through the scotch-tape approach is shown in Figure 3.6 [34]. This protocol is chemical-

free and nanosheets with clear surfaces and high crystal quality can be obtained. The

drawback of this protocol is the low production rate and limited access to the production

parameters due to manual processes.

3.3.1.2 Mechanical Force–Assisted Liquid Exfoliation

To obtain 2D nanomaterials, mechanical force–assisted liquid exfoliation methods emerge

as an appropriate strategy to exfoliate stacked layers of material. In general, the me­

chanical force-assisted liquid exfoliation method is broadly classified on the behalf of

sonication and shear force. The former method involves the sonication of a material

dispersed in a solvent for exfoliation. The suspension obtained after sonication was

further centrifuged to obtain 2D material. The sonication process induces mechanical

forces in the liquid that eventually eliminate or weaken the van der Waals forces between

the layers. This resulted in the formation of 2D materials without interrupting the

covalent bonding of the layers. Furthermore, the exfoliation efficiency can be improved

through the matching surface of layered material and solvent. Hernandez and coworkers

[35] were the first to employ liquid exfoliation via sonication method without utilizing

costly instruments and chemicals. Afterward, this method was modified and surfactant-

assisted liquid exfoliation through sonication was introduced [36]. A graphical re­

presentation of sonication-assisted liquid exfoliation of graphite is presented in

Figure 3.7. However, mechanical force–assisted liquid exfoliation suffers from certain

drawbacks such as low production of mono-layered 2D materials, presence of defects on

the sheets, and smaller lateral size of the exfoliated 2D material sheets.

To overcome these drawbacks, shear force–assisted liquid exfoliation has been in­

troduced [37]. This approach allows the development of high shear rates in the liquid

phase of bulk material. A simple shear force setup includes a mixing head and a rotor to

produce 300 to 800 nm lateral size graphene and few-layered sheets of BPs. The choice of

solvent, shear rate, and polymer additive further improve the exfoliation process. If the

value of the shear rate is below 10⁴ S^–1 then the efficiency of exfoliation is low. As the

shear rate increased above 10⁴ S^–1, the efficiency of exfoliation improves significantly.

Consequently, shear rate plays a vital role to obtain exfoliated 2D material nanosheets.

The shear force–assisted exfoliation technique emerged as a promising protocol to pro­

duce 2D graphene at a large scale by utilizing rotating blade reactors.

3.3.1.3 Liquid Exfoliation Through Ion Intercalation and Ion Exchange

The ion intercalation protocol of liquid exfoliation is based on the idea of cation ion

intercalation in the interlayer of bulk material. This enables weakening or eliminating van

FIGURE 3.6

A schematic representation of nanosheets synthesis

through Scotch-tape protocol. Adapted with permission

[ 34]. Copyright (2012) Elsevier.

2D Materials for Bioelectronics

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