Towards Strain-Tunning Correlations in van der Waals Heterosrtuctures

Two-dimensional (2D) materials and heterostructures offer a rich environment to study and probe quantum phenomena such as electron correlations, superconductivity, ferromagnetism, and topological effects with great tunability. Controllable in-situ strain in 2D materials offers a novel pathway to control those correlated effects. Transition metal dichalcogenides are one class of 2D materials known for their capability of electronic band tuning and promising photonic applications as they exhibit a direct gap at the monolayer limit. While several studies have been performed on commensurate TMD structures, few strain studies have been performed on twisted structures due to the difficulty in fabricating high-quality samples and the techniques available to apply controllable strain. Here, we present a strain study on twisted homobilayers of WSe2 on hexagonal Boron nitride (BN). The twisted WSe2 structure exhibits a moire lattice that alters the electronic band structure and phonon modes. Placing the twisted homobilayers of WSe2/BN structure on flexible polyethylene terephthalate (PET) substrates enables the application of tunable tensile strain along the axis of substrate bending and orthogonal compressive strain due to the Poisson effect. This provides a controllable method to strain the moire superlattice. We then perform ultralow frequency Raman spectroscopy, photoluminescence spectroscopy, and lifetime measurements to characterize any changes with strain. We observe changes in the low-frequency phonon modes which we attribute to changes in the moire wavelength. Furthermore, we extract the strain applied to the homobilayer WSe2 by a Gruneisen parameter analysis on phonon mode shift in the E2g and A1g modes. We measure an applied strain of roughly 1% in our samples. We carry on this analysis to observe shifts in the photoluminescence energy and exciton lifetime. We observe a small decrease in the exciton lifetime, but no significant shift in exciton energy at room temperature. Knowing the exact amount of strain applied to a moire lattice is pivotal to characterizing the strain-tunability in these structures. Our methods allow the characterization of the amount of strain applied and the modification of the moire patterns via low-frequency Raman measurements. Our results could pave the way toward strain-tunable electron interactions in strongly correlated systems and the study of strain-driven phase transitions in van der Waals heterostructures.