Bandgap Engineering of Strained Monolayer and Bilayer MoS2 (1305.3880v2)

Abstract: We report the influence of uniaxial tensile mechanical strain in the range 0-2.2% on the phonon spectra and bandstructures of monolayer and bilayer molybdenum disulfide (MoS2) two-dimensional crystals. First, we employ Raman spectroscopy to observe phonon softening with increased strain, breaking the degeneracy in the E' Raman mode of MoS2, and extract a Gr\"uneisen parameter of ~1.06. Second, using photoluminescence spectroscopy we measure a decrease in the optical band gap of MoS2 that is roughly linear with strain, ~45 meV% strain for monolayer MoS2 and ~120 meV% strain for bilayer MoS2. Third, we observe a pronounced strain-induced decrease in the photoluminescence intensity of monolayer MoS2 that is indicative of the direct-to-indirect transition of the character of the optical band gap of this material at applied strain of ~1.5%, a value supported by first-principles calculations that include excitonic effects. These observations constitute the first demonstration of strain engineering the band structure in the emergent class of two-dimensional crystals, transition-metal dichalcogenides.

• The paper demonstrates that uniaxial tensile strain reduces the bandgap of MoS₂ by 45 meV/% in monolayers and 120 meV/% in bilayers.
• It employs micro-Raman and photoluminescence spectroscopy to reveal phonon softening and a direct-to-indirect bandgap transition at around 1% strain.
• The findings highlight strain engineering as a promising route for tunable electronic and photonic applications in two-dimensional semiconductors.

Bandgap Engineering of Strained Monolayer and Bilayer MoS₂

The paper "Bandgap Engineering of Strained Monolayer and Bilayer MoS₂" explores the effects of uniaxial tensile strain on the phonon spectra and band structures of monolayer and bilayer molybdenum disulfide (MoS₂) crystals. Through a series of sophisticated experiments, this research investigates how mechanical strain can be utilized to alter the electronic properties of these two-dimensional semiconductors, particularly focusing on their bandgaps and photoluminescent characteristics.

The paper employs micro-Raman spectroscopy and photoluminescence spectroscopy to assess the structural and optoelectronic changes in MoS₂ under strain. The authors report that increasing strain leads to phonon softening and a notable reduction in the bandgap energies for both monolayer and bilayer versions. Quantitatively, the photoluminescence spectra reveal a bandgap redshift in monolayer MoS₂ of approximately 45 meV/% strain and 120 meV/% strain for bilayer MoS₂. This strain also induces a transition in the optical bandgap of monolayer MoS₂ from a direct to an indirect character at strains of around 1%, as demonstrated by a discernible decrease in photoluminescence intensity. These results indicate that mechanical strain is an effective tool for modifying the band structures in two-dimensional transition metal dichalcogenides, opening pathways for tunable electronic and photonic applications.

The implications of these findings are consequential. Strain engineering provides a method to tailor the electronic properties of MoS₂ and potentially other similar materials, expanding their applications spectrum in electronics and optoelectronics. Devices such as transistors, photodetectors, and LEDs could be limited by their bandgap properties, yet strain-induced bandgap modulation can circumvent these limitations by facilitating control over the electronic and optical behavior of materials. Additionally, understanding strain effects can lead to devices operating with improved carrier mobility and efficiency in energy conversion applications.

The paper highlights a differential strain response between monolayer and bilayer MoS₂, which is crucial for the development of substrate integration techniques and strain manipulation strategies in nanoscale devices. The reported Grüneisen parameter of ~1.06 for MoS₂ is relatively moderate compared to graphene, pointing to differences in the anharmonic nature of molecular potentials within these materials.

Future research could build on these results by exploring higher strain ranges beyond those investigated in this paper or by experimenting with other transition metal dichalcogenides like MoSe₂, WS₂, or WSe₂. Additionally, the interplay between strain, excitonic behavior, and spin dynamics in these materials remains an exciting avenue, potentially leading to novel quantum phenomena or spintronic applications. In particular, the direct-to-indirect bandgap transition poses intriguing questions about the fundamental optical processes in strained two-dimensional materials.

Overall, this paper contributes significantly to the understanding of strain-driven bandgap modifications in MoS₂. It establishes strain engineering as a versatile technique in the arsenal of materials science, providing a platform for both fundamental research and practical advancements in the development of next-generation electronic and optoelectronic devices.