Ultrafast Electronic Delocalization in CdSe/CdS Quantum Rod Heterostructures

Femtosecond cross-polarized transient grating (CPTG) and polarization anisotropy were used to probe the extent of electronic delocalization in CdSe/CdS quantum rod heterostructures (QRH) with a "dot-in-rod" geometry. The alignment of the bulk valence and conduction band edges of CdSe and CdS suggest a "type I" band configuration, leading to localization of both the electron and hole on the CdSe seed, but size quantization effects make the distinction less clear. Photoexcited electrons in 2.1 and 2.9 nm diameter structures have considerable excess kinetic energy above the CdS conduction band and show clear evidence of electron delocalization into the surrounding shell. However, the dependence of the CPTG decay rate on aspect ratio for 2.9 nm seeded QRHs is minimal, suggesting that the delocalization is mostly isotropic (i.e., not preferentially along the rod length). The rates for the 2.1 and 2.9 nm QRHs fall in line with expected trends based on effective exciton size. The 4.2 nm diameter structures also lack any rod length dependence of the CPTG decay and instead exhibit a biexponential decay that is indicative of coupled pathways for fine structure relaxation, likely due to anisotropic interfacial strain. CPTG is found to serve as a unique tool for determining charge transfer and delocalization in nanoheterostructures, which can rarely be predicted accurately from examination of bulk band offsets.