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A Facile Strategy for the Growth of High-Quality Tungsten Disulfide Crystals Mediated by Oxygen-Deficient Oxide Precursors

Chemical vapor deposition (CVD) has been established as a versatile route for the large-scale synthesis of transition metal dichalcogenides, such as tungsten disulfide (WS2). Yet, the role of the precursor composition on the efficiency of the CVD process remains largely unknown and yet to be explored. Here, we employ Pulsed Laser Deposition (PLD) in a two-stage process to tune the oxygen content in tungsten oxide (WO3-x) precursors and demonstrate that the presence of oxygen vacancies in the precursor films leads to a more facile conversion from WO3-x to WS2. Using a joint study based on ab initio density functional theory (DFT) calculations and experiments, we unravel that the oxygen vacancies in WO3-x can serve as niches through which sulfur atoms enters the lattice and may facilitate an efficient growth of WS2 crystals. By solely modulating the precursor stoichiometry, the photoluminescence emission of WS2 can be greatly enhanced, while the size of WS2 domains increases significantly. Atomic resolution scanning transmission electron microscopy (STEM) reveals that tungsten vacancies are the dominant intrinsic defects in mono- and bilayers WS2. Moreover, our data reveal that grain boundaries in bilayer WS2 emerge upon the coalescence of AA' and AB-oriented crystals, while turbostatic moir\'e patterns originate upon formation of distinct grain boundaries between the bottom and bottom layers. The atomic resolution images show local strain buildup in bilayer WS2 due to competing effects of complex grain boundaries. Our study provides a means to tune the precursor composition in order to control the lateral growth of TMDs, while revealing insights into the different pathways for the formation of grain boundaries in bilayer WS2.

Tungsten disulfide (WS2) is an intriguing material in the layered transition metal dichalcogenides (TMDs) family. WS2 can usually exist in two phases with distinct electronic properties, i.e., the 2H semiconducting phase with a trigonal prismatic geometry and 1T metallic phase with a tetragonal symmetry. 1 In particular, 2H WS2 in its monolayer form is a direct band gap semiconductor, with an energy gap of ~2 eV, as demonstrated both theoretically and experimentally. 2,3 Among two-dimensional 2D TMDs, WS2 monolayers exhibit a high photoluminescence (PL) quantum yield (> 6.3%), which can be further enhanced via doping, 4,5,6 excellent thermal stability, mechanical flexibility 7 and access to valley degree of freedom. 8 Several approaches have been exploited to obtain wafer-scale synthesis of TMD monolayers, including chemical vapor deposition (CVD), 9,10 metal-organic chemical vapor deposition (MOCVD), 11,12 molecular beam epitaxy (MBE), 13 atomic layer deposition (ALD), 14 pulsed laser deposition (PLD). 15,16,17,18 For instance, 19 conventional CVD synthesis of 2D-TMDs involves evaporation of stoichiometric tungsten trioxide (WO3) 4,9,20 or molybdenum trioxide (MoO3) powders. 21 The narrow optimum window of the CVD synthesis requires a precise control of the precursors vapor phase composition, chalcogen flow rate, etc. It has been shown that the conversion of MoO3 to MoS2 evolves via an intermediate step, during which MoO3 is first partially reduced and then sulfurized into MoS2. 22 Moreover, on the basis of previous findings, variation in the oxide precursor composition can greatly impact the growth of WS2 flakes. 23,24 . Recently, it was demonstrated that the epitaxial oxide films grown by PLD can be used to produce quasi-crystalline TMDs with improved device performance. 19 However, despite a vast number of studies on CVD synthesis, the role of the oxygen vacancies of the precursors on the growth of TMD domains remains largely unexplored. This subject is of fundamental importance note only for the synthesis of TMDs, but also because oxygen substitution in basal plane TMDs can open new avenues for engineering of 2D electrocatalysts. 25 In this work, we unravel how the presence of intrinsic oxygen vacancies in the non-stoichiometric tungsten oxides (WO3-x, 0< x<1) grown by PLD leads to a more facile conversion from WO3-x to WS2 films. A two-stage growth process was developed, which employs tunability of the oxygen vacancies in uniform WO3-x precursors to independently control the nucleation, lateral growth and ultimately the WS2 domain size. Our study suggests that native oxygen vacancies in the PLDgrown precursors can serve as active sites through which sulfur atoms enters the lattice and facilitates the growth of WS2 crystals with high photoluminescence (PL) emission and large domain size. Atomic resolution scanning transmission electron microscopy (STEM) images reveal how the single and bilayer WS2 crystal orientation develops for low-temperature (nearly stoichiometric oxides) compared to high-temperature (reduced oxides) grown precursors. By solely modulating the precursor composition, the effect of precursor composition on the surface diffusion and thus lateral growth can be clearly discerned, providing insights into the parameters that control WS2 domain growth.

Pulsed Laser Deposition of WO3-x precursors
To synthesize WS2 crystals, WO3-x (0< x<1) thin films and sulfur powder were used as precursors and reactant materials, respectively (see Materials and Methods). The WO3-x thin films were grown by PLD on (0001)-oriented Al2O3 substrates. To experimentally achieve a modulation of the oxygen vacancy content (VO), WO3-x films were prepared at different substrate temperatures, i.e. 500 ºC, 600 ºC, 700 ºC and 800 ºC and background gases, i.e., oxygen (O2) and argon (Ar). As it will be discussed next, these two parameters determine the extent to which the VO content can be tuned in the precursor films. In contrast to the CVD process, which relies on the use of oxide powder, PLD allows the growth an uniform and epitaxial film as the precursor source. In a second step, the as-grown oxide films were converted into WS2 in a standard CVD furnace at a temperature of 900 ºC, which is a standard temperature for CVD synthesis of WS2. 9 We performed high-resolution X-ray photoelectron spectroscopy (XPS) to investigate the composition of WO3-x films. XPS data show that the WO3-x films grown in O2 at a temperature of 500 ºC are nearly stoichiometric ( Figure 1 and Table S1). The XPS spectrum of the W 4f consists of two spin-orbit doublets corresponding to W 6+ in fully coordinated WO3 (W 4f7/2 binding energy 35.9 eV) and a small fraction of W 5+ in reduced WO3-x (W 4f7/2 binding energy 34.7 eV).
Increasing the substrate temperature to 700 ºC and above, new W 4f doublets appear at binding energies of 33.4 eV and 35.3 eV due to emergence of a lower valence state of W, i.e., W 4+ . The peaks at 41.7 eV and 40.4 eV were attributed to W 5p3/2. The corresponding S 2p spectra are shown in Figures S1 and S2 and the W 4f XPS spectra of O2-grown precursors are shown in Figure S3.
To further modulate the VO content, WO3-x films were grown by ablation of the WO3 target in Ar.
The XPS spectra of the corresponding WO3-x precursors reveal the presence of three distinct valence states of W 6+ , W 5+ and W 4+ corresponding to the W 4f7/2 peak at binding energy of 35.9 eV, 34.1 eV and 32.8 eV, respectively (Figure 1e-h). The evolution of the W chemical state in the WO3-x precursors with the change in the growth temperature and background gas is depicted in Figure 1 i,j. Films grown in O2 at 500 ºC have a very high fraction of WO3 (W 6+ fraction of ~ 0.91), along with a small fraction of W 5+ (~ 0.09). Here, W x+ fraction, x= 4, 5, 6 denotes the ratio of the W 4f peak area (sum of the W4f7/2 and W4f7/2 peak areas) attributed to W in a given valence state (W x+ ) to the total W 4f peak area, i.e., the sum of W 4+ , W 5+ and W 6+ peaks areas. Increasing the growth temperature to 600 ºC, the W 5+ content increases to ~ 0.12. The most significant change occurs at 700 ºC, for which the fitting yields W fraction of W 6+ ~ 0.79, W 5+ ~ 0.10 and W 4+ ~ 0.11. Finally, at the growth temperature of 800 ºC, the W 4+ fraction increases to ~ 0.14. For the films grown in Ar, a significant fraction of W 4+ is already observed at 500 ºC (W 4+~ 0.41) and which increases to ~ 0.50 for oxides grown at 800 ºC.
The XPS analysis demonstrate that the oxygen deficiency (VO) in WO3-x precursors can be greatly tuned from O2 to Ar background gas, and to a degree significantly larger than reported previously. 26 The average composition of the films decreases from ~2.95 to ~2.45 from O2 to Argrown films (see Table I). This significant variation in VO content arises from the different gasphase reactions that occur during propagation of the plasma-plume in gas. The complex phenomena of laser ablation into gas, involving plasma-plume free expansion and shock wave propagation, has been the subject of many theoretical and experimental studies. 27,28,29,30,31 Considering first the case of pulsed laser ablation in the reactive O2 gas, the plasma-plume expansion in O2 leading to the formation and propagation of the shock wave favors the dissociation of the O2 molecules. The interaction of plasma plume species (i.e., W atoms and ions) with oxygen will modify the plume chemistry through formation of molecular WO * excited species 27 . On the contrary, in the case of the inert Ar gas, the plasma chemistry is quantitatively different and determined by the W and O atomic species. On the basis of our findings, we can conclude that for PLD in O2, the gas-phase reactions are an important pathway for the oxygen incorporation in WO3x, which is not the case in Ar gas.
The stability of oxygen vacancies in sub-stoichiometric oxides can be explained through their formation energy determined from density functional theory (DFT) calculations. The formation energy (E F ) of VO was calculated for various suboxides with compositions varying from WO2.944 to WO2.0 (see Figures S7 and S8). In the whole range of oxide compositions, VO was found to have the lowest formation energy for oxides close to stoichiometric WO3 (WO2.944) while the formation energy of VO increases roughly 4 times from WO2.994 to WO2. Moreover, in the whole range of O chemical potential, formation energy of VO in WO2.944 was found to increase from 1.055 eV under an O-poor environment to 1.775 eV under an O-rich environment. This is well aligned with our experimental observations, for which a high VO content was observed in films grown under an O2poor environment and high temperature.
While the theoretical calculations do not account for the stability of the aforementioned oxides, they suggest that oxygen vacancies in WO3-x can easily form under O-poor environment. In particular, oxygen vacancies are a signature of oxide films grown by PLD 32 and can cause an atomic re-arrangement of the WO3 crystal to compensate for the high oxygen deficiencies. 33 Yet, the detailed mechanism of surface reconstruction in sub-stoichiometric tungsten oxide is beyond the scope of the work. A wide range of oxides can form between WO3 and WO2. Among all, W18O49 (WO2.72) and W20O58 (WO2.9) are the most thermodynamically stable phases, and which exhibit metallic characteristics as the Fermi level moves up in the conduction band as compared to the parent WO3 33 , 34 . It is therefore likely that the aforementioned phases either co-exist in the PLD-grown films, or metastable oxides present in the PLD precursors transform into most thermodynamically stable phases during the CVD process. Nanostructures of WO2.72 were previously synthesized by various approaches, such as heating a W foil at high temperature (1600 °C) 35 , photochemical reduction of carbon dioxide by visible light 36 , thermally evaporating WO3 powders, electron-beam induced synthesis 37 , etc.
The X-ray diffraction (XRD) patterns shown in Figure 1k and Figure S4 reveal that monolithically distorted rutile (020)-oriented WO2 films can be grown on (0001) Al2O3 substrate. 38 The (020) reflection of WO2 develops from a very broad to a sharp (020) oriented peak, indicating an optimum window growth of epitaxial-oriented WO2 between 600 ºC and 700 ºC. The b-axis oriented growth of WO2 are similar to that of monoclinic MoO2. 38,39 The WO2 reflection of the film grown at 800 ºC shifts significantly from 36.8º to 37.4º which can probably be assigned to a change the a (002)-preferred oriented WO2 38 .

Conversion from WO3-x to WS2 crystals
The W 4f XPS spectra of WS2 films obtained upon sulfurization of WO3-x precursors grown in Ar are shown in Figure 2. The W 4f XPS spectra of the sulfurized films can be tailored using three doublets, in accordance with different valence states of W (see also Tables S2 and S3 for detailed W 4f and S 2p spectra peak fitting). A similar trend was observed for the precursors deposited in O2. The W 4f XPS spectra of WS2 exhibit a dominant W 4+ doublet (W 4f7/2 binding energy at ~32.6 eV, full width of half maximum (FWHM) ~ 0.7 eV) which corresponds to the hexagonal 2H-WS2 phase 9 with an estimated S/W ratio of ~2. In addition, residual WO3 is evidenced by the presence of the W4f doublet with W 6+ valence state (W 4f7/2 binding energy at ~ 36.0 eV). 40,41 We find that the precursors grown at 500 ⁰C in Ar and sulfurized at 900⁰C have a fraction of WO3 of ~ 0.34 (see Table 1). In contrast, only an oxide fraction of ~ 0.20 was estimated in WS2 based on 800 ⁰C-grown precursors, indicating a nearly complete conversion from oxide to sulfide. In addition, a low-intensity W x+ doublet (binding energy W 4f7/2 at ~ 32.0 eV) is observed in the XPS spectra. Components at lower binding energies with respect to the hexagonal WS2 phase can be either assigned to the 1T-WS2 metallic phase 42,43 or sulfur-deficient films. 42 We hypothesize that the doublet appears due to sulfur-deficient WS2 which may arise from sulfur-deficient grain boundaries, as well as various intrinsic point defects in pristine WS2 42 . We note, however, that the sulfur-deficient WS2 fraction is very low (~ 0.08-0.06) and any variations are within the measurements accuracy (see Table 1). Lastly, the broad contributions at ~ 38.6 eV and ~ 41.6 eV (light green-colored curves in Figure 1) are assigned to the W 5p3/2 peaks. 9,40,41 Since neither S-O bonds nor W-S-O bonds were observed in the XPS spectra (see S 2p XPS spectra in Figure S1), we can conclude that, within the sensitivity of the XPS, the intermediate WOxSy phase is not present in the sulfurized films for Ar-grown WO3-x films.
Our experimental results indicate a more facile sulfurization for oxide precursors with of a high VO content. The sulfurization of WO3 in the presence of H2S has been studied extensively in the past 44 and it was show that the reduction of W valence state (W 6+ to W 5+ ) is the first step in the sulfurization process. In addition, higher bond energy of W-O (∼670 kJ/mol) compared to Mo-O (∼560.2 kJ/mol) requires a higher temperature of conversion for WS2 as compared to MoS2 (typically below 800 0 C when using PLD-grown MoOx films as precursors, not shown). Pathways to reduce the sulfurization temperature were demonstrated via the formation of atomic H in plasma discharge which facilitates W reduction 45 .
To explore the thermodynamics of the process, we have performed DFT calculations to determine the energy required to incorporate S atoms into the bulk WO3 crystal (see Methods and Figure S9 for details). Our calculations indicate that the addition of interstitial sulfur (Si) in WO3 requires a formation energy of 3.44 eV. On the other hand, the formation energy for sulfur atom on an oxygen vacancy site (SO) is only 2.55 eV; hence, incorporation of S atoms into the lattice is more energetically favorable in the presence of oxygen vacancies as compared to the interstitial sites.
Next, we have extended the modelling to investigate the energetics of the sulfurization process on the WO3 (001) surface simulated by a slab model. Our theoretical calculations reveal that adsorbing a sulfur atom at the oxygen site (SO) site yields a negative adsorption energy of -1.256 eV. This suggests that sulfur adsorption on the WO3 (001) surface in the presence of an oxygen vacancy is highly energetically favorable. This allow us to conclude that oxygen vacancies in WO3x can serve as niches through which sulfur atoms enters the lattice and trigger an efficient conversion of WO3-x into WS2.
PL and Raman spectroscopy were used to examine the optical properties of WS2. The PL spectra of WS2 acquired using a 532 nm laser excitation wavelength are shown in Figure 3. Remarkably, the PL spectra differ in peak intensity, peak position and FWHM as a function of the growth temperature of the WO3-x precursors. The overall PL emission of WS2 increases ~40 fold with increasing precursor growth temperature from 500 °C to 800 °C, whereas the FWHM of the PL peak decreases gradually from the 500 °C to 800 °C-based precursors (see Figure 3a, c, e, g). This suggests that the optical quality of WS2 is higher for high temperature-based precursors as compared to the low-temperature counterpart. All peaks marked with asterisk are associated with the sapphire substrate.
To better understand the PL spectra, we consider the contribution of neutral exciton (X), negative exciton or trion (X¯), bi-exciton (XX), and defected-mediated exciton (D) to the overall PL emission of WS2. 46,47,48 The optimal peak fitting was achieved with 100% Lorentzian functions.
The PL spectra of WS2 is dominated by the neutral exciton emission (A exciton), which shits from 2.02 to 1.99 eV with increasing precursor growth temperature from 500°C to 800°C. We also find that the spectral weight of the neutral X exciton decreases. The increase in the A exciton emission along with the decrease of the FHWM of the peak indicates a lower presence of defects in the high temperature-grown samples. 4 Moreover, we observe the emergence of a low-energy emission feature (D), which is attributed to excitons bound to defects. 49 Similar sub-band gap PL features were observed upon ion bombardment of 2D-TMDs. 49,50 The Raman spectra of WS2 display a Raman peak at ~178 cm -1 which corresponds to the longitudinal acoustic LA(M) mode, a first-order Raman active mode of the LA phonons at the edge of the Brillouin zone (the M point) 51 . The dominant Raman feature at ~353 cm -1 can be deconvoluted into three distinct contributions corresponding to the E2g 1 (M) in-plane mode at ~347 cm -1 , 2LA(M), second-order Raman mode of the LA phonons at ~354 cm -1 and E2g 1 (Г) inplane vibration mode of the W and S atoms at ~357.5 cm -1 , in a good agreement with previous work 52 . In addition, the A1g(Γ) peak at ~ 419 cm -1 corresponds to the out-of-plane vibration of the S atoms in WS2. The separation between the A1g(Γ) and E 1 2g(Г) Raman modes (denoted as Δk in for the WS2 films grown using 500 ⁰C, 600 ⁰C, 700 ⁰C and 800 ⁰C Ar-deposited WO3-x precursors, respectively (see Table 2). The data indicate that the sulfurized films consist predominantly of mono-bilayers WS2. A detailed discussion of the Raman peak assignments is given in the Supporting Information.
One remarkable difference is that the I2LA(M)/ IA1g(Γ) ratio is the lowest for WS2 obtained from Ardeposited WO3-x precursors at 500 ⁰C (see Table 2). Small variations in the I2LA(M)/IA1g(Γ) ratio can arise from multilayer regions. We observe, however, that at the excitation wavelength of 532 nm, the A1g(Γ) peak is more enhanced for the 500 ⁰C-based precursors. As it will be discussed next,  Table 2) and thus can be attributed to the reduced amount of defects.
Atomic force microscopy (AFM) images reveal that the size and shape of the WS2 crystals varies For the specimens based on the 700 ⁰C-precursors, the WS2 crystals become more circularlyshaped, of similar grain size of ~200 nm, while the average surface coverage of the monolayer increases as compared to the low-temperature counterpart (see Figures S5 and S6). Finally, increasing the precursor growth temperature to 800 ⁰C, large WS2 domains can be identified. films thickness increases (nucleation density is high), islands merge into a quasi-continuous monolayer, and multilayer regions emerge, while the WS2 crystal size decreases ( Figure S5).
The atomic structure of WS2 was studied using atomic-resolution annular dark-field (ADF) imaging using an aberration-corrected scanning transmission electron microscope (STEM). For this purpose, the WS2 specimens grown on sapphire were transferred onto a TEM grid, as described in Methods. Figure 5 shows the low-resolution STEM images of the as-transferred WS2 films based on 500 and 800 0 C-grown oxide precursors. The intensity of the STEM image is directly related to the atomic number of the atoms (Z) and with the number of atomic layers. 15 , 56 The atomic structure of 2H WS2 can be unambiguously distinguished in Figure 5d, in which sulfur atoms (Z = 16) have much lower intensity than tungsten atoms (Z = 74). The most common point defects identified in monolayer WS2 are single W vacancy (VW) and double W vacancies (V2W) ( Figure   S11), in good agreement with previous work. 57 These intrinsic defects are observed for all specimens, regardless on the precursor synthesis route. We also note that these intrinsic point defects are significantly different from one-step growth PLD 15 or sputtering, 58  The top layer which nucleates on grain A extends over the low-angle GB (7.5 0 ) on the neighboring grain resulting in the formation of a turbostatic moiré pattern (bilayer C). On the contrary, the top layer which nucleates on grain B stops at the high-angle GB. We hypothesize that the formation of the moiré pattern in bilayer WS2 is more energetically favorable for low-angle GB. A zoom-in view of the atomic structure of the GB reveals how the bilayer stacking orientation develops from AB to a moiré pattern without a lattice discontinuity (Figures 6e, S13 and S14). Again, an in-plane translation of the bilayer is observed in the high-resolution STEM image. Here, several competitive factors, i.e., weak interlayer van der Waals forces and intralayer GB results in a buildup strain in the bilayer structure.

CONCLUSIONS
In summary, we have developed a PLD-based synthesis of quasi-continuous mono-and multilayers WS2 films based on sulfurization of WO3-x epitaxial films. We have shown that the composition of WO3-x films determines the morphology and optical properties of WS2 monolayers synthesizes in a CVD process. The presence of oxygen vacancies in tungsten oxide film is highly favorable for successful film conversion. The optimized oxide growth conditions (800 ⁰C deposition temperature using Ar background pressure) results in formation of a stoichiometric 2H-WS2 monolayer film with a large size of crystals, reaching 26 um on the films edges.

Synthesis of WS2 crystals
A customized pulsed laser deposition (PLD) setup consisting of a target carousel equipped with a water cooled baffle (from McAllister Technical Services) and an IR lamp substrate heating system (from AJA International INC) was used to first produce a variety of tungsten oxide precursors which were then sulfurized for conversion to WS2 crystals. The cooling of the target is not critical in this work, but from our experience it is strictly necessary for ablation of sulfide materials.

Growth of precursor epitaxial WOx films by pulsed laser deposition (PLD).
The thin-films were grown by laser ablation of WO3 target (1 inch diameter, 6 mm thickness, 99.95% purity, from Testbourne Ltd.) using a 248 nm KrF laser (pulse duration τ=20 ns) operating at laser frequency of 1 Hz. The target was placed at a distance of ~7 cm from the substrate. Prior growth, the target was ablated using a number of 30 laser pulses. During deposition, the laser beam was rastered across the target using motorized mirrors and the target was rotated at the angular speed of 6 rpm to ensure uniform ablation and consequently films with a good thickness uniformity. The laser fluence on the target was maintained at ~ 1.5 J/cm 2 . Prior deposition, the vacuum chamber was pumped down to base pressure of ~5 × 10 -7 mbar and background gas was introduced manually using a needle valve. To growth of oxide precursors was carried out O (99.9995% purity, Air Liquide Danmark A/S) or Ar (99.9999% purity, Air Liquide Danmark A/S). In all experiments, the gas pressure was maintained at 5 × 10 -3 mbar. The films were grown (0001) Al2O3 and SiO2 (300 nm)/Si substrates of 1 x 1 cm 2 in size cut from large wafers. Before loading the substrates into the PLD chamber, they were sequentially cleaned in acetone, isopropanol (IPA) and deionized water (DI) water for 10 min in each solvent combined with sonication. The substrate were heated using a ramp rate of 15 ⁰C/min to the desired growth temperature and finally cooled down to room temperature using a similar cooling rate of 15 ⁰C/min. The WOx precursors were grown on a temperature range varying from 500 ⁰C to 800 ⁰C using a number of ~40 laser pulses. A minimum number of 30 pulses was required to obtain any deposition.
Sulfurization of WOx films. The sulfurization of WOx precursor films was performed as follows.
(1) Firstly, the precursor films were placed facing up onto a flat graphite plate and loaded in the middle of the three-zone tube furnace of a 3 inch outside diameter. A narrow alumina ceramic boat containing ∼2.0 g of sulfur flakes (≥99.99 % purity, from Sigma Aldrich) was placed outside the main heating area of the quartz tube and heated independently using an external heater. The distance between the ceramic boat and the samples was 35 cm.
(2) The system was pumped down to 5 × 10 -2 mbar and sequentially flushed 5 times by pressuring to 500 mbar with Ar-H2 mixture (5 % volume fraction, 99.9995 % purity, Air Liquide Danmark A/S) to remove residual gases, such as oxygen and water vapor. Then, a 200 sccm Ar-5 % H2 gas flow was introduced as a carrier gas throughout the entire process. The tube furnace was then heated at a rate of 15 ⁰C/min to 900 ⁰C and the sulfurization process took place at 900 ⁰C for 30 min. Moreover, when the furnace temperature reached 900 ⁰C, the external heater was set to 250 ⁰C to produce an adequate sulfur evaporation rate. The system was finally cooled down at a rate of 15 ⁰C/min to 500 ⁰C, and then naturally to room temperature.

Structural characterization of the WS2 films
Raman/PL spectroscopy. Raman and PL spectra were collected using home-built Raman confocal spectroscopy setup using a 532 nm excitation laser. The samples were illuminated through a 40x objective lens (NA=0.75), type Nikon CFI Plan Flour. The spectrometer is a Spectra Pro HRS-750 scanning monochromator from Princeton Instruments equipped with suitable gratings of 300 gr/mm (for PL measurements) and 1800 gr/mm (for Raman acquisition) and a cryogenicallycooled, ultra-low noise Pylon CCD camera, type PyLoN:100BR from Princeton Instruments. The wavelength calibration was carried using an Ar-Ne light source mounted directly on a side entrance slit of the spectrometer. The spectrometer resolution was found to be ∼1 cm -1 at 532 nm. Raman spectra were collected using various integration time and then normalized. The PL spectra were taken at a low laser power below 10 mW to prevent overheating.
Scanning Electron Microscopy (SEM). The morphologies of the WS2 crystals grown on either (0001) sapphire or SiO2/Si substrates were analyzed using a Zeiss Merlin microscope operated using an In Lens detector, acceleration voltages in the 1−2 kV and working distance between 3 and 4 cm.
Atomic Force Microscopy (AFM). AFM measurements were conducted at room temperature using a Bruker Icon AFM microscope.
X-ray Photoelectron Spectroscopy (XPS). The XPS spectra were recorded using a Thermo Fisher    Deconvolution of the Raman spectra using Lorentzian multiple-peak fitting is marked with red curves (major contributions presented).      Table S1, XPS data of WO3-x; Table S2, XPS data of WS2; Figure S1, S 2p core level XPS spectra of WS2; Figure S2, S 2p core level XPS spectra of WS2; Figure S3, W 4f core level XPS spectra of WS2 ; Figure S4, θ −2θ XRD pattern of WO2 on (0001) Al2O3; Figure S5, High-resolution SEM images of WS2; Figure S6, Optical images of WS2 monolayers; Section Raman and PL studies; Figure S7, DFT-calculated formation energy (E F ) of oxygen vacancies in WO3-x oxides; Figure   S8, Equilibrium oxygen vacancy concentration as a function of temperature; Figure S9, Visualization of sulfur interstitial (Si) and sulfur on oxygen site (SO) in WO3; Figure S10, Visualization of systems in the slab calculations; Figure S11, Intrinsic point defects on monolayer WS2; Figure S12, STEM image of 20.5 0 grain boundary in monolayer WS2; Figure S13, Lowresolution STEM image showing a 30.5 0 monolayer; Figure S14, STEM and Fourier-filtered images of bilayer WS2.

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes
Any additional relevant notes should be placed here.