Self-trapping of excitons in PbI2 clusters formed in Pb1-xCdxI2 mixed crystals

Luminescence of PbI2 clusters formed in Pb1-xCdxI2 mixed crystal has been investigated in the range of Pb2+ composition between 20 mol% and 40 mol%: The spectrum consists of a band of free exciton luminescence near the absorption edge and a band with large Stokes-shift. Temperature and excitation energy dependences of the spectra and decay profiles of the luminescence indicate that the low energy band originates from self-trapping of excitons in the clusters. Stability between the free and self-trapped exciton states is governed by the exciton hand width which depends on the size of clusters.