Strain-Induced Indirect-to-Direct Bandgap Transition, Photoluminescence Enhancement, and Linewidth Reduction in Bilayer MoTe₂

Abstract

Two-dimensional (2D) layered materials provide an ideal platform for engineering electronic and optical properties through strain control because of their extremely high mechanical elasticity and sensitive dependence of material properties on mechanical strain. In this paper, a combined experimental and theoretical effort is made to investigate the effects of mechanical strain on various spectral features of bilayer MoTe 2 photoluminescence (PL). We found that bilayer MoTe 2 can be converted from an indirect to a direct bandgap material through strain engineering, resulting in a photoluminescence enhancement by a factor of 2.24. Over 90% of the PL comes from photons emitted by the direct excitons at the maximum strain applied. Importantly, we show that strain effects lead to a reduction of the overall linewidth of PL by as much as 36.6%. We attribute the dramatic decrease of linewidth to a strain-induced complex interplay among various excitonic varieties such as direct bright excitons, trions, and indirect excitons. Our experimental results on direct and indirect exciton emission features are explained by theoretical exciton energies that are based on first-principles electronic band structure calculations. The consistent theory-experimental trend shows that the enhancement of PL and the reduction of linewidth are the consequences of the increasing direct exciton contribution with the increase of strain. Our results demonstrate that strain engineering can lead to a PL quality of the bilayer MoTe 2 comparable to that of the monolayer counterpart. The additional benefit of a longer emission wavelength makes the bilayer MoTe 2 more suitable for silicon-photonics integration due to the reduced silicon absorption.

Imaging Strain-Localized Single-Photon Emitters in Layered GaSe below the Diffraction Limit

“…As the electron beam approaches the apex of tents A and B, the broad band edge emission between 590 nm (∼2.10 eV) and 620 nm (∼2.0 eV) experiences a slight red shift (up to ∼20 nm), and its intensity decreases gradually. This observation of decreasing indirect-exciton emission intensity with increasing strain is common in 2D layered materials. , At the tent apex, the band edge emission almost vanishes. Meanwhile, sub-bandgap CL bands near 700 nm (∼1.77 eV) and 750 nm (∼1.65 eV) are strongly enhanced and red-shifted with increasing strain.…”

“…However, the DFT model does show that the momentum difference between the CBM and VBM for the strained GaSe is larger than that of unstrained GaSe (Supplementary Figure S10b), which might explain the strain-induced decrease in the band edge CL intensity. In addition, the decreasing emission intensities of indirect excitons with increasing strain are common in 2D layered materials. , …”

Unraveling the origin of dominant exciton and band structure details in bilayer MoTe2

“…It is common for 2D materials, after being transferred onto the desired substrates, to exhibit non-flat surfaces with the presence of bubbles 13 . In first-principle bandgap calculations, strain can lead to a decrease in the energy difference between the Q to K valley in the conduction band 14 . Therefore, the strain induced by bubbles was responsible for the smaller energy difference between the direct and indirect bandgaps and the stronger PL emission typically observed in bilayer MoTe2.…”

Dual Design on Hierarchically Hollow MoTe2/C with Ion/Electron Channel Engineering for High-Performance Sodium Storage

“…As a family member of TMDs, molybdenum telluride (MoTe 2 ) is a very promising type of material for electronic or electrochemical applications. − Similar to its analogues such as MoS 2 and MoSe 2 , MoTe 2 also exhibits excellent sodium-storage performance because of its unique layered structure and rich active sites. − Moreover, MoTe 2 has more potential due to its larger interlayer distance (0.699, 0.646, and 0.615 nm for MoTe 2 , MoSe 2 , and MoS 2 , respectively) and weaker interlayer force, which are beneficial for the rapid Na + intercalation/deintercalation . However, bond breakage and layered structure collapse are also issues that cannot be ignored.…”