![]() ![]() However, the practical application of this method is limted by the low dispersed concentration. Compared with mechanical exfoliation, the yield of the 2D material sheets by solvent assistance exfoliation could significantly increase. 30 Generally, solvents with the specific surface tension of ~40 mJ/m 2 are suitable for many materials (such as BN, MoS 2, and WS 2). 30, 31 For the solvent assistance exfoliation process, selecting an appropriate solvent with a specific surface tension is critical because the energy of exfoliation could be minimized when the surface energies of flake and solvent match well therefore, an effective exfoliation could be successfully achieved. 30 Many 2D layered materials (such as MoS 2, WS 2, MoSe 2, MoTe 2, BN, Bi 2Te 3, and so on) could be exfoliated from the bulk crystal counterparts by direct sonication in organic solvents like N-methylpyrrolidone (NMP) and isopropanol (IPA). Solvent-assisted exfoliation was proposed by Coleman et al. Regarding the chemical exfoliation, solvent-assisted exfoliation and ion intercalation assisted exfoliation are two typical approaches. ![]() However, the obvious disadvantage of extremely low yield restricts its large-scale production and applications. 29 The as-prepared layered materials exhibit high crystallinity and clean surfaces, making them attractive for fundamental research. 29 This simple mechanical cleavage method has been used to successfully obtain a variety of 2D layered materials like graphene, boron nitride, dichalcogenides, and so on. Mechanical exfoliation, also called “Scotch tape method”, is to prepare monolayer or few-layer 2D materials by applying an adhesive tape to cleave bulk crystals repeatedly. 1 In general, it can be realized by mechanical or chemical approaches. Preparation of 2D materials Exfoliation methodsĮxfoliation method is a top-down approach to prepare atomically thin 2D materials from the bulk crystals constructed by weak van der Waals interlayer interaction, which was applied to produce graphene in 2004. Two major strategies have been employed to obtain monolayer and few-layer 2D materials: one is the exfoliation of bulk crystals the other is the direct growth through vapor phase deposition methods. Section 3 and 4, respectively, introduce typical 2D piezoelectric and ferroelectric materials and their electronic properties, together with their potential applications. ![]() 20, 21, 22 In section 2 we first summarize the synthetic methods of 2D materials, enumerating the preparation of some representative 2D materials as examples. By now, a range of 2D layered materials have been experimentally confirmed or theoretically predicted as piezoelectric or ferroelectric, 10, 13, 14, 15, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28 like monolayer transition metal dichalcogenides (TMDCs) (such as MoS 2, MoSe 2, WS 2, and WSe 2), 10 group IV monochalcogenides (such as GeSe and SnS) 15, 18, 19, 20 and group III–V binary compounds (such as AlSb, GaP, GaAs, InP, InAs, and InSb). Ferroelectricity is a property of spontaneous electric polarization that can be controlled by an applied external electric field. Piezoelectricity is a property of electric polarization caused by macroscopic strains, a coupling between the mechanical and electrical behaviors to allow efficient mechanical-to-electrical energy conversion. The non-centrosymmetric material is an important research field because of the associated polarization and its potential application. Among them, piezoelectric and ferroelectric 2D materials have drawn a growing interest in recent years. 8 Particularly, the 2D layered materials with non-centrosymmetric structure have great potential for nanoscale electromechanical systems and electronic device applications. Atomically thin 2D materials exhibit a wide range of unique electrical, 2, 3 optical, 4, 5 mechanical 6, and thermal 7 properties, which do not exist in their bulk counterparts, and the outstanding advantages of properties enlighten the development in light-weight and high-performance multifunctional applications. in 2004, 1 worldwide scientific efforts have been focused on wide range of two-dimensional (2D) layered materials, driven by the fundamental interest and their potential applications. Since the successful isolation of graphene by Novoselov et al. ![]()
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