Synthesis of Anisotropic CdSe/CdS Dot-in-Giant-Rod Nanocrystals with Persistent Blue-Shifted Biexciton Emission

Anisotropic single-phase wurtzite CdSe/CdS nanocrystals were synthesized by colloidal chemistry, introducing ZnCl2 to increase the shell growth in the radial direction. As a result, dot-in-giant-rod nanocrystals were obtained, with a core diameter that varied between 3.2 and 7.5 nm and an overall diameter between 15 and 22 nm, corresponding to a 14–26 ML CdS shell. In addition to an extended fluorescence lifetime, typical for CdSe/CdS heteronanocrystals, all samples also yielded a blue-shifted biexciton emission peak. This contrasts with existing data on CdSe/CdS dot-in-rod nanocrystals with a thin shell, which yield a type-I band offset and attractive biexciton interactions for CdSe/CdS with a core larger than about 2.8 nm. However, k·p calculations support the blue shift, with a significant electron delocalization into the CdS shell even for large core diameter. We assign this effect to the influence of strain at the CdSe/CdS interface and associated reduction of the conduction band offset, as well as the buildup of a piezoelectric field along the nanorod long axis. The strain-induced electron–hole separation is particularly effective in large-core nanocrystals, providing a tool to engineer electron and hole wave functions that is complementary to quantum confinement.