Background:
With the continuous research of two-dimensional transition metal carbides and nitrides (MXENES), these materials have attracted a lot of attention due to their excellent metal conductivity, hydrophilicity, dispersion stability, and flexibility. Since the first Mxene (Ti3C2TX) was first discovered by Yury et al. in 2011, the unique combination of physical and chemical properties has led to a wide range of research in fields such as flexible electronics, supercapacitors, catalysis, sensors, aerospace, and micro-nano mechanical devices. However, since MXENES can be subject to mechanical stresses such as tensile, bending, and torsion in practical applications, this can lead to reduced performance.
In order to better understand and optimize the mechanical properties of MXENES, scientists have begun to pay attention to the study of their mechanical properties. In previous studies, experimental studies were carried out on multilayer Ti3C2TX films, and it was found that their tensile strength can reach 670 MPa. However, the experimental results of these multilayer Ti3C2TXs were significantly lower than the theoretical ** value of 20 GPA, which was attributed to the relatively weak interaction between the multilayer 2D sheets. Due to the special structure of MXenes, the mechanical properties of monolayer Ti3C2TX nanosheets become particularly important. However, due to its nanoscale thickness, the traditional atomic force microscopy (AFM) nanoindentation method has some limitations, such as generating uneven stress and strain fields, which makes it difficult to accurately measure the mechanical properties of monolayer Ti3C2TX.
Brief introduction of the results
In order to achieve this,Professor Yan Yabin, Professor Zhang Bowei and Professor Xuan Fuzhen of East China University of Science and TechnologyCo-published a study titled "Elastic Properties and Tensile Strength of 2D Ti3C2TX MX MXENE Monolayers" in Nature Communications**. The purpose of this study was to solve the problem of measuring the mechanical properties of monolayer Ti3C2TX nanosheets by performing uniform loading directly on the 2D material plane through a uniaxial tensile test. Using precision-controlled ion beam focusing (FIB) cutting technology and improved dry transfer technology, the research team successfully prepared high-quality and large-size monolayer Ti3C2TX nanosheets and immobilized them onto a push-pull (PTP) nanomechanical test platform for in-situ tensile experiments. The Young's modulus and tensile strength data of the monolayer Ti3C2TX nanosheets were successfully obtained through experimental measurements. To verify the experimental results, the research team also performed molecular dynamics simulation (MD) theoretical modeling calculations. This study provides an effective nanomechanical testing strategy for other 2D materials produced by mechanical exfoliation, and provides practical guidance for materials that require special mechanical properties, such as Ti3C2TX-based flexible electronic devices.
**Reading guide
In order to successfully transfer monolayer Ti3C2TX nanosheets to PTP devices in in-situ nanomechanical testing, a unique dry transfer method was developed. The method is modified from the previous method. Specifically, the prepared monolayer Ti3C2TX suspension droplets are placed on a 400-mesh copper mesh without a carbon film and vacuum-dried. The monolayer nanosheets are attached to the edge of the copper mesh, which greatly facilitates the subsequent transfer process. One side of the nanosheet is then glued to the mechanical probe by electron beam deposited platinum, and the other three sides are moved by focused ion beam (FIB) cleavage. The resulting nanosheets are transferred to the PTP microdevice in the middle of 25 m stretch area. Due to their monolayer nature, Ti3C2TX nanosheets suspended on nanomechanical devices are almost transparent. By FIB cleavage, the manipulator and Ti3C2TX nanosheets are cut and separated. The significance of this process is that it overcomes the difficulty of transferring monolayer Ti3C2TX nanosheets to a nanomechanical test platform, laying the foundation for subsequent in-situ nanomechanical testing. Through this dry transfer method, the researchers successfully immobilized the Ti3C2TX nanosheets on the PTP microdevice, providing a reliable sample for subsequent mechanical property testing.
Figure 1Schematic diagram of the Ti3C2TX monolayer and the SEM image transfer process
Figure 2 illustrates the key steps in its preparation and characterization. In the SEM image (Figure 2A), the ends of the single-layer Ti3C2TX nanosheets are fixed to the PTP nanomechanical device by electron beam deposited PT, while the suspended portion of the nanosheets is milled by FIB to a shape and size suitable for tensile testing. The thrust exerted on the hemispherical indenter during the experiment is indicated by a red arrow. SEM images of a single-layer Ti3C2TX nanosheet milled by FIB are shown in Figure 2b. The width and length of this nanosheet are 5 m and 2., respectively5 m, the orange arrow indicates the direction of stretching of the sample. Figure 2c shows the cross-section of the fracture edge of the suspended Ti3C2TX nanosheet after mechanical testing by aberration-corrected scanning transmission electron microscopy (AC-STEM). This step verifies the thickness of the monolayer Ti3C2TX. The properties of the Ti3C2TX nanosheets used in the experiments were also confirmed by XRD, XPS, EDX, and elemental mapping. Figure 2D shows TEM images of the Ti3C2TX nanosheets and the corresponding SAED pattern, confirming their high-quality crystal properties and hexagonal carbide structure. These properties do provide the basis for subsequent mechanical property testing.
Figure 2Protocol and characterization of the Ti3C2TX monolayer
Figure 3 shows a comparison of tensile fracture and properties of monolayer Ti3C2TX nanosheets. Using SEM snapshots (Figs. 3A, B), the authors observed that the maximum engineering strain of the monolayer Ti3C2TX nanosheets was up to 36%。The tensile fracture topography shows a typical brittle fracture (Fig. 3c), while the corresponding load-displacement curve reveals a critical stage in the tensile process (Fig. 3d). The slope of the curve provides information about the intrinsic stiffness of the sample and the nanomechanical device, the total stiffness of the sample during the tensile and tensile phases, at different stages. Through precise calculations, the researchers obtained that the actual tensile stiffness and 2D elastic modulus of the single-layer Ti3C2TX nanosheets are about 947., respectively7 nm and 4739 n/m。
Under the assumption of uniaxial stress, the 3D elastic modulus calculated by the model established by the finite element method is about 484 GPA. Several tensile experiments were carried out in the study, but only five were successful due to the difficulty of nanosheet manipulation and the fragility of the monolayer Ti3C2TX. By comparing the experimental measurements with the theoretical values, the researchers found that the measured effective elastic modulus of the monolayer Ti3C2TX nanosheets was close to the theoretical simulation value, which was much higher than the value obtained by the previous nanoindentation method. Compared to other single-layer 2D materials, Ti3C2TX MXene exhibits a high modulus of elasticity, making it a strong choice for micromechanical devices and composites.
Figure 3Comparison of tensile fracture and properties of Ti3C2TX nanosheets
The researchers performed molecular dynamics (MD) simulations in Figure 4 to verify the effect of edge defects on the breaking strength of monolayer Ti3C2TX nanosheets. During the simulation, they established three different types of edge defects and demonstrated the width scale dependence of the samples (see Figure 4A). The atomic structure of Ti3C2TX is arranged in a hexagonal shape and has the inherent material orientation of "armchair" and "zigzag". They simulated tensile experiments on three different width scales of single-layer Ti3C2TX nanosheets, and fixed each end to obtain eighteen breaking strength results (see Fig. 4b). These simulation results provide an important reference for further understanding the mechanical properties of Ti3C2TX nanosheets, and help to reveal their mechanical behavior and application prospects at the micro-nano scale.
Figure 4MD simulation of the breaking strength of Ti3C2TX monolayers at different width scales under edge defects that may be caused by FIB
Summarize the outlook
In conclusion, the authors successfully achieved in-situ mechanical tensile testing of single-layer Ti3C2TX nanosheets in a scanning electron microscope (SEM) using PTP nanomechanical devices. In contrast to the lateral localization test of the AFM nanoindentation test, the PTP device can achieve uniform stretching of the sample in the plane and can reliably measure the mechanical properties of the monolayer Ti3C2TX. The modulus of elasticity of the monolayer Ti3C2TX is 4835 ± 13.2 GPA, close to the theoretical ** value of 502 GPA. The monolayer Ti3C2TX nanosheets exhibit brittle fracture with an average elastic strain of about 32%, which provides an opportunity for the application of Ti3C2TX in elastic strain engineering. In addition, the experimentally measured effective breaking strength is 154 ± 1.92 GPA, with an ideal value of 18The difference between the 4 GPa is attributed to atomic-scale defects at the edge of the sample, and this difference gradually decreases as the sample width scale increases. The influence of edge defects on the fracture strength was quantified by molecular dynamics simulation, and the engineering fracture strength could be improved by adjusting the edge state of the monolayer Ti3C2TX nanosheets.
Bibliographic information
rong, c., su, t., li, z. et al. elastic properties and tensile strength of 2d ti3c2tx mxene monolayers. nat commun 15, 1566 (2024). 10.1038/s41467-024-45657-6