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 MoTe2 photoluminescence (PL). We found that bilayer MoTe2 can be converted from an indirect-to 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-principle 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 MoTe2 comparable to that of the monolayer counterpart. The additional benefit of a longer emission wavelength makes the bilayer MoTe2 more suitable for Silicon-photonics integration due to the reduced Silicon absorption.

中文翻译：

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双层 MoTe2 中应变诱导的间接到直接带隙跃迁、光致发光增强和线宽减小

二维 (2D) 层状材料为通过应变控制设计电子和光学特性提供了理想的平台，因为它们具有极高的机械弹性和材料特性对机械应变的敏感依赖性。在本文中，结合实验和理论努力研究机械应变对双层 MoTe2 光致发光 (PL) 的各种光谱特征的影响。我们发现双层 MoTe2 可以通过应变工程从间接带隙材料转变为直接带隙材料，从而使光致发光提高 2.24 倍。超过 90% 的 PL 来自直接激子在施加的最大应变下发射的光子。重要的是，我们表明应变效应导致 PL 的整体线宽减少多达 36.6%。我们将线宽的急剧减小归因于应变诱导的各种激子类型之间的复杂相互作用，例如直接明亮激子、三重离子和间接激子。我们关于直接和间接激子发射特征的实验结果可以通过基于第一性原理电子能带结构计算的理论激子能量来解释。一致的理论-实验趋势表明，PL的增强和线宽的减小是直接激子贡献随着应变的增加而增加的结果。我们的结果表明，应变工程可以导致双层 MoTe2 的 PL 质量与单层对应物的质量相当。