Band Gap Engineering with Ultralarge Biaxial Strains in Suspended Monolayer MoS2 (1608.04264v1)

Abstract: We demonstrate the continuous and reversible tuning of the optical band gap of suspended monolayer MoS2 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 Gr\"uneisen parameters of two Raman modes in monolayer MoS2. 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.

• The paper demonstrates a reversible 500 meV reduction in the optical band gap of monolayer MoS2 under biaxial strains up to 5.6%.
• It employs pressure differentials across suspended CVD-fabricated membranes to apply controlled strain and monitors effects with photoluminescence and Raman spectroscopy.
• The findings indicate promising applications in wavelength-tunable optoelectronic devices and advanced gas separation technologies.

Band Gap Engineering with Ultralarge Biaxial Strains in MoS2

The paper "Band Gap Engineering with Ultralarge Biaxial Strains in Suspended Monolayer MoS2" presents a thorough investigation into the modulation of the optical band gap of monolayer molybdenum disulfide (MoS2) through biaxial strain induction. This paper is premised on the observation that the optical and electronic properties of two-dimensional materials, such as MoS2, are highly sensitive to mechanical deformation.

Key Findings

1. Biaxial Strain Application: Using chemical vapor deposition (CVD), the researchers fabricated highly impermeable MoS2 membranes that could withstand significant biaxial strains up to 5.6%. The strain was induced by creating a pressure differential across the membranes suspended over cylindrical cavities.
2. Band Gap Modulation: The paper reports a significant reduction in the optical band gap of monolayer MoS2 by approximately 500 meV, equating to over 25% adjustment. This modulation is reversible and linear, with a tuning rate observed at 99 meV/% strain for monolayer MoS2.
3. Photoluminescence and Raman Spectroscopy: Strain effects on the photoluminescence (PL) spectrum were tracked, revealing redshifts in exciton peak positions with increasing strain. The Raman spectroscopy results provided evidence of linear shifts at rates of -1.7 cm^-1% for the A1g mode and -5.2 cm^-1% for the E2g mode, consistent with theoretical predictions.
4. Multilayer MoS2 Observations: The research extended to bilayer and trilayer MoS2 samples prepared by mechanical exfoliation, demonstrating varying shift rates in band gap energies due to differences in layer interactions under strain.
5. Resonant Raman Scattering: At high strains, enhancements in Raman mode intensities, particularly the E2g mode, were attributed to resonant Raman scattering due to tuning higher-level optical transitions closer to the laser excitation energy.

Implications

This work emphasizes the potential of strain engineering to significantly enhance and control material properties for electronic and optoelectronic applications. For example, the ability to fine-tune the band gap of MoS2 by mechanical means opens pathways to designing wavelength-tunable devices such as phototransistors and sensors that may outperform traditional silicon-based counterparts. Additionally, because the CVD-grown MoS2 is also remarkably gas impermeable, it has potential applications in gas separation technologies.

Future Directions

Future research could expand on this work by exploring strain engineering in other two-dimensional materials, potentially extending insights into novel electronic phenomena such as strain-induced magnetism or enhanced piezoelectric effects. Furthermore, integrating strain-engineered MoS2 with other semiconducting materials could lead to hybrid structures with bespoke electronic and optical properties, paving the way for advances in next-generation nanoscale devices.

This paper serves as a pivotal reference for researchers interested in leveraging mechanical strain to modulate two-dimensional material properties, encouraging further exploration in the rapidly evolving field of strain engineering and its applications in advanced nanotechnology.