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Two-dimensional (2D) materials, which are only a few atoms thick, are known to exhibit unique electrical, mechanical and optical properties, which differ considerably from the properties of bulk materials. Some recent studies have also been probing these materials' "transparency" to intermolecular interactions, such as van der Waals (vdW) forces—weak forces arising from fluctuating electrical charges, which prompt the attraction between molecules or surfaces.

Determining the extent to which these forces are screened by atomically thin materials could have important implications for the development of various technologies based on 2D materials.

Researchers at Peking University, Nanjing University of Aeronautics and Astronautics and Tsinghua University recently set out to shed light on whether 2D graphene systems fully transmit, partially screen or block vdW interactions.

Their findings, published in , show that graphene layers in 2D systems screen 15–50% of vdW forces, depending on their thickness, thus providing partial transparency.

"Our study was inspired by a debate known as the 'wetting transparency' of graphene (i.e., ," Zhaohe Dai, senior author of the paper, told Â鶹ÒùÔº.

"Over the past decade, different groups have reported full transparency, partial transparency, or even opacity, leading to considerable confusion. The community has gradually realized that at the heart of this issue lies a deeper and more fundamental question: how does a single-atomic-layer coating modify the surface energy of the underlying substrate?"

Wetting experiments, which probe the interactions of liquids with a solid surface, have proved to be ineffective for directly probing the effects of atomic layer coatings on the surface energy of substrates, due to the complex interactions between liquids and solids. A better approach to probe these effects relies on the direct measurement of vdW forces using well-established classical probes.

As part of their recent study, Dai and his colleagues specifically set out to determine how a thin graphene coating influenced the vdW interactions of an underlying solid substrate. To do this, they used a technique known as colloidal atomic force microscopy (AFM).

"In other words, we set out to clarify how graphene screens—or leaks—the vdW forces of the substrate it covers, thereby quantifying the transparency of graphene to vdW interactions," said Dai.

"To do this, we used an AFM setup. The idea is straightforward: we attached a micrometer-sized silica sphere to the end of a cantilever, creating a calibrated probe whose geometry and stiffness were precisely known."

The researchers brought this probe close to the test surface in conditions of ultra-low humidity (below 10%), which prevented water molecules from disrupting the measurements. They looked at two different samples: one consisting of graphene supported on a silica (SiO₂) substrate and the other of graphene suspended across small circular cavities.

"Using these samples, we performed two complementary types of measurements," explained Dai. "The first are pull-off tests, in which we measured the force required to detach the sphere from the surface.

"By comparing suspended and supported graphene, we could identify how much the underlying SiO₂ substrate contributed to the total adhesion. The second are pull-in tests, in which we monitored how the attractive force between the sphere and the surface increased as the gap between them decreased."

The measurements collected by the researchers yielded consistent results, showing that graphene transmits substrate forces. Yet the pull-in tests yielded the most reliable and quantitative data, which could be used to extract a transparency factor. This is a value indicating how much of the substrate's vdW interaction remains "visible" through a graphene film consisting of N atomic layers.

"The most important finding is that the transparency of graphene to vdW interaction is not a fixed number, but it rather depends on how far the two interacting surfaces are and how thick the graphene film is," said Dai. "For example, when a substrate coated with a single layer of graphene interacts with another surface across a gap of about 5 nanometers, the graphene still 'leaks' roughly 85% of the substrate's vdW force."

This study by Dai and his colleagues could have various implications for future research and development. Firstly, it introduces a consistent theoretical and experimental framework for studying vdW forces in 2D materials. In addition, it highlights the potential of atomic-layer coatings for tuning surface properties, such as adhesion.

The team's methodology and the data they collected could pave the way for further experiments probing the screening capabilities of 2D materials. Ultimately, their work could also inform the design of nanoscale devices that leverage adhesion and the transmission of vdW forces.

"With a quantitative understanding of van der Waals transparency now established, we plan to design silica surfaces (widely used in electronic and photonic devices) with tailored surface properties through atomic-scale coatings," added Dai.

"In particular, we aim to explore how such coatings can tune the surface's affinity to other components, such as functional layers or electrical contacts, thereby improving adhesion, compatibility, and performance in integrated systems."

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