We demonstrate the continuous and reversible tuning of the optical band gap of suspended monolayer MoS 2 membranes by as much as 500 meV by applying very large biaxial strains. By using chemical vapor deposition (CVD) to grow crystals that are highly impermeable to gas, we are able to apply a pressure difference across suspended membranes to induce biaxial strains. We observe the effect of strain on the energy and intensity of the peaks in the photoluminescence (PL) spectrum, and find a linear tuning rate of the optical band gap of 99 meV/%. This method is then used to study the PL spectra of bilayer and trilayer devices under strain, and to find the shift rates and Grüneisen parameters of two Raman modes in monolayer MoS 2 . Finally, we use this result to show that we can apply biaxial strains as large as 5.6% across micron sized areas, and report evidence for the strain tuning of higher level optical transitions.KEYWORDS: Strain engineering, MoS 2 , photoluminescence, bandgap, Raman spectroscopy, biaxial strain 3 The ability to produce materials of truly nanoscale dimensions has revolutionized the potential for modulating or enhancing the physical properties of semiconductors by mechanical strain 1 . Strain engineering is routinely used in semiconductor manufacturing, with essential electrical components such as the silicon transistor or quantum well laser using strain to improve efficiency and performance 2,3 . 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. The combination of this sensitivity with the ultra-high strains possible at the nanoscale could lead to an unprecedented ability to modify the electrical or photonic properties of materials in a continuous and reversible manner.Monolayer MoS 2, a 2D atomic crystal, has been shown in both theory 6,7 and experiment [8][9][10][11][12] to be an ideal candidate for strain engineering. It belongs to the class of 2D transition metal dichalcogonides (TMD's), and as a direct-gap semiconductor 13 has received significant interest as a channel material in transistors 14 , photovoltaics 15 and photodetection 16 devices. It has a breaking strain of 6-11% as measured by nanoindentation, which approaches its maximum theoretical strain limit 17 and classifies it as an ultra-strength material. Its electronic structure has also proven to be highly sensitive to strain, with experiments showing that the optical band gap reduces by ~50 meV/% for 4 uniaxial strain 8,11 , and is predicted to reduce by ~100 meV/% for biaxial strain 18,19 . This reversible modulation of the band...

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...

We measured the work of separation of single and few-layer MoS membranes from a SiO substrate using a mechanical blister test and found a value of 220 ± 35 mJ/m. Our measurements were also used to determine the 2D Young's modulus (E) of a single MoS layer to be 160 ± 40 N/m. We then studied the delamination mechanics of pressurized MoS bubbles, demonstrating both stable and unstable transitions between the bubbles' laminated and delaminated states as the bubbles were inflated. When they were deflated, we observed edge pinning and a snap-in transition that are not accounted for by the previously reported models. We attribute this result to adhesion hysteresis and use our results to estimate the work of adhesion of our membranes to be 42 ± 20 mJ/m.

The role of nanobubbles in selectively controlled ionic transport across fabricated nanoporous graphene membranes is elucidated.

Large arrays of 3-terminal nanoelectromechanical graphene switches are fabricated. The switch is designed with a novel geometry that leads to low actuation voltages and improved mechanical integrity, while reducing adhesion forces, which improves the reliability of the switch. A finite element model including non-linear electromechanics is used to simulate the switching behavior and to deduce a scaling relation between the switching voltage and device dimensions.

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