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Mechanics of spontaneously formed nanoblisters trapped by transferred 2D crystals

Zhaohe Dai's picture

Mechanics of Spontaneously Formed Nanoblisters Trapped by Transferred 2D Crystals. Proceedings of the National Academy of Sciences 2018 - 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.

 

Figure 1: Interfacial blisters between 2D crystals and their supporting substrates. (A) Tapping-mode AFM reveals the complex distribution of HOPG-SiO2 blisters. (Inset) Bright-field optical micrograph where the orange dashed region corresponds with the large AFM image. (White scale bar: 10 μm.) The red, blue, and black dots indicate where Raman measurements were taken. The color bar represents 0–17 nm. (B) A closer look at two monolayer regions from the red dashed region of Fig. 1A. Blisters close to the edges of the graphene are distorted from the typical circular shape. The color bar represents 0–13 nm. (C) By extracting the height profile of each blister, the height and radius is calculated by curve fitting a parabolic function. (D) Blisters for a specific interface show a consistent aspect ratio that is independent of volume.

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LIU WANG's picture

Great Work!

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