Researchers have produced a stable porous membrane that is thinner than a nanometre. This is a 100,000 times thinner than the diameter of a human hair. The membrane consists of two layers of the much exalted ”super material” graphene, a two-dimensional film made of carbon atoms.
Researchers not only succeeded in producing the starting material, a double-layer graphene film with a high level of purity, but they also mastered a technique called focused ion beam milling to etch pores into the graphene film. In this process, which is also used in the production of semiconductors, a beam of helium or gallium ions is controlled with a high level of precision in order to etch away material. The researchers were able to etch pores of a specified number and size into the graphene with unprecedented precision. This process, which could easily take days to complete, took only a few hours in the current work. “This is a breakthrough that enables the nanofabrication of the porous graphene membranes,” explains Ivan Shorubalko, a scientist at Empa that also contributed to the study.
The ultra-thin graphene membrane may one day be used for a range of different purposes, including waterproof clothing. “Our membrane is not only very light and flexible, but it is also a thousand fold more breathable than Goretex,” says Kemal Celebi. The membrane could also potentially be used to separate gaseous mixtures into their constituent parts or to filter impurities from fluids. The researchers were able to demonstrate for the first time that graphene membranes could be suitable for water filtration. The researchers also see a potential use for the membrane in devices used for the accurate measurement of gas and fluid flow rates that are crucial to unveiling the physics around mass transfer at nanoscales and separation of chemical mixtures.
The membrane can thus permeate tiny molecules. Larger molecules or particles, on the other hand, can pass only slowly or not at all. “With a thickness of just two carbon atoms, this is the thinnest porous membrane that is technologically possible to make,” says PhD student Jakob Buchheim, one of the two lead authors of the study.
In order to achieve this level of precision, the researchers had to work with double-layer graphene. “It wouldn’t have been possible for this method to create such a membrane with only one layer because graphene in practice isn’t perfect,” says Park. The material can exhibit certain irregularities in the honeycomb structure of the carbon atoms. Now and again, individual atoms are missing from the structure, which not only impairs the stability of the material but also makes it impossible to etch a high-precision pore onto such a defect. The researchers solved this problem by laying two graphene layers on top of each other. The probability of two defects settling directly above one another is extremely low, explains Park.
A two-dimensional (2D) porous layer can make an ideal membrane for separation of chemical mixtures because its infinitesimal thickness promises ultimate permeation. Graphene—with great mechanical strength, chemical stability, and inherent impermeability—offers a unique 2D system with which to realize this membrane and study the mass transport, if perforated precisely. We report highly efficient mass transfer across physically perforated double-layer graphene, having up to a few million pores with narrowly distributed diameters between less than 10 nanometers and 1 micrometer. The measured transport rates are in agreement with predictions of 2D transport theories. Attributed to its atomic thicknesses, these porous graphene membranes show permeances of gas, liquid, and water vapor far in excess of those shown by finite-thickness membranes, highlighting the ultimate permeation these 2D membranes can provide.