Electrochemical molecular intercalation of crystal powdersOur key idea is to use the liquid metal to construct an adhesive and conductive framework for 2D crystal powders that can function like the regular bulk single crystal for electrochemical molecular intercalation (Fig. 1a–d). The used liquid metal here is Galinstan which consists of 68.5% gallium, 21.5% indium, and 10.0% tin by weight. With a low melting point of −19 °C, it is a conductive metal that can flow like liquid at room temperature. In contrast to conventional liquid metals such as mercury, Galinstan is of much lower toxicity and safer to handle. It is also more chemically reactive than mercury and has a native oxide layer on the surface upon exposure to the ambient air35,36. This surface oxide layer is critical for rendering the capability to disperse inorganic 2D crystal powders in Galinstan, which itself is not expected to interact strongly with these powders. But when stirring in the air, the fresh gallium in Galinstan is rapidly oxidized to ultrathin self-limiting oxides (Ga2O3, ~1 nm). Unlike the liquid metal, the sticky oxide layer can cover the inorganic crystal powders37 and eventually enables the dispersion in the liquid-state Galinstan as a mixture slurry (Fig. 1a, b). As a comparison, when mixed in the air-free glovebox environment to prevent the oxide formation, the powders stay on the surface of liquid metal and do not dissolve into a uniform slurry (Supplementary Fig. 1). The mixing process typically takes a few minutes at room temperature, which is much more convenient and cost-effective than the previous hot-pressing method involving thermal annealing at 1100 °C34. The resulting mixture slurry is much more viscous than the pristine Galinstan and thus can be coated onto various substrates, including silicon, gold, copper, glass, and plastic (Supplementary Fig. 2 and Fig. 1e). For the following study, we will use the gold-plated silicon substrate, as gold and gallium can form AuGa2 intermetallic compound to provide strong adhesion. Then, the mixture slurry of 2D crystal powders and Galinstan coated on the conductive gold-plated substrate is used as the working electrode, resembling the free-standing large single-crystal electrode employed in the standard protocol2,25. The mixture slurry contains a large number of powders uniformly dispersed in a liquid metal framework which serves as a material reservoir for the intercalation reaction.Fig. 1: Liquid-metal-assisted electrochemical intercalation and exfoliation of 2D crystal powders.a–d Schematic illustration of the liquid-metal-assisted electrochemical molecular intercalation of organic cations in 2D layered crystal powders. The liquid metal (Galinstan) was thoroughly mixed with 2D crystal powders into a homogeneous slurry (a). The slurry is then coated onto a conductive gold-plated silicon substrate (b) as the working electrode for the electrochemical molecular intercalation of tetraheptylammonium cations (THA+) (c) and exfoliation (d). WE working electrode, RE reference electrode, CE counter electrode. e Photographs of the pristine GaS crystal powders and pure Galinstan as well as the viscous mixed slurry and slurry-coated substrate. The powder pile, liquid metal droplet, and mixture slurry are ~10 mm in diameter. f The slurry-coated substrate during the electrochemical molecular intercalation. With the completion of the reaction and consumption of all the powders contained in the slurry, the pure liquid metal can be recovered and then used for the next batch of intercalation reaction. The size of the gold-plated silicon substrate is about 12*12 mm. Insets are schematic illustrations of the intercalation and release processes of crystal powders within the mixed slurry. g Photographs of representative 2D ink materials obtained by liquid-metal-assisted electrochemical intercalation and exfoliation of crystal powders. h, i Optical (h) and scanning electron microscopy (i) images of exfoliated 2D MoS2 nanosheets with tunable lateral size of 1–20 µm obtained from source powders of different granule sizes. Scale bars, 20 µm (h) and 1 µm (i).During the electrochemical intercalation process, the negative potential (e.g., −6 V) applied to the liquid metal can electrochemically reduce the surface Ga2O3 to metallic Ga38,39. Then the increased surface tension of Galinstan gradually pushes out the dispersed crystal powders inside the mixture slurry to the metal/electrolyte surface. This process is also confirmed by the macroscopic shrinkage of liquid metal from a spread film to a spherical droplet under applied voltage (Supplementary Fig. 3). The color of liquid metal mixture gradually changes from the original silver to brown (Fig. 1f), signifying the exposure of buried internal powders to electrolyte and the intercalation of alkylammonium cations. The unique advantage of the soft liquid metal is that it can accommodate the dramatic volume expansion during the intercalation of organic alkylammonium cations into crystals while simultaneously maintaining intimate electrical contact with these microcrystal powders40,41. In a standard bulk pellet formed by pressing powders into an entirety, the intercalation and volume expansion of an individual microcrystal might break the pellet structure, resulting in the quick detachment of neighboring powders and a failed powder intercalation. By contrast, the adhesive framework of liquid metal can hold these microcrystal powders throughout the whole intercalation process. When exposed to the electrolyte, the oxide on top is electrochemically reduced to expose the buried microcrystals for intercalation. In the meantime, the oxide on the bottom is not yet reduced and can still connect the microcrystal to the main body of liquid metal. Once the crystals on the surface are fully intercalated and detached, the underlying fresh crystals are then pushed outwards to the solid/electrolyte interface for a continuous intercalation reaction. As a result, the internal 2D layered crystal powders are step by step intercalated until all the powder feedstocks in the mixture slurry are consumed. Finally, the viscous mixture slurry restores the original pure liquid metal state with a shiny silver-like surface and liquid-like fluidity (Fig. 1f). With a much smaller viscosity than the starting mixture slurry, the pure liquid metal shrinks to a droplet-like shape rather than remaining spread over the substrate. It is also worth noting that molecular intercalation and Galinstan oxide reduction would not occur in the absence of external potential (Supplementary Fig. 4). Importantly, the recovered liquid metal can be collected and then mixed with fresh powders for the next batch of intercalation (Supplementary Fig. 5). The recyclability of the liquid metal can greatly reduce the material cost of the overall material production process which is desired for practical applications.After intercalation, the expanded crystals settled down at the vial bottom and can be collected and washed with ethanol before sonication-assisted exfoliation in pure dimethylformamide (DMF) liquid or polyvinylpyrrolidone solution in DMF (PVP/DMF) (Fig. 1f, g). The polymer surfactant PVP is needed for some crystals such as MoS2, HfS2, GaS, and Bi2Se3 to stabilize the exfoliated thin nanosheets, while not for others such as In2Se3, SnS2, and MoO3. After sonicating the intercalated materials for about 5 minutes, a homogeneous colloidal solution of 2D nanosheets dispersed in DMF can be obtained. The powder-based method has offered an additional degree of freedom in tunning the material morphology of the obtained 2D monolayers, which was not achievable in the conventional approach using large-size single crystals. For example, by choosing source powders with various particle sizes/lengths (e.g., 2 µm and 80 µm), the exfoliated 2D nanosheets show an average lateral size of ~1 µm and ~20 µm, respectively (Fig. 1h, i). It suggests that the exfoliated 2D nanosheets may partially inherit the lateral size of the starting crystals, offering the extra capability of tuning the lateral size of exfoliated 2D nanosheets that were previously unattainable. Furthermore, 2D nanosheets with regular geometric shapes can be exfoliated when starting with WS2 and WSe2 hexagonal microcrystals (Supplementary Fig. 6). It has been previously challenging with most liquid-phase exfoliation methods, including LPE or intercalation-assisted exfoliation1,19,25. Typically, with the use of large-size bulk single crystals, the exfoliated 2D nanosheets exhibit irregular geometric shapes, which are mostly determined by the molecular intercalation kinetics and non-directional random lattice breakage caused by ultrasonication waves. By contrast, when using small microcrystals (1–10 µm) as the starting material, the mechanical force-induced breakage is suppressed due to the small lateral size that is comparable to the final product. As a result, the pristine hexagonal shape of starting microcrystals is inherited by the exfoliated nanosheets.Quality assessment of MoS2 nanosheets exfoliated from powderCompared with the conventional protocol of using large-size single crystals, the powder-based intercalation results in 2D nanosheets of comparable morphology, uniformity, and electrical performance. Taking MoS2 as a typical example, when starting from powder consisting of microcrystals (particle size ~10 µm) (Fig. 2a), the exfoliated 2D nanosheets exhibit lateral size of ~0.5–2 µm and thickness of ~2.8 nm (Fig. 2b). Similar to the previous observation in MoS2 and In2Se3, the 2.8 nm thickness indicates the monolayer structure (~0.6 nm) capped with organic ammonium molecules and PVP surfactants (~2.2 nm) which will be further discussed later42,43 (Supplementary Fig. 7). The monolayer purity of the exfoliated MoS2 nanosheets is >98%. These values are comparable to those obtained with conventional large-size single crystals (crystal length ~10 mm) (Fig. 2e, f). The morphology of the monolayer nanosheets is reproducible from batch to batch (Supplementary Fig. 8). Although bulk single crystals have much larger crystal domains, the lateral size of the produced nanosheets after intercalation and exfoliation is mainly determined by the molecular intercalation kinetics and mechanical force induced by ultrasonication. Therefore, the lateral size and thickness of the MoS2 monolayer nanosheets exfoliated from crystal powders are similar to those from large single crystals (Supplementary Fig. 9). As a signature of high-quality monolayers, the liquid-state photoluminescence from the MoS2 monolayer dispersion in solvent was also observed (Fig. 2c, g). The red-light emission at λ = 661 nm (Eg = 1.88 eV) matches the monolayer bandgap of MoS2 crystal because the bilayer and thicker crystals are expected to exhibit much weaker photoluminescence. By assembling the exfoliated monolayers into solid thin films with thermal annealing at 300 °C (Supplementary Fig. 10), the transistors fabricated from two types of MoS2 nanosheets both deliver carrier mobility of ~7–10 cm2·V−1·s−1 and current on/off ratio of ~106 (Fig. 2d, h). This is consistent with the previous report on THAB-exfoliated MoS2 nanosheet thin films2. The field-effect mobility values were extracted from the linear region of the Id–Vgs transfer curves. In specific, it is calculated following the equation μ = gm·L/(W·C·Vds), where μ, gm, L, W, C, and Vds denote field-effect mobility, transconductance, channel length, channel width, gate capacitance, and drain-source voltage, respectively. Also, similar high material and device uniformity was confirmed through the small device-to-device performance variation among 20 individually measured transistors, which is comparable to those previously obtained with large-size single crystals (Fig. 2d, h and Supplementary Fig. 11).Fig. 2: Characterizations of exfoliated MoS2 monolayers from crystal powders.a Photograph of MoS2 crystal powders used for electrochemical molecular intercalation. The average particle size of the powder is ~10 µm. b The atomic force microscopy (AFM) image of 2D monolayer nanosheets exfoliated from crystal powders. Scale bar, 2 µm. c Liquid-state photoluminescence spectrum of powder-exfoliated MoS2 monolayers dispersed in dichloroethane with bis(trifluoromethane)sulfonimide (TFSI) treatment. Insets are the photographs of the ink solution with and without ultraviolet light illumination. The term “a.u.” denotes “arbitrary units”. PL photoluminescence. d Id–Vgs transfer characteristics of 20 individual MoS2 transistors based on 2D nanosheets exfoliated from crystal powders. Inset is the statistical distribution of the mobility values for these transistors. Vds = 1 V. Id, drain current; Vgs, gate-source voltage; Vds, drain-source voltage. e Photograph of a piece of MoS2 bulk single crystal. f The AFM image of 2D monolayer nanosheets exfoliated from the bulk single crystal. Scale bar, 2 µm. g Photoluminescence spectrum of bulk-crystal-exfoliated MoS2 monolayers dispersed in dichloroethane with TFSI treatment. Insets are the photographs of the ink solution with and without ultraviolet light illumination. h Id–Vgs transfer characteristics of 20 individual MoS2 transistors based on 2D nanosheets exfoliated from large-piece single crystal. Inset is the statistical distribution of the mobility values for these transistors. Vds = 1 V. i Transmission electron microscopy (TEM) image of an individual MoS2 nanosheet exfoliated from crystal powders. Inset is the corresponding selected-area electron diffraction pattern of this MoS2 nanosheet. Scale bar, 1 µm. j X-ray photoelectron spectra (XPS) analysis of the MoS2 nanosheets exfoliated from crystal powders. k The thickness distribution of powder-exfoliated MoS2 monolayer nanosheets measured by AFM. Inset is the schematic illustration of an inorganic monolayer capped by organic molecules with a thickness of ~2.8 nm. l Raman spectra of 10 individual MoS2 nanosheets. The red dotted lines indicate the position of the A1g and E2g peaks of the Raman spectra.In addition, the intercalation reaction proceeds faster with micron-size crystal powders than with millimeter-size bulk single crystals, which is mostly determined by the surface area and number of crystal boundaries for molecular intercalation. For example, the powder intercalation completes in ~1 h (weight ~100 mg), which is about 2-3 times faster than that using large-size bulk single crystals of similar weight (~3 h). From the transmission electron microscopy (TEM) analysis, the exfoliated MoS2 2D monolayer is free of structural damage and liquid metal residues (Fig. 2i). The hexagonal electron diffraction pattern matches the (001) crystal plane of MoS2, confirming the intact crystal structure. X-ray photoelectron spectra (XPS) analysis suggests the preservation of the pristine 2H crystal structure and the absence of phase transition to 1 T structure throughout the powder-based electrochemical intercalation and exfoliation (Fig. 2j). Also, the Ga scan confirms the absence of gallium residues from the liquid metal. By analyzing 100 individual nanosheets in AFM images, we have obtained a narrow thickness distribution in which >98% are monolayers with a thickness of ~2.8 nm (Fig. 2k). The monolayer structure was also confirmed by X-ray diffraction (XRD) pattern (Supplementary Fig. 12), in which only the organic/inorganic superlattice diffraction peaks (d ~2.8 nm) were observed while the intrinsic MoS2 crystal peak (d ~0.6 nm) is absent. Raman spectra of the exfoliated nanosheets show a consistent wavenumber separation between E2g and A1g peaks of ~19 cm-1, matching the value for monolayer crystals instead of bilayer or thicker counterparts (Fig. 2l). All these data suggest that the use of liquid metal in the intercalation and exfoliation reaction, compared with the standard intercalation without liquid metal, does not introduce noticeable changes to the morphology, crystal structure, and surface molecular layer of the exfoliated MoS2 monolayers. Together, using readily available and low-cost micron-size crystal powders as the source material, we have realized high-purity 2D semiconductor monolayers with morphology, electrical performance, and material uniformity that resemble those obtained from standard large-size bulk single crystals.A library of 2D electronic material inksSwitching from large-piece bulk single crystal to micron-size powders can greatly expand the scope and applicability of the electrochemical molecular intercalation and exfoliation approach. The high-temperature and time-consuming growth (>100 h) of large single crystals is expensive and requires sophisticated synthetic control. Also these large single crystals may not be available for many 2D materials. By contrast, micron-size crystal powders are much cheaper and readily available for most common 2D layered crystals. As a proof of concept, we have shown that >50 types of 2D nanosheets can be intercalated and exfoliated from corresponding powders, ranging from TMDs (e.g., ZrS2, NbSe2, and MoTe2), main group metal chalcogenides (e.g., InSe, SnSe2, and Bi2Se3), ternary layered crystals (e.g., MnPS3 and ZnIn2S4), layered oxides (e.g., MoO3 and V2O5), to elemental crystals (e.g., graphene and phosphorene) (Fig. 3 and Supplementary Fig. 13). For most TMDs, the exfoliated nanosheets are high-purity monolayers (monolayer purity >90%), including 2D metals/semi-metals such as TiS2, TiSe2, NbS2, NbSe2, TaS2, and TaSe2, and 2D semiconductors such as MoS2, WS2, ZrS2, ZrSe2, HfS2, and HfSe2. By contrast, many bipolar semiconductors such as MoSe2, MoTe2, WSe2, and VSe2 tend to produce multilayer nanosheets, and the exfoliation yield is also lower. Besides, when intercalating 2D powders without significant crystal volume expansion to induce the automatic material detachment from liquid metal (e.g., MoO3, SnS2, and ZnIn2S4), NaHCO3 was added into liquid metal to create abundant internal micropores and thus higher surface area for accessing electrolyte and alkylammonium intercalants. With the addition of NaHCO3, the intercalation and exfoliation yield for these crystals has been greatly improved by about 10 times. Another interesting point is the surface property of these nanosheets and the related dispersion stability in solution. Most 2D nanosheets, such as MoS2 and MoSe2, require PVP polymer surfactant as a capping agent to minimize restacking in organic solvents after exfoliation. However, PVP is not mandatory for some other crystals. For example, we noticed that InSe, In2Se3, MoO3, SnS2, SnSe2, and ZnIn2S4 monolayers can be well exfoliated and stabilized in pure DMF solvent without the addition of polymer surfactant. The exact origin of this discrepancy is not yet clear. In principle, the alkylammonium functionalized nanosheets are expected to disperse in common organic solvents such as DMF, DMSO, and NMP, considering the high solubility of organic alkylammonium molecules in these polar solvents. However, most exfoliated 2D nanosheets do not form a stable dispersion in these solvents without additional surfactants such as PVP. Therefore, it is likely that the molecular configuration of the alkylammonium molecules and their interaction with the inorganic crystalline lattice may differ among diverse 2D layered crystals.Fig. 3: The library of 2D material inks constructed by powder-based electrochemical intercalation and exfoliation.a Overview of metal and non-metal elements (highlighted in purple) that crystalize into layered crystals with chalcogens, phosphorus, and others (>50 types). These crystals can be used for powder-based electrochemical intercalation and exfoliation into 2D nanosheet inks. b Representative optical images of selected 2D nanosheets after exfoliation, ranging from transition metal dichalcogenides (TMDs), main group metal chalcogenides, ternary layered crystals, and layered oxides to elemental crystals. Insets are photographs of each ink solution containing 2D nanosheets dispersed in the solvent. To obtain these large-size nanosheets, the intercalated crystals were exfoliated by manual shaking instead of bath sonication. Scale bars, 5 µm.The electrochemical intercalation of 2D semiconductors with wide bandgap (Eg >2.5 eV) and low electrical conductivity was previously challenging when using large-size bulk single crystals as the source materials due to the difficulty in injecting electrons into the conduction band. Because a respectable electrical conductivity of the bulk crystal itself is the prerequisite for the electrochemical reaction to occur (e.g., MoS2, NbS2, and In2Se3). However, many 2D wide-bandgap semiconductors such as SnS2, GaS, GaSe, MnPS3, and MoO3 (Eg = 2-3 eV) exhibit low electrical conductivity and thus no noticeable electrochemical intercalation reaction. In specific, for crystals with a small bandgap such as MoS2 crystal (Eg = 1.2–1.9 eV), significant volume expansion throughout the entire crystal and successful intercalation reaction can be observed (Fig. 4a). For SnS2 crystal with a medium bandgap (Eg = 2.1 eV), only the top half of the crystal shows obvious volume expansion and the bottom half remains unchanged, signifying a limited intercalation level of the bulk crystal. Therefore, the subsequent exfoliation yield for nanosheets is also low. For 2D semiconductors with an even larger bandgap, such as MoO3 and GaS (Eg = 2.7 eV), a negligible sign of intercalation reaction was observed. Occasionally, only a very small part of the crystal near the metal clip is intercalated and expanded. The distinct intercalation behavior among these crystals can be quantitatively determined by the recorded electrochemical current profiles (Fig. 4b). These wide-bandgap semiconductor typically delivers significantly lower electrochemical current and thus less noticeable intercalation reaction than other crystals. For example, the electrochemical current in bulk MoO3 crystal remains negligible even at an intercalation voltage of − 8 V. This is probably due to the very low electrical conductivity of MoO3 crystal which is also confirmed by the electrical measurement. The intrinsic MoO3 crystal delivers a negligible current compared with SnS2 and MoS2 at similar thickness, which is smaller by about 104–108 times (Fig. 4c). The significantly lower electrical conductivity in the wide-bandgap material may result in a larger voltage drop across the crystal and thus smaller effective voltage for intercalation. It prevents the electron injection to the crystal lattice for the intercalation reaction that occurs at the crystal/electrolyte interface.Fig. 4: The electrochemical intercalation of 2D wide-bandgap semiconductors enabled by liquid metal.a Photographs of MoS2, SnS2, and MoO3 single crystals after the electrochemical intercalation, showing different intercalation levels. The length of crystals is ~8 mm. b Electrochemical current profiles of the intercalation of MoS2, SnS2, and MoO3 single crystals. c Electrical characteristics of intrinsic MoS2, SnS2, and MoO3 thin crystals (~10 nm thickness). d, e Photographs of GaS crystal before and after intercalation in the form of bulk single crystal (d) and powders (e). Insets are the schematic illustration of the electron pathway in these two structures. The length of GaS crystal is ~10 mm. f Optical microscope images of an individual mechanically exfoliated GaS thin flake (~60 nm thickness) before and after intercalation. The flake was contacted with a metal electrode for electrical conduction. Scale bars, 100 µm. The dashed line indicates the intercalation front during the reaction, and the arrows indicate the direction of intercalation. g In situ dark-field optical images of the slurry surface after intercalation for 0 s, 5 s, and 15 s, showing the gradual exposure of buried microcrystal powders to electrolytes for intercalation. Scale bars, 100 µm. The regions labeled by the dashed lines indicate the location of GaS microcrystals. h Photograph of the obtained colloidal solution of GaS 2D nanosheets (200 mL) and the 4-inch thin film deposited on a standard SiO2/Si wafer.This challenge can be addressed by the liquid-metal-assisted intercalation approach based on crystal powders. Wide-bandgap semiconductors such as MoO3, GaSe, and GaS can now be electrochemically intercalated and exfoliated into high-quality 2D nanosheet inks. As a proof of concept, we focus on GaS crystal, which has a large bandgap of 2.6–3.0 eV, similar to MoO3. When using a large-piece bulk single crystal, there is no sign of intercalation even at a high intercalation voltage of −20 V (Fig. 4d). By contrast, when mixing the GaS microcrystal powders with liquid metal for electrochemical intercalation, the volume expansion in crystal and color change from light yellow to brown was observed at −7 V, signifying the successful molecular intercalation of alkylammonium cations into the GaS crystal (Fig. 4e). The discrepancy in intercalation behavior mostly comes from the reduced resistance in powder anchored on liquid metal framework comparing with the bulk crystal. In the electrochemical reaction, the electrons need to travel a long distance from the metal clip to the crystal and then the crystal/electrolyte interface for intercalation reaction. For 2D wide-bandgap semiconductors with high resistance, a significant potential drop across the entire bulk crystal (on a millimeter scale) can be observed, and thus no intercalation reaction would occur. However, such potential drop can be mostly eliminated in the liquid-metal-assisted intercalation reaction. Inside the mixture slurry, the micron-size crystal powders form an intimate electrical contact with the conductive liquid metal framework. The crystal powders (crystal length ~µm) are much smaller than the bulk single crystal (crystal length ~mm), which thus results in a greatly reduced voltage drop. As a result, the electrochemical intercalation of these 2D wide-bandgap semiconductors with low electrical conductivity now becomes feasible.To further understand the mechanism behind this, we have carried out the intercalation of mechanically exfoliated GaS microcrystal (crystal length ~200 µm, thickness ~60 nm). The electrochemical molecular intercalation of these small GaS microcrystals is confirmed and the advance of molecular intercalation front inside the crystal can also be visualized (Fig. 4f). The morphology evolution of GaS crystal powders in liquid metal mixture slurry during the intercalation can be tracked by in situ optical microscopy measurement. With the applied electrochemical potential, the GaS microcrystals buried in liquid crystal are gradually pushed outward and then exposed to the electrolyte for intercalation reaction (Fig. 4g), as discussed in the previous section. This process was also confirmed by the XRD analysis in which the intensity of intrinsic GaS (001) peak and THAB/GaS superlattice peak both grow at this stage (Supplementary Fig. 14), suggesting the emergence of buried GaS microcrystals on the surface and subsequent molecular intercalation. Ascribing to the greatly reduced crystal resistance, the recorded electrochemical current grows higher by ~104 times compared with that using bulk crystal (Supplementary Fig. 15). Following the liquid-metal-assisted intercalation approach using crystal powders, large-scale production of GaS 2D monolayer ink solution and the deposition of uniform 4-inch thin film have been readily achieved (Fig. 4h). These results prove that the electrochemical intercalation reaction of 2D wide-bandgap semiconductors (e.g., GaS, GaSe, and MoO3) that does not occur in previous large-size single crystal can now be enabled by the liquid-metal-assisted method. Importantly, these solution-processable 2D wide-bandgap semiconductors can be used as the dielectric material and further integrated with other 2D semiconductors and 2D metal inks for the fabrication of transistors, memristors, and other electronic devices over a large area and at an affordable cost.Solution-processable integration of van der Waals thin filmsThe creation of a rich library of functional 2D electronic material inks that span from semiconductors, metals, to dielectrics allows for the integration of solution-processable van der Waals thin film electronics. Following the powder-based intercalation and exfoliation approach, both n-type (e.g., MoS2, MoSe2, and WS2) and p-type (e.g., MoSe2, WSe2, and MoTe2) 2D semiconductor inks and thin films have been prepared. For example, the n-type MoS2, MoSe2, and WS2 thin films assembled from the solution-processable 2D inks deliver an electron mobility of ~5–10 cm2·V−1·s−1 and a current on/off ratio of 106 (Fig. 5a, b). On the other hand, MoSe2, WSe2, and MoTe2 typically are bipolar and incline to exhibit n-type transport behavior after exfoliation (Supplementary Fig. 16). But the p-type semiconducting characteristics can be awakened by proper chemical treatment such as bromine for WSe2 nanosheets. The fabricated p-type transistor delivers hole mobility up to ~20 cm2·V−1·s−1 and a current on/off ratio of 106 (Fig. 5c). The solution-processable fabrication of both n-type and p-type transistors defines the material foundation for more complex and practical electronics such as complementary metal-oxide-semiconductor (CMOS) devices. In addition to transistors, memory devices such as memristors can also be fabricated with 2D semiconductor inks. Using Pt and Ti electrodes, switching behavior between high and low-resistance states was observed in solution-processable MoS2, MoSe2, and WSe2 thin films (Supplementary Fig. 17). The transition from high-resistance state to low-resistance state is probably due to the atomic migration of sulfur and selenium defects in TMD crystals under the applied electric field that modulates the Schottky barrier at Pt/TMDs interface5. However, the long-term operation stability of the memristors remains a challenge, which requires further optimization of the device fabrication process, such as bottom electrodes with smaller surface roughness and reduced edge spikes.Fig. 5: All-solution-processable integration of 2D semiconductors, metals, and dielectrics for van der Waals thin-film transistors.a Schematic illustration and optical image of solution-processable MoS2 transistors using standard microfabrication techniques. Scale bar, 100 µm. b, c Representative Id-Vgs transfer characteristics of transistors based on n-type semiconducting MoS2, MoSe2, and WS2 thin films (b) and p-type semiconducting MoSe2, WSe2, and MoTe2 thin films (c). The substrate is 100 nm SiO2/Si substrate, and the electrode is 20/50 nm Cr/Au for (b) and 70 nm Au for (c). Vds = 1 V. Insets are the schematic illustration of the crystal structure of MoS2 (b) and WSe2 (c). d Schematic illustration and optical image of transistors by integrating solution-processable 2D metallic NbS2 as contact electrode and MoS2 thin film as a semiconducting channel. Scale bar, 100 µm. e Electrical characteristics of NbS2, NbSe2, and TaS2 thin films with thickness of ~15 nm, showing high electrical conductivity. f Comparison of Id-Vgs transfer characteristics of MoS2 transistors with solution-processable NbS2 and evaporated Cr/Au electrodes. Vds = 1 V. Inset is the schematic illustration of the crystal structure of stacked MoS2 and NbS2. g Schematic illustration and optical image of transistors using solution-processable 2D GaS thin film as dielectric and MoS2 thin film as a semiconducting channel. Scale bar, 100 µm. h Electrical characterizations of leakage current of GaS, In2Se3, and MnPS3 films. i Comparison of Id-Vgs transfer characteristics of MoS2 transistors with 40-nm-thick solution-processable 2D GaS and conventional 100-nm-thick thermal SiO2 gate dielectric. Vds = 1 V. Inset is the schematic illustration of the crystal structure of stacked MoS2 and GaS.The metal-semiconductor contact is important for delivering optimized current in the transistor. Following the powder-based intercalation and exfoliation approach, 2D metallic/semi-metallic crystals, including NbS2, NbSe2, TaS2, and others, can be prepared as ink materials. The assembled thin films exhibit high electrical conductivity of ~3000 S/cm at room temperature and can thus be used as the electrode material in transistors (Fig. 5d, e). In particular, stable current can be delivered with NbS2, NbSe2, and TaS2 films of thickness down to ~5 nm (Supplementary Fig. 18). At such a small thickness, the conventional evaporated gold and aluminum metal electrodes might experience high risk of failing due to thickness fluctuation and potential film breakage. However, the 2D crystals can maintain high material integrity and associated electrical conductivity even within this thickness range. In a typical case study, we have fabricated a thin-film transistor with solution-processable MoS2 semiconductor channel and NbS2 electrode based on successive ink spin coating process (Supplementary Fig. 19). An electron mobility of 10–15 cm2·V−1·s−1 and current on/off ratio of 106 have been extracted from the Id-Vgs transfer curves (Fig. 5f). These device metrics are comparable to the transistor based on same solution-processable MoS2 channel but with thermally evaporated Cr/Au metal electrodes (µ ~10 cm2·V−1·s−1 and on/off ratio of 106). It demonstrates the potential of using solution-processable 2D metal inks for the fabrication of high-performance and large-area electronic devices.The solution-processable 2D dielectrics remain a challenging topic compared with 2D semiconductors and metals12,44. Our method offers an alternative pathway to the preparation of high-quality 2D dielectric ink materials for further material integration and device fabrication. Here we use the wide-bandgap GaS crystal as a typical example for device fabrication (Fig. 5g). Following the powder-based intercalation with liquid metal and exfoliation in PVP/DMF, the 2D GaS monolayers capped with PVP polymer molecules were obtained. After film deposition, an organic/inorganic hybrid PVP/GaS superlattice structure can be assembled and used as a dielectric layer without further high-temperature thermal annealing process (Supplementary Fig. 20a). The breakdown field strength is about 2-3 MV/cm with the leakage current falling below 10−6 A/cm2 (Fig. 5h). The measured dielectric constant is ~6.6–7.6 for GaS which is higher than the thermal silicon oxide (k ~3.9) (Supplementary Fig. 20b). Similarly, many other PVP-capped 2D nanosheets such as In2Se3 and MnPS3 exhibit decent capacitance and may also serve as the dielectric layer. To investigate the potential of these 2D dielectric materials, we have fabricated a MoS2 transistor with a spin-coated 40-nm-thick GaS layer as the top gate dielectric (Supplementary Fig. 21). Here, the GaS dielectric layer was directly used after spin coating without further thermal annealing or chemical treatment. In contrast to the dielectric layer that requires high-temperature annealing or oxidative UV irradiation12,44,45, this material shows excellent compatibility with most device structures and the current fabrication process in industrial fabrication. Within the applied gate voltage range of − 8 V to 8 V, a current on/off ratio of ~106 was obtained in the MoS2 transistor, which outperforms that with 100-nm-thick thermal SiO2 in the same voltage range (on/off ratio ~5) (Fig. 5i). This is mostly ascribed to the larger dielectric constant and smaller thickness which together result in the higher capacitance. Also, the gate leakage current remains at ~pA level within this voltage range, which is desired as a good dielectric material (Supplementary Fig. 22). Together, we have proposed solution-processable 2D dielectrics with low processing temperature (<100 °C) while simultaneously delivering high gate performance in 2D transistors. It may offer an alternative pathway to the low-cost fabrication of next-generation large-area flexible and wearable 2D electronics in replacement of traditional oxide dielectrics that are processed by costly sputtering or atomic layer deposition.
