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

Abstract

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

Unconventional photoluminescence upconversion from PbS quantum dots

“…7 It has been predicted that NCs in the strong quantum confinement regime, 8 and the lead salts, in particular, 9 should exhibit enhanced coupling to acoustic phonons. This is supported by recent experimental evidence for large homogenous linewidths at room temperature in PbS NCs, 10,11 which have been directly attributed to enhanced acoustic phonon coupling. 12 These predictions and observations together suggest that small lead chalcogenide NCs may, in fact, provide strong PLUC via a direct intrinsic mechanism: that of a large energy uncertainty due to strong dephasing by acoustic phonons.…”

“…12 This model has given insight into single NC contributions to absorption, PL, and PLE data from ensemble measurements. 10,12 In Fig. 3͑a͒ we show that this model provides an excellent fit to our data, including to the entire PLE spectrum with the PLUC component.…”

“…10 Recently, Lifshitz et al have shown that improving the surface passivation of PbSe NCs can remove a similar Stokes shift. 16 As the spectral properties of PbSe NCs and PbS NCs are closely related, 17 it is therefore likely that improved surface passivation of lead PbS NCs in the strong quantum confinement regime could also remove the Stokes shift of the emission, which would then allow PLUC to be used for cooling.…”

“…The peak of the PL spectrum is Stokes shifted from the band-edge exciton peak in the absorption spectrum. 10 Figure 1͑b͒ reveals the evolution of the PLUC signal by exciting at various points across the excitonic absorption peak ͑indicated by vertical arrows͒. As the excitation proceeds toward lower energies, the PLUC signal emerges as the uppermost limit of the spectra ͑indi-cated by an arrow͒.…”

Four-wave-mixing imaging and carrier dynamics of PbS colloidal quantum dots

“…However densityfunctional calculations 54 show the absence of trap states in the CQD gap, which was confirmed experimentally in photoluminescence measurements. 55 Furthermore, the similar dynamics of core-only and core-shell structures indicates that there are no relevant surface trap states, as they are expected to be suppressed by the shell. If relaxation through defect states is ruled out, an intrinsic mechanism for the nanosecond dynamics is phonon-assisted thermalization between the set of eight S-like exciton ground states ͑see sketch of Fig.…”