The introduction of a mobile and polarized organic moiety as a cation in 3D lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the time scales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast 2D vibrational spectroscopy of methylammonium (MA) lead iodide to directly resolve the rotation of the organic cations within the MAPbI₃ lattice. Our results reveal two characteristic time constants of motion. Using ab initio molecular dynamics simulations, we identify these as a fast (∼300 fs) “wobbling-in-a-cone” motion around the crystal axis and a relatively slow (∼3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials.

The weak photon absorption and fast carrier kinetics of graphene limit its application in photodetection. This limitation can be overcome by introducing photosensitive nanostructures to graphene. Herein we report the synthesis of a ZnO nanorod/graphene heterostructure by a facile in situ solution growth method. By combining the attributes of photosensitive ZnO nanorods and highly conductive graphene, we for the first time demonstrate a highly sensitive visible-blind ultraviolet (UV) sensor based on graphene related heterostructure. The photoresponsibility of the UV sensor can reach 22.7 A W⁻¹, which is over 45,000 fold higher than that of single graphene sheet based photodetectors.