Long-Term Exposure of MoS2 to Oxygen and Water Promoted Armchair-to-Zigzag-Directional Line Unzippings

Understanding the long-term stability of MoS2 is important for various optoelectronic applications. Herein, we show that the long-term exposure to an oxygen atmosphere for up to a few months results in zigzag (zz)-directional line unzipping of the MoS2 basal plane. In contrast to exposure to dry or humid N2 atmospheres, dry O2 treatment promotes the initial formation of line defects, mainly along the armchair (ac) direction, and humid O2 treatment further promotes ac line unzipping near edges. Further incubation of MoS2 for a few months in an O2 atmosphere results in massive zz-directional line unzipping. The photoluminescence and the strain-doping plot based on two prominent bands in the Raman spectrum show that, in contrast to dry-N2-treated MoS2, the O2-treated MoS2 primarily exhibits hole doping, whereas humid-O2-treated MoS2 mainly exists in a neutral charge state with tension. This study provides a guideline for MoS2 preservation and a further method for generating controlled defects.


Introduction
Molybdenum disulfide (MoS 2 ) is a representative of the transition metal dichalcogenide (TMDC) MX 2 family which has sandwiched layer structures with the transition metal M (groups four, five, and six atoms) located between chalcogen atoms X (S, Se, and Te). Owing to the presence of a band gap associated with its few-atom-thick layers held together by van der Waals forces, MoS 2 possesses interesting optical [1][2][3], spin-valley polarization [4,5], and catalytic properties [6][7][8]. In the context of its optical properties, neutral MoS 2 displays two features known as A (~1.89 eV) and B (~2.08 eV) excitons [1,2,9], which are associated with direct transitions from the highest spin-split valence bands to the lowest conduction bands. Furthermore, the A exciton has subcomponents in the form of a charge-neutral exciton band A o at 1.89 eV and a lower-lying charged exciton (trion) band A − at 1.86 eV, whose relative intensities and positions are dependent on the doping [10][11][12] and strain (ε) [11,[13][14][15][16] of MoS 2 . Applications of the optoelectronic properties of MoS 2 require an understanding of its long-term stability.
MoS 2 is amenable to both electron (n) or hole (p) doping [10,11] and it can possess strain (ε) [11,[13][14][15][16]. The effects of environmentally abundant oxygen and water on the optoelectronic properties of MoS 2 have been studied [17][18][19], but how these species affect the doping and ε properties of MoS 2 under ambient conditions remains largely unknown, and this is an important issue. Treatments under harsh oxidative conditions such as with oxygen plasma [18], UV-ozone [20], and high-temperature annealing (>300 • C) [18,21,22] are known to induce oxidation of the basal plane of MoS 2 . Following these treatments, the basal plane of MoS 2 exhibits reduced photoluminescence (PL), while edges and cracked regions display increased PL, suggesting that oxidation plays different roles in governing the properties of the basal plane and periphery. Moreover, O 2 incubation of up to one year leads to random cracks and defects [19]. In contrast, physisorption of oxygen by MoS 2 activated at 450 • C under vacuum results in a large basal-plane PL enhancement [17] caused by MoS 2 -to-O 2 charge transfer. Because chemisorbed oxygen modulates the optical properties of MoS 2 differently from physisorbed oxygen, it is important to gain an understanding of how chemical oxidation controls the doping and ε, which in turn govern the band-gap structure of MoS 2 [12].
Theoretical study [23] suggests that the basal plane of MoS 2 exhibits a large kinetic barrier (i.e., 1.6 eV) for O 2 chemisorption, whereas vacancies (i.e., sulfur vacancies) at the surface of MoS 2 reduce the barrier to 0.8 eV. The experimentally determined activation energy for bulk MoS 2 oxidation (0.54 eV and 0.79 eV) [22,24] is somewhat lower than the theoretically predicted value. However, recent scanning tunneling microscope measurements [25] show that point-like oxygen-substitution reactions producing oxygenated MoS 2 occur spontaneously, even under ambient conditions. Along with the fact that Moterminated edges of MoS 2 readily react with O 2 [26], these findings indicate that oxygenated MoS 2 possesses various point defects which are randomly distributed over the plane.
The investigation described below was designed to address this issue. For this purpose, chemical vapor deposition (CVD)-grown MoS 2 crystals were exposed to four different conditions for two weeks: N 2 , N 2 with 75% relative humidity (N 2 -75RH), O 2 , and O 2 with 75% relative humidity (O 2 -75RH). Using various methods, including Raman spectroscopy, photoluminescence (PL) spectroscopy, and atomic force microscopy (AFM), we observed that the basal plane of MoS 2 possessing tensile strain (ε T ) associated with its preparation using CVD, undergoes zigzag (zz)-directional unzipping upon long-term exposure to an oxygen atmosphere. Specifically, during the initial phases of exposure to O 2 and O 2 -75RH, MoS 2 crystals display initial macroscopic armchair (ac)-directional micrometer-scale defects near triangular edges. Following increased exposure times of up to a few months, the initial ac-directional defects near the periphery change into zz-directional unzipping in the basal plane. This unique unzipping behavior is associated with the susceptibility of S defects in the basal plane to tension caused by oxidation. Moreover, Raman and PL spectroscopic studies show that changes occurring in the optoelectronic properties of MoS 2 upon chemical oxidation are a consequence of changes in ε T and doping.

Materials and Instrumentation
MoO 3 (product no. 267856, ACS reagents, purity ≥ 99.5%) and sulfur (product no. 13803, purity ≥ 99.5%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sulfur was further recrystallized using vacuum sublimation at ca. 10 −3 torr, as described previously [27]. Sodium cholate (SC), with a purity of over 98%, was purchased from TCI (Tokyo, Japan) and used as a surfactant and an adhesion promoter to a silicon substrate. Deionized water with a resistivity greater than 18 MΩ was used. All gases, including N 2 , Ar, and O 2 with purities of over 99.99% were obtained from Donga Gas (Jinju, Korea). The 285-nm thick SiO 2 /Si substrates (lot no. 7400397-601, Shin-Etsu, Tokyo, Japan) were spin-coated with MoO 3 and converted to MoS 2 . The as-received wafer was cut into pieces of size 1.0 × 1.0 cm 2 and further rinsed with methanol, acetone, and isopropanol while undergoing bath sonication, then subjected to drying with a N 2 stream. Optical microscope (OM) images were obtained using an upright fluorescence microscope (BX-51, Olympus, Tokyo, Japan) with a CMOS camera (3.6 µm/pixel, 1280 × 1024 pixels, part no. DCC1645C, Thorlabs, Newton, NJ, USA) and a light-emitting diode light source (cold white color, part no. MCWHL2, Thorlabs) with an optional dichroic cube set (MWG, Olympus), which could image the 660 nm PL of MoS 2 . SEM images were acquired using field-emission SEMs (SU8000, Hitachi or 7610f-plus, JEOL Ltd., Tokyo, Japan) operating at an acceleration voltage of 5 kV.

Precursor Preparation
MoS 2 growth involved the use of SC as adhesion promoter and surfactant for MoO 3 powder, according to the procedure described in the literature [28]. Aqueous MoO 3 dispersion was prepared by sonication using SC as a surfactant. Briefly, 20 mM MoO 3 was added to 1 wt. % SC in 100 mL of water. The suspension was subjected to bath sonication for 1 h (70 W, Branson1519, Brookfield, CT, USA) followed by tip sonication for 2 h (300 W amplitude, probe tip diameter: 13 mm, VCX 750, Sonics and Materials, Newtown, CT, USA). The dispersion was centrifuged using a table-top centrifuge (Wisespin CF-10, Daihan Scientific Co, Ltd., Wonju-si, Korea) to collect an 80% supernatant. 100 µL of the MoO 3 dispersion was spin-coated at 3000 rpm for 80 s by drop-casting on precleaned SiO 2 on a Si substrate. Sulfur was prepared by vacuum sublimation. Briefly, 1 g of sulfur was placed at the bottom of a sublimation kit with a cold finger. The sublimation temperature was set to 200 • C under vacuum (10 −3 torr).

Growth of MoS 2 Crystal
MoS 2 was prepared by using a hot-wall CVD apparatus [29] operating at atmospheric pressure, with the aforementioned spin-coated MoO 3 and the sublimed sulfur powder as precursors, modified from the previous study [27]. Prior to the MoS 2 growth, a quartz tube in a tube furnace was pre-annealed for 30 min at 1000 • C with a 20 standard cubic centimeter per min (sccm) Ar flow to remove any physisorbed water, in a CVD chamber. After cooling, the MoO 3 spin-coated substrate was loaded into the hot zone of a tube furnace with the SiO 2 side face up, and an alumina boat containing 30 mg of sulfur was placed in an upstream position. Crystal growth was conducted at 750 • C for 20 min, at which time the alumina boat containing the sulfur reached~220 • C. The substrate was then cooled to room temperature.

Environmental Control of MoS 2
Four different environments (i.e., N 2 , N 2 -75RH, O 2 , and O 2 -75RH) were prepared using a Schlenk line technique and a septum-capped vial, with precautions as follows. For the N 2 -75RH or O 2 -75RH environments, a vial containing saturated brine solution maintaining 75RH [30] was bubbled with the N 2 (or O 2 ) for 90 min using a syringe to remove any O 2 dissolved in the solution. For the N 2 or O 2 environments, a vial containing a moisture-absorbing silica gel was initially flame-dried thoroughly with a hand-held torch while a vacuum (10 −3 torr) was pulled, and the sample was subsequently treated with a cycle of N 2 (or O 2 ) purging/vacuum, at least four times. Each sample was incubated for two weeks prior to any measurements. A further one month of incubation was conducted for observation of long-term changes.

Raman and PL Measurements
The samples were loaded into an environmental chamber (TS1000V-17/3 with T96-S, Linkam Scientific Instruments Ltd., Redhill, UK) which allowed observation of the sample through a coverslip while measurements were made. Raman and PL spectra were obtained using a micro-Raman spectroscopy setup with a backscattering geometry, as described in the literature [27,29,31]. Briefly, a spectrometer (Triax 320 with 1800 gr/mm) and coversliptolerant 40× objective (UPlanSApo, N.A.: 0.95, Olympus) were utilized to obtain the Raman and PL spectra. Calibration for Raman spectroscopy was conducted with multiple Hg/Ar lamp peaks using a light source (HG-1, Ocean Optics, Oxford, UK), according to the procedure described in the literature [29]. The Si peak at 520.89 cm −1 was used as an internal reference and an intensity-normalizing peak. Laser power was maintained below 0.06 mW to minimize any light-induced damage. The obtained Raman and PL spectra were further deconvoluted with Lorentzian fitting. Especially in the PL spectra, A • , A − , and B excitons were fitted to an unrestricted position and area.

AFM Measurements
AFM height and phase images were obtained by using a tapping mode with an NX-10 AFM (Park Systems, Suwon, Korea). Al-coated silicon cantilevers (force constant: 37 N/m, resonance frequency: 300 kHz, ACTA-20, App Nano, Mountain View, CA, USA) were used. Typically, 512 × 512 pixels for a 40 µm length were routinely acquired at a speed of 0.2 Hz. The XEI program (Park Systems, Korea) was used to flatten topographies along the fast axis of scan using a polynomial, by excluding speckles of size 5 nm.

Results
CVD-grown triangular MoS 2 crystals were single crystals terminated with zz edges [37] and were used to probe the effects of oxidation reactions on the morphological and optoelectronic properties. The MoS 2 crystals were grown by CVD, using the procedure developed in our previous investigation [27], starting with MoO 3 and sublimed sulfur (see Materials and Methods section). First, a well-dispersed aqueous MoO 3 dispersion containing 1 wt. % sodium cholate (SC) as a surfactant and adhesion promoter [28] was spin-coated onto a 285 nm thick SiO 2 /Si substrate. After annealing at 1000 • C to eliminate adsorbates, a quartz tube was loaded with the MoO 3 -coated substrate and a boat containing freshly sublimed sulfur. A 20 sccm Ar flow was used as a carrier gas and the temperature of the hot zone was raised to 750 • C for 20 min, to promote MoO 3 reduction with sulfur. The growth of the MoS 2 crystals was terminated by cooling the tube to room temperature for 40 min while maintaining the Ar flow.
The Initial characterization of the as-grown MoS 2 as a control was conducted using various methods, including optical microscopy (OM), atomic force microscopy (AFM), photoluminescence (PL) imaging/spectroscopy, and Raman spectroscopy (see Figure 1A-E). Inspection of the representative CVD-grown MoS 2 via the OM image ( Figure 1A) shows the MoS 2 crystal as a ca. 46 µm long equilateral triangle with uniform contrast. The corresponding AFM height image ( Figure 1B) shows that the crystal has a clean surface and a 0.70 nm edge height, indicating a monolayer of MoS 2 [37]. Notably, the PL image ( Figure 1C) shows a gradient of PL brightening from the center to the peripheral regions. Other researchers [16,36] have found that such gradual PL intensity (I PL ) and peak position (λ) changes, as well as shifts in the Raman bands in CVD-grown MoS 2 , occur when proceeding from the center to the peripheral regions owing to differences in tensile strain (ε T ) caused by thermal expansion coefficient differences between the Si substrate and the MoS 2 . Since biaxial ε T shifts the excitonic A band by −99 meV/% [11], the observed λ of 675 nm in the PL spectrum at the center ( Figure 1D) suggests the presence of substantial ε T (i.e., 0.4%) compared to that at peripheral regions where the λ is ca. 660 nm. The positions of the two characteristic Raman bands of the in-plane vibration E 1 2g and out-of-plane vibration A 1g are known to be sensitive to ε T , and E 1 2g undergoes larger downfield shifts compared to A 1g with increasing ε T [11,[13][14][15][16]38]. The Raman spectrum of the central MoS 2 region ( Figure 1E) contains E 1 2g at 383.1 cm −1 and A 1g at 406.2 cm −1 [39], whereas the spectra at the peripheries contain upshifted 383.6 cm −1 and 406.8 cm −1 bands, which is in good agreement with reported spectra of MoS 2 under ε T . The results suggest that ε T is a major contributor to the anisotropy present in the PL and Raman spectra of the as-prepared MoS 2 crystal. Immediately after characterization, the MoS 2 crystals were incubated under the four different atmospheres N 2 , N 2 -75RH, O 2 , and O 2 -75RH for two weeks (see Figure S1A in the Supplementary Materials (SM) for schematics of the environmental incubation processes).
terials 2022, 12, x FOR PEER REVIEW 5 of 14 region ( Figure 1E) contains E 1 2g at 383.1 cm −1 and A1g at 406.2 cm −1 [39], whereas the spectra at the peripheries contain upshifted 383.6 cm −1 and 406.8 cm −1 bands, which is in good agreement with reported spectra of MoS2 under εT. The results suggest that εT is a major contributor to the anisotropy present in the PL and Raman spectra of the as-prepared MoS2 crystal. Immediately after characterization, the MoS2 crystals were incubated under the four different atmospheres N2, N2-75RH, O2, and O2-75RH for two weeks (see Figure  S1A in the Supplementary Materials (SM) for schematics of the environmental incubation processes).  [19] that aging of CVD-grown MoS2 and WS2 under ambient environmental conditions leads to random cracks and defects. This result suggests that the conditions for CVD growth affect the oxidative defects. Furthermore, the O2-75RH-treated sample ( Figure 2d) shows that unzipping occurs from the edges in the ac directions. The inset in Figure 2d shows that unzipping, instead of occurring at ac line defects, occurs at the edges, and that crack directions are at 120° from each other (see red arrows for the intersection). This result  [19] that aging of CVD-grown MoS 2 and WS 2 under ambient environmental conditions leads to random cracks and defects. This result suggests that the conditions for CVD growth affect the oxidative defects. Furthermore, the O 2 -75RH-treated sample (Figure 2d) shows that unzipping occurs from the edges in the ac directions. The inset in Figure 2d shows that unzipping, instead of occurring at ac line defects, occurs at the edges, and that crack directions are at 120 • from each other (see red arrows for the intersection). This result  1 μm for the inset of (d).
Because unzipping typically begins at the mechanically weakest points, the line defects and the subsequent unzipping are likely to be correlated with the εT of the MoS2, which in turn is associated with the optical properties of the MoS2 crystal. Figure 3A-D show the corresponding PL images of the samples. The images show that the IPL for the N2-treated crystal gradually increases from the center to the periphery, suggesting that the εT behavior is similar to that of as-prepared MoS2. Interestingly, the N2-75RH sample ( Figure 3B) has a uniform IPL at both the center and periphery. A humid environment is known to form entrapped water between graphene and the substrate [40][41][42] or MoS2 and the substrate [43]. To confirm that such entrapped water is related to the uniform IPL, AFM height measurements of the basal plane from the N2-75RH sample ( Figure S3A) were made. Water was entrapped evenly over the sample, and water entrapped regions is 0.5 nm thick and a few micrometers wide ( Figure S3B) were entrapped between the MoS2 and the substrate. This was a phenomenon not seen in the N2-treated sample. These results are in agreement with the previous study, which demonstrated that a monolayer of water adhered under the MoS2 surface to a thickness of ~0.5 nm [43], as seen in other two-dimensional materials such as graphene [40][41][42]. This result suggests that the increased height induced by water and the subsequent strain might increase εT and lead to the observed uniform IPL over the MoS2 crystal. Because unzipping typically begins at the mechanically weakest points, the line defects and the subsequent unzipping are likely to be correlated with the ε T of the MoS 2 , which in turn is associated with the optical properties of the MoS 2 crystal. Figure 3A-D show the corresponding PL images of the samples. The images show that the I PL for the N 2treated crystal gradually increases from the center to the periphery, suggesting that the ε T behavior is similar to that of as-prepared MoS 2 . Interestingly, the N 2 -75RH sample ( Figure 3B) has a uniform I PL at both the center and periphery. A humid environment is known to form entrapped water between graphene and the substrate [40][41][42] or MoS 2 and the substrate [43]. To confirm that such entrapped water is related to the uniform I PL , AFM height measurements of the basal plane from the N 2 -75RH sample ( Figure S3A) were made. Water was entrapped evenly over the sample, and water entrapped regions is 0.5 nm thick and a few micrometers wide ( Figure S3B) were entrapped between the MoS 2 and the substrate. This was a phenomenon not seen in the N 2 -treated sample. These results are in agreement with the previous study, which demonstrated that a monolayer of water adhered under the MoS 2 surface to a thickness of~0.5 nm [43], as seen in other two-dimensional materials such as graphene [40][41][42]. This result suggests that the increased height induced by water and the subsequent strain might increase ε T and lead to the observed uniform I PL over the MoS 2 crystal.  The PL image from the O2-treated sample ( Figure 3C) shows that IPL gradually increased from the periphery to the center, a trend that is opposite to that occurring in the N2-treated sample. The low IPL near the periphery overlaps with the AFM-observed line defects present in the O2-treated MoS2. These results suggest that, unlike for samples treated using harsh oxidations [18,20,22], physisorbed O2 at the center of the crystal actually results in an enhancement of IPL, which is in accordance with the previous report [17]. In contrast, chemically oxygenated species evidenced by line defects form near the periphery and result in the reduced IPL. Similar to the O2 sample, the crystal incubated under the O2-75RH condition ( Figure 3D) displays an IPL that is brighter at the center and dimmer and more irregular at the periphery. Such spatial inhomogeneity originates from unzipping and folding of the MoS2, as evidenced by comparing the PL and AFM images.
Qualitative information was gained about doping and εT by analyzing the PL spectra from the central ( Figure 3E) and peripheral ( Figure 3F) regions of the incubated MoS2 crystals. Inspection of the spectra of the basal planes ( Figure 3E) treated using humidified atmospheres (i.e., N2-75RH and O2-75RH) show that the λ values of the A bands display a bathochromic shift from 24 to 32 meV compared with those treated with N2 and O2 only. Moreover, the basal plane of the O2-treated sample exhibits an IPL about 11 times larger than that of the crystal exposed to N2. The spectra from the peripheries ( Figure 3F) show that while the peripheral IPLs of the O2 and O2-75RH samples were lower than those associated with the basal plane, the peripheral IPL of the N2 sample was increased and that of N2-75RH was unchanged. These results suggest that the center and periphery of the MoS2 crystal experience different degrees of doping and εT.
The λ and IPL of A o and A − peaks in the PL spectra of MoS2 are dependent upon ε and n, and this serves as a foundation for deciphering the roles that O2 and H2O play in forming peripheral defects and unzipping [12,36]. To elucidate these values, the PL spectra at the central positions of the MoS2 crystals were deconvoluted using Lorentzians (shaded area), as shown in Figure S4. As evidenced by the dashed lines, the λ values and peak areas of A o and A − underwent a systematic change for crystals treated using each condition. For example, among the four samples, the dry N2-and O2-treated samples exhibited the most blue-shifted λ values for the A o and A − peaks, suggesting that they are associated The PL image from the O 2 -treated sample ( Figure 3C) shows that I PL gradually increased from the periphery to the center, a trend that is opposite to that occurring in the N 2 -treated sample. The low I PL near the periphery overlaps with the AFM-observed line defects present in the O 2 -treated MoS 2 . These results suggest that, unlike for samples treated using harsh oxidations [18,20,22], physisorbed O 2 at the center of the crystal actually results in an enhancement of I PL , which is in accordance with the previous report [17]. In contrast, chemically oxygenated species evidenced by line defects form near the periphery and result in the reduced I PL . Similar to the O 2 sample, the crystal incubated under the O 2 -75RH condition ( Figure 3D) displays an I PL that is brighter at the center and dimmer and more irregular at the periphery. Such spatial inhomogeneity originates from unzipping and folding of the MoS 2 , as evidenced by comparing the PL and AFM images.
Qualitative information was gained about doping and ε T by analyzing the PL spectra from the central ( Figure 3E) and peripheral ( Figure 3F) regions of the incubated MoS 2 crystals. Inspection of the spectra of the basal planes ( Figure 3E) treated using humidified atmospheres (i.e., N 2 -75RH and O 2 -75RH) show that the λ values of the A bands display a bathochromic shift from 24 to 32 meV compared with those treated with N 2 and O 2 only. Moreover, the basal plane of the O 2 -treated sample exhibits an I PL about 11 times larger than that of the crystal exposed to N 2 . The spectra from the peripheries ( Figure 3F) show that while the peripheral I PL s of the O 2 and O 2 -75RH samples were lower than those associated with the basal plane, the peripheral I PL of the N 2 sample was increased and that of N 2 -75RH was unchanged. These results suggest that the center and periphery of the MoS 2 crystal experience different degrees of doping and ε T .
The λ and I PL of A o and A − peaks in the PL spectra of MoS 2 are dependent upon ε and n, and this serves as a foundation for deciphering the roles that O 2 and H 2 O play in forming peripheral defects and unzipping [12,36]. To elucidate these values, the PL spectra at the central positions of the MoS 2 crystals were deconvoluted using Lorentzians (shaded area), as shown in Figure S4. As evidenced by the dashed lines, the λ values and peak areas of A o and A − underwent a systematic change for crystals treated using each condition. For example, among the four samples, the dry N 2 -and O 2 -treated samples exhibited the most blue-shifted λ values for the A o and A − peaks, suggesting that they are associated  Figure 3G) bands of samples treated using humid atmospheres exhibited proportional red shifts. A comparison of the relative areas of the A o and A − bands ( Figure 3F), which provides information about the charge state of MoS 2 [10], shows that peak areas are much greater in the spectra of O 2 and O 2 -75RH samples. Moreover, the comparison shows that water induces an increase in the population of the charge-neutral A o state, and O 2 causes an increase in the area of the charged A − state. This result is opposite to that of a previous study suggesting that water has a p-doping effect [43], and suggests that water promotes neutral exciton formation while physisorbed O 2 facilitates trion formation. Similar analysis of the PL spectra of peripheral regions ( Figure S5A-C) shows that analogous but lesser shifts in λ occur ( Figure S5B) and that the relative areas of A o and A − bands are relatively smaller compared to those in the spectra of basal planes. These observations in peripheral regions are in line with doping created by line defects and unzipping [19].
Raman spectroscopy is a powerful tool for gaining an understanding of the quantitative ε and nor p-doping density of MoS 2 , because these parameters are closely related to chemical bond strengthening or weakening, which alters the vibrational behavior. A previous study [35] showed that ε and n contributions in graphene are quantitatively associated with two Raman bands (i.e., G and 2D modes). A similar concept has been utilized to evaluate ε and n for monolayer MoS 2 [32][33][34]. To apply this treatment, we chose the frequencies from the spectra of suspended MoS 2 as an origin for the unperturbed, pristine state [44,45]. Utilizing the published values for ε and n of monolayer MoS 2 [11,46], an ε-n coordinate system in units of % and 1 × 10 12 cm −2 (Figure 4c) was devised in a coordinate framework comprising two prominent Raman bands (E 1 2g , A 1g ). Specifically, Figure 4c is a plot of ε (black dashed line) and n (red dashed line), with an origin O = (E 1 2g , A 1g ) = (385.3, 404.8), corresponding to pristine MoS 2 . In addition, ε C values are compressive strains, and n and p denote n and p doping, respectively. Representative crystal spectra acquired from the center and periphery are shown in Figure 4a,b. It is noteworthy that Raman bands near 275 cm −1 corresponding to MoO 3 are not present [21]. As shown in Figure 4c, the central (peripheral) position of the N 2 sample displays two bands at 381.8 (383.1) and 403.8 (403.8) cm −1 . In terms of strain doping, the central part has ε-n coordinates of (0.34, −0.4), and the peripheral part has coordinates of (0.2, 0.7). This result suggests that while ε T decreases from the center to the periphery, in accordance with previous findings [27,36], a slight pto n-doping transition occurs simultaneously. The doping of the N 2 -treated sample is likely to be related to the interaction of the basal MoS 2 with the SiO 2 /Si substrate, which acts as a p dopant [29,35]. Compared to the N 2 -treated sample, the N 2 -75RH sample (orange) exhibited much higher ε T values (from 0.37 to 0.42%) at both the central and peripheral positions and experienced only a negligible doping density change. This result further supports the idea of the presence of water-induced tensile strain. This result underscores the advantage of utilizing an ε-n plot for analyzing the spectroscopic data, because otherwise the p-doping and ε T results are both included in a similar downshift in vibrational frequencies [11].
Inspection of the ε-n plot shows that the two prominent Raman bands (E 1 2g and A 1g ) associated with the central and peripheral regions of the O 2 -and N 2 -treated MoS 2 crystals exhibited nearly similar movement along the p-doping axis. This observation suggests that O 2 treatment results in p-doping, which is in agreement with the occurrence of charge transfer from MoS 2 to physisorbed O 2 [17]. Finally, the O 2 -75RH treated sample has ε-n coordinates of (0.  Inspection of the ε-n plot shows that the two prominent Raman bands (E 1 2g and A1g) associated with the central and peripheral regions of the O2-and N2-treated MoS2 crystals exhibited nearly similar movement along the p-doping axis. This observation suggests that O2 treatment results in p-doping, which is in agreement with the occurrence of charge transfer from MoS2 to physisorbed O2 [17]. Finally, the O2-75RH treated sample has ε-n coordinates of (0.4, −1) and (0.25, 1) for the central and peripheral positions, respectively. The different doping densities with different signs observed for the central and peripheral regions are closely connected to the existence of peripheral unzipping and folding, which decrease εT. The increase in εT and the charge neutralization taking place in changing from O2 to O2-75RH, in conjunction with the PL results, indicate that treatments with O2 and H2O lead predominantly to p-doping and εT, respectively.
The results suggest that regardless of the environmental conditions used, MoS2 samples possess considerable εT, albeit with different doping densities. However, only O2treated crystals experienced line defects and unzipping. This observation prompted us to perform an experiment in which MoS2 crystals were exposed to O2 and O2-75RH environments for three months. Figure 5A,D show the respective AFM phase images, facilitating the visualization of unzipping. Remarkably, both samples show zz-directional line unzipping with respect to zz-terminated edges. The O2-treated sample had a wider unzipping width compared to the O2-75RH-treated MoS2. Unzippings occurred at 120° with respect to each other. Although unzipping near the periphery is ac-directional with respect to the edge, it changes to the zz direction in the basal plane. This finding stands in stark contrast to the etched triangular pit of exfoliated MoS2 prepared by high-temperature annealing (i.e., 300 °C) [21,47] and the random cracks incubated at room temperature [19]. Inspection of the normalized PL spectra of the O2-incubated crystal ( Figure 5B) shows that both unzipping and basal positions occur at ~670 nm, which corresponds to the near-absence of tension. Similarly, both Raman spectra ( Figure 5C) show similar interpeak separations (i.e., ~25 cm −1 ). A similar unzipping behavior but associated with a larger difference in εT was observed for the O2-75RH-treated sample. In this case, the PL peak position ( Figure  5E) of the unzipped region (black circle in Figure 5D) displayed a large blue shift (20 nm) compared to that from the basal plane, showing that the εT was relieved. Raman spectra Here, ε T and ε C stand for tensile and compressive strains, respectively, and n and p denote n and p doping, respectively. Solid lines between Raman points are drawn for grouping the same samples.
The results suggest that regardless of the environmental conditions used, MoS 2 samples possess considerable ε T , albeit with different doping densities. However, only O 2treated crystals experienced line defects and unzipping. This observation prompted us to perform an experiment in which MoS 2 crystals were exposed to O 2 and O 2 -75RH environments for three months. Figure 5A,D show the respective AFM phase images, facilitating the visualization of unzipping. Remarkably, both samples show zz-directional line unzipping with respect to zz-terminated edges. The O 2 -treated sample had a wider unzipping width compared to the O 2 -75RH-treated MoS 2 . Unzippings occurred at 120 • with respect to each other. Although unzipping near the periphery is ac-directional with respect to the edge, it changes to the zz direction in the basal plane. This finding stands in stark contrast to the etched triangular pit of exfoliated MoS 2 prepared by high-temperature annealing (i.e., 300 • C) [21,47] and the random cracks incubated at room temperature [19]. Inspection of the normalized PL spectra of the O 2 -incubated crystal ( Figure 5B) shows that both unzipping and basal positions occur at~670 nm, which corresponds to the near-absence of tension. Similarly, both Raman spectra ( Figure 5C) show similar interpeak separations (i.e., 25 cm −1 ). A similar unzipping behavior but associated with a larger difference in ε T was observed for the O 2 -75RH-treated sample. In this case, the PL peak position ( Figure 5E) of the unzipped region (black circle in Figure 5D) displayed a large blue shift (20 nm) compared to that from the basal plane, showing that the ε T was relieved. Raman spectra analysis ( Figure 5F) further supported the fact that the basal plane has a larger ε T (larger interpeak separation) compared to that for the O 2 -treated crystal.
These defects are different from the defects that can exist in the as-grown state. Figure S6A,B show PL images of the as-prepared MoS 2 sample and the same sample after incubation. Bright PL originates from the crack or unzipping regions, showing an increase in line defects. However, the existing cracks in the as-prepared sample appear to be random, in contrast to the precise zz-directional unzipping after O 2 -75RH treatment. The AFM phase image ( Figure S6C) clearly shows that the O 2 -75RH-treated sample had precise turns and angles of unzipping with respect to the edges. In addition, proceeding from the edges to the center slowly changed the unzipping direction from ac to zz lines. Figure S7 shows the AFM phase images from O 2 - (Figure S7A-C) and O 2 -75RH-treated ( Figure S7D-F) samples. Irrespective of the presence of H 2 O, nearly all the ac unzipping at the edges changed into the zz direction in the basal plane within a few micrometers. Interestingly, the width of the ac unzipping is much less than that of the zz unzipping, presumably owing to the ε T difference in the central and peripheral regions. Along with the PL and Raman studies, this finding strongly supports the fact that directional unzipping is dependent on ε.
Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 14 analysis ( Figure 5F) further supported the fact that the basal plane has a larger εT (larger interpeak separation) compared to that for the O2-treated crystal. These defects are different from the defects that can exist in the as-grown state. Figure  S6A,B show PL images of the as-prepared MoS2 sample and the same sample after incubation. Bright PL originates from the crack or unzipping regions, showing an increase in line defects. However, the existing cracks in the as-prepared sample appear to be random, in contrast to the precise zz-directional unzipping after O2-75RH treatment. The AFM phase image ( Figure S6C) clearly shows that the O2-75RH-treated sample had precise turns and angles of unzipping with respect to the edges. In addition, proceeding from the edges to the center slowly changed the unzipping direction from ac to zz lines. Figure S7 shows the AFM phase images from O2- (Figure S7A-C) and O2-75RH-treated ( Figure S7D-F) samples. Irrespective of the presence of H2O, nearly all the ac unzipping at the edges changed into the zz direction in the basal plane within a few micrometers. Interestingly, the width of the ac unzipping is much less than that of the zz unzipping, presumably owing to the εT difference in the central and peripheral regions. Along with the PL and Raman studies, this finding strongly supports the fact that directional unzipping is dependent on ε.
The AFM image ( Figure 6A) reveals details of the zz-directional line unzipping and origin. The O2-treated sample shows 45 nm wide unzipping. The width is persistent along the unzipped segments. In addition, the 120° turns are very sharp. Since the observed typical εT is in the range of 0.2-0.4%, the width of the MoS2 grain extends by 20-40 nm, The AFM image ( Figure 6A) reveals details of the zz-directional line unzipping and origin. The O 2 -treated sample shows 45 nm wide unzipping. The width is persistent along the unzipped segments. In addition, the 120 • turns are very sharp. Since the observed typical ε T is in the range of 0.2-0.4%, the width of the MoS 2 grain extends by 20-40 nm, which accounts for the few unzippings with a 45 nm width. Similarly, the AFM phase image of the O 2 -75RH sample ( Figure 6B), which has similar ε T , displays somewhat similar line width (i.e., 30 nm). As evidenced by Figure S7, typical unzipping occurs in two or three lines along the 10 µm wide MoS 2 crystals, in good agreement with the observed line unzipping. Moreover, MoS 2 with a few layers also has similar unzipping. Figure S7A-C shows the AFM and corresponding OM images of MoS 2 with a few layers. The fewlayered regions have less line unzipping compared to single-layered MoS 2 . In addition, line unzipping is much more random, as indicated by the yellow arrows. We speculate that this effect of fewer and more random unzippings originates from the lesser ε T exerted on few-layered MoS 2 .
three lines along the 10 μm wide MoS2 crystals, in good agreement with the observed line unzipping. Moreover, MoS2 with a few layers also has similar unzipping. Figure S7A-C shows the AFM and corresponding OM images of MoS2 with a few layers. The few-layered regions have less line unzipping compared to single-layered MoS2. In addition, line unzipping is much more random, as indicated by the yellow arrows. We speculate that this effect of fewer and more random unzippings originates from the lesser εT exerted on few-layered MoS2. The remaining question to be addressed is that of how O2 treatment causes MoS2 to unzip along the zz directions. Raman spectroscopic analysis did not show the presence of detectable signals associated with MoO3. In addition, the energy dispersive spectrum (EDS) using scanning electron microscopy (SEM) was also used to attempt to probe the nature of line defects, and this only showed strong Si and O signals from the substrate ( Figure S8A-C). Mo-terminated edges readily produce oxygenated Mo. Because the Mo-O bond length (ca. 2.1 Å ) [23] is shorter than that (ca. 2.4 Å ) of Mo-S, additional εT should exist near the substitution defect sites, facilitating initial unzipping. Proceeding to zz unzipping seems to be associated with abundant S vacancies in CVD-grown MoS2 [48], which are expected to be much higher than the S vacancies present in exfoliated MoS2 (i.e., ranging from 5 × 10 12 to 5 × 10 13 cm −2 ) [49]. The S vacancies accumulate and are transformed into O-substituted defects with high density up to 1 × 10 15 cm −2 upon long-term exposure to ambient conditions [25]. Furthermore, the transmission electron microscopy study [50] showed that, at a high e-beam dose, S vacancies are formed owing to the excision of S atoms. Then, ac line defects up to a few tens of nanometers form as a result of the accumulation of S vacancies by adjacent S diffusion in the MoS2 sheet before forming a zz unzipping. Therefore, the formation and accumulation of S vacancies represent a possible model for the formation of directional unzipping near the center. Directional unzipping change from ac to zz is likely to be associated with a larger εT in the basal plane than at the peripheries.

Conclusions
In summary, in the study described above, we found that initial armchair-directional line defects and subsequent zigzag-directional line unzipping occurred upon treatment with O2. Moreover, we showed that these phenomena originate from tension in the chemical-vapor-deposition-grown monolayer MoS2 crystals, caused by the thermal expansion coefficient difference with the substrate. The O2-treated MoS2 crystal exhibited armchairdirectional line defects, and the inclusion of water in the incubation atmosphere resulted The remaining question to be addressed is that of how O 2 treatment causes MoS 2 to unzip along the zz directions. Raman spectroscopic analysis did not show the presence of detectable signals associated with MoO 3 . In addition, the energy dispersive spectrum (EDS) using scanning electron microscopy (SEM) was also used to attempt to probe the nature of line defects, and this only showed strong Si and O signals from the substrate ( Figure S8A-C). Mo-terminated edges readily produce oxygenated Mo. Because the Mo-O bond length (ca. 2.1 Å) [23] is shorter than that (ca. 2.4 Å) of Mo-S, additional ε T should exist near the substitution defect sites, facilitating initial unzipping. Proceeding to zz unzipping seems to be associated with abundant S vacancies in CVD-grown MoS 2 [48], which are expected to be much higher than the S vacancies present in exfoliated MoS 2 (i.e., ranging from 5 × 10 12 to 5 × 10 13 cm −2 ) [49]. The S vacancies accumulate and are transformed into O-substituted defects with high density up to 1 × 10 15 cm −2 upon long-term exposure to ambient conditions [25]. Furthermore, the transmission electron microscopy study [50] showed that, at a high e-beam dose, S vacancies are formed owing to the excision of S atoms. Then, ac line defects up to a few tens of nanometers form as a result of the accumulation of S vacancies by adjacent S diffusion in the MoS 2 sheet before forming a zz unzipping. Therefore, the formation and accumulation of S vacancies represent a possible model for the formation of directional unzipping near the center. Directional unzipping change from ac to zz is likely to be associated with a larger ε T in the basal plane than at the peripheries.

Conclusions
In summary, in the study described above, we found that initial armchair-directional line defects and subsequent zigzag-directional line unzipping occurred upon treatment with O 2 . Moreover, we showed that these phenomena originate from tension in the chemical-vapor-deposition-grown monolayer MoS 2 crystals, caused by the thermal expansion coefficient difference with the substrate. The O 2 -treated MoS 2 crystal exhibited armchair-directional line defects, and the inclusion of water in the incubation atmosphere resulted in further unzipping and folding of MoS 2 . Raman and photoluminescence spectroscopic studies revealed that different prevailing tensions exist in MoS 2 grown by CVD under the four different conditions. Oxygenated defects, along with tension, further facilitated zigzag line unzipping in the MoS 2 basal plane upon long-term exposure to an O 2 atmosphere. The observations provide a potential strategy for directionally selective engineering of the MoS 2 basal plane as part of efforts to prepare novel building blocks such as MoS 2 nanoribbons [51]. In addition, the analysis developed for assessing the net contributions of O 2 and H 2 O utilizing a strain-doping plot should be useful for the understanding of redox and catalytic effects.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12101706/s1. Figure S1: Schematics of environmental incubations for MoS 2 -containing substrates; Figure S2: AFM phase images of the sample; Figure S3: AFM height images of entrapped water; Figure S4: A • and A − contributions to PL spectra from central regions of MoS 2 ; Figure S5: PL spectrum analysis of the peripheral regions of MoS 2 treated by different environments; Figure S6: PL image and AFM phase image of MoS 2 before and after O 2 -75RH treatment for three months; Figure S7: The ac-to-zz directional unzipping change, proceeding from edges to center; Figure S8: EDS of selected area from zz unzipped samples.