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Register a new userVan der Waals interaction affects wrinkle formation in two-dimensional materials
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Nonlinear mechanics of solids is an exciting field that encompasses both beautiful mathematics, such as the emergence of instabilities and the formation of complex patterns, as well as multiple applications. Two-dimensional crystals and van der Waals (vdW) heterostructures allow revisiting this field on the atomic level, allowing much finer control over the parameters and offering atomistic interpretation of experimental observations. In this work, we consider the formation of instabilities consisting of radially-oriented wrinkles around mono- and few-layer bubbles in two-dimensional vdW heterostructures. Interestingly, the shape and wavelength of the wrinkles depend not only on the thickness of the two-dimensional crystal forming the bubble, but also on the atomistic structure of the interface between the bubble and the substrate, which can be controlled by their relative orientation. We argue that the periodic nature of these patterns emanates from an energetic balance between the resistance of the top membrane to bending, which favors large wavelength of wrinkles, and the membrane-substrate vdW attraction, which favors small wrinkle amplitude. Employing the classical Winkler foundation model of elasticity theory, we show that the number of radial wrinkles conveys a valuable relationship between the bending rigidity of the top membrane and the strength of the vdW interaction. Armed with this relationship, we use our data to demonstrate a nontrivial dependence of the bending rigidity on the number of layers in the top membrane, which shows two different regimes driven by slippage between the layers, and a high sensitivity of the vdW force to the alignment between the substrate and the membrane.
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Even if individual two-dimensional materials own various interesting and unexpected properties, the stacking of such layers leads to van der Waals solids which unite the characteristics of two dimensions with novel features originating from the interlayer interactions. In this topical review, we cover fabrication and characterization of van der Waals heterosructures with a focus on heterobilayers made of monolayers of semiconducting transition metal dichalcogenides. Experimental and theoretical techniques to investigate those heterobilayers are introduced. Most recent findings focusing on different transition metal dichalcogenides heterostructures are presented and possible optical transitions between different valleys, appearance of moire patterns and signatures of moire excitons are discussed. The fascinating and fast growing research on van der Waals hetero-bilayers provide promising insights required for their application as emerging quantum-nano materials.
Two-dimensional (2D) materials exhibit a number of improved mechanical, optical, electronic properties compared to their bulk counterparts. The absence of dangling bonds in the cleaved surfaces of these materials allows combining different 2D materials into van der Waals heterostructures to fabricate p-n junctions, photodetectors, 2D-2D ohmic contacts that show unexpected performances. These intriguing results are regularly summarized in comprehensive reviews. A strategy to tailor their properties even further and to observe novel quantum phenomena consists in the fabrication of superlattices whose unit cell is formed either by two dissimilar 2D materials or by a 2D material subjected to a periodical perturbation, each component contributing with different characteristics. Furthermore, in a 2D materials-based superlattice, the interlayer interaction between the layers mediated by van der Waals forces constitutes a key parameter to tune the global properties of the superlattice. The above-mentioned factors reflect the potential to devise countless combinations of van der Waals 2D materials based superlattices. In the present feature article, we explain in detail the state-of-the-art of 2D materials-based superlattices and we describe the different methods to fabricate them, classified as vertical stacking, intercalation with atoms or molecules, moire patterning, strain engineering and lithographic design. We also aim to highlight some of the specific applications for each type of superlattices.
The van der Waals (vdW) force is a ubiquitous short-range interaction between atoms and molecules that underlies many fundamental phenomena. Early pairwise additive theories pioneered by Keesom, Debye, and London suggested the force to be monotonically attractive for separations larger than the vdW contact distance. However, seminal work by Lifshitz et al. predicted that quantum fluctuations can change the sign of vdW interactions from attractive to repulsive. Although recent experiments carried out in fluid environment have demonstrated the long-range counterpart the Casimir repulsion, it remains controversial whether the vdW repulsion exists, or is sufficiently strong to alter solid-state properties. Here we show that the atomic thickness and birefringent nature of two-dimensional (2D) materials, arising from their anisotropic dielectric responses, make them a versatile medium to tailor the many-body Lifshitz-vdW interactions at solid-state interfaces. Based on our theoretical prediction, we experimentally examine two heterointerface systems in which the vdW repulsion becomes comparable to the two-body attraction. We demonstrate that the in-plane movement of gold atoms on a sheet of freestanding graphene becomes nearly frictionless at room temperature. Repulsion between molecular solid and gold across graphene results in a new polymorph with enlarged out-of-plane lattice spacings. The possibility of creating repulsive energy barriers in nanoscale proximity to an uncharged solid surface offers technological opportunities such as single-molecule actuation and atomic assembly.
The synthesis of one-dimensional van der Waals heterostructures was realized recently, which opens up new possibilities for prospective applications in electronics and optoelectronics. The even reduced dimension will enable novel properties and further miniaturization beyond the capabilities of its two-dimensional counterparts have revealed. The natural doping results in p-type electrical characteristics for semiconducting single-walled carbon nanotubes, while n-type for molybdenum disulfide with conventional noble metal contacts. Therefore, we demonstrate here a one-dimensional heterostructure nanotube of 11-nm-wide, with the coaxial assembly of semiconducting single-walled carbon nanotube, insulating boron nitride nanotube, and semiconducting molybdenum disulfide nanotube which induces a radial semiconductor-insulator-semiconductor heterojunction. When opposite potential polarity was applied on semiconducting single-walled carbon nanotube and molybdenum disulfide nanotube, respectively, the rectifying effect was materialized.
We study the interaction energy between two graphene nanoribbons by first principles calculations, including van der Waals interactions and spin polarization. For ultranarrow zigzag nanoribbons, the direct stacking is even more stable than Bernal, competing in energy for wider ribbons. This behavior is due to the magnetic interaction between edge states. We relate the reduction of the magnetization in zigzag nanoribbons with increasing ribbon width to the structural changes produced by the magnetic interaction, and show that when deposited on a substrate, zigzag bilayer ribbons remain magnetic for larger widths.
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