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= 2D Nanopores =

Two-dimensional (2D) material-based solid-state nanopores or 2D nanopores are nanometre (nm) size holes or apertures in atomically thin membranes. Their size can range 50 Å to a few tens of nanometers. The main advantage of using 2D material is the thickness of the membrane, usually sub-nanometer that can provide high spatial resolution over thicker silicon-based or polymer-based membranes. The 2D materials usually used are graphene, MoS2, hBN, WS2, MXenes, and other hybrid 2D materials.

History
The first reports on 2D nanopore devices are in 2010s, where monolayer and multi-layer graphene nanopores were demonstrated for DNA translocation. However, due to hydrophobic nature of the graphene there were issues such as sticking or clogging of the DNA in the graphene pore and high noise of the graphene made it unsuitable for translocation experiments. The transition metal dichalcogenide - molybdenum disulfide in a monolayer form was shown to be a better alternative to the graphene. There was also less sticking of DNA to the MoS2 monolayer.

Principle of working
The nanopore technology is inspired by biological nanopores used for DNA sequencing. In essence, it is similar to a Coulter counter technique used for counting and sorting of the red blood cells. Typically, a 2D membrane is suspended on a large aperture that separates two compartments. Then, a nanopore is created in the suspended part either using high-energy electron beam or other method such as electrochemical reaction or chemical dielectric breakdown. Usually this nanopore is very small compared to the suspending aperture. The nanopore or nanopores in the 2D membrane is separates two compartments filled with salt and buffer solution (e.g. KCl). A pair of electrodes (generally Ag/AgCl) generates a voltage bias that forces the oppositely charged ions to move through either side of the compartment. This transport of the ions through the pore generates electric current that can then be measured using a current-amplifier.

The redox reaction occurs that can be summed up as follows:

Ag(s) + Cl− → AgCl(s) + e− (Oxidation reaction occurring at Anode (+))

AgCl(s) + e− → Ag(s) + Cl− (Reduction reaction occurring at Cathode(-))

Device Architecture
A 2D nanopore device consists of three aspects: (1) supporting membrane with a larger aperture size (~100-200 nm), (2) a 2D material completely suspended over this aperture, and finally (3) a nanopore or multiple nanopores (sub-nm to few tens of nanometers) in the suspended region.

Microfabrication
Usually, a silicon-based wafer (100 mm) is coated with silicon-oxide (60 nm) and silicon nitride (low stress, 20 nm). Using photolithography and dry-etching technology, the back-side of the wafer is opened and using electron-beam lithography and dry-etching a nanopore aperture is created on the front-side of the wafer. Piranha cleaning is used to achieve clean surface of nanopore substrates.

The nanopores in 2D material can be created using different methods. High-energy electron beam in Transmission electron microscope as well as in-situ electrochemical reaction are the most well-known methods to create nanopores in 2D materials.

Dry stamping Technique
Usually, the 2D material is transferred by exfoliating the crystal or from the growth substrate to the nanopore device. In case of exfoliation, the 2D material is first exfoliated using a tape onto a silicon-based substrate. Based on the optical contrast, one can select the crystal with different number of layers. the crystal is the coated with a polymer such as PMMA, or directly picked up using a PDMS stamp. Once coated, the underlying substrate is etched and the 2D material alongwith the polymer is fished out and transferred onto the device. The material aligned over the substrate and later the polymer is dissolved using organic solvents such as acetone.

Wet transfer Technique
In case of CVD grown material, polymer such as PMMA is coated on the material and fished out by etching the growth substrate (using KOH) or water. The polymer/2D material is then aligned precisely on the nanopore device and the polymer is dissolved using acetone. While other technique has focused directly growing of MoS2 over the silicon nitride membranes.

Wafer-scale Transfer Technique
Recently, using polymer-based approaches a complete wafer-scale transfer is achieved. The advantage of such technique is the production of a large number of nanopore devices in one batch by employing the wafer-scale thin-film transfer method. With this approach, a high transfer efficiency (>70%) was achieved.

Biosensing
The 2D nanopores have been majorly employed for biomolecular detection of DNA, RNA and proteins. For single-nucleotide detection of the DNA, ionic-liquids were used in combination of KCl to decrease the translocation speed of the nucleotide.

Nanopower generation
Monolayer MoS2 has been demonstrated as osmotic nanopower generator. An osmotically induced current was produced from a salt gradient, capable of generation of ~106 watts per square metre due to the atomic thickness of the MoS2 membrane (~0.7 nm).

Defect Engineering
Creation of defects and nanopores in 2D material is important to understand various phenomenon occurring at the nanoscale especially in ionic and molecular filtration. Ultra-small atomic defects have been engineered in the 2D material for understanding the ion transport phenomenon.

Challenges
One of the major challenge related to the practical application of 2D materials-based nanopores is the noise in the system. The low frequency noise (<1000 Hz) also known as 1/f noise (flicker noise) dominates the noise spectrum. The origin of such noise is not clear and some studies show that the mechanical fluctuations of the thin suspended membrane while other studies show that the noise might arise from incomplete wetting of the pore. the main signal is buried within this noise.

Another challenge in the nanopore DNA sequencing using 2D nanopores is the speed of the translocating DNA (translocation velocity). With 2D nanopores, the measured translocation velocity is about 20k-100k base pairs per millisecond, whereas a required velocity needs to be <100 base pairs per millisecond.