Charge carrier cascade in Type II CdSe-CdTe graded core-shell interface

We report the synthesis, structural characterisation and charge separation behaviour in an interface graded Type II CdSe-CdTe core-shell nanostructure. The gradation was accomplished by the use of two layers consisting of an alloyed composition with increasing Te composition between the CdSe core and finally a CdTe shell grown by the successive ionic layer adsorption and reaction (SILAR) method. The effect of gradation was analysed by steady state UV-Vis absorption, photoluminescence (PL) spectroscopy, time-resolved luminescence and femtosecond transient absorption spectroscopy and was found to be immensely beneficial in improving the quantum yield as compared to an ungraded core-shell. Improvement in charge separation was further ascertained by temperature dependent luminescence studies and time correlated single photon counting studies. Better charge separation behaviour accompanied by a more robust PL yield is indicative of better surface passivation and band alignment for charge carrier funnelling. The reduction in stress was further verified by Raman studies where the Raman peak position was used as an index for stress in the film.