Role of ZnS Segment on Charge Carrier Dynamics and Photoluminescence Property of CdSe@CdS/ZnS Quantum Rods

Growing a wide band gap shell on bare core and/or core@shell materials is a fascinating idea for improving the photoluminescence (PL) efficiency and stability. An epitaxially grown shell adds another degree of complexity to the system and modulates the excited-state relaxation dynamics, which remain poorly understood. Employing time-resolved PL and femtosecond transient absorption (TA) spectroscopy, we present a thorough study on charge carrier dynamics of CdSe@CdS and CdSe@CdS/ZnS quantum rods (QRs). Various excitation wavelengths were used to identify the contribution of individual segment toward the optical properties of the QRs. Our femtosecond TA measurements provide a clear evidence of excitation migration from CdS as well as ZnS to CdSe core within few picoseconds of photoexcitation. The excitons recombine faster in the CdSe moiety of the CdSe@CdS/ZnS than that of the CdSe@CdS QRs via an extra decay path. The interband trap states that are created via the formation of extended defects because of lattice strain relaxation (or ion exchange during the formation of ZnS segment) in CdSe@CdS/ZnS QRs could provide the additional decay channel leading to low PL intensity and quantum yield. We believe that our study will help to develop a strategy for enhancing the PL efficiency through energy funneling across semiconductor heterojunctions and to understand the charge carrier dynamics in nanoheterostructures