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Article

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

by
Xuemin Luo
,
Yanhui Jiao
,
Hang Li
,
Qi Liu
,
Jinfeng Liu
,
Mingwei Wang
and
Yong Liu
*
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Materials Science and Engineering (ISMSE), State Wuhan University of Technology, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
Materials 2024, 17(10), 2190; https://doi.org/10.3390/ma17102190
Submission received: 12 March 2024 / Revised: 16 April 2024 / Accepted: 18 April 2024 / Published: 7 May 2024
(This article belongs to the Special Issue Synthesis, Properties, and Applications of Low-Dimensional Transition Metal Oxides and Selenides)
Browse Figures
Versions Notes

Abstract

:
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.

1. 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 MoS2, MoSe2, WS2, and WSe2 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 (H2) introduction during the CVD process with metal precursors like WO3, MoO3, and the chalcogens precursors like Se and S, contributing to the crystalline quality enhancement of the resultant TMDs. Zhang et al. employed H2-assisted low-pressure chemical vapor deposition (LPCVD) to synthesize WS2. This achieved a morphological transition from serrated to straight-edged triangular single-layer WS2 sheets, preserving their monocrystalline structure. They posited that H2 integration modulates via kinetic effects, facilitating the formation of thermodynamically stable equilateral triangular structures [24]. Subsequent investigations by McCreary et al. into H2-assisted growth of WS2 revealed that incorporating H2 within an Ar atmosphere not only significantly enhanced the photoluminescence intensity, thereby elevating the optical characteristics of WS2, but also efficiently minimized the presence of the WO3 precursor, which in turn inhibited the oxidative etching observed in monolayer WS2 [25]. Sheng et al. investigated the impact of the H2/Ar ratio on the growth of large-area WS2 films. They revealed that H2 in the reaction not only accelerates the process but also mitigates oxidative damage [26]. Ji et al. discovered that H2 introduced during growth etches multilayer nuclei on monolayer WS2, effectively impeding the genesis of multilayer WS2. Furthermore, the study elucidated that defects within the H2-WS2 grains were suppressed, with H2 facilitating the rectification of lattice defects in WS2. Due to the energetically unstable nature of these defects, they are readily eradicated by H2 during growth, aiding in the WS2 lattice reconstitution and thereby enhancing the physical properties of TMDs monolayers grown with H2 assistance [27]. These studies conducted a qualitative investigation into the role of hydrogen gas (H2) during the growth dynamics of TMDs, underscoring the critical function of hydrogen in the fabrication of high-quality TMDs via CVD method.
WSe2 has gathered substantial attention for its unique electronic and optical attributes. It has shown superior photoresponsivity behavior, high carrier mobility, and unique spin–orbit 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 WS2, MoS2, and MoSe2 [33], research on WSe2 growth via hydrogen-assisted CVD is relatively scarce. Given WSe2’s significant advantages in performance and potential applications, this study focuses on the growth mechanism of monolayer WSe2 under H2-assisted CVD, particularly examining the effect of H2 flow rate on the morphology of the products on the substrate. A H2/Ar gas mixture with 10% H2 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, H2 causes the decomposition of existing monolayer WSe2. Additionally, Raman and photoluminescence spectroscopy, along with AFM results, confirmed the monolayer nature of the synthesized WSe2. SEM and XPS analyses provided insights into the elemental composition and valence states, while HRTEM confirmed WSe2’s high crystal quality. This research offers a new strategy for precisely controlling H2/Ar carrier gas flow rate to grow high-quality monolayer WSe2, potentially advancing the practical application of WSe2-based optoelectronic devices.

2. Materials and Methods

2.1. Materials

Tungsten trioxide powder (WO3, 99.99% Aladdin, Shanghai, China) and selenium powder (Se, 99%, Aladdin, Shanghai, China) were used as growth precursors. Acetone (AR, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), anhydrous ethanol (AR, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and deionized water were, respectively, utilized for the purification of Si/SiO2 substrates (Hefei Kejing Materials Technology Co., Ltd., Hefei, China) on which the materials were synthesized. Argon (Ar gas, 99.999%,Wuhan Newradar Special Gas Co., Ltd., Wuhan, China) was employed as a purging gas and as a protective carrier gas during the temperature ramping stages, while a hydrogen–argon mixture gas (H2/Ar, 99.999%, Wuhan Newradar Special Gas Co., Ltd., Wuhan, China) serves as the carrier gas for the growth phase.

2.2. The Growth of WSe2

Using a single-zone tube furnace, WSe2 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 WO3 powder and 820 mg of Se powder. In the experiment, the WO3 powder was placed 8 mm from the end of the alumina boat, and the SiO2/Si substrate was vertically positioned at the same end of the boat near the WO3 side, ensuring the SiO2 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 WO3 powder and the growth substrates into the boat, we carefully placed it in the center of the tube furnace, ensuring the WO3 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 28.3~31.2 sccm H2/Ar gas mixed carrier gas containing 10% H2, 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 WSe2 samples after cooling to room temperature.

2.3. 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 = 1.54056 Å, 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 WSe2 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 WSe2 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 WSe2. 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/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.

3. 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.
WO3 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 (H2) was introduced during the growth phase to promote the reduction of WO3 to the more volatile form, WO3−x [34]. This was followed by the transport of gaseous Se and WO3−x to the substrate’s surface via carrier gas, triggering intricate reactions that culminate in the deposition of solid WSe2. 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 WO3 reached the growth temperature of WSe2 at 950 °C, a suitable regulated H2/Ar gas mixture was introduced, quickly filling the entire sealed space, and reducing the WO3 powder to volatile WO3−x. The WO3−x gas, facilitated by the carrier gas, migrated to the substrate, forming WO3−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 WO3−x nanoparticles, yielding WSe2 samples. The comprehensive reaction for this phase is delineated as follows:
3Se(s) + WO3(s) + H2(g)→WSe2(s) + H2O(g) + SeO2(g)
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 H2/Ar mixed gas containing 10% H2 was utilized as the carrier gas for the WSe2 growth phase. Adjusting the carrier gas’s flow rate yielded WSe2 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 WSe2, with WO3−x being the main product. Further increments in flow rate led to complete conversion of some nucleation sites to WSe2, 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 H2/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 increased further, as shown in Figure 2g, with a detailed view in Figure S2, although there were still some partially selenized particles on some WSe2 flakes, partial decomposition and edge thickening of WSe2 monolayers were observed, with extensive decomposition and the loss of sharp triangular edges upon further increases, as seen in Figure 2h. Excessive H2 flow as shown in Figure 2i, however, led to significant decomposition of WSe2, severely impairing its structural integrity and crystallinity. The results show that excess hydrogen can decompose the monolayer WSe2 already present on the substrate.
Figure 3a illustrates multiple monolayer WSe2 crystals with regular triangular structures grown experimentally on SiO2/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 WSe2 to the growth substrate, an incompletely cleaned substrate and lattice mismatch between WSe2 and SiO2/Si substrates could limit the final lateral dimension of the produced WSe2. To further confirm the morphology of WSe2, we observed a single WSe2 at a higher magnification, as shown in Figure 3b. The observed WSe2, 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 WSe2 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 (E12g mode) and the out-of-plane vibrations of Se atoms (A1g mode) in monolayer WSe2, respectively [34,38]. The frequency difference between these two peaks is 10.818 cm−1. Under illumination with a 532 nm laser, WSe2 produced a strong luminescence peak near 770 nm, corresponding to the A exciton absorption of monolayer WSe2, indicating its direct bandgap characteristic [29,39,40]. It is noteworthy that bilayer and thicker WSe2 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 WSe2. In the PL spectrum of bilayer WSe2 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 WSe2, with a thickness of 0.88 nm, which is consistent with the thickness reported in the literature for monolayer WSe2 prepared by CVD method. This thickness is slightly greater than that of monolayer WSe2 obtained by mechanical exfoliation (0.7 nm) [29,41]. The observed discrepancy in thickness might be attributed to the lattice mismatch between WSe2 and the SiO2/Si growth substrate, coupled with potential surface states present on the CVD-prepared samples.
The chemically synthesized WSe2 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 WSe2 samples, revealing two prominent peaks at 32.6 eV and 34.8 eV, assignable to the 4f7/2 and 4f5/2 orbitals of W4+ in WSe2 [42]. This spectral fitting evidence a notable transition of the peak from W6+ in the WO3 precursor to W4+, validating the valence change. Additionally, peaks of lesser intensity near 38.2 eV and 35.6 eV were 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 (W6+) in tungsten trioxide (WO3) and tetravalent tungsten (W4+) 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 amelioration of the crystal quality of the samples. The optical spectra of WSe2 before and after annealing are shown in Figure S7, which are consistent with XPS results.
To investigate the microscopic crystal structure of WSe2, 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 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-crystalline nature of the WSe2 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 WSe2. The X-ray diffraction (XRD) analysis of the monolayer WSe2, 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 WSe2 (as shown in Figure 5h), where W and Se atoms are alternately arranged to form hexagonal rings [46].

4. Conclusions

In this study, we successfully synthesized monolayer WSe2 with a regular triangular morphology using a H2-assisted CVD technique. The experiment utilized a H2/Ar mixed gas, with the H2 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 H2/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 WO3−x nucleation sites. When the carrier gas flow rate was optimized within a specific range, minor adjustments did not affect the achievement of monolayer WSe2 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 H2/Ar carrier gas flow rate intensified the impact of H2 in the carrier gas on the final product, particularly in decomposing the already formed monolayer WSe2, a phenomenon that became more pronounced with increased carrier gas flow rates. This study not only provides significant insights for the preparation of monolayer WSe2 via H2-assisted CVD but also serves as a reference for the fabrication of other monolayer TMDs materials such as WS2, MoS2, and MoSe2, laying the groundwork for the future application of these materials in optoelectronic devices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma17102190/s1, Table S1: TMDs materials were prepared by CVD method; Figure S1: The effect of growth temperature on WSe2 growth. (a) 900 °C, (b) 925 °C, (c) 950 °C and (d) 975 °C; Figure S2: The optical microscope images at higher magnifications related to Figure 2; Figure S3: Supplement to the effect of carrier gas flow rate on products on substrate.(a) 27.6 sccm, (b) 27.9 sccm, (c) 28.3 sccm, (d) 31.2 sccm, (e) 31.6 sccm and (f) 31.9 sccm; Figure S4: PL spectrum of WSe2. (a) bilayer WSe2, (b) thickness-dependent PL spectra of WSe2; Figure S5: The comprehensive WSe2 spectrum acquired through XPS testing; Figure S6: The XPS fitting analysis of W element in WSe2 films, stored under non-vacuum conditions for an extended period exceeding one month; Figure S7: WSe2 Optical spectra of samples before and after annealing. (a) the photoluminescence spectra of samples, (b) and (c) the Raman spectra of samples; Figure S8: WSe2 elemental distribution spectrum conducted on the sample corresponding to Figure 5a under HAADF-STEM mode; Figure S9: The XRD analysis of monolayer WSe2. References [20,25,26,32,34,35,43,45,47,48,49,50,51] are cited in the Supplementary Materials.

Author Contributions

Y.L. conceived the project and was responsible for the material synthetic design. X.L, Y.J., H.L., Q.L., J.L. and M.W. performed the experiments, sample testing, data collection, and analysis. X.L and Y.L. discussed the data and performed formal analyses. X.L. and Y.L. wrote, reviewed, and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (52072281, Yong Liu), the Major Program of the National Natural Science Foundation of China (22293021, Yong Liu), and the National Innovation and Entrepreneurship Training Program for College Students (No. S202210497011, Yong Liu). Yong Liu gratefully acknowledges the Youth Innovation Research Fund project of the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology.

Data Availability Statement

Data are available in a publicly accessible repository that does not issue DOIs or upon request from the corresponding author.

Acknowledgments

The authors acknowledge the support for and help with the testing platform provided by the School of Materials Science and Engineering of Wuhan University of Technology and the State Key Laboratory of New Materials Composite Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

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Materials 17 02190 g001
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.
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.
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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. Influence of carrier gas flow rate on products on substrate. The scales of (a,d) are 100 μm, and the rest are 50 μm.
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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. 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.
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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 W4+ 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. 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 W4+ 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).
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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. 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.
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Table 1. Parameters for each stage in the synthesis of WSe2 via multi-step H2-assisted CVD.
Table 1. Parameters for each stage in the synthesis of WSe2 via multi-step H2-assisted CVD.
Time
(min)		StageDuration
(min)		Ar
(sccm)		H2/Ar
(sccm)		Temperature
(°C)
0–30Ⅰ30300025–25
30–90Ⅱ60100025–950
90–98Ⅲ8028.3~31.2950–950
98–200Ⅳ1021000950–200
200–215Ⅴ1500200–25
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Luo, X.; Jiao, Y.; Li, H.; Liu, Q.; Liu, J.; Wang, M.; Liu, Y. Impact of Carrier Gas Flow Rate on the Synthesis of Monolayer WSe2 via Hydrogen-Assisted Chemical Vapor Deposition. Materials 2024, 17, 2190. https://doi.org/10.3390/ma17102190

AMA Style

Luo X, Jiao Y, Li H, Liu Q, Liu J, Wang M, Liu Y. Impact of Carrier Gas Flow Rate on the Synthesis of Monolayer WSe2 via Hydrogen-Assisted Chemical Vapor Deposition. Materials. 2024; 17(10):2190. https://doi.org/10.3390/ma17102190

Chicago/Turabian Style

Luo, Xuemin, Yanhui Jiao, Hang Li, Qi Liu, Jinfeng Liu, Mingwei Wang, and Yong Liu. 2024. "Impact of Carrier Gas Flow Rate on the Synthesis of Monolayer WSe2 via Hydrogen-Assisted Chemical Vapor Deposition" Materials 17, no. 10: 2190. https://doi.org/10.3390/ma17102190

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