Stereo-Hindrance Engineering of A Cation toward <110>-Oriented 2D Perovskite with Minimized Tilting and High-Performance X-Ray Detection

2D <100>-oriented Dion-Jacobson or Ruddlesden-Popper perovskites are widely recognized as promising candidates for optoelectronic applications. However, the large interlayer spacing significantly hinders the carrier transport. <110>-oriented 2D perovskites naturally exhibit reduced interlayer spacings, but the tilting of metal halide octahedra is typically serious and leads to poor charge transport. Herein, a <110>-oriented 2D perovskite EPZPbBr(4) (EPZ = 1-ethylpiperazine) with minimized tilting is designed through A-site stereo-hindrance engineering. The piperazine functional group enters the space enclosed by the three [PbBr6](4-) octahedra, pushing Pb & horbar;Br & horbar;Pb closer to a straight line (maximum Pb & horbar;Br & horbar;Pb angle approximate to 180 degrees), suppressing the tilting as well as electron-phonon coupling. Meanwhile, the ethyl group is located between layers and contributes an extremely reduced effective interlayer distance (2.22 & Aring;), further facilitating the carrier transport. As a result, EPZPbBr(4) simultaneously demonstrates high mu tau product (1.8 x 10(-3) cm(2) V-1) and large resistivity (2.17 x 10(10) Omega cm). The assembled X-ray detector achieves low dark current of 1.02 x 10(-10) A cm(-2) and high sensitivity of 1240 mu C Gy(-1) cm(-2) under the same bias voltage. The realized specific detectivity (ratio of sensitivity to noise current density, 1.23 x 10(8) mu C Gy(-1) cm(-1) A(-1/2)) is the highest among all reported perovskite X-ray detectors.