Universal Radiative Lifetimes in the Long-Lived Luminescence of Si Quantum Dots

Silicon quantum dots (SiQDs) represent a perspective light emitting material. Here, we show that their typical long-lived photoluminescence can be fully radiative. However, despite the fully radiative nature, the overall yield of light emission from SiQDs is still hindered by dark QDs, the understanding of which is only very limited so far. To address this problem, first, we experimentally quantify the dependence of radiative lifetimes on the emission photon energy and show that this dependence is universal across different types of samples and different laboratories. Second, we use this dependence to quantify the internal photoluminescence quantum yield using simply the emission-photon energy dependence of measured photoluminescence (PL) lifetimes as the input. The knowledge of the internal quantum yield then lets us determine the relative population of dark SiQDs if the external quantum yield is known. The application of our approach to the decoupling of the influence of nonradiative processes and dark quantum dots can be easily applied by other researchers, which will shed more light on the mechanism of PL quenching in dark QDs. Besides focusing on dark QDs, we observe that the PL decays of SiQDs can be non-single-exponential despite being fully radiative and suggest the natural variation of radiative lifetimes as a possible mechanism responsible for the non-single-exponential character of PL decay. Lastly, we emphasize the importance of average lifetimes as the quantity characterizing PL decays, especially in the case of non-single-exponential PL decays in the absence of a generally accepted physical model explaining the PL dynamics.