**1. Introduction**

The discovery of single-layer graphene in 2004 by Novoselov and Geim has shown that it is highly possible to exfoliate stable, single-atom or single-polyhedral-thick two-dimensional (2D) materials from van der Waals solids, and these 2D materials could exhibit unique and fascinat‐ ed physical properties, such as ultrahigh carrier mobility at room temperature (∼10,000 cm<sup>2</sup> V −1s−1), quantum hall effect, large theoretical specific surface area (2630 m2 g−1), excellent optical transparency (∼97.7%) and so on [1, 2]. This great discovery helps Novoselov and Geim win Nobel Prize in 2010. The success of graphene arouses intensive interests in 2D materials in the

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world. As a consequence, more and more 2D materials have been synthesized successfully, showing great promise for in many applications.

Ideal 2D materials belong to those with only one atom or several atoms thickness and infinite lateral size. The reliable synthesis of single- and few-layer 2D materials is an essential first step for characterizing the layer-dependent changes in their properties, as well as for providing pathways of their integration into a multitude of applications [1]. For instances, when the thickness of metal Co was reduced to only one or several atoms, the catalytic activity of carbon dioxide reduction would be improved greatly when comparing to bulk metal Co [3]. Singlelayer MoS2 has been synthesized by mechanical exfoliation strategy and exhibits excellent performance in the field of gas sensors and phototransistors. It is worth noting that when bulk MoS2 is exfoliated into monolayer, the type of band gap would change from indirect type to direct one [1, 4]. Another important example is that single-layered Co(OH)2 can realize an ultrahigh energy density in all-solid-state asymmetric supercapacitor [5].

The emergence of these novel properties is the driving force for the rapid development of research in ultrathin 2D nanosheets, which has been on the forefront of scientific disciplines including chemistry, physics, materials, science, medicine and biology. However, the quest for methods of producing 2D materials with controlled thickness and lateral size has been always a challenging subject. This may be caused by the anisotropic crystal growth and strong chemical bonds in crystal structure. The common classification of crystalline structures according to the type of chemical bonds could be divided into van der Waals solids, layered ionic solids and nonlayered materials [1]. Every synthetic strategy has its own merits and demerits in preparing different kind of materials. Therefore, in this chapter, we concentrate on the different synthetic methods for synthesizing two-dimensional crystals. According to the principle of generating two-dimensional materials, we can divide the synthetic strategies into top-down and bottom-up strategies. The distinction between these two general classifi‐ cations is based on the processes involved in the creation of the nanometer-sized structures [6]. In the bottom-up approach, nanoscale materials are constructed from atomic or molecular precursors that are allowed to react and grow in size or self-assemble into more complex structures. By contrast, the top-down approach carves nanoscale structures by controlled removal of materials from larger or bulk solids.
