Fig. 1: Structural and optical characteristics of CsPbI₃ perovskite NCs.

a Orthorhombic perovskite crystal structure of CsPbI₃ NCs. Room-temperature absorption and PL spectra of an ensemble of CsPbI₃ NCs dispersed in hexane. The vertical arrow at 3.1 eV indicates the excitation energy chosen for the acquisition of this PL spectrum. The width (FWHM) of the PL spectrum is ~85 meV. The green histogram displays the distribution of emission energies of 58 single NCs at 4 K. b Annular bright-field scanning transmission electron microscopy image of CsPbI₃ NCs. c Size histogram (left panel) and histogram of aspect ratios (right panel) for these NCs. The black curve is a Gaussian distribution centered at 11.2 nm, with a full-width at half-maximum (FWHM) of 2.4 nm. The mean aspect ratio is 1.13. d Spectral trajectory of a single NC at 4 K, built with 33 consecutive PL spectra recorded over 1 s at an excitation intensity of ~50 W cm⁻². It shows a spectral jump together with a change of spectral structure associated to a transition between exciton emission and trion (charged exciton) emission. e PL spectrum of the same single NC at 4 K, integrated over 57 spectra of 1 s extracted from d. It displays the spectral fingerprints of the exciton (X) band-edge exciton fine structure with its LO-phonon replica, trion (X*) and biexciton (XX) emissions. The correspondence between the XX and X multiplets is shown in the inset. As depicted in the energy level diagram, the transition lines connect the bright biexciton state |XX> with zero angular momentum to the non-degenerate exciton triplet |X>, and the latter to the NC ground state |G>.