mo protonation bond by electron beam irradiation during the ADF-STEM observations. We have, however, observed no accelerated structural damage relative to pristine MoS2, which strongly supports the hypothesis that the healed S atoms form strong bonds with the surrounding Mo atoms without any intervention by hydrogen. From our atomic-scale observation of the S vacancy healing phenomenon, we report that the S vacancy sites are directly filled by the excess S atoms supplied by the TFSI molecules. Previous theoretical studies suggested that after losing a H+ due to its strong acidity, TFSI molecules can dissociate into the ionic compound [CF3-SO2-N-CF3]– and a reactive SO2 molecule,48 and also that SO2 molecule can dissociate into an S atom and CO2, where the S atom adsorbs on the graphite surface49. Based on these previous theoretical predictions, the energy landscape for the S vacancy healing process was explored through DFT calculations. From these calculations, we found that the formation of SO2 + (CF3-SO2N-CF3) is energetically more favorable than the formation of CF3-SO2-N-SO2-CF3 by 0.73 eV, which indicates that the dissociation of an SO2 molecule from a TFSI anion is plausible (the complete energy landscape is given in Figure S13). To explore the energy landscape of the Vs healing process, a single S atom was removed from a 1L-MoS2 4×4×1 supercell composed of 16 Mo and 32 S atoms (corresponding to a vacancy concentration of approximately 3%), and a TFSI molecule (without the H atom considering the super acidic nature of TFSI) was placed approximately 10 Å away from the 1L-MoS2 supercell, as shown in Figure 5. Then, the total energy of the system was evaluated after the defective 1L-MoS2 and TFSI were fully relaxed until the forces on all atoms were below 0.01 eV/Å. For the second step, the S vacancy site was occupied by the S atom of an SO2 molecule, and the remaining CF3-N-SO2-CF3 was fully relaxed near the 1L-MoS2 surface. An activation energy of ~approximately 0.55 eV was 11


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needed for this second process compared to the initial configuration. For the last step, an O2 molecule was dissociated from the SO2 molecule, and both this O2 molecule and the remaining CF3-N-SO2-CF3 are physically adsorbed on the perfect 1L-MoS2 supercell through van der Waals interactions. After atomic relaxation, this final configuration was energetically more favorable by approximately 0.89 eV than the transition state and by approximately 0.34 eV than the initial TFSI configuration, which strongly supports the premise that the S atoms were supplied by TFSI and that their direct filling process is energetically favorable. Furthermore, our DFT calculations confirm that the remaining CF3-N-SO2-CF3 compound are still physically adsorbed on the perfect 1L-MoS2 surface even after the S vacancy healing process, which is in good agreement with the EDX experimental results. We performed DFT calculations of the adsorption by other possible dissociated components of TFSI than SO2, such as CF3, CF3-SO2, and CF3-SO2-NH, and found that the adsorption of these components on 1L-MoS2 with a S vacancy commonly induced defective midgap states which quenches the PL (Figure S14). These control calculation results support further the premise that the S vacancies are directly healed by the S atoms of SO2 molecules. Furthermore, for the VS2 healing process, we found that the conversion of VS2 to VS is energetically favorable by approximately 0.61 eV, which shows that VS2 healing is also possible (Figure S15). Our interpretation based on the optical and ADF-STEM measurements and the DFT results is consistent with the results of the control experiment using F4TCNQ, where no repair of the S vacancies was observed due to the absence of available S atoms in the F4TCNQ molecule. Furthermore, the results for the hexagonal 1L-WS2 grains suggested that only S vacancies were repaired, while other types of defects, such as W vacancies and larger void defects, were not repaired after the TFSI treatment. Our mechanism now explains why 12


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previous the TFSI treatment did not effectively increase the PL of 1L-MoSe2 and 1L-WSe2.29 From the combined results of optical characterization, ADF-STEM analysis and DFT calculations of pristine and TFSI-treated 1L-MoS2, we found that the S vacancies of CVDgrown 1L-MoS2 can be effectively repaired by the S atoms from decomposed TFSI molecules. The observed process of S vacancy atomic healing provides a definitive explanation for the mechanism of the PL increase observed for 1L-MoS2 following TFSI treatment and suggests that new lattice repair mechanisms can be used, depending on finding the right chemicals, for other monolayer semiconductors including Se-based TMDs.

Methods. 1L-TMDs synthesis and transfer onto SiO2/Si substrates and TEM grids: 1L-MoS2 and Hexagonal shaped 1L-WS2 with different defect domains in a single grain were grown on SiO2/Si substrates by the CVD method following previously reported protocol.10,11,19 These samples were transferred to clean SiO2/Si substrates and Au TEM grids with 1.2 µm holes (Quantifoil substrate, Ted Pella, Inc.) by the wet transfer method using an aqueous HF solution as the SiO2 etchant and poly (methyl methacrylate) (PMMA).10,11 The PMMA/TMD film on TEM grids was air dried 24 h before removing the PMMA film to achieve good adsorption of TMD on the grid. Chemical treatment: The samples on SiO2/Si substrates and TEM grids were chemically treated by following the previously published method.24,27,29 In brief, 10-3 M solutions of TFSI (Sigma Aldrich) and F4TCNQ (Sigma Aldrich) were prepared in a 9:1 mixture of 1,2-dichlorobenzene and 1,2-dichloromethane. The substrate or TEM grid with samples was immersed in one of these two chemical solutions in tightly sealed vials and heated at 100°C for 15 min on a hot plate. After that, the samples were taken out of the vials

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and dried under a stream of N2 gas. Prior to the STEM measurements, the samples were heated at 180°C for 15 min to remove other organic residues. Optical characterization and XPS measurements:, A lab-made laser confocal microscope equipped with a 0.9 NA objective lens, spectrometer and cooled charge-coupled device was used for confocal PL and Raman imaging and spectroscopy.10,11,50 514 nm laser was used for the PL and Raman measurements with a typical acquisition time of 5 ms and 15 ms per pixel in the spectral images with 150 µW and 420 µW power for 1L-MoS2 on SiO2/Si substrate respectively, while for 1L-WS2, they were 15 µw and 350 µW. Similarly, the PL measurements of the samples on the TEM grids were carried out by a manufactured confocal microscope (Alpha-300s, WITec Instrument GmbH) using a 0.9 NA objective lens with 532 nm laser illumination of 1 µW power. XPS was conducted with a PHI 5000 Versa II XPS with a monochromatic Al X-ray line. ADF-STEM imaging, simulations, and vacancy quantifications: ADF-STEM imaging was performed on an aberration-corrected (S)TEM (ARM200CF, JEOL Ltd.) working at 60 and 80 kV with a probe semiangle of 23 mrad and a detector angle range of ~45 to 180 mrad. Possible damage under electron irradiation was carefully monitored from the serial image recording of the atomic structure of MoS2.51 The structure was found to be intact for up to ~45 s of irradiation, even at an accelerating voltage of 80 kV and a probe current of 25 pA (see Figure S16 and the Supporting Video 1 for more details on the effect of electron beam irradiation). Under this probing condition, the electron dose for high-resolution ADF-STEM imaging was estimated to be ~1.6 x 108 e-/nm2. All the ADF-STEM images used to quantify the S content in the 1L-MoS2 samples were recorded at a scanning rate of 8 µs/pix for a 1024 x 1024 pixel image (8.4 s total acquisition time), which was substantially less than the irradiation time that triggered the radiation-induced structural change. To render our S 14


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quantification results statistically meaningful, we performed multiple measurements over at least ten ADF-STEM images for all the MoS2 samples prepared from different batches. Five representative quantification results were displayed for each MoS2 sample in the respective main figures after confirming that there was no meaningful change in the image intensity quantification. To reduce the error of the intensity analysis, the random baseline noise in the ADF-STEM images was reduced by the Wiener filtering method implemented in a commercial software program (HREM Filter Pro, HREM research Ltd.). ADF-STEM image simulations were performed using the multislice method in the QSTEM software package52 with the same microscope parameters to most closely approximate the experimental conditions. To quantify the S vacancy density in the MoS2 samples before and after the chemical treatment, integrated intensity mapping of every atomic column of Mo and S in the ADF-STEM images using the center of mass tracking method was carried out with commercial software called qHAADF (HREM Research Ltd.), and the histograms were obtained. The values of the intensities as which the atomic sites can be assigned as VS2, Vs, S2, and Mo were specified. The two-dimensional distributions of the VS and VS2 defects were mapped by comparing their intensities with the intensity of a double S site as a reference. Although the peaks in the histogram are distinct, some overlap between them is observed due to the contrast fluctuations owing to possible surface contamination of the sample, the detection noise behavior, instrumental and environmental instabilities and the thermal diffuse scattering of atoms,53 which are unavoidable and contribute to the error in the assignment of the atoms. To verify the vacancy measurement, the simulated ADF-STEM images modeled with the same S vacancy concentration as that determined in the experiment were also quantified, and the results were compared to the experimental results. To detect the chemical species in the pristine and TFSI-treated 1L-MoS2 samples, EDX in ADF-STEM imaging 15


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mode was performed with a highly focused electron probe and a dual-type silicon drift detector (SDD) EDX detector (JED-2300T, JEOL Ltd.) having a large effective solid angle (~1.2 sr.). Each detector has an effective X-ray sensing area of 100 mm2, thus improving the collection efficiency by up to ~10% of the total X-ray signals (4π sr.). To determine if any changes occurred in the chemical state and electronic structure of the 1L-MoS2 sample before and after the chemical treatment, EELS (Gatan GIF Quantum ER 965) of the S L edges was performed in ADF imaging mode. Computational Methods: All calculations were carried out using DFT with the plane wave-based Vienna ab initio simulation package.54 We used the projector augmented-wave method of Blöchl55 in the implementation of Kresse and Joubert.56 The generalized gradient approximation was employed for the exchange-correlation functional. In addition, the semiempirical DFT-D2 method57 was chosen for the description of the long-range weak van der Waals interactions between the molecules and 1L-MoS2. A 4×4×1 supercell consisting of 16 Mo and 31 S atoms was considered to calculate the energy landscape associated with the S vacancy healing process. We used an energy cut-off for the plane wave of 550 eV and Γcentered 2×2×2 k-point meshes for the total energy calculations. The calculations were converged in energy to 10−6 eV/cell, and the structures were allowed to relax until the forces were less than 1×10−2 eV/Å. A vacuum layer of 30 Å was inserted perpendicular to the MoS2 surface to avoid spurious interlayer interactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website at DOI: XPS spectra; Comparison of experimental and simulated STEM images for reliability; STEM 16


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characterization for samples from different batch of growth before and after TFSI treatment; STEM-EDX characterization; Comparison of PL, Raman and STEM images of pristine and F4TCNQ (TFSI) treated 1L-MoS2 (1L-WS2 with different defect domains); DFT calculated energy landscapes for dissociation of TFSI molecule and possible mechanisms of adsorption of its components for defect healing; Monitoring of electron beam induced structural damage in TFSI-treated 1L-MoS2 (Supporting Video 1); Estimation of quantum yield of the sample.

AUTHOR INFORMATON Corresponding Authors *E-mail: [email protected]. Phone: +82-51-510-2227 *E-mail: [email protected]. Phone: +82-31-299-4059 *E-mail: [email protected]. Phone: +82-31-299-4054

Authors Contributions †

S. R., W. C. and S. J. contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by IBS-R011-D1. YMK was also partially supported by the Creative Materials Discovery Program through the NRF (National Research Foundation of Korea) grant (NRF-2015M3D1A1070672) in Korea. JL and JK acknowledge the support of the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIP) (NRF-2015R1C1A1A01053810 (JL), NRF-2018R1D1A1B07042917 (JK)).

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Figure 1. PL characterization. (a) Optical microscopy images of three triangular grains of 1LMoS2 prepared on a TEM grid. The PL intensity images of the 1L-MoS2 grains are overlaid as insets. The scale bars in the PL images represent 5 µm. (b) PL spectra of pristine (black curve) and TFSI-treated (red curves) 1L-MoS2. The PL images and spectra of the pristine and TFSI-treated 1L-MoS2 samples were taken directly of the 1L-MoS2 grains prepared on the TEM grid. The number in each panel indicates the sample number shown in Fig. 1a. The inset is a schematic of a TFSI molecule on 1L-MoS2.

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Figure 2. Raman characterization and estimation of the lattice parameters by DFT. (a) Raman spectra of pristine 1L-MoS2 (top panel) and TFSI-treated 1L-MoS2 (bottom panel). The peak position maps of the E′ and A′1 modes of the TFSI-treated 1L-MoS2 and pristine 1L-MoS2 are shown as insets. The scale bars in the insets represent 5 µm. A redshift and blueshift in the E′ and A′1 modes by 1.1 cm-1 and 0.6 cm-1, respectively, with TFSI treatment is noted. (b) 23


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Schematic of the 1L-MoS2 lattice with (i) one di-S vacancy (VS2), (ii) one mono-S vacancy (VS) and (iii) no vacancies (pristine 1L-MoS2 lattice). The atomic positions and interatomic distances were determined from the results of the geometry optimization calculations by DFT.

Figure 3. Quantification of S and the vacancy concentrations in pristine 1L-MoS2. (a) ADFSTEM images of five pristine 1L-MoS2 samples (top panels) and the distribution maps (bottom panels) of the VS (yellow dot) and VS2 (red dot) defects measured from the corresponding ADF-STEM images. Note that a noticeable antisite defect, MoS2, is observed and is denoted by an arrow on the ADF-STEM images. (b) Site-assigned image intensity histogram of ADF-STEM image no. 1 in panel a. The solid lines are Gaussian fits of the intensity peaks. (c) Measured concentrations of the total VS, VS2, and S in the five pristine 24


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MoS2 samples. Note that the concentrations of VS, VS2, and S in the five ADF-STEM images have variations of only 0.60%, 0.21% and 0.63%, respectively, which indicates that the densities of S vacancies in the MoS2 grains from the same growth batch are very similar.

Figure 4. Quantification of S and the vacancy concentrations in TFSI-treated 1L-MoS2. (a) ADF-STEM images of the five TFSI-treated 1L-MoS2 (top panels) and the distribution maps (bottom panels) of the VS (yellow dot) and VS2 (red dot) defects measured from the corresponding ADF-STEM images. Note that a noticeable antisite defect, MoS2, is observed and is denoted by an arrow on the ADF-STEM images. (b) Site-assigned image intensity histogram of ADF-STEM image no. 1 in panel a. The solid lines are Gaussian fits of the intensity peaks. (c) Measured concentrations of the total VS, VS2, and S in the five TFSItreated MoS2 samples. 25


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Figure 5. TFSI-induced S vacancy healing mechanism. DFT calculation of the energy landscape for the S vacancy healing process.

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“For TOC only”

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Atomic Observation of Filling Vacancies in Monolayer Transition Metal

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