Heterostrain Engineering Twisted and Non-Twisted 2D Bilayers with Process-Induced Strain*

Two-dimensional (2D) bilayers have exotic (opto)electronic properties that may be controlled with strain and twist angle between the two layers. Industrially, process induced strain engineering has long been used to strain Si to enhance electron or hole mobility by capping transistors with stressed thin films. We have applied the same concept in 2D systems by evaporating optically transparent thin films with high stress onto graphene and MoS₂¹. We observed that 2D systems have nonhomogeneous strain transfer in the c-axis, where the interlayer coupling of the given system dictates how many layers strain can propagate from the top strained layer². Graphene and MoS₂ have weak interlayer coupling, therefore heterostrain is present in a bilayer structure as no strain can be transferred to the bottom layer (this layer is fixed to the substrate). Here, we discuss using Raman spectroscopic mapping to probe heterostrain in graphene and MoS₂ homobilayers. We then investigate twisted bilayer graphene structures, where we probe heterostrain by observing shifts in twist induced Raman modes (R-band) with varying thin film force application. Since these techniques are robust at low temperature, this can potentially be used to probe correlated electron physics with respect to heterostrain.

[1] T. Peña et al., 2D Mater. 8, 045001 (2021).

[2] S. A. Chowdhury, et. al.  J. Eng. Mater. Technol. 144, 011006 (2021).