Perseverance of direct bandgap in multilayer 2D PbI2 under an experimental strain up to 7.69%

The two-dimensional (2D) materials are naturally suitable for various flexible 2D optoelectronic devices, in which the direct band gap perseverance is crucial because the flexibility deformations often cause a bandgap transition and thus break performance of the devices. Most of 2D transition metal dichalcogenides (TMDs) materials such as monolayer MoS2, WS2 and MoSe2 have been thought to be not suitable for flexible optoelectronic devices due to their direct-to-indirect bandgap transition even under a small strain (similar to 1%-2%) for any flexibility deformations. So far, only 2D phosphorene has been theoretically predicted to be able to keep direct bandgap property under a large strain. Here we report a 2D material lead iodide (PbI2) mutilayer with a direct band gap and find by photoluminescence (PL) measurements that it maintains a direct bandgap nature under a large experimental strain up to 7.69%. Theoretical simulations support and explain well our experimental results.