Band Gap Engineering with Ultralarge Biaxial Strains in Suspended Monolayer MoS₂

Abstract

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...

Two-dimensional square buckled Rashba lead chalcogenides

“…As we have shown in DFT simulations, this buckling can be controlled through application of moderate strains of ≤ 10%, which are achievable in the current 2D experiments [33,34]. A similar system showing such tunability is LaOBiS 2 [35].…”

Suppressing Nucleation in Metal–Organic Chemical Vapor Deposition of MoS2 Monolayers by Alkali Metal Halides

“…position values, a tensile strain difference of ∼0.3% was estimated, which is consistent with the E′ position shift of a sample after a transfer onto a c-sapphire substrate ( Figure S4a), and what is expected from the thermal expansion coefficient mismatch between bulk MoS 2 and sapphire. 8,15 For chemical state analysis, micro-XPS was performed on the islands before and after transferring the entire film onto the SiO 2 /Si substrate. From the survey spectra (Figure 3a), the main contaminant on the as-grown MoS 2 appears to be alkali metals originating from the alkali metal halides used for the growth.…”

Band Gap Engineering of Hybrid 2-D Nanomaterials Some Recent Developments

“…Impermeability to gas of CVD grown crystals was used to apply a pressure difference across suspended membranes to induce biaxial strains for determining 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 99meV/%. These results show that as large as 5.6% strain could be applied across micron-sized areas showing strain tuning of higher-level optical transitions [75].…”