Over the last decade, fully inorganic lead halide perovskite nanocrystals (NCs) have received a lot of attention as active materials for photonic and optoelectronic devices. Despite their high sensitivity to ambient conditions typically inducing irreversible degradation mechanisms, some experiments have evidenced reversible environmental effects, clearing the way for their application as active materials for resistive and optical sensors. In particular, the sensitivity of CsPbBr3 NC thin films to ambient air was demonstrated, noticeable as reversible modulation of the PL and ASE intensities, which is a sign of physical perovskite–air interactions, ruling out degradation effects. The air's humidity determines the solvation of the surface and the hydrophilic ligand's head group, resulting in the formation of surface trap states that modulate the emitted PL intensity; in a vacuum, water molecules can be desorbed, restoring pristine conditions. Moreover, the stimulated emission demonstrated a higher sensitivity (up to 6.5 times higher) to ambient air compared to that of the spontaneous emission, opening the way for the realization of ASE-based optical gas sensors with perovskites.
Since the PL and stability properties of the NCs strongly depend on their surface chemistry and, in particular, on the surfactant molecules used to passivate the surface defects, we performed a systematic investigation of the effects of the NC capping ligands on the ASE and sensing properties of CsPbBr3 thin films. In particular, our experiments were performed on four different samples, representatives of three generations of capping ligands: oleic acid and oleylamine (OAc/OAm) as the first, didodecyldimethylammonium-bromide (DDAB) as the second, and 3-(N,N-dimethyloctadecylammonio)propanesulfonate (ASC18) and lecithin as the third. The lowest ASE threshold was reported for the lecithin-capped NC sample, together with the strongest sensitivity to air. On the other hand, the OA-capped sample, which showed one of the highest ASE thresholds and the lowest sensitivity to air, was demonstrated to be highly stable under strong laser irradiation.
