Self-trapped excitons in ultrathin PbI2 nanocrystals embedded in a polymer

We have found broad band luminescence due to a self-trapped exciton (STE) and disappearance of a free exciton luminescence in ultrathin nanocrystals of PbI2 embedded in ethylene-methacrylic acid copolymer at 77K. This fact suggests that the STE becomes stable because of a rise in the lowest free exciton level due to size quantization. The Stokes shift in the two- and three- monolayer nanocrystals is found to be larger in comparison with that in more than four monolayer nanocrystals, from luminescence spectra in the size selective excitation. From this larger Stokes shift, it is imagined that a quasi-two-dimensional STE strongly interacts with an acoustic phonon characteristic of the ultrathin crystal. Moreover we observed temporal behavior of the STE luminescence in various excitation intensities. In very weak excitation, the STE luminescence shows a slow single exponential decay with a time constant of 330 mu s. This means that an isolated STE in a nanocrystal is in a spin-triplet state and annihilates radiatively. In strong excitation, the STE luminescence decays faster and shows non-exponential decay. From this fact and the additional fact that the quantum efficiency of the STE luminescence is independent of the excitation intensity, it is thought that wave functions of the nearest two STEs overlap with each other, resulting in a singlet-triplet mixing caused by a spin-orbit interaction.