Ultrafast Charge Carrier Delocalization in CdSe/CdS Quasi-Type II and CdS/CdSe Inverted Type I Core–Shell: A Structural Analysis through Carrier-Quenching Study

Abstract

We have employed femtosecond transient absorption spectrocopy to monitor charge carrier delocalization in CdSe/CdS quasi-type II and CdS/CdSe inverted type I core–shell nanocrystals (NCs). Interestingly, CdSe and CdS QD pairs can make both type I and quasi-type II core–shell structures, depending on their band alignment and charge carrier localization. Steady-state optical absorption and luminescence studies show a gradual red-shift in both optical absorption and emission spectra in CdSe/CdS core–shell with increasing CdS shell thickness. The luminescence quantum yield in CdSe/CdS core–shell drastically increases with shell thickness. Notably, CdS/CdSe inverted core–shell shows a huge red-shift both in absorption and luminescence which closely matches with the band edge photoluminescence (PL) of pure CdSe QDs (shell). However, the luminescence quantum yield does not change much with shell thickness. Depending on their band energy level alignment, the charge carrier (electron and hole) delocalization in bo...