Gas flow through atomic-scale apertures

Abstract

Gas flows are often analyzed with the theoretical descriptions formulated over a century ago and constantly challenged by the emerging architectures of narrow channels, slits, and apertures. Here, we report atomic-scale defects in two-dimensional (2D) materials as apertures for gas flows at the ultimate quasi-0D atomic limit. We establish that pristine monolayer tungsten disulfide (WS2) membranes act as atomically thin barriers to gas transport. Atomic vacancies from missing tungsten (W) sites are made in freestanding (WS2) monolayers by focused ion beam irradiation and characterized using aberration-corrected transmission electron microscopy. WS2 monolayers with atomic apertures are mechanically sturdy and showed fast helium flow. We propose a simple yet robust method for confirming the formation of atomic apertures over large areas using gas flows, an essential step for pursuing their prospective applications in various domains including molecular separation, single quantum emitters, sensing and monitoring of gases at ultralow concentrations.

Exponentially selective molecular sieving through angstrom pores

“…Two-dimensional (2D) membranes with a high density of angstrom-scale pores can be made by engineering defects in 2D crystals 1 – 9 or, perhaps more realistically in terms of applications, by growing intrinsically porous crystals such as, e.g., graphynes 10 – 12 . Interest in angstroporous 2D materials is strongly stimulated by potential applications, particularly for gas separation as an alternative to polymeric membranes employed by industry 3 , 13 .…”

“…At present, this optimistic assessment is based mostly on theoretical modeling. Experimental clarity has so far been achieved only for the classical regime of d P > d K where the flow is governed by the Knudsen equation, and the resulting modest selectivities arise from differences in thermal velocities of gases having different molecular masses m 7 – 9 , 16 . For smaller pores with d P ≈ d K , S up to 10–100 have been reported for monolayer graphene 5 , 8 , and even higher selectivities (∼10 4 ) were found for some defects with an estimated diameter of ∼3.5 Å in bilayer graphene 4 .…”

“…Still, this is many orders of magnitude smaller than S predicted for the activated-transport regime, d P < d K 1 , 14 , 15 . Little remains known about the latter regime, which has proven to be extremely difficult to reach in experiment 5 , 8 , 9 . Indeed, even monovacancies in dichalcogenide monolayers were suggested to exhibit the conventional Knudsen flow 9 .…”

“…Little remains known about the latter regime, which has proven to be extremely difficult to reach in experiment 5 , 8 , 9 . Indeed, even monovacancies in dichalcogenide monolayers were suggested to exhibit the conventional Knudsen flow 9 . The experimental difficulties and lack of understanding are further exacerbated by prohibitive computational costs of simulating molecular permeation in the activated regime 17 – 20 .…”

Mass Transfer in Boronate Ester 2D COF Single Crystals

“…Indeed, perforated single‐layer materials are featured by pore densities of less than 10 13 cm −2 whereas their areal porosity, i.e., the integral open area relative to the overall membrane area, rarely exceeds a few percent. [ 6–8 ] On the other hand, there exist inherently porous planar nanomaterials, including 2D covalent organic frameworks (COFs) and 2D polymers (2D COFs in monolayer form), that represent ordered fishing net‐like structures with monodisperse openings located next to one another. [ 9 ] The pore dimensions in the 2D COFs can be easily tailored via changing their building blocks as well as by adopting different packing configurations.…”