**Author details**

zone is heated up to *T*<sup>2</sup> = 1000°C for typically 10∼90 min under 10 sccm hydrogen atmosphere at a pressure of 350 mTorr after mounting the copper foil. Copper foil (25 μm, Alfa Aesar) is used as the metal catalyst substrate. Before the growth of h-BN, copper foil is annealed at 1000°C for 20 min under 10 sccm H2 to grow the copper grain and to obtain a smooth surface. AFM of as-obtained sample provides direct evidence for single-layer h-BN synthesized by chemical vapor deposition method (**Figure 10b**). AFM image and the corresponding height distribution inset in **Figure 10b** show that the measured height is about 0.42 nm, consistent with monolayer thickness (c-axis spacing for h-BN is ∼0.32 nm). In addition, TEM image of monolayer h-BN clearly demonstrates that the number of layer of h-BN is one (**Figure 10c**). Moreover, for a complete h-BN layer (**Figure 10d**), the wrinkles of h-BN film (indicated by a yellow arrow) under scanning electron microscopy (SEM) can be clearly seen. These wrinkles are characteristic of h-BN and graphene films due to the negative thermal expansion coeffi‐ cients of h-BN and graphene. All of the results undoubtedly confirm that h-BN with monolayer

Synthesis of transition metal dichalcogenides (TMDs) using CVD is the cutting edge research area in recent years. Lee [46] synthesized single-layer MoS2 by this method recently. As shown in **Figure 10c**, ultrathin MoS2 nanosheets with smooth surface are observed with AFM. The cross-sectional height in inset of **Figure 10c** reveals that the thickness of MoS2 film is ∼0.72 nm,

With the advantages of very high surface-to-volume ratios and special electronic properties in two-dimensional materials, ultrathin nanosheets are very promising in catalysis, supercapa‐ citors, photoconductive materials, batteries and magneto-optical components. Therefore, it is imperative to exploit more technologically advanced synthesis strategies to obtain high quality, large size, ultrathin two-dimensional materials. At present, thinner, larger size, high productive and quality nanosheets are still the chief purpose for scientists. Usually, the thinner nanosheets give a higher catalytic performance and other special properties. Besides these, controllable thickness, architectures assembled by nanosheets and composite nanostructure, such as quantum-dot/nanosheets, sandwich structure and modification on two-dimensional

In summary, synthetic strategies, such as top-down and bottom-up method, have been employed for synthesizing two-dimensional crystals. The top-down strategy includes micro‐ mechanical exfoliation, ultrasonic exfoliation, lithium-intercalated and exfoliation and ionchange exfoliation. Using top-down strategies, one is able to obtain high-quality and large-size two-dimensional crystal conveniently. Even so, this method is only appropriate for those materials whose bulk crystals are layered. Comparatively, bottom-up strategy could overcome this shortage. Bottom-up strategy is composed of wet chemical method, microwave-assisted chemical, topological conversion strategy and chemical vapor deposition method. All these strategies may present different features and questions for preparing two-dimensional

nanosheets, also provide new opportunities for material science.

thickness have been successfully synthesized.

16 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

which corresponds to a monolayer MoS2.

**4. Perspective and conclusions**

Jianghao Wang1 , Guangshe Li2 and Liping Li1\*

\*Address all correspondence to: Lipingli@fjirsm.ac.cn

1 Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Fuzhou, P.R. China

2 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chem‐ istry, Jilin University, Changchun, P.R. China
