Inhomogeneous Strain Release during Bending of WS2 on Flexible Substrates

Two-dimensional (2D) materials hold great promise in flexible electronics, but the weak van der Waals interlayer bonding may pose a problem during bending, where easy interlayer sliding can occur. Furthermore, thin films of rigid materials are often observed to delaminate from soft substrates during straining. Here, we study the influence of substrate strain on some of the heterostructure configurations we expect to find in devices, composed of three common 2D materials: graphene, tungsten disulfide, and boron nitride. We used photoluminescence (PL) spectroscopy to measure changes in the heterostructures with strain applied in situ. All heterostructures were fabricated directly on polymer substrates, using materials synthesized by chemical vapor deposition. We observed an inhomogeneous release of strain in all structures, leading to a nonrecoverable broadening of the PL peak and shift of the bandgap. This suggests the need for preconditioning devices before service to ensure stable behavior. A gradual time-dependent relaxation of strain between strain cycles was characterized using time-dependent measurements-an effect which could lead to drift of device behavior during operation. Furthermore, possible degradation was assessed by performing the strain and relax the cycle up to 200 times, where we found little further change after the initial shifts had stabilized. These results have important ramifications for devices fabricated from these and other 2D materials, as they suggest extra processing steps and considerations that must be taken to achieve consistent and stable properties.