Liquid-phase deposition of exfoliated 2D nanosheets is the basis for emerging technologies that include writable electronic inks, molecular barriers, selective membranes, and protective coatings against fouling or corrosion. These nanosheet thin films have complex internal structures that are discontinuous assemblies of irregularly tiled micron-scale sheets held together by van der Waals (vdW) forces. On stiff substrates, nanosheet vdW films are stable to many common stresses, but can fail by internal delamination under shear stress associated with handling or abrasion. This "re-exfoliation" pathway is an intrinsic feature of stacked vdW films and can limit nanosheet-based technologies. Here we investigate the shear stability of graphene oxide and MoSe2 nanosheet vdW films through lap shear experiments on polymer-nanosheet-polymer laminates. These sandwich laminate structures fail in mixed cohesive and interfacial mode with critical shear forces from 40 - 140 kPa and fracture energies ranging from 0.2 - 6 J/m2. Surprisingly these energies are higher than delamination energies reported for smooth peeling of ordered stacks of continuous 2D sheets, which we propose is due to energy dissipation and chaotic crack motion during nanosheet film disassembly at the crack tip. Experiment results also show that film thickness plays a key role in determining critical shear force (maximum load before failure) and dissipated energy for different nanosheet vdW films. Using a mechanical model with an edge crack in the thin nanosheet film, we propose a shear-to-tensile failure mode transition to explain a maximum in critical shear force for graphene oxide films but not MoSe2 films. This transition reflects a weakening of the substrate confinement effect and increasing rotational deformation near the film edge as the film thickness increases. For graphene oxide, the critical shear force can be increased by electrostatic cross-linking achieved through interlayer incorporation of metal cations. These results have important implications for the stability of functional devices that employ 2D nanosheet coatings.

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支撑二维纳米片范德华薄膜的剪切破坏

剥离二维纳米片的液相沉积是新兴技术的基础，包括可写电子墨水、分子屏障、选择性膜和防止污染或腐蚀的保护涂层。这些纳米片薄膜具有复杂的内部结构，它们是由范德华力 (vdW) 保持在一起的不规则平铺的微米级片材的不连续组件。在坚硬的基板上，纳米片 vdW 薄膜对许多常见应力是稳定的，但在与处理或磨损相关的剪切应力下会因内部分层而失效。这种“再剥离”途径是堆叠 vdW 薄膜的固有特征，可能会限制基于纳米片的技术。在这里，我们通过聚合物-纳米片-聚合物层压板的搭接剪切实验研究了氧化石墨烯和 MoSe2 纳米片 vdW 薄膜的剪切稳定性。这些夹层层压结构在混合粘性和界面模式下失效，临界剪切力为 40 - 140 kPa，断裂能范围为 0.2 - 6 J/m2。令人惊讶的是，这些能量高于报道的用于连续二维片材有序堆叠平滑剥离的分层能量，我们认为这是由于裂纹尖端处纳米片膜拆卸过程中的能量耗散和无序裂纹运动。实验结果还表明，薄膜厚度在确定不同纳米片 vdW 薄膜的临界剪切力（失效前的最大载荷）和耗散能量方面起着关键作用。使用在薄纳米片薄膜中具有边缘裂纹的机械模型，我们提出了一种剪切-拉伸失效模式转变来解释氧化石墨烯薄膜的临界剪切力的最大值，而不是 MoSe2 薄膜。这种转变反映了基材限制效应的减弱以及随着薄膜厚度的增加，薄膜边缘附近的旋转变形增加。对于氧化石墨烯，临界剪切力可以通过通过层间掺入金属阳离子实现的静电交联来增加。这些结果对采用二维纳米片涂层的功能器件的稳定性具有重要意义。这种转变反映了基材限制效应的减弱以及随着薄膜厚度的增加，薄膜边缘附近的旋转变形增加。对于氧化石墨烯，临界剪切力可以通过通过层间掺入金属阳离子实现的静电交联来增加。这些结果对采用二维纳米片涂层的功能器件的稳定性具有重要意义。这种转变反映了基材限制效应的减弱以及随着薄膜厚度的增加，薄膜边缘附近的旋转变形增加。对于氧化石墨烯，临界剪切力可以通过通过层间掺入金属阳离子实现的静电交联来增加。这些结果对采用二维纳米片涂层的功能器件的稳定性具有重要意义。