Tailoring Moiré Wavelength in van der Waals Heterostructures via Interfacial Heterostrain from Thin-Film Stress Capping Layers

Due to their high deformability, two-dimensional (2D) materials are ideal candidates for using strain engineering to manipulate the electronic structure and material properties. One system ripe for strain engineering is the moiré superlattice created by stacking nearly commensurate 2D layers. Moiré systems show diverse physics, which are extremely sensitive to the structure and size of the moiré superlattice such as “magic angle” superconductivity in bilayer graphene or strongly bound interlayer excitons in reconstructed transition metal dichalcogenide heterostructure superlattices. Most studies use interlayer twist to generate the moiré superlattice. However, generating an interlayer heterostrain has been proposed as a more flexible tool for inducing the moiré superlattice by modulating the relative lattice constant between the layers. Developing new strategies to induce and tailor heterostrains will open up new dimensions of materials design in moiré superlattices.<br/><br/>An enormous opportunity comes by borrowing techniques developed for strain engineering in the semiconductor industry to apply strain to 2D materials by depositing thin film stress capping layers i.e., MgO, MgF₂, etc. [1]. These approaches apply the strain from the top down, suggesting the enticing possibility for controlled heterostrain in 2D heterostructures.<br/><br/>Here, we investigate strain transfer across the van der Waals interface in twisted MoS₂-WSe₂ heterostructures. We deposited MgO through a shadow mask onto a MoS₂-WSe₂heterostructure, to generate a patterned array of squares with regions covered and not covered by the stressor. We used Raman spectroscopy to measure the E¹_2g and A_1g peak positions between stressed and unstressed regions on the heterostructure and deconvolved the applied strain and doping effects in the MoS₂ layer by extracting the relative shift in both peaks. To quantify the heterostrain between the constituent layers, we deposited the stressor on two opposite stacking configurations: MoS₂(top)-WSe₂(bottom) and WSe₂(top)-MoS₂(bottom). At the center of a square with 30 nm thick MgO, when MoS₂ is the top layer, the applied tensile strain reaches up to 0.8%. In contrast, when MoS₂is the bottom layer, tensile strain is limited to less than 0.2%, demonstrating that strain transfer to the bottom layer is minimal due to slip at the 2D interface. The magnitude of the applied strain scales with the thickness of the stressor layer, showing the ability to tailor the heterostrain in 2D heterostructures by more than 0.6%/layer.<br/><br/>To understand how quickly the interlayer heterostrain equilibrates away from the localized stressor layer, we performed hyperspectral Raman mapping and generated spatial maps of the local strain and doping. We found that for the top layer, there is a sharp drop in strain at the edge of the stressor, but some strain extends significantly beyond the edge, taking up to 12 μm to reach equilibrium generating an in-plane heterostrain gradient of ~0.06% strain/μm. This indicates that the low friction at van der Waals interfaces allow heterostrains to propagate for long distances.<br/><br/>Heterostrains show strong potential for tailoring the moiré superlattice wavelength: a 1% strain in an aligned bilayer is equivalent to a 0.6 degree twist, while 1% strain can modify the moiré wavelength in 1° twisted bilayer by more 10%. This is the regime where much of the most interesting physics and interfacial mechanics occurs. We will explore the potential for using dark field electron microscopy or scan probe techniques to directly measure modifications to the moiré wavelength induced by heterostrain.<br/><br/>Taken together, heterostrains via stress capping layers show many new opportunities for spatially patterned or anisotropic moiré superlattices.<br/><br/>References:<br/>[1] Peña, T., et al., Strain engineering 2D MoS₂ with thin film stress capping layers. <i>2D Materials</i> 2021, 8, 045001.