Researchers from China's Peking University, Nanjing University of Aeronautics and Astronautics and Tsinghua University have gained new insights into the van der Waals (vdW) transparency of graphene, a property that determines how it mediates molecular interactions at the nanoscale.
Understanding how atomically thin materials transmit or screen intermolecular forces, particularly vdW interactions, is crucial for developing nanofluidic and microelectromechanical systems. Although previous experiments have investigated this phenomenon, results have been contradictory, leaving uncertainty over whether materials like graphene behave as transparent transmitters or as barriers to vdW forces. To address this, the research team used colloidal atomic force microscopy (AFM), a precision technique that measures adhesion and attractive forces through a microsphere-tipped cantilever probe. Their experiments, performed under ultra-dry conditions, compared graphene supported on silicon dioxide (SiO₂) substrates with graphene suspended across microcavities. Two complementary measurement modes - pull-off and pull-in tests - enabled them to quantify the extent to which substrate forces pass through or are screened by graphene layers.
The data revealed that vdW forces cannot be simply calculated as the sum of contributions from graphene and its substrate. For suspended graphene, the team observed that its effective surface energy can exceed that of graphene supported on a substrate. Moreover, pull-in measurements showed that graphene films one to five atomic layers thick screen approximately 15-50 percent of the substrate’s intrinsic vdW interaction. These findings match theoretical predictions from Lifshitz theory, suggesting that graphene only partially transmits vdW forces.
According to senior author Zhaohe Dai, the transparency of graphene to vdW interactions depends strongly on both the separation between surfaces and the thickness of the graphene film. For example, when two surfaces are separated by about five nanometers, a single graphene layer still allows roughly 85 percent of the substrate’s vdW force to pass through.
Beyond resolving long-standing inconsistencies in experimental results, the study provides a consistent framework for understanding how atomic-scale coatings influence surface energy and adhesion. This could inform future engineering of nanostructures and devices that rely on precisely tuned interfacial properties.
The team plans to extend this understanding toward the design of silica-based surfaces with adjustable adhesion, using atomic-layer coatings to improve compatibility and performance in electronic and photonic systems.
