Radiative Decay Time Analysis

From the study of the radiative lifetime, it is thus possible to extract information on the localization parameter More precisely, in the framework of the wavefunction described by Equation 2 and granted that 2 azo, the oscillator strength is given by [61, 62]  

Let us first assume that the center of mass of the exciton is only laterally confined by the AlN barrier (in other words, no supplemental localization inside the QD occurs). In that case, we need to estimate the exciton center of mass extension parameter. In a simple approach, we can consider that the exciton is laterally confined by infinite barriers in a cylinder that has the height of the QD and a diameter corresponding to the diameter of the top facet of the QD. Let us note that this model tends to underestimate the lateral extent of the confined wavefunction. For the center of the QD size distribution (height 1.6 nm, diameter 19 nm), the parameter deduced from this simple approach is 3.4 nm. Equations 3 and 4 thus yield/ = 85 and r = 24 ps. This is clearly a much faster decay than the experimentally measured decay, and thus shows that the expected giant oscillator strength regime is not reached in our structures. 

They are better described by a collection of independent emitters with decay times varying between 70 and 600 ps. To test this hypothesis, we performed time-resolved microphotoluminescence to probe the decay of single QDs. 

Analyzing the temperature dependence of the PL line width in the framework of a quasi-2D wavefunction model and combining it with time-resolved PL experiments, it is concluded that excitons are laterally localized at a length scale smaller than the dot size. No direct correlation was found between monoexponential decay time constants and dot size. As a consequence, it appears that the localization of exciton itself does not simply depend on dot size, making the nature of localization in these nonpolar QDs an issue still open till date. 

We acknowledge the technical assistance of J. Cure and Y. Genuist for epitaxial growth. We thank M. Terrier and Dr D. Peyrade for their help in processing the samples for single dot spectroscopy. The additional help of Dr J. Bleuse for time-resolved spectroscopy measurements is gratefully acknowledged. 

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Energy-level gaps and both radiative and radiationless decay constants have been obtained for a series of Os complexes containing 7r-conjugated ligands. This has been done by computer analysis of the intensity and decay time of the photoluminescence. An ion-parent coupling model has been used to rationalize the empirical parameters and the emitting levels shown to have CT con-... 

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