Highly efficient luminescence from a hybrid state found in strongly quantum confined PbSnanocrystals

Abstract

We report that high quality PbS nanocrystals, synthesized in the strong quantumconfinement regime, have quantum yields as high as 70% at room temperature. We use acombination of modelling and photoluminescence up-conversion to show that we obtain anearly monodisperse size distribution. Nevertheless, the emission displays a largenonresonant Stokes shift. The magnitude of the Stokes shift is found to be directlyproportional to the degree of quantum confinement, from which we establish that theemission results from the recombination of one quantum confined charge carrier with onelocalized or surface-trapped charge carrier. Furthermore, the surface state energy is foundto lie outside the bulk bandgap so that surface-related emission only commences forstrongly quantum confined nanocrystals, thus highlighting a regime where improved surfacepassivation becomes necessary.