Fig. 3. Anhamonicity in FAPbX₃ PNCs.

a The FFT power spectrum of the FAPbBr₃ PNCs at 510 nm and FAPbI₃ PNCs at 695 nm probe, respectively, computed over the first 10 ps time delay. The vertical dot-dashed line at 75 cm⁻¹ separates two regions of interest. The data shown are averaged over two datasets obtained from twice-repeated experimental observations. b Schematic of bending and stretching modes (red and blue arrows, respectively) in FAPbBr₃. c Schematic illustration of potential energy as a function of internuclear distance along with Pb-X bending (A-mode) coordinate, showing the extent of anharmonic character in FAPbBr₃ and FAPbI₃, as interpreted from the FFT analysis for the A-mode. D_e stands for the bond dissociation energy.