Many novel transport phenomena are observed in graphene nanochannels with ultrahigh surface flatness and nano- or sub-nanoscale constraints. Two critical physical parameters, surface slip length, and surface charge, play a vital role in the channel transport process. However, effectively controlling these parameters under such tight constraints remains a significant challenge. Here, it is developed a novel method that combines oxygen ion etching and layer-by-layer assembly of 2D material, to prepare graphene nanochannels. During the assembly process, defects are introduced into the graphene surface via oxygen ion etching. A significantly higher conductivity is observed for the pristine graphene channels compared to those with defects on both the upper and lower surfaces. Consistent with this observation, the conductivity of graphene channels with defects on only one surface falls between the two aforementioned values. Combined with theoretical analysis, the conductivity difference is attributed to the surface slip inhibition due to the introduced defects, and the change of surface charge, both caused by oxygen ion etching. By introducing defects, a new method is uncovered for fine-tuning ion transport in graphene nanochannels. Graphene nanochannels are prepared by oxygen ion etching and layer-by-layer assembly. The conductivity of the pristine graphene channel is one order of magnitude higher than that of the graphene channel with defects on both the upper and lower surfaces. Meanwhile, the conductivity of the graphene channel with defects on only one surface is observed to be intermediate between the two aforementioned. The introduction of defects provides a new method to adjust ion transport by changing surface charge and slip in the graphene channel. image

Confining DNA in nanochannels is an important approach to studying its structure and transportation dynamics. Graphene nanochannels are particularly attractive for studying DNA confinement due to their atomic flatness, precise height control, and excellent mechanical strength. Here, using femtosecond laser etching and wetting transfer, we fabricate graphene nanochannels down to less than 4.3 nm in height, with the length-to-height ratios up to 103. These channels exhibit high stability, low noise, and self-cleaning ability during the long-term ionic current recording. We report a clear linear relationship between DNA length and the residence time in the channel and further utilize this relationship to differentiate DNA fragments based on their lengths, ranging widely from 200 bps to 48.5 kbps. The graphene nanochannel presented here provides a potential platform for label-free analyses and reveals fundamental insights into the conformational dynamics of DNA and proteins in confined space.

Two-dimensional material nanochannels with molecular-scale confinement can be constructed by Van der Waals assembly and show unexpected fluid transport phenomena. The crystal structure of the channel surface plays a key role in controlling fluid transportation, and many strange properties are explored in these confined channels. Here, we use black phosphorus as the channel surface to enable ion transport along a specific crystal orientation. We observed a significant nonlinear and anisotropic ion transport phenomenon in the black phosphorus nanochannels. Theoretical results revealed an anisotropy of ion transport energy barrier on the black phosphorus surface, with the minimum energy barrier along the armchair direction approximately ten times larger than that along the zigzag direction. This difference in energy barrier affects the electrophoretic and electroosmotic transport of ions in the channel. This anisotropic transport, which depends on the orientation of the crystal, may provide new approaches to controlling the transport of fluids.

Highly stable nanopores with precise size and film thickness are essential for ultrasensitive molecular biosensors and efficient ion filters. However, the stability of both solid-state nanopores and biological nanopores is of a few hours. Even for two-dimensional (2D) materials with excellent properties, it is very difficult to prepare nanopores with stability for more than 1 day. Here, in mechanically exfoliated 2D materials with excellent quality, highly stable nanopores have been fabricated. Through the manual control of defect density and laser spot irradiation, the breakdown process is more controllable. The nanopore diameter can be precisely adjusted from 1 to 10 nm. The 2 nm thick graphene nanopore (with a diameter of 2 nm) maintains good consistency in conductivity during the 1-month immersion in KCI (1 M) solution. The pore size changes less than 0.7% per day. We found that 2D material nanopores can produce a strong rectification ratio (similar to 50) due to the morphology undulation. In addition, our devices can recognize modified 5 nm Au nanoparticles, showing great potential for single-molecule detection. This work can guide the development of high-quality ion rectifier devices and biosensors based on 2D material nanopores.