Organic Spacers Modulated Low Dose X-Ray Detection in Hybrid Halide 2D Perovskites: Unveiling Exciton Dynamics

In this study, we investigate how modulating organic spacers in perovskites influences their X-ray detection performance and reveal the mechanism of low-dose detection with high sensitivity using femtosecond-transient absorption spectroscopy (fs-TAS). Particularly, we employ N,N,N',N'-tetramethyl-1,4-phenylenediammonium (TMPDA) and N,N-dimethylphenylene-p-diammonium (DPDA) as organic spacers to synthesize 2D perovskite single crystals (SCs). We find that DPDA-based SCs exhibit reduced interplanar spacing between inorganic layers, leading to increased lattice packing. Density functional theory (DFT) results indicate the reduced effective mass and lower lattice distortion in (DPDA)PbBr₄ suppressing the formation of self-trapped exciton (STEs) and electron-phonon coupling and enhancing carrier delocalization in these SCs. Further, X-ray detection measurements reveal that (DPDA)PbBr₄ demonstrates higher sensitivity than (TMPDA)PbBr₄, attributed to its enhanced carrier delocalization, and higher mobility-lifetime product. The limit of detection (LoD) for (DPDA)PbBr₄ is determined to be 13 nGy/s, significantly lower than both commercial detectors and state-of-the-art perovskite-based X-ray detectors. Furthermore, fs-TAS study reveals that (DPDA)PbBr₄ crystals exhibit prolonged hot STE cooling and decay lifetimes, which directly correlate with their higher sensitivity. This study highlights the impact of organic spacers on X-ray detection performance, providing a framework for designing ultra-low LoD detectors essential for health and security applications.