Membranes act as selective barriers and play an important role in processes such as cellular compartmentalization and industrial-scale chemical and gas purification. The ideal membrane should be as thin as possible to maximize flux, mechanically robust to prevent fracture, and have well-defined pore sizes to increase selectivity. Graphene is an excellent starting point for developing size-selective membranes because of its atomic thickness, high mechanical strength, relative inertness and impermeability to all standard gases. However, pores that can exclude larger molecules but allow smaller molecules to pass through would have to be introduced into the material. Here, we show that ultraviolet-induced oxidative etching can create pores in micrometre-sized graphene membranes, and the resulting membranes can be used as molecular sieves. A pressurized blister test and mechanical resonance are used to measure the transport of a range of gases (H(2), CO(2), Ar, N(2), CH(4) and SF(6)) through the pores. The experimentally measured leak rate, separation factors and Raman spectrum agree well with models based on effusion through a small number of ångstrom-sized pores.

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An ability to precisely regulate the quantity and location of molecular flux is of value in applications such as nanoscale 3D printing, catalysis, and sensor design 1-4 .Barrier materials containing pores with molecular dimensions have previously been used to manipulate molecular compositions in the gas phase, but have so far been 2 unable to offer controlled gas transport through individual pores [5][6][7][8][9][10][11][12][13][14][15][16][17][18] . Here, we show that gas flux through discrete angstrom-sized pores in monolayer graphene can be detected and then controlled using nanometer-sized gold clusters, which are formed on the surface of the graphene and can migrate and partially block a pore. In samples without gold clusters, we observe stochastic switching of the magnitude of the gas permeance, which we attribute to molecular rearrangements of the pore.Our molecular valves could be used, for example, to develop unique approaches to molecular synthesis that are based on the controllable switching of a molecular gas flux, reminiscent of ion channels in biological cell membranes and solid state nanopores 19 .We studied 2 types of angstrom pore molecular valves: a porous single layer of suspended graphene with no gold nanoclusters on its surface (PSLG) and a porous single layer of suspended graphene on top of which we evaporated gold nanoclusters (PSLGAuNCs). To fabricate both types of devices, we start with suspended pristine monolayer graphene which is impermeable to all gases 20 and defect free 21 . The graphene is mechanically exfoliated over predefined etched wells in a silicon substrate with 90 nm of thermal silicon oxide on top. This forms a graphene-sealed microcavity which confines a ~µm 3 volume of gas underneath the suspended graphene. We use 2 techniques to introduce molecular-sized pores. The first method uses a voltage pulse applied by a metallized AFM tip 22 . Figure 1a illustrates the method with a ~300 nm diameter pore created in the centre of a graphene membrane by applying a voltage pulse of -5V for 100 ms. 3A pressurized blister test is used to determine the leak rate out of the graphene sealed microcavity 23 . The microcavity is filled with pure H2 or N2 at 300-400 kPa and the graphene is bulged up due to the pressure difference across it. An example for an unetched pristine sample pressurized with N2 is shown in Figure 1b. In this instance, after a voltage pulse of -9 V for 2 s to the centre of the membrane a single pore is created-we found that the voltage and time needed to introduce a pore varied depending on the AFM tip used, thus the difference in sizes between Fig. 1a and 1c. Immediately after a pore is formed, the deflection drops and the graphene is flat except for a few wrinkles introduced by the process (Fig. 1c). The AFM image shows no detectable pore meaning that the pore is smaller than the resolution of the AFM. For the PSLG-AuNCs samples, gold atoms are evaporated onto the graphene. Figure 1d shows the graphene sample in Fig. 1c after gold evaporation and repressurization...

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