Structural and Optical Properties of Tungsten Disulfide Nanoscale Films Grown by Sulfurization from W and WO₃

Abstract

Tungsten disulfide (WS₂) was prepared from W metal and WO₃ by ion beam sputtering and sulfurization in a different number of layers, including monolayer, bilayer, six-layer, and nine-layer. To obtain better crystallinity, the nine-layer of WS₂ was also prepared from W metal and sulfurized in a furnace at different temperatures (800, 850, 900, and 950 °C). X-ray diffraction revealed that WS₂ has a 2-H crystal structure and the crystallinity improved with increasing sulfurization temperature, while the crystallinity of WS₂ sulfurized from WO₃ (WS₂-WO₃) is better than that sulfurized from W-metal (WS₂-W). Raman spectra show that the full-width at half maximum (FWHM) of WS₂-WO₃ is narrower than that of WS₂-W. We demonstrate that high-quality monocrystalline WS₂ thin films can be prepared at wafer scale by sulfurization of WO₃. The photoluminescence of the WS₂ monolayer is strongly enhanced and centered at 1.98 eV. The transmittance of the WS₂ monolayer exceeds 80%, and the measured band gap is 1.9 eV, as shown by ultraviolet-visible-infrared spectroscopy.

1. Introduction

Tungsten disulfide (WS₂) is a two-dimensional (2-D) material consisting of a covalently bonded sheet of W atoms filled between two trigonal sheets of S atoms. WS₂ is mainly composed of three structures, hexagonal (2-H), trigonal (1T), or rhombohedral (3R) phase. Among them, the 2H phase structure is relatively stable and exhibits better optical properties [1]. The interlayers of WS₂ are bound only by weak van der Waals forces, and the interlayer spacing is ~0.6 nm [2,3].

WS₂ has been extensively studied due to its unique layer-dependent properties, such as the ability to absorb 5–10% of incident sunlight [4], and its unique band structure. Monolayer WS₂ exhibits a direct band gap [5] of 1.98 eV [6], while multilayer WS₂ shows an indirect band gap [7] of about 1.3 eV [8,9]. This phenomenon can be attributed to the lack of Coulomb repulsion between the p_z orbitals of the chalcogenide elements in adjacent layers, leading to stabilization of the Γ-state valence band [10]. WS₂ has been investigated for applications such as solar cells [11], hydrogen evolution reactions (HER) [12], electrocatalysis [13], batteries [14], and transistors [15]. However, the efficiency of most of the devices remains low. Therefore, a better understanding of the formation and physical properties of WS₂ is essential. Many efforts have been made to improve the application value of WS₂, such as controlling the formation mechanism, including studying the growth mode of the films to obtain a uniform, large-area [16], and well-oriented crystals. There are three common methods for depositing WS₂ thin films: the stripping method [17], the chemical vapor deposition (CVD) [18,19], and the direct vulcanization synthesis method [20]. In this study, we seek to obtain high-quality WS₂ films by comparing tungsten (W) metal and oxide (WO₃) sulfurization processes. The structural and optical properties of the WS₂ thin films were investigated in detail for future applications in next-generation optoelectronic devices.

2. Experimental

The W and WO₃ films were prepared on c-axis Al₂O₃ (1 cm², provided by Bangjie Material Technology). First, the Al₂O₃ substrate was ultrasonicated in acetone for 5 min to remove dirt and then rinsed with methanol. The cleaned c-axis sapphire was then inserted into the ion beam sputtering (IBS, Commonwealth Scientific) chamber for the growth of W or WO₃ films. Subsequently, the grown W or WO₃ films were sulfurized to obtain WS₂ films, as schematically shown in Figure S1a,b of the Supplementary Document. The sputtering of W films was performed at a base pressure of about 5 × 10⁻⁶ Torr. Ar (99.999% purity) was introduced into the chamber at a flow rate of 5 sccm to initiate sputtering process. On the other hand, the WO₃ films were grown by flowing Ar and oxygen at flow rates of 5 and 3 sccm, respectively. The sulfurization of the W and WO₃ films was carried out in a horizontal quartz-tube furnace. The W or WO₃ films were placed in the center of the quartz-tube furnace while about 3 g of sulfur (99.999% purity) was placed next to the tube lid. The tube was then evacuated to 5 × 10⁻² Torr. During the sulfurization process, nitrogen gas (99.999%) flowed into the chamber while maintaining a pressure of 0.7 Torr. In this work, the sulfurization process is maintained at 900 °C for thickness-dependent studies. To study the crystallinity behavior, the nine-layer sample of WS₂-W was sulfurized at different temperatures of 800, 850, 900, and 950 °C. All sulfurization processes were carried out for 20–30 min and cooled naturally to room temperature. The elemental compositions of the WS₂ films were examined using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific with an Al Kα light source). The binding energies were referenced to the NIST-XPS database. The WS₂ phase was identified by micro-Raman (Horiba Jobin Yvon Lab RAM HR) equipped with an 1800-cycle grating, an objective lens with 100× magnification, and a laser wavelength of 532 nm. X-ray absorption near-edge spectroscopy (XANES) was performed to identify the local electron structure of WS₂. The XANES was performed at the 07A1 beamline of the National Synchrotron Radiation Research Center (Hsinchu, Taiwan). The crystal structure of WS₂ films was examined by X-ray diffraction (XRD, D8 by Bruker AXS Gmbh) equipped with a Cu 2 keV light source. A high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2100F CS STEM) equipped with a dual-beam focus ion beam was conducted to study the microstructure and thickness at the atomic scale. The electrons were accelerated at 12 kV and a magnification of 500 K. The TEM was also equipped with an energy-dispersive X-ray spectrometer (EDS). Further analysis was performed using micro-photoluminescence spectroscopy (PL) to study the band gap of WS₂ and ultraviolet-visible-infrared spectroscopy to understand the light absorption properties of the samples.

3. Results and Discussion

3.1. Structural Analysis

Figure 1a,b shows the XRD of WS₂-W at different sputtering thicknesses and sulfurization temperatures. All the XRD peaks marked with an asterisk match the substrate Al₂O₃ peaks. The presence of lattice planes (002), (004), (006), and (008) suggests that the structure of WS₂ is a 2H (hexagonal) phase [5]. Moreover, the interlayer spacing of the WS₂ was estimated to be ~0.62 nm using the Bragg equation, which agrees with the theory and previous results [21,22].

With increasing film thickness, the intensity of XRD signals (002), (004), (006), and (008) increases, while the FWHM of each peak is reduced, as shown in Figure 1a, indicating that the crystallinity of the films improves with the number of WS₂ layers. The absence of the (002) peak for the monolayer of WS₂ is due to the diffraction limit. The tungsten films were sulfurized at different temperatures of 800, 850, 900, and 950 °C to obtain the optimum temperature for formation WS₂ (nine-layer sample). The corresponding XRD results are shown in Figure 1b, indicating that the crystallinity of WS₂ improves with increasing sulfurization temperature. Additionally, to study the local structure of WS₂, the X-ray absorption near-edge structure (XANES) was also performed at W L₃-edge. Figure 1c exhibits the spectrum of WS₂ with tungsten and tungsten trioxide as references. The XANES of WS₂ agrees with the previous result [23]. In addition, it also confirms that the WS₂ (black path) has a 2H (hexagonal) structure.

TEM investigation has been carried out to observe the atomic stacking of WS₂ (nine-layer sample). In order to improve the conductivity of the film, about 5 nm of platinum was deposited on the surface of WS₂. Platinum was also used to protect the sample from oxidation. Figure 1c shows the TEM image of the nine-layer of WS₂ stacked along the c-axis with the platinum capping layer. The total thickness of the WS₂ layer was estimated to be ~5.37 nm with an interplanar spacing of 0.62 nm (Figure 1d), which agrees with the XRD result.

3.2. Elemental Composition Analysis

The elemental composition of WS₂ was qualitatively examined by X-ray photoelectron spectroscopy (XPS). Figure 2 shows high-resolution XPS spectra in W⁴⁺ and S²⁻ energy regions. Figure 2a exhibits the tungsten signals at 32.6, 34.8, and 38.2 eV, which can be ascribed to W 4f_7/2, W 4f_5/2, and W 5f_3/2, respectively, with a spin-orbit splitting of $\Delta E_P$(4f_7/2 − 4f_5/2) = 2.2 eV. The existence of high-intensity signals at 32.6 and 34.8 eV also suggests the formation of 2H-WS₂, which concurs with previous reports [24,25]. Figure 2b shows the spectrum of the doublet S 2p at 163.5 eV and 162.3 eV, representing S 2p_1/2 and S 2p_3/2, respectively [26,27]. These peaks are attributed to the divalent sulfide ions (S²⁻), which are also associated with the formation of 2H-WS₂. The elemental composition of WS₂ was estimated to be 56.0, 28.7, and 15.3% (in atomic percentage) for S 2p, W 4f, and O 1s, in order. The oxygen signal is from the sapphire substrate. Therefore, the W and S ratio is very close to 2:1, confirming that the samples are in good stoichiometry. Additionally, the elemental composition of WS₂ was also qualitatively examined using EDS (Figure S2).

3.3. Optical Properties

3.3.1. Micro-PL Spectroscopy

Micro-PL has been performed to study the optical and electronic properties of WS₂. It is found that the PL signal comes from the energy gap of the direct transition when the thickness is reduced below two atomic layers. The PL of WS₂ is strongly suppressed as thickness increases above two atomic layers, in which the indirect band gap is completely formed.

Figure 3 shows the PL peaks of the monolayer and bilayer of WS₂. The peaks are centered at 1.98 and 1.95 eV, respectively, in good agreement with previous results (2.1–1.9 eV) [28] and with the DFT-LDA calculations for the monolayer and the bilayer of WS₂ [29,30]. The PL intensity shows a slight redshift (~3 eV) and a threefold decrease in intensity as the layer increases from the monolayer to the bilayer of WS₂ [3]. The peak shift is likely due to the proportional relationship between layer number and carrier concentration, which also contributes to Coulomb scattering [31]. This result is in line with the theoretical prediction that the WS₂ monolayer has a direct band gap at the Γ point [32].

3.3.2. Ultraviolet-Visible-Infrared Spectroscopy

The optical properties of WS₂ were further investigated using an ultraviolet-visible-infrared (UV/visible/NIR) spectrophotometer, as shown in Figure 4. It was observed that the transmittance of WS₂ increased from 40% (nine-layer) to over 80% (monolayer) in the UV-visible region (Figure 4a). The band gap of the WS₂ monolayer is about 1.9 eV, as estimated from the Tauc plot of the absorption spectrum (Figure 4b).

3.4. Comparison of WS₂-W and WS₂-WO₃

3.4.1. XRD of WS₂-W and WS₂-WO₃

In order to obtain high-quality WS₂ films, we compared the sulfurization of W and WO₃ films, termed as WS₂-W and WS₂-WO₃, respectively. The samples were sulfurized side by side at 900 °C. The XRD results are shown in Figure 5. It was revealed that the diffraction peaks of WS₂-WO₃ are sharper and narrower than those of WS₂-W with similar thickness, suggesting that the crystallinity of WS₂-WO₃ is better than that of WS₂-W.

3.4.2. Raman Spectra of WS₂-W and WS₂-WO₃

Raman spectroscopy is widely used for elemental analysis, layer stacking order, and doping effect of transition metal dichalcogenides [33]. Figure 6 shows the Raman spectra of WS₂-W and WS₂-WO₃ with the different number of stacked layers.

Figure 6a,b shows two main Raman peaks centered at ~354 and ~420 cm⁻¹, respectively. These Raman peaks agree with previous work [21,34,35]. It was observed that the A_1g peaks overlap with the Raman peaks of sapphire (380 and 417cm⁻¹) [36] when the Raman peaks were deconvoluted using a multi-peak Lorentzian fitting method [37]. Deconvolution of Raman peaks also revealed three strong peaks, which are $E_{2g}^1$ (in-plane displacement of W and S), A_1g (out-plane displacement of S-S atoms), and 2LA (M) (second order Raman peak), which are centered at ~356, ~418, and ~351 cm⁻¹_, respectively. Furthermore, the approximate distance between A_1g and $E_{2g}^1$ of WS₂-W is ~61.8, 63.8, 61.6, and 64.1 cm⁻¹ for monolayer, bilayer, six-layer, and nine-layer samples, respectively, which is not much different from the distance for WS₂-WO₃ of about 61.8, 63.7, 63.2, 62.7 in the same order. Figure 6c shows the $I_{2LA}/I_{A_{1g}}$ intensity ratio of WS₂-W reduces from the monolayer (1.8) to the nine-layer (1.3). However, WS₂-WO₃ shows a slightly different trend, where the intensity ratio increases from the monolayer (1.3) to the bilayer (1.8), and then decreases to the nine-layer (1.3). The results suggest that the second-order Raman peak intensity ($I_{2LA}$) is almost twice that of the first-order $I_{A_{1g}}$. This is consistent with the previous results [38,39]. Additionally, for both WS₂-W and WS₂-WO₃ samples, as the film thickness decreases, the A_1g signals show a slight red shift of about ~2.3 cm⁻¹ (Figure 6d). The redshift of A_1g is likely associated with reducing the atomic restoring forces. When the number of WS₂ layers is decreased, the long-range Coulomb interaction between effective charges and dielectric screening is enhanced, leading to increased restoring force between the S-S atoms [3].

Due to the dielectric screening effect, the Raman signal is sensitive to the interactions between atoms in the interlayer and long-range Coulomb interactions [40]. The Raman wavevector and the intensity of WS₂ are strongly correlated with the layer thickness at the nanoscale level. In this work, we also used Raman spectra to calibrate the thickness of WS₂ as a monolayer, bilayer, six-layer, and nine-layer [31,39]. The Raman spectra confirmed that WS₂ was well-formed as WS₂-W and WS₂-WO₃. It is clear that the peak intensity and FWHM of $E_{2g}^1$ for WS₂-WO₃ are narrower than that of WS₂-W (Table S1 in the Supplementary Materials). The average estimation of FWHM of WS₂-WO₃ is ~7.2; however, the WS₂-W is ~10.1, suggesting WS₂-WO₃ has better crystallinity than WS₂-W. This is consistent with the XRD results (Figure 5). This phenomenon can be attributed to the fact that sulfur can replace oxygen more effectively at a relatively lower temperature [41], while WS₂ formed from tungsten metal requires a higher temperature to facilitate metal–sulfur bonding [42]. As a result, the defect density in WS₂-WO₃ is likely to be lower than in WS₂-W.

4. Conclusions

We report a study of nanoscale WS₂ films with various number of layers, prepared by sulfurization of W and WO₃ at different temperatures (800 to 950 °C). The WS₂ films were inferred to be the 2H phase and c-axis oriented. The XRD shows that the crystal quality of the WS₂ films improved with increasing sulfurization temperature. The photoluminescence of the monolayer of WS₂ is strongly enhanced and centered at 1.98 eV. The transmittance of the monolayer WS₂ exceeds 80% and the band gap is 1.9 eV, revealed by ultraviolet-visible-infrared spectroscopy. The Raman analysis shows that the FWHM of WS₂-WO₃ is narrower than that of WS₂-W, indicating the structure of WS₂-WO₃ is superior to that of WS₂-W, which is in good agreement with the X-ray diffraction result. We conclude that a large-area, high-quality WS₂ film can be prepared by sulfurizing WO₃. The results are promising for applications in next-generation optoelectronic devices.