Impact of Carrier Gas Flow Rate on the Synthesis of Monolayer WSe2 via Hydrogen-Assisted Chemical Vapor Deposition

Transition metal dichalcogenides (TMDs), particularly monolayer TMDs with direct bandgap properties, are key to advancing optoelectronic device technology. WSe2 stands out due to its adjustable carrier transport, making it a prime candidate for optoelectronic applications. This study explores monolayer WSe2 synthesis via H2-assisted CVD, focusing on how carrier gas flow rate affects WSe2 quality. A comprehensive characterization of monolayer WSe2 was conducted using OM (optical microscope), Raman spectroscopy, PL spectroscopy, AFM, SEM, XPS, HRTEM, and XRD. It was found that H2 incorporation and flow rate critically influence WSe2’s growth and structural integrity, with low flow rates favoring precursor concentration for product formation and high rates causing disintegration of existing structures. This research accentuates the significance of fine-tuning the carrier gas flow rate for optimizing monolayer WSe2 synthesis, offering insights for fabricating monolayer TMDs like WS2, MoSe2, and MoS2, and facilitating their broader integration into optoelectronic devices.


Introduction
Since the inaugural laboratory synthesis of graphene in 2004 [1], the domain of two-dimensional materials has sparked a widespread exploration frenzy within the global scientific research community, attributed to their extraordinary physicochemical attributes [2,3].These materials, with their two-dimensional structure just a few atoms thick, demonstrate exceptional surface activity, electronic and optical properties, and noteworthy mechanical stability.With the rapid development of this field, numerous two-dimensional materials such as hexagonal boron nitride (hBN), black phosphorus (also known as "black phosphene"), transition metal dichalcogenides (TMDs), and two-dimensional perovskites have been sequentially unveiled and thoroughly explored [4,5].They manifest immense potential in diverse application areas including energy conversion, data storage, and optoelectronic device fabrication.In particular, TMDs have emerged as a focal point of scientific inquiry, distinguished by their superior electronic, optical, and mechanical performance levels [6].Their exhibited tunable bandgap range of 1-2 eV reveals the feasibility of subtle electronic structural modifications via layer number adjustments or the application of external pressures, enabling a shift from indirect to direct bandgaps [7].For instance, monolayer forms of MoS 2 , MoSe 2 , WS 2 , and WSe 2 manifest direct bandgaps, conferring advantages in photovoltaic conversion and optical signal processing [8,9], thereby propelling the development of TMDs in applications such as sensor, photodetection, storage devices, and biomedicine [10][11][12][13][14][15].
Mechanical exfoliation techniques for fabricating TMDs are predominantly utilized in the realm of catalysis and foundational studies on the intrinsic attributes of materials [16].In contrast, chemical vapor deposition (CVD) has been proven to be an effective method for fabricating high-quality TMDs with scalable domain size, controllable thickness, and superior electronic properties [17,18].Consequently, the fabrication of uniformly distributed and high-quality TMDs has emerged as a focal point in research.Numerous researchers have delved into the pivotal parameters affecting TMDs synthesis via CVD, encompassing variables such as the temperature of growth [19], growth duration, the amount of precursor [20], and the distance between the sources and the growth substrate, achieving noteworthy results [21][22][23].
Notably, recent studies have incrementally uncovered the pivotal influence of hydrogen gas (H 2 ) introduction during the CVD process with metal precursors like WO 3 , MoO 3 , and the chalcogens precursors like Se and S, contributing to the crystalline quality enhancement of the resultant TMDs.Zhang et al. employed H 2 -assisted low-pressure chemical vapor deposition (LPCVD) to synthesize WS 2 .This achieved a morphological transition from serrated to straight-edged triangular single-layer WS 2 sheets, preserving their monocrystalline structure.They posited that H 2 integration modulates via kinetic effects, facilitating the formation of thermodynamically stable equilateral triangular structures [24].Subsequent investigations by McCreary et al. into H 2 -assisted growth of WS 2 revealed that incorporating H 2 within an Ar atmosphere not only significantly enhanced the photoluminescence intensity, thereby elevating the optical characteristics of WS 2 , but also efficiently minimized the presence of the WO 3 precursor, which in turn inhibited the oxidative etching observed in monolayer WS 2 [25].Sheng et al. investigated the impact of the H 2 /Ar ratio on the growth of large-area WS 2 films.They revealed that H 2 in the reaction not only accelerates the process but also mitigates oxidative damage [26].Ji et al. discovered that H 2 introduced during growth etches multilayer nuclei on monolayer WS 2 , effectively impeding the genesis of multilayer WS 2 .Furthermore, the study elucidated that defects within the H 2 -WS 2 grains were suppressed, with H 2 facilitating the rectification of lattice defects in WS 2 .Due to the energetically unstable nature of these defects, they are readily eradicated by H 2 during growth, aiding in the WS 2 lattice reconstitution and thereby enhancing the physical properties of TMDs monolayers grown with H 2 assistance [27].These studies conducted a qualitative investigation into the role of hydrogen gas (H 2 ) during the growth dynamics of TMDs, underscoring the critical function of hydrogen in the fabrication of high-quality TMDs via CVD method.
WSe 2 has gathered substantial attention for its unique electronic and optical attributes.It has shown superior photoresponsivity behavior, high carrier mobility, and unique spinorbit interaction effects [28][29][30].More significantly, its ambipolar conductivity allows it to be used as either an n-type or p-type semiconductor-considerably broadening its applicability in electronic and optoelectronic devices, signaling its vast potential in the future landscape of nanoelectronics and optoelectronics [31,32].Compared to other TMDs materials like WS 2 , MoS 2 , and MoSe 2 [33], research on WSe 2 growth via hydrogen-assisted CVD is relatively scarce.Given WSe 2 's significant advantages in performance and potential applications, this study focuses on the growth mechanism of monolayer WSe 2 under H 2 -assisted CVD, particularly examining the effect of H 2 flow rate on the morphology of the products on the substrate.A H 2 /Ar gas mixture with 10% H 2 was used as the carrier gas during the growth phase.An analysis of optical microscopy images of samples obtained under different carrier gas flow rates revealed the crucial impact of flow rate on the growth process.At lower flow rates, the concentration of precursors delivered to the substrate surface dominates product formation, while at higher flow rates, H 2 causes the decomposition of existing monolayer WSe 2 .Additionally, Raman and photoluminescence spectroscopy, along with AFM results, confirmed the monolayer nature of the synthesized WSe 2 .SEM and XPS analyses provided insights into the elemental composition and valence states, while HRTEM confirmed WSe 2 's high crystal quality.This research offers a new strategy for precisely controlling H 2 /Ar carrier gas flow rate to grow high-quality monolayer WSe 2 , potentially advancing the practical application of WSe 2 -based optoelectronic devices.

The Growth of WSe 2
Using a single-zone tube furnace, WSe 2 materials were successfully prepared through chemical vapor deposition (CVD) under atmospheric pressure.Prior to growth, the substrates were ultrasonically cleaned with acetone, ethanol, and deionized water, with each solvent cleaning lasting for 15 min.We weighed out approximately 3600 mg of WO 3 powder and 820 mg of Se powder.In the experiment, the WO 3 powder was placed 8 mm from the end of the alumina boat, and the SiO 2 /Si substrate was vertically positioned at the same end of the boat near the WO 3 side, ensuring the SiO 2 side faced outward.Additionally, the Se powder was placed in the center of another boat, positioned 16 cm from the heating center at a temperature of 220 • C.After loading the WO 3 powder and the growth substrates into the boat, we carefully placed it in the center of the tube furnace, ensuring the WO 3 powder was at the heating center.At room temperature, 300 sccm of Ar gas was injected into the quartz tube, continuing for 30 min.Next, with 100 sccm of Ar carrier gas flowing, the tube furnace temperature was raised to 950 • C within 60 min.Upon reaching 950 • C, we shut off the Ar gas and introduced a 28.3~31.2sccm H 2 /Ar gas mixed carrier gas containing 10% H 2 , maintaining the temperature for 8 min.Finally, we allowed the tube furnace to naturally cool to below 200 • C, opened the lid, and removed the WSe 2 samples after cooling to room temperature.

Characterization
Optical microscope (OM) images were obtained using a Sunwoo RX50M microscope (Yuyao, China).Scanning Electron Microscope (SEM) images were captured with a JEM-1400Plus (JEOL, Beijing, China) at an acceleration voltage of 5 kV.Corresponding elemental analysis was performed using a 20 kV acceleration voltage.Photoluminescence (PL) and Raman spectra were measured using a Horiba Raman microscope (Irvine, CA, USA) with a 532 nm laser beam.Atomic Force Microscopy (AFM) image was acquired using a Bruker Dimension FastScan AFM (Billerica, MA, USA) in knockdown mode.X-ray Photoelectron Spectroscopy (XPS) spectra were obtained using a Thermo Scientific Kα XPS spectrometer (Waltham, MA, USA) equipped with a monochromatic Al-Kα X-ray source.X-ray diffraction (XRD) analysis was conducted using a Bruker D8 ADVANCE X-Ray diffractometer (Bruker, Karlsruhe, Germany) with Cu-Kα radiation (λ = 1.54056Å, 40 kV and 40 mA), with the diffraction angle 2θ range set from 10 to 60 • , a scan speed of 0.5 s per step, and a step size of 0.05 • .The model used for Transmission Electron Microscopy was a JEOL JEM-1400 Plus electron microscope (JEOL, Beijing, China).
For HRTEM analysis, the WSe 2 sample was transferred from the growth substrate to the copper grids for electron microscopy using a PMMA-assisted wet transfer technique.Initially, 2 g of NaOH and 50 mL of deionized water were measured to prepare a 1 M NaOH solution.Subsequently, the substrate containing WSe 2 was placed at the center of a spin coater, 10 µL of PMMA solution was dropped onto it, and it was spin-coated at 2000 rpm for 60 s.The sample was then transferred to a hotplate heated at 85 • C for 15 min to remove residual solvent and to solidify the interface between PMMA and WSe 2 .Afterwards, the sample was immersed in the previously mentioned NaOH solution and wet-etched at 85 • C for 2.5~3 h.During this process, the PMMA/WSe 2 film floated on the surface of the solution, was subsequently scooped out, and washed several times with deionized water.After drying, the sample was placed on the copper grid, and an appropriate amount of acetone was dropped onto it to remove the PMMA layer, leaving a pure WSe 2 sample on the copper grid.

Results and Discussion
Figure 1 illustrates the synthesis of WSe 2 on SiO 2 /Si substrates via a single-zone tube furnace, utilizing selenium (Se) and tungsten trioxide (WO 3 ) powders as sources for selenium and tungsten, respectively.The cross-sectional atomic structure of the monolayer WSe 2 shows the W atom is sandwiched between two Se atoms, forming a Se-W-Se sandwich structure.The Se powder was placed in an alumina boat upstream of the furnace, and its temperature was controlled by adjusting the distance to the heating center.WO 3 powder was placed in another alumina boat in the heating area, and the thoroughly cleaned 285 nm SiO 2 /Si substrate was positioned vertically to the direction of gas flow, approximately 8 mm from the WO 3 powder, to ensure uniform reaction on the substrate.The entire reaction was conducted under ambient pressure with an alternating flow of argon and a hydrogen-argon mixture (10% H 2 ).This experiment hinged on the hightemperature sublimation of precursors, their transportation to the substrate via carrier gas, and subsequent solid product formation.In the initial phase of our experiments, we extensively explored the impact of growth temperature of WSe 2 , as detailed in Figure S1.
During the experiments to investigate the effect of carrier gas flow rate, we maintained a constant growth temperature at 950 • C. The substrate's outcomes, influenced by the flow rate, ranged from partially reduced WO 3−x grains to regular triangular monolayers of WSe 2 and hydrogen-decomposed monolayer WSe 2 , with an in-depth mechanism and product analysis provided subsequently.As depicted in Figure 1b, the temperature profile within the furnace core throughout the CVD experiment was characterized by a multi-stage process, encompassing pre-purification, ramp-up, growth, cooling, and sample retrieval, detailed across five phases with respective parameters outlined in Table 1.
Materials 2024, 17, x FOR PEER REVIEW 4 of 13 remove residual solvent and to solidify the interface between PMMA and WSe2.Afterwards, the sample was immersed in the previously mentioned NaOH solution and wetetched at 85 °C for 2.5~3 h.During this process, the PMMA/WSe2 film floated on the surface of the solution, was subsequently scooped out, and washed several times with deionized water.After drying, the sample was placed on the copper grid, and an appropriate amount of acetone was dropped onto it to remove the PMMA layer, leaving a pure WSe2 sample on the copper grid.

Results and Discussion
Figure 1 illustrates the synthesis of WSe2 on SiO2/Si substrates via a single-zone tube furnace, utilizing selenium (Se) and tungsten trioxide (WO3) powders as sources for selenium and tungsten, respectively.The cross-sectional atomic structure of the monolayer WSe2 shows the W atom is sandwiched between two Se atoms, forming a Se-W-Se sandwich structure.The Se powder was placed in an alumina boat upstream of the furnace, and its temperature was controlled by adjusting the distance to the heating center.WO3 powder was placed in another alumina boat in the heating area, and the thoroughly cleaned 285 nm SiO2/Si substrate was positioned vertically to the direction of gas flow, approximately 8 mm from the WO3 powder, to ensure uniform reaction on the substrate.The entire reaction was conducted under ambient pressure with an alternating flow of argon and a hydrogen-argon mixture (10% H2).This experiment hinged on the high-temperature sublimation of precursors, their transportation to the substrate via carrier gas, and subsequent solid product formation.In the initial phase of our experiments, we extensively explored the impact of growth temperature of WSe2, as detailed in Figure S1.
During the experiments to investigate the effect of carrier gas flow rate, we maintained a constant growth temperature at 950 °C.The substrate's outcomes, influenced by the flow rate, ranged from partially reduced WO3−x grains to regular triangular monolayers of WSe2 and hydrogen-decomposed monolayer WSe2, with an in-depth mechanism and product analysis provided subsequently.As depicted in Figure 1b, the temperature profile within the furnace core throughout the CVD experiment was characterized by a multi-stage process, encompassing pre-purification, ramp-up, growth, cooling, and sample retrieval, detailed across five phases with respective parameters outlined in Table 1.WO 3 exhibits good thermal stability, which poses a challenge for its reduction or decomposition by Se, a reductant with limited efficacy, under elevated temperatures.To address this, hydrogen (H 2 ) was introduced during the growth phase to promote the reduction of WO 3 to the more volatile form, WO 3−x [34].This was followed by the transport of gaseous Se and WO 3−x to the substrate's surface via carrier gas, triggering intricate reactions that culminate in the deposition of solid WSe 2 .The synthesis was conducted within a quartz tube of 35 mm diameter, optimizing the homogenous delivery of the carrier and precursor gases to the substrate due to the tube's constrained diameter.When the heating zone containing WO 3 reached the growth temperature of WSe 2 at 950 • C, a suitable regulated H 2 /Ar gas mixture was introduced, quickly filling the entire sealed space, and reducing the WO 3 powder to volatile WO 3−x .The WO 3−x gas, facilitated by the carrier gas, migrated to the substrate, forming WO 3−x nanoparticles-the nucleation sites.Concurrently, the Se powder reached its sublimation point (approximately 220 • C), transforming into vapor and being conveyed to the substrate surface, where it reacted with WO 3−x nanoparticles, yielding WSe 2 samples.The comprehensive reaction for this phase is delineated as follows: Prior investigations have corroborated that the Gibbs free energy of this reaction is negative at the operational temperature, denoting the spontaneity of this substitution reaction [35].
In our study, a H 2 /Ar mixed gas containing 10% H 2 was utilized as the carrier gas for the WSe 2 growth phase.Adjusting the carrier gas's flow rate yielded WSe 2 under various conditions depicted in Figure 2. When the flow rate of the carrier gas was too low, the Se vapor could not reach the substrate surface to react, forming the nanoparticles shown in Figure 2a, with a detailed view in Figure S2; the adjustments of the carrier gas flow rate across a wider range are further illustrated in Figure S3.Increasing the flow rate, Figure 2b illustrates the nucleation sites' periphery turning blue due to limited selenium diffusion to the substrate, producing a small amount of WSe 2 , with WO 3−x being the main product.Further increments in flow rate led to complete conversion of some nucleation sites to WSe 2 , depicted by small triangles in Figure 2c, though most of the surface was still covered with particles.In other words, under relatively low carrier gas flow rates, the predominance of Se vapor concentration delivered to the substrate via the H 2 /Ar carrier gas emerged as the critical determinant in steering the reaction dynamics, subsequently affecting the composition of the resultant product.The effects of carrier gas on the transport of Se precursors have also been reported in the existing literature [36].An optimal increase in flow rate resulted in numerous regular triangular monolayers of WSe2, approximately 100 µm in size, as shown in Figure 2d-f, indicating sufficient Se delivery and conversion of most nucleation sites to WSe2, albeit with some unreacted sites.Upon optimizing the carrier gas flow rate to a precise range, fine-tuning ceased to impact the synthesis of monolayer WSe2 characterized by uniform triangular configurations.This suggests that within this confined parameter range, the production of the targeted samples remains consistent, even with very slight alterations in flow rate.As the flow rate was An optimal increase in flow rate resulted in numerous regular triangular monolayers of WSe 2 , approximately 100 µm in size, as shown in Figure 2d-f, indicating sufficient Se delivery and conversion of most nucleation sites to WSe 2 , albeit with some unreacted sites.Upon optimizing the carrier gas flow rate to a precise range, fine-tuning ceased to impact the synthesis of monolayer WSe 2 characterized by uniform triangular configurations.This suggests that within this confined parameter range, the production of the targeted samples remains consistent, even with very slight alterations in flow rate.As the flow rate was increased further, as shown in Figure 2g, with a detailed view in Figure S2, although there were still some partially selenized particles on some WSe 2 flakes, partial decomposition and edge thickening of WSe 2 monolayers were observed, with extensive decomposition and the loss of sharp triangular edges upon further increases, as seen in Figure 2h.Excessive H 2 flow as shown in Figure 2i, however, led to significant decomposition of WSe 2 , severely impairing its structural integrity and crystallinity.The results show that excess hydrogen can decompose the monolayer WSe 2 already present on the substrate.
Figure 3a illustrates multiple monolayer WSe 2 crystals with regular triangular structures grown experimentally on SiO 2 /Si substrates.In this image, each triangle's lateral dimension exceeds 50 µm and can reach up to 120 µm.Due to the sensitivity of WSe 2 to the growth substrate, an incompletely cleaned substrate and lattice mismatch between WSe 2 and SiO 2 /Si substrates could limit the final lateral dimension of the produced WSe 2 .To further confirm the morphology of WSe 2 , we observed a single WSe 2 at a higher magnification, as shown in Figure 3b.The observed WSe 2 , approximately 45 µm in size, exhibits a regular equilateral triangular structure with atomically sharp edges, indicating good crystallinity.Figure 3c presents an enlarged morphology of a single WSe 2 obtained using Scanning Electron Microscopy (SEM), with a lateral dimension of about 25 µm.It displays a perfect triangular structure consistent with Figure 3b, with a smooth surface.It is well known that Raman spectroscopy and photoluminescence (PL) spectroscopy are crucial tools for analyzing TMDs materials, where Raman spectroscopy is primarily used for analyzing atomic vibration modes and doping levels, and it also aids in identifying the layer count of TMDs materials such as WSe2, WS2, and MoS2 [37,38].Photoluminescence spectroscopy offers information about the bandgap energy of materials, and notably, for monolayer TMDs materials like WSe2, WS2, and MoS2, PL test results are significant indicators of the direct bandgap.To confirm the structure of the WSe2 sample, we conducted Raman and photoluminescence tests using a laser with a 532 nm wavelength, as shown in Figure 3d,e.The Raman spectrum displayed two distinct peaks at 248.220 cm −1 and 259.038 cm −1 , corresponding to the in-plane vibrations of W and Se atoms (E 1 2g It is well known that Raman spectroscopy and photoluminescence (PL) spectroscopy are crucial tools for analyzing TMDs materials, where Raman spectroscopy is primarily used for analyzing atomic vibration modes and doping levels, and it also aids in identifying the layer count of TMDs materials such as WSe 2 , WS 2 , and MoS 2 [37,38].Photoluminescence spectroscopy offers information about the bandgap energy of materials, and notably, for monolayer TMDs materials like WSe 2 , WS 2 , and MoS 2 , PL test results are significant indicators of the direct bandgap.To confirm the structure of the WSe 2 sample, we conducted Raman and photoluminescence tests using a laser with a 532 nm wavelength, as shown in Figure 3d,e.The Raman spectrum displayed two distinct peaks at 248.220 cm −1 and 259.038 cm −1 , corresponding to the in-plane vibrations of W and Se atoms (E 1 2g mode) and the out-of-plane vibrations of Se atoms (A 1g mode) in monolayer WSe 2 , respectively [34,38].The frequency difference between these two peaks is 10.818 cm −1 .Under illumination with a 532 nm laser, WSe 2 produced a strong luminescence peak near 770 nm, corresponding to the A exciton absorption of monolayer WSe 2 , indicating its direct bandgap characteristic [29,39,40].It is noteworthy that bilayer and thicker WSe 2 usually exhibit an extra indirect bandgap transition peaks at a higher wavelength [34].Our PL test results showed only a direct bandgap transition peak near 770 nm, with no extra indirect bandgap emission, consistent with recent reports on monolayer WSe 2 .In the PL spectrum of bilayer WSe 2 shown in Supplementary Figure S4, There is an additional peak at a higher wavelength in addition to the indirect transition peak.Figure 3f shows the Atomic Force Microscopy (AFM) image of WSe 2 , with a thickness of 0.88 nm, which is consistent with the thickness reported in the literature for monolayer WSe 2 prepared by CVD method.This thickness is slightly greater than that of monolayer WSe 2 obtained by mechanical exfoliation (0.7 nm) [29,41].The observed discrepancy in thickness might be attributed to the lattice mismatch between WSe 2 and the SiO 2 /Si growth substrate, coupled with potential surface states present on the CVD-prepared samples.
The chemically synthesized WSe 2 sample was subjected to X-ray Photoelectron Spectroscopy (XPS) for compositional analysis.Figure 4a illustrates the results from the spectral fitting of tungsten in the WSe 2 samples, revealing two prominent peaks at 32.6 eV and 34.8 eV, assignable to the 4f 7/2 and 4f 5/2 orbitals of W 4+ in WSe 2 [42].This spectral fitting evidence a notable transition of the peak from W 6+ in the WO 3 precursor to W 4+ , validating the valence change.Additionally, peaks of lesser intensity near 38.2 eV and 35.6 eV were detected, corresponding to the metal oxide WO x [25].We hypothesize that the presence of trace amounts WO x nanoparticles on the WSe 2 surface is attributed to the incomplete reduction in the metal oxide precursor WO 3 during the synthesis of WSe 2 .Figure 4b details the fitting analysis for selenium in WSe 2 , where the binding energies for Se 3d 5/2 and 3d 3/2 are identified at 55.68 eV and 54.78 eV, respectively.Supplementary Figure S5 showcases the comprehensive WSe 2 spectra acquired through XPS testing, featuring four elements: tungsten and selenium from the monolayer WSe 2 , alongside silicon and oxygen from the SiO 2 substrate [43].The analytical outcomes from our XPS data on the experimentally derived WSe 2 align with those of pure phase WSe 2 reported in the existing literature.
To elucidate the elemental composition of the synthesized WSe 2 samples, Energy Dispersive Spectroscopy (EDS) analyses were conducted utilizing a SEM at an acceleration voltage of 20 kV, as illustrated in Figure 4c.The data, depicted in the inset of Figure 4d, reveal a Se/W ratio of 1.988 in the WSe 2 samples, closely aligning with the theoretical stoichiometric ratio of 2:1.This minor deviation from the stoichiometric ratio is ascribed to the presence of selenium vacancies, a prevalent surface defect in WSe 2 synthesized via CVD, leading to a marginally reduced selenium content.
Significantly, the XPS examination of monolayer WSe 2 films, stored for an extended period exceeding one month and depicted in Supplementary Figure S6, identified the emergence of two novel peaks at 33.8 eV and 36.0 eV within the tungsten spectrum.Correlation with the literature suggests these peaks are attributed to the transitional states between hexavalent tungsten (W 6+ ) in tungsten trioxide (WO 3 ) and tetravalent tungsten (W 4+ ) in WSe 2 , indicative of the signals produced by partially oxidized WSe 2 [25].Subsequently, an annealing intervention was applied to this specimen (situated at the center of heating at 200 • C, subjected to preliminary purification within the tube furnace, and maintained under a 100 sccm flow of argon gas for 300 min).The XPS analysis post-annealing, aligning with the observations in Figure 4a, substantiates that (1) specimens conserved in nonvacuum conditions undergo partial oxidation by environmental oxygen over time, and (2) the annealing process significantly purges impurities, culminating in the amelioration of the crystal quality of the samples.The optical spectra of WSe 2 before and after annealing are shown in Figure S7, which are consistent with XPS results.
detected, corresponding to the metal oxide WOx [25].We hypothesize that the presence of trace amounts WOx nanoparticles on the WSe2 surface is attributed to the incomplete reduction in the metal oxide precursor WO3 during the synthesis of WSe2. Figure 4b details the fitting analysis for selenium in WSe2, where the binding energies for Se 3d5/2 and 3d3/2 are identified at 55.68 eV and 54.78 eV, respectively.Supplementary Figure S5 showcases the comprehensive WSe2 spectra acquired through XPS testing, featuring four elements: tungsten and selenium from the monolayer WSe2, alongside silicon and oxygen from the SiO2 substrate [43].The analytical outcomes from our XPS data on the experimentally derived WSe2 align with those of pure phase WSe2 reported in the existing literature.To elucidate the elemental composition of the synthesized WSe2 samples, Energy Dispersive Spectroscopy (EDS) analyses were conducted utilizing a SEM at an acceleration voltage of 20 kV, as illustrated in Figure 4c.The data, depicted in the inset of Figure 4d, reveal a Se/W ratio of 1.988 in the WSe2 samples, closely aligning with the theoretical stoichiometric ratio of 2:1.This minor deviation from the stoichiometric ratio is ascribed to the presence of selenium vacancies, a prevalent surface defect in WSe2 synthesized via CVD, leading to a marginally reduced selenium content.
Significantly, the XPS examination of monolayer WSe2 films, stored for an extended period exceeding one month and depicted in Supplementary Figure S6, identified the emergence of two novel peaks at 33.8 eV and 36.0 eV within the tungsten spectrum.Correlation with the literature suggests these peaks are attributed to the transitional states between hexavalent tungsten (W 6+ ) in tungsten trioxide (WO3) and tetravalent tungsten (W 4+ ) in WSe2, indicative of the signals produced by partially oxidized WSe2 [25].Subsequently, an annealing intervention was applied to this specimen (situated at the center of heating at 200 °C, subjected to preliminary purification within the tube furnace, and maintained under a 100 sccm flow of argon gas for 300 min).The XPS analysis post-annealing, aligning with the observations in Figure 4a, substantiates that (1) specimens conserved in non-vacuum conditions undergo partial oxidation by environmental oxygen over time, and (2) the annealing process significantly purges impurities, culminating in the To investigate the microscopic crystal structure of WSe 2 , the High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), the High-Resolution Transmission Microscope (HRTEM), and the Selective Area Electron Diffraction (SAED) studies were conducted.In our HAADF-STEM analysis, we concentrated on smaller domains to facilitate a comprehensive examination of the entire domain.The HAADF-STEM image, as shown in Figure 5a, reveals the regular triangular morphology of the WSe 2 samples.To further elucidate the chemical composition within these domains, we employed EDS for elemental mapping of the triangular domains.These mapping images indicate a nearly uniform distribution of W and Se elements throughout the triangular domains (as seen in Figure 5b,c).The corresponding surface scans (Figure 5b-d) and line scan (Figure 5e) indicate a uniform distribution of W and Se elements on the sample surface, confirming the high quality of the prepared WSe 2 sample.Supplementary Figure S8 presents the corresponding EDS elemental spectra, which show good compatibility with WSe 2 .
HRTEM was used to further analyze the microstructure of WSe 2 , as shown in Figure 5f,g.A detailed element distribution spectrum is shown in Supplementary Information Figure S8.In Figure 5f, the SAED test on the region outlined by the red dashed box reveals the typical SAED pattern of 2H-WSe 2 crystals (as illustrated in the inset of Figure 5f).The hexagonal symmetry of the diffraction spots from the (100) plane corresponds to the hexagonal symmetry of the WSe 2 lattice structure in the [001] zone axis, confirming the single-crystalline nature of the WSe 2 sample with a hexagonal lattice structure.In Figure 5g, the magnified view within the red dashed box, when correlated with the previous diffraction peak analysis, reveals that the interplanar spacing of the (100) plane is 0.283 nm [44].This measurement consistent with the standard values for WSe 2 .The X-ray diffraction (XRD) analysis of the monolayer WSe 2 , presented in Supplementary Figure S9, identified the (002), (006), and (008) crystallographic planes, indicative of its layered structure [35,45].This observation is in alignment with findings obtained through HRTEM.Additionally, Figure 5g clearly depicts the six-membered ring structure, aligning with the inherent hexagonal lattice structure of 2H-phase WSe 2 (as shown in Figure 5h), where W and Se atoms are alternately arranged to form hexagonal rings [46].
HAADF-STEM image, as shown in Figure 5a, reveals the regular triangular morphology of the WSe2 samples.To further elucidate the chemical composition within these domains, we employed EDS for elemental mapping of the triangular domains.These mapping images indicate a nearly uniform distribution of W and Se elements throughout the triangular domains (as seen in Figure 5b,c).The corresponding surface scans (Figure 5b-d) and line scan (Figure 5e) indicate a uniform distribution of W and Se elements on the sample surface, confirming the high quality of the prepared WSe2 sample.Supplementary Figure S8 presents the corresponding EDS elemental spectra, which show good compatibility with WSe2.HRTEM was used to further analyze the microstructure of WSe2, as shown in Figure 5f,g.A detailed element distribution spectrum is shown in Supplementary Information Figure S8.In Figure 5f, the SAED test on the region outlined by the red dashed box reveals the typical SAED pattern of 2H-WSe2 crystals (as illustrated in the inset of Figure 5f).The hexagonal symmetry of the diffraction spots from the (100) plane corresponds to the hexagonal symmetry of the WSe2 lattice structure in the [001] zone axis, confirming the single-

Conclusions
In this study, we successfully synthesized monolayer WSe 2 with a regular triangular morphology using a H 2 -assisted CVD technique.The experiment utilized a H 2 /Ar mixed gas, with the H 2 ratio fixed at 10%, as the carrier gas during the growth phase.Our findings indicate that slight adjustments to the carrier gas flow rate during the growth stage significantly impact the composition of the products formed on the substrate.Specifically, at lower carrier gas flow rates, the concentration of Se vapor transported to the substrate by the H 2 /Ar carrier gas became the dominant factor driving the reaction, thereby influencing the final product composition.As the carrier gas flow rate increased, the concentration of Se vapor involved in the reaction rose, enhancing the selenization of the WO 3−x nucleation sites.When the carrier gas flow rate was optimized within a specific range, minor adjustments did not affect the achievement of monolayer WSe 2 with a regular triangular morphology, indicating that a stable preparation of the desired sample could be maintained with minor flow rate adjustments within this narrow window.However, further increasing the H 2 /Ar carrier gas flow rate intensified the impact of H 2 in the carrier gas on the final product, particularly in decomposing the already formed monolayer WSe 2 , a phenomenon that became more pronounced with increased carrier gas flow rates.This study not only provides significant insights for the preparation of monolayer WSe 2 via H 2 -assisted CVD but also serves as a reference for the fabrication of other monolayer TMDs materials such as

Figure 1 .
Figure 1.Atmospheric CVD synthesis of WSe2.(a) Schematic illustration of the single temperature zone tube furnace CVD system used to synthesize of WSe2 on the SiO2/Si substrate.Illustration of WSe2 growing on SiO2/Si substrate and cross-sectional atomic structure of monolayer WSe2, where yellow and gray spheres represent W and Se atoms, respectively.(b) Temperature programming process of heating center.

Table 1 .
Parameters for each stage in the synthesis of WSe2 via multi-step H2-assisted CVD.

Figure 1 .
Figure 1.Atmospheric CVD synthesis of WSe 2 .(a) Schematic illustration of the single temperature zone tube furnace CVD system used to synthesize of WSe 2 on the SiO 2 /Si substrate.Illustration of WSe 2 growing on SiO 2 /Si substrate and cross-sectional atomic structure of monolayer WSe 2 , where yellow and gray spheres represent W and Se atoms, respectively.(b) Temperature programming process of heating center.

Materials 2024 , 13 Figure 2 .
Figure 2. Influence of carrier gas flow rate on products on substrate.The scales of (a,d) are 100 µm, and the rest are 50 µm.

Figure 2 .
Figure 2. Influence of carrier gas flow rate on products on substrate.The scales of (a,d) are 100 µm, and the rest are 50 µm.

Materials 2024 , 13 Figure 3 .
Figure 3.A series of characteristics of monolayer WSe2.(a) Monolayer WSe2 grown on SiO2/Si substrates, featuring multiple regular triangular structures.(b,c) The magnified optical microscopy image and SEM image of an individual WSe2, respectively.(d) The Raman spectrum of monolayer WSe2, with insets at 248.220 cm −1 and 259.038 cm −1 illustrating the atomic models of in-plane vibrations of W atoms and Se atoms and out-of-plane vibrations of Se atoms in monolayer WSe2.(e) Shows the photoluminescence spectrum of monolayer WSe2.(f) The AFM image from a WSe2 triangular flake with an inset showing the height distribution along the marked line in the image.

Figure 3 .
Figure 3.A series of characteristics of monolayer WSe 2 .(a) Monolayer WSe 2 grown on SiO 2 /Si substrates, featuring multiple regular triangular structures.(b,c) The magnified optical microscopy image and SEM image of an individual WSe 2 , respectively.(d) The Raman spectrum of monolayer WSe 2 , with insets at 248.220 cm −1 and 259.038 cm −1 illustrating the atomic models of in-plane vibrations of W atoms and Se atoms and out-of-plane vibrations of Se atoms in monolayer WSe 2 .(e) Shows the photoluminescence spectrum of monolayer WSe 2 .(f) The AFM image from a WSe 2 triangular flake with an inset showing the height distribution along the marked line in the image.

Figure 4 .
Figure 4.The XPS spectra and SEM-EDS analysis of monolayer WSe2.(a) The W 4f spectrum reveals two peaks of high intensity at 32.6 eV and 34.8 eV, attributable to the 4f7/2 and 4f5/2 orbitals of W 4+ within WSe2.(b) In the Se 3d spectrum, the binding energies of Se 3d5/2 and 3d3/2 are located at 55.7 eV and 54.8 eV, respectively.(c) The image acquired through SEM-EDS.(d) The elemental analysis results of the image in (c).

Figure 4 .
Figure 4.The XPS spectra and SEM-EDS analysis of monolayer WSe 2 .(a) The W 4f spectrum reveals two peaks of high intensity at 32.6 eV and 34.8 eV, attributable to the 4f 7/2 and 4f 5/2 orbitals of W 4+ within WSe 2 .(b) In the Se 3d spectrum, the binding energies of Se 3d 5/2 and 3d 3/2 are located at 55.7 eV and 54.8 eV, respectively.(c) The image acquired through SEM-EDS.(d) The elemental analysis results of the image in (c).

Figure 5 .
Figure 5.The surface element distribution and microstructural analysis of WSe2.(a) The HAADF-STEM image of WSe2.(b,c) The distribution of W and Se elements in the area shown in (a), respectively.(d) An overlay view of the HAADF image with the element distribution.(e) The line-scan results of the elements, following the direction of the red arrow marked in (d).(f) The HRTEM image of a monolayer WSe2.The inset shows the SAED pattern along the [001] zone axis, corresponding to the region highlighted by the red dashed box.(g) An enlarged view of the area within the red dashed box in (f), where the blue spheres represent W atoms and the red ones represent Se atoms.(h) A schematic diagram of the planar structure of monolayer WSe2.

Figure 5 .
Figure 5.The surface element distribution and microstructural analysis of WSe 2 .(a) The HAADF-STEM image of WSe 2 .(b,c) The distribution of W and Se elements in the area shown in (a), respectively.(d) An overlay view of the HAADF image with the element distribution.(e) The line-scan results of the elements, following the direction of the red arrow marked in (d).(f) The HRTEM image of a monolayer WSe 2 .The inset shows the SAED pattern along the [001] zone axis, corresponding to the region highlighted by the red dashed box.(g) An enlarged view of the area within the red dashed box in (f), where the blue spheres represent W atoms and the red ones represent Se atoms.(h) A schematic diagram of the planar structure of monolayer WSe 2 .

Table 1 .
Parameters for each stage in the synthesis of WSe 2 via multi-step H 2 -assisted CVD.