2D materials and van der Waals heterostructures
TLDR
Two-dimensional heterostructures with extended range of functionalities yields a range of possible applications, and spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system.Abstract:
BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III-V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two-dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One-atom-thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero-overlap semimetal. The family of 2D crystals has grown to includes metals (e.g., NbSe 2 ), semiconductors (e.g., MoS 2 ), and insulators [e.g., hexagonal boron nitride (hBN)]. Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long-range order, raises many thought-provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together), such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first-order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest-mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light-emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large-area crystals grown by chemical vapor deposition (CVD), direct growth of heterostructures by CVD or physical epitaxy, or one-step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable.read more
Citations
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Mechanical properties of graphene and graphene-based nanocomposites
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Huanyu Jin,Chunxian Guo,Xin Liu,Jinlong Liu,Anthony Vasileff,Yan Jiao,Yao Zheng,Shi-Zhang Qiao +7 more
TL;DR: The fundamental relationships between electronic structure, adsorption energy, and apparent activity for a wide variety of 2D electrocatalysts are described with the goal of providing a better understanding of these emerging nanomaterials at the atomic level.
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Strong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substrates.
Manuel Bonilla,Sadhu Kolekar,Yujing Ma,Horacio Coy Diaz,Vijaysankar Kalappattil,Raja Das,Tatiana Eggers,Humberto R. Gutierrez,Manh-Huong Phan,Matthias Batzill +9 more
TL;DR: Reducing the dimensionality of paramagnetic V Se2 results in the emergence of ferromagnetism that is observed in a monolayer and up to room temperature, making VSe2 an attractive material for van der Waals spintronics applications.
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Raman spectroscopy of graphene-based materials and its applications in related devices.
TL;DR: The essential Raman scattering processes of the entire first- and second-order modes in intrinsic graphene are described and the extensive capabilities of Raman spectroscopy for the investigation of the fundamental properties of graphene under external perturbations are described.
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Approaching the Schottky-Mott limit in van der Waals metal-semiconductor junctions.
Yuan Liu,Yuan Liu,Jian Guo,Enbo Zhu,Lei Liao,Sung-Joon Lee,Mengning Ding,Imran Shakir,Vincent Gambin,Yu Huang,Xiangfeng Duan +10 more
TL;DR: The creation of van der Waals metal–semiconductor junctions is reported, in which atomically flat metal thin films are laminated onto two-dimensional semiconductors without direct chemical bonding, creating an interface that is essentially free from chemical disorder and Fermi-level pinning.
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