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Page 8 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

the exfoliation process. Such issues may hinder the obtainment of ideal 2D nanosheets.

The “bottom-up” strategy [Figure 3] involves synthesizing 2D nanosheets through chemical reactions from
specific precursors under given conditions, which is theoretically feasible for all types of 2D nanosheets.
Chemical vapor deposition (CVD) [Figure 3A] growth and physical vapor deposition (PVD) [Figure 3B]
growth are reliable routes for synthesizing highly crystalline 2D nanosheets such as graphene, MXene, h-
BN, TMDs, and others, with large-area uniformity [45-47] . However, transferring the crack-free as-prepared 2D
nanosheets from the substrate remains challenging [48-50] . As an alternative, wet-chemical synthesis
[Figure 3C] is widely employed due to its good controllability, reproducibility, and scalability, especially for
preparing those crystalline porous materials made from predesigned skeletons such as zeolites, MOFs, and
COFs. Jeon et al. successfully produced high-aspect-ratio MFI-type zeolite nanosheets using a nanocrystal-
[51]
seeded growth method that was triggered by a single rotational intergrowth . These highly oriented MFI
nanosheets with straight vertical micropores allow the secondary growth of thin and defect-free MFI
[51]
membranes with extraordinary performance for separating xylene isomers . Makiura et al. reported a
procedure for the rational modular assembly of MOF nanosheets with perfect orientation on a solid
[52]
substrate by integrating the layer-by-layer growth and the Langmuir-Blodgett methods . Similarly,
Rodenas et al. presented a new approach to produce self-standing intact MOF nanosheets that relied on
[53]
the diffusion-mediated modulation of the MOF growth kinetics. The synthesis medium involves three
vertically arranged liquid layers, where a topmost solution of cations and a bottom solution of linker
precursors diffuse into the intermediate solvent layer, causing a slow supply of the MOF nutrients to form
MOF nanosheets in a highly diluted medium. On the other hand, COF nanosheets can either grow on
various substrates under solvothermal conditions or at solid-liquid/liquid-liquid/air-liquid interfaces via
interfacial polymerizations [54,55] . An excellent representative work was done by Kandambeth et al., who
successfully demonstrated the fabrication of various flexible, continuous, and defect-free COFs by casting
and baking the mixed solution containing organic linkers and co-reagents . In addition, the “bottom-up”
[56]
approach can also be used to directly fabricate porous graphene with a well-defined pore structure by
selecting appropriate rigid molecular building blocks as monomers for the organic synthesis .
[57]
Perforation on 2D nanosheets
Unlike intrinsically porous 2D nanosheets, the perfect monolayer nonporous 2D nanosheets are almost
impermeable, requiring the artificial drilling of out-of-plane nanoscale holes to serve as nanochannels.
Continuous experimental endeavors have been made to develop various perforation techniques to realize
this goal, including physical and chemical methods such as focused electron beam, bombardment/focused
ion beam, oxygen plasma etching, ultraviolet-induced oxidative etching, and chemical etching [Figure 4].
For example, Fischbein et al. showed that graphene nanosheets could be controllably nano-sculpted using
the focused electron beam ablation technique with few nanometer precisions [Figure 4A] . Similarly,
[58]
Koenig et al. utilized ultraviolet-induced oxidative etching [Figure 4B] to create angstrom-sized pores in the
pristine graphene membrane, which were used as molecular sieves and exhibited selective gas transport
[59]
capabilities .
The focused ion beam perforation method is another effective technique [Figure 4C], which was
successfully employed by Celebi et al. to produce narrowly-distributed pore sizes ranging from < 10 nm to
1 μm in free-standing graphene . Later, Russo et al. created pore nucleation sites on graphene using an
[60]
argon ion beam followed by edge-selective electron recoil sputtering, yielding graphene nanopores with
radii as small as 3 Å, all without the use of focused beams . However, the effective areas of these
[61]
nanoporous graphenes are limited to the micrometer scale. To overcome this limitation, O’Hern et al.
introduced isolated and reactive defects into the graphene lattice through ion bombardment, which were
then enlarged by oxidative etching to produce permeable pores with diameters of 0.40 ± 0.24 nm and