In this study, we demonstrated the on-water surface synthesis of crystalline mechanically interlocked monolayer (MI-M2DP) and bilayer (MI-B2DP) 2DP films by incorporating macrocyclic molecules (MCMs) into the backbones. Through cooperative host-guest assembly between MCMs (cucurbit[8]uril (CB8) with a single cavity or nor-seco-cucurbit[10]uril (ns-CB10) with double cavities) and 1,1'-bis(4-aminophenyl)-[4,4'-bipyridine]-1,1'-diium chloride (V-2NH2) molecules, we achieved precise control over the number of interlocking layers. The resulting films were characterized by imaging and X-ray scattering techniques, confirming their crystallinity. Using strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) and atomic force microscopy (AFM) nanoindentation techniques22,23, the effective Young's modulus (EYoung) of MI-M2DP and MI-B2DP were systematically examined, showing ultrahigh elastic modulus. Theoretical calculations were conducted to elucidate the underlying mechanism governing the observed layer-dependent mechanical behaviour. The MI-B2DP film was further integrated as the membrane for seawater desalination to demonstrate its practical utility. This study sheds light on the controlled synthesis of crystalline 2DPs at the monolayer or bilayer level and provides potential avenues to address the challenges of exploring the interlayer structure-property relationships.
MCMs are of interest as supramolecular scaffolds for constructing linear polymers and crosslinked polymer networks through host-guest chemistry. A key feature of MCMs is their pronounced steric bulk, which disrupts π-π stacking between adjacent polymer backbones. This characteristic presents a unique opportunity to suppress layer stacking in 2DPs and enables structural control in the out-of-plane direction. Leveraging this property, we propose that MCMs containing one or more host cavities could serve as programmable spacers to regulate interlayer interactions and guide the synthesis of interlocked 2DPs with defined layer numbers and in-plane periodicity. To explore this concept, we employed cucurbiturils as model MCMs. CB8, which features a single host cavity, was used to suppress interlayer interaction and confine 2D polymerization to a monolayer (MI-M2DP). For bilayer formation, we designed and synthesized ns-CB10 via the condensation between glycoluril and formaldehyde (Supplementary Figs. 1-3). The resulting ns-CB10 with two adjacent cavities (∼6.5 Å diameter; Supplementary Fig. 4) is capable of hosting two guest molecules. This dual-cavity architecture allows for precise spatial alignment of monomeric units across two stacked layers, thereby offering a molecular-level design principle for constructing bilayer 2DP (MI-B2DP).
The synthesis of MI-M2DP and MI-B2DP using a surfactant monolayer-assisted interfacial synthesis (SMAIS) method on the water surface is illustrated in Fig. 1a,b. First, monomers V-CB8 and V-CB10 were synthesized in aqueous solutions by incorporating CB8 and ns-CB10 into the backbone of V-2NH, respectively, as building blocks for creating MI-M2DP and MI-B2DP (step 1). The successful formation of V-CB8 and V-CB10 was confirmed by UV-visible absorption and H NMR studies (Supplementary Figs. 5-7). Then, a monolayer of sodium oleyl sulfate (SOS) was prepared on the water surface (step 2), followed by the injection of 1 ml mixed aqueous solution of trifluoromethanesulfonic acid (TfOH, 7.4 µmol) and V-CB8 (2.4 µmol; or V-CB10 for MI-B2DP synthesis) into the water subphase (pH ≈ 1.3). The electrostatic interaction between the SOS monolayer and V-CB8 (or V-CB10) drives their adsorption on the water surface within 2 h (step 3; Supplementary Figs. 8-10). Subsequently, 1 ml aqueous solution of 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (Tp, 1.6 µmol) was added to the sublayer of the system to initiate the 2D polycondensation via a Schiff-base reaction (step 4). The polymerization was then kept undisturbed at room temperature for 1 day, affording a pale-yellow film with a scalable lateral size (from ∼12.6 to ∼154.1 cm) on the water surface (step 5; Supplementary Figs. 11 and 12). The reaction was extended to 7 days, aiming to monitor thickness evolution and to demonstrate the critical role of MCMs in controlling the layer number of 2DPs.
Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy shows that the stretching vibration of N-H (∼3,323 cm) from V-CB8 and V-CB10, and -CHO (∼1,640 cm) from compound Tp completely disappeared after polycondensation (Supplementary Figs. 13 and 14), suggesting the complete conversion of monomers into 2DPs. Compared to multilayer 2DP without using MCMs (ML2DP), the characteristic FTIR peaks of -CH- (2,945 cm) and C=O (1,716 cm) from V-CB8 and V-CB10 monomers can be observed in MI-M2DP and MI-B2DP, supporting the successful embedding of CB8 and ns-CB10 into the 2DP networks. The model reaction results also reveal that the MCMs remain firmly integrated into the viologen moieties even after the on-water surface reaction (Supplementary Figs. 15-17). Furthermore, the chemical structure and composition of MI-M2DP and MI-B2DP were confirmed by surface-enhanced Raman and X-ray photoelectron spectroscopy (XPS) characterizations (Supplementary Figs. 18-22 and Supplementary Table 1). Energy dispersive X-ray (EDX) mapping also reveals a homogeneous distribution of carbon, nitrogen, oxygen and fluorine in both 2DP films (Supplementary Figs. 23 and 24).
Optical microscopy and scanning electron microscopy (SEM) images show the macroscopically homogeneous nature of MI-M2DP and MI-B2DP films (Supplementary Fig. 25). As shown in Fig. 1c and Supplementary Figs. 26 and 27, both MI-M2DP and MI-B2DP films can suspend over the holes (lateral size, ∼20 μm) on a transmission electron microscopy (TEM) grid without rupturing, indicative of their excellent mechanical stability. AFM analysis of MI-M2DP and MI-B2DP films shows a root mean square roughness of 0.18 nm and 0.27 nm in an area of 10 × 10 µm (Supplementary Fig. 28). The thicknesses of MI-M2DP and MI-B2DP are determined to be ∼1.7 and 2.1 nm, respectively, aligning with the anticipated values for the monolayer and bilayer structures (Fig. 1d and Supplementary Figs. 29 and 30). In contrast to the observed increase in thickness over time for ML2DP (from 2.0 nm after 1 day to 11.2 nm after 7 days), the thicknesses of MI-M2DP and MI-B2DP are maintained (Fig. 1e and Supplementary Figs. 31-36). These results suggest that the bulky MCMs with single or dual cavities prevent π-π stacking in 2DPs, enabling precise control over the layer numbers from monolayer to bilayers.
To date, characterizing the crystal structure of monolayer and bilayer 2DPs through TEM and synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS) remains challenging due to their sensitivity to high-energy radiation. To mitigate the structural degradation caused by electron-radiation-induced knock-on damage, electrostatic charging and chemical etching, we lowered the voltage and electron dose rate of TEM, and applied a graphene encapsulation method to enhance their radiation resistance (G/MI-M2DP/G and G/MI-B2DP/G), as shown in Supplementary Figs. 37-43. High-resolution TEM (HRTEM) resolves the hexagonal structure of MI-M2DP with a lattice parameter of a = b = 44.5 Å, γ = 120° (Fig. 2a,b and Supplementary Fig. 44). The selected-area electron diffraction (SAED) pattern of G/MI-M2DP/G shows a weak diffraction ring at 0.45 nm (d spacing, 22.2 Å) attributed to the (110) crystal plane (Supplementary Fig. 40), indicating a polycrystalline nature. To probe the macroscopic structural order of MI-M2DP, we further performed GIWAXS measurement on a 20-layer MI-M2DP film prepared through layer-by-layer (LBL) assembly. The in-plane reflection ring at Q = 0.17 Å (that is, d spacing, 37.0 Å) agrees well with the (100) plane of MI-M2DP (Fig. 2c,d and Supplementary Fig. 43), confirming its in-plane crystal structure.
The same characterizations were then carried out for MI-B2DP samples (Fig. 2e). The hexagonal lattice of MI-B2DP (a = b = 44.5 Å, γ = 120°) was observed in HRTEM images (Fig. 2f and Supplementary Fig. 44). The SAED pattern shows an obvious reflection at 0.45 nm (d spacing, 22.2 Å), which can be assigned to the (110) plane of MI-B2DP (Supplementary Fig. 41). The GIWAXS pattern displays a diffraction ring at Q = 0.17 Å (that is, d spacing, 37.0 Å), corresponding to the (100) in-plane parameter of MI-B2DP. Additionally, a weak reflection peak at Q = 1.86 Å suggests an interlayer distance of 3.4 Å between MI-B2DP bilayers (Fig. 2g,h and Supplementary Fig. 43). These results validate the successful synthesis of polycrystalline MI-M2DP and MI-B2DP films, lending further credence to the feasibility of utilizing MCMs for modulating the out-of-plane structure of 2DPs.
To determine the mechanical properties of MI-M2DP and MI-B2DP, the SIEBIMM technique was initially used. The synthesized 2DP films were horizontally transferred onto a polydimethylsiloxane (PDMS) elastomeric support and strained using a motorized strain device, resulting in the formation of a regular wrinkling pattern perpendicular to the strain direction in 2DPs (Supplementary Fig. 45). Three samples, namely, MI-M2DP, two-layer stacked MI-M2DP (2×MI-M2DP) and MI-B2DP, were measured to investigate the impact of interlayer interactions on the mechanical properties (Fig. 3a,b). AFM topographical images show regular wrinkle patterns for all samples (Fig. 3c), indicating their high quality and suitability for SIEBIMM. The wavelengths of the wrinkles in MI-M2DP, 2×MI-M2DP and MI-B2DP, calculated by a Python-based calculation method and cross-checked by 2D Fourier-transformation, were 274 ± 10, 188 ± 7 and 402 ± 31 nm, respectively (Supplementary Figs. 46 and 47). The E was thus evaluated using the regular wrinkling wavelength, film thicknesses (Supplementary Fig. 48 and Supplementary Table 2) and the mechanical stiffness of the PDMS substrate (2.06 MPa), as described by the following equation.
To verify the mechanical properties of these synthetic films, we further conducted AFM nanoindentation measurements. To this end, the film samples were transferred onto SiN substrates with 1-μm circular holes (Supplementary Fig. 53). As shown in Fig. 3e, we plotted the non-linear load versus deflection (F-δ) curves for MI-M2DP, 2×MI-M2DP and MI-B2DP films, respectively, which were fitted using a cubic F-δ relationship (R > 0.99):
Next, we attempted to investigate the fracture behaviour upon applying higher loads (Supplementary Figs. 56-60). The films remained suspended on the hole, with nanoscale fracture occurring only in the area of direct contact with the AFM tip. The F-δ curves of the MI-M2DP, 2×MI-M2DP and MI-B2DP films were recorded to determine their fracture loads (F; Fig. 3h). The maximum fracture stresses () of the films were calculated using the formula of the linearly elastic membrane under a spherical indenter:
where E is the 2D elastic modulus of the suspended film, and r is the radius of the AFM tip, measured as ∼13.5 nm from the SEM image (Supplementary Fig. 61). The breaking strengths (σ) of the MI-M2DP, 2×MI-M2DP and MI-B2DP films were determined to be 13 ± 3, 6 ± 2 and 19 ± 3 GPa, respectively, according to the equation σ = . (Fig. 3i). Compared to the weak vdW interaction in 2×MI-M2DP, the mechanical interlocking in MI-B2DP tightly integrates the two layers, forming a composite structure that enhances its mechanical properties. The breaking strength of MI-B2DP is the highest among layer-stacked 2DP, 2D COF, 2D MOF films, and CNMs (Fig. 3j).
Notably, for both SIEBIMM and AFM nanoindentation, the MI-B2DP reveals higher E compared with MI-M2DP. This stands as a unique phenomenon because, typically, when stacking monolayers into bilayer 2D materials, interlayer sliding impedes the collective engagement of the layers in the stress-strain process, leading to mechanical relaxation and subsequent degradation of their E (Fig. 4a). However, the mechanically interlocked structure tightly integrates the layers, forming a composite layer that enables simultaneous contributions to the mechanical strength of 2DP films (Fig. 4b). To gain insight into the underlying mechanism of the interlayer sliding in these bilayer systems, we calculated the energy penalty as shown in Supplementary Fig. 62. The sliding energy of MI-B2DP is consistently higher (by up to 200 kcal mol) compared with 2×MI-M2DP, indicating stronger interlayer coupling in the mechanically interlocked structures. To understand the structural reinforcement in MI-B2DP enabled by MCMs, we further performed classical molecular dynamics (MD) simulations to investigate the stress-strain behaviour of 2×MI-M2DP and MI-B2DP. In the simulations, the top layer of 2×MI-M2DP was weakly coupled to the bottom layer, enabling its free movement during the stress loading. As shown in Fig. 4c,d and Supplementary Figs. 63-67, the interlocked MI-B2DP possesses an enhanced in-plane stress response to applied strain, characterized by a higher E (E = (216 ± 1) × 10 GPa) compared with 2×MI-M2DP (E = (77 ± 1) × 10GPa), which supports the experimental findings. The weak vdW interaction in 2×MI-M2DP facilitates the interlayer sliding, thereby reducing the contribution of the chemical bonds in the top layer to the overall stiffness and modulus of the film. In contrast, the synthetic MI-B2DP is highly interconnected via MCMs throughout its thickness, which effectively prevents the interlayer sliding. This interconnection ensures that all covalent bonds in the interlocked bilayers and the MCMs simultaneously contribute to the stress response, leading to the reinforced stiffness and modulus of MI-B2DP.
To evaluate the practical utility of the enhanced mechanical properties, we assessed the cation separation performance of both 2×MI-M2DP and MI-B2DP films. The as-prepared films were mounted between two reservoirs filled with 0.2 M NaCl solution (feed part) and 2 M sucrose solution (permeate part), in which substantial stress was applied to the films (Fig. 4e,f). After five filtration cycles (72 h per cycle), the Na rejection rate of 2×MI-M2DP declined sharply by ∼97.5%, indicative of structural disruption (Fig. 4g). In contrast, MI-B2DP maintained a nearly unchanged rejection rate, with only a ∼2.6% decrease over the same period. These results highlight the critical role of mechanical interlocking in preserving structural integrity under prolonged operation, thereby enabling sustained ion-separation performance.