Morphology and temperature dependence of a dual excitonic emissive 2D bromoplumbate hybrid perovskite: the key role of crystal edges

Halide perovskites exhibiting broad band emission have attracted increasing attention since their discovery in 2014 because of their potential in lighting applications. Here, we report a new bromoplumbate organic-inorganic perovskite (C4-E)(2)PbBr4 where C4-E+ is the ethyl-butyrate-ammonium cation crystallizing in the polar space group Ccm2(1) at room temperature. This layered perovskite undergoes two reversible structural transitions at 45 degrees C and 180 degrees C assigned to a structural change and a rare congruent melting, respectively. Its emission properties are morphology dependent since crystals exhibit whitish emission due to a broad band (BB) extending in the whole visible range beside two excitonic emissions (HE and LE), while as thin films obtained by spin-coating the HE excitonic emission is by far the main component. PL measurements performed at different incident angles and different configurations, together with fluorescence microscopy analysis, reveal that the LE emission originates from the edges of crystals and HE from the bulk. PL measurements along with the temperature also revealed the effects of the structural transition on the two excitonic peaks, being partially quenched and red-shifted to 100-130 meV for temperatures above 45 degrees C, while minor changes of the BB emission was observed through the transition. This acentric material exhibits a strong SHG signal at room temperature, similar to the one exhibited by urea.