Molecular valves for controlling gas phase transport made from discrete ångström-sized pores in graphene

Abstract

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...

Band Gap Engineering with Ultralarge Biaxial Strains in Suspended Monolayer MoS2

“…Nano-structured materials are particularly suited to this technique, as they are often able to remain elastic when subject to strains many times larger than their bulk counterparts can withstand 4 . For instance, bulk silicon fractures when strained to just 1.2%, whereas silicon nanowires can reach strains of as much as 3.5% 5 . Parameters such as the band gap energy or carrier mobility of a semiconductor, which are often crucial to the electronic or photonic device performance, can be highly sensitive to the application of only small strains.…”

Analysis of Time-Varying, Stochastic Gas Transport through Graphene Membranes

“…For this work, we used a rearrangement of this equation that enabled extracting values from the data using a linear leastsquares fit. 30 With these relations, we follow the permeation behavior of the gas over the time just from the AFM measurements of deflection versus time. can be utilized to investigate the transport mechanism by examining the dependence of the flux versus pressure, as done in Figure 2d, corresponding to the data in Figure 2a.…”

“…Nanopore transport with molecular sieving is confirmed as the mechanism by showing that the ratio of permeance values exceeds Knudsen selectivities, which are determined by the inverse square root of molecular weight ratio for the series of gases. 30 The linearity rules out Poiseuille flow through a larger orifice which would demonstrate a quadratic dependence on pressure.…”

“…The transport through the pore is limited by the energy barrier the molecule experiences when passing through the pore, as the separation factors between gases exceed the molecular weight based Knudsen selectivities from an effusion mechanism. 30 This barrier energy is different for each molecule-pore combination, and the differences in the barrier energy between molecules are the source of the observed high selectivities. We explain the observed permeance switching as the result of a small rearrangement in the pore, in essence a small chemical change, that alters the energy barrier that the gas molecules experience passing through the pore.…”

Локальная Симметрия Канала Для Транспорта Атомов, Молекул И Его Внутренние Размеры

“…45879.17140 Для переноса инородных атомов (ионов, молекул) в различных телах большую роль играют транспортные каналы, поперечные размеры которых сопоставимы или превышают в разы размеры транспорти-руемых атомов (ионов, молекул) [1][2][3][4][5]. Такими каналами могут быть различные молекулярные структуры в биологии [4], кристаллографи-ческие каналы [5] (каналы с открытым типом кристаллографической структуры, например кварц, вода), треки от заряженных частиц, дис-локации [1][2][3][4][5]. В частности, проникновение с поверхности и пере-мещение атомов гелия в кристаллографических телах обусловлены дислокационно-динамической диффузией [6,7].…”