Synthesis and Investigation of ReSe₂ Thin Films Obtained from Magnetron Sputtered Re and ReO_x

Abstract

The promise of two-dimensional (2D) rhenium diselenide (ReSe₂) in electronics and optoelectronics has sparked considerable interest in this material. However, achieving the growth of high-quality ReSe₂ thin films on a wafer scale remains a significant challenge. In this study, we adopted a two-step method to produce ReSe₂ thin films by combining magnetron sputtering of Re and ReO_x onto flat substrates with subsequent selenization via atmospheric pressure chemical vapor transport (CVT). After analyzing the produced films using X-ray diffraction to identify the crystalline phase in formed thin film and scanning electron microscopy (SEM) to examine surface morphology, it was determined that the suitable temperature range for the 15 min selenization process with CVT is 650 °C–750 °C. Further investigation of these optimally produced ReSe₂ thin films included atomic force microscopy (AFM), X-ray photoelectron spectroscopy, and Raman spectroscopy. The bulk electrical analysis of these films and AFM and SEM surface morphology revealed a strong reliance on the type of precursor material used for their synthesis, whereas optical measurements indicated a potential for the films in non-linear optics applications, irrespective of the precursor or temperature used. This study not only provides a new pathway for the growth of ReSe₂ films but also sheds light on the synthesis approaches of other 2D transition metal dichalcogenide materials.

1. Introduction

ReSe₂, belonging to the transition metal dichalcogenide (TMD) group, possesses a unique set of properties, including a direct bandgap, high carrier mobility, and robust spin–orbit coupling [1,2,3,4,5]. Unlike group-VI TMDs (molybdenum- or tungsten-based ones), which stabilize in the highly symmetric 2H structure, ReSe₂ crystallizes in the 1T triclinic structure (space group $P\overline{1}$ or No 2) with reduced in-plane crystal symmetry [6]. The structure of ReSe₂ includes four formula units per unit cell with lattice constants a = 0.6602 nm, b = 0.6716 nm, and c = 0.6728 nm and unit cell angles α = 91.82°, β = 104.9°, and γ = 118.94°, where the Re-chain direction corresponds to the b axis [7] (Figure A1). Let us separately note that in the published ReSe₂ structural studies, different orders of angles and vectors are encountered due to the low symmetry of the structure, making the choice of the order of axes arbitrary. The latest investigations support that ReSe₂ has a distorted CdCl₂-type structure that is isostructural with a ReS₂ compound [3,7,8], rather than a Cd(OH)₂-type structure, as was suggested by Alkock and Kjekshus in ref. [9].

This low-symmetry structure gives rise to pronounced in-plane anisotropic optical, electrical, and phonon properties, influencing also thermal conductivity and other vibrational properties, offering considerable promise for creating advanced photonic, electronic, and optoelectronic devices that transcend conventional isotropic 2D materials [4,10,11,12,13]. Additionally, the weak interlayer coupling allows ReSe₂ bulk to act like electronically and vibrationally decoupled monolayers, resulting in a notable layer-independent nature across various properties [14,15].

The unique structural and electronic properties of ReSe₂ extend beyond the realm of electronics, showcasing remarkable catalytic activity for various reactions [16]. Its layered structure provides abundant edge sites, acting as active centers for catalytic processes [17]. Studies reveal its exceptional performance in hydrogen evolution reactions (HERs), offering a promising avenue for clean and sustainable energy generation [17,18,19,20,21]. The multi-layer architecture of ReSe₂ is well-suited to energy storage applications. Its interlayer spacing readily accommodates lithium ions, making it a promising candidate for next-generation lithium-ion batteries. Beyond lithium-ion batteries, ReSe₂ exhibits potential for supercapacitors, leveraging its excellent charge–discharge rates and long cycle life [22]. These characteristics position ReSe₂ as a valuable contributor to the development of efficient and sustainable energy storage systems [22].

Materials with nonlinear optical properties are foundational to the field of nonlinear optics (NLOs), enabling a range of practical applications like optical switching, mode-locking, and optical limiting [13,23]. Optical nonlinearity occurs when a material’s characteristics, such as absorptivity and refractive index, change with the intensity of the electric field. Over the years, scientists have discovered a variety of materials in the TMD class that exhibit nonlinear optical properties, with ReSe₂ being a notable example [13,23]. The unique attributes of ReSe₂, such as its direct bandgap and strong spin–orbit coupling, make it an attractive candidate for cutting-edge NLO technologies, offering researchers and engineers a versatile platform to explore and develop leading-edge optical devices [13,23,24].

In this work, we detail a two-step approach to synthesizing ReSe₂ thin films. This study aimed to develop a reliable, scalable method for ReSe₂ synthesis. The first step involves magnetron sputtering of Re/ReO_x onto a flat substrate, followed by selenization via atmospheric pressure chemical vapor transport. By comparing films synthesized from both precursor materials—Re and ReO_x—we sought to validate the robustness of the methodology and understand how the choice of the precursor impacts various properties of the films. Magnetron sputtering and CVT are both widely used and scalable techniques, suggesting that this method for ReSe₂ synthesis holds promise for future scalability. To determine the quality of the structural properties of the synthesized films, comprehensive characterization techniques were utilized for the obtained samples’ description. These techniques included X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) for chemical composition identification, scanning electron microscopy (SEM) for surface morphology analysis, atomic force microscopy (AFM) for detailed surface topography investigation, Raman spectroscopy for vibrational properties characterization, and electrical measurements using the Van der Pauw configuration. Additionally, the optical properties were investigated through transmittance and reflectance measurements, and the nonlinear optical properties were characterized using the Z-scan method.

2. Experimental Details

2.1. Synthesis of ReSe₂ Thin Films

To find the optimal method for producing high-quality ReSe₂ thin films in the framework of this study, we investigated the influence of different precursors (metallic rhenium Re and rhenium oxide ReO_x) as well as two substrates: Si (100) wafers with thermally grown 300 nm SiO₂ from Semiconductor Wafer, Inc. (Hsinchu, Taiwan) and sapphire (c-plane) from Biotain Crystal Co. (Longyan, China). A two-step approach was employed, starting with the deposition of thin rhenium (Re) and rhenium oxide (ReO_x) films with thicknesses ranging between ~10 and 70 nm on both substrates of a ~1 cm² square (cut by a diamond scriber) size at room temperature by reactive DC magnetron sputtering of a metallic rhenium target (99.9%) in an Ar or mixed Ar/O₂ (20 sccm: 10 sccm) atmosphere (5 × 10⁻³ torr, 100 W DC power)[25]. Prior to the magnetron deposition of the precursor thin films, the substrates underwent a cleaning process. This involved immersing them in an ultrasonic bath with acetone for 3 min, followed by a subsequent 3 min ultrasonic bath in deionized water (DIW). After the ultrasonic cleaning, the substrates were blow-dried using nitrogen (N₂) gas to ensure the removal of any residual contaminants. These deposited precursor films served as the foundation for the subsequent conversion to ReSe₂ via CVT.

For the CVT process, the configuration of the setup, illustrated in Figure 1c, was operated at atmospheric pressure using a 5% H₂/Ar gas mixture. The temperature range tested was from 550 °C to 1000 °C, with a process duration of 15 min (with a pre-heated furnace), and selenium powder (99.99%, Sigma Aldrich, St. Louis, MO, USA) placed at the intake end of the carrier gas. The furnace used here exhibited a temperature gradient, maintaining the target temperature primarily within the central 5 cm region, beyond which, the temperature decreased. Depending on the selected process temperature, the selenium powder was positioned in a zone where the temperature reached 350–400 °C to facilitate its evaporation, to be carried by the flow of the carrier gas to placed samples. A relatively wide range of temperatures was selected to find the ideal temperature range and conduct subsequent, in-depth analyses of the samples produced within the found optimal temperature range.

2.2. Characterization

High-resolution images of the film surface were obtained using a scanning electron microscope (SEM-FIB Lyra, Tescan, Brno, Czechia) to reveal the morphology and potential artifacts. To further investigate the surface morphology and its electrical profile across the film, experiments were conducted using a PARK NX10 atomic force microscope (Suwon, Korea) in intermittent-contact AFM mode. These measurements employed a platinum cantilever with a tip radius of 30 nm and a setpoint force of 640 nN. The bias voltage applied to the sample was 0.02 V, a value chosen to accommodate the observed conductivity of the samples. Using the same AFM tool, thicknesses of the synthesized ReSe₂ films were also measured (Figure 2). During this study, the impact of precursor thickness on selenization was also briefly investigated. Two thicknesses were examined: a thin layer (≤15 nm) and a thick layer (≥50 nm) for both precursors (Re metal and ReO_x), as shown in Figure 2 with hatched bars. XRD analysis then confirmed that a 15 min selenization process was of sufficient duration for both the thin and thick precursor materials treated at 650 °C and 750 °C (Figure A2). Following this, all the other chemical, morphological, electrical, and optical characterizations were performed on the prepared thick ReSe₂ films.

X-ray diffraction (powder diffractometer Rigaku Miniflex 600, Tokyo, Japan) with monochromatic Cu Kα irradiation (λ = 1.5406 Å) was used to study the phase composition and the patterns obtained were analyzed using PDXL2 software (2.8.3.0 version). Micro-Raman spectroscopy measurements were performed with a 532 nm laser source using a TriVista 777 confocal Raman system (Princeton Instruments, Trenton, NJ, USA) equipped with an upright Olympus microscope with an Olympus UISe MPlanN 100×/0.90 objective and a continuous-wave single frequency. X-ray photoelectron spectroscopy measurements were performed using the ESCALAB Xi (ThermoFisher, Waltham, MA, USA) spectrometer. An Al Kα X-ray tube with an energy of 1486 eV was used as an excitation source, the size of the analyzed sample area was 650 μm × 100 μm, and the angle between the analyzer and the sample surface was 90°. The samples were not sputter-cleaned prior to the measurements. An electron gun was used to perform charge compensation. The base pressure during the spectra acquisition was better than 10⁻⁵ Pa.

Furthermore, the electrical resistivity of the produced thin films was measured in the Van der Pauw configuration using a Hall effect system, HMS5000 (Ecopia Hall Effect Measurement Systems, Anyang, South Korea). Four small ohmic contacts at the corners of the samples were made of high-purity silver paint (SPI-PAINT, West Chester, PA, USA). The film’s transmittance and reflectance in the range of 330–2500 nm were determined by a Cary 7000 spectrophotometer (Agilent, Santa Clara, CA, USA). The samples were placed at an angle of 6° against the incident beam, and the detector was placed at 180° behind the sample to measure the transmittance and at 12° in front of the sample to measure the specular reflectance. The optical band gap was determined using the Tauc plot for the direct allowed optical transition. It was determined at the energy where the extrapolation of the linear part of the curve intersected with the linear fit of the curve in the energy range of 0.50 to 1.25 eV below the optical absorption edge. In addition, the Z-scan method was employed to characterize the nonlinear optical properties. As the laser source, an ORPHEUS-HP + PHAROS PH2 femtosecond laser with a repetition rate of 500 kHz and pulse width of 150 fs at 900 nm was used. To focus the laser beam, a lens with a focal length of 110 mm was used, resulting in a calculated Rayleigh length of 1.2 mm for this experimental setup.

3. Results and Discussion

3.1. XRD and SEM Results

To determine the optimal synthesis temperature for producing ReSe₂ thin films using an atmospheric pressure CVT setup, the range of 550 °C–1000 °C was investigated for both precursor materials Re metal and ReO_x. ReSe₂ peaks are shown in Figure 3 with the obtained XRD patterns. From the XRD patterns, it was found that at a lower temperature (550 °C), the Re metallic precursor did not react completely with selenium. At a higher temperature (1000 °C), the reduction of ReSe₂ films was observed for both precursors. This was likely due to the use of a 5% H₂/Ar carrier gas mixture, which could be seen from the appearance of the shoulder of metallic Re to the left of the (003) ReSe₂ peak. As was estimated by Rietveld refinement performed by Profex code [26] in the sample obtained from the ReO_x precursor films, the reduction of ReSe₂ films at 1000 °C was observed to be much more pronounced: the refinement indicated a possible metallic Re phase [27] content of no more than 10% for the sample obtained from Re and a clear presence of the metallic Re phase at 50% and higher for the sample obtained from the ReO_x precursor (see

Table A1. Estimation of % content of ReSe_2, Re, ReO₂, and ReO₃ in films produced from metallic Re and ReO_x using Rietveld refinement.

	From Metallic Re Precursor					From ReOx Precursor
Temp. °C	550	650	750	850	1000	550	650	750	850	1000
ReSe2	100	99.53	99.30	98.67	92.04	76.50	94.71	96.61	88.80	28.10
Re	0	0.43	0.75	0.33	7.13	3.24	3.92	2.95	7.91	71.30
ReO2	0	0.00	0.00	0.00	0.00	15.10	0.70	0.44	2.00	0.00
ReO3	0	0.43	0.94	0.99	0.83	5.20	0.67	0.00	1.31	0.60

for details). Here, it is important to note that in this CVT process, the presence of H₂ in carrier gas also boosted the reaction and subsequent selenization rate by producing highly reactive H₂Se gas, and, in addition to this, H₂ also played a role in limiting the formation of any undesired oxides while cooling down the synthesized films.

The obtained XRD patterns (Figure 3) matched well with ICDD card no. 04-007-1113 and previous studies conducted on this material [3], indicating the presence of highly crystalline films of ReSe₂ in a distorted so-called 1T′ structure with a triclinic symmetry $P\overline{1}$ in the temperature range of 650 °C–850 °C. The Rietveld refinement also confirmed the possibility of the presence of ReO₂ [27] and ReO₃ [28] phases only in minimal quantities for all samples except the film produced from ReO_x at 550 °C. At the same time, for both series of samples, the Rietveld refinement required pronounced ReSe₂ phase reconstruction with lattice constant changes (up to 0.1 Å) and significant atomic shifts in the ReSe₂ phase (0.3 Å and more), which were larger for the spectra of samples obtained at high temperatures. This may indirectly have indicated the amorphization of the film or the formation of metallic Re islands at high temperatures or may have been related to the interaction of the film with the substrate and surface effects.

To further investigate the selenization temperatures and their effects on the surface morphology of the produced ReSe₂ films, SEM (Figure 4) images were taken. Samples synthesized at lower temperatures were discarded as they showed partially converted precursor materials from the XRD analysis (Figure 3).

SEM images revealed that as the temperature was increased, surface roughness became more noticeable for both precursors. The films synthesized from the Re metal precursor, which displayed an increase in out-of-plane filamentary structures with increasing temperature and at the highest synthesis temperature tested in this study (1000 °C), particularly displayed a trend of surface crystal formation for films derived from both precursor materials. We hypothesized that the reason for out-of-plane filamentary structure formation in the case of Re precursor-based films could have been due to the high density of the Re metal when it was subjected to such a process (here, selenization), where two extra atoms (Se) were added per one atom of Re; it needed space to expand and when, due to various reasons (interlayer forces), the expansion was restricted, it developed these out-of-plane features to relax the constraining tensions in the film. With the increasing temperature of the selenization, this behavior seemed to become more prominent, resulting in higher density/elevations of these features when compared to lower temperature conversions (Figure 4).

Based on these initial findings, the synthesis temperatures were constrained to two optimal points—650 °C and 750 °C—going forward to study the effects of temperature variation on the synthesized films’ properties from different aspects. The samples obtained on a silicon substrate were used to find the optimal synthesis temperature range. Further AFM and optical measurements were performed for the films grown on sapphire only. XRD and SEM analyses confirmed that changing the substrate did not impact the growth process of ReSe₂ thin films in the case of using identical synthesis parameters (Figure A3).

3.2. AFM Data

AFM findings (Figure 5) revealed a highly conductive, fine-grained surface, with crystallite sizes typically ranging from 50 to 100 nm, varying with the sample. The AFM topography results (Figure 5a,c,e,g) for ReSe₂ films synthesized at 650 °C and 750 °C from Re and ReO_x precursors showed notable differences. At 650 °C, Re-derived films exhibited a rough surface with significant roughness, whereas films from ReO_x presented a more uniform and smoother texture. When the synthesis temperature was increased to 750 °C, the Re-derived films displayed pronounced out-of-plane filamentary structures, indicating substantial changes in surface morphology, while ReO_x-derived films remained relatively smooth but with slightly increased roughness compared to at the lower temperature.

The conductivity mapping results further highlighted the differences in the electronic properties of the films. At 650 °C, the c-AFM mapping for Re-derived films showed a varied distribution of conductivity, corresponding to the rough surface morphology observed. In contrast, the ReO_x-derived films exhibited a more homogeneous conductivity map, reflecting their smoother surface. At 750 °C, the Re-derived films continued to show significant variations in conductivity due to their complex surface features, while the ReO_x-derived films maintained a more uniform conductivity distribution, consistent with their relatively smoother surface.

These AFM images collectively demonstrate how the precursor material and synthesis temperature influenced the surface morphology and electronic properties of ReSe₂ films. Films synthesized from Re metal exhibited more pronounced surface roughness and varied conductivity, particularly at higher temperatures, indicating significant structural and electronic modifications. In contrast, films derived from ReO_x maintained a smoother surface and more uniform conductivity, suggesting a more stable growth process under the same conditions. These findings highlight the importance of precursor selection and temperature control in tailoring the properties of ReSe₂ films for specific applications.

3.3. XPS and Raman Analysis

XPS analysis was employed to ascertain the chemical states of the elements in ReSe₂ films synthesized from both metal and oxide precursors at 650 °C. The XPS spectra (Figure 6) revealed the presence of only Re and Se elements, alongside organic surface contaminants such as carbon and oxygen. High-resolution spectra of Re 4f and Se 3d peaks were obtained and calibrated against the adventitious C 1s peak at 284.8 eV. The ReSe₂ prepared from both precursor types yielded identical XPS spectra. The Re 4f scan exhibited a peak with a single doublet. To minimize error during peak fitting, a spin–orbit splitting Δ_7/2−5/2 of 2.43 eV was fixed between the Re 4f_7/2 and 4f_5/2 components of the doublet, with an area ratio of 4:3. The Re 4f_7/2 peak was located at approximately 41.8 eV, indicating a valence state of 4⁺ in the ReSe₂ compound. Conversely, the Se 3d peak displayed two doublets (spin-orbit splitting Δ_5/2−3/2 = 0.86 eV, area ratio 0.735). The first doublet, with the Se 3d_3/2 at 54.7 eV, corresponded to the 2− chemical state in ReSe₂, while the second doublet, with the Se 3d_5/2 at 55.2 eV, suggested the presence of elemental Se on the selenide surface [3,21,29,30].

Raman spectroscopy was also employed to further verify the composition and structure of the ReSe₂ films synthesized at 650 °C. The Raman spectra (Figure 6e) of these films revealed distinct vibrational modes corresponding to the E_g and A_g modes of ReSe₂, aligning well with previously reported studies [10,31]. In this case, the E_g and A_g modes were characteristic of the in-plane and out-of-plane vibrations of the Re and Se atoms in the ReSe₂ crystal lattice. The consistency of these Raman peaks with literature values confirmed the successful formation of ReSe₂, with the expected crystalline structure and chemical composition.

3.4. Optical Measurements

Figure 7 shows the transmittance, absorbance, and specular reflectance of ReSe₂ films synthesized on c-plane sapphire, measured across the spectrum from 500 to 2500 nm. A notable trend emerged, revealing that ReSe₂ films synthesized from ReO_x exhibited higher transparency in the infrared region, specifically within the range of approximately 1100 to 2500 nm (Figure 7a). The higher transmittance was attributed to significantly lower absorbance within the relevant spectrum, as depicted in Figure 7b. The observed optical properties demonstrated a clear correlation with temperature-dependent electrical resistivity measurements (Figure 8c), indicating that films produced from ReO_x were semiconductors (dρ/dT < 0), while those from Re exhibited higher conductivity and metallic behavior as a degenerate semiconductor, with a slight increase in resistivity with temperature (dρ/dT > 0) due to increased lattice phonon scattering. Consequently, the pronounced absorbance observed in general was attributed to the higher concentration of charge carriers. The type of conductivity, carrier concentration, and mobility of the samples could not be determined from the measurements due to inconsistent Hall voltages. This inconsistency was caused by either the carrier mobility being too low (<1 cm²(Vs)⁻¹) or the carrier concentration being too high (>10²¹ cm⁻³), both of which fell outside the measurable value ranges of the system, while those from Re exhibited higher conductivity and metallic behavior. The activation energy E_a derived from Figure 8c for the semiconducting films derived from ReO_x was E_a = 92 meV at 650 °C and E_a = 131 meV at 750 °C. The metallic behavior of the WSe₂ films produced from the W metal precursor was also observed in a previous study, suggesting a trend in this synthesis methodology [32]. Additionally, the selenization temperature influenced the optical properties in both cases of ReO_x and Re precursors, with a more pronounced effect noted in the case of ReO_x. Figure 7c presents the corresponding specular reflectance of the films. Since reflectance is influenced not only by, for example, charge carrier concentration, but also by surface roughness, a comprehensive interpretation and clear correlation between reflectance and deposition parameters are challenging. However, the more intense reflection observed at >1200 nm in ReSe₂ films synthesized from ReO_x at 650 °C compared to 750 °C was related to the lesser roughness, as confirmed by SEM (Figure 4) and AFM (Figure 5) analyses, which increased specular reflectance.

All the as-synthesized films exhibited a dark grey color and were highly absorbed (70–80%) in the visible light range (Figure 7b) due to the small direct optical band gap of approximately 1.25 eV determined from the Tauc plots depicted in Figure 8a,b. These obtained values are consistent with those reported in the literature [13,33].

Furthermore, when using the Z-scan method to assess nonlinear optical properties, the closed-aperture measurements were fitted using a standard Z-scan equation [34]:

$${\mathrm{T}}_{\mathrm{OA}}=1+{\displaystyle \frac{4\times \mathsf{\Delta }\mathsf{\Phi }\times \frac{\mathrm{z}}{{\mathrm{z}}_{\mathrm{R}}}}{\left(\frac{{\mathrm{z}}^2}{{{\mathrm{z}}_{\mathrm{R}}}^2}+9\right)\times \left(\frac{{\mathrm{z}}^2}{{{\mathrm{z}}_{\mathrm{R}}}^2}+1\right)}}$$

(1)

where ΔΦ = n₂ × k × I × L_eff is the induced phase change, n₂ is the Kerr coefficient, k is the wave number, I is the optical intensity, L_eff = (1−exp(α₀ × L))/α₀ is the effective length, α₀ is the optical absorption, L is the thickness, z is the sample position, and z_R is the Rayleigh length. To calculate the nonlinear absorption coefficient from open-aperture data, the following expression was used:

$${\mathrm{T}}_{\mathrm{CA}}=\sum _{\mathrm{n}=0}^{\infty }{\displaystyle \frac{{\left(\mathsf{\beta }\times \mathrm{I}\times {\mathrm{L}}_{\mathrm{eff}}\right)}^{\mathrm{n}}}{{\left(\frac{{\mathrm{z}}^2}{{{\mathrm{z}}_{\mathrm{R}}}^2}+1\right)}^{\mathrm{n}}{\times \left(\mathrm{n}+1\right)}^{3/2}}}$$

(2)

where β is nonlinear absorption. In the case of absorption saturation, this equation can only be used for low laser intensities (I < I_s), in which case, β = −α₀/I_s, where I_s is saturation intensity [35]. A sample of sapphire substrate was used as a reference measurement. The measured value was n₂ = (2.16 ± 0.42) × 10⁻¹⁶ cm²/W, which corresponded to values demonstrated in the literature [36]. No nonlinear absorption effect was observed.

A Kerr signal was consistently observed (Figure 9) for all ReSe₂ samples synthesized in the suggested optimal temperature range of 650 °C–750 °C, of value n₂ = (−1.23 ± 0.65) × 10⁻² cm²/W, exhibiting minimal variation depending on the preparation procedure. Previous studies of ReSe₂/PVA thin films have presented Kerr values at 1560 nm (n₂ = −2.81 × 10⁻² cm²/W) [13] and 1900 nm (n₂ = −6.3 × 10⁻² cm²/W) [23]. The current findings align with these previous studies and indicate that the Kerr value decreases with wavelength in the infrared region.

Regarding saturation absorption, the value for samples produced from Re was β = (−950 ± 290) cm/GW and for the sample produced from ReO_x it was β = (−730 ± 330) cm/GW. The SA coefficient has been demonstrated to exhibit a dependency on the layer number for ReSe₂, ranging from −4156 cm/GW for a single layer (0.71 nm) to −878 cm/GW for 42 layers (bulk sample; 29.81 nm) at 800 nm [24]. The acquired values in this work align well with literature findings for bulk samples. For longer wavelengths, the SA value was measured for the ReSe₂/PVA thin films and increased to −5670 cm/GW at 1560 nm [13] and −13,800 cm/GW at 1900 nm [23], indicating an increase in SA values for longer wavelengths. The saturation absorption values of these samples indicated that they could be applicable for Q-switching or mode-locking in laser systems, as 2D materials have been shown to play a key role in ultrafast laser systems [37]. To implement these samples in practice, either samples closer to a few layers (thickness < 7 nm, as fewer layers lead to higher nonlinearities [24]) or fabricating them in the form of the ReSe₂/PVA structure should be considered [13].

4. Conclusions

In this study, we synthesized and compared ReSe₂ thin films from Re metal and ReO_x precursors deposited using DC magnetron sputtering followed by atmospheric pressure CVT selenization. XRD, Raman spectroscopy, and XPS analyses confirmed the ReSe₂ phase for both precursors, with optimal synthesis temperatures between 650 °C and 750 °C. SEM and AFM revealed out-of-plane elongated structures in thin films from the Re metal precursor as the temperature increased, in contrast to the ReO_x-derived films. Despite the chemical similarity, the ReSe₂ films prepared from Re metal had higher surface roughness, potentially beneficial for catalytic applications such as electrocatalytic HER. Optical properties were largely unaffected by the synthesis temperature, although Re metal films were less sensitive to changes. Non-linear optical measurements showed no significant correlation with synthesis parameters and indicated a potential for the films in non-linear optics applications, such as Q-switching and mode-locking in laser systems. Importantly, semiconductor behavior was observed only in films from ReO_x, highlighting the impact of precursor choice on electronic properties and the potential for tailoring material characteristics through controlled synthesis.