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Register a new userTwo-dimensional covalent crystals by chemical conversion of thin van der Waals materials
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Added by
Rahul Raveendran Nair
Publication date
2019
fields
Physics
and research's language is
English
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Most of the studied two-dimensional (2D) materials have been obtained by exfoliation of van der Waals crystals. Recently, there has been growing interest in fabricating synthetic 2D crystals which have no layered bulk analogues. These efforts have been focused mainly on the surface growth of molecules in high vacuum. Here, we report an approach to making 2D crystals of covalent solids by chemical conversion of van der Waals layers. As an example, we use 2D indium selenide (InSe) obtained by exfoliation and converted it by direct fluorination into indium fluoride (InF3), which has a non-layered, rhombohedral structure and therefore cannot be possibly obtained by exfoliation. The conversion of InSe into InF3 is found to be feasible for thicknesses down to three layers of InSe, and the obtained stable InF3 layers are doped with selenium. We study this new 2D material by optical, electron transport and Raman measurements and show that it is a semiconductor with a direct bandgap of 2.2 eV, exhibiting high optical transparency across the visible and infrared spectral ranges. We also demonstrate the scalability of our approach by chemical conversion of large-area, thin InSe laminates obtained by liquid exfoliation into InF3 films. The concept of chemical conversion of cleavable thin van der Waals crystals into covalently-bonded non-cleavable ones opens exciting prospects for synthesizing a wide variety of novel atomically thin covalent crystals.
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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.
Three-dimensional epitaxial heterostructures are based on covalently-bonded interfaces, whereas those from 2-dimensional (2D) materials exhibit van der Waals interactions. Under the right conditions, however, material structures with mixed interfacial van der Waals and covalent bonding may be realized. Atomically thin layers formed at the epitaxial graphene (EG)/silicon carbide (SiC) interface indicate that EG/SiC interfaces provide this unique environment and enable synthesis of a rich palette of 2D materials not accessible with traditional techniques. Here, we demonstrate a method termed confinement heteroepitaxy (CHet), to realize air-stable, structurally unique, crystalline 2D-Ga, In, and Sn at the EG/SiC interface. The first intercalant layer is covalently-bonded to the SiC, and is accompanied by a vertical bonding gradient that ends with van der Waals interactions. Such structures break out of plane centrosymmetry, thereby introducing atomically thin, non-centrosymmetric 2D allotropes of 3D materials as a foundation for tunable superconductivity, topological states, and plasmonic properties.
The exfoliation of two naturally occurring van der Waals minerals, graphite and molybdenite, arouse an unprecedented level of interest by the scientific community and shaped a whole new field of research: 2D materials research. Several years later, the family of van der Waals materials that can be exfoliated to isolate 2D materials keeps growing, but most of them are synthetic. Interestingly, in nature plenty of naturally occurring van der Waals minerals can be found with a wide range of chemical compositions and crystal structures whose properties are mostly unexplored so far. This Perspective aims to provide an overview of different families of van der Waals minerals to stimulate their exploration in the 2D limit.
The recently emerged organohalide perovskites (e.g., CH3NH3PbI3) have drawn intense attention for high efficiency solar cells. However, with a considerable solubility in many solvents, these perovskites are not typically compatible with conventional lithography processes for more complicated device fabrications that are important for both fundamental studies and technological applications. Here we report the creation of novel heterojunction devices based on perovskites and two-dimensional (2D) crystals by taking advantage of the layered characteristic of lead iodide (PbI2) and vapor phase intercalation. We show a graphene/perovskite/graphene vertical stack can deliver a highest photoresponsivity of ~950 A/W and photoconductive gain of ~2200, and a graphene/WSe2/perovskite/graphene heterojunction can display a high on/off ratio (~10^6) transistor behavior with distinct gate-tunable diode characteristics and open-circuit voltages. Such unique perovskite-2D heterostructures have significant potential for future optoelectronic research and can enable broad possibilities with compositional tunability of organohalide perovskites and the versatility offered by diverse 2D materials.
In recent years, noncollinear topological textures have long gained increasing research attentions for their high values of both fundamental researches and potential applications. The recent discovery of intrinsic orders in magnetic and polar two-dimensional van der Waals materials provides a new ideal platform for the investigation of noncollinear topological textures. Here, we review the theoretical and experimental progresses on noncollinear topological textures in two-dimensional van der Waals materials in very recent years. During these years, magnetic skyrmions of both Bloch and Neel types have been observed experimentally in a few two-dimensional van der Waals materials and related heterostructures. Concurrently, more theoretic predictions basing on various mechanisms have been reported about different noncollinear topological textures in two-dimensional van der Waals materials, such as skyrmions, bimerons, anti-biskyrmions and skyrmionium, which are still waiting to be confirmed in experiments. Besides, noncollinear topological electric dipole orders have also been predicted in two-dimensional van der Waals materials. Taking advantage of the intrinsic two-dimensional nature and high integratability, the two-dimensional van der Waals materials will play an important role in the investigation on noncollinear topological textures in both magnetic and polar systems.
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