Interfacial Degradation and Pattern Evolution of Exfoliated Graphene by Cyclic Mechanical Loading

Abstract

mechanical properties. [1,2] They possess an extremely large in-plane stiffness but a very small bending stiffness. Therefore, out-of-plane deformations such as ripples, wrinkles, folds, and crumples are frequently observed in the process of preparation, transfer, and application of 2D materials. [3,4] The surface deformations are initially regarded as troublesome issues because they may affect the mechanical reliability of 2D materials. However, further research shows that the appearance of surface deformations can adjust band gap, change chemical reaction activity, and enhance surface performance of 2D materials, which are beneficial for a wide range of applications in photoelectronic devices. [5][6][7] The strain is now treated as a new degree of freedom for tailoring electrical, optical, and chemical properties of 2D materials. This so called "strain engineering" usually needs to transfer 2D materials on polymer substrates for exerting strain more effectively. Furthermore, 2D material-polymer systems can be extensively applied in various high-tech fields including flexible electronics, multifunctional coatings, and nanocomposites. The 2D materials are often in contact with the polymer substrates through weak van der Waals (vdW) interactions. The interfacial effect between 2D materials and polymer substrates has received increasing interest recently both for fundamental researches and engineering applications. [8,9] For 2D material-polymer systems, the strain can be conveniently exerted and well controlled by thermal annealing [10][11][12][13][14] or mechanical loading. [15][16][17][18][19][20][21][22] As the coefficient of thermal expansion of 2D materials is much smaller than that of polymers, the temperature increase (decrease) of the system induces an isotropic tensile (compressive) strain in the 2D materials. Unidirectional mechanical stretching (releasing) or bending outward (inward) generates a uniaxial tensile (compressive) strain. The tensile strain usually leads to interfacial slippage when the strain is beyond a certain value (≈0.8%) due to the weak vdW interactions. [15][16][17] On the other hand, the compressive strain can cause prominent out-of-plane deformations such as wrinkles, buckles, and folds. The isotropic compression by thermal annealing induces wrinkle networks with preferential alignments along the crystal orientation of 2D materials. [11,12] The uniaxial compression generates parallel straight wrinkles or folds perpendicular to the loading direction. [18][19][20] For uniaxialThe interfacial interactions between 2D materials and polymer substrates receive increasing interest due to the surge of flexible electronics, multifunctional coatings, and nanocomposites. Although the strain effect on electrical, optical, and mechanical properties of 2D materials is extensively investigated, understanding the interfacial mechanics of 2D material-polymer systems by dynamic loading is still a challenge. Here, the interfacial degradation and pattern evolution of mechanically exfoliated single-and fe...