Ultra-thin two-dimensional (2D) transition metal dichalcogenides (TMDs), exhibiting unique electronic properties that are absent in bulk due to the geometrical confinement, distinct crystal symmetry, and natural out-of-plane self-terminating surfaces without dangling bonds, have demonstrated immense potential for diverse applications, including electronics, optoelectronics, and catalysis.^[1–4] Depending on the selection of metal and chalcogen species, TMDs can be semiconducting (e.g., MoS₂, WSe₂), metallic (e.g., NbS₂, TaSe₂), or semimetallic (e.g., WTe₂). Combining with other 2D materials with diverse insulating (h-BN), metallic (graphene), and superconducting (NbSe₂) properties, TMDs nanosheets with layer-dependent band gaps would be very attractive building blocks for large-area printed and flexible thin-film devices.^[5,6]

A wide variety of TMDs nanosheets can be produced by liquid-phase exfoliation. This technique brings considerable advantages: This method is robust, can be carried out in ambient conditions, is scalable, allows the preparation of films, hybrids, and composites, and may facilitate processing by using standard technologies such as reel-to-reel manufacturing to apply in large-area thin-film electronics.^[7,8] Chemical intercalation, as a widely-used liquid-phase exfoliation method mostly using n-butyllithium (n-BuLi) as a lithiation agent, is a versatile method to prepare few-layer TMDs nanosheets. The troublesome issues with this method are that the lithium intercalation is time-consuming and extremely sensitive to the environmental conditions. Furthermore, the residual lithium ions induce structural deformations in some TMDs, typically the formation of 1T phase, resulting in poor electrical performance.^[9,10] The ultrasonication assisted liquid exfoliation method developed by Coleman et al. can facilely prepare nanosheets from various TMDs bulk materials.^[4] Specifically, the surface energies of TMDs are about 46–65 mJ⋅m⁻²,^[11] which are close to 51.6 mJ⋅m⁻² and 52.3 mJ⋅m⁻² of isopropanol and acetone.^[12] Thus, the ultrasonic exfoliation of TMDs is commonly performed in organic solvents with matching surface energies. However, few-layer TMDs nanosheets cannot be easily obtained using this method and the exfoliated nanosheets often exhibit a broad thickness distribution due to the strong van der Waals force between the individual layers, which leads to the poor film quality and unsatisfactory thin-film electrical performance.^[13] Nevertheless, it is difficult to prepare well-dispersed few-layer TMDs nanosheets by sonication in the most commonly used solvent, water, with a large surface energy of ∼ 100 mJ⋅m⁻².^[12] Therefore, though great progress in the preparation of few or single-layer TMD nanosheets has been achieved, more facile, rapid, and versatile preparation methods for preparing high-quality aqueous-processable 2D semiconductor nanosheets are still in great demand.

In this work, we report a general approach to fast preparing highly uniform and phase-pure 2H-WSe₂ semiconducting nanosheets, which involves the pre-treatment of bulk WSe₂ crystals with supercritical carbon dioxide (SC-CO₂) exfoliation, following a mild sonication and exfoliation process in water with the assistance of a surfactant. The aqueous exfoliation of layered materials is considered as a promising technology because it allows a wide range of film-processing techniques to be applied. By precisely controlling the ionic surfactant concentration, we obtained phase-pure, few-layer 2H-WSe₂ semiconducting nanosheets with a narrow thickness and size distribution which was steadily dispersive in an aqueous solution for a long time. These WSe₂ nanosheets were then further processed into large-scale thin-film electronics by self-assembly method.

All materials were purchased from Sigma-Aldrich and used without further purification. The conductivity of pure water is 18 MΩ. Bulk WSe₂ crystal powders were pretreated by SC-CO₂ exfoliation firstly. 500 g of as-received WSe₂ powders were placed in a chamber with a heater and a temperature controller. CO₂ was then liquified and added into this chamber and kept at a pressure of 10 MPa. Simultaneously, the temperature was raised to 55 °C. The WSe₂ powders were immersed in SC-CO₂ and stirred for 30 min. Then the SC-CO₂ was rapidly depressurized by opening the outlet valve.

The ultrasonic exfoliation of the pretreated WSe₂ flakes was performed by adding 20 g SC-CO₂ pretreated flakes to 400 mL of sodium deoxycholate (NaDC) aqueous solution (10 mg/mL) in a double-walled beaker. These samples were sonicated continuously for 1 h using a horn probe sonic tip (450 W, 75 % amplitude, pulse rate 6 s on 2 s off). The beaker was connected to a cooling system that allowed for cold water (20 °C) to flow around the dispersion during sonification. Later, 400 mL of dispersion were centrifuged at 3000 rpm for 15 min. The sediments were discarded and the supernatant was subjected to further centrifugation of 15 min at 13000 rpm. The sediments were washed by pure water for 3 times until the residual NaDC was removed completely.

The self-assembly method was used to realize the layer-by-layer fabrication of thin film of WSe₂ nanosheets. The WSe₂ nanosheets ink was centrifugated and re-dispersed in pure water for three times to remove residual NaDC. The precipitate was dispersed in hexylamine and dropped in the hexane/ethylene glycol mixed solution. The hexylamine solution well located at the interface between hexane layer and ethylene glycol layer due to their different solubilities and densities. After complete volatilization of hexane, a SiO₂/Si or PET substrate was dipped in the ethylene glycol and the WSe₂ film was deposited on the substrate by slowly removing the ethylene glycol. The obtained WSe₂ film was treated by a mild thermal annealing (200 °C) to remove the residual solvent and enhanced the binding strength among the nanosheets.

In this work, the crystal structure of WSe₂ was analyzed through x-ray diffraction (XRD, D8 ADVANCE). Raman spectrums were collected using Raman microspectrometer (InVia Reflex, Renishaw, UK). The chemical composition of the as-prepared products was examined by x-ray photoelectron spectroscopy (XPS, R4000). The morphology of the as-prepared samples was observed using scanning electron microscope (SEM, Hitachi S-4800), and transmission electron microscope (TEM, Tecnai F20) operating at 200 kV. High angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) image was obtained using JEM ARM200F TEM/STEM. The atomic force microscopy (AFM) images were obtained by using a Veeco NanoScope IV with a silicon cantilever in tapping mode. The back-gate thin-film transistors were fabricated on SiO₂/Si substrate with a 300-nm-thick oxide layer following standard photolithography and e-beam evaporation of Ti/Au (6 nm/60 nm) source/drain electrodes. The measurements of the transport characteristic were conducted at room temperature under ambient conditions by a semiconductor characterization system (Keithley 4200 SCS) with a Lakeshore probe station.

CO₂ is commonly used as a supercritical fluid medium for its nonflammable, nontoxic nature and easily accessible critical conditions (T_c = 31.3 °C, P_c = 7.38 MPa).^[14] The WSe₂ bulk powders were treated by the supercritical CO₂ as the first step in our method. As shown in Fig. 1(a), most of the bulk WSe₂ crystals are regular hexagon with a lateral size of over 10 μm and thickness of 500 nm. However, after the supercritical treatment, the obtained WSe₂ flakes show an irregular morphology with a lateral size of about 2–5 μm and thickness of 50 nm. The edges of the WSe₂ crystals crack heavily as shown in Fig. 1(b). Since SC-CO₂ is a low surface tension fluid with high diffusivity, during the immersion and stirring of WSe₂ in SC-CO₂, CO₂ molecules with a size of 0.23 nm can easily diffuse into the interlayer space of WSe₂ with a layer distance of 0.65 nm.^[14–16] After the rapid depressurization process, the intercalated CO₂ molecules widen the distance between the WSe₂ layers due to their expanded volume, which is greatly helpful to the further liquid sonication exfoliation.^[15] To determine the crystalline structures, the XRD patterns were measured before and after supercritical exfoliation as shown in Fig. 1(c). After the supercritical exfoliation, all major observed diffraction peaks are consistent with those in bulk WSe₂ powders. But there is a decreasing dominant peak at around 13.8°, which is related to the (002) plane (ICDD card No. 38-1388). It indicates that bulk WSe₂ powders with high planar crystal structure are exfoliated.^[17] Furthermore, the magnified XRD patterns of the (002) peaks show that the (002) peak of the SC-CO₂ pretreated WSe₂ flakes shifts to lower angle, which means that the interlayer distance is expanded because of the supercritical exfoliation.^[18] Though supercritical exfoliation can be used to fabricate few-layer graphene and MoS₂ nanosheets,^[16,20] few-layer WSe₂ nanosheets with a stronger van deer Waals force have to be prepared with repetitious or long-time treatments. In order to efficiently exfoliate WSe₂ nanosheets and well disperse them in water, sonication exfoliation in aqueous solution was explored to obtain few-layer WSe₂ nanosheets in a short time.

Fig. 1.

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	Fig. 1. SEM images of (a) original WSe2 powders, and (b) SC-CO2 pre-treated WSe2 powders. (c) XRD patterns of original and SC-CO2 pretreated WSe2 powders. (d) Magnified XRD patterns of the (002) peaks in (c).

Sonication waves generate unstable cavitation bubbles that collapse into high-energy jets, which are large enough to break up layered crystals and produce exfoliated nanosheets.^[21] The key problem is to prevent rapid reaggregation of the nanosheets due to the mismatch of the surface energy between TMDs and water. To solve this problem, NaDC, a kind of anionic detergent, was selected to decrease the surface energy of water and stabilize the nanosheets as an ionic surfactant. The NaDC molecular chain is about 1.5 nm long resulting in an enough distance to decrease the binding force between the nanosheets since the interlayer spacings of most TMDs are about 0.7 nm. The equilibrium contact angles of different solvents on the WSe₂ substrate can decrease from 49.2° of pure water to 26.3° of NaDC aqueous solution, and to 11.2° of 50 % ethanol aqueous solution, which indicates that the surface energy of water can effectively regulate by NaDC and ethanol. As a comparation, N-methyl pyrrolidone (NMP) with the contact angle of 6.1° was also used to sonicate WSe₂ powder. The pretreated WSe₂ powders were sonicated for 1 h in NaDC aqueous solution, pure water, 50 % ethanol aqueous solution, and NMP, respectively. Among them, the best quality of the as-prepared WSe₂ nanosheets is obtained in the NaDC aqueous solution, a better case is in 50 % ethanol aqueous solution, and the worst case is in water or NMP. These results confirm that a solvent with a match surface energy is of benefit to the liquid exfoliation of TMDs. As shown in Fig. 2(a), the resultant WSe₂ nanosheets can well disperse in NaDC aqueous solution with a concentration of 5 mg/mL and stay stable as a uniform colloidal suspension for more than six months. The concentration of the WSe₂ nanosheets could be tuned from 0.1 mg/mL to 10 mg/mL by centrifugation and redispersion in proper solvents for specific applications. The typical TEM image of few-layered WSe₂ nanosheets prepared in the NaDC aqueous solution is shown in Fig. 2(b). The well-dispersed WSe₂ nanosheets have an average size of 59.7 nm determined over 200 nanosheets by the major axis and a size distribution of less than 10 %. The samples sonicated in other solvents show thick nanosheets or a poor dispersivity. Atomic force microscopy was used for assessing the thicknesses of these 2D WSe₂ layers and a clear height profile across the nanosheet of 5.0 nm was observed, corresponding to 5–7 layers of WSe₂ nanosheets as shown in Fig. 2(c). It should be noted that if the supercritical exfoliation is absent, WSe₂ nanosheets are too large and thick after direct sonication exfoliation even under a stronger sonication exfoliation treatment. It indicates that supercritical exfoliation as the pretreatment step, is vital for the production of few-layer WSe₂ nanosheets with high-quality. High-resolution scanning TEM (STEM) was performed on an aberration corrected scanning transmission electron microscope for investigating the structure of the WSe₂ nanosheets. Owing to the different atomic arrangements of 2H and 1T structures, their direct identification is possible with HAADF STEM imaging. Figure 2(d) shows the HAADF STEM image of the WSe₂ nanosheets. The lattice spacing of 0.28 nm corresponds to the (100) plane of 2H-WSe₂, which agrees with the results from the fast Fourier transform (FFT) pattern. In Fig. 2(e), the signal intensity of atoms along the red line shows the same value since the signals from the Se sites in the top layer overlap with those from the W sites in the bottom layer in 2H-WSe₂ along the electron beam direction. Moreover, the crystalline structures are still intact after the sonication exfoliation, which indicates that the molecular-assisted sonication can exfoliate the WSe₂ powder at low sonication power. It should be noted that the mild sonication exfoliation method in the NaDC aqueous solution can also be applied for the preparation of other 2D crystals nanosheets.

Fig. 2.

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Fig. 2. (a) Photograph of exfoliated WSe2 nanosheets dispersed in aqueous NaDC solution. (b) TEM and (c) AFM images of WSe2 nanosheets, the inset pattern in (c) is the cross-sectional height profile. (d) HAADF STEM image of WSe2 nanosheets, the FFT pattern is shown in the inset. (e) The signal intensity of atoms along the red line in (d) showing the same contrast value, corresponding to the lattice constant of 0.33 nm.

The as-prepared WSe₂ nanosheets were further washed over three times to remove any impurities. The Raman spectrum of the WSe₂ nanosheets is shown in Fig. 3(a). Three characteristic peaks at 135 cm⁻¹, 249 cm⁻¹, and 308 cm⁻¹ are assigned to E_1g, , and modes, which are in good agreement with those of mechanically exfoliated few-layer samples, suggesting the absence of significant strain and defect in our samples.^[22] Additionally, the chemical composition of the exfoliated WSe₂ was examined by XPS. In Fig. 3(b), two obvious peaks at 34.28 eV and 32.08 eV are attributed to W 4f_5/2 and W 4f_7/2, respectively. The small peak at 38.1 eV is attributed to W 5p_3/2. The peaks at 54.18 eV and 54.38 eV in Fig. 3(c) correspond to Se 3d_5/2 and Se 3d_3/2, respectively, confirming the formation of Se²⁻, which agrees with the spectrum of pristine 2H-WSe₂.^[22] The lack of any Na signal in XPS spectrum can be proved by Fig. S6. These results indicate that the WSe₂ nanosheets are neither oxidized nor covalently functionalized with impurity species such as NaDC after the supercritical exfoliation and sonication exfoliation.

Fig. 3.

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	Fig. 3. (a) Raman spectrum of WSe2 nanosheets. XPS spectra of (b) W 4f and (c) Se 3d.

Based on the stable WSe₂ nanosheet ink, large-area thin films can be prepared on diverse substrates using various solution-processing approaches such as spin-coating, self-assembly, and dipping coating. Alternatively, the ink solution may be applied to produce thin films of larger area by using the industrial roll-to-roll coating process. Herein, considering the well-dispersed state of the WSe₂ nanosheets with a size of dozens of nanometers, we tried to obtain a more smooth film by using a self-assembly technique at two-phase interface.^[22–24] We realized the high-quality 2D semiconducting thin films with a large area of 10 mm × 10 mm by the layer-by-layer self-assembly technique as shown in Fig. 4(a). Figure 4(b) shows the boundaries of the different self-assembly layers with the thicknesses of 100 nm, 150 nm, and 200 nm for the first, second, and third layers, respectively. The XRD pattern of the resulted thin film exhibits only {001} planes (Fig. 4(c)), indicating the well-controlled assembly of the WSe₂ nanosheets along the c axis of the crystal and the broad-area plane-to-plane contacts between the nanosheets. The high-quality WSe₂ semiconducting thin films with different self-assembly layers can be further processed using standard photolithography to create desired patterns for the fabrication of arrays of devices. It should be noted that a continuous film is difficult to form using a spin-coating process with our WSe₂ nanosheets. To explore the potential of the thin films for device applications, we fabricated back-gate transistors on 300-nm-thick SiO₂/Si substrate based on the self-assembled WSe₂ thin films. The Ti (5 nm)/Au (60 nm) electrodes were defined directly on the WSe₂ thin films through a shadow mask by thermal evaporation (as shown in Fig. 4(d)). The channel width (W) and equivalent channel length (L) of the device are 20 μm and 2000 μm, respectively.

Fig. 4.

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Fig. 4. (a) Optical image of a smooth thin film of WSe2 nanosheets. (b) Optical image of layer-by-layer self-assembly thin film of WSe2 nanosheets. Numbers 1–3 indicate the layers of WSe2 films, the regions with different colors show the films with different thicknesses. (c) XRD pattern of the resulting thin film displaying only {001} planes. (d) Schematic configuration and optical images of the back-gate WSe2 thin-film transistors on the 300-nm-thick SiO2/Si substrate.

Typical output curve and transfer curves of the field-effect transistors are shown in Figs. 5(a) and 5(b), respectively, where V_gs is the gate voltage, I_ds is the source–drain current, and V_ds is the source–drain bias. The linear output curves in Fig. 5(a) demonstrate the formation of ohmic contacts in the WSe₂/Au electrodes. The WSe₂ transistor exhibits a p-type transport behavior with an on/off ratio of ∼ 2.8 and an on-state current of ∼ 12 nA at V_ds = 10 V. The weak gate tunability may be due to the large thickness of the WSe₂ film. The field-effect electron mobility in the transistor was extracted from the linear regime of the transfer curve using the standard formula , where W is the width of the channel, L is the length of the channel, C_g is the capacitance per unit area of the gate dielectric, V_ds is the applied source–drain bias, and d I_ds/dV_g is the slope of the linear part of the transfer plot. From the transfer characteristics, the extracted field effect carrier mobility is about 0.12 cm²⋅V⁻¹⋅s⁻¹. The carrier mobility in our large-area WSe₂ thin films obtained by the facile solution-processed method is higher than that reported in previous solution processed and MOCVD fabricated WSe₂ thin films (∼ 0.08 cm²⋅V⁻¹⋅s⁻¹).^[26,27] While it is smaller than that reported in few-layer WSe₂ nanosheets obtained by chemical vapor deposition (CVD; 7.3 cm²⋅V⁻¹⋅s⁻¹).^[28] Though the performance of our solution-processed device could be improved by further optimization, the present results indicate that the contacts between WSe₂ nanosheets in the large-area thin film do not greatly hinder the transport properties of the thin film, owing to the charge transport across the stacked nanosheets due to the broad-area dangling-bond-free plane-to-plane contacts between them. Such contacts are advantageous compared with the point-to-point contacts in zero-dimensional quantum-dot thin films and in one-dimensional nanowire thin films, which, as mentioned previously, have considerable interfacial dangling bonds and chemical disorder. With a stable ink solution and a relatively low processing temperature, thin-film devices can also readily be prepared on flexible plastic substrates with comparable electronic performance such as polyethylene terephthalate (PET) and polyimide (PI), which demonstrates the potential of the 2D semiconductor ink for flexible and wearable electronics.

Fig. 5.

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	Fig. 5. (a) Typical output characteristics and (b) transfer characteristics of the WSe2 transistors. (c) Photocurrent under different laser powers and (d) transient photoresponsivity characteristics of the WSe2 transistors. All measurements were carried out with 650 nm wavelength under a zero gate–source bias.

Since WSe₂ has a direct bandgap of 1.6 eV in monolayer and an indirect bandgap of 1.2 eV in bulk, it has not only been widely used for field-effect transistors (FETs) but also for optoelectronic applications owing to its tunable energy bands, strong optical absorption, and ultrafast charge transfer. We extracted the photocurrent and photoresponsivity of the WSe₂ thin film transistor and analyzed its transient response as shown in Figs. 5(c) and 5(d). The photocurrent at room temperature is defined as the difference between the currents measured in dark state and under light illumination, i.e., I_ph = I_illumination – I_dark. Figure 5(c) presents the drain to source current I_ds as a function of the source–drain bias V_ds under different illumination powers from 0 (dark state) to 69 mW of a λ = 650 nm laser source. These data were collected in absence of gate voltage (V_gs = 0 V). The I_ds displays a linear dependence on V_ds under different illumination powers, which is primarily attributed to the significant reduction of Schottky barriers allowing the photo-generated carriers to transport efficiently through the WSe₂/metal interface. The photocurrent can arrive at 4 nA at V_ds = 1 V and increases with the light power, which demonstrates that the electron–hole pair generation is almost proportional to the absorbed photon flux.^[28] The photoresponsivity R is the ratio between I_ph and the applied illumination power P. The time-resolved photoresponse of the WSe₂ thin film is displayed in Fig. 5(d). The maximum responsivity reaches 0.26 μA/W at V_ds = 5 V and 0.05 μA/W at V_ds = 1 V. The response time and the recovery time are 0.2 s and 0.3 s at V_ds = 5 V according to the rising and falling edges, respectively. While, the response time and the recovery time are 0.3 s and 0.3 s at V_ds = 1 V. Thus, the WSe₂ thin film is demonstrated to display significant potential for future optoelectronic applications.

We demonstrate a rapid and scalable method to prepare WSe₂ nanosheets. After treatment with SC-CO₂, WSe₂ flakes are partially exfoliated with enlarged layer distance. Subsequently, the SC-CO₂ pretreated WSe₂ flakes are exfoliated into few-layer nanosheets by ultrasonication in water, resulting in a well dispersive colloid solution without any precipitate even after half a year. The crystal structures and physical properties are still intact after the exfoliation process. Good dispersivity and uniformity enable these nanosheets to self-assembly into thickness-controlled thin films for scalable fabrication of large-area arrays of thin-film transistors. Furthermore, our SC-CO₂ treatment and sonication exfoliation strategy can generally be applied to a wide range of layered 2D materials, to establish a library of 2D-material inks with diverse properties. Our study thus provides a robust pathway to scalable production of high-quality nanosheets for large-area electronics and optoelectronics.