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

Abstract

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.

Why do Si quantum dots with stronger fast emission have lower external photoluminescence quantum yield?

“…5 and Table 2 limit the spectral tunability of the long-lived PL component. In our previous work, 38 we showed that the long-lived PL channel can be tuned in a wide range of emission photon energies. Other reports clearly suggest 11,14–20 that one important factor which limits the spectral tunability of the long-lived component is the exposure to oxygen.…”

“…Since the oscillator strength of the transition decreases as the -space overlap of the electron–hole wavefunction decreases, the latter transitions are relatively slow (≈100 μs) and are responsible for the classical red emission in SiQDs, 7,45,46 which we investigated in more detail in our previous article. 38 In an ideal hypothetical case, the PL measured in an SiQD sample should reflect these two radiative pathways, as sketched in the schematic image in Fig. 1c.…”

“…aSamples marked with an asterisk (*) were shown to be fully radiative in the long-lived PL channel ( η slow = 1) in our previous work, 38 double asterisk (**) shows that the sample underwent a deagglomeration treatment. Photoluminescence quantum yield (QY), the characterization of the short-lived component (average lifetimes τ av and dispersion parameters β ) as well as the spectral position of the PL maximum for the short- and long-lived ( λ short/long max ) component, respectively, are listed.…”

“…The PL decay measurements were made as a function of wavelength and were transformed into emission photon energies in eVs including the 1/ λ 2 spectral correction factor. 42 Using a method described earlier, 38,41 we took into account also the onset of PL in the PL decay curves, which results in much more accurate fits. The emission-photon-energy dependent series of PL decays I ( λ , t ) was thus fitted using a convolution of a Gaussian curve and a fitting function.…”

“…We used the same set of samples as in our previous publication. 38 In brief, we used SiQDs synthesized in a noncommercial flow-through reactor with low-pressure nonthermal plasma, denoted as NTP:SiQDs. For subsequent hydrosilylation (NTP:SiQDs:C), the synthesized powder was collected directly to a dodecene-filled vial in the synthesis reactor and the vial was then placed onto a hot plate with an aluminum heating nest.…”