Tailored Synthesis of Heterogenous 2D TMDs and Their Spectroscopic Characterization

Two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) show great potential across various applications. However, synthesizing large-scale structures poses challenges owing to the intricate growth parameters, forming unexpected hybrid film structures. Thus, precision in synthesis and thorough structural analysis are essential aspects. In this study, we successfully synthesized large-scale structured 2D transition metal dichalcogenides (TMDs) via chemical vapor deposition using metal oxide (WO3 and MoO3) thin films and a diluted H2S precursor, individual MoS2, WS2 films and various MoS2/WS2 hybrid films (Type I: MoxW1−xS2 alloy; Type II: MoS2/WS2 vdWH; Type III: MoS2 dots/WS2). Structural analyses, including optical microscopy, Raman spectroscopy, transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy, and cross-sectional imaging revealed that the A1g and E2g modes of WS2 and MoS2 were sensitive to structural variations, enabling hybrid structure differentiation. Type II showed minimal changes in the MoS2′s A1g mode, while Types I and III exhibited a ~2.8 cm−1 blue shift. Furthermore, the A1g mode of WS2 in Type I displayed a 1.4 cm−1 red shift. These variations agreed with the TEM-observed microstructural features, demonstrating strain effects on the MoS2–WS2 interfaces. Our study provides insights into the structural features of diverse hybrid TMD materials, facilitating their differentiation through Raman spectroscopy.


Introduction
Transition metal dichalcogenides (TMDs), akin to two-dimensional (2D) layered structures like graphene, have attracted considerable attention and have been widely applied-from insulators and semiconductors to superconductors-owing to their diverse properties [1][2][3].Recent studies have highlighted the application scope of heterostructures formed by vertically or horizontally stacking different types of single TMD materials, showcasing their potential in the development of optoelectronic systems, solar cells, and nanoelectronic devices [4][5][6][7][8][9][10][11].Vertical van der Waals heterostructures (vdWHs), e.g., MoS 2 /WS 2 structures, are particularly noteworthy.Engineered through material selection and junction design, vdWHs operate through phenomena such as light-induced separation or recombination of electron-hole pairs, driving active research in photodetection, photocatalysis, and development of solar cells [5,7,8].While mechanical exfoliation and transfer processes can be employed for vdWH synthesis and characterization [12,13], films obtained through mechanical exfoliation exhibit low yields and have inconsistent thicknesses and nonuniform size distribution, hindering the use of such methods in large-scale production.Additionally, interlayer interfaces are susceptible to contamination during transfer.In contrast, chemical vapor deposition (CVD) provides a facile route for vdWH synthesis by controlling variables such as precursors, temperature, and gas atmosphere.CVD allows easy thickness modulation, scalability, and clean interface formation during synthesis, mitigating contamination issues.As a result, CVD has become the primary method for synthesizing TMD-based vdWHs [14][15][16][17].Various vertical heterostructures, such as WS 2 /MoS 2 , directly synthesized via CVD, have been reported [18][19][20].
One of the challenges in the synthesis of vdWHs is the limited feedback for establishing optimal synthesis conditions post-microstructure examination.During CVD-based synthesis, variations in temperature, atmosphere, and precursor supply may result in the formation of not only 2D TMD films but also TMD hybridized structures with diverse configurations, resembling alloys or quantum dots, rather than vdWHs.Surface observations through techniques such as optical microscopy (OM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) can discern morphological differences; however, distinguishing between alloyed structures and vdWHs is challenging.Although XPS provides insights into elemental composition and chemical bonding, the differentiation of the structural differences in vertically stacked samples remains arduous.Hence, the precise analysis of fine structures can be achieved by directly observing sample interfaces through transmission electron microscopy (TEM).Given that properties vary with structures, clear distinctions in the detailed structures of CVD-synthesized TMD heterostructures are crucial for optimizing vdWH synthesis conditions.However, conducting TEM analysis for every sample is inefficient.
In this study, we propose a more general analytical method by comparing and analyzing Raman spectra with peaks belonging to the A 1g and E 2g modes vibrating in vertical and horizontal directions with TEM images to identify the microstructures.Two-dimensional films, such as TMDs, are sensitive to strain at interfaces, leading to the transition of phonon vibration modes at the interface.When materials such as MoS 2 and WS 2 exhibit different microstructures in alloys and vdWHs, the strain distribution at the interface may differ.We designed and synthesized various TMD heterostructures, performing Raman spectroscopy to analyze the behavior of phonons under strain in the A 1g and E 2g modes at the interface, allowing the clear distinction of microstructural differences.Furthermore, we confirmed the identification of microstructures by analyzing the phonon behavior arising from strain differences at the interface of TMD heterostructures synthesized via CVD on SiO 2 /Si substrates.For validation, we synthesized various MoS 2 /WS 2 hybrid films (Type I: Mo x W 1−x S 2 alloy; Type II: MoS 2 /WS 2 vdWH; Type III: MoS 2 dots/WS 2 ) via CVD.The fine microstructures of the synthesized TMD films' heterostructures were examined by preparing TEM specimens through a focused ion beam.Raman mapping of these microstructures was also performed.By analyzing variations in the mobility of the A 1g and E 2g modes in relation to differences in microstructures, we were able to establish correlations between the observed changes in phonon behavior and specific microstructural characteristics.

Preparation of TMO Thin Films (WO 3 , MoO 3 ) on A SiO 2 Substrate
A 100 mm-sized 300 nm SiO 2 /Si substrate was cleaned with acetone and isopropyl alcohol in an ultrasonic cleaner for 1 h.Subsequently, the cleaned substrate was positioned on the rotation stage within a thermal evaporator system.Once the base pressure of the chamber dropped below 1 × 10 −6 Torr, the transition metal oxide (TMO) was evaporated by thermal heating.The TMO was deposited on a SiO 2 substrate at a rate of 0.1 Å/s and at room temperature.

Synthesis of TMD (MoS 2 , WS 2 ) Thin Films under H 2 S-Ar Mixed Gas
Crystalline TMD (MoS 2 , WS 2 ) thin films were grown in a CVD system.The thermally deposited TMO (MoO 3 , WO 3 ) film on the SiO 2 substrate was placed at the center of a quartz tube in a furnace.Then, the pressure of the quartz tube was decreased to less than 1 × 10 −3 Torr, and the quartz tube was purged with Ar (200 sccm).Next, the manual valve was shut until the pressure reached 350 Torr, and the furnace was then heated to 800 • C at a rate of 25 • C min −1 .Upon reaching 800 • C, the furnace was purged for 10 min with H 2 S gas (0.6 Torr).Subsequently, the chamber was left to naturally cool to room temperature.

Transfer of Hetero-TMD Thin Film onto the TEM Grid
The TMD samples synthesized on the Si substrate were immersed in a 2 mol NaOH aqueous solution for 30 s on a hot plate heated to 80 • C. Upon placing the TMD sample diagonally into deionized water, the TMD thin film separates from the substrate and remains on the water surface.The floating TMD thin film was then carefully transferred onto a TEM grid, left to dry naturally in a desiccator, and was further dried in an oven at 100 • C for 3 h.

Characterization
To investigate the microstructure of the hetero-TMD thin films, TEM was conducted using a spherical aberration-corrected transmission electron microscope (Thermo Fisher Scientific, Waltham, MA, USA, Titan G2 60-300) equipped with a monochromator at an accelerating voltage of 80 kV.Measurements were performed at this acceleration voltage to minimize any knock-on damage and contamination caused by the beam.X-ray photoelectron spectroscopy (XPS) analyses were performed using a K-Alpha XPS system and a monochromatic Al Ka X-ray radiation source (Thermo Fisher Scientific).The elements composing the TMO and TMD thin films (W4f, Mo3d, S2p, C1s, and O1s) were measured by irradiating X-rays with a pass energy of 50 eV in an area of 400 µm 2 .Raman spectroscopy (Renishaw, Wotton-under-Edge, Glos, UK, Renishaw Invia Reflex) was performed using a 514 nm laser with an 1800/mm grating to observe the phonon modes of the TMD thin films (WS 2 and MoS 2 ).Recent studies have revealed that the E 2g and A 1g peaks in the Raman spectrum of MoS 2 are influenced by mechanical strain and changes in charge carrier concentration, resulting in shifts in their frequencies [13,21].To examine this, we performed a correlation analysis between the E 2g and A 1g peaks of the MoS 2 thin films to study the influence of the hetero-TMD structure on the mechanical deformation of the samples.For this analysis, Raman mapping was conducted on a 60 µm × 60 µm area at 1 µm intervals.Additionally, deconvolution of the E 2g and A 1g peaks in the Raman spectrum of the hetero-TMD thin films, specifically WS 2 and MoS 2 , was conducted to extract their respective frequencies.

Results and Discussion
Figure 1 shows the synthesis and characteristics of the three samples.The metal oxide thin films (MoO 3 or WO 3 , Sigma Aldrich, St. Louis, MI, USA) were deposited on SiO 2 substrates using a thermal evaporator.During the deposition of the MoO 3 film, efforts were made to create a patterned structure on the SiO 2 substrate.A TEM grid served as a shadow mask to create a honeycomb-shaped pattern, ensuring precise positioning for comparative analyses.The use of metal oxides instead of metals is advantageous owing to their lower melting points, enabling reduced processing temperatures.The TMD samples used for analysis were prepared by loading the MoO 3 and WO 3 films into a CVD chamber, lowering the pressure using a vacuum pump, and supplying diluted H 2 S gas (2% H 2 S diluted in Ar) at 800 • C under 300 Torr pressure with argon for 10 min.This method was used to synthesize the combination depicted in Figure 1.The number of TMD layers is affected by the thickness of the metal oxide; by adjusting the thickness (number of layers) of TMD in proportion to the thickness of TMO, we sulfurized TMO (MoO 3 , WO 3 ) of 1, 3, and 5 nm thicknesses to produce the corresponding TMD (MoS 2 , WS 2 ).After synthesis, the Raman spectrum was confirmed (Figure S1 and Table S1).The positions and ratios of the E 2g and A 1g peaks show that the number of TMD layers increases with the oxide thickness.The results from Figure S1 indicate the feasibility of synthesizing TMD from TMO of 1 nm thickness.However, as shown in Figure S2, the TEM observations reveal that WS 2 films synthesized from 1 nm WO 3 exhibit a non-uniform structure with small grain sizes and numerous holes.In contrast, WS 2 films synthesized using a 3 nm-thick WO 3 precursor exhibit a relatively uniform film shape.Furthermore, examination of the folded regions in the WS 2 film indicated the presence of four layers.Therefore, in the synthesis of heterogenous TMD, the thickness of the etch metal oxide was set at 3 nm for conducting the experiments.
thicknesses to produce the corresponding TMD (MoS2, WS2).After synthesis, the Raman spectrum was confirmed (Figure S1 and Table S1).The positions and ratios of the E2g and A1g peaks show that the number of TMD layers increases with the oxide thickness.The results from Figure S1 indicate the feasibility of synthesizing TMD from TMO of 1 nm thickness.However, as shown in Figure S2, the TEM observations reveal that WS2 films synthesized from 1 nm WO3 exhibit a non-uniform structure with small grain sizes and numerous holes.In contrast, WS2 films synthesized using a 3 nm-thick WO3 precursor exhibit a relatively uniform film shape.Furthermore, examination of the folded regions in the WS2 film indicated the presence of four layers.Therefore, in the synthesis of heterogenous TMD, the thickness of the etch metal oxide was set at 3 nm for conducting the experiments.First, for MoxW1-xS2 (Type I), we deposited WO3 and MoO3 films sequentially, each with a thickness of 3 nm, onto the substrate using a thermal evaporator.Following this, they were loaded into a chamber and synthesized at 800 ℃ using dilute H2S gas for sulfurization.Second, the fabrication of MoS2/WS2 vdWH (Type II) involved a two-step synthesis.Initially, we sulfurized only the WO3 film (3 nm) to form the WS2 film.Then, we deposited MoO3 (3 nm) onto the WS2 film, which was subsequently sulfurized.Finally, for the fabrication of MoS2 dots/WS2 (Type III), after synthesizing the WS2 film, the quartz substrate (thickness = 2 mm) was positioned in an inverted orientation.A second layer of transition metal oxide, MoO3 film, was placed using a quartz substrate as a spacer.The MoO3, which vaporized at 800 °C under a low pressure of 600 mTorr, reacted with the H2S on the WS2 surface, resulting in its conversion into droplet-shaped MoS2.
Figure 2 shows the optical images of the Type I, II and III samples, along with the mapping results of the intensity of the E2g Raman peak signals for MoS2 and WS2.Noticeable shading differences are evident in the OM images (Figure 2a) for each sample.To analyze these differences, Raman mapping was conducted at a resolution of 1 μm over a 60 μm × 60 μm area centered around the edges of the samples patterned in a hexagonal grid structure.The mapping results revealed clear boundaries between the MoS2 and WS2, indicating no horizontal growth during the sulfurization of the MoO3 film.Furthermore, First, for Mo x W 1−x S 2 (Type I), we deposited WO 3 and MoO 3 films sequentially, each with a thickness of 3 nm, onto the substrate using a thermal evaporator.Following this, they were loaded into a chamber and synthesized at 800 • C using dilute H 2 S gas for sulfurization.Second, the fabrication of MoS 2 /WS 2 vdWH (Type II) involved a two-step synthesis.Initially, we sulfurized only the WO 3 film (3 nm) to form the WS 2 film.Then, we deposited MoO 3 (3 nm) onto the WS 2 film, which was subsequently sulfurized.Finally, for the fabrication of MoS 2 dots/WS 2 (Type III), after synthesizing the WS 2 film, the quartz substrate (thickness = 2 mm) was positioned in an inverted orientation.A second layer of transition metal oxide, MoO 3 film, was placed using a quartz substrate as a spacer.The MoO 3 , which vaporized at 800 • C under a low pressure of 600 mTorr, reacted with the H 2 S on the WS 2 surface, resulting in its conversion into droplet-shaped MoS 2 .
Figure 2 shows the optical images of the Type I, II and III samples, along with the mapping results of the intensity of the E 2g Raman peak signals for MoS 2 and WS 2 .Noticeable shading differences are evident in the OM images (Figure 2a) for each sample.To analyze these differences, Raman mapping was conducted at a resolution of 1 µm over a 60 µm × 60 µm area centered around the edges of the samples patterned in a hexagonal grid structure.The mapping results revealed clear boundaries between the MoS 2 and WS 2 , indicating no horizontal growth during the sulfurization of the MoO 3 film.Furthermore, the Type I, II and III samples exhibited both E 2g and A 1g peaks for the MoS 2 and WS 2 , enabling easy differentiation of the MoS 2 and WS 2 films [20].Nevertheless, variations in the peak positions and signal intensities of the E 2g and A 1g modes were observed in the Raman spectra, as depicted in Figure 2b, depending on the synthesis method.The differences in color observed in the OM images and Raman spectra among the Type I, II and III samples signify distinct morphological or structural states.
the Type I, II and III samples exhibited both E2g and A1g peaks for the MoS2 and WS2, enabling easy differentiation of the MoS2 and WS2 films [20].Nevertheless, variations in the peak positions and signal intensities of the E2g and A1g modes were observed in the Raman spectra, as depicted in Figure 2b, depending on the synthesis method.The differences in color observed in the OM images and Raman spectra among the Type I, II and III samples signify distinct morphological or structural states.XPS analysis was performed to investigate the chemical composition and binding energy of the MoS2 and WS2 thin films synthesized from metal oxides along with Type I, II and III samples.All the XPS spectra were based on the C1s peak (284.5 eV) to correct for charging effects.The XPS spectral changes before and after TMD synthesis of the metal oxide thin films are shown in Figure S3.In the W4f spectrum of the WO3 thin film, the doublet peaks of 37.6 and 35.8 eV correspond to W4f5/2 and W4f7/2, respectively, and indicate the W 6+ of WO3.After sulfidation into a WS2 thin film, the W 6+ phase decreases in the W4f spectrum, generating a W 4+ doublet peak corresponding to the hexagonal (2H) phase of WS2 located at 34.6 and 32.5 eV [22].In the MoO3 thin film (Mo3d spectrum), the Mo 5+ phase located at 235.1 and 231.9 eV, and the Mo 6+ phase located at 235.5 and 232.4 eV, corresponding to the oxide state, were confirmed.However, after sulfurization, the oxide phase decreased and appeared together with the Mo 4+ peak, whereas the S2s peak was located at 226.8 eV, indicating the formation of MoS2.The Mo 4+ phase was divided into a tetragonal (1T) phase corresponding to 232.6 and 229.7 eV and a 2H phase corresponding to 233.1 and 229.8 eV through peak deconvolution [23][24][25].The MoS2 thin film was analyzed in a form in which the 1T and 2H phases coexist.And S2p peak was located at 163.25 and 162.1 eV.The ratio of transition metal to sulfur in WS2 and MoS2 films showed an approximate ratio of 1:2.
Figure 3 shows the XPS spectra of the MoS2/WS2 hybrid thin films: (a) W4f, (b) Mo3d, and (c) S2p of Type I, II and III samples.In Figure 3a, unlike in the WS2 thin film, doublet peaks at 38.6 and 36.8 eV are added in the W4f spectrum of Types I, II and III, corresponding to the peaks of Mo4p [26].Except for the W4f XPS spectrum, no significant changes were observed in the Mo3d and S2p XPS spectra.The binding energy and elemental ratios for these XPS peaks are summarized in Table S2.Type II exhibits a ratio of 1:1.7 for the (Mo + W):S elements that make up MoS2 and WS2.These XPS quantitative analysis results XPS analysis was performed to investigate the chemical composition and binding energy of the MoS 2 and WS 2 thin films synthesized from metal oxides along with Type I, II and III samples.All the XPS spectra were based on the C1s peak (284.5 eV) to correct for charging effects.The XPS spectral changes before and after TMD synthesis of the metal oxide thin films are shown in Figure S3.In the W4f spectrum of the WO 3 thin film, the doublet peaks of 37.6 and 35.8 eV correspond to W4f 5/2 and W4f 7/2 , respectively, and indicate the W 6+ of WO 3 .After sulfidation into a WS 2 thin film, the W 6+ phase decreases in the W4f spectrum, generating a W 4+ doublet peak corresponding to the hexagonal (2H) phase of WS 2 located at 34.6 and 32.5 eV [22].In the MoO 3 thin film (Mo3d spectrum), the Mo 5+ phase located at 235.1 and 231.9 eV, and the Mo 6+ phase located at 235.5 and 232.4 eV, corresponding to the oxide state, were confirmed.However, after sulfurization, the oxide phase decreased and appeared together with the Mo 4+ peak, whereas the S2s peak was located at 226.8 eV, indicating the formation of MoS 2 .The Mo 4+ phase was divided into a tetragonal (1T) phase corresponding to 232.6 and 229.7 eV and a 2H phase corresponding to 233.1 and 229.8 eV through peak deconvolution [23][24][25].The MoS 2 thin film was analyzed in a form in which the 1T and 2H phases coexist.And S2p peak was located at 163.25 and 162.1 eV.The ratio of transition metal to sulfur in WS 2 and MoS 2 films showed an approximate ratio of 1:2.
Figure 3 shows the XPS spectra of the MoS 2 /WS 2 hybrid thin films: (a) W4f, (b) Mo3d, and (c) S2p of Type I, II and III samples.In Figure 3a, unlike in the WS 2 thin film, doublet peaks at 38.6 and 36.8 eV are added in the W4f spectrum of Types I, II and III, corresponding to the peaks of Mo4p [26].Except for the W4f XPS spectrum, no significant changes were observed in the Mo3d and S2p XPS spectra.The binding energy and elemental ratios for these XPS peaks are summarized in Table S2.Type II exhibits a ratio of 1:1.7 for the (Mo + W):S elements that make up MoS 2 and WS 2 .These XPS quantitative analysis results indicate a sulfur deficiency occurred during the synthesis process of the MoS 2 /WS 2 hybrid films.
indicate a sulfur deficiency occurred during the synthesis process of the MoS2/WS2 hybrid films.For the investigation of the fine structures of MoS2 and WS2 within the Type I, II and III samples, we conducted focused ion beam (FIB) processing on the thin sections of the samples and observed the cross sections using high-resolution TEM (Figure 4).For the investigation of the fine structures of MoS 2 and WS 2 within the Type I, II and III samples, we conducted focused ion beam (FIB) processing on the thin sections of the samples and observed the cross sections using high-resolution TEM (Figure 4).For the investigation of the fine structures of MoS2 and WS2 within the Type I, II and III samples, we conducted focused ion beam (FIB) processing on the thin sections of the samples and observed the cross sections using high-resolution TEM (Figure 4).   Figure 4 presents the cross-sectional views of the Type I, II and III samples, showing energy-dispersive X-ray spectroscopy (EDS) mapping images of Mo (in blue) and W (in orange) obtained through scanning transmission electron microscopy (STEM) and highresolution transmission electron microscopy (HRTEM).For Type I, the EDS mapping showed a mixed distribution of Mo and W, indicating that the film had a Mo x W 1−x S 2 alloy structure.The cross-sectional HRTEM image of Type I exhibits ambiguous shading, making it difficult to distinguish the boundary between the MoS 2 and WS 2 layers.In contrast, for Type II, the EDS map illustrates the vertical separation of Mo and W, whereas the high-resolution TEM image reveals distinct layer-by-layer structures between the WS 2 and MoS 2 .In the case of Type III, the HRTEM image and EDS mapping confirmed the formation of MoS 2 with a droplet-like dot structure, with a size of 20-30 nm, on the WS 2 film.This phenomenon resulted from the poor wetting properties of the MoS 2 on the WS 2 surface in high-temperature and low-pressure environments.The comparison of WS 2 and MoS 2 revealed a darker appearance for the WS 2 region, attributed to the difference in atomic numbers between Mo and W. Each film consisted of 4-5 layers.The EDS mapping results and cross-sectional HRTEM images indicated that the synthesized three types of MoS 2 /WS 2 hybrid films had alloy-, vdWH-, and dot-like fine structures.
To accurately analyze the structural differences observed through TEM and their correlation with the Raman spectroscopic characteristics, we conducted a detailed analysis by fitting the position distribution and gap of the E 2g and A 1g modes from the mapping data in Figure 2a.The results of this analysis are summarized in Table 1.The data used for the analysis in Figure 5 and Table 1 were exclusively extracted from the MoS 2 /WS 2 hybrid film areas within the hexagonal grid pattern regions, excluding the areas outside the hexagonal grid pattern.Figure 5a illustrates the distribution of the A 1g peak positions of the MoS 2 and WS 2 in the Raman spectra represented by histograms.The colors correspond to Type I (red), Type II (blue), and Type III (magenta), with the green and gray lines indicating MoS 2 and WS 2 , respectively.Regarding the MoS 2 A 1g mode at 407.6 cm −1 , Type II exhibited a red shift of 0.5 cm −1 , while Types I and III showed blue shifts of 2.8 cm −1 .For the WS 2 , referencing the A 1g mode at 418.9 cm −1 , Types I and II displayed red shifts of 1.4 and 0.2 cm −1 , respectively, whereas Type III exhibited a 0.2 cm −1 blue shift.Figure 5b demonstrates the distribution of the E 2g peak positions for MoS 2 and WS 2 in the histograms using the same color scheme as in Figure 5a.For the MoS 2 E 2g mode at 383.6 cm −1 , Types I, II, and III showcased red shifts of 4.1, 1.5, and 2.6 cm −1 , respectively.Regarding the WS 2 E 2g mode at 357.2 cm −1 , Types I, II and III displayed red shifts of 0.9, 2.4, and 0.8 cm −1 , respectively.Shifts observed in the E 2g and A 1g modes of MoS 2 and WS 2 in the Raman spectra are typically attributed to doping or strain.However, in the MoS 2 /WS 2 hybrid film analyzed in this study, there were no opportunities or processes for doping during the CVD synthesis.Considering the distinct microstructures observed via TEM (as described in Figure 4), it is likely that the shifts in the E 2g and A 1g modes in the Raman spectra are attributed to the strain exerted between MoS 2 and WS 2 at the interface.Examining the A 1g mode of the MoS 2 , the peak of Type I indicates a mixture of MoS 2 and WS 2 (as in Figure 4a), resulting in a blue shift of 2.8 cm −1 , while Type III, characterized by MoS 2 dot structures, experiences a vertical strain, causing a 2.8 cm −1 blue shift.Conversely, Type II possesses a clear vdWH structure at the MoS 2 -WS 2 boundary, experiencing minimal strain, resulting in only a subtle 0.5 cm −1 red shift.Regarding the WS 2 A 1g mode, Types II and III displayed minor shifts, indicating the presence of a nearly negligible strain.Type II exhibits a horizontally uniform interface in the vdWH structure, whereas Type III creates local areas for the MoS 2 dots, contributing to the interface.In contrast, a noticeable 1.4 cm −1 red shift was observed for Type I, owing to vertical strain induced by the alloy microstructure.Additionally, the E 2g modes of MoS 2 and WS 2 displayed red shifts in Types I, II and III.The horizontally acting strain in all the samples aligns well with the structural characteristics.Notably, the standard deviation of the red shift in Type I was smaller than that of the other samples, implying a uniformly distributed horizontal strain attributed to vdWHs.Figure 5c shows the correlation plot of MoS 2 ′ s E 2g and A 1g modes [13,21].Types I and III exhibited scattered features at strain levels between 1.0% and 0.75%, whereas Type II showed a consistent distribution at 0.5% strain.These analytical findings indicate the capability to differentiate the microstructure of TMD hybrid films by analyzing the strain-induced behavior in the E 2g and A 1g modes of the Raman spectra.
showed a mixed distribution of Mo and W, indicating that the film had a MoxW1-xS2 alloy structure.The cross-sectional HRTEM image of Type I exhibits ambiguous shading, making it difficult to distinguish the boundary between the MoS2 and WS2 layers.In contrast, for Type II, the EDS map illustrates the vertical separation of Mo and W, whereas the highresolution TEM image reveals distinct layer-by-layer structures between the WS2 and MoS2.In the case of Type III, the HRTEM image and EDS mapping confirmed the formation of MoS2 with a droplet-like dot structure, with a size of 20-30 nm, on the WS2 film.This phenomenon resulted from the poor wetting properties of the MoS2 on the WS2 surface in high-temperature and low-pressure environments.The comparison of WS2 and MoS2 revealed a darker appearance for the WS2 region, attributed to the difference in atomic numbers between Mo and W. Each film consisted of 4-5 layers.The EDS mapping results and cross-sectional HRTEM images indicated that the synthesized three types of MoS2/WS2 hybrid films had alloy-, vdWH-, and dot-like fine structures.
To accurately analyze the structural differences observed through TEM and their correlation with the Raman spectroscopic characteristics, we conducted a detailed analysis by fitting the position distribution and gap of the E2g and A1g modes from the mapping data in Figure 2a.The results of this analysis are summarized in Table 1.The data used for the analysis in Figure 5 and Table 1 were exclusively extracted from the MoS2/WS2 hybrid film areas within the hexagonal grid pattern regions, excluding the areas outside the hexagonal grid pattern.

Conclusions
In this study, we synthesized MoS 2 and WS 2 individual single TMD films and MoS 2 /WS 2 hybrid films, namely, Type I (Mo x W 1−x S 2 alloy), Type II (MoS 2 /WS 2 vdWH), and Type III (MoS 2 dots/WS 2 ), via CVD using metal oxide (WO 3 and MoO 3 ) films and dilute H 2 S gas as precursors.The synthesized MoS 2 /WS 2 hybrid films were characterized via OM and Raman mapping to confirm variations in their structural composition.Microstructural analysis for structure identification was conducted through TEM with EDS and cross-sectional imaging.Furthermore, Raman mapping facilitated a detailed comparison and analysis of the phonon behavior induced by strain at the MoS 2 and WS 2 interfaces.Based on their structural characteristics, distinct variations in the A 1g and E 2g modes were observed in the MoS 2 /WS 2 hybrid films compared to those in the individual MoS 2 and WS 2 TMD films.Type II showed minimal changes in MoS 2 A 1g mode, while Type I and III exhibited a ~2.8 cm −1 blue shift.In contrast, the WS 2 A 1g mode in Type I showed a 1.4 cm −1 red shift, whereas Types II and III showed negligible or slight changes in peak positions.The alignment observed between strain-induced changes in the A 1g mode and structural features of MoS 2 /WS 2 hybrid films underscores the application scope of Raman spectroscopy

Figure 1 .
Figure 1.Schematic of TMD synthesis and structural diagrams of Types I, II and III.

Figure 1 .
Figure 1.Schematic of TMD synthesis and structural diagrams of Types I, II and III.

Figure 2 .
Figure 2. (a) Intensity mapping of the E 2g mode at 383 cm −1 (MoS 2 E 2g mode) and 355 cm −1 (WS 2 E 2g mode) for each structure of the MoS 2 /WS 2 hybrid thin film.(b) Representative Raman spectra of each sample.

Figure 4 .
Figure 4. (a-c) STEM-EDS images and (d-f) cross-sectional TEM images of Types I, II and III.In the EDS map, blue corresponds to Mo, and orange corresponds to W elements.

Figure 4
Figure 4 presents the cross-sectional views of the Type I, II and III samples, showing energy-dispersive X-ray spectroscopy (EDS) mapping images of Mo (in blue) and W (in orange) obtained through scanning transmission electron microscopy (STEM) and high-

Nanomaterials 2024 ,
14, 248 6 of 10 indicate a sulfur deficiency occurred during the synthesis process of the MoS2/WS2 hybrid films.

Figure 4 .
Figure 4. (a-c) STEM-EDS images and (d-f) cross-sectional TEM images of Types I, II and III.In the EDS map, blue corresponds to Mo, and orange corresponds to W elements.

Figure 4
Figure 4 presents the cross-sectional views of the Type I, II and III samples, showing energy-dispersive X-ray spectroscopy (EDS) mapping images of Mo (in blue) and W (in orange) obtained through scanning transmission electron microscopy (STEM) and high-

Figure 4 .
Figure 4. (a-c) STEM-EDS images and (d-f) cross-sectional TEM images of Types I, II and III.In the EDS map, blue corresponds to Mo, and orange corresponds to W elements.

Figure 5 .
Figure 5. Histogram of the (a) A1g and (b) E2g peak positions for MoS2 and WS2 in Types I, II and III.(c) Correlative plot of the A1g and E2g peak positions for MoS2 to evaluate biaxial strain and charge doping distributions in Type I, II and III films.Red, blue and magenta colors indicate Type I (MoxW1-xS2 alloy), Type II (MoS2/WS2 vdWH), and Type III (MoS2 Dots/WS2) films, respectively.

Table 1 .Figure 5 .
Figure 5. Histogram of the (a) A 1g and (b) E 2g peak positions for MoS 2 and WS 2 in Types I, II and III.(c) Correlative plot of the A 1g and E 2g peak positions for MoS 2 to evaluate biaxial strain and charge doping distributions in Type I, II and III films.Red, blue and magenta colors indicate Type I (Mo x W 1−x S 2 alloy), Type II (MoS 2 /WS 2 vdWH), and Type III (MoS 2 Dots/WS 2 ) films, respectively.

Table 1 .
Positions of the E 2g and A 1g peaks of MoS 2 and WS 2 in Type I, II and III films.