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Preparation of 2D Materials

Yue Tang and Hua Xu

Shaanxi Normal University, Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Xi'an, 710119, P. R. China

Two-dimensional (2D) materials, an emerging new class of nanomaterials with rich structures and remarkable properties, would bring many transformative technologies and applications [1]. Since the discovery of graphene for the first time in 2004, the 2D material family has expanded dramatically to include insulators (hexagonal boron nitride [h-BN]), semiconductors (most transition metal dichalcogenides [TMDCs], black phosphorus [BP], and tellurium [Te]), semi-metals (some TMDCs and graphene), metals (transition metal carbides and nitrides [MXenes]), superconductors (NbSe2), and topological insulators (Bi2Se3 and Bi2Te3) [2, 3]. The atomic thickness and dangling free surface of 2D materials, together with their superior optical, electrical, magnetism, thermal, and mechanical properties, endow them with great promise for applications in optical communication, electronics, optoelectronics, spintronics, memory, thermoelectric, and energy conversation and storage devices [4, 5].

As highlighted by the famous nanomaterial scientist Zhongfan Liu, "preparation determines the future" is an inexorable law for all materials. In the past decade, a series of preparation technologies have been developed to fabricate 2D materials for satisfying the requirements of their fundamental studies and various applications. In view of the layered structure of 2D materials, the primary preparation technologies can be divided into two major types: top-down and bottom-up approaches. In this chapter, we will introduce the recently developed preparation technologies for 2D materials, including two top-down approaches (mechanical exfoliation and liquid exfoliation) and one bottom-up approach (vapor phase growth). Here, we give more space to introduce single crystal growth, thickness control, and phase control in the vapor phase growth of 2D materials.

Figure 1.1 Mechanical exfoliation preparation of 2D materials. (a) Schematic diagram of mechanical cleavage process and optical micrograph of one of the graphene flakes on the SiO2/Si substrate with different thicknesses.

Source: Reproduced with permission from Yuan Huang et al. [7]/American Chemical Society.

(b) Schematic diagram of the Au-assisted mechanical exfoliation process and corresponding optical microscope (OM) images of obtained vdW-layered 2D materials.

Source: Yuan Huang et al. [8]/Springer Nature/CC BY 4.0.

1.1 Mechanical Exfoliation of 2D Materials

In 2004, Geim and Novosolov firstly prepared monolayer graphene by exfoliating bulk graphite using Scotch tape [6]. The schematic exfoliation method and the obtained 1-4-layer graphene are shown in Figure 1.1a [7]. Since then, mechanical cleavage, commonly referred to as the Scotch tape method and involving no chemical reactions, is considered to be the simplest and best approach to obtain large-area, high-quality 2D materials that retain their pristine structures and properties. Until 2010, Heinz's group firstly extended the mechanical cleavage method to prepare monolayer MoS2 and discovered the indirect to direct bandgap transition with the thickness changing from bulk to monolayer [9]. Then, A. Kis's group fabricated the field effect transistor based on the exfoliated monolayer MoS2 and achieved room-temperature current on/off ratios of 108 and ultralow standby power dissipation [10]. Later, mechanical cleavage techniques were widely used to produce dozens of 2D transition metal dichalcogenides (TMDCs: WS2, WSe2, MoSe2, ReS2, etc.) [11]. However, with the increasing requirements on thickness and domain size of 2D materials for device fabrication and property exploration, cleavage technology pursues efficient preparation of 2D materials with large areas and high quality. Hence, many optimized mechanical cleavage techniques have been developed in recent years. For example, Huang et al. developed a universal Au-assisted mechanical cleavage technique, as shown in Figure 1.1b [8]. Theoretical calculation indicates that Au and many 2D materials can form quasi-covalent bonds, which are larger than van der Waals (vdW) interactions but smaller than covalent bonds. In the experiment, a thin Au layer was firstly deposited onto a substrate covered with a thin Ti or Cr adhesion layer, and a freshly cleaved bulk crystal on tape was brought into contact with the Au layer. Then the adhesive tape was placed on the outward side of the crystal, and gentle pressure was applied to establish a good crystal/Au contact. Finally, peel off the tape to remove the major portion of the crystal, leaving monolayer or few-layer flakes on the Au surface. Using this approach, they obtained large-area monolayer flakes on the Au surface, including MoS2, FeSe, PtSe2, PtTe2, PdTe2, and CrSiTe3. Using the above exfoliation methods, a series of new 2D-layered materials have been successfully prepared, such as Fe3GeTe2, MnBi2Te4, CrOCl, NbSe2, and NbOCl2, [12-16] which exhibit superior ferromagnetic, superconduction and nonlinear optic properties. In a word, mechanical cleavage technology has been widely utilized to prepare 2D-layered materials for studying their fundamental properties.

However, the methods mentioned above are restricted to 2D-layered materials in which the interlayer interactions are dominated by weak vdW force, and thus these methods are not applicable to materials in which interlayer interactions are dominated by non-vdW force. In view of this, Zhang et al. developed a new mechanical cleavage strategy [17]. The polished metallic surface is oxidized under a controlled environment to enable the growth of hexagonal metal oxides (h-MO). These high-crystalline h-MO with layered structures, without ionic dopants or vacancies, can be easily exfoliated by stamping them onto the target substrates. This cleavage strategy was firstly applied to prepare the three-dimensional (3D) transition metal-based h-MO (TiO2, Fe2O3, and Ni2O3), and it could be readily extended to prepare a variety of other metal oxides for exploring their novel 2D quantum properties. Most recently, Fengxia Geng's group reported a calendaring pretreatment mechanically exfoliate approach to prepare 2D materials from non-vdW structures [18]. On the basis of the traditional scotch tape method, an external mechanical force was applied to the non-vdW materials. This approach involves laterally sliding in the closely packed neighboring layers to transform the structure from a stable to a metastable phase, weakening the interlayer binding. Using this approach, a variety of 2D materials have been prepared, including metals (Bi, Sb), semiconductors (SnO, V2O5, Bi2O2Se), and superconducting compounds (KV3Sb5). This method for mechanically exfoliating non-vdW materials increases the availability of 2D materials for the exploration of their physical characteristics and potential applications. To sum up, using the mechanical cleavage method can prepare most of the target 2D materials with high crystal quality, but the samples acquired via this approach also possess several problems, such as irregular morphology, uncontrollable thickness, small domain size, and low yield. Therefore, the present mechanical cleavage technology for 2D material preparation still faces great challenges, which limits its basic research to practical application. It is worth to further optimize the present cleavage methods and explore novel cleavage technologies to achieve 2D materials with controllable layers and sizes.

1.2 Liquid-Phase Exfoliation of 2D Materials

As demonstrated above, the preparation of 2D materials via mechanical exfoliation is limited by the inevitable low yield and uncontrollable thickness, domain size, and morphology. To resolve these issues, liquid-phase exfoliation (LPE) was developed to prepare 2D materials, which has the advantages of high efficiency, large scale, and better controllability [19-23]. The LPE of 2D materials can be divided into two primary approaches: direct exfoliation and chemical exfoliation [24]. Direct exfoliation methods include sonication-assisted exfoliation [SAE] and shear exfoliation. Chemical exfoliation methods include chemical intercalation and electrochemical intercalation [25, 26].

For the SAE method, layered bulk crystal was firstly dispersed in a solvent, followed by material exfoliation via ultrasonic energy and removal of the non-exfoliated material via centrifugation [27]. The dispersion is then centrifuged and purified to achieve 2D nanosheets with uniform domain size and thickness. Moreover, a shear exfoliation method has been widely used to disperse the pretreated layered crystals, which enables the production of high-quality 2D nanosheets while avoiding...