Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712.
Department of Aerospace Engineering and Engineering Mechanics, Center for Mechanics of Solids, Structures and Materials, The University of Texas at Austin, Austin, TX 78712.
Proc Natl Acad Sci U S A. 2018 Jul 31;115(31):7884-7889. doi: 10.1073/pnas.1801551115. Epub 2018 Jul 13.
Layered systems of 2D crystals and heterostructures are widely explored for new physics and devices. In many cases, monolayer or few-layer 2D crystals are transferred to a target substrate including other 2D crystals, and nanometer-scale blisters form spontaneously between the 2D crystal and its substrate. Such nanoblisters are often recognized as an indicator of good adhesion, but there is no consensus on the contents inside the blisters. While gas-filled blisters have been modeled and measured by bulge tests, applying such models to spontaneously formed nanoblisters yielded unrealistically low adhesion energy values between the 2D crystal and its substrate. Typically, gas-filled blisters are fully deflated within hours or days. In contrast, we found that the height of the spontaneously formed nanoblisters dropped only by 20-30% after 3 mo, indicating that probably liquid instead of gas is trapped in them. We therefore developed a simple scaling law and a rigorous theoretical model for liquid-filled nanoblisters, which predicts that the interfacial work of adhesion is related to the fourth power of the aspect ratio of the nanoblister and depends on the surface tension of the liquid. Our model was verified by molecular dynamics simulations, and the adhesion energy values obtained for the measured nanoblisters are in good agreement with those reported in the literature. This model can be applied to estimate the pressure inside the nanoblisters and the work of adhesion for a variety of 2D interfaces, which provides important implications for the fabrication and deformability of 2D heterostructures and devices.
二维晶体和异质结构的层状系统被广泛探索用于新物理和器件。在许多情况下，单层或少数层二维晶体被转移到目标衬底上，包括其他二维晶体，并且二维晶体与其衬底之间会自发形成纳米级的气泡。这种纳米气泡通常被认为是良好附着力的指标，但对于气泡内的内容物还没有共识。虽然充气气泡已经通过鼓胀试验进行了建模和测量，但将这些模型应用于自发形成的纳米气泡会导致二维晶体与其衬底之间的附着力能值低得不切实际。通常，充气气泡会在数小时或数天内完全瘪掉。相比之下，我们发现自发形成的纳米气泡的高度仅在 3 个月后下降了 20-30%，这表明可能是液体而不是气体被困在其中。因此，我们开发了一种用于填充液体的纳米气泡的简单比例定律和严格理论模型，该模型表明界面粘附功与纳米气泡纵横比的四次方成正比，并取决于液体的表面张力。我们的模型通过分子动力学模拟得到了验证，并且为测量的纳米气泡获得的粘附能值与文献中报道的值非常吻合。该模型可用于估计纳米气泡内的压力和各种二维界面的附着力，这对于二维异质结构和器件的制造和可变形性具有重要意义。
