Elastic properties of 2D Ti3C2T x MXene monolayers and bilayers

Abstract

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are a large class of materials that are finding numerous applications ranging from energy storage and electromagnetic interference shielding to water purification and antibacterial coatings. Yet, despite the fact that more than 20 different MXenes have been synthesized, the mechanical properties of a MXene monolayer have not been experimentally studied. We measured the elastic properties of monolayers and bilayers of the most important MXene material to date, Ti₃C₂T _x (T _x stands for surface termination). We developed a method for preparing well-strained membranes of Ti₃C₂T _x monolayers and bilayers, and performed their nanoindentation with the tip of an atomic force microscope to record the force-displacement curves. The effective Young's modulus of a single layer of Ti₃C₂T _x was found to be 0.33 ± 0.03 TPa, which is the highest among the mean values reported in nanoindentation experiments for other solution-processed 2D materials, including graphene oxide. This work opens a pathway for investigating the mechanical properties of monolayers and bilayers of other MXenes and extends the already broad range of MXenes' applications to structural composites, protective coatings, nanoresonators, and membranes that require materials with exceptional mechanical properties.

Fig. 1. Preparation of MXene membranes.

(A) Structure of a Ti₃C₂T_x monolayer. Yellow spheres, Ti; black spheres, C; red spheres, O; gray spheres, H. (B) Scheme of the polydimethylsiloxane (PDMS)–assisted transfer of MXene flake on a Si/SiO₂ substrate with prefabricated microwells. See text for details. (C) SEM image of a Ti₃C₂T_x flake covering an array of circular wells in a Si/SiO₂ substrate with diameters of 0.82 μm. (D) Noncontact AFM image of Ti₃C₂T_x membranes. (E and F) Height profiles along the dashed blue (E) and red (F) lines shown in (D).

Fig. 2. Elastic response and indentation test results.

(A) Scheme of nanoindentation of a suspended Ti₃C₂T_x membrane with an AFM tip. (B) Force-deflection curves of a bilayer Ti₃C₂T_x flake at different loads. The bottom inset is a detailed view of the same curves showing the center of origin. The top inset shows AFM image of the fractured membrane. (C) Comparison of loading curves for monolayer (1L) and bilayer (2L) Ti₃C₂T_x membranes and the least squares fit to the experimental indentation curves by Eq. 1. Hole diameter is 820 nm. The inset shows the same experimental curve for bilayer Ti₃C₂T_x in logarithmic coordinates. The curve shows a linear behavior in the first 10 nm of indentation (blue line) and approaches the cubic behavior at high loads (red line). (D) Histogram of elastic stiffness for monolayer and bilayer membranes. Solid lines represent Gaussian fits to the data. (E) Histogram of pretensions of monolayer membranes. (F) Histogram and Gaussian distribution of breaking forces for monolayer membranes. Tip radius is 7 nm.

Fig. 3. Comparison of indentation tests on Ti₃C₂T_x with other 2D materials.

(A) Comparison of experimental F-δ curves for monolayer graphene and Ti₃C₂T_x membranes. (B) Comparison of effective Young’s moduli for several 2D materials: GO (12), rGO (13), MoS₂ (5), h-BN (8), and graphene (1). In this chart, we compare values produced on membranes of monolayer 2D materials in similar nanoindentation experiments.