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.

2D materials and van der Waals heterostructures

We present a method for synthesizing MoS2/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports,(1, 2) a much lower growth temperature of 400 °C is required for this procedure. The chemical vapor deposition of MoS2 on the graphene surface gives rise to single crystalline hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temperature, which was then followed by post annealing. At an elevated temperature, the precursor self-assembles to form MoS2 flakes epitaxially on the graphene surface via thermal decomposition. With higher amount of precursor delivered onto the graphene surface, a continuous MoS2 film on graphene can be obtained. This simple chemical vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface funct...

https://www.phys.sinica.edu.tw/TIGP-NANO/Course/2013_Spring/ClassNote/MoS2.pdf

van der Waals Epitaxy of MoS2 Layers Using Graphene As Growth Templates

Control of preferred orientation for ZnOx films: control of self-texture

Van der Waals epitaxy—a new epitaxial growth method for a highly lattice-mismatched system

This study reports the design and fabrication of ultrathin MoS2 nanosheets@metal organic framework-derived N-doped carbon nanowall array hybrids on flexible carbon cloth (CC@CN@MoS2) as a free-standing anode for high-performance sodium ion batteries. When evaluated as an anode for sodium ion battery, the as-fabricated CC@CN@MoS2 electrode exhibits a high capacity (653.9 mA h g−1 of the second cycle and 619.2 mA h g−1 after 100 cycles at 200 mA g−1), excellent rate capability, and long cycling life stability (265 mA h g−1 at 1 A g−1 after 1000 cycles). The excellent electrochemical performance can be attributed to the unique 2D hybrid structures, in which the ultrathin MoS2 nanosheets with expanded interlayers can provide shortened ion diffusion paths and favorable Na+ insertion/extraction space, and the porous N-doped carbon nanowall arrays on flexible carbon cloth are able to improve the conductivity and maintain the structural integrity. Moreover, the N-doping-induced defects also make them favorable for the effective storage of sodium ions, which enables the enhanced capacity and rate performance of MoS2.

Ultrathin MoS2 Nanosheets@Metal Organic Framework-Derived N-Doped Carbon Nanowall Arrays as Sodium Ion Battery Anode with Superior Cycling Life and Rate Capability

Fabrication and characterization of heterostructures with subnanometer thickness

Summary Abstract: Fabrication of ultrathin heterostructures with van der Waals epitaxy

A good quality of monolayer NbSe2 film has been successfully grown on a cleaved face of 2H-MoS2 by molecular beam epitaxy. This has become possible because of van der Waals growth nature in the present system. The electronic structure of the prepared film has been measured by low-energy electron energy loss spectroscopy. Its electronic structure has been found to be very close to that of bulk NbSe2, which indicates that the electronic structure of a layered material is predominantly determined by intralayer interactions.

K. Sunouchi

Papers

Fabrication and characterization of heterostructures with subnanometer thickness

Summary Abstract: Fabrication of ultrathin heterostructures with van der Waals epitaxy

Electronic Structure of a Monolayer NbSe2 Film Grown Heteroepitaxially on the Cleaved Face of 2H-MoS2

Heteroepitaxy onto Surfaces with No Dangling Bonds -Heteroepitaxy of Selenium on Cleaved Faces of Tellurium-