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 (∼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.