In biological systems, Ångstrom-scale channels are crucial for a multitude of cellular functions, ensuring the regulated flow of molecules and ions. These channels have ramifications ranging from cellular trafficking to molecular filtration across membranes. While understanding their function at an atomic level has been challenging, recent advancements are paving the way for a deeper exploration of artificial nanofluidic transport systems. The regime of fluidic transport at angstrom-scale is intriguing and there has been significant interest in scientific community to study fluidic behaviour in synthetic Ångstrom-scale channels. This research not only enhances our fundamental understanding of molecular transport, but also has implications for innovations in nanotechnology, materials science, membranes, and biotechnology.

The relevance of Ångstrom-scale Channels

Ångstrom-scale channels, quantified in units even smaller than nanometres, have emerged as pivotal tools for studying fluidic behaviour at scales previously unattainable. These channels allow for detailed analyses of gas, ion, and water transport, promising to provide fresh perspectives on their interactions at ultra-confined dimensions and theories that govern mass transport at atomic scales.

The Challenge of Nanofabrication

The task of fabricating devices at Ångstrom-scale presents its own set of challenges. Traditional nanofabrication techniques, even though advanced, face certain limitations. Notably, the surface roughness of widely used materials often impedes the desired precision. However, recent advancements in two-dimensional (2D) materials have given us the tools to overcome these hurdles.

A Breakthrough in Channel Fabrication

In a ground-breaking development, Geim, Radha, and co-workers have successfully crafted Ångstrom-scale slit-like channels with meticulous control over their dimensions using 2D materials. These channels serve as a window into the world of fluidics at the angstrom-scale, allowing us to observe and understand fluidic properties like never before. In our present paper, the detailed fabrication method for these channels is being reported.

Ångstrom-scale Channels: A How-To Guide

Fabrication of Ångstrom-scale channels with 2D materials as building blocks requires a rigorous approach. This protocol outlines the comprehensive procedure for creating these channels, which can be singular or many in parallel, using layered crystals for atomic-scale precision.

1. Fabricating the Substrate: The initial step involves the development of a substrate, typically utilizing silicon nitride. This substrate, typically made of silicon nitride, provides the mechanical support stability of the channels to be made in subsequent steps.
2. Preparing the 2D material flakes: Next, we prepare the building blocks from layered crystals, known as flakes. These flakes, mere atomic layers thick, are the essence of our channels. The layered crystals that are commonly used include graphene, hexagonal boron nitride, molybdenum disulphide or mica flakes. Most importantly, these 2D crystal flakes hold the key to the atomic-scale precision we seek as the basal planes in 2D crystals are atomically smooth and their thickness can be varied from one atom thick to multiple of such layers (nanometres).
3. Spacer Layer: This layer determines the height of the channels. Whether a single layer or a few, this layer (flake) of 2D material sets the atomic-scale control.
4. Flake Transfers: A crucial step requiring precision, this involves the transfer of flakes onto the substrate. Proper alignment is imperative at the atomic level. This step requires the steady hands and precision of a surgeon, ensuring that the atomic layers align well without any gaps in between the layers.
5. Assembly: The van der Waals forces hold these 2D crystal layers together. Like an atomic-scale Lego^Ò set, the layer are assembled to form Ångstrom-scale channels on the substrate, silicon nitride membrane.
6. Post-Processing: This step involves annealing and cutting off the edges of the channels so as to ensure the fabricated channels meet the requisite specifications for scientific exploration.

Our fluidic channel devices as illustrated in Figure 1 use tri-crystal stacks of 2D materials (flakes). The middle layer is patterned with electron beam lithography to create parallel stripes, forming Å-scale channels. The spacers are made of single to a few layers of 2D material, determining capillary height. These devices offer atomic-scale control over dimensions and smooth channel walls. They are placed on a silicon nitride membrane with a micrometre-sized rectangular hole on a silicon substrate.

Considerations and Challenges

It is important to note that this procedure requires expertise in clean room environments and a solid understanding of nanofabrication. The entire process spans 7 to 13 days, necessitating commitment and a meticulous approach.

Applications and Importance

The Ångstrom-scale channels, once fabricated, serve as platforms for numerous scientific investigations. Their small scale permits detailed studies into molecular transport, ionic interactions, and atomic-level fluid dynamics. They enable researchers to closely observe phenomena such as the movement of individual layer of water molecules, ion interactions, and the behaviour of various gases. The development and study of Ångstrom-scale channels holds significant promise in advancing our understanding of molecular and ionic interactions in atomic-scale confinement (Figure 2). As we delve deeper into this domain, the prospect of unveiling new scientific insights remains high. The meticulous process of channel fabrication requires expertise but paves the way for ground-breaking scientific discoveries in the nanoscale world.