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Page 10 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

[69]
[19]
In addition to graphene, other porous 2D nanosheets, such as h-BN , MoS 2 [70,71] , and g-C N , can be
4
3
obtained through artificial perforation approaches. The atomically thin 2D nanosheets hosting either
intrinsical or artificial nanopores impart outstanding molecular transporting and sieving capabilities,
making them promising materials for the direct fabrication of selectively permeable membranes with
abundant nanofluidic channels.
Nanochannel membrane assembly strategies
In-plane parallel capillaries are another type of nanochannels that can be constructed by assembling
[72]
isolated atomic planes of 2D nanosheets into van der Waals heterostructures made layer by layer in a
precisely chosen sequence, namely van der Waals assembly [Figure 5A]. A typical stacking procedure starts
by isolating micrometer-sized 2D nanosheets on top of a thin supporting film as one brick for the Lego wall,
which are then put face-down onto a chosen target with the supporting film removed or dissolved. This
process is repeated until the desired stack is assembled. While covalent solid bonds provide in-plane
stability of 2D nanosheets, relatively weak, van der Waals-like forces are sufficient to keep the stack
together. Radha has made outstanding contributions to this technique, having fabricated narrow and
smooth capillaries through van der Waals assembly, with atomically flat graphene sheets at the top and
bottom separated by spacers with a precisely controlled number of layers . They found that the water
[73]
transport through the channels created by this method was characterized by an exceptionally fast flow that
can be attributed to high capillary pressures and large slip lengths and can be associated with the structural
ordering degree of nanoconfined water. They also investigated hydrated ion transport through ultimately
narrow slits with dimensions approaching the size of small ions and water molecules . The ions with
[74]
hydrated diameters larger than the slit size can still permeate through by distortions of their hydration
shells. The mobility of ions under angstrom-scale confinement showed a notable dependence on the electric
charges inside channels or at their entries. Using the same method, 2D channels made from graphene,
MXene, and h-BN allowed helium gas flow that is orders of magnitude faster than expected from theory,
whereas similar 2D channels made from MoS exhibited much slower permeation that remains well
2
[75]
described by Knudsen diffusion . It demonstrated the ballistic molecular transport effect that surface
scattering could be either diffuse or specular, dependent on the fine details of the atomic landscape of the
surface. The van der Waals assembly technique opens up an avenue to making capillaries with channel sizes
tunable to angstrom precision and controllable transport properties through a wide choice of available 2D
materials as channel walls. These series of 2D nanochannels in the forms of in-plane parallel capillaries
provide an ideal experimental platform for offering a new understanding of many excellent nanofluidic
observations, such as the unexpectedly high transport of thermal protons [76,77] ,the qualitatively different
[78]
coordination of square ice , the anomalously low dielectric constant of confined water , and the
[79]
transistor-like electrohydrodynamic effect of hydrated ions .
[80]
The assembly of uniform 2D layered films from well-dispersed 2D nanosheets provides an ideal platform
for constructing densely packed nanochannels for nanofluidic transport due to the large aspect ratio with
atomic thickness and micron lateral dimension. Furthermore, the spaces, including in-plane slits (or
defects) and plane-to-plane interlayer galleries within the 2D laminates, create numerous channels for
nanofluidic transport. Among various feasible strategies, solution-assisted assembly offers a general, facile,
and scalable way for high-throughput constructing densely packed nanochannels from 2D materials. The
internal forces, such as electrostatic and van der Waals attractive interactions that are existed inside the 2D
laminate, together with external forces, such as compressive, centrifugal, and shear forces that are applied
outside the 2D laminate, are largely responsible for tuning the 2D building blocks into the ordered 2D
[81]
laminar membranes . Predominantly used solution-assisted assembly methods include pressure/vacuum
filtrating, coating, and layer-by-layer assembling.