The Two-Photon Absorption Cross-Section Studies of CsPbX (X = I, Br, Cl) Nanocrystals.

The CsPbX nanocrystals (NCs) with X = I, Br, Cl, or the mixture of Br:I and Br:Cl in a 1:1 ratio were synthesized and characterized by TEM, DLS, and XRD. Recrystallization of the small luminescent NCs in the metastable cubic phase into bigger orthorhombic nanocrystals was monitored by XRD and identified as the main cause of the nanocolloid coagulation. The recrystallization also leads to a decrease in the photoluminescence quantum yield (QY) of the colloidal solution and shortening of the emission lifetime. The two-photon absorption cross-section σ values calculated from femtosecond Z-scan measurements were compared with those obtained based on the two-photon excited emission technique. These two techniques were shown to be equivalent with the cross-section values calculated per molar mass of CsPbX perovskite being in the range of 10-200 GM depending on the halide anions X. The σ values recalculated for the mole of the NCs were in the range of 10-10 GM, which is in good agreement with values previously reported elsewhere and the σ/M parameter was in the range of 0.01 to 0.33. This study shows the perovskite NCs to be a good nonlinear material with the third-order nonlinear optical susceptibility χ of the NCs on the order of 10 esu.