Experimental and theoretical determination of van der Waals forces between and within two-dimensional materials

Van der Waals (vdW) forces play a crucial role in matter, for example, they drive the formation of two-dimensional multilayered functional devices with novel electronic, magnetic, and optical properties. Noncovalent vdW forces are well understood when interactions involve small molecules or macroscopic bodies. The situation is much less obvious when at least one of the interacting objects has nanoscale size. In this project, we developed a novel force microscopy setup which allowed a direct experimental confirmation of strongly nonadditive van der Waals forces from graphenic nanostacks (and other 2D materials). Surprisingly, we observed nonadditivity of van der Waals interactions with a macroscopic tip at macroscopic (> 1 micrometer) separations from a nanostructured surface. This sets a new record in terms of force measurements and clearly demonstrates the importance of quantum-size effects in macroscopic systems.
The overall objectives of the project were:
• (O1) Deliver measurements of vdW forces in 2D samples. Present a clarification of interaction ranges, thickness-dependent scaling laws, comparison between samples, and between nano- and macro- measurements.
• (O2) Deliver experimental data on the binding energies between 2D materials.
• (O3) Plan theoretical models explaining scaling laws of vdW interaction at the nano- and macro-scale, and show their application for samples with different numbers, types, and quality of 2D materials.

Van der Waals (vdW) forces play a crucial role in matter, for example, they drive the formation of two-dimensional multilayered functional devices with novel electronic, magnetic, and optical properties. Noncovalent vdW forces are well understood when interactions involve small molecules or macroscopic bodies. The situation is much less obvious when at least one of the interacting objects has nanoscale size. Crucial questions remain about the strength and the distance scaling of vdW forces at the nanoscale. For example, vdW forces between graphene and solid surfaces lead to a surprisingly extended and strong μm-scale adhesive stress upon delamination. In contrast, a layer of graphene adsorbed on SiO2 exhibits a “Faraday cage” effect, effectively eliminating the vdW force between a sharp microscope tip and the SiO2 surface. In addition, theoretical models predict that wavelike charge density fluctuations are responsible for the emergence of nonadditive modifications of the power laws that govern vdW interactions at the nanoscale.
In this project, we developed a novel force microscopy setup which allows the alignment of the underlying surface with a macroscopic tipless Si cantilever. Using this setup, we could provide a direct experimental confirmation of strongly nonadditive van der Waals forces from graphenic nanostacks (and other 2D materials). Surprisingly, we observed nonadditivity of van der Waals interactions with a macroscopic tip at macroscopic (> 1 micrometer) separations from a nanostructured surface. This sets a new record in terms of force measurements and clearly demonstrates the importance of quantum-size effects in macroscopic systems.
There has been made significant progress in sharing the results of our research: i) We have submitted a manuscript to Nature, combining findings from WP1 and WP3. This manuscript, titled [Direct Measurement of Nonadditive van der Waals Forces From Graphene Nanostacks], can be accessed as a preprint at (https://www.researchsquare.com/article/rs-1155685/v1) and ii) we are preparing a detailed manuscript on our experimental setup's technical aspects, targeting respected journals in chemical physics or physical chemistry.

In this project, we set a new record in terms of force measurements and clearly demonstrates the importance of quantum-size effects in macroscopic systems. These results are currently under review in Nature. In the link provided herein (https://www.researchsquare.com/article/rs-1155685/v1) the preprint can be found.
Our research findings have significant socio-economic impact and broader societal implications. Specifically, our work opens doors to the development of experimental tools that can deepen our understanding of complex material interactions. This is particularly relevant due to the sensitivity of dielectric properties to material thickness and the presence of non-additive interactions in nanoparticles. The potential applications of our research extend beyond the laboratory and can have far-reaching effects on various areas: Advanced Materials Development: Our findings provide a foundation for the creation of new materials with tailored properties, which can be utilized in industries such as electronics, materials science, and manufacturing. This can lead to the development of more efficient and advanced technologies, contributing to economic growth. Nanotechnology Advancements: Understanding non-additive interactions in nanoparticles is crucial for the design and engineering of nanoscale materials and devices. This can drive innovation in fields like nanomedicine, nanoelectronics, and energy storage, potentially improving healthcare, electronics, and energy sustainability. Microscopic Robotics and Colloidal Particles: The research may find applications in the development of functional 2D colloidal micro-Janus particles and interacting microscopic robots. These advancements could revolutionize fields like robotics, automation, and healthcare by enabling the creation of highly specialized and controllable micro- and nanoscale systems. Microscopic Optical Devices: Our work also has implications for the self-assembly of microscopic optical devices driven by long-range van der Waals (vdW) and Casimir forces. This has the potential to lead to the creation of innovative optical technologies with applications in telecommunications, imaging, and sensing. In summary, our research not only deepens our scientific understanding but also has the potential to drive advancements in technology and industry, fostering economic growth and benefiting society as a whole.

Periodic Reporting for period 1 - vdWForcesIn2D (Experimental and theoretical determination of van der Waals forces between and within two-dimensional materials)

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