Surface-Engineered Methylammonium Lead Bromide Single Crystals: A Platform for Fluorescent Security Tags and Photodetector Applications

Surface/interface engineering of methylammonium lead bromide (MAPbBr₃) single crystals (SCs) is crucial for carrier generation/recombination, separation, and transportation, thereby enabling superior optoelectronics. Herein, a surface-engineered crystallization technique is presented in which a thin polycrystalline MAPbBr₃ layer is in situ grown on the surface of bulk mm-sized MAPbBr₃ SCs. The growth direction and the atomic distribution of both the polycrystalline surface layer and the single crystalline core have been analyzed by damage-free atomic-resolution transmission electron microscopy (TEM). The polycrystalline nature of the crystals gives rise to bright green emission under UV light. Scanning probe microscopy results show that the current at the emissive MAPbBr₃ film/MAPbBr₃ SC surface is five times higher than that at a non-emissive surface when illuminated. Lateral photodetectors based on MAPbBr₃ film/MAPbBr₃ SCs can achieve excellent detectivities (≈10¹³ Jones) and high photoresponsivities (≈20 A W⁻¹). Furthermore, a semiconductor-based fluorescence quick response (FQR) code is patterned on the same bright green-emitting surface of the MAPbBr₃ film/MAPbBr₃ SC using a picosecond laser pulse, with a scalability of 3 × 3 mm². An FQR code integrated with a photodetector may serve as a potential double security tag in bank cards, passports, door security, etc.