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[(a) and (b)] Pillars of different heights with luminescent dioxide islands on top, etched in silicon substrate. [(c) and (d)] Corresponding time-integrated photoluminescence images, taken with the same exposure time. (a) Closer proximity of the luminescent layer to the substrate leads to (c) less collected signals. Irregular bright spots are conglomerates of nanocrystals left on the surface after the etching.
(Color online) Luminescence yield as a function of pillar height for (a) pillars with diameters less than a wavelength for two excitation regimes and (b) large structures with sizes much more than a wavelength; here the effect of height is not seen because of very small aspect ratios. A PL spectrum from the nanocrystals is shown in the inset.
(Color online) Luminescence decay measurements: (a) stretched-exponential decay is seen with different parameters for nanocrystals on the substrate and on top of the highest pillars; (b) instantaneous lifetime is extracted from (a); the relative change is shown on the inset and reveals a value of .
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