Synthesis of Wrinkled MoS₂ Thin Films Using a Two-Step Method Consisting of Magnetron Sputtering and Sulfurization in a Confined Space

Abstract

Considering the increasing need for sustainable and economical energy storage solutions, the integration of layered materials such as MoS₂ into these systems represents an important step toward enhancing energy sustainability and efficiency. Exploring environmentally responsible fabrication techniques, this study assesses wrinkled MoS₂ thin films synthesized from distinct Mo and MoS₂ targets, followed by sulfurization conducted in a graphite box. We utilized magnetron sputtering to deposit precursor Mo and MoS₂ films on Si substrates, achieving thicknesses below 20 nm. This novel approach decreases sulfur by up to tenfold during sulfurization due to the confined space technique, contributing also to avoiding the formation of toxic gases such as SO₂ or the necessity of using H₂S, aligning with sustainable materials development. Thinner MoS₂ layers were obtained post-sulfurization from the MoS₂ precursors, as shown by X-ray reflectometry. Raman spectroscopy and grazing X-ray diffraction analyses confirmed the amorphous nature of the as-deposited films. Post-sulfurization, both types of films exhibited crystalline hexagonal MoS₂ phases, with the sulfurized Mo showing a polycrystalline nature with a (100) orientation and sulfurized MoS₂ displaying a (00L) preferred orientation. The X-ray photoelectron spectroscopy results supported a Mo:S ratio of 1:2 on the surface of the films obtained using the MoS₂ precursor films, confirming the stoichiometry obtained by means of energy dispersive X-ray spectroscopy. Scanning electron microscopy and atomic force microscopy images revealed micrometer-sized clusters potentially formed during rapid cooling post-sulfurization, with an increased average roughness. These results open the way for the further exploration of wrinkled MoS₂ thin films in advanced energy storage technologies.

1. Introduction

The rapid expansion of the electric vehicle and portable electronic device markets has escalated the demand for energy storage systems with a high energy density and a substantial power output [1,2]. In response, significant research efforts have been directed towards developing efficient energy storage devices, such as batteries and supercapacitors [3,4]. Among various materials, MoS₂, a two-dimensional layered inorganic material, has emerged as a good candidate for energy conversion and storage applications due to its adjustable bandgap and unique electronic and physicochemical properties [5,6,7,8,9]. In particular, the layered structure of MoS₂ promotes efficient electron and ion transport, desirable for high-performance energy storage devices, and its bandgap manipulation enhances electrical conductivity and electrochemical properties. These characteristics are highly beneficial in increasing energy density, improving charge–discharge rates, enhancing cycle stability in battery technology, and boosting capacitance and energy efficiency in supercapacitors [10,11,12,13,14]. Molybdenum disulfide has also been widely as a solid lubricant [15]. Its combination with other tribological materials such as DLC [16] and high-wear-resistance Ti₃C₂T_x was recently discussed [17,18,19].

Given the growing demand for environmentally friendly and cost-effective energy storage systems, MoS₂ integration into these systems is a significant advance towards achieving energy sustainability and efficiency. The unique structure of layered materials, including MoS₂, offers advantages in energy storage applications due to their high surface area-to-volume ratio and observable characteristics within a few atomic layers [20,21]. However, challenges such as poor cycle stability due to structural deterioration during charge/discharge cycles have prompted research into nanostructuring MoS₂ to enhance ion kinetics, thus improving overall battery performance [13,22]. The development of such nanostructured MoS₂ anodes is crucial for overcoming the limitations of bulk MoS₂, particularly in terms of cycling stability and rate capability, thereby making it more viable for practical applications in advanced batteries and supercapacitors [23].

Regarding material synthesis, magnetron sputtering is a superior technique for depositing thin films, offering uniformity, smoothness, and precise control over layer thickness, essential for creating high-quality MoS₂ thin films [24,25]. However, the reliance of CVD on chemical precursors can pose more sustainability challenges in terms of chemical handling, safety, and disposal. Moreover, magnetron sputtering is more material-efficient, which is beneficial for sustainability. Additionally, it can easily be used for doping to improve the properties of MoS₂ coatings [26]. Wu et al. [27] demonstrated the potential of this method to obtain thin MoS₂ films through the sequential deposition of Mo layers and controlled sulfurization [28,29]. During the sulfurization, there is competition between Mo oxide segregation leading to the formation of small clusters and the sulfurization reaction leading to planar MoS₂. The most common sulfurization methods involve conducting annealing in H₂S- or S₂-containing atmospheres. However, H₂S is a toxic gas, while the contamination of the medium becomes an important issue when the annealing is conducted in a S₂ environment due to the formation of SO₂ [30,31].

In this study, MoS₂ thin films were synthesized in a two-step process consisting of magnetron sputtering deposition of two types of precursors, namely Mo and MoS₂ layers, followed by sulfurization in a graphite box. This novel approach significantly reduces the S quantity up to ten times during sulfurization due to the confined space and represents an important advancement in the controlled synthesis of nanostructured MoS₂ thin films on Si/SiO₂ substrates. It also contributes to avoiding the formation of toxic gases such as SO₂ or the necessity of using H₂S for sulfurization, aligning with the need for efficient and sustainable energy storage solutions.

2. Materials and Methods

The MoS₂ films were obtained using a two-step process involving the deposition of a precursor thin film of either Mo or MoS₂, followed by sulfurization in a confined space. The method is depicted in Figure 1. In the first step, molybdenum and molybdenum disulfide precursor thin films were deposited using a custom-built magnetron sputtering system [32]. Commercial Mo and MoS₂ targets (Mateck, Jülich, Germany, 99.9% purity, 5 cm in diameter and 3 mm thickness) were used. The process was initiated in a vacuum of 6.5 × 10⁻⁶ Torr. Argon was introduced at a constant flow of 30 sccm and the chamber pressure increased to 6 × 10⁻³ Torr, which was kept constant during the deposition. The Mo target received a power of 26 W from a T&C Power Conversion AG 0313 RF source (Rochester, NY, USA) for 1886 s, while the MoS₂ target received 18 W for 1785 s from the same RF source. The used Si substrates with a 90 nm SiO₂ layer were continuously rotated during deposition for thickness uniformity.

Post-deposition, in the second step, the Mo and MoS₂ films underwent a sulfurization process. Enclosed in a graphite box with 0.025 g of sulfur, the samples were placed in an MTI GSL 1600X (MTI Corporation, Richmond, CA, USA) quartz tube furnace. The system was vacuumed and pressurized with argon to 0.03 MPa. The thermal treatment was performed at 800 °C for 45 min, with a ramp and cooling rate of 10 °C/min.

The samples were subjected to detailed morphological and elemental analysis using a Zeiss Gemini 500 Field Emission Scanning Electron Microscope (SEM) equipped with a Bruker energy dispersive X-ray (EDX) spectrometer (Bruker, Billerica, MA, USA). The crystalline structures of the samples were investigated using X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXRD). This analysis was performed using a Rigaku SmartLab diffractometer (Rigaku Corporation, Tokyo, Japan), set in a parallel beam arrangement, provided with copper Kα radiation (λ = 1.54187 Å) and a HyPix-3000 2D Hybrid Pixel Array Detector (Rigaku Corporation, Tokyo, Japan) used in 0D and 1D modes. The diffractograms were recorded from 5° to 65° (2θ) with a step size of 0.04° and the measuring time per step was 2 s for GIXRD and 40 s for XRD (θ-2θ Bragg–Brentano geometry). The incident angle was precisely adjusted to enhance diffraction conditions for GIXRD. The background of the diffractograms was subtracted. X-ray reflectometry (XRR) patterns were captured using the same equipment. The thickness and mass density of the samples were accurately determined using Leptos software (version 4.01 from Bruker), which allows for the alignment of simulated XRR curves with the experimentally obtained data. Additional micro-Raman (m-Raman) structural examination was carried out with a LabRam HR Evolution spectrometer (Horiba Jobin Yvon, Palaiseau, France) equipped with a Syncerity OE detector. The measurements were performed by focusing a He-Ne laser (λ = 633 nm) on the surface of the samples in backscattering geometry with an Olympus MPlanN 100×/0.90 numerical aperture using a diffraction grating of 1800 lines/mm. The spectra were recorded in the range 50–600 cm^–1 for an acquisition time of 10 s and 25 accumulations with a neutral density filter of 50% and 5% of the maximum laser power for a confocal hole of 100 μm for the target and thin films, respectively. This corresponds to a laser power of 750 and 75 μW, respectively. The maximum visible spot that excites the laser corresponded to the Airy spot was ~0.85 μm. Atomic force microscopy (AFM) imaging was performed using an NT-MDT Aura Ntegra Prima atomic force microscope (Amsterdam, Netherlands) in a non-contact mode. The surface of the films was characterized by means of X-ray photoelectron spectroscopy (XPS) using a monochromatized Al Kα source at 1486.6 eV and a power of 144 W (12 kV × 12 mA), employing a Kratos Axis Ultra spectrometer (Kratos Analytical Ltd., Manchester, UK). The analyzer was operated in the fixed analyzer transmission (FAT) mode, and the high-resolution spectra were acquired at 40 eV pass energy and 0.05 eV energy step. The analysis of the S 2p and Mo 3d doublets was performed after spectral deconvolution with Voigt profiles including two Lorentzian components. Both the spectral deconvolution extraction of the Shirley background and the data analysis were performed using IGOR Pro 9 software.

3. Results

3.1. X-ray Reflectometry (XRR) Measurements

The XRR diagrams and the corresponding fitted curves for the Mo and MoS₂ precursor thin films are depicted in Figure 2(a1,a2). We determined the thickness of the Mo precursor layer to be 14.4 nm and its average mass density to be 8.0 g/cm³, compared to 10.28 g/cm³ [33] for a bulk Mo monocrystal. For the MoS₂ precursor layer, the values obtained were 18.0 nm for the thickness and an average mass density of 4.3 g/cm³, against 5.06 g/cm³ [33] for a bulk MoS₂ monocrystal. The observed variations in mass density of the Mo and MoS₂ thin films, as compared to their bulk monocrystal counterparts, can be attributed to structural differences, the presence of defects or voids, surface roughness, and deviations in film composition.

The XRR diagrams for the sulfurized Mo and MoS₂ thin films are shown in Figure 2(b1,b2). The thickness of the Mo film is 28 nm. Theoretically, a thickness of approximately 55 nm was expected if the as-deposited Mo film would have been entirely sulfurized. In contrast, the sulfurized MoS₂ layer showed a uniform thickness of 13 nm, compared with a theoretical expectation of around 18 nm. The observed discrepancy in thickness and structure between the theoretical expectations and the experimental results can be attributed to several factors. In the case of the Mo layer, the formation of a thick 28 nm layer indicates a significant reaction during sulfurization, leading to an expanded or altered structure. This might be due to the diffusion of sulfur into the Mo layer or the formation of an additional MoS₂ layer on top of the original Mo. For the sulfurized MoS₂ layer, the reduction in thickness from the expected value might be due to the loss of material and an increase in density during sulfurization. The uniformity of this layer suggests a more controlled and consistent reaction compared to the Mo layer.

3.2. X-ray Diffraction

The X-ray diffraction diagrams at grazing incidence for the Mo (Figure 3(a1)) and MoS₂ (Figure 3(b1)) precursor thin films were obtained at an incidence angle of 0.3°. The broad peaks observed in both diagrams suggest that each of the layers possesses an amorphous, disordered structure. This is in contrast with the distinct crystalline peaks of the substrate, which are clearly discernible. The amorphous nature indicated by the GIXRD analysis complements the findings from the XRR measurements, explaining the lower density as compared to their crystalline monocrystal forms.

The GIXRD diagrams, performed with the detector in 0D mode, and XRD diagrams, obtained using Bragg–Brentano geometry with the detector in 1D mode, for sulfurized Mo and MoS₂ precursors are displayed in Figure 3(a2,b2), and (a3,b3), respectively. In both cases, the data confirm the successful synthesis of the hexagonal MoS₂ (h-MoS₂) phase according to PDF 00-037-1492, highlighting the effectiveness of the sulfurization process. Sulfurized Mo exhibits a polycrystalline structure with a preferential (100) orientation. Most of the MoS₂ crystallites have layers in the plane (002) that are not parallel to the substrate but perpendicular.

On the other hand, the sulfurized MoS₂ precursor shows a preferential (00L) orientation. In this case, the MoS₂ layers are parallel to the substrate. The polycrystalline nature of the sulfurized Mo film and the (100) preferential orientation suggest a distinct structural rearrangement post-sulfurization. This perpendicular arrangement might generate an anisotropic electronic transport mechanism that could influence the charge carrier mobility and mechanical stress distribution within the layer, potentially impacting its application in electronic devices, but beneficial for applications such as catalysis and energy storage. In contrast, the preferential (00L) orientation, parallel to the substrate, of the MoS₂ crystallites in sulfurized MoS₂, indicative of a layered structure, is beneficial for applications requiring a high surface area, as in electronic devices, where the directionality of electronic and ionic transport plays an important role in device performance.

3.3. Micro Raman Spectroscopy

The hexagonal MoS₂ exhibits four Raman peaks: ${\mathrm{E}}_{2\mathrm{g}}^2$ at 32 cm⁻¹, which requires high spectral resolution Raman spectrometers for detection and corresponds to the interlayer vibration within MoS₂; E_1g at 286 cm⁻¹ (forbidden in backscattering geometry on surfaces perpendicular to the c-axis); ${\mathrm{E}}_{2\mathrm{g}}^1$ at 381 cm⁻¹; and A_1g at 408 cm⁻¹ [34]. Other peaks in the m-Raman spectra arise due to the resonance phenomenon between the excitation laser radiation (633 nm) and the MoS₂ band gap. This resonance, alongside the coupling between electronic transitions (correlated with excitonic states) and phonon modes, significantly amplifies the second- or higher-order Raman scattering.

The m-Raman spectra of the as-deposited Mo and MoS₂ thin films did not show any Raman peaks, indicating that they are completely amorphous, as confirmed by the X-ray diffraction diagrams (Figure 3(a1,b1)).

The m-Raman spectra of the sulfurized Mo sample (Figure 4a, blue curve) and sulfurized MoS₂ sample (Figure 4a, red curve) confirm that sulfurization led to the formation of the hexagonal crystalline MoS₂ phase. The red curve is similar to the spectrum of the solid MoS₂ sputtering target (Figure 4b). The A_1g peak shows a shoulder at 402 cm⁻¹; consequently, the frequency difference (δ) of the two Raman modes, ${\mathrm{E}}_{2\mathrm{g}}^1$ and A_1g, decreases from 28 cm⁻¹ to 22 cm⁻¹. This means that on average, there are two categories of crystallites thicknesses in each sample. The majority category is represented by thick crystallites containing more than four MoS₂ layers, while the second category is formed by three MoS₂ layers [35].

Due to the resonance conditions, second-order Raman bands at 123 cm⁻¹, 150 cm⁻¹, 177 cm⁻¹, 188 cm⁻¹, 227 cm⁻¹, 230 cm⁻¹, 417 cm⁻¹, 453 cm⁻¹, 464 cm⁻¹, and 564 cm⁻¹ [36,37,38] are also visible. The work of Livneh et al. [37] facilitates the symmetry assignments of these peaks:

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A_1g(Γ)—E_1g(Γ) for the peak at 123 cm⁻¹;

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${\mathrm{E}}_{2\mathrm{g}}^1$(K₂)—LA’(K) for the peak at 150 cm⁻¹, where LA’(K) is the longitudinal quasi-acoustic optical phonon in K point of the hexagonal Brillouin zone. A different process involves the creation of one phonon while absorbing another;

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A_1g(M)—LA(M) for the peak at 177 cm⁻¹, where LA(M) is the longitudinal acoustic phonon in M point of the hexagonal Brillouin zone. The asymmetric line shape arising from phonon dispersion of LA mode demonstrates a concave dispersion towards the M point [39]. Notably, the characteristic MoS₂ modes, A_1g and E¹_2g, show no dispersion in the Γ-M direction;

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${\mathrm{E}}_{2\mathrm{g}}^1$(M₂)—ZA’(M) for the peak at 188 cm⁻¹, which exhibits an exponential temperature dependence, detectable even at liquid nitrogen temperatures [36];

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A degenerate van Hove singularity (vHs) is located between the K and M points for the peak at 227 cm⁻¹, while its intensity is proportional to the disorder degree [34]. The diminished intensity of this peak in the sulfurized MoS₂ film, compared to the sulfurized Mo film, can be attributed to the higher crystalline quality and reduced disorder and defects in the MoS₂ film post-sulfurization;

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LA’(M) for the peak at 230 cm⁻¹;

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A_1g(M) for the peak at 417 cm⁻¹ [36];

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The most pronounced band among the second-order Raman peaks, at 453 cm⁻¹, is attributed to the overlap of the vHs from two phonon branches located between the K and M points, each resonating near the frequency of 227 cm⁻¹. The peak at 453 cm⁻¹ exhibits significantly greater intensity in the sulfurized MoS₂ film. This could be due to increased resonance effects between the laser excitation and the MoS₂ band gap. Additionally, the intensity variations of this band inversely correlate with defect concentrations [38,40] (for the first-order peak, at 227 cm⁻¹, the intensity is proportional with defect concentrations). Accordingly, the sulfurized MoS₂ film has a relatively low defect density within the MoS₂ layers, enhancing the resonance Raman effect;

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LA’(M) + LA(M) for the peak at 464 cm⁻¹;

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A_1g(M) + TA’(M) for the peak at 564 cm⁻¹, where TA’(M) is a transversal quasi-acoustic optical phonon in M point of the hexagonal Brillouin zone.

3.4. Scanning Electron Microscopy

The scanning electron microscopy images obtained on sulfurized Mo films (Figure 5a) indicate elongated formations ranging from 1 to 7 microns in length on the surface of the samples, as compared with the as-deposited samples, which display a dense, compact surface with minimal roughness for both Mo and MoS₂ thin films. The sulfurized MoS₂ sample (Figure 5b) exhibits a less uniform surface, composed of small nanometric clusters from 1 to 10 microns in length. These formations are attributed to a phenomenon known as ‘surface wrinkling’, occurring when rapid cooling from 800 °C to room temperature takes place. The ‘surface wrinkling’ [41] phenomenon observed in both samples is a significant aspect of the cooling process following high-temperature treatments. This feature indicates the presence of mechanical stresses and structural rearrangements occurring during rapid cooling.

Using EDX, we measured the composition of the sulfurized Mo and MoS₂ thin films, and the results are shown in

Table 1. Composition of sulfurized Mo and MoS₂ thin films after sulfurization.

Precursor	Composition
Mo	Mo40S60
MoS2	Mo34S66

. For the sulfurized Mo sample, the composition is Mo₄₀S₆₀, while for the sulfurized MoS₂ sample, it is Mo₃₄S₆₆. The results show a substantial incorporation of sulfur in the Mo film, indicating a successful sulfurization process. However, the Mo-rich nature of this composition compared to the ideal Mo:S ratio of 1:2 (or Mo_33.33S_66.67 in atomic percentages) suggests the incomplete incorporation of sulfur. This could be due to various factors such as the initial thickness of the Mo layer, the sulfurization temperature, or the duration of the sulfurization process. The sulfurized MoS₂ (Mo₃₄S₆₆) is very close to the ideal stoichiometry, indicating that the sulfurization process was highly effective.

3.5. Atomic Force Microscopy

Figure 6 shows the AFM image obtained for the sulfurized MoS₂ sample, where elongated formations are visible on the surface, corroborating the observations made in the SEM analysis. Utilizing a cantilever equipped with a sharp tip to probe the surface, we determined the surface roughness to be significantly high, measured at 20 nm. This measurement reveals an intriguing aspect of the material morphology, namely that the surface roughness surpasses the film thickness, which was previously estimated to be 13 nm post-sulfurization. Typically, a roughness greater than the film thickness suggests a highly textured surface with pronounced topographical features that extend beyond the general plane of the film. Such a condition could impact the film’s physical properties and its interaction with the environment, potentially affecting applications where surface area and texture play important roles, such as in catalysis, sensing, or energy storage devices. The pronounced roughness indicates a complex surface morphology that may influence the electronic, mechanical, and chemical behavior of the MoS₂ thin film.

3.6. X-ray Photoelectron Spectroscopy

Since the thin film obtained from the MoS₂ target and subsequently thermally treated in a sulfur atmosphere proved to be of high quality and homogeneous when characterized by “bulk techniques”, the quality of the surface and its stability were also checked by means of photoemission spectroscopy. The results are presented in Figure 7. The Mo 3d spectrum was deconvoluted into four components for Mo and a singlet corresponding to S 2s at a binding energy of 226.7 eV. The component at higher binding energies of 232.7 eV is attributed to MoO₃. The other three components at binding energies of 229.2 eV, 229.5 eV, and 230 eV are associated with S-deficient MoS₂, bulk MoS₂, and surface MoS₂, respectively. The S 2p spectrum was deconvoluted into a single component attributed to S²⁻ at a binding energy of 162.3 eV, showing that all sulfur participates in Mo-S bonds and that there is no unreacted S-S.

Considering that the three main contributions of Mo 3d are related to S, the Mo:S ratio on the analyzed surface is approximately 1:2, as indicated by the ratio of the integral amplitudes corrected for photoionization cross-section, a result also confirmed by the identification of the thin film composition through EDX.

4. Discussion

The two-step process employed herein, utilizing magnetron sputtering and a confined space for sulfurization [42], exemplifies a significant advancement towards sustainability. This method, characterized by a small energy footprint and a high material efficiency demonstrates an energy- and resource-conserving approach to material synthesis. By optimizing the annealing conditions, including the innovative use of a graphite box for sulfurization, this study not only ensures the precise control of the chemical environment but also minimizes the potential waste and environmental impact. Moreover, this approach eliminates the necessity of using toxic H₂S gas. Such a methodology is important in advancing the MoS₂ material synthesis towards more sustainable practices.

Supercapacitors and batteries, characterized by their compact size, low thickness, and mechanical robustness, are essential for integration into flexible electronics. Utilizing materials with wrinkled structures as electrodes can fulfill these requirements. The increased specific surface area and minimized height of wrinkled films enhance their synergy with electrochemically active materials, boosting performance. Moreover, such supercapacitors demonstrate stable operation under mechanical stress due to the added flexibility provided by the wrinkles. Additionally, lithium and lithium–sulfur batteries significantly benefit from the unique properties of wrinkled electrode materials, including high stretchability and specific surface area, thus enhancing their performance.

Compared to traditional capacitors, electrodes prepared from thin films with wrinkled structures demonstrate superior qualities in supercapacitors [43,44,45,46,47,48,49]. Wrinkles notably reduce electrode resistance, a fundamental requirement for high-performing supercapacitors. Moreover, integrating supercapacitors into wearable electronics demands resilience under mechanical strains [50], making the development of wrinkled structures essential. Additionally, wrinkled structures serve to anchor transition metal oxides or metal nanoparticles onto thin films, enhancing specific capacitance. Thus, these thin, flexible, and eco-friendly supercapacitors hold great potential for energy storage and supply applications, leveraging the unique advantages of wrinkled electrode architectures.

High-energy lithium-ion and lithium–sulfur (Li-S) batteries are at the forefront of next-generation energy storage technologies, despite facing certain challenges. The quest for a high specific capacity has led to increased interest in Li-alloy-based batteries [51,52]. In Li-S batteries, the generation of lithium polysulfides during cycling reduces sulfur utilization and diminishes the amount of active material, limiting battery performance [53]. The use of wrinkled sheets to encapsulate sulfur leverages their ample nanopores to increase specific surface area and polysulfide absorption. The wrinkles act as physical barriers, mitigating polysulfide shuttle effects and significantly boosting cyclability. Batteries incorporating wrinkled thin films, serving both as active holders and flexible supporters, emerge as viable candidates for future energy storage solutions, capitalizing on the unique mechanical and chemical properties of wrinkled structures to overcome longstanding challenges.

5. Conclusions

This study successfully produced wrinkled MoS₂ thin films using a novel sustainable two-step approach, namely the sputtering of a precursor Mo or MoS₂ film followed by sulfurization in a graphite box. Better results were obtained when MoS₂ precursor thin films are used, suggesting that the existence of S in the film increases the efficiency of the sulfurization process. These thin films were characterized as being textured using X-ray diffraction, and the MoS₂ crystallites grew with their (00L) planes preferentially oriented parallel to the substrate surface. Micro-Raman spectroscopy confirmed the hexagonal structure and revealed a relatively low defect density within the layer. Additionally, surface morphology assessments via SEM and AFM also illustrated varied textures, with notable surface wrinkling resulting from thermal processing dynamics. Chemical composition examinations through XPS confirmed a Mo:S ratio near 1:2, suggestive of MoS₂ synthesis. The wrinkles obtained on the film surfaces post-sulfurization pave the way for tailored application in energy storage, further highlighting the role of advanced materials in the development of sustainable technological solutions.