Shear properties and stable wrinkle resistance in 2D Ti₃C₂T_x MXene monolayers

Abstract

Two-dimensional (2D) transition metal carbides or nitrides (MXenes), represented by Ti₃C₂T_x, exhibit exceptional mechanical and electrical properties, making them ideal candidates in aerospace, structural membranes, and flexible electronics. Understanding the mechanical properties of 2D Ti₃C₂T_x under various load conditions is crucial for ensuring structural integrity and performance reliability. Here, we conduct direct in-situ shear test of monolayer Ti₃C₂T_x nanosheets using a push-to-shear strategy under the scanning electron microscope. Contrary to the expected ultra-low shear performance and wrinkle instability of 2D materials, we report a high in-plane shear modulus of 0.279 ± 0.007 TPa for single-crystal monolayer Ti₃C₂T_x, along with stable wrinkle resistance. Additionally, the monolayer Ti₃C₂T_x exhibits a shear strength as high as ~18.6 GPa and an average shear strain of ~8.6%. This work provides direct measurement of the intrinsic shear properties of monolayer Ti₃C₂T_x and highlights the promising potential of Ti₃C₂T_x in maintaining device performance and reliability.

Introduction

Two-dimensional (2D) titanium carbide (Ti₃C₂T_x MXene) nanosheets have promising applications in aerospace¹, intelligent sensors², flexible electronics³, and micro- and nanoelectromechanical devices (MEMSs and NEMSs)^4,5, owing to their excellent mechanical properties, metallic conductivity, and solution-processability^6,7. In practical applications, the mechanical stress experienced by component units can significantly influence the performance and stability of functional nanodevices^8,9. Consequently, a comprehensive understanding of the mechanical properties of 2D Ti₃C₂T_x under various loading conditions is essential. While the mechanical properties of monolayer Ti₃C₂T_x under uniaxial tension have been extensively investigated through experiments and atomic simulation methods^10,11, the in-plane mechanical properties of monolayer Ti₃C₂T_x under shear loading remain unexplored. This gap is largely due to the limitations of micro- and nanomechanical testing techniques, as well as the absence of standardized testing protocols. Notably, the shear modulus quantifies the material’s resistance to shear deformation under stress. A detailed investigation of the shear modulus and mechanical behavior of 2D Ti₃C₂T_x under shear loading will provide valuable insights for the structural design of 2D flexible electronics devices¹².

Currently, the related work primarily focuses on investigating the interfacial shear modulus and interlayer shear stress of monolayer 2D materials, including tip-based adhesion measurement^13,14, pressurized bubbling method^15,16, and atomic force microscopy (AFM) nanoindentation strategy^17,18. However, these methods make it difficult to reveal the in-plane shear behavior of 2D materials and obscure their intrinsic shear properties. Specifically, the tip-based adhesion measurement relies on the interaction between the probe with the sample surface. Guo’s team systematically studied the shear mechanical properties of the surface and interface of 2D materials using graphite-wrapped AFM probes, but could not directly measure the shear mechanical properties of the 2D materials themselves¹⁹. The pressurized bubbling method employs uniform bubbles on the surface of 2D materials to assess the shear mechanical properties. For instance, an interfacial shear stress of 1.64 MPa was determined by bubble loading monolayer graphene on a silicon oxide substrate²⁰. This technique, however, is affected by the unstable interfacial force between the 2D material and the substrate, necessitating an accurate assessment of the airtightness of the 2D material. Additionally, the inhomogeneity of bubble expansion and stress concentration impacts the measurement accuracy²¹. The AFM nanoindentation strategy involves loading different layers of 2D materials suspended on a porous substrate, with the interlayer shear force determined by comparing and analyzing the load-displacement curves²². However, nanoindentation method induces both uneven shear stress and tensile stress in the transverse test area of the 2D material, and different indenter sizes and indentation locations contribute to significant uncertainty in the results²³. In general, due to the limitations of current testing techniques, the intrinsic shear properties and mechanical behavior of 2D materials under in-plane shear loading are often overlooked. As highlighted in recent in-plane shear studies, internal shear deformation plays a crucial role in dictating both the mechanical stability and functional properties of atomically thin materials²⁴. Nevertheless, quantitative investigations of solution-processed Ti₃C₂T_x remain scarce. Therefore, there is an urgent need for a direct, quantitative, and reliable approach to study the in-plane shear properties of monolayer Ti₃C₂T_x nanosheets.

In this work, we synthesized high-quality, large-sized monolayer Ti₃C₂T_x nanosheets, which were fixed onto the nanomechanical testing device using improved dry transfer method and precisely controlled focused ion beam (FIB) technique, as previously reported by us¹⁰. The “Push-to-Shear” (PTS) strategy was employed to apply in-plane shear loading. The shear modulus and shear strength of monolayer Ti₃C₂T_x nanosheets were directly measured, and the stable wrinkle resistance was observed, which differs from the behavior of monolayer graphene. Additionally, the experimental observations of in situ shear in monolayer Ti₃C₂T_x were validated through simulations. Our findings show that MXene can break the conventional understanding of ultra-low shear properties and wrinkle instability typically expected in 2D atomically thin materials. This work demonstrates that 2D Ti₃C₂T_x MXene holds significant potential for applications in structural composite films and flexible electronics devices that require specific mechanical properties and robust performance.

Results

Transfer of monolayer Ti₃C₂T_x samples

To successfully conduct in situ shear testing, a crucial prerequisite is the transfer of high-quality monolayer Ti₃C₂T_x nanosheets to the nanomechanical testing device without causing damage. Unlike the transfer process for large-area 2D material synthesized via chemical vapor deposition (CVD), such as graphene and MoS₂, which can be quickly and easily transferred by simply removing the polymer protective layer^25,26. 2D monolayer Ti₃C₂T_x nanosheets are obtained by mechanical exfoliation in solution (the synthesis steps are detailed in Supplementary Fig. 1), which significantly increases the difficulty of sample transfer. Therefore, we developed an improved dry transfer method. In detail, the monolayer Ti₃C₂T_x supernatant was dripped onto the bare copper mesh and dried under vacuum. Monolayer nanosheets will be suspended on the edge of bare copper mesh (Fig. 1a), thus avoiding the difficult transfer problem caused by the van der Waals force between the sample and the substrate, significantly facilitating the subsequent transfer process. Then, a rectangular sample was obtained by attaching the manipulator to one side of the sample via electron-beam (e-beam) platinum (Pt) deposition, while the other three sides were cut using FIB technique. The resulting sample was carefully transferred to the test area at the center of the nanomechanical device using the manipulator (Fig. 1b). The 2D monolayer Ti₃C₂T_x nanosheet was then subjected to in-plane shear testing using the “push-to-shear” strategy (Fig. 1c). Notably, in order to avoid curling of the samples during the transfer process, we reinforced the sample with e-beam Pt deposition around its edges (Fig. 1d). Ti₃C₂T_x nanosheets suspended on the PTS nanomechanical device are nearly transparent due to their monolayer nature. We used FIB to cut and shape the monolayer Ti₃C₂T_x nanosheets in the test area, and deposited e-beam Pt on both ends of the sample, which can avoid the slippage between the sample and the PTS device during the shear test (Fig. 1e). It is worth noting that the PTS device was originally designed for uniaxial tensile loading (commonly referred to as push-to-pull loading). Here, it is employed to perform shear loading on monolayer Ti₃C₂T_x for shear strength measurement, with a discussion on the validity of the shear testing to demonstrate that the specimen experienced pure shear deformation. As shown in Supplementary Fig. 2a, b, the PTS device consists of a movable plate (purple region) and a fixed plate (gray region). When the hemispherical indenter is loaded, the purple movable plate translates downward to apply force on the specimen. Under loading, the entire region beneath the hemispherical indenter constitutes the effective testing area of the specimen (the boxed orange region), while the surrounding springs ensure the stability of the device. During the loading process, the gap width remains unchanged (Supplementary Fig. 2c), enabling one side of the specimen to be fixed while a tangential displacement is applied to the opposite side. Consequently, the specimen at the device side gap undergoes pure shear deformation. SEM images taken during the test further show that the gap width does not change (Supplementary Fig. 2d), confirming that the specimen experiences pure shear deformation, consistent with the testing requirement of fixing one side while applying a parallel tangential displacement to the other side.

Fig. 1: Schematic diagram of the transfer process of monolayer Ti₃C₂T_x nanosheet.

a Transfer of monolayer Ti₃C₂T_x nanosheets suspended at the edge of a copper mesh using a mechanical probe and FIB cutting. b The monolayer Ti₃C₂T_x is transferred to the test area in the middle of the PTS nanomechanical device by a mechanical probe and e-beam Pt deposition. c In situ shear testing of monolayer Ti₃C₂T_x via the “push-to-shear” strategy of the PTS nanomechanical device. d SEM images (repeated independently with similar structures for four times) of monolayer Ti₃C₂T_x nanosheet during mechanical probe transfer. Sample edges are treated with e-beam Pt deposition to prevent sample curling. e SEM image (repeated independently with similar structures for four times) of monolayer Ti₃C₂T_x on the test area of the PTS device, with the ends of the samples immobilized by e-beam Pt deposition, and the samples above the gap of the test area milled and shaped by FIB.

Characterization of monolayer Ti₃C₂T_x samples

As shown in the SEM image (Fig. 2a), the ends of the monolayer Ti₃C₂T_x were immobilized onto the test area of the PTS nanomechanical device by e-beam Pt deposition. The nanosheet suspended above the gap was milled using FIB to achieve the required shape and size for shear testing. A pushing force is applied to the hemispherical indenter on the PTS device via the probe (indicated by red arrows), and four springs (indicated by purple arrows) convert the pushing into shearing on the sample. The in-plane shear test of monolayer Ti₃C₂T_x nanosheets is achieved through the “push-to-shear” mechanism. The loading rate of the probe is 10 nm/s, and the load-displacement data can be recorded during the loading process using a mechanical sensor in the planar probe. Figure 2b shows the SEM image of monolayer Ti₃C₂T_x after FIB cutting; the length and width of the Ti₃C₂T_x nanosheet are 4.5 and 5.5 μm, respectively. The cyan arrow indicates the in-plane shear direction of the sample.

Fig. 2: Characterization of monolayer Ti₃C₂T_x during in situ shear experiments.

a The probe applies compression to the hemispherical indenter on the PTS, and four springs convert pulling force into shearing force on the sample by the “push-to-shear” mechanism. b The magnified image of the red region in (a) displays the monolayer Ti₃C₂T_x in the shear test area on the PTS device, with sample cut into rectangles by FIB, and the cyan arrow indicating the shearing direction. c TEM image (repeated independently with similar structures for four times) of the edge region of the tested Ti₃C₂T_x after FIB cutting, and the corresponding SAED pattern. d Monolayer Ti₃C₂T_x samples suspended at the edge of the copper mesh prior to transfer operations, and corresponding crystalline SAED patterns. e TEM image (repeated independently with similar structures for four times) of the center region of the tested Ti₃C₂T_x, and the corresponding SAED pattern. f STEM image (repeated independently with similar structures for four times) of the fracture edge region of the tested Ti₃C₂T_x, and the zoomed-in red rectangular box exhibits the fracture cross-section of the tested sample. Black arrows indicate two C atomic layers, cyan arrows indicate three Ti atomic layers, and orange arrows indicate surface groups. g AFM images (repeated independently with similar structures for four times) of monolayer Ti₃C₂T_x nanosheets, corresponding curves are thickness values.

To ensure the accuracy of test results, it is essential to maintain the high quality of the samples during the transfer process. Therefore, we thoroughly characterized the monolayer Ti₃C₂T_x nanosheets both before and after the transfer. The properties of Ti₃C₂T_x were confirmed during the preparation process through X-ray diffraction (XRD, Supplementary Fig. 3), X-ray photoelectron spectroscopy (XPS, Supplementary Figs. 4 and 5), and energy dispersive X-ray spectroscopy (EDS, Supplementary Fig. 6). Prior to sample transfer, the high quality of the suspended Ti₃C₂T_x was verified using transmission electron microscopy (TEM) and EDS mapping (Supplementary Figs. 7 and 8). The corresponding selected area electron diffraction (SAED) pattern confirming the single-crystal nature of Ti₃C₂T_x (Fig. 2d, Supplementary Fig. 9). To minimize potential damage to the sample during FIB milling, we employed an ultra-low accelerating voltage of 2 kV for cutting. As shown in Fig. 2c, the corresponding SAED from the edge area of the Ti₃C₂T_x nanosheet confirms that the crystal structure remains intact after FIB cutting, and the sample retains its high-quality morphology. In addition, the TEM image of the central area of the tested Ti₃C₂T_x on the PTS device, along with the corresponding SAED pattern, also confirms the high quality and single-crystal properties of the Ti₃C₂T_x nanosheets after transfer (Fig. 2e). It is worth noting that during the transfer process of Ti₃C₂T_x nanosheets, the lateral size of the sample is typically larger than 10 μm due to the reduction in size caused by the Pt deposition and FIB cutting. Supplementary Fig. 10 shows that the average lateral size of the Ti₃C₂T_x nanosheets is about 11.9 μm, which provides ample size for the sample transfer process.

The thickness of a monolayer Ti₃C₂T_x is a key factor for the subsequent calculation of its in-plane shear properties, and therefore, accurate determination of its thickness is essential. Although AFM is commonly used to measure the thickness of 2D materials, the presence of water molecules and adsorbates on the surface of the Ti₃C₂T_x nanosheets, as well as the tapping and contact mode of the AFM and the interaction between the AFM tip and the Ti₃C₂T_x surface, can often lead to an overestimation of the monolayer thickness^25,27. Similarly, XRD measurements of the thickness of monolayer samples can be influenced by the amount and chemical properties of embedded substances, resulting in inaccurate measurements²⁸. Therefore, STEM characterization of the cross-section of a monolayer Ti₃C₂T_x nanosheet after fracture, combined with theoretical models, provides the most accurate method for determining the thickness (Fig. 2f). Furthermore, we used AFM to confirm the monolayer nature of Ti₃C₂T_x nanosheets. As shown in Fig. 2g, the thickness of the Ti₃C₂T_x nanosheet was found to be 1.5 nm. It is noteworthy that in other studies, the “nominal” thickness of the monolayer 2D material has been used as a calculation parameter rather than the thickness measured by AFM²⁹. For instance, the AFM-measured thickness of monolayer graphene typically ranges from 0.4 to 1.7 nm, which significantly overestimates the actual thickness of monolayer graphene³⁰. In studies evaluating the mechanical properties of monolayer graphene, the nominal thickness of 0.335 nm is typically used as the calculation parameter³¹. Similarly, in this study, we used the nominal thickness of 0.98 nm for monolayer Ti₃C₂T_x nanosheet as the parameter for subsequent calculations of shear modulus. Specifically, we employed STEM to obtain high-resolution images of the cross-section of monolayer Ti₃C₂T_x. By analyzing the distinct boundary of the monolayer Ti₃C₂T_x in the STEM images, we were able to precisely measure its thickness, confirming that the material’s thickness is approximately 0.98 nm. To further validate the result obtained from the STEM images, we combined a theoretical model to derive the thickness of the monolayer Ti₃C₂T_x. The model predictions were consistent with the experimental results, further supporting the thickness value of 0.98 nm. Notably, this value has been widely used in the literature^10,27,32. Therefore, this value is supported by both STEM measurements and theoretical models (Fig. 2f).

In situ shear test of monolayer Ti3C2Tx nanosheets

To study the in-plane shear performance and mechanical behavior of monolayer Ti₃C₂T_x nanosheets, displacement-controlled in situ shear tests were conducted using a PTS device within the SEM chamber. During the shear test, it is necessary to avoid slippage between the sample and the device, as this could affect the experimental results. Therefore, we deposited a Pt strip on both ends of the sample for reinforcement, ensuring that both ends of the sample remained securely adhered to the device. As a result, no slippage was observed throughout the experiment. The entire experimental process was observed and recorded in real time (Supplementary Movie 1).

Before the shear test, the probe was positioned near the top of the hemispherical indenter on the PTS device and aligned on the same horizontal plane. The load-displacement curve began to be recorded as soon as the probe made contact with the PTS device (Fig. 3a). In the initial stage (as indicated by the red arrow), the monolayer Ti₃C₂T_x nanosheets were subjected to in-plane shear force and began to shear in situ. In mechanical testing of 2D materials, enlarging the lateral dimensions of the specimen (while keeping the thickness constant) generally mitigates size-dependent effects and yields measurements that better reflect the intrinsic mechanical properties³³. To validate this, we performed preliminary experiments and simulations on samples with varying widths, and found that the measured shear response approached the intrinsic behavior when the sample width exceeded 5 μm (Supplementary Figs. 11 and 12, Supplementary Tables 1 and 2). Considering the reliability of the measurements, the initial dimensions of monolayer Ti₃C₂T_x, the inevitable size loss during the experimental process, and the challenges associated with transfer. The actual dimensions of the sample in the test area were 4.5 μm in length and 5.5 μm in width, with a monolayer thickness of 0.98 nm. By subtracting the inherent stiffness of the PTS device and incorporating these dimensional parameters, the actual 3D in-plane shear modulus of the monolayer Ti₃C₂T_x was calculated to be ~278.8 GPa. In the second stage, a gradual decrease in the actual shear stiffness of the sample was observed (as indicated by the green arrow). However, the sample did not exhibit the expected wrinkling instability behavior of 2D material, and still maintained strong wrinkle resistance. In the third stage, the monolayer Ti₃C₂T_x was further subjected to shear loading, and the effective shear modulus eventually stabilized at ~110.7 GPa. Ultimately, the sustained shear load led to brittle fracture of the sample starting from the position marked with a purple asterisk, after which the probe began to unload. It should be noted that the effective shear modulus in this context denotes a representative engineering shear modulus derived from the actual experimental conditions. Through SEM snapshots of the samples before and after shear testing, the maximum shear strain of the monolayer Ti₃C₂T_x can be obtained to be 8.9% (Fig. 3b), which illustrates the potential of Ti₃C₂T_x in device applications. Determining the shear modulus of monolayer Ti₃C₂T_x nanosheets remains highly challenging, as it is essential to ensure the integrity and high quality of the monolayer 2D material during the transfer process. Although multiple monolayer Ti₃C₂T_x nanosheets were used in our study, only seven tests were successfully performed. This is primarily due to the inherently fragile structure of monolayer 2D materials, which makes them highly susceptible to damage during the transfer process. Additionally, surface contamination is often introduced during transfer, which can adversely affect the material’s properties and compromise the accuracy of subsequent test results. Therefore, the key to improving the success rate and reproducibility of testing 2D materials lies in the ability of researchers to select the appropriate material transfer technique based on the specific properties of the material, ensure sample cleanliness, avoid sample damage, precisely secure the sample, and choose suitable testing methods^34,35. Table 1 summarizes the geometry, shear modulus, maximum shear strain, and shear fracture strength of the tested samples with a width of 5.5 μm. The average shear modulus in the first stage is 278.8 ± 7.4 GPa, the average shear modulus in the third stage remains at 110.7 ± 9.7 GPa, and the average shear fracture strength is 18.6 ± 2.0 GPa. Notably, in conventional wet-transfer methods with polymer protection layers, 2D materials are anchored onto nano-mechanical devices through van der Waals interactions. Pre-strain is often introduced due to capillary contraction, surface tension during the dissolution of the polymer protection layer, and the interfacial van der Waals forces³⁶. In contrast, in this work, monolayer Ti₃C₂T_x was dry-transferred using a micromanipulator under SEM, enabling precise pickup and release of the flakes. The subsequent localized Pt fixation at the sample edges imposed minimal disturbance on the suspended area, thereby substantially reducing pre-strain effects. Prior to the formal tests, a small cyclic load was applied to release the pre-strain introduced during the transfer and fixation processes. Specifically, sample #4 was subjected to 19 loading cycles at 8–10% of the maximum load (Supplementary Fig. 13), which effectively eliminated the pre-strain. The obtained results were consistent with those of the other three samples, confirming that pre-strain had no effect on the measurements. Furthermore, STEM imaging and geometric phase analysis (GPA) mapping of the samples prior to testing also verified the absence of pre-strain (Supplementary Fig. 14).

Fig. 3: Shear properties of monolayer Ti₃C₂T_x nanosheets.

a The measured load-displacement curves during shear loading, inserted into the formula (see section “ In situ SEM shear testing” for details) can be used to calculate the 2D and 3D shear modulus G of the nanosheets, and the shear fracture strength τ, where C, W, b, h, δ, γ are the shear stiffness, the gap length of the PTS device, width, and thickness of the sample, relative displacement, and shear strain, respectively. b The tested samples are sheared in-plane until fracture with a maximum strain at fracture of 8.9%. Source data are provided as a Source data file.

Table 1 Shear properties of monolayer Ti₃C₂T_x under in-plane shear loading

Discussion

In general, monolayer 2D material exhibit low shear resistance under in-plane shear loading, leading to the formation of numerous wrinkles^37,38. These wrinkles can significantly affect the structural integrity and performance reliability of the 2D material^39,40. For instance, monolayer graphene demonstrates a low in-plane shear modulus of ~70 GPa under shear stress, while also generating periodic wrinkles^24,41. In practical applications, 2D material-based functional devices subjected to complex loading conditions, such as shear, suffer from instability due to individual 2D nanosheet wrinkle formation. This wrinkle instability impedes electron and phonon transport, resulting in degraded device performance^42,43. In macroscopic 2D material assembled films, the low wrinkle resistance of monolayer graphene nanosheets reduces the compactness and orientation of the assemblies, ultimately leading to a deterioration in both the mechanical and electrical properties of macroscopic films⁴⁴. Despite substantial efforts by researchers to enhance the wrinkle resistance of 2D materials to improve their structural stability and performance, the results have remained less than satisfactory. Currently, researchers have succeeded in integrating wrinkle-free, single-crystal monolayer graphene onto metal foil substrates or semiconductor wafers, which exhibit uniform field-effect transistor performance and high carrier mobility⁴⁵. However, these graphene films are mainly useful in laboratory settings, relying on the stacking of “perfect” layers. In the field of structural composite films, Cheng et al. have focused on eliminating wrinkles in 2D nanosheets to enhance the mechanical strength and electrical conductivity of macroscopic films^46,47. Despite these efforts, the exceptional physicochemical properties of individual 2D materials at the nanoscale have not been fully achieved. Excitingly, our study demonstrates that the in-plane shear modulus of monolayer Ti₃C₂T_x is as high as ~278.8 GPa. Although the effective shear modulus decreased slightly during the loading process, it still remains at ~110.7 GPa, and exhibits excellent wrinkle resistance, significantly outperforming the shear properties of monolayer graphene (Fig. 4a). The high shear modulus and wrinkle resistance of monolayer Ti₃C₂T_x suggest that Ti₃C₂T_x nanosheets are an ideal material for next-generation flexible electronics, MEMS, and structural composite films^48,49. Recently, researchers have leveraged the high shear stiffness and wrinkle resistance of monolayer Ti₃C₂T_x to provide additional mechanical protection for monolayer graphene, enhancing the mechanical properties and stability of graphene films through a nano-confined water assembly strategy⁵⁰. Our research confirms that monolayer Ti₃C₂T_x can indeed enhance the mechanical properties and stability of structural membranes.

Fig. 4: MD simulations and performance comparison of monolayer Ti₃C₂T_x and monolayer graphene under shear loading.

a Comparison of in-plane shear modulus of monolayer graphene²⁴ and monolayer Ti₃C₂T_x. Data are presented as mean ± standard deviation from four independent experiments. The detailed data corresponding to the sample numbers are listed in Table 1. b MD simulation model of monolayer Ti₃C₂T_x, where purple atoms represent Ti atoms, black atoms represent C atoms, and pink atoms represent surface groups. c MD simulation results of monolayer Ti₃C₂T_x under shear loading, with color changes showing the localized strain distribution on the sample surface. d MD simulation of monolayer graphene during shear loading, with color variations demonstrating the overall periodic wrinkles and instabilities of the monolayer graphene.

In this study, the shear modulus of monolayer graphene is chosen as the reference, based on the practical application scenarios of 2D materials, and graphene is also a typical benchmark in the research of the mechanical properties of 2D materials. Specifically, the large-scale fabrication of nanomaterials into self-supporting films with both high electrical conductivity and excellent mechanical properties is key to advancing the development of various applications, such as flexible electronics, electromechanical nano-devices, supercapacitors, sensors, and electromagnetic interference shielding^51,52. However, the core challenge in achieving this goal lies in finding nanomaterials that combine both high strength and excellent conductivity⁵³. Among existing 2D material systems, hexagonal boron nitride is non-conductive⁵⁴, and transition metal dichalcogenides (TMDs, such as MoS₂) have relatively low conductivity⁵⁵. Notably, monolayer graphene possesses both remarkably high conductivity and strength, and is widely regarded as the strongest known material to date, making it applicable in flexible electronics, energy storage, and electromagnetic shielding⁵⁶. However, the poor dispersion of graphene during processing limits its large-scale applications⁵⁷. In the exploration of other 2D materials with excellent elasticity and conductivity, MXenes have emerged as ideal candidates. Among them, Ti₃C₂T_x is the earliest discovered and most extensively studied MXene material. Solution-processed Ti₃C₂T_x exhibits conductivity significantly higher than that of graphene and demonstrates impressive electrochemical performance, offering great potential for applications in energy storage and flexible electronics. Monolayer Ti₃C₂T_x nanosheets are excellent building blocks for constructing micro-/nanoelectromechanical devices and wearable electronics^58,59. In the realm of 2D materials, the robust mechanical properties and solution processability of Ti₃C₂T_x MXene make it a promising alternative to 2D materials such as graphene. It is worth emphasizing that monolayer graphene is often regarded as a “prototype system” in the research of 2D materials, as its mechanical properties are representative and universal, making it suitable as a comparative benchmark^27,29,60. Based on the aforementioned reasons, we selected monolayer graphene as the reference in this study and performed a comparative analysis with monolayer Ti₃C₂T_x to highlight the performance advantages of Ti₃C₂T_x and support the research discussion.

The effective Young’s modulus of monolayer Ti₃C₂T_x is ~484 GPa¹⁰, which is on the same order of magnitude as its in-plane shear modulus. In contrast, monolayer graphene has an effective Young’s modulus of ~1 TPa²⁵, but its in-plane shear modulus is only 70 GPa²⁴, a difference of three orders of magnitude. This discrepancy is attributed to the distinct chemical structures of monolayer Ti₃C₂T_x and monolayer graphene, which enable monolayer Ti₃C₂T_x to resist wrinkle instability through different mechanisms when subjected to in-plane shear stress. Specifically, monolayer graphene consists of a single layer of carbon atoms arranged in a hexagonal structure, with each carbon atom covalently bonded to its neighbors via sp² hybrid orbitals⁶¹. When subjected to in-plane shear, monolayer graphene relies solely on the flexible deformation of individual in-plane C-C bonds and lacks additional bonding reinforcement, resulting in a low in-plane shear modulus. Furthermore, the energy generated by in-plane shear stress is dissipated through out-of-plane buckling, leading to the formation of periodic wrinkles⁶². In contrast, monolayer Ti₃C₂T_x has a more complex atomic structure. It consists of five primary atomic layers, composed of alternating transition metal Ti atoms and C atoms (Supplementary Fig. 15a), with strong covalent and ionic bonds forming a compact, layered atomic structure⁶³. Under in-plane shear stress, the strong bonding between individual atomic layers enhances the mechanical stability of monolayer Ti₃C₂T_x. Unlike the out-of-plane deformation of monolayer graphene, the deformation of monolayer Ti₃C₂T_x remains in-plane (Supplementary Fig. 16), which can be attributed to its high bending stiffness (The ability to resist out-of-plane deformation), approximately 123.4 times that of monolayer graphene (Supplementary Fig. 17). Due to the significant challenges of experimentally observing out-of-plane deformation from the sample cross-section in real time, we have confirmed through MD simulations that the deformation of the sample remains in-plane. The interactions between these atomic layers can effectively transfer the shear stress, contributing to a high in-plane shear modulus and strong wrinkle resistance^64,65. To understand the mechanism behind the slight secondary decrease in the shear modulus of monolayer Ti₃C₂T_x, we use molecular dynamics (MD) simulation to replicate the experimental observations (Fig. 4b, Supplementary Movie 2). The strain distribution results from the MD simulation show that internal strain is generated during the shear process, propagating locally and incrementally (Fig. 4c). This behavior contrasts with that observed in monolayer graphene, where periodic wrinkling and overall structural instability occur under shear loading (Fig. 4d). Additionally, monolayer Ti₃C₂T_x can endure large shear strain, whereas monolayer graphene experiences overall wrinkling instability under small strain (Supplementary Movie 3). Before shear loading, the surface of the monolayer Ti₃C₂T_x was flat, and no localized strain was observed (Supplementary Fig. 14). In stage 1, the overall structure of monolayer Ti₃C₂T_x undergoes uniform shear deformation, with no noticeable localized strain, which reliably reflects the intrinsic shear modulus of the sample. In stage 2, after localized strain generation (Supplementary Fig. 18), the shear modulus of the monolayer Ti₃C₂T_x decreased to a certain extent. But the shear modulus still remained at a high level until shear fracture in stage 3, and no structural instability occurred. Notably, the effective shear modulus in stage 2 lies between that of stage 1 and stage 3, analogous to the yield point in conventional mechanical tests. In stage 3, with continued shear loading, the strong bonding between the atomic layers of Ti₃C₂T_x effectively redistributes the localized stress (Supplementary Fig. 15b), preventing macroscopic wrinkles triggered by large-scale energy accumulation, until shear fracture. This phenomenon is further corroborated by the stress-strain curve from MD simulations: at the onset of shear loading, the monolayer Ti₃C₂T_x underwent uniform shear deformation, with a simulated shear modulus of ~294 GPa, maintaining a high shear modulus consistent with the experimental observations (Supplementary Table 2). As the loading progressed, however, the atoms in the Ti₃C₂T_x lattice were severely compressed, localized strain emerged (Supplementary Fig. 19), and the shear modulus of the monolayer Ti₃C₂T_x exhibited a slight decline (Supplementary Fig. 12a), in agreement with the experimental trend. This unique behavior is responsible for the high shear stiffness of monolayer Ti₃C₂T_x. It is worth noting that this phenomenon and the shear properties are also supported by first-principles calculations (Supplementary Fig. 20, Supplementary Table 3). In other words, after localized strain generation, the five main atomic layers of Ti₃C₂T_x dissipate more energy during the shearing process, maintaining its shear resistance properties at a high level, which aligns with the observations from in situ shearing experiments. As is well known, manipulating terminal groups on MXenes in experimental settings is challenging. To address this issue, we employed MD simulations to investigate the shear properties of monolayer Ti₃C₂T_x with different terminal group concentrations (Supplementary Fig. 21). Despite variations in terminal group concentrations, the shear modulus and fracture strength of monolayer Ti₃C₂T_x remain within reasonable ranges, consistent with experimental values.

In summary, we have successfully implemented in situ in-plane shear experiments on monolayer Ti₃C₂T_x nanosheets by developing optimized transfer technique and PTS nanomechanical device. This approach overcomes the limitations of conventional micro- and nanomechanical techniques, which can only measure the surface-interfacial shear modulus and interlayer shear stress of 2D materials. The PTS device allows for direct and reliable determination of the intrinsic shear properties of monolayer Ti₃C₂T_x. The in-plane shear modulus of monolayer Ti₃C₂T_x is 278.8 ± 7.4 GPa, and the effective shear modulus shows a slight decrease during loading due to localized strain, but remains relatively high at 110.7 ± 9.7 GPa, which is in the same order of magnitude as its effective Young’s modulus ~484 GPa. This study highlights the strong mechanical properties of monolayer Ti₃C₂T_x under various loading conditions, demonstrating its potential for practical engineering applications, including nanoelectromechanical devices, structural composite films, flexible electronics, and other applications that require solution-processable materials with excellent mechanical properties. Moreover, monolayer Ti₃C₂T_x exhibits an effective shear fracture strength of 18.6 ± 2.0 GPa, an average shear fracture strain of ~8.6%, and stable wrinkle resistance, significantly outperforming the low shear performance and wrinkle instability of monolayer graphene. This work confirms the unique mechanical properties of monolayer Ti₃C₂T_x, distinguishing it from other 2D materials and offering valuable insights for maintaining the structural integrity and performance stability of 2D material-based functional devices in future applications.

Methods

Sample preparation for shear tests

Due to the size requirement of the test area of the PTS device and the size loss during the transfer process, the improved minimally intensive layer delamination (MILD) method is required to synthesize large-size monolayer Ti₃C₂T_x nanosheets (Supplementary Fig. 1). Specifically, we selected 200 mesh (74 μm) of MAX phase Ti₃AlC₂, and used the mild etching method of HCl + LiF: 40 ml of 6 mol/L HCl and 1.6 g LiF were mixed in a polytetrafluoroethylene (PTFE) beaker, stirred with a magnetic stirrer for 15 min to completely dissolve, then slowly add 1 g Ti₃AlC₂, seal the beaker with a sealing film. The PTFE beaker was sealed with sealing film, and several small holes were pricked on the surface to exclude the gas escaping from the reaction. The Ti₃AlC₂ was etched in a water bath at 35 °C for 30 h. After the reaction was completed, the solution was washed twice with 1 mol/L HCl to remove the excess LiF, and then washed several times with deionized water until the pH of the solution was neutral. The obtained solution was subjected to vacuum-assisted filtration, the precipitate was collected, and dried in vacuum at 50 °C for 12 h. The dried multilayer Ti₃C₂T_x powder was redispersed with 50 ml of deionized water, and the solution was treated with a spin shaker for 30 min, and centrifuged at 684.8 × g for 30 min. The dark green upper colloid solution was taken out, which was the high-quality and large-size monolayer Ti₃C₂T_x suspension.

In situ SEM shear testing

In situ shear tests of monolayer Ti₃C₂T_x nanosheets were carried out in the chamber of Carl Zeiss CossBeam340 SEM, utilizing the Brukers-Hysitron PI88 picindenter (Supplementary Fig. 22a) to load the hemispherical indenter of the PTS device (Supplementary Fig. 22b). Prior to testing, an air calibration of the PI88 was required, followed by pushing force applied to the PTS device using a displacement control of 10 nm/s. The PTS device converts pushing force into shear force on the sample through four springs. During testing, the Pico-Indenter applied force electrostatically while simultaneously measuring displacement capacitively, enabling load and displacement data to be recorded with sub-μN and sub-nm resolution, respectively. The integrated controller provided feedback stability and high-speed data acquisition, allowing real-time synchronization of load-displacement curves with in situ SEM movie imaging. The PTS device also acted as a strain sensor and energy buffer, ensuring accurate strain measurement during tensile loading. The accuracy of the mechanical parameters was ensured by minimizing instrument compliance, thermal drift, and alignment errors. The load-displacement data in the experiment were recorded by the mechanical sensors on the PI88 probe, and the in situ shearing process was also recorded in real time by movie, and the shear strain of the sample could be measured directly from the in situ shearing movie. The boundary conditions of the tested specimen are illustrated in Supplementary Fig. 23, where the locations of the Pt strips are clearly specified, including the no-slip positions and the defined testing area. The slope of the load-displacement curve experimentally determined is the total shear stiffness of the sample and the PTS device. By subtracting the inherent stiffness of the PTS device, the calculation of the monolayer Ti₃C₂T_x shear performance can be achieved. Specifically, the 3D shear modulus of the monolayer Ti₃C₂T_x sample: ${G}_{3D}=C\cdot \frac{W}{b\cdot h}$, C, W, b, and h in the formula represent the shear stiffness of the sample in the test area, the gap length of the PTS device, width, and thickness of the sample, respectively. Here, the PTS gap length W is 4.5 μm, and the sample width b is 5.5 μm. The thickness of the sample is selected as the nominal thickness of 0.98 nm for monolayer Ti₃C₂T_x, which is supported by STEM and DFT theoretical models, and is consistent with the thickness selection principle of the monolayer graphene. The shear strength: $\tau=\frac{F}{b\cdot h}$, and shear strain: $\gamma=\frac{\delta }{W}$, F in the formula is the shear force applied to the sample, and δ is the relative displacement on both sides of the sample. It can be calculated that the ultimate shear strength is ~18.6 GPa and the maximum shear strain is ~8.6%. The 2D shear modulus G_2D of monolayer Ti₃C₂T_x is listed in Supplementary Table 4. In practical applications, the effect of thickness plays a crucial role in determining the mechanical performance. Therefore, adopting the 3D shear modulus G_3D provides a more comprehensive representation of the mechanical properties of monolayer Ti₃C₂T_x. Accordingly, the 3D shear modulus G_3D is employed in the present discussion.

Molecular dynamics (MD) simulation

To verify the mechanical behavior of the monolayer Ti₃C₂T_x sample during shear loading and to understand the reasons for the slight decrease in the shear modulus, we use MD simulations to reproduce the experimental phenomenon. All simulations are performed on a large-scale atomic/molecule massively parallel simulator (LAMMPS). The modeling sizes of the monolayer Ti₃C₂T_x are 400 Å × 320 Å, and the upper and lower edges of the monolayer Ti₃C₂T_x are fixed to apply shear strain at a constant rate of 0.001 ps⁻¹. The temperature is maintained at 300 K, and the temperature rise during loading is within 10 K. The COMB3 potential function is used to set the interaction forces between atoms in the Ti₃C₂T_x model. The Open Visualization Tool (OVITO) is used to visualize the results of in situ shear simulations of monolayer Ti₃C₂T_x.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.