Spectral and Nonlinear Optical Properties of Quasi-Type II CdSe/CdS Nanotadpoles

CdSe/CdS semiconductor heterojunction nanocrystals (NCs) have important applications in various optoelectronic devices and biological science. However, the relationship between their geometry and photophysical properties has not been fully elucidated. In this work, we synthesized two kinds of water-soluble CdSe/CdS NCs with different geometries but similar volumes, that is, nanotadpoles (NTs) and nanorods (NRs). Compared with NRs, NTs exhibit higher photoluminescence quantum yields and longer lifetimes. In addition, the biexciton Auger lifetime and hot exciton cooling time of NTs are much longer than those of NRs. Measurements of the femtosecond transient absorption spectrum reveal that the NTs demonstrate reduced overlapping of electron-hole wave functions compared with NRs, resulting in a more prominent quasi-type II electronic structure. Consequently, significant differences in photophysical properties between NRs and NTs are observed. Although NTs and NRs have comparable low-temperature excitonic properties, the multiphoton absorption action cross sections of the former are much higher. This work is important for optimizing the photophysical properties of semiconductor heterojunction NCs, thereby increasing their applicability.