Molten glass-mediated conditional CVD growth of MoS2 monolayers and effect of surface treatment on their optical properties

In the rapidly developing field of optoelectronics, the utilization of transition-metal dichalcogenides with adjustable band gaps holds great promise. MoS2, in particular, has garnered considerable attention owing to its versatility. However, a persistent challenge is to establish a simple, reliable and scalable method for large-scale synthesis of continuous monolayer films. In this study, we report the growth of continuous large-area monolayer MoS2 films using a glass-assisted chemical vapor deposition (CVD) process. High-quality monolayer films were achieved by precisely controlling carrier gas flow and sulfur vaporization with a customized CVD system. Additionally, we explored the impact of chemical treatment using lithium bistrifluoromethylsulfonylamine (Li-TFSI) salt on the optical properties of monolayer MoS2 crystals. To investigate the evolution of excitonic characteristics, we conditionally grew monolayer MoS2 flakes by controlling sulfur evaporation. We reported two scenarios on MoS2 films and flakes based on substrate-related strain and defect density. Our findings revealed that high-quality monolayer MoS2 films exhibited lower treatment efficiency due to substrate-induced surface strain. whereas defective monolayer MoS2 flakes demonstrated a higher treatment sensitivity due to the p-doping effect. The Li-TFSI-induced changes in exciton density were elucidated through photoluminescence, Raman, and x-ray photoelectron spectroscopy results. Furthermore, we demonstrated treatment-related healing in flakes under variable laser excitation power. The advancements highlighted in our study carry significant implications for the scalable fabrication of diverse optoelectronic devices, potentially paving the way for widespread real-world applications.


Introduction
Since the discovery of two-dimensional (2D) graphene, 2D materials have recently garnered great attention owing to their intriguing optoelectrical and mechanical properties [1][2][3][4][5].Among the 2D materials, molybdenum disulfide (MoS 2 ), a semiconductor with remarkable electrical and optical properties, including native n-type behavior and a lower dielectric constant, has tremendous promise for next-generation technological uses, particularly in photonics, energy storage, spintronics, and sensors [3,[6][7][8].Scaling, memory limitations, and uncontrollable leakage current issues below 5 nm gate length in silicon-based transistors have made MoS 2 transistors a promising candidate in integrated circuits [9,10].Recently, Wu et al achieved MoS 2 field-effect transistors with a 0.34 nm gate length in extraordinary architecture [11].
However, a viable method for synthesizing MoS 2 crystals with enhanced optical properties is necessary to realize future applications.Inhomogeneous films with non-uniform thickness, remnants of unreacted transition metal oxide precursors, and sulfur vacancies can lead to lower-quality films, resulting in non-scalable device characteristics [15].Particularly, continuous, reproducible films with fewer scattering centers are vital for scalable optoelectronic applications [16].Moreover, luminescent devices need more radiative recombination centers to obtain a strong current [17].The development of large-area monolayer MoS 2 crystals has received a lot of attention since the manufacture of these devices can be more compatible with the conventional complementary metal oxide semiconductor fabrication technique.At this point, chemical vapor deposition (CVD) and metal-organic CVD (MOCVD) offer repeatable and scalable growth conditions for high-quality monolayer MoS2 crystals [18][19][20].Compared to the MOCVD technique, CVD is a good candidate for relatively low-cost, large-area MoS 2 crystal growth [21].Solid-phase molybdenum trioxide (MoO 3 ) and sulfur (S) powder are mostly used as precursors to synthesize MoS 2 films.The ratio of chalcogen to transition metal oxide precursors is highly dependent on the location of the substrate, carrier gas concentration, vapor pressure, and temperature, making it difficult to control the uniformity of the MoS 2 crystals.Moreover, CVD-grown MoS 2 crystals exhibit significantly lower photoluminescence (PL) emission efficiency (PL quantum yield (QY) ≈%0.01-6) due to the presence of sulfur vacancies leading to nonradiative recombination.Comprehending the mechanisms behind PL emission is a pivotal performance metric that directly determines the maximum efficiency of devices, and this understanding is crucial as it heavily influences the optical and electronic characteristics of 2D MoS 2 .
While numerous efforts have been made, there is still a need for an effective, repeatable, and cost-efficient method to produce highly crystalline monolayer MoS 2 .Yu et al demonstrated the fabrication of wafer-scale epitaxial MoS 2 films on single-crystalline sapphire using homemade CVD systems [22].They employed two independently positioned quartz tubes for each precursor to regulate carrier gas pathways and introduced a small amount of O 2 with Ar to balance the growth rate, which resulted in the formation of two domain orientations of 0 • and 60 • , connected by a 60 • domain boundary.Yang et al successfully synthesized a 6 inch monolayer MoS2 film with a flake size of about 400 µm on soda-lime glass by using Mo foil and sulfur as precursors under Ar/O 2 gas flows at a ratio of 50/6 sccm [23].
In recent years, the chemical passivation of surface defects, especially sulfur vacancies, has proven to be a facile and effective method for enhancing the optical properties of MoS 2 crystals.Notably, Amani and Javey documented the use of a super acid, trifluoromethanesulfonimide (H-TFSI), resulting in a remarkable up to 190-fold increase in PL quantum yield (PL QY) efficiency by passivating sulfur vacancies [24].Despite its advantageous features, the corrosive nature of H-TFSI limits its applicability in optoelectronic applications, causing damage to both the 2D material and contacts [25].Moreover, there is no consensus in the literature regarding the mechanism by which the H-TFSI chemical process passivates sulfur vacancies [26].Amani et al proposed that sulfur vacancies are filled by sulfur adatoms on the monolayer MoS 2 surface through a TFSI-induced hydrogenation process [27].In contrast, Yamada et al found that in a strongly acidic proton (H+) environment, ultraviolet radiation-activated photocarriers were formed in monolayer MoS 2 .They demonstrated that the reaction with environmental molecules produces O 2 and OH radicals, partially passivating surface sulfur vacancies [28].Yamada et al suggested that H+ can consume electrons around sulfur vacancies and passivate the defect sites of the TFSI anion [28].As an alternative to H-TFSI, the use of ionic salts such as lithium bistrifluoromethylsulfonylamine (Li-TFSI) salt yielded a higher PL QY compared to H-TFSI, without damaging the components of the device [25].Unlike H-TFSI, Li-TFSI enables operations in an atmospheric environment instead of a glovebox.According to the theoretical study by Li et al, sulfur vacancy regions were identified as the most suitable adsorption sites for all adatoms.They demonstrated that the concentration of Li adatoms on MoS 2 was higher than that of H adatoms, leading to the superior suppression of trion formation and enhanced PL of transition metal dichalcogenides through Li-TFSI chemical treatment.
In this study, to control carrier gas flow and eliminate system contamination, we constructed a homemade reactor with coaxial quartz tubes and a thermal holder, enabling the continuous growth of large-area MoS 2 films and flakes by employing a molten glass-assisted CVD process.In glass-assisted CVD growth approach, molten glass serves as a promoter, supplying a sodium source that facilitates the vaporization of MoO 3 , and reduces precursor contamination as compared to the conventional NaCl-assisted process, enhances the growth rate by reducing formation energy and establishes repeatable, straightforward routes for the growth of high-quality MoS 2 crystals [23,[29][30][31].Subsequently, to investigate and enhance the optical properties of MoS 2 crystals, we treated the crystals with Li-TFSI salt.The pristine monolayer MoS 2 and Li-TFSI-treated crystals underwent comprehensive optical and structural characterization using techniques such as optical microscopy (OM), scanning electron microscopy (SEM), Raman and PL spectroscopy, x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM).Our findings indicate good film uniformity on the substrate surface, and we explored the PL properties of CVD-grown monolayer MoS 2 films and flakes, examining the evolution of A-exciton, trion, and B-exciton emissions under variable excitation power.This study reveals promising implications for MoS 2 -based optoelectronic devices, potentially paving the way for widespread industrial applications.

2D monolayer MoS 2 growth
Prior to the synthesis, the SiO 2 /Si substrates and soda lime lamellas with areas of 1 cm × 1 cm were cleaned using a sequential process of acetone, ethanol, and DI water in an ultrasonic bath.Cleaned SiO 2 /Si substrates, lamellas, S powder (300 mg), and MoO 3 powder (0.4 mg) were placed upstream at the same plane within the CVD system.Specifically, S powder was placed 15 cm away from the MoO 3 powder, and the substrate was positioned 3 cm away from the MoO 3 powder at the railed CVD system (see figure S1).Soda lime lamellas on the graphite foil were placed near the substrate in a small quartz tube with a diameter of 2 cm.The small quartz tube was placed in the center of a larger quartz tube with a diameter of 5 cm.Before heating, the furnace was evacuated to 5 × 10 −2 mbar and purged with 500 sccm of Ar gas to reach ambient pressure.This purging process was maintained for 5 min.Subsequently, the system was ramped up to 750 • C within 30 min and held at this temperature for the growth of MoS 2 for 5 min under atmospheric pressure.After completing the growth process, the MoS 2 film was rapidly cooled down to room temperature by opening the furnace cover.

Li-TFSI treatment
High-purity methanol was chosen as the solvent for the Li-TFSI treatment process.The MoS 2 samples were immersed into a concentrated solution of the chemical (0.02 M) for 40 min.After that, the samples were gently dried using a nitrogen flow.

Characterization of 2D MoS 2 crystals
2D MoS 2 crystals were thoroughly characterized using Raman and PL spectroscopy, OM, SEM, and XPS.Confocal Raman spectroscopy was conducted using a spectrometer (WITec) equipped with a laser source emitting at an excitation wavelength of 532 nm.Thermo Scientific K-Alpha was employed for XPS analyses on the samples.The SEM images were acquired using an FEI Nova NanoSEM 430.

Results and discussion
The schematic image in figure 1(a) illustrates the homemade CVD system, depicting the positions of the substrate and precursor utilized for synthesizing monolayer MoS 2 on SiO 2 /Si substrate.To stabilize gas flow, control S vaporization, and maintain a clean environment with minimal contamination, the alumina boat containing precursors and substrates was enclosed in a smaller quartz tube with a diameter of 2 cm, suspended within the outer quartz tube of the CVD system with a diameter of 5.0 cm.Refractive holders were employed for thermal stabilization inside the inner tube [32].Our process is optimized for three substrates with lateral sizes of 1 cm × 1 cm, one of which is a SiO 2 /Si substrate, and the other two are glasses on graphite foil at the same plane.Figure 1(b) illustrates the temperature evolution profile of the CVD reactor over time.The central region of the reactor was heated from room temperature to 750 • C with a ramping rate of 25 • C min −1 , while the lower temperature region was maintained at 200 • C with a ramping rate of 12 • C min −1 .After a growth duration of 5 min, the furnace lid was opened to cool the system immediately.The growth temperature is set at 750 • C, close to the glass's melting point, to preserve the natural morphology of soda-lime glasses.Detailed information regarding the growth technique is provided in the method section.
Soda-lime glass lamellas, primarily composed of sodium ions as a catalyzer, were employed as promoters in this study to synthesize monolayer MoS 2 film, consistent with previous research [18,23,29,33].The sodium concentration in the lamellas is critical in the molten glass-assisted process [31].The soda-lime glasses used in this work, containing approximately 10% sodium, are favorable for MoS 2 growth.Additionally, energy-dispersive x-ray spectroscopy investigations, in addition to silicon and oxygen host atoms, revealed that the glass also included atomic concentrations of 9.47% sodium, 0.76% calcium, 1.69% magnesium, and 0.12% aluminum.Previous studies have shown that sodium ions enhance the growth rate by reducing the energy barrier for MoS 2 growth and promote film formation by forming an intermediate product with a lower melting point than MoO 3 [34][35][36].The CVD process can be outlined in five key steps: (a) sublimation of precursors and transport by a carrier gas; (b) diffusion towards the substrate; (c) surface adsorption; (d) spreading of adatoms across the surface; and (e) the occurrence of the reaction to form flakes/film [37].The efficiency of precursor transport is directly dependent on the carrier gas.An important factor affecting the nucleation density is the movement of the carrier gas around the substrate.The gas flow direction causes a change in the Mo:S ratio, affecting the distribution of the flakes on the surface and their crystal orientation, shape, and thickness [38].Achieving an optimal Mo:S ratio alone is not enough for obtaining MoS 2 flakes/film; it also depends on precise transport with the carrier gas in terms of speed and direction.Initially, argon was employed as the carrier gas at a flow rate of 150 sccm, leading to intense nucleation on the substrate surface and resulting in a multilayer film (figure S2(a)).To observe the evolution from flakes to film formation, the argon flow rate was reduced to 100 sccm.Under this condition, the periphery of the substrate displayed monolayer MoS 2 flakes, while the central region exhibited uniform multilayer films (figure S2(b)).Further reduction of the argon flow rate to 50 sccm yielded a condition of uniformly grown monolayer MoS 2 film, as depicted in figures S2(c), 1(c) and (d).This meticulous adjustment of the carrier gas is imperative for achieving continuous and uniform 1L MoS 2 film.Figure 1(c) displays OM image of a uniform MoS 2 film on 1 × 1 cm 2 SiO 2 /Si substrate, and figure 1(d) presents an SEM image of the MoS 2 film.It is important to note that experiments conducted without the use of glass did not yield MoS 2 crystals, indicating conditional growth (see figure S3).Detailed information regarding the growth configuration is provided in the SI document.
To assess the uniformity of the films, the PL and Raman spectra were acquired from multiple random points on the substrates, as illustrated in figure 2(a).Results were obtained in various areas from a 1 × 1 cm 2 surface, and similar results were observed for each area.Figures 2(b) and (c) show the PL and Raman spectra of the monolayer MoS 2 film recorded at ten different points (1-10) given in figure 2(a).The PL spectra obtained from the ten points exhibit the A exciton at 1.89 eV and 200 µW, indicating a monolayer crystal across the large area, as depicted in figure S4(a).The PL centers of MoS 2 are positioned as follows; A exciton at 1.89 eV, trion at 1.86 eV, and B exciton at 2.02 eV. Figure S4(b) further demonstrates good film uniformity with comparable full width at half-maximum (FWHM), aligning with the characteristics observed for high-quality monolayer MoS 2 [39].In addition, a significantly high PL peak intensity was obtained without any surface modification.This enhancement can be attributed to the passivation effect of sodium ions on sulfur vacancies.Wang et al showed that sulfur vacancies serve as nonradiative recombination centers, leading to PL quenching based on charge density distribution analysis [40].Their findings suggest that the filling of sulfur vacancies by sodium atoms eliminates trap density levels in MoS 2 , enhancing the emission of excitons.
The Raman spectra presented in figure 2(c) reveal peak positions corresponding to the in-plane (E 1 2g ) and out-of-plane (A 1g ) vibration modes of MoS 2 , located at 385.58 and 405.20 cm −1 , respectively.The difference between the E 1 2g and A 1g peaks (∆) is approximately 19.62 cm −1 , a characteristic feature indicating that the as-grown MoS 2 film is a monolayer.The corresponding FWHM values for the E 1 2g mode exhibit a small deviation, with an average value of 6.9 cm −1 , as shown in figure 2(d).The FWHM values of the A 1g modes are presented in figure S4(c).Analysis of the Raman spectra reveals that the peaks exhibit no split or shift, suggesting that the monolayer MoS 2 film demonstrates a low defect density [24].
We used XPS to gain insight into the chemical composition of our MoS 2 film.As illustrated in figure 3, the XPS peaks for Mo 4+ 3d were prominently observed at 229.5 eV (Mo 3d 5/2 ) and 233.6 eV (Mo 3d 3/2 ), while the peaks for S 2p were located at 162.3 eV (2p 3/2 ) and 163.5 eV (2p 1/2 ), as depicted in figure 3(a).The results validate the expected stoichiometry of our as-grown monolayer, aligning with relevant literature [41].Subsequently, we utilized angle-resolved XPS (ARXPS) to investigate the depth profile of the sample.Measurements were conducted at different incident angles of 10 • , 20 • , 30 • , and 40 • .Intriguingly, the atomic ratios of Mo 4+ and S exhibited minimal variation with increasing incidence angle, as shown in figures 3(b) and (c), respectively.This observation suggests that the top surface of the synthesized MoS 2 film has not undergone significant contamination or deterioration due to surface oxidation.
After optimizing our custom CVD system and confirming the monolayer MoS 2 film, we conducted chemical treatment to observe the evolution of PL emission characteristics.The treatment was carried out in the ambient atmosphere using Li-TFSI ionic salt stored in an argon-filled glovebox.We performed PL, Raman, and XPS techniques to understand the effect of the Li-TFSI treatment on the characteristics of CVD-grown monolayer MoS 2 crystals.To elucidate the mechanism behind the excitonic changes, each PL spectrum underwent a fitting process using multi-peak Lorentzian analysis (figure 4(a)).Our pristine MoS 2 films exhibited three distinct and strong PL emission peaks at 1.88, 1.86, and 2.02 eV, corresponding to excitons (A), trions (A − ), and B excitons (B), respectively.In contrast, Li-TFSI treatment caused a dramatic decrease in the PL intensity of MoS 2 films, as shown in figure 4(a).Additionally, a redshift was observed in  the A, A -, and B exciton energies to 1.87, 1.85, and 2.00 eV, respectively.Alongside this decrease in PL peak intensity, we observed an increase in the integrated intensity ratio of I(A − )/I(A) from 0.22 to 0.34.
For a detailed investigation of changes in PL emission, Raman spectra were conducted for both pristine and Li-TFSI treated films (figure 4(b)).The 383.61 cm −1 in-plane E 1 2g mode of pristine MoS 2 corresponds to the in-plane vibration of Mo-S atoms, while the 403.81 cm −1 A 1g Raman mode arises from the out-of-plane vibration of S atoms in opposing directions and is sensitive to doping-induced electron density.Post-treatment, A 1g modes displayed a slight blueshift to 404.87, whereas the E 1 2g mode shifted to 385.41 cm −1 .The increase in the A 1g peak position implies p-doping through Li-TFSI treatment, resulting in a reduction of electron density within MoS 2 [42].However, the shift in the E 1 2g mode is mainly due to the development of strain stemming from defect passivation and surface-related effects.The observed increase in the integrated intensity ratio of I(A − )/I(A), indicative of an increasing charge density, suggests that the shift in the A 1g mode is likely due to cation absorption rather than defect-related p-doping [25].This indicates that cations and sulfur vacancies interact weakly due to surface strain.To compare the changes in optical properties resulting from treatment, we collected PL mapping data from both pristine and treated surfaces.This data demonstrates the relative uniformity in PL intensity after the treatment (See figure S5).
Moreover, the FWHM of the E 1 2g mode increased from 6.11 cm −1 to 6.65 cm −1 , whereas the FWHM of A 1g slightly decreased from 7.58 cm −1 to 7.26 cm −1 after Li-TFSI treatment (figure S6).However, these changes are not sufficient to explain the decrease in PL intensity.Javey et al showed that substrate-related surface strain negatively affected PL emission efficiency with TFSI [27,43].Additionally, the defect structure of the MoS 2 crystal significantly affects the passivation efficiency of Li-TFSI.Under growth conditions that lead to a sulfur deficiency, prevalent defects in the crystal consist predominantly of sulfur vacancies, making them more responsive to Li-TFSI treatment.The growth conditions employed result in MoS 2 films with a low defect density and strong A exciton emission.In conditions abundant with sulfur, defects primarily stem from the cation and are not susceptible to passivation by TFSI [27].
To comprehend the mechanism behind the intriguing PL behavior, we altered the CVD growth conditions by changing the sulfur position to explore the contribution of sulfur deficiency.In this case, we obtained monolayer MoS 2 flakes (figure S7) with a lower A exciton energy compared to the MoS 2 film and a significant redshift in the A exciton energy from 1.890 to 1.832 eV, indicating more defective crystals.The trion energy also decreased from 1.855 to 1.797 eV.After applying Li-TFSI under these conditions, we observed a strong enhancement in PL intensity and a significant blueshift in the A exciton energy from 1.822 to 1.868 eV.The trion energy increased from 1.797 to 1.840 eV.In addition, we observed a decrease in the I(A − )/I(A) ratio from 0.309 to 0.220, along with a dramatic increase in PL intensity, as shown in figure 5(a).In Raman measurements, remarkable variations were observed: the E 2g mode peak position shifted from 384. 19 cm −1 to 385.42 cm −1 , whereas the A 1g mode peak position shifted from 403.74 cm −1 to 405.08 cm −1 , as shown in figure 5(b).
Furthermore, the FWHM of the E 1 2g mode exhibited a negligible decrease from 6.57 cm −1 to 6.56 cm −1 , whereas the FWHM of A 1g exhibited a decrease from 7.44 cm −1 to 7.02 cm −1 after Li-TFSI treatment (figure S6).To gain insight into the changes in optical properties resulting from the treatments, we present PL mapping data from pristine and treated flakes, demonstrating an enhancement in PL intensity and relative uniformity post-treatment (see figure S8).In this regard, the emergence of tensile strain occurs due to an expansion in the in-plane lattice parameters, simultaneously leading to the healing of sulfur vacancies that tend to lower electron density.Consequently, the shift in the E 2g mode primarily stems from the development of tensile strain associated with sulfur vacancy healing.Conversely, the A 1g mode is more responsive to charge doping than the E 2g mode.Hence, the blueshift in the A 1g mode implies the introduction of p-doping through Li-TFSI treatment, attributable to the remediation of sulfur vacancies and the reduction of excess electrons in MoS 2 .
We then conducted AFM measurements on the same MoS 2 flake both before and after the Li-TFSI treatment.The OM images did not reveal any noticeable changes, which is consistent with the findings in the literature [44].However, the AFM images showed a slight decrease in root mean square roughness from 0.39 to 0.36 nm after the treatment, indicating an enhancement in surface uniformity, as illustrated in figure S9.
We further conducted XPS analysis on our MoS 2 flakes to confirm the contributions of Li-TFSI via p-doping, as seen in figure 6.The XPS analysis does not reveal any alteration in the stoichiometric ratio Typically, variations in excitation intensity affecting PL intensity serve as reliable markers for identifying the Li-TFSI healing effect on MoS 2 flakes.In essence, variations in the integral PL intensity, denoted as I, exhibit a dependency represented by a power law concerning the laser excitation power, P, as I ∝ P θ [48,49].A value of θ around 1 signifies a transition to excitons, whereas a substantially lower θ suggests a recombination involving defects [50].If trions and neutral excitons coexist, the intensity increase should show a superlinear pattern with a θ value between 1 and 2. However, the specific value of θ for each PL band depends on various conditions, including substrate type, defect concentration, and crystal quality of monolayers.Figure 7 presents the correlation between the integral intensity of PL and laser power after and before treatment in the MoS 2 flake.We achieved an increase in the θ value from 0.894 to 1.425 due to the healing of MoS 2 crystals.
The chemical treatment has been observed to exhibit different tendencies for MoS 2 film and flakes, and thus the mechanism of Li-TFSI treatment remains elusive.Recent studies have shown that treating MoS 2 with Li-TFSI significantly boosts PL emission by addressing defects and p-doping.However, some studies have reported that treated MoS 2 by Li-TFSI leads to a decrease in PL emission owing to substrate-related strain and defect-free MoS 2 crystals [51].In the first treatment approach, Li-TFSI helps mitigate electron density in MoS 2 , especially in areas with sulfur vacancies, while also reducing electron density via highly electronegative fluorine atoms in Li-TFSI.As a result, we observed that the efficiency of Li-TFSI treatment highly depends on surface strain and defect densities.While our monolayer MoS 2 films showed poor treatment performance due to a highly crystalline structure and substrate-related surface strain, our defective monolayer MoS 2 flakes, grown by controlling sulfur evaporation using our custom CVD system, showed obvious healing via Li-TFSI treatment.P-doping in monolayer MoS 2 flakes was verified by PL, Raman, XPS, and power-dependent PL measurements.Understanding these effects from chemical treatment is crucial since the optical and electronic characteristics of 2D MoS 2 heavily rely on these alterations.

Conclusion
To summarize, we have successfully fabricated a monolayer MoS 2 film on a 1 × 1 cm 2 SiO 2 /Si substrate using a custom-built CVD system.Our optical and structural investigations confirm the production of a high-quality monolayer MoS 2 film, achieved through the optimization of growth conditions facilitated by a glass-assisted CVD technique.To comprehensively characterize our MoS 2 films, we conducted PL, Raman, XPS, and power-dependent PL measurements on both pristine and Li-TFSI-treated MoS 2 films and flakes.Our investigations encompassed the analysis of strain and doping effects resulting from chemical treatment.PL results revealed that lower defective crystals and substrate strain lead to a significant reduction in Li-TFSI treatment efficiency.Raman results also showed that in the film case, strain effects dominated compared to the flake case.XPS and power-dependent PL measurements confirmed the p-doping mechanism on MoS 2 flakes.These observations underscore the intricate interplay between crystal quality and substrate-induced strain during chemical treatment of the film.Our findings demonstrate that the monolayer MoS 2 films synthesized in this study are exceptionally well-suited for integration into a wide range of optoelectronic applications.

Figure 1 .
Figure 1.Growth of 1L MoS2 film on SiO2/Si; (a) schematic of the CVD setup, (b) heating profile of MoO3 and S precursors during 1L MoS2 growth, (c) photo of 1 × 1 cm 2 and OM image of uniform 1L MoS2 film on SiO2/Si substrate, (d) SEM image of 1L MoS2 film.

Figure 2 .
Figure 2. Spectroscopic characterizations of 1L MoS2 film synthesized; (a) OM images of large area 1L MoS2 film (b) PL spectra of monolayer 1L MoS2, (c) Raman spectra of monolayer 1L MoS2 film at 10 different locations marked in (a) indicating a high-quality uniform monolayer MoS2, (d) E2g peak positions and the corresponding FWHM values collected from the Raman spectra in (b).

Figure 3 .
Figure 3. Compositional characterizations of 1L MoS2 film synthesized.XPS spectra of monolayer MoS2 film, indicating (a) the Mo 3d and S 2p binding energies of MoS2; atomic ratios of (b) Mo 4+ and (c) S at different incident angles via angle-resolved XPS spectra.

Figure 4 .
Figure 4. Evolution of A exciton, trion, and B exciton peaks in case of (a) Li-TFSI treatment MoS2 film and pristine MoS2 film, (b) Raman spectra of Li-TFSI treatment MoS2 film and pristine MoS2 film.

Figure 7 .
Figure 7. PL intensity of pristine and Li-TFSI treated MoS2 flake as a function of the laser power plotted on a log-log scale.