Exciton recycling in graded gap nanocrystal structures

We propose a cascaded energy transfer structure made of semiconductor nanocrystals. Funnel-like band gap profiles are realized applying layer-by-layer assembly to CdTe nanocrystals of distinct sizes. Optical excitations efficiently transfer along the band gap gradient and are finally captured by the largest nanocrystals. The photoluminescence yield from the center layer is surprisingly high. The stepwise passing on of excitation energy avoids hot carriers, and the funnel structure recycles otherwise trapped and lost electron-hole pairs.