3.3.1 Top-Down Synthesis Protocols

A mono or few-layered 2D material can be synthesized through top-down synthesis

protocols. These protocols eliminate the van der Waals interactions in between the

stacked layers of 2D materials. Several top-down synthesis protocols have been reported

such as Scotch-tape, mechanical force-assisted liquid exfoliation, liquid exfoliation

through ion intercalation and ion exchange, liquid exfoliation through oxidation/re­

duction, and liquid exfoliation through etching [30].

3.3.1.1 Scotch-Tape Protocol

Scotch-tape protocol is a conventional route to synthesize nanosheets of 2D materials.

This method is also named as mechanical cleavage method. In this protocol, nanosheets

are peeled off from the stacked layers of 2D materials by eliminating/weakening the van

der Waals forces. The nanosheets are obtained in such a way that the in-plane covalent

bonds remained unaffected. Scotch-tape protocol was coined by Geim and Novoselov

[31] in 2004. They were the first to cleave a mono-layered graphene sheet from graphite.

After this remarkable contribution, the scotch-tape protocol is used to prepare a wide

range of 2D materials including h-BN, TMDs, antimonene, CuInP2S6, and BP from their

bulk counterparts [32,33]. A schematic representation of graphene nanosheets synthesis

FIGURE 3.5

Absorption spectra of MoS2 thin films having thicknesses varying from 1.3–7.6 nm. Inset shows the energy of

excitation peak with respect to film thickness. Adapted with permission [ 25]. Copyright (2011) American

Chemical Society.

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