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Jin‐Woo Jung, Hyeon‐Seo Choi, Young‐Jun Lee, Youngjae Kim, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Min‐Yeong Choi, Jae Hyuck Jang, Hee‐Suk Chung, Dohun Kim, Youngwook Kim, Chang‐Hee Cho

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[Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via <i>h</i>‐BN Encapsulations](https://mdr.nims.go.jp/datasets/9204d4ba-e2f9-456d-bfcf-900449133c64)

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Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h‐BN EncapsulationsRESEARCH ARTICLEwww.advancedscience.comDefect Passivation of 2D Semiconductors by FixatingChemisorbed Oxygen Molecules via h-BN EncapsulationsJin-Woo Jung, Hyeon-Seo Choi, Young-Jun Lee, Youngjae Kim, Takashi Taniguchi,Kenji Watanabe, Min-Yeong Choi, Jae Hyuck Jang, Hee-Suk Chung, Dohun Kim,Youngwook Kim, and Chang-Hee Cho*Hexagonal boron nitride (h-BN) is a key ingredient for various 2D van derWaals heterostructure devices, but the exact role of h-BN encapsulation inrelation to the internal defects of 2D semiconductors remains unclear. Here, itis reported that h-BN encapsulation greatly removes the defect-related gapstates by stabilizing the chemisorbed oxygen molecules onto the defects ofmonolayer WS2 crystals. Electron energy loss spectroscopy (EELS) combinedwith theoretical analysis clearly confirms that the oxygen molecules arechemisorbed onto the defects of WS2 crystals and are fixated by h-BNencapsulation, with excluding a possibility of oxygen molecules trapped inbubbles or wrinkles formed at the interface between WS2 and h-BN. Opticalspectroscopic studies show that h-BN encapsulation prevents the desorptionof oxygen molecules over various excitation and ambient conditions, resultingin a greatly lowered and stabilized free electron density in monolayer WS2crystals. This suppresses the exciton annihilation processes by two orders ofmagnitude compared to that of bare WS2. Furthermore, the valley polarizationbecomes robust against the various excitation and ambient conditions in theh-BN encapsulated WS2 crystals.1. IntroductionMonolayer transition metal dichalcogenides (TMDs) haveemerged as a platform to examine various exciton species suchas trions, biexcitons, interlayer excitons, and moiré excitons dueJ.-W. Jung, H.-S. Choi, Y.-J. Lee, D. Kim, Y. Kim, C.-H. ChoDepartment of Physics and ChemistryDaegu Gyeongbuk Institute of Science and Technology (DGIST)Daegu 42988, South KoreaE-mail: chcho@dgist.ac.krY. KimSchool of PhysicsKorea Institute for Advanced Study (KIAS)Seoul 02455, South KoreaThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/advs.202310197© 2024 The Authors. Advanced Science published by Wiley-VCH GmbH.This is an open access article under the terms of the Creative CommonsAttribution License, which permits use, distribution and reproduction inany medium, provided the original work is properly cited.DOI: 10.1002/advs.202310197to the strong inter-particle interactions.[1−4]Furthermore, the valley-dependent opticalselection rules given by the broken inver-sion symmetry enable the selective gen-eration of excitons in the particular val-ley (+K or −K) using circularly polarizedlight,[5] providing the opportunity for ap-plications toward valleytronic devices. How-ever, the external disorders in close proxim-ity of monolayer TMDs such as substrate-induced surface roughness and absorbatescan significantly alter the excitonic prop-erties of two-dimensional (2D) TMD ma-terials, hindering the observation of theunique properties in monolayer TMDs.[6,7]In an attempt to reduce the disorderfrom substrates, it has been proposedto encapsulate TMD materials usinghexagonal boron nitride (h-BN) layers.[8,9]Recently, it has been shown that h-BN en-capsulation enables to observe the intrinsicoptical properties of monolayer TMDs,including the excitonic linewidth withhomogeneous broadening limit[9] and the suppression of excitonannihilation processes.[10] As the origin, the reduced substratedisorders have often been suggested in the previous works,[9,10]but the excitonic properties of TMDs on h-BN substratesshow large discrepancies from those of TMDs encapsulated byT. TaniguchiInternational Center for Materials NanoarchitectonicsNational Institute for Materials ScienceTsukuba 305-0044, JapanK. WatanabeResearch Center for Functional MaterialsNational Institute for Materials ScienceTsukuba 305-0044, JapanM.-Y. Choi, J. H. Jang, H.-S. ChungElectron Microscopy and Spectroscopy TeamKorea Basic Science InstituteDaejeon 34133, South KoreaJ. H. JangGraduate School of Analytic Science and TechnologyChungnam National UniversityDaejeon 34134, South KoreaAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (1 of 10)http://www.advancedscience.commailto:chcho@dgist.ac.krhttps://doi.org/10.1002/advs.202310197http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadvs.202310197&domain=pdf&date_stamp=2024-03-17www.advancedsciencenews.com www.advancedscience.comh-BN.[11,12] On the other hand, previous investigations haveshown that the oxygen molecules on the surface of TMDsalso alter the electronic and optical properties of 2D TMDmaterials.[13−18] The adsorption of the oxygen molecules occurson the defects with relatively lower kinetic barrier rather thanthe perfect sites of TMD materials,[18] and the oxygen moleculesunlike other molecules can only be chemisorbed at chalcogenvacancies due to isovalent valence electrons (two unpaired elec-trons) with the chalcogen atom.[14,15] This chemisorption of oxy-gen molecules at the defect sites removes the defect-related gapstates without significantly altering the electronic band structuresof TMD materials.[13,14] Thus, the oxygen molecules, that are sup-plied during the exposure of TMDs into the atmosphere, can sig-nificantly change the properties of defect-related states throughthe chemical adsorption process. In this regard, h-BN encapsu-lation can play a crucial role in the defect states of TMDs, inwhich the h-BN layers fixate the adsorbed oxygen molecules onthe TMD defects and facilitate the interaction between the oxy-gen molecules and the defect states. However, the role of h-BNencapsulation in relation to the defects of TMDs remains unex-plored.In this work, we found that h-BN encapsulation stabilizesthe chemisorbed oxygen molecules on the defect sites of mono-layer WS2 crystals, which greatly passivates the defect-relatedgap states along with the decrease in the free electron density.Electron energy loss spectroscopy (EELS) combined with the-oretical analysis clearly reveals that the oxygen molecules arechemisorbed onto the defects of WS2 crystals and are fixated by h-BN encapsulation, that excludes a possibility of oxygen moleculestrapped in bubbles or wrinkles formed at the interface betweenWS2 and h-BN. Optical spectroscopic studies show that h-BN en-capsulation prevents the desorption of oxygen molecules over var-ious excitation and ambient conditions, resulting in a greatly low-ered and stabilized free electron density in monolayer WS2 crys-tals. This suppresses the exciton annihilation processes by twoorders of magnitude compared to that of bare WS2. Furthermore,due to the stabilized free electron density in the h-BN encapsu-lated WS2 crystals, the valley polarization becomes robust againstthe elevated excitation condition.2. Results and DiscussionFigure 1a shows a schematic illustration showing that the fixatedoxygen molecules by the h-BN layers effectively passivate defectsof the WS2. We considered the chemisorption type, where theoxygen molecule chemically bonds to three surrounding tung-sten atoms, which is the most common configuration for theoxygen chemisorption (inset image of Figure 1a) in the S-basedTMDs.[14−17] The chemisorbed oxygen molecules at the chalco-gen vacancies can also be dissociated into two oxygen atoms,leading to a dissociative chemisorption, which occupies the sul-fur vacancies with the dissociated oxygen atoms.[13−15] However,in the case of the WS2 used in our study, the kinetic barrier forthe O2-chemisorption (0.56 eV) is lower than that for the O2-dissociative chemisorption process (0.76 eV). It is estimated thatthe probability of the O2-chemisorption is 1000 times higher thanthat of the dissociative chemisorpiton (see Figure S1, Support-ing Information). Thus, the oxygen chemisorption on the mono-layer WS2 crystals would have the final configuration of the O2-chemisorption rather than the O2-dissociative chemisorption.[14]The major molecules such as N2, O2, and H2O in air can beweakly physisorbed at both the pristine surface and defect sitesof WS2. However, this physisorption has virtually no influenceon the electronic and optical properties of the WS2 monolayerdue to easy desorption of physisorbed molecules.[14] In addi-tion, our first-principle calculations demonstrate that the oxy-gen molecules can only be chemisorbed onto the defects (sul-fur vacancy) and attain a fully stable chemisorption state, indi-cating that the oxygen molecules can be majorly adsorbed ontothe WS2 in the air. The detailed theoretical calculation resultson molecular interactions with the sulfur vacancy and pristinesurface of WS2 are provided in Sections S2 and S3 (Support-ing Information). To investigate the role of oxygen fixation inthe excitonic properties of monolayer WS2 with excluding theeffects of disorders induced by the substrates, we studied h-BNencapsulated WS2 crystals suspended on line trenches with alinewidth of 1.8 μm in comparison with bare WS2, as shownin Figure 1b,d. Scanning electron microscope images confirmthe suspended structures for both the bare (Figure 1b) and h-BN encapsulated (Figure 1d) WS2 crystals on the line trenches(see Figure S4, Supporting Information). The monolayered WS2crystals used in this study were grown on sapphire substrates us-ing a chemical vapor deposition (CVD) method.[19] Mechanicallyexfoliated h-BN flakes with a thickness of ≈40 nm were usedas the encapsulating layers in the h-BN/WS2/h-BN structures.The detailed sample preparation processes are described in themethods. Figure 1c,e display the spatial photoluminescence pro-files of the bare (Figure 1c) and h-BN encapsulated (Figure 1e)WS2 suspended on the line trenches, respectively. The steady-state photoluminescence measurements were carried out at a lowlevel of excitation (≈0.065 kW cm−2) to rule out the heating ef-fect. It is worth noting that the photoluminescence intensity be-comes stronger in the suspended regions than in the supportedregions for both the bare and h-BN encapsulated WS2 crystalsdue to the enhanced local field effect by optical interference inthe trench region.[7] To confirm the exciton species, the photolu-minescence spectra were measured at a cryogenic temperatureof 77 K under a vacuum level of ≈1 × 10−5 Torr, as shown inFigure 1f. For the h-BN encapsulated WS2, the neutral exciton(X0) and the trion (X−) are identified at energies of 2.042 and2.001 eV, respectively, while the bare WS2 shows three speciesof the neutral exciton (X0), trion (X−), and defect-related trappedexciton (L) at 2.087, 2.042, and 2.026 eV, respectively. The energyof neutral exciton was assigned by measuring the differential re-flectance spectra (see Figure S5, Supporting Information), andthose of trion and defect-related trapped exciton were identifiedby the energy differences from the neutral exciton.[20,21] Com-pared to the bare WS2 showing the emission prevailed by thetrion and the defect-related trapped exciton, the h-BN encapsu-lated WS2 exhibits the predominant emission from the neutralexciton with homogeneous linewidth of the 6 meV,[9] indicat-ing that the defect-induced free electrons and inhomogeneousbroadening are substantially reduced by the encapsulating WS2with h-BN layers. These results indicate that the oxygen fixationby h-BN encapsulation can play a crucial role in the defect re-moval with reducing the free electron density. To directly confirmthe fixation effect of the adsorbed oxygen molecules on the de-fects, the change in excitonic spectra was monitored under theAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (2 of 10) 21983844, 2024, 22, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202310197 by National Institute For, Wiley Online Library on [16/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 1. a) Schematic illustration showing the chemisorbed oxygen molecules anchored by the h-BN encapsulation. Right inset image represents thedetailed atomic configuration of the chemisorbed oxygen molecule at the sulfur vacancy. b) Scanning electron microscope image of the bare WS2 crystalson the line trenches. c) Spatial photoluminescence profile measured from the bare WS2 on the line trench. d) Scanning electron microscope image ofthe h-BN encapsulated WS2 crystals on the line trench. e) Spatial photoluminescence profile measured from the h-BN encapsulated WS2 on the linetrenches. The yellow and black dashed lines marked in (b,d) and (c,e) indicate the boundary of the suspended and supported regions. The scale bars of(b,d) are 1 μm. f) Photoluminescence spectra for the bare (top panel) and h-BN encapsulated (bottom panel) WS2 measured at a cryogenic temperatureof 77 K under a vacuum level of ≈1 × 10−5 Torr. g) Photoluminescence spectra for the bare (top panel) and h-BN encapsulated (bottom panel) WS2measured under different ambient conditions of air and vacuum (T = 300 K). Each photoluminescence spectrum was fitted by Lorentzian functions. Theblack, olive, and blue dashed lines represent the neutral exciton (X0), trion (X−), and defect-related trapped exciton (L) states, respectively.different ambient conditions of air and vacuum, as shown inFigure 1g (see Figure S6, Supporting Information). Striking fea-tures are observed for the bare WS2 crystals, showing that thespectral weight of neutral excitons is predominant over that oftrions under ambient air condition, whereas that of trions be-comes larger than that of neutral excitons under vacuum. Theseresults indicate that the oxygen adsorbates on the defect sitesare released by changing the ambient condition from air to vac-uum, raising the density of free electrons in the bare WS2 crystalsunder vacuum.[16,17] As shown in Figure S7 (Supporting Infor-mation), the h-BN encapsulated WS2 samples fabricated underan inert (N2) environment exhibit much stronger trion intensity(higher free electron concentration) compared to that of the h-BN encapsulated WS2 fabricated in the air. The spectral featureis very similar to that of the bare WS2 measured in the vacuumenvironment (see top panels in Figure 1f,g). Furthermore, the ex-foliated monolayer WS2 and WSe2 with a lower density of chalco-gen vacancies give rise to a less change in the free electron den-sity against the variation of the ambient conditions, implying thatthe oxygen molecules are mostly adsorbed on the chalcogen va-cancies (see Figure S8, Supporting Information). In contrast, theh-BN encapsulated WS2 crystals exhibit almost the same spec-tra prevailed by the neutral excitons regardless of the ambientconditions, highlighting that the h-BN encapsulation effectivelyremoves the internal defects by stabilizing the oxygen moleculesadsorbed onto WS2 crystals.The EELS analysis for oxygen K-edge confirms that the h-BNencapsulation anchors the chemisorbed oxygen molecules ontothe defects of monolayer WS2 crystals. Figure 2a displays theEELS spectra of the oxygen K-edge for the h-BN encapsulatedWS2, bare WS2, and h-BN flake crystals. The h-BN encapsulatedWS2 crystals show the oxygen K-edge peaks centered at 538 and556 eV, whereas any oxygen-related features are not observed forthe bare WS2 and the h-BN crystals. This suggests that the ad-sorbed oxygen molecules on WS2 are fixated by the h-BN encap-sulation (see Figure S9, Supporting Information). Figure 2b–eshows the EELS maps for the boron (b), nitrogen (c), oxygen (d)K-edge, and the sulfur (e) L-edge measured from the h-BN en-capsulated WS2. Note that the elemental signal displayed for apixel in the EELS map is the sum of the elemental signals de-tected through 2D scanning using an electron beam with a spa-tial resolution of 1 Å over an area of 20 nm × 20 nm. The spatialAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (3 of 10) 21983844, 2024, 22, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202310197 by National Institute For, Wiley Online Library on [16/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 2. a) Experimental oxygen K-edge EELS spectra for the h-BN encapsulated WS2, bare WS2, and h-BN flake crystals. b–e) The EELS maps for theboron (b), nitrogen (c), oxygen (d) K-edge, and the sulfur (e) L-edge measured from h-BN encapsulated WS2. The scale bars are 100 nm. f) The calculatedoxygen K-edge EELS spectrum 𝜖2(𝜔) for a physisorbed oxygen molecule on the pristine WS2. g) The calculated oxygen K-edge EELS spectrum 𝜖2(𝜔) (reddashed) for a chemisorbed oxygen molecule on the sulfur vacancy (O2-SV) of the WS2. The experimental oxygen K-edge spectrum (blue line) for theh-BN encapsulated WS2 is in good agreement with the calculation result (red dashed). The insets of (f,g) represent the real-space orbital distributions𝜌(r)E of the 𝜋* (E ≈ 530 eV) and 𝜎* (E ≈ 539 eV) for (f) and the A (E ≈ 538 eV) and B (E ≈ 555 eV) for (g), respectively.EELS maps confirm that the oxygen K-edge signal is markedlyweak compared to those of other elements, and is randomly dis-tributed in the 2D plane of h-BN encapsulated WS2 samples. Thisstrongly indicates that the oxygen molecules are chemisorbed onthe randomly distributed local defects in WS2 crystals (see FigureS10, Supporting Information).To discover the adsorption types of the adsorbed oxygenmolecules by h-BN encapsulation, we demonstrate the theoret-ical EELS spectra of oxygen K-edge for the physisorbed oxygenmolecule on the pristine WS2 (Figure 2f) and chemisorbed oxy-gen molecule at the sulfur vacancy site of the WS2 (Figure 2g). Tounderstand the underlying physics for the EELS results, we per-formed theoretical EELS calculations based on the first-principlescalculations implemented in the full-potential linearized planewave (FLAPW) + local orbitals with the ELK code. Here, we usedthe pseudo core-hole method, where the self-consistent calcula-tion is made in terms of one of the oxygen nuclei constrainedto be positively charged (+1e) and an additional electron (−1e)is simultaneously constrained to occupy the conduction orbitals.After the self-consistent calculation, the Kohn-Sham orbitals be-come well defined, and then we compute the photon-absorptionmatrix elements between the core orbital (s-orbital) of the oxygenAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (4 of 10) 21983844, 2024, 22, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202310197 by National Institute For, Wiley Online Library on [16/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comnuclei and the unoccupied conduction bands, which correspondsto the dielectric function for EELS spectra (see Section S11, Sup-porting Information for more detail).The calculated EELS spectrum for the physisorbed oxygenmolecule on the pristine WS2 indicates the two main peaks at530 and 539 eV, which originate from the transition of the coreelectrons to two kinds of trivial hybridized states in the oxy-gen molecule, featured as anti-bonding orbital 𝜋* (top) and 𝜎*(bottom) distributions, respectively (see the inset of Figure 2f),whereas the calculated oxygen K-edge peaks for the chemisorbedoxygen molecule appear at the energy loss positions of 538 and555 eV (labelled as A and B on the spectrum), respectively. Sincethe core-hole excitation makes the oxygen molecule to have anasymmetric potential, the 𝜋* orbital distribution in the inset re-flects asymmetric densities. As shown in Figure 2g, the calculatedEELS spectrum (red dashed) for a chemisorbed oxygen moleculeis in good agreement with that of the h-BN encapsulated WS2(blue line), indicating that the fixated oxygen molecules by theh-BN encapsulation are chemisorbed at the defect sites of theWS2. The anti-bonding orbital 𝜋* peak (530 eV) is absent forthe chemisorbed oxygen molecule in the calculated EELS spec-trum. The disappearance of 𝜋-bonding is commonly interpretedas a transformation of bonding sequences.[22] In our study, itis found that the hybridizations between the oxygen moleculeand the surrounding tungsten atoms directly suppress the 𝜋*peak. As shown in the top inset of Figure 2g, the real-space or-bital distribution for the peak A resembles an orbital shape ofthe 𝜎* peak for the physisorbed oxygen molecule, indicating thatthe hybridizations between the oxygen molecule and the tung-sten atoms induce 𝜎* antibonding energy state similar to thatof the physisorbed oxygen molecule. In contrast, for the peak B,the real-space orbitals show highly delocalized distribution forboth the oxygen molecule and the WS2 crystal (the bottom insetof Figure 2g), implying that the peak B is due to the transitionof the core electrons to continuum bands contributed from boththe oxygen molecules and the WS2 crystals.The investigation of the exciton recombination processesagainst the excitation power for the bare and h-BN encapsu-lated WS2 crystals reveals that the oxygen fixation suppressesthe nonradiative decay for the neutral excitons by stabilizing thefree electron density of the WS2. Figure 3a,b shows the double-logarithmic plots of the neutral exciton (X0) emission intensityof the bare and h-BN encapsulated WS2 crystals as a function ofthe excitation power density under ambient air and vacuum con-ditions at room temperature, respectively. The bare WS2 crystalin air exhibits a sublinear increase with an exponent of 0.71 ina power-law (I ∝ P𝛼 , P: excitation power), while that in vacuumshows a smaller exponent of 0.32. The power dependence canbe understood by introducing a simple rate equation model forthe steady-state photoluminescence. The rate equation for exci-ton generation and recombination can be described by[23,24]G =nX𝜏X+ TnX ne + AnX ne + 𝛾n2X (1)where nX is the neutral exciton density, 𝜏X is the exciton lifetime,ne is the electron density, T is the trion formation coefficient, Ais the exciton-electron Auger coefficient, and 𝛾 is the exciton–exciton annihilation coefficient. At our excitation power range,Figure 3. a,b) Neutral exciton (X0) emission intensity of the bare (a) andh-BN encapsulated (b) WS2 crystals as a function of the excitation powerdensity at room temperature. The blue diamond and pink circle symbolsindicate the power-dependent exciton emission intensity measured underambient air and vacuum conditions. c) Photoluminescence intensity ratio(X−/X0) of the trion to the neutral exciton as a function of the excitationpower density. d) Free electron density as a function of the excitation powerdensity by using the mass action law. The pink square (blue triangle) andolive diamond (purple circle) symbols represent the X−/X0 and free elec-tron density of the bare and h-BN encapsulated WS2 under vacuum (air)ambient conditions, respectively. The error bars in (a−d) indicate the stan-dard deviation of the measured data.the exciton density is estimated to be from the low 1010 to the low1011 cm−2. In this range of exciton density, it has been known thatthe exciton-exciton annihilation process becomes negligible.[25]Furthermore, the recent work has shown that the photogeneratedexcitons are mostly converted to the trions in this range, indicat-ing that the Auger process can also be neglected.[24] Thus, thepredominant nonradiative decay channel of the neutral excitonsis the exciton-to-trion conversion process (see Figure S13, Sup-porting Information). When the free electron density increaseswith the power dependence of P𝜇 , the neutral excitons are con-verted to the trions in proportional to P𝜇 , and the neutral exci-ton emission intensity follows the power-law of P1−𝜇 (see Sec-tion S12, Supporting Information). The sublinear increase in theexciton emission in bare WS2 indicates that the free electrondensity increases with increasing excitation power density by re-leasing oxygen molecules chemisorbed on defect sites, promot-ing the trion conversion process. The smaller exponent furtherconfirms the easier desorption under ambient vacuum condi-tion. Interestingly, the h-BN encapsulated WS2 shows a linearincrease with an exponent of 0.99 for both the air and vacuumambient conditions, as shown in Figure 3b. This indicates thatAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (5 of 10) 21983844, 2024, 22, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202310197 by National Institute For, Wiley Online Library on [16/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comh-BN encapsulation results in predominant neutral exciton emis-sion over the decay channels without generating free electrons bydesorption.[16,24] Figure 3c shows the photoluminescence inten-sity ratio (X−/X0) of the trion to the neutral exciton as a func-tion of the excitation power density. The X−/X0 of bare WS2 mea-sured under ambient vacuum condition steeply increases from0.38 to 4.18, while the X−/X0 under the air environment gradu-ally increases from 0.25 to 0.50 over the excitation power densityrange from 0.03 to 0.66 kW cm−2. These further evidences thatthe desorption process, which induces free electrons and exciton-to-trion conversion, is much facilitated in vacuum than in air. Instriking contrast, the X−/X0 of h-BN encapsulated WS2 is keptalmost constant at ≈0.02 over the excitation power densities un-der both the ambient air and vacuum conditions, which is greatlyreduced by more than one order of magnitude compared to thatof the bare WS2. This clearly indicates that h-BN encapsulationprevents the desorption of oxygen molecules by the laser illumi-nation, resulting in predominant recombination by neutral exci-tons. By considering the intensity weight (IX−∕Itotal) of the trionin the photoluminescence spectra, we estimated the free electrondensity as a function of the excitation power density by using themass action law (see Section S14, Supporting Information).[26,27]As shown in Figure 3d, the free electron density exhibits a verysimilar trend as X−/X0. For bare WS2, the free electron den-sity drastically increases and reaches 2.66 × 1013 cm−2 at an ex-citation power density of 0.66 kW cm−2 in vacuum, which isclose to the Mott density (≈1 × 1014 cm−2),[28] while gradually in-creasing to 7.38 × 1012 cm−2 at an excitation power density of4.99 kW cm−2 in air. However, regardless of the ambient con-ditions, the free electron density of h-BN encapsulated WS2 ismaintained at ≈9 × 1010 cm−2 with increasing excitation power,resulting from the fixation of chemisorbed oxygen molecules byh-BN layers. Additonally, using the bare and h-BN encapsulatedWS2 capacitor devices, we estimated the free electron densitiesin the bare and h-BN encapsulated WS2 at the vacuum and airenvironments. For the bare WS2, the electron densities were es-timated to be 1.15 × 1013 cm−2 (vacuum) and 3.69 × 1012 cm−2(air) at Vg≅0 V (gate voltage), respectively, while those of h-BNencapsulated WS2 were estimated to be 2.05 × 1011 cm−2 atboth the vaccum and air environments. These electron densi-ties were in good agreement with those estimated using themass-action law. From the difference in free electron densitiesin-between the vacuum and air environments, we estimated thenumber of desorbed/adsorbed oxygen molecules on the sulfurvacanies to be 7.17 × 1012 cm−2 (see Section S15, SupportingInformation).The nonradiative decay channel of neutral excitons can be at-tributed to exciton-to-trion conversion at our excitation powerrange. From the rate Equation (1), the total recombination rate(RT) can be described by RT = R0 + RAnX, where R0 is thedensity-independent recombination rate and RA is the excitonannihilation rate constant due to the exciton-to-trion conversionprocess. The role of the oxygen fixation can be quantitativelyunderstood by estimating the exciton annihilation rate constant(RA). To measure the exciton lifetime with increasing excitationpower, we carried out time-resolved photoluminescence spec-troscopy. For bare WS2, the photoluminescence decay curves ofneutral excitons under air ambient condition show a steep short-ening of the decay time from 74 to 43 ps with increasing pumpfluence from 27 to 959 nJ cm−2, as shown in Figure 4a. However,the exciton decay curves for h-BN encapsulated WS2 show a slightdecrease in the lifetime above the pump fluence of 421 nJ cm−2,as shown in Figure 4b. It is noteworthy that the exciton lifetime inh-BN encapsulated WS2 becomes longer than that of bare WS2,showing exciton lifetime of 136 ps in air at a pump fluence of27 nJ cm−2. The decay spectra at vacuum are shown in FigureS16 (Supporting Information). The increase in exciton lifetimein the h-BN encapsulated WS2 strongly suggests that the non-radiative decay by the trion formation, which occurs on a veryfast time scale of a few ps, is significantly inhibited due to thepassivation of defects by the oxygen fixation.[23,29,30] To quanti-tatively evaluate the exciton annihilation rate constant (RA), theexciton density-induced recombination rate (𝜏−1) was plotted asa function of the exciton density using the measured exciton life-times (see Section S17, Supporting Information).[30] Note thatthe exciton density was estimated by calculating the net absorp-tion in the monolayer WS2 for the pump fluence.[31,32] As pre-sented in Figure 4c,d, the h-BN encapsulated WS2 crystals ex-hibit an exciton annihilation rate constant of 8.3 × 10−3 cm2 s−1(7.8 × 10−3 cm2 s−1), while the bare WS2 crystals show a valueof 8.0 × 10−2 cm2 s−1 (3.2 × 10−1 cm2 s−1) under air (vacuum)ambient conditions. As expected, the exciton annihilation rateconstant of the h-BN encapsulated WS2 is remarkably reducedby approximately two orders of magnitude compared to that ofthe bare WS2. These results are attributed to the suppressionof exciton-to-trion conversion process in the h-BN encapsulatedWS2 due to the greatly lowered and stabilized free electron den-sity. This fact can be additionally verified by investigating the de-cay dynamics of neutral excitons with an electrostatic doping inthe h-BN encapsulated WS2 capacitor devices at a fixted excia-tion power. Figure 4e shows the gate-voltage-dependent photolu-minescence spectral map in the h-BN encapsulated WS2 capaci-tor devices, showing that the charge neutral point is determinedat Vg≅0 V. The increase in the gate voltage (Vg > 0 V) gives riseto the gradual decrease in the emission intensity for the neutralexciton and, simultaneously, the increase in the emission inten-sity for the trion. This indicates that the increase in free electrondensity facilitates the exciton-to-trion conversion process, leadingto the nonradiative decay of neutral excitons. The gate-voltage-dependent photoluminescence decay curves of neutral excitonsalso clearly show that the increase in the free electron densitypromotes the exciton annihiliation. As shown in Figure 4f, thedecay time of neutral excitons in the h-BN encapsulated WS2 ca-pacitor devices becomes steeply shorten from 139 to 36 ps forincreasing the gate voltage from 0.5 to 0.9 V. The exciton annihi-lation rate constant with the electrostatic doping is estimated tobe 1.3 × 10−1 cm2 s−1 (see Section S18, Supporting Information),which is similar to that of bare WS2 under the vacuum ambientcondition.The h-BN encapsulation gives rise to an almost constant levelof the free electron density in WS2 crystals for elevated excitationpowers, which can provide a stable and robust valley polarizationagainst various excitation conditions. In contrast, for bare WS2,the drastic increase in free electron density with increasingexcitation would cause a large variation in the valley polarizationdue to the change in decay dynamics of the neutral excitonscaused by the exciton-to-trion conversion.[33,34] To investigatethe effect of h-BN encapsulation on the valley polarization, weAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (6 of 10) 21983844, 2024, 22, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202310197 by National Institute For, Wiley Online Library on [16/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 4. a,b) Photoluminescence decay curves of the neutral excitons measured as a function of the energy fluence for bare (a) and h-BN encapsulated(b) WS2 under ambient air condition. c,d) Exciton density-induced recombination rate (𝜏−1) for the bare (c) and h-BN encapsulated (d) WS2 underambient air and vacuum conditions, resulting in the exciton annihilation rate constant (RA) due to exciton-to-trion conversion process by a linear fit. e)Gate-voltage-dependent photoluminescence spectral map of the h-BN encapsulated WS2 capacitor devices. The black arrow indicates the charge neutralpoint of the h-BN encapsulated WS2 capacitor device. f) Gate-voltage-dependent photoluminescence decay curves of the neutral excitons in the h-BNencapsulated WS2 capacitor device.carried out circular polarization-resolved photoluminescencemeasurements as a function of the excitation power density at77 K. Figure 5a show the circularly polarized photoluminescencespectra measured from the bare (top panel) and h-BN encapsu-lated (bottom panel) WS2, respectively. As a result, the degree ofvalley polarization with increasing excitation power shows a verydifferent trend for the bare and h-BN encapsulated WS2 crystals,as shown in Figure 5b. An important distinction is that the h-BNencapsulated WS2 exhibits a stable valley polarization ratio ata constant level, whereas that of the bare WS2 shows a largeAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (7 of 10) 21983844, 2024, 22, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202310197 by National Institute For, Wiley Online Library on [16/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 5. a) Circularly polarized photoluminescence spectra for the bare (top panel) and h-BN encapsulated (bottom panel) WS2 measured at 77 K. b)Degree of valley polarization for the exciton emission in the bare and h-BN encapsulated WS2 taken as a function of the excitation power density. c)Degree of valley polarization for the neutral excitons in the h-BN encapsulated WS2 capacitor devices taken as a function of the gate voltage. The errorbars marked in (b,c) exhibit the standard deviation of the measured valley polarization values.variation by changing the excitation power. The valley polariza-tion can be described by the following equation PV = P0/(1 +2𝜏X/𝜏V) ,[35] where P0 is the initial valley polarization, 𝜏X is thevalley exciton lifetime, and 𝜏V is the valley relaxation time. Theinitial valley polarization (P0) given by the optical selection rulescan be assumed to be unity,[5] meaning that the valley polariza-tion (PV) is then mainly governed by the competition betweenthe exciton lifetime and the valley relaxation time.[35] As shownin Figure 5b, the degree of valley polarization of bare WS2 is 1.5times higher than that of h-BN encapsulated WS2 at the lowestexcitation power density of 0.17 kW cm−2. This can be attributedto the fact that in bare WS2, the neutral excitons decay rapidlywithin the valley, rather than an intervalley scattering, due to ashortened exciton lifetime caused by a higher exciton-to-trionconversion rate. In addition, the large variation in the valley po-larization of bare WS2 can be understood as a result of the changein the exciton lifetime by the accelerated trion conversion processwith increasing excitation power, as shown in Figure 4a,c.[33,34]Accordingly, the valley polarization can be increased in the bareWS2 for elevated excitation powers. For the h-BN encapsulatedWS2, however, the exciton-to-trion conversion and the excitonlifetime are maintained at almost constant levels, resulting ina stable valley polarization ratio for elevated excitation powers.The large variation in the valley polarization is also observed inthe h-BN encapsulated WS2 capacitor devices with the electro-static doping (Figure 5c), showing the drastic decrease in theexciton lifetime as the gate voltage increases (see Section S19,Supporting Information). This also confirms that the change inthe free electron density leads to the large variation in the valleypolarization.3. ConclusionIn conclusion, we have demonstrated that h-BN encapsula-tion greatly removes the defect-related gap states by stabilizingthe chemisorbed oxygen molecules onto the defects of mono-layer WS2 crystals, that are provided during the interactions be-tween WS2 and atmosphere. It is clearly shown that the oxygenmolecules are chemisorbed onto the defects of WS2 crystals andare fixated by h-BN encapsulation with excluding a possibilityof oxygen molecules trapped in bubbles or wrinkles formed atthe interface between WS2 and h-BN, as confirmed by the EELSstudy. Optical spectroscopic studies show that h-BN encapsula-tion prevents the desorption of oxygen molecules over variousexcitation and ambient conditions, resulting in a greatly loweredand stabilized free electron density in monolayer WS2 crystals.This suppresses the exciton annihilation processes by two ordersof magnitude compared to that of bare WS2. Furthermore, dueto the stabilized free electron density in the h-BN encapsulatedWS2 crystals, the valley polarization becomes robust against thevarious excitation and ambient conditions. Our findings provideinsight into the role of h-BN encapsulation and open up the pos-sibility to control the defect states in 2D semiconductors throughadsorbate-engineered 2D heterostructures.4. Experimental SectionSample Preparation: The monolayer WS2 crystals were grown on asapphire substrate by chemical vapor deposition methods,[19] and theh-BN flakes with a thickness of ≈40 nm were prepared by mechanicalAdv. Sci. 2024, 11, 2310197 © 2024 The Authors. Advanced Science published by Wiley-VCH GmbH2310197 (8 of 10) 21983844, 2024, 22, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/advs.202310197 by National Institute For, Wiley Online Library on [16/06/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comexfoliations from bulk h-BN single crystals. The h-BN encapsulated WS2structure was fabricated by sequential pick-up processes using the dryvan der Waals stacking method. Elvacite resin or polycarbonate (PC)was utilized as a polymer stamp for the pick-up of layered materials. Theassembled h-BN/WS2/h-BN structures and picked-up WS2 crystals werereleased onto the line trenches by melting the polymer stamp, wherethe line trenches were fabricated through conventional photolithographyand reactive ion etching processes using 400-nm-thick SiO2-coatedSi substrates. The polymer stamps were removed by immersing thefabricated sample in chloroform. Finally, both the h-BN encapsulated andthe bare WS2 onto line trenches were annealed at 350 °C to improve thecoupling between the stacked layers and remove transfer residues.Optical Measurements: The steady-state and time-resolved photolu-minescence measurements were performed using a home-built confocalmicrophotoluminescence system. Using a 40× (0.6 NA) objective (Nikon),the excitation beam was focused, and the signal from the samples wascollected through an optical fiber on the focal image plane. For the steady-state photoluminescence measurements, an argon-ion laser with a wave-length of 457.9 nm (continuous wave) was used as an excitation source.The photoluminescence spectra were resolved by a spectrometer (ActonSpectraPro 500i with 0.5 m focal length and 1200 grooves/mm grating)equipped with a charge-coupled device (CCD) detector (Princeton Instru-ments, 512 × 2048 pixels). For the circular polarization measurements, acombination of linear polarizer and quarter waveplate was used to gen-erate circularly polarized excitation light, while another pair of linear po-larizer and quarter waveplate was set for a polarization analyzer beforecollecting the signal through the slit of the spectrometer. The degree of val-ley polarization is defined as 𝜌 = [(PL(𝜎+) − PL(𝜎−)]/[PL(𝜎+) + PL(𝜎−)],where PL(𝜎±) are the photoluminescence intensity for 𝜎+ and 𝜎− polar-ized light components under excitation with a 𝜎+ or 𝜎− polarized laserbeam. The time-resolved photoluminescence measurements were carriedout using a picosecond pulsed diode laser (PicoQuant, LDH-P-FA-355)with a wavelength of 355 nm (FWHM = 56 ps) and repetition rate of40 MHz. The exciton lifetimes were measured using a hybrid photomulti-plier detector (PicoQuant, PMA hybrid series) and a time-correlated singlephoton counting system (PicoQuant).Scanning Transmission Electron Microscopy (STEM) and EELS Analysis:STEM was used by Monochromated ARM-200F (NEO-ARM) in Korea Ba-sic Science Institute (KBSI) operated at 200 kV. Gatan imaging filter (GIF)Continuum HR-1066 spectrometer was used to collect electron energy lossspectraSupporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was supported by the Basic Science Research Program(2019R1A2C1088525) and the BrainLink program (RS-2023-00236798)through the National Research Foundation of Korea, by the DGIST R&DProgram (23-CoE-NT-01 and 23-HRHR+-03) funded by the Ministry ofScience and ICT of the Korean Government. K.W. and T.T. acknowledgesupport from JSPS KAKENHI (Grant Numbers 19H05790, 20H00354, and21H05233) and A3 Foresight by JSPS. Y.K. was supported by National Re-search Foundation of Korea (2020R1C1C1006914). J.H.J. and H.S.C. weresupported by the Technology Innovation Program (20010542) funded bythe MOTIE, Korea. The authors thank the computational support from theCenter for Advanced Compuation (CAC) at Korea Insitute for AdvancedStudy (KIAS). Y.K. supported by a KIAS individual Grant (PG088601).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the cor-responding author upon reasonable request.Keywordschemisorption, defect passivation, hexagonal boron nitride, oxygenmolecule, transition metal dichalcogenideReceived: January 14, 2024Revised: February 25, 2024Published online: March 17, 2024[1] Y. You, X. X. Zhang, T. C. Berkelbach, M. S. Hybertsen, D. R.Reichman, T. F. Heinz, Nat. Phys. 2015, 11, 477.[2] J. G. Kim, W. S. Yun, S. Jo, J. Lee, C. H. Cho, Sci. Rep. 2016, 6, 29813.[3] C. Jin, E. C. Regan, A. 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See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.com Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h-BN Encapsulations 1. Introduction 2. Results and Discussion 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords