二维材料力学行为的压痕测试

准确了解二维材料的力学性能对于推动其应用具有重要意义, 无基底压痕技术是目前最广泛采用的二维材料力学性能测试方法之一, 本文综述了二维材料压痕研究的最新进展以及所面临的问题, 并对将来的研究工作进行了展望.无基底压痕技术是将二维材料转移到带有沟槽或柱形孔的基底上, 制备二维材料"梁"和"鼓"模型, 然后利用原子力显微镜测量其在压针作用下的载荷--位移关系, 最后通过基于连续介质薄膜导出的压痕响应分析模型拟合实验结果, 估算出二维材料的弹性模量和本征强度.由于二维材料的厚度远小于连续介质薄膜, 来自于压头以及基底孔侧壁的范德华力对二维材料的压痕响应具有显著影响, 造成二维材料与传统压痕分析模型中的基本假设不符, 导致不能准确预测二维材料的弹性模量; 另外, 由于传统压痕模型无法准确描述二维材料在大变形下的非线性行为, 以及由缺陷等引起的应力集中, 导致由压痕测试表征的二维材料(特别是多晶二维材料)本征强度具有较大的偏差. 因此, 一方面需要正确了解由压痕技术获得的二维材料力学性能, 另一方面还需对目前的研究方法做进一步的改进和完善. 

Indentation tests investigation of mechanical behavior of two-dimensional materials

Fully understanding the mechanical properties of two-dimensional (2D) materials is highly important for developing applications based on 2D-materials. Free-standing indentation (FSI) is currently the most common method to measure the mechanical properties of 2D-materials. The present work reviewed the state-of-the-art FSI investigations of 2D-mateirals, the issues to be considered, and some future works in this area. In FSI tests, 2D-materials are firstly transferred on the top of the substrate with cylindrical holes to create beam/drum-type samples, and atomic force microscopy (AFM) is then used to measure the indentation load-displacement relationship of these samples. Finally, the mechanical properties, including the elastic modulus and intrinsic strength, can be determined by fitting the experimental results as the indentation analysis model is developed on the basis of the continuum thin film. However, since the thickness of 2D-materials is far less than that of the continuum thin film, the van der Waals (vdW) adhesion interactions from the AFM-tip and the side-wall of substrate hole have a strong influence on the indentation response, which leads to measurement inaccuracy of the elastic modulus of 2D-materials from FSI tests. In addition, the nonlinear response of 2D-materials under large deformation as well as the stress concentration created by defects cannot be effectively described by the conventional indentation analysis model, and thus, the intrinsic stress of 2D-materials cannot be accurately determined, especially for the poly-crystalline 2D-materials. Therefore, we should correctly understand the present experimental results from FSI tests, and in the meantime, it is very necessary to further improve the FSI technique for measuring the mechanical properties of 2D-materials.