Controllable Growth of Large-Scale Continuous ReS₂ Atomic Layers

Abstract

In recent years, two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received significant attention due to their exceptional electrical and optical properties. Among these 2D materials, ReS₂ distinguishes itself through its unique optical and conductance anisotropy. Despite concerted efforts to produce high-quality ReS₂, the unique interlayer decoupling properties pose substantial challenges in growing large-area ReS₂ thin films, with the preparation of single layers proving even more complex. In this work, large-scale continuous monolayer and bilayer ReS₂ films were successfully grown on mica substrates using low-pressure chemical vapor deposition (LPCVD). Photodetectors were fabricated using the prepared high-quality ReS₂ films, and the devices presented stable photoresponse and enhanced response sensitivity. The production of continuous ReS₂ atomic layers heralds promising prospects for large-scale integrated circuits and advances the practical application of optoelectronics based on 2D layered materials.

1. Introduction

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) are emerging semiconductors for next-generation electronics and optoelectronics due to their excellent properties [1,2,3,4]. Different from graphene, most sulfide- and selenide-based TMDs exhibit bandgaps spanning the visible to near-infrared spectral range [5,6]. In particular, MoS₂ and WS₂, which are among the most extensively studied 2D materials, demonstrate direct bandgap semiconductor characteristics when reduced to monolayer thickness. Therefore, these materials have attracted significant interest for their light–matter interactions and shown enormous potential in the application of optoelectronic devices.

With the development of exploration for new 2D materials, ReS₂ emerges as a standout candidate among numerous 2D materials which benefits from its distinct optical and electrical anisotropies [7,8]. The low crystal symmetry of ReS₂ imparts a unique energy band structure, leading to strong anisotropy in effective mass, conductance, mobility, and thermal conductivity. The laser cavity prepared using the nonlinear saturable absorption characteristics of ReS₂ achieves a modulation depth of 44% and a saturation level of 8.4 MW/cm² [9,10,11,12]. ReS₂ also demonstrates glorious polarization sensitivity, making it suitable for linear dichroic photodetectors [13,14,15]. Additionally, ReS₂ has minimal dependence on thickness for its optical bandgap [16,17,18,19], making it the optimal choice for photodetector applications. A polycrystalline ReS₂-based photodetector exhibits a responsivity of 4 × 10⁻³ A/W and an external quantum efficiency (EQE) of 0.99% [20]. Furthermore, ReS₂ shows promising possibilities for facilitating the exploration of unusual physical phenomena and contributing to the design of innovative functional devices. Therefore, the synthesis of large-scale and high-quality ReS₂ atomic layers is indispensable.

The emergence of chemical vapor deposition (CVD) as a synthesis technique signals promising avenues for large-scale manufacturing. However, CVD-synthesized ReS₂ typically forms discrete micro-scale dendritic structures [21,22,23] instead of continuous films. The asymmetric and out-of-plane growth of ReS₂ can be ascribed to its distorted 1T (1T’) structure and weaker interlayer interaction. Most instances of ReS₂ growth through CVD occur under atmospheric pressure, thus limiting the control of the thickness of the synthesized samples and posing challenges to repeatable large-scale growth [24,25,26]. The growth of ReS₂ thin films using low-pressure CVD is also reported; however, the samples are thicker bilayer or multilayer ones [27,28]. For example, Liu et al. successfully manufactured a large-area thin-layer ReS₂ film under reduced pressure [20]. However, studies on the growth of high-quality and large-area ReS₂ films with single-layer thickness under low pressure remain a challenge.

In this work, we achieved the growth of large-area monolayer and bilayer ReS₂ thin films on mica utilizing LPCVD. The conditions facilitated the growth of both monolayer and bilayer ReS₂ films within a wide temperature range. Furthermore, we constructed ReS₂ photodetectors, and the devices showed excellent performance. This study proposes a novel method for producing large-scale ReS₂ atomic layers, fostering broader practical application possibilities for ReS₂ electronic and optoelectronic devices.

2. Materials and Methods

2.1. Growth of ReS₂

The growth of monolayer ReS₂ was conducted in a two-temperature-zone LPCVD system, utilizing a quartz tube nesting in another quartz tube. ReO₃ (1~2 mg) and S (1.5~2 g) powder were employed as precursors in the internal tube for the growth of ReS₂. S powder was located in a quartz boat in low-temperature zone I. Mica substrate (1 cm × 1 cm) was arranged in the internal tube on a quartz plate in high-temperature zone II. An open-ended quartz boat with ReO₃ precursor was situated within the same temperature zone adjacent to the mica substrate. A layer of molecular sieve (Na₂O·Al₂O₃·2SiO₂·4.5H₂O) powder with a pore size of 0.4 nm covered the surface of ReO₃ powder to regulate the release rate of the ReO₃ source. During the growth process, the pressure of the quartz tube was approximately 1 Torr with 100 sccm high-purity Ar as carrier gas. The temperature of zone I was raised to 200 °C, and the temperature for zone II was raised to 700 °C and 600 °C for the growth of monolayer and bilayer ReS₂, respectively. The growth process was sustained for 8 min. Following the growth phase, the samples cooled naturally to room temperature inside the furnace.

2.2. Characterizations of Samples

The characterization of ReS₂ samples was performed with optical microscope (BX-51M, Olympus, Tokyo, Japan), atomic force microscope (Multimode8, Bruker, Bilerika, MA, USA), and transmission electron microscope (TecnaiG2 F30 S-TWIN, FEI, Hillsboro, OR, USA). Carbon films supported on copper grids served as the medium for TEM characterization, to which the ReS₂ layer was transferred. The Raman spectra and PL spectra were determined employing a laser Raman spectrometer (inVia96Q857, Renishaw, Gloucestershire, UK) with an excitation laser wavelength of 532 nm.

2.3. Fabrication and Measurement of Photodetectors

Devices were fabricated by transferring ReS₂ onto SiO₂/Si substrate with pre-deposited Cr/Au (30 nm/100 nm) electrodes. The volt-ampere characteristic curve and the photoresponse of ReS₂ devices under various laser irradiation conditions were tested using a multi-functional precision electronic timer (GCI-73, Daheng, Beijing, China) and a digital source meter (2635B, Keithley, Cleveland, OH, USA).

3. Results and Discussion

3.1. Growth and Characterization of ReS₂ Film

Large-area uniform monolayer and bilayer ReS₂ films were grown using LPCVD in an argon gas atmosphere with S powder and ReO₃ powder as sulfur and rhenium reaction sources, and the experimental setup is schematically illustrated in Figure 1a. Compared with atmospheric pressure CVD, LPCVD offers more precise control over the carrier gas, a cleaner reaction atmosphere, and reduced gas turbulence, which accelerates the transfer rate of gaseous reactants and enhances the reaction rate of thin film formation. Fluorophlogopite Mica (KMg₃AlSi₃O₁₀F₂) is employed as a substrate which has been proven to be a suitable candidate for crystal growth due to its inert and dangling-bond-free surface, as well as matched symmetry. Prior to growth, the freshly cracked mica was pre-treated via annealing in air to remove any remaining K⁺ ions on the surface and reduce the number of heterogeneous nucleation sites. This pre-treatment facilitates the lateral growth mode and promotes the epitaxial growth of large-area and high-quality ReS₂ films [29]. It is worth noting that the positions of ReO₃, sulfur precursors, and mica substrates were strategically arranged. The sulfur powder was located upstream in zone I and the ReO₃ and mica substrates were placed downstream in zone II. The distance between sulfur and ReO₃ is 25 cm, and the distance between ReO₃ and the center of the substrates is 3 cm. During the process of growth, the mica substrates were placed horizontally on the sample holder. Compared with placing the substrate above the reaction source, positioning the substrate downstream horizontally helps to overcome the issue of uneven growth caused by the evaporation of the reaction source, as shown in Figure S1a. This substrate layout promotes the growth of large, uniform, and flat films, maximizing the utilization of mica substrates. It is notable that by employing this substrate placement method, ReS₂ films can be synthesized over multiple mica substrates simultaneously, thereby greatly improving the synthesis efficiency of ReS₂ films.

In addition, we adopted a dual gas intake system for the growth of ReS₂, and a photo of the dual gas intake is presented in Figure S1b. The quality of film is improved thanks to the designed gas intake system, which ensures more stable and uniform gas flow. When only a single gas intake is used, turbulence often occurs in the high-temperature region, leading to uneven growth, as shown in Figure S1c. Note that molecular sieves served to regulate the evaporation rate of ReO₃, which effectively suppressed the nucleation density of ReS₂. When the temperature exceeds 400 °C, ReO₃ decomposes into evaporable Re₂O₇ and non-volatile ReO₂ through the disproportionation reaction [25]. The equation of the reaction is:

3ReO₃ → Re₂O₇ + ReO₂

(1)

Rapidly volatilizing Re₂O₇ is partially sulfurized and adsorbs on the substrate surface, furnishing numerous nucleation sites for the subsequent growth of ReS₂. The amount of precursors was also skillfully adjusted to ensure uniform growth conditions. We finally fixed the growth parameter, including the amount and ratio of reaction sources, the distance between each reaction source and the substrate, and the flow rate of carrier gas, while changing the growth temperature of ReS₂. Based on the contrast experiments, it was determined that the monolayer and bilayer film could be optimally produced at growth temperatures of approximately 700 °C and 600 °C, respectively.

A photograph of monolayer and bilayer ReS₂ films and bare mica substrate is presented in Figure 1b and the ReS₂ atomic layers are clean and homogeneous. Figure 1c is an optical microscope image for the ReS₂ monolayer grown on mica substrate with an artificial scratch made by tweezers, and the film shows a smooth and uniform surface. The color contrast between the ReS₂ film and the exposed mica substrate is evident, which confirms the presence of a single-layer ReS₂ film on the mica surface. Note that the thickness of the ReS₂ film can only be roughly judged in advance using color comparison. The specific thickness of the film needs to be further tested with atomic force microscope (AFM) measurements. Optical microscope images of bilayer ReS₂ films are shown in Figure S2a and the film also exhibits clean and uniform morphology. For more information on the growth of ReS₂ on other substrates, please refer to Figure S2b,c. ReS₂ films transferred to SiO₂, whether through the water-assisted method [30] (Figure 1d) or employing the wet transfer method [31,32] (Figure S2d), exhibit an intact and uniform appearance under the optical microscope, confirming their excellent stability and mechanical strength.

An AFM image of the ReS₂ film on mica is presented in Figure 1e, and the thickness of the ReS₂ film is determined by measuring the height of the step at the scratched site, which is approximately 1 nm. It has been documented that the thickness of monolayer ReS₂ exfoliated from bulk crystals is 0.7 nm, suggesting that the fabricated ReS₂ thin film comprises one single layer [17]. The discrepancy in the thickness between the grown ReS₂ and the exfoliated ReS₂ results from the small gap between the grown ReS₂ and the mica substrates. In addition to the gap on the interface, the roughness on the surface of the large-area continuous grown film also prompts the slight increase in the thickness. Please also see the AFM image of the bilayer ReS₂ film shown in Figure S3. Both optical microscopy images and AFM characterizations substantiate the successful synthesis of large-area ReS₂ films comprising highly uniform monolayers or bilayers.

Raman spectroscopy has emerged as a vital tool for the rapid and lossless identification of layer numbers and for the evaluation of crystal quality for synthesized materials. The Raman signal of ReS₂ was collected from the sample grown on mica substrate, revealing approximately 18 vibrational modes in the spectrum. Theoretically, these modes can be classified into in-plane (E_g), out-of-plane (A_g), and coupled in-plane and out-of-plane (C_p) vibrational modes. Vibrations below 250 cm⁻¹ are attributable to the movement of Re atoms, while those above 250 cm⁻¹ primarily result from the motion of S atoms [33]. In the case of monolayer ReS₂, the phonon dispersion spectrum predicts the most pronounced intralayer E_g-like and A_g-like Raman modes [34]. Figure 2a is the Raman spectra for monolayer and bilayer ReS₂. Five peaks, located at 151.7, 161.5, 213.6, 236.4, and 309 cm⁻¹, correspond to the in-plane vibrations E_g, and two peaks at 135.3 and 141.8 cm⁻¹ correspond to the out-of-plane vibrations A_g. In comparison with the previously reported Raman spectrum of ReS₂ monolayers, the Raman peaks of the grown monolayer ReS₂ in this work align closely with those typical Raman peaks, as presented in Table S1 [35,36]. In addition, the remaining high-frequency Raman peaks arise from the slight vibrations of S atoms and coupled in-plane and out-of-plane vibration modes of Re atoms [33]. Note that the peak located at 197.5 cm⁻¹ is attributed to the signal from the mica substrate, as evidenced in Supplementary Figure S4 for the measurement of the Raman spectra of mica and the comparison of the Raman signal for pristine ReS₂ on the mica substrate and transferred ReS₂ on the silicon substrate. In summary, the synthesized ReS₂ sample demonstrates well-defined Raman peaks, which indicates the excellent crystal quality of the samples.

Moreover, the ReS₂ samples demonstrated intense photoluminescence (PL) emission at room temperature. Figure 2b is the PL spectra of monolayer and bilayer ReS₂ films, and the positions of the PL peaks for both samples are approximately 1.73 eV. The photoluminescence intensity experiences a mild decrease for the ReS₂ monolayer compared with that of the ReS₂ bilayer, which attributes to the direct bandgap and interlayer decoupling features of ReS₂. Contrary to traditional Mo- and W-based TMDs, the intensity of the PL peak in ReS₂ films does not decline with the increased layer number [17].

Transmission electron microscopy (TEM) was employed for further analysis of the crystal structure of ReS₂ films. Due to the weak adhesive force between the ReS₂ film and mica substrate, the water-assisted transfer method was adopted to move the ReS₂ monolayer onto a carbon-film-coated copper grid. Figure 2c is a TEM image of the ReS₂ film, showcasing a neat, consistent, and expansive continuous monolayer film. The presence of the creases in the film akin to the edge folding phenomenon observed in TEM analyses of other two-dimensional materials, such as MoS₂ and graphene [37], implies that the creases may be an artifact of the transfer process. Figure 2d presents an enlarged image obtained with fast Fourier transform (FFT) of the TEM selection area. The image reveals that the crystal structure of ReS₂ follows a parallelogram-shaped 1T’ structure in accordance with the Re₄ cluster growth mode [38]. Two distinct diffraction rings were displayed in the selected area electron diffraction (SAED) pattern, corresponding to the (100) and (020) crystal planes in the 1T’ structure, providing evidence of numerous randomly oriented grains within the film [20]. Please also refer to Figure S5 for more TEM images of ReS₂ films. This observation further confirms that the coalescence of these arbitrarily oriented ReS₂ grains leads to the formation of uniform polycrystalline films. For the purpose of discerning the grain size, high-resolution transmission electron microscopy (HRTEM) measurements of the ReS₂ film were performed. Figure S6 presents the HRTEM image, and the average grain size of ReS₂ is about 3.8 nm. Furthermore, the orientations of grains are random without preferred directions.

The constituent elements of the synthesized samples were analyzed using an energy dispersive spectrometer (EDS) to determine their type and content. As shown in Figure 2e, the signals of Re and S were easily detected. Note that the signal of the Cu element comes from the copper grid. The atomic percentage of Re and S were 32.41% and 67.59%, respectively, with a ratio of approximately 1:2. The results of EDS confirm that the sample is ReS₂. Please also see Table S2 for more information. EDS element mapping is presented in Figure 2f, which demonstrates the high quality of the low-pressure CVD-grown ReS₂ with Re and S elements evenly distributed throughout the entire sample.

3.2. Optoelectronic Properties of ReS₂ Films

In order to figure out the optoelectronic properties of the as-grown ReS₂ atomic layers, we fabricated ReS₂ photodetectors by implementing a water-assisted transfer technique. The fabrication process is depicted in Figure 3a, and Figure 3b is the photograph of the triaxial displacement system. The ReS₂ film grown on a mica substrate can be easily detached from the substrate utilizing the water-assisted method owing to the weak binding force between ReS₂ and mica. Notably, Figure 3c illustrates the peeled ReS₂ film in deionized (DI) water, which remarkably retains its integrity and cleanliness. Subsequently, Polydimethylsiloxane (PDMS) film was employed to conglutinate the ReS₂ film from the surface of DI water. When attaching the ReS₂ film with PDMS, it is crucial to handle the ReS₂ film with care, which helps to prevent the film from curling or folding during the transfer process. The PDMS film containing the ReS₂ film was then affixed to a glass slide, which is shown in Figure 3d. Due to the strong bonding force between the ReS₂ and silicon substrate, it is effortless to transfer the ReS₂ film from PDMS onto silicon substrate. Note that the strong binding force between ReS₂ and SiO₂ substrate here is a relative concept. Compared with the binding force between ReS₂ and PDMS, the binding force between ReS₂ and SiO₂ is stronger. Therefore, the ReS₂ film attached on the PDMS is easily transferred onto SiO₂. The origin of the interaction between ReS₂ and the substrate comes from the van der Waals force, which is the attraction caused by the uneven distribution of charges between molecules. The difference in bonding forces may attribute to the diversity in the distribution of charges between different molecules. The fabrication of ReS₂ photodetectors was realized through the directional transfer of ReS₂ onto the target location on silicon wafers with prefabricated electrodes using the triaxial displacement system, and Figure 3e presents a photograph of the ReS₂ photodetector with Cr/Au electrodes on a silicon substrate. Notably, this technique effectively minimizes potential contaminations from the fabrication process of devices, such as photoresist for photolithography and polymethyl methacrylate (PMMA) for wet transfer. Furthermore, this method simplifies the fabrication process and reduces the manufacturing cost of optoelectronic devices. The water-assisted transfer technique allows for the reuse of the mica substrate after exfoliation. Large-area, uniform, and continuous ReS₂ films can still be grown on the used mica substrates, which significantly saves the consumption of substrates. Figure S7 presents an AFM image of the ReS₂ film grown on a reusable mica substrate. In addition, the water-assisted transfer method also offers convenience for TEM characterization. It is important to note that the thin layer of ReS₂ may be susceptible to damage during the wet transfer process, particularly causing issues with the adhesion of films onto the copper grid. To overcome this challenge, the water-assisted transfer method enables the direct capture of large-area ReS₂ films using copper grids. This approach reduces the difficulty of preparing TEM samples and facilitates the characterization of the morphology and lattice structure of samples.

Compared with ReS₂ monolayers, few-layer and bulk-like ReS₂ has been exploited in high photoresponsivity detectors operating within the visible range owing to their higher light absorption along with the existence of trap states [39,40,41]. Therefore, the as-grown bilayer ReS₂ films were used to fabricate photodetectors. ReS₂ photodetectors were prepared directly on a SiO₂/Si substrate with prefabricated Cr/Au (30/100 nm) electrodes using the water-assisted transfer technique. The SiO₂/Si substrate consists of P-type heavily doped silicon with a 300 nm SiO₂ oxide layer on its surface. Figure 4a depicts a schematic diagram of the device, while Figure 4b presents an optical microscope image of the device. The channel length of the device is 5 μm. The I-V characteristic curve of the ReS₂ photodetector in Figure 4c exhibits a linear shape and voltage symmetry, suggesting the formation of reliable ohmic contact between ReS₂ and the electrodes. Subsequently, we annealed the devices in the Ar atmosphere, resulting in a significant improvement in the magnitude of the current. The noted improvement is attributable to the annealing effect. Annealing of the prepared ReS₂ thin film results in an increase in the grain sizes of ReS₂. This phenomenon occurs as smaller grains show the tendency to combine and form larger grains at high temperatures. The grain size plays a significant role in the conductivity because the energy barrier forms at the grain boundary. The amount of grain boundary decreases with the enlargement of grain sizes; thus, the energy barriers decrease. Consequently, the reduction in energy barriers enhances the mobility of electrons, thus improving the conductivity of ReS₂ [42]. Additionally, we encapsulated the ReS₂ photodetectors using PMMA. Compared with exposed devices, the encapsulated ReS₂ devices exhibit equally outstanding electrical performances and photoelectric responses. Figure S8 shows the optical microscope image and I-V curves of the device after packaging. Similarly, an annealing treatment was also performed on the encapsulated ReS₂ photodetector, resulting in an improvement in the current as well. Encapsulation technology not only prevents environmental pollution to the material but also inhibits the oxidation of the sample.

Previous studies on the optoelectronic properties of ReS₂ mainly focused on single-crystal ReS₂, while relatively little attention was paid to the properties of polycrystalline ReS₂ [23,43,44]. In this work, high-power laser photodetections were conducted for polycrystalline bilayer ReS₂ films utilizing a laser with a wavelength of 450 nm, which is illustrated in Figure 4d. The switching time of the laser was set to be 10 s with optical power densities of 340, 300, 190, 90, and 20 mW/cm², and the bias voltage was maintained at 2 V. The ReS₂ photodetector exhibited a consistent, stable, and repeatable response to the laser light. Despite the potential for high-power lasers to damage optoelectronic devices composed of two-dimensional materials, the ReS₂ photodetectors prepared in this work demonstrated commendable stability under high-power lasers.

For photodetectors, responsivity (R_λ) is defined as R_λ = I_ph/PS, where I_ph represents the generated photocurrent (I_ph = I_light − I_dark) and P is the optical power density impinging on the active region (S) of the device. The external quantum efficiency is calculated with the equation EQE = hcR_λ/eλ, where the parameters h, c, λ, and e are the Plank constant, speed of light, wavelength of incident light, and charge of electron, respectively. In our study, I_ph, P, and S were determined to be 11.7 nA, 340 mW/cm², and 100 μm², respectively. Consequently, under the incidence of the laser with a wavelength of 450 nm, the maximum R_λ for the ReS₂ photodetector was determined to be 3.4 × 10⁻² A/W and the value of EQE is 9.38%. As discernible from Figure 4e, the device displays a relaxation time in response to the high-power laser. The rise/decay times of the photodetector, defined as the time for the current to increase/decrease from 10% to 90% of the steady value, are 4.3 s and 3.7 s, respectively, which is presented in Figure 4e. This phenomenon can be attributed to the properties of polycrystalline films, which are characterized by a multitude of grain boundaries. Defects around these grain boundaries form trap states, decelerating the migration of photogenerated carriers and thereby diminishing the response speed. The relationship between the photocurrent I_ph and the incident laser power density P can be described with the formula I_ph = P^α, where α represents fitting parameters. Figure 4f shows the relationship between the device photocurrent and incident laser power density when an external voltage of 2 V is applied. Notably, the optimal fitting result yields α as 0.93. The exponent α typically falls within the range of 0 to 1, reflecting complex processes that encompass the generation, separation, and recombination of photogenerated electron–hole pairs [45].

4. Conclusions

In conclusion, this work achieves successful growth of large-scale ReS₂ atomic layers on mica substrates using low-pressure chemical vapor deposition. Morphology characterizations have confirmed the excellent cleanliness, integrality, and homogeneity of the continuous films. Moreover, ReS₂ photodetectors were fabricated employing the water-assisted transfer technique. Demonstrating a distinct photoresponse under a laser bias voltage of 2.0 V at a wavelength of 450 nm, the ReS₂ photodetector exhibits a rise time of 4.3 s, a decay time of 3.7 s, a photoresponse rate of 3.4 × 10⁻² A/W and an external quantum efficiency of 9.38%. Remarkably, it retains its stability even under the exposure of high-power lasers, showcasing the consistent optoelectronic performance, which makes ReS₂ a promising candidate for practical application in high-performance photodetectors.