力学特性是二维材料最基本的属性之一。当前人们对二维材料的弹性性质研究比较充分，但对它们的断裂行为和层间力学作用的研究则缺乏足够的实验方法和理论上的支持。事实上，二维材料的断裂和层间滑移是二维材料和器件在实际应用中失效的重要原因。因此，充分理解二维材料的力学特性，特别是其断裂行为和层间力学作用，是十分重要的科学问题。本论文针对这一关键问题，通过纳米推压技术和拉曼光谱技术系统研究了MoTe2、MoS2等二维材料的力学性质和层间力学特性，揭示出其断裂行为和层间力学作用与界面能量、晶格对称性之间的关系。本论文的研究对基础研究和应用研究都具有重要的意义，主要内容包括以下三部分：（一）多相MoTe2的力学性质和断裂行为研究。建立了纳米推压方法，系统研究了面内各向同性的2H-MoTe2与面内各向异性的1T’和Td-MoTe2，发现三相MoTe2的弹性模量差异在15%以内，但2H-MoTe2的断裂强度比1T’及Td-MoTe2大一倍以上，且其断口形貌分别呈三叉形和直线形，与晶体对称性存在显著的对应关系，结合第一性原理计算，揭示出多相MoTe2的断裂行为遵循弹性能和边界能最小原理。这一研究加深了人们对各向同性和各向异性二维材料力学行为的认识。（二）转角双层MoS2层间力学作用与堆叠角度的关系。提出了纳米推压法测试二维材料层间力学作用的实验方法，对应的滑移边界条件模型相比传统的固定边界条件模型具有更高的普适性；将该模型用于测试转角双层MoS2层间力学特性，结合分子动力学（MD）模拟，发现MoS2层间的剪切模量与堆叠角度无依赖关系。该结论说明，双层MoS2的界面结合强度与堆叠角度无关，这对于二维垂直异质结的研究具有重要参考意义。（三）十六烷基三甲基溴化铵（CTAB）插层MoS2的弱层间耦合作用和异常拉曼行为研究。发现CTAB-MoS2的超晶格异质结构出现异常拉曼行为，其面外振动的A1g峰极弱且出现显著红移。由于插层物削弱层间耦合作用，块体CTAB-MoS2具有类似于单层的性质。进一步研究表明，CTAB与MoS2的相互作用抑制了A1g振动模式，而强烈的电子掺杂则使得A1g峰红移。这种通过插层调制二维材料层间作用并对其进行电子重掺杂的方式，有望用于下一代柔性电子器件的开发。
The mechanical properties are regarded as fundamental factors of 2-dimentional (2D) materials. The elastic properties of 2D materials have been widely studied, while the fracture behavior and interlayer interactions are not fully understood, which lack both the experimental approaches and corresponding theories. In fact, the failure of 2D materials and devices is mainly caused by the fracture and interlayer sliding. Hence, it is of vital importance to understand the mechanical behavior of strained 2D materials, especially the facture behaviors and interlayer interaction. To clarify these important issues, we studied the mechanical properties and interlayer coupling of typical 2D materials like MoTe2 and MoS2, by means of nanoindentation and Raman spectra, and uncovered the fracture behavior and interlayer coupling in regard to the edge formation energy and lattice symmetry. These studies deepen the understanding of mechanical properties of 2D materials in both fundamental science and application studies. The main contents of this dissertation are as follows: (1) The elastic properties and fracture behaviors of polymorphic MoTe2. In this work, the nanoindentation experiments are applied to study the mechanical properties of isotropic 2H-MoTe2 and anisotropic 1T’- and Td-MoTe2. We have discovered that the elastic moduli of three phases of MoTe2 are almost identical, while the fracture strength of 2H-MoTe2 is over 100% larger than those of 1T’- and Td-MoTe2. The fracture pattern shows clear correspondence with crystal structure. Density functional theory (DFT) calculations indicates that the fracture behavior is dependent on the elastic energy and edge formation energy. This work has deepen our understanding of elastic properties and fracture behaviors of isotropic and anisotropic 2D materials. (2) Probing the twist-angle-dependence on interlayer interaction at twisted-bilayer-MoS2 (TBLM). Here we put forward a new experimental configuration based on nanoindentation to probe the interlayer interaction at 2D materials interfaces, and a shear boundary mechanical model is established correspondingly. Compared with the traditional fixed boundary model, the shear boundary model has shown greater adaptation regardless of the materials and interfaces. We applied our experimental configuration to probe the interlayer interaction of TBLM, and discovered that the shear stress at MoS2 interfaces is twist-angle-insensitive. This conclusion is further proved by our molecular dynamic (MD) simulation. The fact that the interlayer interaction is independent on the twist angle provides good indications for the studies of vertical van der Waals (vdWs) heterostructures. (3) The weak interlayer coupling and abnormal Raman behavior in CTAB intercalated MoS2. In the CTAB-MoS2 superlattice, we have discovered abnormal Raman behavior: the A1g mode that reflects the out-of-plane vibration is strongly suppressed and red-shifted. The existence of organic intercalation has weakened the interlayer interaction of MoS2 layers, making the bulk CTAB-MoS2 behavior like a monolayer MoS2. In-depth analysis has indicated that the interaction between CTAB and MoS2 has restrained the A1g vibrational mode, while the strong electron doping has triggered the red-shift of this peak. It is a promising idea to tailor the interlayer interaction and induce strong doping with intercalation approaches, which may find its applications in next generation flexible electronics.
